Seismic facies and hydrocarbon potential of carbonate reservoirs … · 2019-04-28 · Seismic facies and hydrocarbon potential of carbonate reservoirs in ramp settings 3 Classification

Seismic facies and hydrocarbon potential of carbonate reservoirs in ramp settings

Alexander Wunderlich

Institute for Geology, Gustav-Zeuner-Straße 12, 09599 Freiberg, Germany

Abstract. Carbonate ramps have a gently dipping surface less than 1° and show homoclinal or distally steepend morphologies. Carbonate rocks make up only 20% of the sedimentary rock record yet account for more than 60% of the world’s proven hydrocarbon reservoirs. Reservoirs in ramp settings make up only 10 to 15% of the whole carbonate reservoirs but show more subtle play types than many rimmed shelves, with wide opportunities for stratigraphic and structural trapping and lateral variations in reservoirs quality. 3-D seismic imaging represent a robust methology for the interpretation of carbonate reservoirs and structures. Accurate seismic imaging of the reservoir architecture has become an important predictive tool for reservoir characterization because it helped to build 3-D geological framework within which depositional facies can be distributed in time and space. Calculation of volume-based attributes produced new volumes of data in addition to reflectivity volumes to extract 3-D geometries within the reservoir.

Introduction

The original concept of the carbonate ramp (Ahr, 1973), as an alternative to the steep-sloped, reef-rimmed shelf, was a carbonate slope with a low-gradient (<1°) from shoreline to basin (Burchette and Wright, 1992).

Petroleum exploration in 1960s (e.g. Smackover Fromation, Bishop, 1969) led the oil industry to the develop regional facies maps that showed extensive grainstone belts along and parallel to the shoreline. Detailed analysis of seismic lines from carbonate platforms from across the globe revealed major differences. The most puzzling of these was the absence of a distinct shelf margin (shelf-slope break) from seismic profiles. This shelf margin could be seen in the modern Bahamas, Barrier Reef and other classic carbonate localities. Therefore facies models of Wilson (1970) were calibrated to this specific morphology. The absence of this shelf-slope break feature suggested that standard facies models could not be applied to such platforms. Therefore another set of models as created that evolved into the present-day ramp model(Fig. 2).

2 Alexander Wunderlich

Fig.1. A vintage-1973 sktech illustrating the physiography of a carbonate shelf (modified after Burchette and Wright, 1992).

Fig.2. A vintage-1973 sketch illustrating the physiography of a carbonate ramp (modified after Burchette and Wright, 1992).

Seismic facies and hydrocarbon potential of carbonate reservoirs in ramp settings 3

Classification of ramp settings

Read (1985) developed six types based on energy regimes and the distribution of shallow-water facies. Burchette and Wright (1992) subdivided ramp settings according to the degree of wave, tide and storm activity with reference to the depostional processes (Fig.3).

The inner ramp is the zone above fair-weather wave base where wave and current activity are almost continuous (Reading, 1996). This zone is dominated by sand shoals or organic barriers and like siliciclastic shorelines they are marked by shoreface, beach, lagoonal and tidal environments. The mid-ramp zone lies between fair-weather wave base and storm wave base where the sea floor is affected by storm waves but not by fair-weather waves. According to that, sediments show evidence of frequent storm reworking (Burchette and Wright, 1992). Graded beds (e.g. tempestites) and hummocky cross-stratification characterize these deposits (Reading, 1996). Aigner (1984), Burchette (1987) and Faulkner (1988) recognized distinct proximal to distal trends within the succession. The outer-ramp zone extends from below the normal wave base to the basin floor. Storm-generated currents produce graded units or erode and rework the sediment. In deeper zones restricted bottom conditions appear, producing suboxic or anoxic environments.

Fig.3. The main environmental subdivisions on a homoclinal, carbonate ramp. The pycnocline is not always originally present or identifiable in the rock record (from Burchette & Wright, 1992).

Carbonate Seismic Facies Analysis

The appearance of carbonate rocks in general in seismic data contains a lot of different information. The seismic data can contain evidence about their original


depositional environment, lithofacies, diagenesis, and source rock and reservoir potential (Macurda, 1997).

To get these very usefull informations, it is necessary to have methods, which produces them. While surface mapping, paleontology, palynology, oxygen and carbon isotope studies, petrography, X-ray diffraction, gravity, magnetics, and core and well log analyses all play essential roles in oil and gas exploration, a vast majority of the data is obtaoined by reflection seismology. In addition to being relatively inexpensive, reflection seismology provides the only practical method of obtaining a 3-D image of the earth’s susbsurface and of course of carbonate environments (Palaz and Marfurt, 1997).

