Carbonate Grainstone Geobody Occurrence and Organization

Charlie Kerans
Jackson School of Geosciences, University of Texas at Austin, Austin, TX

Skeletal, peloidal, and oolitic grainstones are critical elements of many of the world’s carbonate reservoirs. The four-dimensional distribution of grainstone systems often controls the rate and efficiency of hydrocarbon recovery. Most carbonate grainstones are assumed to occur near the shelf margin in flat-topped platforms or in the ramp-crest area of low-angle foreshore-shoreface successions, but recent studies have also shown abundant occurrence of carbonate grainstones in toe-of-slope and basin-floor settings. Equally important to platform morphology and current regime is the combined short-term and long-term accommodation controls that drive the sequence architecture and 4D arrangement of grainstone shoals.

Examination of grainstone reservoirs from a range of reservoirs and outcrop analogs allows differentiation in terms of both physical dimensions and accommodation setting. Data from Carboniferous, Permian, Jurassic, and Cretaceous outcrops and reservoirs illustrates a dominance of grainstones in 3rd-4th order systems tracts. Within an individual sequence, TST grainstones are more prone to tide-dominated architecture whereas HST grainstones illustrate wave-dominated geometries. Continuity of grainstone bodies in a broad sense is inversely correlated with magnitude of high-frequency eustatic amplitude, with individual grainstone bodies (those occurring within a single high-frequency cycle) rarely exceeding 1 km in dip dimension in icehouse systems, 2-3 km in transitional, and more than 4 km in greenhouse systems. Forced regressive systems in general illustrate amalgamated wave-dominated geobodies with strongly preferred strike orientation and a low degree of compartmentalization.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery