--> Abstract: Facies and Lithostratigraphy of the Upper Cretaceous (Turonian-Cenomanian) Eagle Ford Formation, Central and South Texas, by Ryan Harbor, Stephen C. Ruppel, and Tucker Hentz; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Facies and Lithostratigraphy of the Upper Cretaceous (Turonian-Cenomanian) Eagle Ford Formation, Central and South Texas

Ryan Harbor1; Stephen C. Ruppel1; Tucker Hentz1

(1) Bureau of Economic Geology, University of Texas, Austin, TX.

The Eagle Ford Formation is a well-known source rock for both sandstone (Woodbine, East Texas Basin) and carbonate (Austin and Buda) reservoirs in central and south Texas. Yet, only minor research has been published on the character of Eagle Ford facies. Recent discoveries have demonstrated that source rocks, such as the Eagle Ford, are capable of producing economic volumes of gas and oil. At the same time, variations in well producibility indicate that these rocks, like conventional reservoirs, display considerable geological heterogeneity. Understanding the types, controls, and distribution of these heterogeneities requires in-depth rock-based studies.

To characterize facies, depositional processes, and system geometry of the Eagle Ford, we investigated more than 50 cores from 30 counties for depositional texture, sedimentary structure, and fossil assemblages. These studies were supported by light and electron microscopy, analysis of elemental chemistry and mineralogy, and determination of organ carbon distribution and thermal maturity.

Eagle Ford deposition occurred during a marine transgressive-regressive event that coincided with Oceanic Anoxic Event II. The Eagle Ford can be divided into lower (transgressive) and upper (regressive) members. Primary lower Eagle Ford facies comprise intergradational (1) laminated and unlaminated, organic-rich, calcareous mudrocks, and (2) unlaminated, fissile, siliceous mudrocks. Carbonate content is associated dominantly with planktic globigerinid foraminifera and nanoplankton. Locally these rocks show evidence of early diagenetic calcite recrystallization. The upper Eagle Ford also displays significant facies complexity; dominant facies include laminated (cycle-base?), calcareous, organic-rich mudrocks and lime mudstones, burrowed (cycle-capping?), organic-lean lime mudstones, and volcanic ash beds. Upper Eagle Ford rocks are distinctly gradational into the overlying Austin Formation.

Facies complexities in the Eagle Ford stem from complicated and interrelated processes of sediment production and distribution, diagenesis, and water column chemistry. Integrated studies of cores have begun to shed light on the both the controls of facies formation and their spatial distribution. These findings are critical for developing predictive models of the distribution of key reservoir properties including pore distribution and abundance, permeability, hydrocarbon distribution, and reservoir producibility.