Bed-Scale Facies Variability of Outer Shelf Deposits in a Gas-Shale Reservoir Analog: Cretaceous Eagle Ford Formation, Del Rio, SW Texas

Minisini, Daniel ¹; Bergman, Steven ²; Macaulay, Calum ²
(1)Clastic Team, Shell, Houston, TX. (2) Shell, Houston, TX.

Using measured sections, sedimentological descriptions, photomosaics, thin sections and GR logs, this study characterizes various scales of lithological heterogeneity in the outer shelf deposits of the Eagle Ford Fm., a cropping out formation analog for producing shale-gas reservoirs in the adjacent Maverick Basin. Lately, the Maverick Basin has become a target for unconventional shale-gas production in Cretaceous carbonate and clastic deposits, including Eagle Ford Fm. In SW Texas, the Eagle Ford shale is 70 m thick and is exposed in outcrops up to 500 m allowing correlations for 80 km.

Large-scale vertical heterogeneity is associated with alternating clay-rich shales, carbonate-rich shales and limestones. Each of these facies record considerable small-scale heterogeneity. Some reddish clay-rich shale present mm-scale laminations and/or interbedded layers of white chalk, specific units of the carbonate-rich shales include in-situ Inoceramus shells, limestones have nodular or tabular aspects, and register massive or cross-bedding laminations. Beds are rarely bioturbated and, in general, have sharp contacts in the lower units and gradual contacts in the upper units.

Secondary depositional features generate high vertical variability and determine markers for stratigraphic correlations: i.e., mass-transport and turbidite deposits, thin claystone beds, chert layers, fossil-rich units and volcanoclastic layers. Some of these depositional elements show lateral facies variations: e.g., 1) mass-transport deposits exhibit decreasing the amount of rip-up clasts and become thinner basinward, hence helping the understanding of fluid flow direction, 2) limestone beds exhibit variable amount of carbonate content and well-defined elongated concretions, suggesting preferential locations for diagenetic processes, 3) clay-rich shales are black where the outcrops are higher and become reddish where the outcrop are lower, indicating that weathering is an important factor when studying shales in the field.

The stratigraphy of the Eagle Ford suggests an environment of deposition subject to intense variations in quantity and quality of sediment delivery, depositional processes, fauna colonization and organic content. The observations of bed-scale facies variability in the outcrop may help the understanding of the correlatable facies which are producing gas in the subsurface and define exploration strategy.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.