Eagle Ford Facies Architecture: Interplay of Proximal and Distal Carbonate and Siliciclastic Sediment Sources

Like most mudrock systems, the Eagle Ford Formation of the South Texas Gulf Coast Basin is a product of varying sediment input from several sources, each of whose dominance varied through time. Understanding these sources, their products, and their distribution is key to developing realistic models of heterogeneity in unconventional reservoirs.

We conducted integrated studies of the Eagle Ford and related units in South Texas using outcrops and subsurface cores and logs to define the sources, types, and timing of major sediment input into the basin. This work shows that Eagle Ford facies are a function of three primary sediment sources (1) a high-accommodation carbonate platform to the southwest, (2) a low-accommodation fluvial deltaic system to the northeast, and (3) an intervening, low accommodation carbonate platform. Each of these source areas displays distinct assemblages of proximal facies that reflect local accommodation and sediment availability. Proximal facies along the southwest margin of the trend comprise carbonate grain density flows and debris flows and mudrocks deposited in a steepened outer platform setting. Equivalent facies at the northeast end of the trend are dominated by interbedded coarse siliciclastics and mudrocks that accumulated on a shallow shelf. Proximal areas of the intervening, low-accommodation platform contain interbedded redeposited carbonates and clay-rich, calcareous mudrocks deposited on a low-relief ramp. Distal facies comprise laminated organic-rich globigerinid/coccolith mudrocks across much of the shelf. Sealevel, changes in sediment flux, sea floor redeposition, and diagenesis all contribute both systematic and non-systematic variations in facies distribution. Facies stacking patterns define cyclic alternations of proximal and distal deposition that may be attributable to sealevel oscillations. However, attempts to correlate and define the continuity of these deposits are hampered by the effects of local diagenesis and bottom current reworking and the limited resolution of wireline logs. Nevertheless, comparison of rock-based depositional facies with other rock properties, such as mineralogy, pore types, sizes, and distribution, and organic matter types and distribution, indicate a fundamental link to reservoir properties. More precise definition of facies architecture requires innovative methods such as whole rock chemostratigraphy and a better understanding of the processes and results of diagenesis.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California