ABSTRACT

The Jurassic Norphlet Formation of the Deep-Water Eastern Gulf of Mexico: A

Sedimentologic Investigation of Aeolian Facies, their Reservoir Characteristics, and their Depositional History

Scott W. Douglas, M.S.

Thesis Chairperson: Stephen I. Dworkin, Ph.D.

This study defined the sedimentary facies, reservoir architecture, and depositional

history of the Jurassic aeolian Norphlet Formation in the deep-water Eastern Gulf of

Mexico. Seven depositional facies were identified and grouped into four facies

associations, three of which being primary aeolian facies: 1) Grainflow, exhibiting the

highest permeability and dip angles; 2) Wind-Ripple, exhibiting moderate permeability

and dip angles; and 3) Wet Interdune, exhibiting the lowest permeability and dip angles.

Associated extradunal fluvial facies were identified in the lower Norphlet interval. All

facies displayed heterogeneous attributes of porosity and permeability. Permeability has

been reduced by various diagenetic cements in the Norphlet.

Stratigraphically equivalent aeolian stabilization surfaces were identified within

the Norphlet. The reconstructed Norphlet depositional model demonstrates a relatively

dry, mature, northwestward migrating aeolian erg with the Middle Ground Arch highland

sediment source to the east, and presumably an ancestral Gulf of Mexico to the west.

Page bearing signatures is kept on file in the Graduate School.

The Jurassic Norphlet Formation of the Deep-Water Eastern Gulf of Mexico: A Sedimentologic Investigation of Aeolian Facies, their Reservoir Characteristics, and their

Depositional History

by

Scott W. Douglas, B.S.

A Thesis

Approved by the Department of Geology

___________________________________ Steven G. Driese, Ph.D., Chairperson

Submitted to the Graduate Faculty of

Baylor University in Partial Fulfillment of the Requirements for the Degree

of Master of Science

Approved by the Thesis Committee

___________________________________

Stephen I. Dworkin, Ph.D., Chairperson

___________________________________ John B. Wagner, Ph.D.

___________________________________

Steven G. Driese, Ph.D.

___________________________________ Stacy C. Atchley, Ph.D.

___________________________________

Sascha Usenko, Ph.D.

Accepted by the Graduate School December 2010

___________________________________

J. Larry Lyon, Ph.D., Dean

Copyright © 2010 Scott W. Douglas

All rights reserved

TABLE OF CONTENTS

LIST OF FIGURES v

LIST OF TABLES vii

ACKNOWLEDGMENTS viii

CHAPTER ONE – Introduction 1

Overview and Objectives 1

Norphlet Geologic Setting 3

Dataset and Approach 9

CHAPTER TWO – Depositional Facies 11

Aeolian Facies Descriptions 13

Facies 1: Avalanche-Dominated Grainflow 13

Facies 2: Wind-Ripple-Interbedded Grainflow 14

Facies 3: Cross-laminated Wind-Ripple 14

Facies 4: Planar-laminated Wind-Ripple 16

Facies 5: Damp Interdune 16

Facies 6: Wet Interdune 16

Facies 7: Reworked Aeolian 18

Norphlet Facies Associations 19

Depositional Facies Summary 20

CHAPTER THREE – Analysis of Reservoir Characteristics 21

Facies-Controlled Porosity and Permeability 21

Norphlet Detrital Texture Analysis 24

iii

Grain Size 24

Grain Sorting 24

Grain Rounding 25

Textural Summary 25

Norphlet Diagenesis 27

Reservoir Characteristics Summary 31

CHAPTER FOUR – Norphlet Depositional History 33

Seismic Analysis 33

Core-Log Correlation 35

Sedimentary Provenance 37

Depositional History 39

Dune Morphology and Wind Regime 40

Norphlet Analogs 41

Depositional Models 43

CHAPTER FIVE – Conclusions 47

APPENDIX – Supplementary Figures 50

REFERENCES 54

iv

LIST OF FIGURES

1. Norphlet Formation study area map 2

2. Stratigraphic column of study interval 3

3. Late Jurassic paleogeographic map 4

4. Depositional model of Norphlet 5

5. Seismic isopach map of dunes in Mobile Bay 6

6. Norphlet paleogeographic reconstruction 7

7. Jurassic Oxfordian tectonic map of the Gulf of Mexico region 8

8. Schematic diagram of aeolian deposits 13

9. Navajo Sandstone Outcrop showing aeolian Facies 1-4 15

10. Modern Namibia dune showing aeolian facies formation 15

11. Modern Namibia interdune with crinkly beds of (Facies 5) 17

12. Modern Great Sand Dunes ephemeral flooding in interdune in (Facies 6) 17

13. Modern Namibia coastal region where seawater is reworking aeolian (Facies 7) 18

14. Norphlet porosity and permeability against facies 22

15. Facies controlled porosity and permeability at Mobile Bay 23

16. Permeability heterogeneity in Rotliegendes sandstone core 23

17. Relation of sandstone texture to porosity and permeability 26

18. Representative thin section photos of diagenetic cements 28

19. Norphlet diagenetic phase chart 30

20. Seismic profile through wells in study area 34

21. Thin section with detrital constituents in Norphlet 38

v

22. Norphlet sandstone classification ternary diagrams 38

23. Progression of Norphlet dune morphology change 41

24. Plot of grainflow thickness against dune slipface height 42

25. Namibia modern analog 44

26. Cedar Mesa ancient analog 45

27. Cedar Mesa analog to lateral field changes 45

28. Norphlet depositional model 46

A1.1 Aeolian facies summary table 51

A1.2 Aeolian processes of deposition diagram 52

A1.3 Schematic diagram of aeolian bounding surface 52

A1.4 Schematic diagram of modern dune morphologies 53

vi

LIST OF TABLES

1. Core-described facies table 12

vii

viii

ACKNOWLEDGMENTS

First and foremost, I want to thank Nexen Petroleum USA, Inc. for providing me

with the project dataset used for my thesis. This project would not have been possible

without it. More specifically, I want to thank Dr. John Wagner, Chief Geologist at

Nexen, who personally set me up with this research and with an internship allowing me

access to all the data necessary for the project. Dr. Wagner has been a great mentor

throughout the entire project. Thanks to the many others at Nexen who contributed to

different parts of the project during my internship, including Tom Walker, Nils Andresen,

Jim Colliton, John Davis. I also want to acknowledge Reed Glasmann at Willamette

Geological Service.

Of course, this thesis project would not be possible without the many people at

Baylor University who helped me write and organize my thesis properly and provided

insight to the content as well. Most notably, I want to thank Dr. Steve Dworkin, my

advisor at Baylor, who took me on as a student and worked tirelessly with me on editing

and organizing my thesis, and who was always open to discuss ideas for its direction.

Thanks to Dr. Stacy Atchley, who helped with the organization and flow of my thesis.

Last but not least, I want to thank Dr. Steven Driese for being supportive throughout the

writing process and who provided geologic insight on my project.

CHAPTER ONE

Introduction

Overview and Objectives

Ancient aeolian depositional systems like the Norphlet Formation of the Gulf

Coast United States have been studied extensively because of their thick accumulations in

the rock record, large aerial extent, and potential to store large volumes of hydrocarbons.

Aeolian deposits have proven to be effective reservoirs due to preservation of high

porosity with interconnected pore throats and high net-to-gross sand proportions.

However, even though aeolian systems are primarily sand, they are characterized by an

internal heterogeneity that is challenging to understand due to rapid changes in both

vertical and horizontal stratification. These rapid changes occur at various scales,

ranging from small incremental changes (laminations) between grainflow and wind-

ripple deposits to changes between dune and interdune facies.

