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To Frack, Follow Heterohelicids’ Track

Abstract

With an estimated average production of 1.364 million bpd, the Eagle Ford still contains undiscovered 8.5 billion barrels of oil. The Cenomanian-Turonian Eagle Ford Group of South Texas was deposited during a transgressive cycle at the southern end of the Western Interior Seaway (WIS) that connected the Tethyan and Boreal seas. The Eagle Ford Group represents a classic example of lateral and vertical facies heterogeneities within a mixed carbonate-clastic mudrock system. These have been blamed for variations in wells producibility, which requires an application of variety of completion techniques. Some of the foraminiferal associated facies of the Eagle Ford include massive argillaceous foraminiferal mudrock, laminated argillaceous foraminiferal mudrock, laminated foraminiferal wackestone, cross-laminated foraminiferal packstone/grainstone, and nodular foraminiferal packstone/grainstone. Hedbergellids (trochospiral) and/or Heterohelicids (biserial) make the most the foraminiferal component of these facies, which also accounts for the increased carbonate content. The high calcite content in the Eagle Ford contributes to the rock brittleness, which is critical for hydraulic and acid fracking. Foraminiferal assemblages have been widely applied to interpret the paleoceanography of WIS, including at its southern aperture during the deposition of Eagle Ford and equivalent units, which are partially associated with OAE2. Several studies suggest a covariant relationship between the abundance of Heterohelicids (e.g., Planoheterohelix spp.) and TOC values. The increase in the relative abundance of biserial taxa indicates high surface primary productivity, associated with an expanded oxygen minimum zone and increased organic matter preservation. Initially, primary productivity in southern Texas may have been driven by bathymetry-induced upwelling and restriction, caused by Comanche Platform, between the WIS and Tethys. As sea-level rose, the margin of the Comanche Platform was a localized upwelling cell that enhanced the preservation of more organic matter. These paleoceanographic events are associated with abundance increase and dominance of dwarfed Protoheterohelix spp., which represent the most productive facies in the Lower and Upper Eagle Ford Group. Tracking Heterohelicid dominance events, by understanding and enhancing paleoceanographic models of the basin, will eventually facilitate the decision of picking the sweet spots for fracking.