Hedberg 1998 Models for Understanding Risk 1

Copyright © 2000 by The American Association of Petroleum Geologists. All Rights reserved.Paper presented at the AAPG Hedberg Research Conference, "Integration of Geologic Modelsfor Understanding Risk in the Gulf of Mexico," September 20–24, 1998 (Galveston, Texas).

Deep Structure and Evolution of the Gulf of Mexico Basin, Based on15-second Record Seismic with 2-D Pre-stack Depth Migration, and Risking

a Frontier Hydrocarbon Play in the Sub-salt Foldbelts

David J. Hall, PennzEnergySteven M. Murray, PennzEnergy

Kenneth W. Mohn, TGS-NOPEC Geophysical Company

Interpretation and mapping of new regional 15-secondrecord seismic grids in the Louisiana and Texas shelfand slope reveal new details of structure andstratigraphic architecture underneath a massivecanopy of allochthonous salt. We present apreliminary interpretation of the structuralconfiguration under the salt. Although interpretationof the depth structure at the Mid-Cretaceous sequenceboundary and younger events under the salt isdifficult, iterative flattening of successive horizons intime during the interpretation contributes toconfidence in the resulting depth mapping.Conversion to depth of the time interpretation filesfollows a straightforward procedure using isochronand interval velocity layering to develop approximatesub-sea depth contours. Conversion to depth is criticalfor realistic understanding of the early developmentand subsequent structural evolution under the salt.

In Walker Ridge, the south-southwest trendingstructural continuation of the Mississippi fan foldbelt(Fig.1) underneath the salt is best imaged on prestackdepth migrated long regional lines. Attractive depthstructures, now largely leased, represent a frontiersub-salt opportunity currently widely recognized inthe industry. The mapped structures apparently do notcontinue to the west to connect with the Perdidofoldbelt (Fig.2) in Alaminos Canyon. This reinforcesearlier suggestions that the formation of the two foldbelts occurred in two separate events, one in theOligocene (Perdido), and the second in theMiocene(Mississippi fan). Conversely, in KeathleyCanyon, a relatively unstructured sub-salt section canbe mapped with little if any similar compressionaldeformation. Most attractive plays here are truncatedagainst deep-rooted salt walls that may provide

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migration pathways from buried source rocks of lowerTertiary and Mesozoic age.We attribute the location and orientation of thefoldbelts to the original seaward depositional limit ofLouann salt (Fig.3) coupled with the gradual migrationof Cenozoic depocenters from the west(Oligocene) tothe east (Miocene). Compressional structures in thePort Isabel foldbelt(Fig.4) may have a somewhatdifferent origin; high-angle reverse faults withcomplex associated faulting contrast sharply with themore symmetrical double-plunging anticlines in thePerdido and Mississippi fan foldbelts. There is goodevidence on the newer seismic data for a deepstructural detachment underlying the western PortIsabel growth fault systems. This detachment surfaceappears to tie back to the listric fault systems offshoreNorth Padre Island and Mustang Island. Back-to-backor “tee-pee” structures, in may respects identical tothose in the salt-free shelf and slope offshore Nigeria,may be a result of less salt in the original depositionalpackage in this part of the Gulf of Mexico. Moreregionally, original salt thicknesses of 4 km or moreseem to have been isolated to an east-northeasttrending depo-trough. Thick salt later becameremobilized to form allochthonous salt sheets. Fromthe Sigsbee scarp, the sheets thicken to the north andnorthwest, largely limiting the currently practical sub-salt exploration play in the vicinity of the Sigsbee scarpto a zone about 20-30 km behind the scarp or its buriedequivalent.

The structural evolution of areas of thinner salt farthernorth of the Sigsbee scarp(Fig.5) appears to be relatedto the complex interplay between slope sedimentationand ductile salt flow. Attractive exploration targetsbetween 5000 and 7000 meters sub-sea are widespread.Still unresolved are 1) the nature of the connectionbetween the fold belts and the relatively unstructuredsection at deep levels in the upper and shelf, 2) thelithostratigraphy of the pre-salt section and 30 theinfluence or lack of influence of basement structures inthe overlying Mesozoic and Cenozoic structuralcomplexes. With respect to evaluating and risking thesub-salt foldbelt play(s), the lack of reliable seismichydrocarbon indicators, the lack of well control andthe large number of individual structural prospectssuggest modifications of traditional approaches arerequired The normal critical parameters:″Does the playeven exist,?” must all be evaluated as quantitatively aspossible. The Neptune discovery in Atwater Valleyand the BAHA discovery in Alaminos Canyon suggestthe answer to the last question is yes.Major shortcomings in the current generation of riskanalyses are illustrated by the sub-salt foldbellt play.All the seismic uncertainties are typically capturedunder geological categories, such as probability ofstructure and hydrocarbon trapping. We suggest thedifferences in uncertainty among projects variouslyutilizing depth migration, evaluation and modeling ofconverted waves and multiples are significant andmerit independent ranking as well.

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Figure 1 Map showing salt tectonism of the U.S. Gulf Coast Basin (After John Lopez, NOGS 1995

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Figure 2 Perdido Foldbelt in Alaminos Canyon (Line E on Map) Seismic data courtesy of TGS-NOPEC and GECO-PRAKLA

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Figure 3 Sigsbee Escarpment, Walker Ridge area showing allochthonous Louann Salt(line H onmap) Depositional limit of autochthonous salt Seismic data courtesy of TGS-NOPEC and GECO-PRAKLA

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Figure 5 Louisiana shelf area, allochthonous salt with disharmonic structures below the salt(Line A on Map)

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Figure 5 Deep structural detachment underlying the western Port Isabel growth fault systems(Line D on Map)