The Jurassic Norphlet Formation is a widespread buried aeolian sandstone first

discovered in Norphlet, Arkansas in 1922 with an aerial extent stretching thousands of

square kilometers across the states of Louisiana, Mississippi, Alabama, and Florida, and

into the waters of the Gulf of Mexico (Mancini et al., 1985). The full extent of the

Jurassic aeolian system is not known, although the boundaries onshore are fairly well

constrained. Hydrocarbon accumulation is attributed to the Jurassic Smackover

Formation, which acts as both the source and seal for underlying Norphlet aeolian

reservoirs (Tew et al., 1991). Drilling and exploration in the deep-waters of the Eastern

Gulf of Mexico utilizing high-resolution 3D seismic data has led to further seaward

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extension of the prolific aeolian system into water depths greater than 5000 feet (Godo et

al., 2006). This study summarizes the published information of the Norphlet Formation

located in the subsurface of the deep-water Eastern Gulf of Mexico (Figure 1).

Figure 1. Norphlet Formation study area in the deep-water Eastern Gulf of Mexico. Depths in the study area are around 2000 meters, and about 150 km from the Mississippi River Delta (modified from Salvador, 1991)

Although well studied onshore and offshore (e.g., McBride, 1981; Mancini et al.

1985, 1986, 1990; Tew et al., 1991; Dixon et al., 1989; Story, 1998; Kugler and Mink,

1999; Ajdukiewicz et al., 2010), little is known about the Norphlet beneath the deep-

waters of the Eastern Gulf of Mexico. Determining the relationships between facies and

reservoir properties such as porosity, permeability, texture, and petrophysical response

will aid in future production of these aeolian reservoirs. Additionally, the ability to

predict lateral facies extent will assist in further exploration of the deep-water Norphlet.

This thesis will investigate the following general objectives about the Norphlet in

the deep-water Eastern Gulf of Mexico: 1) Characterize the aeolian depositional facies

within the Norphlet; 2) Establish relationships between facies and reservoir

2

characteristics; 3) Understand effect of diagenesis on facies porosity and permeability; 4)

Determine the depositional setting of the Norphlet and reconstruct the paleo-dune field

(erg) based on existing data.

Norphlet Geologic Setting

During the late Triassic, rifting of the supercontinent Pangea was initiated and the

Gulf of Mexico basin began to form (Jacques and Clegg, 2002). Rifting continued

through the middle Jurassic, allowing seawater to enter the restricted ancestral Gulf of

Mexico. These waters evaporated leaving behind thick evaporite deposits known today

as the Louann Salt (Figure 2; Salvador, 1991; Bird et al., 2005).

Figure 2. Stratigraphic column of geologic units found in the eastern Gulf of Mexico region with study interval highlighted. The thick Louann Salt evaporites are in red. The Norphlet aeolian sandstone is in yellow. The overlying Smackover carbonates are in blue (modified from Mancini et al., 1986).

3

A regional paleogeographic reconstruction (Blakey, 2010) during late Jurassic

time (170 Ma) shows the ancestral Gulf of Mexico restricted from the open ocean (Figure

3). A eustatic regression following the deposition of the Louann evaporites during the

late Jurassic resulted in an ideal arid environment for the Norphlet Formation (Oxfordian

stage, 160 Ma) aeolian deposits to develop along the exposed continental shelf (Figures 2

and 3; Mancini et al., 1990).

Figure 3. Regional Paleogeographic map during the Mid-Late Jurassic. Pangea is breaking apart allowing waters to enter the ancestral Gulf of Mexico and evaporate forming the Louann Salt. The Norphlet Formation is not developing at this time. Close proximity to paleo-equator indicates a warm climate during this time (170 ma; modified from Blakey, 2010).

The Norphlet Formation shows the marked change in sand thickness from a few

feet to around 1000 feet, depending on the proximity to the Norphlet erg margin. Thicker

deposits are located within the erg axis. Mancini et al. (1985) reconstructed a

depositional model of the Norphlet at Mobile Bay (Figure 4). The southern Appalachian

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Mountains were paleo-topographic highs during this time that provided sediment to the

Norphlet system (Figure 4; Tew et al., 1991). Norphlet alluvial and fluvial sediments

deposited in close proximity to these mountains. Wind processes transported and

deposited aeolian sediment across the arid landscape towards the southwest (Figure 4B;

Mancini et al., 1985, 1986).

Figure 4. Depositional reconstruction of Norphlet from Appalachian highlands to Mobile Bay during the Jurassic showing structural highlands shedding alluvial sediments to the southwest, which are reworked by aeolian processes (Mancini et al., 1985, 1986).

Seismic mapping of the Norphlet aeolian deposits at Mobile Bay (Story, 1998;

Ajdukiewicz et al., 2010) shows linear dune morphologies preserved within the

subsurface oriented north-south indicating a northward paleo-wind-direction (Figure 5).

5

Trends within this linear dune system have permeability values that are higher when

parallel to the linear dune crests (Figure 5; Story, 1998).

5 km

Figure 5. Seismic isopach map showing linear dune bedform pattern at Mobile Bay. Anisotropy is demonstrated by these morphologies, with higher permeability parallel to the dune crests (Story, 1998).

The depositional model of the Norphlet and suggested aeolian dune extent shown

in Figure 4 will be extended seaward due to the recent Norphlet discoveries within the

deep-waters of the Eastern Gulf of Mexico. A new paleogeographic reconstruction is

proposed based on the interpretation of known Norphlet maps From Mancini et al.,

(1985, 1990) and a recent publication by Lovell, 2010 that extends the Norphlet further

south (Figure 6). Within the study area, the nearby basement structure, known as the

Middle Ground Arch, was previously documented and mapped by Dobson and Buffler

(1991). The Middle Ground Arch was a Jurassic highland area, which is interpreted to

have provided a sediment source for this southern Norphlet system. Alluvial, fluvial, and

aeolian sediments are interpreted to be deposited adjacent to the highland areas associated

with the arch (Figure 6). Much is still unknown about the boundaries of the system,

6

particularly where the coastline between the Norphlet aeolian deposits and the Jurassic

Gulf of Mexico Sea is, or if it is present at this time.

Figure 6. Paleogeographic map of the Norphlet about 160 Ma (Lovell, 2010). The Norphlet includes alluvial, fluvial, and aeolian deposits shed off of Jurassic mountain areas. Connectivity between each proven Norphlet aeolian system is uncertain. In addition, the location of the Norphlet coastline is also unknown. The map also shows source terrains providing sediment to the system.

During the late Oxfordian Stage (158 Ma; Figure 2), the Norphlet Formation was

buried and eroded by a transgression associated with the Smackover Formation (Mancini

7

et al., 1990). The Smackover is characterized by shallow water carbonates, whose sharp

basal contact suggests reworking and removal of an unknown amount of the Norphlet

deposits during this transgression. The breakup of Pangea around this time increased

accommodation space within the Gulf of Mexico basin, allowing for thick accumulations

of sediment to fill the basin throughout the Mesozoic and Cenozoic. The weight of these

sediments caused structural deformation of the ductile Louann Salt throughout much of

the Gulf of Mexico (Salvador, 1991). Salt remobilization created many structural

adjustments within and fragmentation of the deep-water Norphlet deposits, which are

located overtop thick Louann Salt deposits (Figures 2 and 7). The extent of the Louann

Salt is large, as the Gulf of Mexico is actively spreading apart during the Oxfordian

(Figure 7).

Figure 7. Tectonic Map of Gulf of Mexico during the Late Jurassic Oxfordian Stage. Pangea was splitting apart along the active ridges (red) allowing for marine waters to enter into a newly formed Gulf of Mexico and to evaporate, forming the Louann Salt (Modified from Pindell and Kennan, 2000).

8

Dataset and Approach

In addition to the published data available on the Norphlet, this project included

observations from whole core and petrophysical data. Petrophysical data included: 1)

gamma-ray logs that measure gamma radiation; 2) neutron porosity and bulk density

logs, which measure pore space; 3) formation resistivity logs, which measures resistance

to flow; and 4) Oil-Based MicroImage (OBMI) logs, which are used to generate dip

direction and azimuth diagrams. This project also utilized data gathered from high-

resolution three-dimensional seismic surveys of the area.

The project approach included of the following steps:

1) Compile literature from historical aeolian deposits, with emphasis on previously

studied Norphlet successions, in order to develop a clear understanding of aeolian

systems, particularly within the Norphlet.

