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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 28 (1978), Pages 93-100

Sand-Body Geometry and Reservoir Potential of Recent Clastic Depositional Systems of the Central Texas Coast

Thomas C. Connaly, Jr. (1)

ABSTRACT

The central Texas coast exhibits dip-oriented sand bodies, the Colorado and Brazos Rivers and Brown Cedar Cut, a tidal pass, and strike-oriented sands of the deltas of these rivers and the associated Matagorda Peninsula barrier-strandplain complex. The dip-oriented sand systems are depositing sediment by lateral migration of the fluvial channels of the rivers and the tidal and flood exchange between East Matagorda Bay and the Gulf of Mexico at Brown Cedar Cut. The strike-oriented sand bodies are developed in response to the marine energy of the receiving basin, the Gulf of Mexico. Strong southeast trade winds and occasional tropical storms in the Gulf have imparted a northeast-southwest trend to the barrier-peninsula and to the deltaic systems.

The facies relationships, geologic framework, and active processes in the area have formed predictable porosity trends. Relatively coarse and well-sorted channel sands have excellent porosity and lateral permeability. On the other hand, bar crest sands are muddy, contain shells, and tend to be burrowed. Crevasse splay sands are generally silty and iron-cemented. Shelly sands of the barrier-peninsula complex can develop excellent secondary moldic porosity by fresh-water solution of aragonitic shell material.

High potential exists for hydrocarbon accumulations in these sand bodies. The deltaic platform, washover fan complexes associated with the barrier peninsula, and abandoned fluvial meander belt sands offer the best reservoir possibilities. Organic-rich terrigenous muds underlie the sands as a result of the post-Wisconsin sea level rise. The protected bay-lagoon-barrier facies tract formed a setting for low-energy mud accumulation. As the drowned Pleistocene river valleys were filled, high-energy sand environments developed on the platform of lagoonal muds. Overlying muds of the delta plain, flood basin, and natural levee environments provide a hydrocarbon seal and enhance the preservation potential of the sand bodies.


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