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Shale Microfacies: Eagle Ford Group (Cenomanian-Turonian) North-Central Texas Outcrops and Subsurface Equivalents
The Eagle Ford Group is a mixed siliciclastic/carbonate unit that records a Late Cretaceous (Cenomanian to Turonian) transgression. A major condensed interval (cycle 2.5 of the UZA-2 supercycle) occurs within the Eagle Ford Shale. Eagle Ford strata are overlain disconformably by the Austin Chalk. The Austin/Eagle Ford contact represents the Turonian/Coniacian boundary (89 MA). The Cenomanian/Turonian (92 MA) boundary occurs within the Eagle Ford Group. Regionally, Eagle Ford strata consist of two major depositional units: a lower (transgressive) unit dominated by dark well-laminated shales (exhibiting only minor evidence of bioturbation); and an upper (regressive) unit consisting of thinly interstratified (high-frequency cycles) shales, limestones, and carbonaceous quartzose siltstones. Petrographic characterization of shales from Eagle Ford outcrops near Austin, Waco and Dallas, Texas and subsurface equivalents in a core (Getty #1 J. T. Wilson) from LaSalle County, Texas reveals six microfacies: 1) pyritic shales; 2) phosphatic shales; 3) bentonitic shales; 4) fossiliferous shales; 5) silty (quartzose) shales; and 6) bituminous claystone and shales. Transgressive Eagle Ford shales consist mainly of microfacies 1, 4, and 6. The Eagle Ford condensed interval consists of microfacies 1, 2 and 3, and highstand Eagle Ford shales are comprised of microfacies 4 and 5. Eagle Ford microfacies exhibit distinctive sedimentological aspects and have source rock characteristics that vary systematically within a sequence stratigraphic framework. Transgressive Eagle Ford shales (occurring below the condensed interval) have optimum oil-prone source potential, whereas overlying regressive shales are gas-prone. Transgressive and condensed Eagle Ford strata represent poorly oxygenated low-energy, (below storm wave base) marine paleoenvironments of deposition. In contrast, overlying regressive Eagle Ford lithofacies accumulated in higher energy (above storm wave base), well-oxygenated, shallow marine paleoenvironments.
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