2) Describe the facies observed in core using conventional core description

techniques. This involves determining the sedimentary features that separate each

facies from one another.

3) Understand the diagenesis of the Norphlet with respect to reservoir properties.

4) Tie petrophysical data into core-calibrated facies to determine relationships

between facies and well log responses.

5) Use the high-resolution three-dimensional seismic data to correlate and map out

the shape and size of the Norphlet subsurface extent.

6) Identify both modern (Namib Desert, southwestern Africa) and ancient (Permian

Cedar Mesa Sandstone, southeastern Utah) aeolian analogs in order to better

understand the lateral and vertical changes we may expect within the Norphlet

9

system. These analogs will aid in predicting dune morphology, determining the

lateral extent, and better understanding the overall sedimentary architecture.

7) Reconstruct a depositional model of the migrating dune field, including the

original dune height from grainflow thickness, paleo-directional depositional

trend, and overall lateral distribution of the erg axis vs. erg margins within the

aeolian environment.

10

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CHAPTER TWO

Depositional Facies

Depositional facies present are all aeolian and included avalanche-dominated

grainflow, wind-ripple influenced grainflow, cross-laminated wind-ripple, planar-

laminated wind-ripple (dry interdune), damp interdune, wet interdune, and reworked

aeolian (Table 1). A wide variety of aeolian dune, interdune, and associated extradunal

depositional facies similar to those in the Norphlet have been observed by Glennie

(1970), McKee (1979), Ahlbrandt and Fryberger (1981a), Ahlbrandt and Fryberger

(1981b), Fryberger et al. (1983), Driese (1985), Rubin (1987), Fryberger et al. (1990a),

Fryberger et al. (1990b), Kocurek (1991), and Mountney (2006a), (complete list in Figure

A1.1).

Depositional facies were combined into four facies associations based on

similarities between sedimentary stratification and environmental interpretation. These

include the Grainflow, Wind-Ripple, Wet Interdune, and Reworked Aeolian Facies

Associations (Table 1). A schematic diagram representing these facies associations’

relative location to each other in aeolian systems is shown in Figure 8.

In addition to the depositional facies characterized of dunes (grainflow, wind

ripple, wet interdune), extradunal facies (e.g., wadi, sabkha, sand sheet, fluvial, alluvial

fan, etc.) are also believed to be associated with the Norphlet Formation and are likely

laterally adjacent to the aeolian facies.

Table 1. Facies Summary Table for the Norphlet Formation. Facies names and detailed descriptions are given in text. Seven facies identified in core were combined into four facies association based on similar stratification patterns.

Facies Number Depositional Facies

Facies Stratification (Interpretation) Abbreviation Facies Associations

1 Fine- to medium-grained, cm-scale, planar-tabular cross-bedded sandstone (20-35o dip)

Avalanche-Dominated Grainflow (High-Angle Dune)

GA

2 Fine- to medium-grained, mm- to cm-scale, cross-bedded sandstone (10-25o dip)

Grainflow with interbedded Wind-Ripple (Low-Angle Dune)

GW

1- Grainflow (G)

3 Fine- to medium-grained, mm-scale, cross-laminated, inversely graded sandstone (pinstripe, 5-10o dip)

Wind-Ripple cross-laminated (Dune Plinth)

WRc

4 Fine- to medium-grained, mm-scale, planar-laminated, inversely graded sandstone (pinstripe, 0-4o dip)

Wind-Ripple planar-laminated (Dry Interdune)

WRp

2- Wind Ripple (WR)

5 Very fine- to coarse-grained, poorly sorted, mm- to cm-scale, crinkly-laminated argillaceous sandstone

Wavy or crinkly laminated water-table influenced deposits (Damp Interdune)

DI

6 Fine- to coarse-grained, mm-scale, rippleform cross-laminated sandstone

Current ripple ephemeral flood or fluvial deposits (Wet Interdune)

WI

3- Wet Interdune (WI)

7 Fine- to medium-grained, mm- to cm-scale, irregularly deformed, cross-bedded sandstone

Marine flooded and reworked aeolian deposits (Reworked Aeolian)

RW 4- Reworked (RW)

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Figure 8. Grainflow, wind-ripple, and interdune deposits represented within an aeolian system. Wind-ripple deposits occur along any surface, but are preserved at the toe-sets of the grainflow deposits. Grainfall is reworked into grainfall deposits as the avalanche down a steep dune slope (Modified from Wagner and Moiola, 2010)

Aeolian Facies Descriptions

Facies descriptions illustrate changes in facies, facies associations, grain size,

sedimentary structures, lithology, and bed dip angles. Higher angle dune sets are

sometimes punctuated with slump features, which may be due to over-steepening,

wetting events, or even earthquakes (Doe and Dott, 1980). Each facies will be discussed

in detail in this chapter. Drying-upward cycles of sediment accumulation are represented,

which will be discussed further in Chapter Four.

Facies 1: Avalanche-Dominated Grainflow

Facies 1 is a fine- to medium-grained, cm-scale, planar-tabular cross-bedded

sandstone (Table 1). Facies 1 is characterized by strata that dip at high angles (>20o)

with fine- to medium-grained, cm-scale, sometimes faintly inversely graded grainflow

beds commonly separated by a single thin, fine-grained lamina of grainfall deposits.

Regular alternations of grainflow and grainfall layers in Facies 1 are observed in outcrop

(Figure 9). Facies 1 is interpreted as representing the high-angle slipface of a migrating

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dune (Figure 10). Interpretations of this facies are based on similar observations by

Hunter (1977), Mckee (1979), Ahlbrandt and Fryberger (1981a), and Mountney (2006a).

Facies 2: Wind-Ripple-Interbedded Grainflow

Facies 2 is a fine- to medium-grained, mm- to cm-scale, cross-bedded sandstone

(Table 1). Facies 2 is characterized by strata that dip at moderate to high angles (10-25o),

with fine- to medium-grained, cm-scale grainflow beds interbedded with inversely graded

wind-ripple laminations. Irregular alternations of grainflow and wind ripple beds in

Facies 2 are observed in outcrop (Figure 9). Facies 2 is interpreted to be lower slipface

deposits along the lee slope of dune sets where grainflow tongues are interfingering with

the wind-ripple strata of the plinth of the dune (observed by Hunter, 1977; Mountney,

2006a).

Facies 3: Cross-Laminated Wind Ripple

Facies 3 is a fine- to medium-grained, mm-scale, cross-laminated, inversely

graded sandstone (Table 1). Facies 3 is characterized by low to moderate angle (5-10o)

inversely graded wind-ripple laminations. Inclined wind-ripple strata of Facies 3 are

observed in outcrop (Figure 9). Facies 3 is interpreted to be plinth deposits (Mountney,

2006a) dominated by wind ripple strata formed along the lower part of the migrating

dune. This facies would be found below where grainflows are pinching out into the

wind-ripple strata, but above flat lying interdune strata (Figure 9; Ahlbrandt and

Fryberger, 1981a; Fryberger et al., 1990a; Kocurek, 1991).

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Figure 9. Ancient outcrop of Navajo Sandstone (Jurassic) with Facies 1-4 represented. Facies 1, Avalanche-Dominated Grainflow, represents high-angle dune deposits. Facies 2, Grainflow with Interbedded Wind-Ripple represents the moderate angle dune deposits. Facies 3, Cross-Laminated Wind-Ripple, represents moderate angle wind-ripple deposits. Planar Laminated Wind-Ripple represents low angle wind-ripple deposits.

Figure 10. Modern example of aeolian facies. Dune is migrating over dry-interdune (wind-ripple). Grainfall deposits are blown over dune crest, deposit on the leeward side of the dune, and avalanching (grainflow) carries the grainfall deposits downslope (Namibia; modified from Mountney, 2006a). For a detailed diagram of modern aeolian processes, see Figure A1.2.

