Coupling Reservoir-Quality Grainstone Deposition With Source Rock Distribution Around OAE 1A, Central Texas: A Holistic Approach

Abstract

Oceanic anoxic events (OAE) are regional to global exacerbated episodes of depleted oxygen in the ocean, and are usually accompanied by ocean acidification. These events are triggered by a variety of preconditions, with high atmospheric pCO₂ being a chief cause. An increase in global temperature and eustatic sea level rise may also accompany OAE. Commensurate with OAE, the benthic carbonate factory often limits or ceases sediment production, while the generation, deposition, and preservation of organic matter increases. Eventually the oceans revert to a state of equilibrium, and carbonate production returns. During the return to equilibrium, there is a period of supersaturation with respect to calcite within the oceans, which results in an interval of intense carbonate deposition (OAE recovery) that are typically characterized by precipitation of ooids and a preponderance of microbial biocalcifiers and heterozoans. The focus on OAE in the past has typically been on the organic-rich shales as an unconventional resource, but the potential for conventional plays should not be overlooked, particularly when OAE recovery facies are considered part of the larger OAE depositional spectrum. A better understanding of OAE and recovery could improve predictability of source and reservoir distribution, especially in Mesozoic carbonates. This study utilized outcrop, core, and seismic data to characterize the Early Aptian OAE (OAE 1A) recovery in Central Texas for the construction of a conceptual depositional model. OAE 1A in the western interior is marked stratigraphically by the Pine Island shale, and is followed by the recovery facies of the James/Cow Creek limestones. The James/Cow Creek forms a carbonate ramp system deposited in between the larger Sligo and Stuart City rimmed shelf systems of the Cretaceous of central Texas. Outcrop, core, and reflection seismic data have allowed for a proximal-to-distal reconstruction of facies tracts, including strandplain deposits dominated by bivalve grainstones, lagoonal deposits dominated by oncoid rudstones, and outer ramp buildups primarily composed of corals. The reversion of the depositional system from rimmed shelf to ramp is accompanied by a shift in skeletal allochem production from a photozoan and oligotrophic assemblage to a grainy, monomineralic heterozoan assemblage.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015