The Evolving Role of Sequence Stratigraphy in
Carbonate Reservoir Characterization
Charlie Kerans Senior Research Scientist Senior Technical Advisor Bureau of Economic Geology John A.& Katherine G. Jackson School of Geosciences The University of Texas at Austin
The advent of carbonate sequence stratigraphy has played a major role in the development of modern high-resolution stratigraphic frameworks to be used as a front end for reservoir characterization and simulation. In retrospect it should have come as no surprise that stratigraphic and "facies" models with chronostratigraphic resolutions two orders of magnitude greater in resolution would provide the basis for rethinking the way we characterize reservoirs, but the acceptance of sequence stratigraphy into the realm of the production geologist, petrophysicist, and reservoir engineer, has been a slow process. When seismic stratigraphy and later sequence stratigraphic concepts were introduced in the 70's and early 80's, the emphasis for this interpretation methodology was as an exploration tool for evaluation of frontier basins. The fast velocities inherent in carbonate sections, and the common setting of many carbonate reservoirs in broad platform-top positions with little depositional topography limited our ability to apply concepts of seismic sequence stratigraphy as founded in stratal geometry at the reservoir scale.
The Vail sequence stratigraphic model proved far more robust than the original seismic stratigraphic approach, providing a predictive model for reservoir, source, and seal distribution as controlled by a combination of eustatic driver, sediment supply, subsidence, and climate. It was this predictive sequence model that through the 80's and 90's was run through the ringer using subsurface core, well and seismic data as well as detailed and regional outcrop studies and found to be a revolutionary tool. The role of sequence stratigraphic studies in carbonate reservoirs is a story that is still developing, but already has many success stories. Early applications of sequence concepts to carbonate fields provided some significant impacts. Exxon's reinterpretation of the Shuaiba reservoir at Bu Hasa, first presented internally in the early 80's, is still strongly influencing the interpretation and model construction of the reservoir. Acceptance for sequence-based reservoir models was boosted significantly in the engineering community when parasequence-scale mapping and simulation studies showed that flooding surfaces of these individual cycles corresponded to flow units (barriers and baffles). Engineers could be heard in hallways discussing what a TST and HST meant.
Intensive analysis of outcrop data, computer-based forward stratigraphic models, and cyclostratigraphic approaches have led to the construction of a robust set of interpretive tools for constructing 3D models of carbonate outcrops and reservoirs. In addition to the predictive elements of the original Vail model, workers have added stacking pattern analysis, progradation/aggradation ratios, facies partitioning, and Milankovitch calibration, and rock-fabric/geometric analysis, to the original sequence stratigraphic tool kit. Using these approaches, major advances have occurred in constructing stratigraphic frameworks for carbonate reservoirs. Important challenges remain in the areas of translating these models into engineering models, and ensuring model veracity, but it is clear that sequence stratigraphy has given a boost to the characterization of many of the world's largest reservoirs.