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Facies 4: Planar-Laminated Wind Ripple

Facies 4 is a fine- to medium-grained, mm-scale, planar-laminated, inversely

graded sandstone (Table 1). Facies 4 is characterized by flat lying (0-4o), inversely

graded wind-ripple laminations. Flat-laying wind-ripple strata in Facies 4 are observed in

outcrop (Figure 9). Facies 4 is interpreted to be a dry interdune deposit where wind-

ripples dominate, and there is little to no water table influence (Figure 10). Some thicker

deposits of Facies 4 may be interpreted as a sandsheet (Fryberger et al, 1990a; Ahlbrandt

and Fryberger, 1981a; Mountney, 2006a; Hunter, 1977).

Facies 5: Damp Interdune (Crinkly-Laminated Sandstone)

Facies 5 is a very fine- to coarse-grained, poorly sorted, mm- to cm-scale, crinkly-

laminated argillaceous sandstone (Table 1). Facies 5 is characterized by wavy or crinkly

laminated, low angle beds representing water-influenced deposits. Facies 5 is interpreted

to have formed in a damp interdune where capillary forces of the water table caused the

surface to be damp, trapping windblown suspended silt and clay (Ahlbrandt and

Fryberger, 1981a, b; Fryberger et al., 1990a). Evaporation of water on the interdune

surface during deposition likely caused the beds to become crinkled and deformed

(Figure 11; Ahlbrandt and Fryberger, 1981b). This facies also includes damp adhesion

ripples, similar to the deposits documented in Namibia by Kocurek and Fielder (1982).

Facies 6: Wet Interdune (Rippleform-Laminated Sandstone)

Facies 6 is a fine- to coarse-grained, mm-scale, rippleform-laminated sandstone

(Table 1). Facies 6 is characterized by low angle, cross-laminated, current-ripple

bedforms, with a wide range in grain size, from fine to coarse sand. Facies 6 is

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Figure 11. Modern example of a damp interdune (Facies 5). The crinkly bedded nature of the interdune is due to capillary action of the water table deforming the depositional surface (Namibia; http://www.imaginature.nl/pages/Southern%20Africa%203.html)

Figure 12. Modern example of a wet interdune (Facies 6). An ephemeral flood is depositing sediments over the interdune surface. These flood deposits are short lived and are quickly buried by aeolian deposits overtop (Great Sand Dunes, CO; http://mansurovs.com/category/digital-photography/wallpapers).

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interpreted to represent a wet interdune environment, where ephemeral fluvial (wadi)

processes deposited sediment during a flash flood event (Figure 12), based on similar

interpretations by Mountney (2006b).

Facies 7: Marine Reworked Aeolian (Irregular Cross-Bedded Sandstone)

Facies 7 is a fine- to medium-grained, mm- to cm-scale, irregularly deformed,

rippleform cross-bedded sandstone (Table 1). It is characteristically irregularly

deposited, cross-bedded sandstone that appears to have aeolian and marine (subaqueous)

elements. Identified only in the uppermost part of the Norphlet, Facies 7 is interpreted to

be aeolian dune sand that has been reworked by marine processes (Figure 13; Fryberger

et al., 1990b) during the flooding of the Smackover Formation sea-level rise. This upper

reworked zone is also identified in the Norphlet in updip onshore and offshore

environments (Tew et al., 1991).

Figure 13. Modern example of reworked aeolian (Facies 7). Seawater is reworking aeolian sands of a dune field along the coast (Namibia; http://www.ecdc.co.za/fromabove/photographers-of-the-eastern-cape.asp)

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Norphlet Facies Associations

The seven identified depositional facies were grouped into four facies associations

in order to combine small-scale variants of similar stratification types (sedimentary

fabrics) and processes of formation. This facilitates the correlation of facies to reservoir

characteristics (next section) and permits assessment of large-scale distribution of the

facies (Chapter Four). Facies Association 1 is Grainflow, combining high angle dune

deposits of Facies 1 and 2. Facies Association 2 is Wind-Ripple, combining all wind-

ripple deposits of Facies 3 and 4. Facies Association 3 is Wet Interdune, combining

water-influenced interdune deposits of Facies 5 and 6. Facies Association 4 is the same

as Facies 7, Reworked Aeolian. The percentage of each aeolian facies association in the

Norphlet suggests much thicker accumulations of the Grainflow Facies Association than

Wind-Ripple Facies Association, and subsequently even less Wet Interdune Facies

Association.

The Grainflow Facies Association is more abundant which indicates potentially

more dune development (dryer system). Conversely, the presence of wet interdune facies

indicates that environmental conditions were wetter due perhaps to a lower sediment

supply (Fryberger et al., 1983). The percentage distribution, however, may not

accurately represent percentages for these facies associations due to the low preservation

potential of aeolian deposits. Truncation from dune migration and deflation during

periods of non-deposition has likely skewed the percentage of preserved facies within the

Norphlet (Loope, 1985).

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Depositional Facies Summary

Seven depositional facies were identified within the Norphlet, which were

grouped into four facies associations. The Grainflow Facies Association (1) is a fine

grained, high-angle (with a consistent dip direction on dip log), cross-stratified sandstone.

The Wind-Ripple Facies Association (2) is a medium grained, low to moderate angle (on

dip log), laminated sandstone. The Wet Interdune Facies Association (3) is a medium to

course grained, low-angle (on dip log), crinkly laminated, argillaceous (often slightly

higher gamma-ray response) sandstone. The Reworked Facies Association (4) is a fine

grained, random low to moderate angle (on dip log), irregularly bedded sandstone.

Because of similar sandstone lithology, differences in neutron-density, bulk-density and

gamma-ray response between facies were small. No relationship was found between

facies and resistivity logs because its resolution is too course to depict sedimentary

differences in the sandstone. Other non-aeolian facies are interpreted to represent an

extradunal fluvial system that is either interfingering with, or is a precursor to, the

Norphlet Formation.

20

CHAPTER THREE

Analysis of Reservoir Characteristics

In this section, the relationship between depositional facies and the reservoir

characteristics 1) porosity, 2) permeability, 3) texture, and 4) diagenesis are analyzed. If

a correlation between key reservoir characteristics and sedimentary facies can be

established, it will aid in understanding reservoir quality. Facies that display higher

porosity and permeability and more mature textural attributes can be used to predict

where best to target future hydrocarbon recovery in the Norphlet. Porosity, permeability,

and texture commonly display a strong relationship to facies types at other localities (e.g.

Beard and Weyl, 1973; Dixon et al., 1989; Mancini et al., 1990).

Facies-Controlled Porosity and Permeability

The total range and average porosity and permeability values for each facies

within the Norphlet was calculated, but focus will be placed on the four grouped facies

associations. The relationship between each facies association and porosity and

permeability in the Norphlet is illustrated in the scatter plot in Figure 14. When these

facies are combined into facies associations, a clear distinction between porosity and

permeability values can be observed. The highest porosity and permeability are found

within the Grainflow Facies Association, followed by the Wind-Ripple Facies

Association (Figure 14), and the lowest reservoir quality is found within the Wet

Interdune Facies Association. These results match facies porosity and permeability

defined in the Norphlet at Mobile Bay, as grainflow cross-lamination facies display the

21

highest porosity and permeability, wind-ripple cross-lamination facies display moderate

values, and interdune facies displaying the lowest values (Figure 15).

Overlap of core facies porosity and permeability is attributed primarily to aeolian

facies heterogeneity, which was shown by Weber (1979) to have large ranges in

permeability over short (bed scale) vertical and horizontal distances (Figure 16).

Figure 14. Abundance of measured porosity and permeability with respect to facies. The yellow confidence ellipses represents Grainflow Facies Association. The red confidence ellipses represents Wind-Ripple Facies Association. There is a clear distinction between Grainflow and Wind-Ripple Facies Associations.

22

Figure 15. Facies controlled porosity and permeability defined in the Norphlet fields at Mobile Bay. Grainflow cross-lamination facies display the highest porosity and permeability, wind-ripple cross-lamination facies display moderate values, and interdune facies displaying the lowest values (Dixon et al., 1989)

115 md

7 md

Figure16. Permeability measurements ranging from 2 to 115 md over 6 inches of the Rotliegendes Sandstone core. This figure illustrates depositional heterogeneity in short segments (Weber, 1979).

