Cyclic Drivers for Limestone/Marl Sequences, Eagle Ford Shale (Cenomanian-Turonian,) South Texas
Kosanke, Tobi; Denne, Richard; Campion, Kirt; Egenhoff, Sven; Spaw, Joan; Breyer, John
Oil production from the Eagle Ford Shale (Cenomanian-Turonian) exceeded 300,000 barrels per day in July 2012 and helped push oil production in Texas to its highest levels in more than twenty years. The Eagle Ford consists of cyclic interbeds of organic-rich marls and limestones containing abundant coccoliths and planktic foraminifera. Some suggested drivers for the cyclicity include complex interactions of eustacy, plankton productivity, clastic input, volcanic activity and bottom currents. These explanations overlook the substantial amounts of diagenetic calcite in the limestones. Diagenesis has been suggested as the primary source of rhythmic limestone-marl successions in other stratigraphic successions. Analysis of Eagle Ford Shale intervals from multiple wells focusing on representative one-meter cycles sampled at decimeter intervals reveals the importance of bottom currents and diagenesis in forming the limestone-marl interbeds. Ripple and scour lamina sets dominate the Eagle Ford mudrocks. Planktonic foraminifera fill ripple troughs and small scale scours. Burrows have created additional disruptions to laminae as seen in both thin section and computed tomography (CT) scan analyses. Planar horizontal lamination that would result from grains settling from suspension without subsequent reworking are absent. The highest concentrations of planktic foraminifera and inoceramid fragments—the most common silt-sized and coarser grains in the limestones—are typically found in winnowed lag deposits. These lag deposits are characterized by calcite overgrowth cements on the inoceramid prisms. The rocks containing the highest volume and weight % calcite are composed almost entirely of neomorphic spar and microspar. The lack of compaction within the recrystallized limestones evidenced by the condition of the visible planktic foraminifera suggests early diagenesis. The absolute abundances of nannofossils and microfossils can vary but no significant changes in the overall assemblage of fossils occurs within individual cycles, implying that changes in biologic productivity are not the main factor driving the cyclicity.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013