Recent advances in seismic data acquisition, processing, and visualization techniques provide the opportunity to image carbonate reservoir architecture with unprecented resolution. However, the analysis of seismic is a developing metodologhy to quantify the volumes and rock properties of carbonate rocks. The additional advantage of seismic data is that the imaged deposits can be displayed at various stratigraphic levels and so it is possible to document the evolution of the depositional environment through time. The paleogeomorphology can now be accurately imaged for carbonate systems constructed by extinct reef builders that have no modern analogs. This capability offers the unique opportunity to exploit 3-D images for questions regarding the growth pattern of different reef communities, their paleoecology and reservoir heterogeneities in ancient systems, for example ramp systems.

In the past decade, 3-D reflection seismology has replaced 2-D seismology almost entirely in the seismic industry, and helped to understand the reflectors within carbonate reservoirs. One very usefall advantages is the more accurate imaging in 3-D seismology. With the help of depth-slices it is possible get horizontal planes of the carbonate platform. And with inclusion of out-of-plane diffractor it is possible to get more informations about the dip of the circumjacent layers.

Variations of seismic reflections patterns of carbonate sediments result from the combined effect of several parameters (Eberli et.al). These parameters can be grouped into three categories that have a direct influence on the seismic image and are the same for every carbonate depositional environment: a) physical properties of the rock, in particular sonic velocity and bulk density, which together define acoustic impedance; b) scale and geometrical relations of the seismic survey; c) technical parameters and quality of the seismic survey.

Physical Properties

Porosity is the most important physical factor that influences velocity. Vp and Vc increase with decreasing porosity, but large departures from this general trend are possible. Choquette and Pray (1970) classified carbonate porosities into 15 basic types which, combined with other elements, provide a detailed geologic characterization. The most important and frequently observed types of porosity are


interparticle, intercrystalline, moldic, vug, intraparticle, fracture and fenestral (Wang, 1997).

Fig.4. Velocities (Vp and Vs) versus porosity in 173 gassaturated carbonate core samples. Overall, dolomites show higher velocities than limestones at a given porosity

Geometrical relations

There are four basic reflections configurations found in seismic data. These are parallel or subparallel, prograding, mounded or draped, and onlap fill (Macurda, 1997).

Parallel or subparalle reflections (Fig. 4) imply that during the deposition a regional increase in accommodation potential was essential and that there is nothing inherent in the reflection which gives us the information about how fast or slow it was. Progradational reflections are really good to differentiate. Progradation takes many forms as for example sigmoid, oblique (Fig. 5) and shingled reflections. And all of these have a consequence for reservoir prediction (Macurda, 1997). Mounded and draped reflections assume to be reefs (Fig. 6) or buildups. But the question is first, if they are real geological feature or if they are just geophysical artifacts like sideswipes, noise trains or overmigration. Seismic patterns of onlap and onlap fill imply a termination of low-angle strata against a steeper stratigraphic surface (Catuneanu, 2002). The onlap pattern is most commonly found in two settings: at the base of a slope (Fig. 7) or on the shelf according to transgression.


Fig.4. Seismic profile from the northeastern Gulf of Mexico, perpendicular to the Early Cretaceous shelf edge, showing parallel and subparallel reflections (modified after Macurda, 1997).

Fig.5.Northwest-southeast seismic profile across the Campeche Bank north of the Yucatan Peninsula, Mexico. The shelf margin is to the left. A series of inclined oblique reflections (1) record northwestward progradation of this carbonate shelf platform. Flat reflections (2) mark shelfal carbonate sediments (modified after Macurda, 1997).


Fig.6. Seismic profile depicting Cretaceous Sligo buildups (1,2) on the shelf margin of the northeastern Gulf of Mexico (modified after Macurda, 1997).

Hydrocarbon Potential and Economic Aspect

The focus by explorationists lies on depositional models for petroleum exploration and development in carbonate rimmed shelves or in large isolated buildups than in ramp system (Burchette and Wright,1992). Carbonate ramps often form more subtle play types than many rimmed shelves, with wide opportunities for stratigraphic and structural trapping and lateral variations in reservoirs quality. Little is known about factors that control the distribution of source and reservoir rocks in ramp settings.