23

Norphlet Detrital Texture Analysis

Depositional facies all display relationships to the textural characteristics of grain

size, grain sorting, and grain rounding within the Norphlet indicating the processes of

deposition control the grain populations within the sandstone. The sandstones have grain

sizes that range from very fine- to coarse-sand, based on the Udden-Wentworth scale

(Wentworth, 1922). The Norphlet Sandstone displays an upper-fine to lower-medium

grain size average, moderately well sorting, and is typically subrounded to sububangular.

Grain Size

Grain-size distributions in the Norphlet display unique relationships with respect

to facies. The Grainflow Facies Association is concentrated in the upper fine- to lower

medium-grained size category, and have small amounts of very fine or very coarse sand.

The Wind-Ripple Facies Association spans the size categories of very fine to coarse sand,

and has a distinct bimodal grain size distribution in the fine and medium sand sizes. This

is attributed to the alternating finer and coarser, mm-scale, stratified nature of wind-ripple

deposits. The poorly sorted nature of the Wet Interdune Facies Association shows a wide

range of grain sizes. This range is attributed to finer material being trapped by water

along the interdune surface and courser material being deposited by ephemeral streams

across the interdune surface (Albrandt and Fryberger, 1981b).

Grain Sorting

Grain sorting ranges from well-sorted to very poorly sorted, with moderately

well-sorted sand most common. The Grainflow Facies Association displays the best

sorting, the Wind-Ripple Facies Association is more moderately sorted, and the Wet

24

Interdune Facies Association has the poorest sorting. The Reworked Facies Association,

although there are few samples, has a similar well-sorted nature as the grainflow facies.

Grain Rounding

Grain rounding in the Norphlet sands ranges from well-rounded to angular, with

the average being subrounded to subangular. The relationship between facies and grain

rounding indicates that the Grainflow Facies Association is mostly well-rounded,

whereas Wind-Ripple Facies Association is mostly subrounded to subangular, and the

Wet Interdune Facies Association is subangular to angular.

Textural Summary

The textural attributes of the Norphlet Sandstone observed in this study are within

the typical range of aeolian sand (Bagnold, 1941), although the sand grains are a little

coarser grained than is typical. The well-sorted and well-rounded nature of the sand is

due to the aeolian processes of saltation (Schenk, 1983; Bagnold, 1941). The reason so

few grains have coarse or very fine sizes has been explained by Bagnold (1941), who

stated that it is difficult for wind to carry coarse, heavy sediment in saltation across an

erg; and conversely, the wind holds the finer grain sizes suspended in the air over long

distances.

The Grainflow Facies Association has a consistent upper fine to lower medium

grain-size, is well-sorted, and is well-rounded, which identifies it as being the most

texturally mature sandstones (Schenk, 1983). The Wind-Ripple Facies Association has a

bimodal grain-size distribution, with moderate sorting and rounding indicating less

texturally maturity. The Wet Interdune Facies Association is the least mature and

25

commonly displays a very poorly sorted nature. Fine-grained sediment more readily

becomes incorporated into the Wet Interdune Facies Association because the water traps

fine sediment. The Wet Interdune Facies Association will trap coarse sediments from

wind as well. The maturity of sandstone has been shown experimentally to be directly

related to porosity and permeability. Sandstones with better sorting and coarser grain

sizes typically display higher porosity and permeability values (Figure 17; Beard and

Weyl, 1973; Nagtegaal, 1978). This parallels facies-controlled porosity and permeability

data of the Norphlet sandstones.

Figure 17. Experiments on different grain sizes and sorting characteristics in sandstones indicate higher values of porosity and permeability for coarser, more well-sorted sands (Beard and Weyl, 1973)

26

Norphlet Diagenesis

Norphlet sandstones have similar diagenetic constituents, which include the

following cement phases:

1) Chlorite: Chlorite cement is present throughout the entire interval of study, and is the

most abundant cement. Chlorite occurs as books and rosettes that nucleate on sand

grains and grow tangentially into intergranular areas. The amount of chlorite

precipitated is somewhat random throughout the core (Figure 18A).

2) Illite: Illite has a mostly mesh-like (tangential) and platy occurrence. Illite occurs

randomly throughout the reservoir in relatively low abundances and is always found

associated with chlorite cement (Figure 18B), indicating that if formed after chlorite.

Sources of chlorite and illite authigenic clays are likely from deep, high-temperature

brines dissolving local volcanic rocks (Taylor et al., 2010).

3) Quartz: Quartz cement occurs as small euhedral crystals that nucleate on detrital

quartz grains. The amount of quartz cement in the Norphlet is low, and its abundance

is inversely proportion to the abundance of chlorite cement. Quartz cement

precipitated on detrital quartz grains only in locations where chlorite coats are

missing. This suggests that quartz cement formed after chlorite (Taylor et al., 2010;

Figure 18C and D).

4) Calcite and anhydrite: Calcite and anhydrite occur in low abundance throughout the

Norphlet reservoir. These cements are most common in patches near damp and wet

interdune facies, and are interpreted to have formed as early evaporate minerals as the

interdune waters dried up (McBride, 1981; Driese, 1985).

27

Figure 18. Diagenetic phases within the Norphlet. A) Thick chlorite filling pores in some areas, open pore-space in blue (Taylor et al., 2010). B) Tangential illite found in Norphlet (Ajdukiewicz et al., 2010). C) Authigenic quartz common in areas lacking chlorite (Taylor et al., 2010). D) SEM photograph showing authigenic quartz crystals growing out of windows without chlorite coating (Taylor et al., 2010). E) Dissolution of feldspar and VRF common (Ajdukiewicz et al., 2010). F) Stylolitization from pressure solution occurring within the Norphlet (Ajdukiewicz et al., 2010). 5) Hematite: Detrital sand grains are coated with a thin layer of iron-oxide in the lower

part of the Norphlet.

28

6) Pyrite: Pyrite occurs as opaque, gold-colored patchy clusters. It occurs in very small

amounts throughout the reservoir.

Secondary porosity formed from the dissolution of detrital grains and occurs

sporadically throughout the Norphlet (Figures 18A and E). Secondary porosity can be

identified by the oversized pores that contain the rimming cements. Secondary porosity

is very effective at increasing porosity. Compaction is evident throughout the Norphlet

interval as well. Grains have become tightly packed in some areas, particularly in the

lower part of the Norphlet (Figure 18F). Fused grain boundaries are common (Figure

18B) and result from pressure solution during deep burial. In addition, stylolites have

formed in some intervals due to extreme pressure solution (Figure 18F).

The petrographic analysis of cements, dissolution, and compaction in the Norphlet

has revealed many diagenetic phases, which have formed since initial sand deposition.

Refer to the paragenetic sequence presented in Figure 19 for the relative timing of the

major diagenetic phases. The diagenetic events in the Norphlet are explained below:

1) During early burial, calcite, anhydrite, and possibly halite filled the pore space to an

unknown amount in the sandstone. It is also probable these cements were at least

partially syndepositional. During later stages of diagenesis, these cements were

mostly dissolved away (McBride, 1981; Driese, 1985).

2) Chlorite cement occurred pervasively throughout the Norphlet Formation, and covers

most detrital grains with thick, fibrous coatings (Figure 18A).

3) Illite cement occurred in close association with chlorite precipitation, and covers the

chlorite with a mesh-like appearance (Figure 18B).

29

Figure 19. Paragenetic sequence of the diagenetic phases present in the Norphlet in Mobile Bay and onshore wells. Black bars represent confident stages of paragenesis. A diagenetic phases occur within the deep water Norphlet extension (Ajdukiewicz et al., 2010). 4) As burial continued, dissolution of detrital grains, particularly Volcanic Rock

Fragments (VRFs) resulted in the formation of secondary porosity (Figures 18A and

E).

