Surface to Subsurface Correlation of Eagle Ford Equivalent Strata From West to South Texas
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2017, Unconventional Resources Technology Conference (URTeC). Upper Cretaceous (Cenomanian-Turonian) Eagle Ford Group equivalent strata in west Texas (Ojinaga, Boquillas Formations) can be correlated with subsurface Eagle Ford Group strata in south Texas using hand-held spectral gamma-ray logs on outcrops, biostratigraphy, stable isotopes and U/Pb geochronology. These correlations indicate that some of these units are incompletely recorded even though they do not preserve evidence for long-term exposure. The Ojinaga Formation outcrops in west Texas records only Cenomanian strata based on its high uranium abundance throughout and biostratigraphy. Multiple ash beds correspond to thorium spikes. Sedimentary structures are limited to hummocky cross stratification (HCS) within the lower 30 feet. In Big Bend National Park two detailed measured sections (Hot Springs and Ernst Tinaja) of Boquillas Formation strata also suggest much of the Cenomanian is recorded but the Turonian portion is only partially preserved; specifically, neither of these sections preserves strata equivalent to Ocean Anoxic Event 2 (OAE2) that spans the Cenomanian-Turonian boundary. The Lower Eagle Ford Group of each section has elevated uranium abundance and contains abundant HCS. The Upper Eagle Ford Group at both sections has little uranium and few wave generated structures. At Lozier Canyon in Terrell County (west Texas) approximately 80 miles (130 km) to the east of Big Bend National Park, an almost complete section of the Eagle Ford Group is well preserved. Here the Lower Eagle Ford Group has abundant uranium and molybdenum (Mo), whereas the Upper Eagle Ford Group has much less of these two elements. The Lower Eagle Ford Group here contains abundant HCS and transitions upward into bioturbated skeletal wackestone/packstone. The stable isotope excursion associated with OAE2 is truncated at this section indicating a short-lived unconformity in this section. Subsurface correlation of multiple cores and wireline logs of the Eagle Ford Group in south and west Texas indicate that its thickness increases to the south into the Maverick Basin. The Lower Eagle Ford Group commonly has abundant uranium and locally preserved abundant thorium spikes. HCS is common in many of the cores of the Eagle Ford Group and bioturbation is rare. Biostratigraphy and stable isotope analysis indicates that many of these cores, even though they are thicker do not preserve the entire OAE2. Surface to subsurface correlations indicate that the Eagle Ford Group was deposited across a broad area of Texas that deepened to the south and west. Abundant HCS and increased bioturbation updip on the shelf indicate much deposition there occurred within storm wave base. Geochemical proxies (U, Mo) indicate much of the deposition occurred within a zone of shallow-water euxinia. Lastly, subtle surfaces record prolonged periods of non-deposition, the processes forming these surfaces are heretofore unknown as there is little or no physical evidence for subaerial exposure.
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Proceedings of the 5th Unconventional Resources Technology Conference