Low-energy ramps are sparse in potential reservoir facies unless downslope buildups or incipient organic rims are developed (Read, 1985), or they lie in situations where the timing of diagenesis in relation to petroleum migration have been particularly favourable (Burchette and Wright, 1992). High-energy ramps in contrast commonly possess a wide range of reefoid facies and carbonate sandbody types.


Reservoirs in organic build-ups

Organic build-ups are a common location for petroleum reservoirs (Fig. 7). These features have exhibited a wide range of organic and sedimentary facies through time and this diversity is reflected in variations in their reservoir potential. Buildup shape and location may reflect tectonic, topographic or hydrographic control (Burchette and Wright, 1992). The thickness of ramp buildup reservoirs is seldom greater than 100-200m.

Fig.7. Miocene buildup, Luconia Province, Malaysia. This buildup is not situated on a carbonate ramp, instead it is on platform. But the main geometries are the same (modified after Masaferro et al., 2004)

Example 1: Morgan, W.A, 1985; Silurian Reservoirs in Upward-Shoaling Cycles of the Hunton Group, Mt. Everette and Southwest Reeding Fields, Kingfisher County, Oklahoma

This Oil-field is characterized by two main productive facies. One of them is a skeletal buildup (Fig. 8) from the Clarita Formation (Hunton Group).

Porosity within the Clarita buildup facies ranges from 0 to 15% and is mainly biomoldic and solution-enhanced. The trap is formed by a combination of updip loss of porosity (associated with a facies change from porous, dolomitized crinoid-dominated packstone and wackestone to non-porous arthropod packstone and wackestones of the shallow shelf facies). The trap is also formed by an overlying seal provided by the non-porous deep ramp facies of the overlying Henryhouse Formation (Morgen, 1985).


Fig.8. Depositional model for the Clarita Formation. During the Late Silurian, crinoid-rich skeletal build-ups accumulated near a shelf edge. Subaerial exposure resulted in local solution brecciation of some build-ups, and probably was influential in their dolomitization. Vertical scale is exaggerated (modified after Morgan, 1985).

Reservoirs in grain-to wackestone dominated ramps

Grainstone reservoirs in ramp settings are widespread and occur in a number of variations. The composition of the grainstone sediments may also vary (e.g. oolitic or bioclastic), depending on the age and the location of the ramp (e.g. whether windward or leeward). Shoreline carbonate sandbodies to major detached shoal complexes or shoal complexes over offshore highs represent the major reservoir facies . Nevertheless, a general characteristic of many ramp grainstone reservoirs is that they are relatively thin or layered, seldom with more than a few tens of metres of reservoir facies in any one zone, and commonly of wide lateral extent (Burchette and Wright, 1992).


Example 2: Lindsay and Kendall, 1985, Depositional facies, Diagensis, and Reservoir Character of Mississippian Cyclic Carbonates in the Mission Canyon Formation, Little Knife Field, Williston Basin, North Dakota

This oil-field is characterized by a dolomitized skeletal wackestone and pellet wackestone-packstone with an average porosity of 14% and an average permeability of 30md. Key beds were identified on the basis of openhole well logs, which permitted subdivision of the section into six informal zones designated as A, B, C, D, E and F (Lindsay and Kendall, 1985). Zone D is the main productive portion (Fig. 9) due to the well developed porosity.

Fig.9. Idealized depositional setting of Mission Canyon Formation at Little Knife Field. Zone D within the transitional open to restricted marine facies is the main productive portion (modified after Lindsay and Kendall, 1985).

Conclusion

(1) Carbonate ramps have a gently sloping surface with a dip of less than 1°. They can also be subdivided into inner-, mid-, and outer-ramp environments. (2) Carbonate ramps have characteristic combinations of seismic facies that aid in their recognition and in the evaluation of their hydrocarbon potential. (3) Ramps and their associated sediments form prolific petroleum source and reservoir systems and offer a range of subtle stratigraphic play types and lateral facies variations (Burchette and Wright, 1992). The reservoirs can be found in mid- or outer ramp isolated buildups. Grainstone and packstone reser-voirs are common as well, but they show difficult reservoir heterogeneity. (4) Seismic imaging of carbonate depositional architectur (in 2-D as well as in 3-D) has seen marked improvement over the last 10 years and has allowed interpreters to better delineate the complex histories of carbonate platform sequences.


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Seismic facies and hydrocarbon potential of carbonate reservoirs … · 2019-04-28 · Seismic facies and hydrocarbon potential of carbonate reservoirs in ramp settings 3 Classification