5) Late-stage, silica-rich pore fluids permeated the Norphlet, resulting in the

precipitation of small amounts of authigenic quartz where chlorite cement was thin

(Figures 18C and D).

30

6) Although compaction was occurring throughout the burial history, major pressure

solution and stylolitization began to occur at greater depths during late stages of

diagenesis (Figure 18F).

Diagenetic phases within the Norphlet Sandstone have altered the porosity and

permeability in the many ways throughout the burial history. For example, porosity is

unusually well preserved for a sandstone that has been buried over four miles deep. The

reason for the preservation of the pore space is most likely due to the early precipitation

of cements that kept intergranular areas open (McBride, 1981). Evaporite minerals, such

as calcite, anhydrite, and halite, were the likely culprits that may have prevented

destruction of InterGranular Volume (IGV) in some areas of the reservoir (Paxton et al.,

2002). These cements were probably dissolved away after the sandstone had been deeply

buried.

Because chlorite is the most abundant cement, it is likely the reason for much of

the porosity reduction in the Norphlet (Dixon et al., 1989). However, it has prevented

quartz cement from forming and occluding the porosity. This remarkable relationship is

thought to be one of the primary reasons for the high effective porosity within the

Norphlet Formation (McBride, 1981). In general, diagenetic phases did not preferentially

affect specific facies within the Norphlet.

Reservoir Characteristics Summary

Analysis of the Norphlet sandstone revealed that depositional facies are the

primary control of porosity and permeability throughout the system. The Grainflow

Facies Association has the highest porosity and permeability, followed by the Wind-

Ripple and Wet Interdune Facies Associations. The Grainflow Facies Association is also

31

the most texturally mature, which parallels the fact that it has the highest porosity and

permeability of the Facies Associations. Diagenesis showed little preference towards

facies, and has caused decreased porosity and permeability in the entire Norphlet. Calcite

has prevented early compaction in this system during burial and was later dissolved

away. Chlorite has significantly preserved porosity occlusion from quartz overgrowth.

32

CHAPTER FOUR

Norphlet Depositional History

Developing an accurate composite depositional model for the Norphlet in the

deep-water Eastern Gulf of Mexico is difficult because very little is known about the

lateral extent of the Norphlet Formation in the deep-waters of the Gulf of Mexico.

Regional mapping of the lateral extent of the Norphlet Formation utilizing three-

dimensional seismic data will aid in the interpretation.

Additionally, grain composition within the Norphlet sandstone can be used to aid

in determination of provenance (Lovell, 2010). This provides information on the location

of the source(s) of the sediment and the distances that it traveled. Comparative analogs,

both modern and ancient, are also discussed to help understand the depositional setting of

the Norphlet.

Seismic Analysis

The seismic line displayed in Figure 20 includes two well locations for the study

area. The Norphlet interval identified in seismic covers roughly one seismic wave

making depositional facies identification at a seismic scale difficult due to low seismic

resolution.

The Norphlet has been fragmented into geographically isolated “pods” in this

region due to Louann Salt movement and formation of salt diapir structures (Figure 20).

This salt remobilization occurred post depositionally as a result of the accumulation of

thick packages of sediment in the Gulf of Mexico since the Late Jurassic, causing the

present day Louann salt to vary in thickness drastically (Figure 20). Variations in salt

33

Figure 20. A) Seismic profile through the Norphlet trend in the Eastern Gulf of Mexico. The top of the Norphlet is shown in green. The line is in time-depth, and represents roughly 22,000 feet from the seafloor to the base of the salt. The Louann salt (orange) forms a domed diapir beneath the Norphlet and Smackover Formations. The distance between these two wells is about 15 miles (Chowdhury, 2009).

thickness are also syndepositional as rifting caused transitional continental crust to thin

and sink, allowing more accommodation space in some areas (Salvador, 1991).

Difference in accommodation space is at least part of the reason for variations in deep-

water Norphlet locality thickness as the thicker deposits on the western side of the field

would have deposited in a topographic low (Kugler and Mink, 1999).

34

Additionally, differences in sediment preservation also contributed to the

thickness variations. The consistent thickness of the Smackover Formation and the sharp,

erosive nature of the contact between the underlying Norphlet Formation give evidence

for removal of Norphlet sediment during sea-level transgression, with erosion depending

on the basin configuration at the time of transgression. Dune morphology observed in

Mobile Bay (Figure 5; Story, 1998; Ajdukiewicz et al., 2010) is not visible in the

Norphlet of the deep-water Eastern Gulf of Mexico, which is due to the Smackover

transgression flattening field topography. Finally, thickness changes of the Norphlet are

can be tied to the erg-margin proximity, as aeolian deposits often thicken towards the erg-

axis (Mountney, 2006b).

Core-Log Correlation

A number of bounding or truncation surfaces can be seen in the Norphlet cores,

which mark sharp changes in aeolian facies. The significance of aeolian bounding

surfaces can be determined from their lateral continuity (Brookfield, 1977). Determining

the stratigraphic significance and the spatial extent of these surfaces is important for

understanding reservoir continuity. Because dune sediment preservation is so low in

aeolian systems (usually 20-30%; Bagnold, 1941; Loope, 1985), the origin of aeolian

bounding surfaces has been a subject of considerable debate over the years. These

surfaces are best interpreted by examining modern dune fields and large, well exposed

ancient outcrops (McKee and Moiola, 1975) and are controlled by factors such as wind

regime, sediment supply, water-table influence, and climatic changes (Rubin, 1987;

Kocurek and Havholm, 1993). Early workers identified and established many different

types of aeolian bounding surfaces (Stokes, 1968; Brooksfield, 1977; Kocurek and Dott,

35

1981; Kocurek, 1988) including a set of hierarchical bounding surfaces ranging from

first- to fourth-order, each representing different amounts of geologic time. More current

aeolian bounding surface terminology, which describes the processes of surface

formation, is described by Fryberger et al. (1990, 1993), and Mountney (2006a). These

surfaces include reactivation surfaces formed from brief periods of wind direction

change (seasonality), stacking surfaces formed from dune sets migrating overtop one

another, stabilization surfaces formed from periods of aeolian deflation to the water table,

and supersurfaces, which represent a drastic change to non-aeolian deposits during wetter

conditions controlled by allocyclic changes (Fryberger, 1990, Mountney, 2006a; Figure

A1.3). Although all surfaces are likely to exist in the Norphlet, only stabilization

surfaces and supersurfaces are laterally continuous enough to correlate stratigraphically

across a regional or sub-regional area. Even these surfaces, however, are difficult to

interpret from core.

The Norphlet-Smackover transition in all wells marks a major change in climate

and associated deposition. This results in the development of a supersurface, which was

used as a datum for stratigraphic analysis. This surface is sharp, indicating significant

erosion of the Norphlet erg during Smackover sea-level transgression. Stratigraphically

equivalent cycles, bounded at the top and base by regionally significant stabilization

surfaces can be interpreted within the Norphlet. These surfaces were defined by a course

grained basal lag commonly followed by thick (10 ft) low-angle deposits (Wet Interdune

or Wind-Ripple Facies Associations) and succeeded by an upward-“drying” trend which

commonly fined upward. Drying upwards cycles were observed progressive changes in

accumulation each defined by successively thicker dune sets, increased Grainflow Facies

36

Association abundance, and decreased Wet Interdune and Wind Ripple Facies

Associations abundance. Cycles exhibited similar thicknesses across the field.

Stabilization surfaces mark periods of erg deflation down to the water table.

Surfaces with extensive, thick deposits of wind-ripple or wet interdune elements may

denote more prolonged periods of aeolian dampening caused by a high water table. It is

possible that these surfaces are not correlative or significant, considering sparse data

density and the inability to trace these surfaces out laterally. In addition, these surfaces

lack features such as soil development, plant roots and animal burrows that would

suggest prolonged deflation (Mountney, 2006b). Nonetheless, the repeating cyclic drying

nature of these sediments cannot be ignored. If, in fact, these surfaces are significant,

they will certainly act as reservoir baffles closer to the erg margins.

