Stratal Geometries and Patterns of Platform Carbonates: The Cretaceous of Oman
Henk Droste, Mia Van Steenwinkel, 2004. "Stratal Geometries and Patterns of Platform Carbonates: The Cretaceous of Oman", Seismic Imaging of Carbonate Reservoirs and Systems, Gregor P. Eberli, Jose Luis Masaferro, J. F. “Rick” Sarg
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Extensive carbonate platforms covered the eastern part of the Arabian Plate during Mesozoic times. The interior parts of these platforms are commonly visualized as undifferentiated, extensive shallow-water areas, where carbonates accumulate by aggradation. This view is based on the fact that individual shallowing-upward carbonate packages are laterally extensive.
The improvement of seismic quality and resolution, however, reveal internal geometries within the carbonates. The Cretaceous carbonate platform of Oman, for example, shows a complex internal architecture, rather than a “layer-cake” configuration. Recognition of these stratal geometries has important implications for prospective hydrocarbon discovery and development in these sequences.
The aim of this study is a better understanding of the internal architecture of carbonate platforms, which can guide exploratory-play evaluation and at the same time field-scale reservoir modeling and production performance of carbonates in general. Two examples—and two scales—have been chosen to highlight the internal complexity of these carbonate systems:
Habshan Formation: large-scale Arabian Plate margin configuration (approximately 300 m thick, progradation of more than 250 km);
Natih Formation: smaller-scale intrashelf carbonates configuration (approximately 50–100 m thick, progradation of more than 50–60 km).
These examples are based on good-quality, high-density seismic data, closely spaced wells and excellent outcrop exposures.
Arabian Plate Margin Configuration (Habshan Formation): The Cretaceous carbonate platform was initiated in central Oman during the major transgression over the base Cretaceous unconformity. After a rapid progradation of some 250 km to the north and northeast, the platform edge aggraded, leading to the development of a 700-m-thick platform succession.
A well-developed clinoform complex occurs in the prograding lower part of the platform sequence, represented by the “Habshan system.” This clinoform complex consists of a series of forward-built stratigraphic packages of 10–20 km width, each showing a change from low- to high-angle clinoforms. These packages are thought to represent third-order sea level cycles superimposed on a second-order regressive trend. The variation in clinoform dip angle is interpreted to reflect changes in accommodation space available during relative sea level lowstands and highstands. It is also associated with variations in sediment fabric. The high-angle clinoforms developed during platform aggradation in times of sea level highstand. They are composed of thick sequences of porous and permeable shallow-water-derived grainstones and packstones. The low-angle clinoforms represent forced regressive wedges formed during lowering sea level. They are composed of muddy, deeper-water calcareous shales, with platform-derived porous units.
Intrashelf Carbonates Configuration (e.g., Natih Formation): In the younger platform interior carbonates, seismic data reveal the presence of similar but smaller-scale clino-form complexes and the occurrence of intraplatform basins. Mapping of clinoform belts and directions of progradation in the platform interior Natih E shows that this extensive carbonate member consists of several separate platforms, which merged by lateral accretion. The platforms started to grow in areas with relatively low subsidence rates, such as basement highs and salt domes, following a regional rise in sea level. Merging of the platforms was not always complete and in the intervening areas relict intraplatform basins developed, which were later filled with shales. Similar to the large Habshan clinoform system, the clinoforms of the younger platform interior show cyclic variations in slope angle, associated with variations in sediment composition and related to relative change in sea level.
In addition to the internal features, the top of the Natih A Platform is characterized by uplift, karstification, and erosion. An extensive system of deeply incised meandering fluvial channel systems is observed on three-dimensional seismic.
Generally, the integration of seismic and well data is crucial to the recognition of inclined stratal geometries and the diachronic character of lithologic units. This recognition can have important implications for hydrocarbon prospectivity and reservoir development. At exploration scale, it allows the definition of stratigraphic trapping potential. In addition, risks and opportunities for reservoir and seal can be better evaluated. At development scale, it will guide well-log correlations away from the “layer-cake” model. This way, the understanding and prediction of reservoir heterogeneities, continuity, sweep efficiency, early high water cut, and water flood can be improved and field development plans optimized.