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An Integrated Stratigraphy of the Cenomanian-Turonian Eagle Ford Formation, Texas, USA – Part 2


The Eagle Ford Fm is a prolific source rock and active shale gas and liquid rich unconventional play encompassing the Cenomanian-Turonian oceanic anoxic event 2 (OAE-2) and deposited near the margin of the Gulf of Mexico Carbonate Shelf also in a retroarc foreland basin setting behind the Cordilleran orogen. Shell has been conducting research on road cuts along US Highway 90, and natural outcrops in the vicinity of Del Rio and Austin, Texas that provide excellent exposures of the Buda, Eagle Ford (locally known as Boquillas Fm) and the Austin Chalk Formations. In addition, Shell has drilled two cores, Shell IONA-1 and Shell INNES-1, behind these outcrops, recovering almost 1,000ft of continuous core from the Austin Chalk through the Eagle Ford and Buda Fms. The Eagle Ford Fm consists of a succession of calcite-rich mudstones (marls, limestones, and bentonites), which has been interpreted within a sequence stratigraphic framework, including parasequence bounded marl-limestone couplets. We provide biological, sedimentological, mineralogic, geochemical and statistical analyses of these marl-limestone couplets that raise doubts over the application of conventional sequence stratigraphy to these sediments. In contrast, variations in paleoclimate, volcanism, and paleoceanographic regime (all of which could be independent of global eustacy and local tectonics), with subsequent diagenesis, seem to have played more important role in controlling the deposition and preservation of these sediments. These results have important implications for the interpretation of organic-rich shale systems.