Sediment Provenance

As shown by the recent work of Taylor et al. (2010) and Ajdukiewicz et al. (2010)

Petrographic studies of the Norphlet sandstones reveal important characteristics about

detrital components. These detrital components define the source(s) of the Norphlet

sediments and provide information on the distance that the sediment may have traveled

before deposition. Quartz grains are mostly monocrystalline and nonundulose, K-

feldspar (orthoclase) is the dominant feldspar, and lithic fragments are dominantly

volcanic in origin. These detrital components are common in thin section throughout the

Norphlet interval (Figure 21). The Norphlet Sandstone from Mobile Bay and onshore

wells plot as arkosic sandstones (Figure 22).

37

Figure 21. Detrital constituents present in the Norphlet include Quartz (Qtz), K-feldspar (Kspar) and Volcanic Rock Fragments (VRF). Constituents in this thin section are typical throughout the Norphlet (Taylor et al., 2010)

Figure 22. Sandstone classification of all point-count data for Mobile Bay and Onshore Wells. Rock is much more arkosic than the Norphlet in the Deep Water Eastern Gulf of Mexico (Ajdukiewicz et al., 2010).

Volcanic rock fragments (VRFs) are more abundant within the Norphlet

Formation than in most aeolian sandstones (Pye and Tsoar, 2009), which typically

contain much higher percentages of quartz. Lithic fragments are the best indicator of

sediment provenance. The abundance of these fragments indicates close proximity to the

38

source area, because lithic fragments readily breakdown during prolonged transport.

Well and seismic data to the east provided by Salvador (1991) and Dobson and Buffler

(1991) indicate that beneath Florida’s present day western gulf shelf there is a Jurassic

highland area, termed the Middle Ground Arch, that is composed of extrusive volcanic

basalts and andesites, which likely provided sediment during Norphlet deposition.

Depositional History

Work done by Mancini et al. (1985, 1990) at the Mobile Bay fields indicates the

lower part of the Norphlet represents a fluvial or distal alluvial fan environment, with a

number of ephemeral wadi elements. Although these accumulations are roughly 100

miles away, a similar facies assemblage containing wadi elements and lacking aeolian

facies may exist throughout most of the lower part of the Norphlet Erg System. Soil

development, muddy laminations and sandstones with mud intraclasts would all be

indications of a lower alluvial fan or fluvial overbank environment (Blakey et al., 1996;

Kocurek, 1981). The natural progression of aeolian deposition is interrupted and buried

by a sea-level transgression represented by the deposition of the shallow water

Smackover carbonates, likely scouring the upper Norphlet interval.

The Norphlet varies in thickness throughout the erg system and ranges in

thickness from a few feet onshore to 1200 feet in Mobile Bay. The reasons for the drastic

thickness difference are due to both the erosive Smackover transition, which would have

scoured the Norphlet differentially, and differences in accommodation space during time

of deposition. Aeolian systems are commonly deposited within a basin or topographic

low (Ahlbrandt and Fryberger, 1981a). Tectonic deformation from salt loading onto the

Louann probably had some syndepositional effects; however, structural influence from

39

salt tectonics largely did not affect the system until later Mesozoic and Cenozoic

sediment accumulation, adding an enormous load on top of the thick, ductile salt deposits

(Salvador, 1991).

Dune Morphology and Wind Regime

Many researchers have described modern dune morphologies (e.g., McKee, 1979;

Fryberger and Goudie, 1981; Lancaster, 1989; Mountney, 2006a). A complied list of

these morphologies can be found in Figure A1.4. Work by Albrandt and Fryberger

(1981a) and Fryberger and Dean (1979) studied aeolian dip-angle relationships to dune

morphologies and concluded that unidirectional dip azimuths signify a unidirectional

paleo-wind direction. A consistent dip direction in the dune facies of the Norphlet

indicates that the Norphlet was accumulating from dominantly southeasterly winds.

Unidirectional winds commonly form barchan, barchanoid, and transverse dune types,

depending on sediment supply, wind velocity, and seasonal variability (Albrandt and

Fryberger, 1981a; Rubin, 1987; Mountney, 2006a). This hierarchical progression of dune

forms with increasing sediment supply is interpreted to be the general setting of the

Norphlet system (Figure 23). Generally, the lower-angle grainflow sets represent isolated

barchan dunes, whereas the higher-angle dune sets represent migrating transverse or

barchanoid dune complexes (Fryberger and Dean, 1979).

Kocurek and Dott (1981) constructed a plot that relates stacked sets and grainflow

bed thickness to the original dune height (Figure 24). Although their results were

somewhat inconclusive and variable, the concept appears to be correct. This plot (Figure

24) was applied to Norphlet sandstones and indicates that the original dune height ranges

between 5 to10 meters for most of the dunes. Thicknesses vary quite a bit from set to set

40

2000 m

C

1000 m

B

50 m

A

Figure 23. Hypothetical interpretation of hierarchical progression in the Norphlet dune field from erg margin to erg axis. Dune forms increase in size, amalgamate together, and interdunes decrease in abundance. A) Barchan dunes (gomyclass.com/wind.html); B) Barchanoid dune ridges (Google Earth, 2010); C) Transverse dune ridges (Google Earth, 2010)

throughout the core, indicating a range in original dune heights. This was likely caused

by changes in sediment supply and seasonality (Rubin, 1987).

Norphlet Analogs

The Norphlet in Mobile Bay and onshore areas displays a northwest-southeast

trending “band” of aeolian deposits. This sand sea was bounded by highlands to the east

(Appalachian Mountains) and a restricted Jurassic sea to the west as the Gulf of Mexico

41

Figure 24. Maximum cross-strata thickness plotted against height of the dune slipface in the Little Sahara Dune Field indicates that thicker grainflows match larger dune slipfaces (Kocurek and Dott, 1981).

was opening up (Mancini, 1985, 1990). One of the more important questions about the

Norphlet Formation in the deep-water Eastern Gulf of Mexico is whether it was

physically connected to the Norphlet onshore and in Mobile Bay. The scale of an aeolian

system is difficult to determine with limited well/core data and a lack of outcrops. The

Namib Desert along the southwestern coast of Africa provides a potential analog of a

modern aeolian system with similar geographic constraints. Given that this system has

continuous dunes for around 500 miles of north-south African coastline, the system is

identical in geographic length to the total known distribution of the Norphlet Formation.

Because this elongate band of Norphlet deposits is bounded to east by highlands and to

the west by the ancestral Gulf of Mexico, the depositional setting is also similar to the

Namib Desert. The extent of the Namib Desert and a detailed overlay of the

environments of deposition are illustrated in Figure 25.

42

Another way of gaining an understanding the Norphlet is by comparison with well

exposed ancient aeolian deposits. For example, the Cedar Mesa Formation in

southeastern Utah is a lower Permian succession of aeolian sediments that has very

similar stratification patterns and cross-bed thicknesses to the Norphlet Formation. The

dune bedforms in the Cedar Mesa have mostly unidirectional dips (Mountney, 2006b),

identical to those in the Norphlet. An outcrop view of the Cedar Mesa Sandstone depicts

these similarities in stratification, with stacking dune set thicknesses also comparable to

those observed in the Norphlet cores (Figure 26). The central erg sediments in the Cedar

Mesa Sandstone are mostly continuous dune deposits, with a few interdune environments

represented (Figure 27A). Dune sets are thinner at the erg margins and interfinger with

extradunal fluvial facies (Figure 27B; Mountney, 2006b). This proximal-to-distal facies-

tract relationship is probable within the Norphlet System. Finally, deposition of the

Cedar Mesa was punctuated by deflationary surfaces throughout its depositional history

(Loope 1985, Mountney, 2006b). These surfaces may be analogous to the stabilization

surfaces identified in the Norphlet, and tracing them may aid in understanding the lateral

scale of the system.

Depositional Model

The depositional model illustrated in Figure 28 depicts the Norphlet during later

stages of widespread aeolian deposition. The model represents a time of maximum

aeolian growth just before the Smackover seas transgressed over the erg and buried the

entire system.

