Chapter 7: Carbonate Sequence Stratigraphy and its Application to Hydrocarbon Exploration and Reservoir Development
Carbonate strata differ fundamentally from siliciclastic strata and require their own set of sequence stratigraphic and facies models to aid in hydrocarbon exploration and reservoir development. Significant differences that impact sequence stratigraphic models include the following: (1) the largely autochthonous marine deposition of carbonates in the shallow-water photic zone; (2) secular changes in carbonate biota and mineralogy; (3) higher impedance of carbonates; (4)greater range of platform slope angles in carbonates due to early cementation, organic binding, and coarser grain size; (5) the virtual termination of platform growth and associated rapid freshwater cementation of carbonates during sea level lowstands; and (6)vigorous off-platform shedding of excess carbonate sediment during sea level highstands. Because carbonate deposition is linked to the photic zone, carbonates are regarded as better recorders of accommodation (sea level + subsidence) changes than siliciclastics.
Carbonate platforms are composed of a spatial and temporal hierarchy of stratal units, including parasequences, sequences, composite sequences, and supersequences, which records repetitive and cyclic changes in accommodation and sediment supply. Stratal units are recognized by their position in the stratal hierarchy and by bounding surface type and stacking patterns. Stacking patterns (landward-, vertical-, and seaward-stepping) describe changes in the geographic position and character of stratal units with respect to thickness, stratal geometry, facies types and distributions, and bounding surface type. The character of stratal units and hierarchies is a function of platform geometry, subsidence rate, amplitude and frequency of eustatic sea level fluctuations, siliciclastic supply, and environmental factors such as marine energy and climate. Furthermore, the character of smaller scale stratal units is a function of position within the larger scale stratal units. The recognition and analysis of stratal hierarchies is limited by the type, quality, location, and resolution of available data. Where possible, the analysis of stacking patterns within a stratal hierarchy is the key to improved correlation, better prediction of reservoir, source and seal facies distribution, geometry and continuity, and prediction of unconformities and related diagenesis that impact reservoir quality.