43

Figure 25. The Namib Desert located along the coast of Namibia is comparable in scale to the Norphlet aeolian system. Overlaying environments of deposition and the three wells give an idea for scale of these aeolian systems (images modified from Google Earth, 2010).

44

45

Figure 26. Comparison of the Norphlet core with an outcrop of the Cedar Mesa. Bounding surfaces (yellow lines), grainflow facies (GF), and wind-ripple facies (WR) are shown. A hypothetical core taken from the Cedar Mesa between two bounding surfaces shows similarities can be seen between the Norphlet and Cedar Mesa Sandstones in facies types, stratification patterns, cross-bed thicknesses, bedding thicknesses, and dune set thicknesses. The difficulty of understanding these systems distributions without outcrop exposure to trace out lateral and vertical changes in the system is apparent.

A

B

20 m

10 m

Figure 27. Cedar Mesa outcrops at two different localities A) Cedar Mesa erg axis (central erg) displaying thick stacked dune sets with high net to gross sand proportions. B) Cedar Mesa erg margin displaying thin dune sets with many interdune and interfingering fluvial intervals. The distance between these two Cedar mesa outcrops is roughly 20 km, which is similar to the distance separating the Norphlet wells.

46

Figure 28. Schematic depositional model of the Norphlet in the deep water Eastern Gulf of Mexico in the Late Jurassic Oxfordian Stage. Highlands are in close proximity with alluvial deposits shedding from them. Distal alluvial deposits interfinger with the aeolian system. The aeolian system is more widespread, maturing upward and laterally towards the central erg.

CHAPTER FIVE

Conclusions

1) Seven aeolian depositional facies were identified within the Norphlet Formation,

which display a range of dune and interdune elements. These facies are combined

into four facies associations defined as Grainflow, Wind-Ripple, Wet Interdune, and

Reworked Aeolian. A variety of non-aeolian facies (fluvial/wadi) recognized by

previous workers (Mancini et al., 1985, 1986) occur in the lower Norphlet and may

reflect regional changes in the system to extradunal environments of deposition.

2) Grainflow facies displayed the highest porosity and permeability, wind-ripple facies

displayed moderate values, and wet interdune facies displayed the lowest values.

Porosity and permeability values show heterogeneity between and within each facies.

3) Textural changes in the Norphlet are facies-controlled. The Grainflow Facies

Association is texturally the most mature, the Wind-Ripple Facies Association has

bimodal grain-size distribution with moderate maturity, and the Wet Interdune Facies

Association is poorly sorted and relatively immature. The Reworked Aeolian Facies

Association is similar to grainflow in textural attributes.

4) Diagenetic constituents include calcite, anhydrite, quartz, chlorite clay, and illite clay,

all of which affect reservoir quality differently. Cementation showed no relationship

to facies. Early evaporates may have preserved intergranular volume (IGV). Chlorite

cement reduced porosity and permeability, but prevented quartz precipitation from

occluding pore space (Taylor et al., 2010; Ajdukiewicz et al., 2010).

47

5) Seismic analysis defined the full extent of the Norphlet Formation to deeper waters of

Eastern Gulf of Mexico. It is structurally fragmented into separate pods, primarily

due to salt tectonics, which resulted from sediment loading on the widespread and

thick, subjacent Louann Salt.

6) Detrital grains in the Norphlet contain a high abundance of VRFs, indicating

relatively close proximity to a volcanic source rock.

7) Aeolian deposits along the erg margin may interfinger with the fluvial extradunal

facies (and/or bury a Norphlet precursor) and eventually dominate the system during

later stages of Norphlet deposition. Aeolian deposition is eventually interrupted by

the Smackover sea-level rise, which buried and eroded the system severely.

8) Depositional reconstruction of the Norphlet in the Gulf of Mexico suggests that wind

direction was dominantly from the southeast throughout deposition. Based on

modern and ancient analogues, the system is forming mostly barchanoid and

transverse dune forms on the order of around 10 meters high. Determining the

Norphlet connectivity from onshore to Mobile Bay to deepwater would stretch the

dune field out nearly 500 miles, which is a similar scale to Namibia.

Future work on the Norphlet Formation in the deep-water Eastern Gulf of Mexico

should involve utilizing higher-resolution seismic data to better delineate the system’s

morphology on a regional scale. Petrophysical, core, and thin section analysis from

current and future wells will provide a better understanding of the architectural elements

within the dune environments and better constrain facies reservoir characteristics.

Finally, understanding the geographic extent of the Norphlet by determining 1) the

connectivity of the deep-water Norphlet to the deposits in the north, and 2) the Upper

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Jurassic coastline to the Norphlet deposits (if one exists) would be invaluable to future

exploration on a more regional scale.

APPENDIX

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APPENDIX

Supplemental Figures

Arid Environments  Aeolian Stratification  Facies and Sedimentary Structures  Other 

Dune High‐angle 

Grainfall  Fine grained, thin, mm‐scale laminated Sandstone    

     Grainflow (Avalanche) 

Fine‐medium grained, massive, cm‐scale planar tablular to trough cross‐strata, inverse and normal graded Sandstone    

  Low‐angle  Translatent 

Fine‐medium grained, thin, mm‐scale pin stripe laminated Sandstone    

Interdune  Dry 

Wind‐Ripple 

Rippleform Fine‐medium grained, thin, mm‐ to cm‐scale rippleform laminated Sandstone    

   Damp  Adhesion Ripples poorly sorted, wavy‐laminated Argillaceous Sandstone 

minor rooting, burrows, salt crusts 

     Dessication Polygons/Mudcracks 

mud‐silty laminations with infilled sand in cracks    

   Wet  ephemeral flood/wadi rippleform laminated, course grained, poorly sorted sandstone    

      paleosol poorly to non‐stratified argillaceous siltstone and mudstone 

common rooting, burrowing, walking traces; Also Extradune 

      lacustrine/pond  dark laminated shales  Also Extradune 

      Carbonate  carbonate cemented bedding    

  Evap‐oritic  Sabkha 

crinky to wavy laminated poorly sorted sandstone and evaporites 

minor rooting, burrowing, walking traces; Also Extradune 

      Evaporites (playa) bedded halite or anhydrite to interlaminated with argillaceous sandstone  Also Extradune 

Extradune     Sandsheet (zibar)  Horizontally bedded sandstone    

      Marine Reworked rippleform laminated, irrgularly deformed cross‐stratified sandstone 

deformed aeolian bedding 

      fluvial channel normally graded ripple and/or cross‐bedded sandstone    

      overbank cravasse splay interbedded fine silts and muds with thin, sandy sheets/lobes    

      sandy red beds  hematite stained arkosic sandstones (oxidized)    

      Upper (proximal) alluvial fan  course conglomerates and debris flows    

     Lower (distal) alluvial fan (plain)  fine interbedded muds and silts 

common rooting, burrowing, walking traces 

Defromation structures  soft sediment deformation  Convolute, wavy beds or bedsets 

syndepositional slump features 

Figure A1.1. Facies summary table displaying a compiled list of facies found within or associated with aeolian systems. Facies in table supported by McKee (1979), Ahlbrandt and Fryberger (1981a), Ahlbrandt and Fryberger (1981b), Driese (1985), Rubin (1987), Fryberger et al. (1990), Kocurek (1991), and Mountney (2006a).

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Wind Direction

Figure A1.2. Diagram of modern processes aeolian dune migration. Wind direction forms wind ripples on windward slope picking up sediment and depositing it as grainfall on the leeward slope. Over-steepening of the leeward slope leads to Avalanching, or grainflow.

Figure A1.3. Various types of bounding surfaces defined by Mountney (2006a), including reactivation surfaces, superimposition surfaces, interdune migration surfaces, and supersurfaces.

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Figure A1.4. Illustrations of various modern dune types. Arrows indicate wind direction (Mountney, 2006a). Dune forms A, B, and C are common for unidirectional winds, such as those interpreted in the Norphlet.

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