Lateral Continuity of the Eagle Ford Group Strata in Lozier Canyon and Antonio Creek, Terrell County, Texas

Abstract

Understanding of the local lateral heterogeneity within the Eagle Ford Group, a prolific mudstone reservoir on the Texas Gulf Coast, is hindered by a lack of well-preserved outcrops in close proximity to one another. Misinformation or over simplistic assumptions about relevant horizontal reservoir heterogeneities can lead to sub-optimal or uneconomical exploitation. High-resolution correlation of individual beds in the Eagle Ford Group over several miles in Lozier Canyon and Antonio Creek in Terrell County, West Texas, was used to document lateral variation in thickness, composition, sedimentary structures, and gamma ray response of these strata on a local scale. Physical tracing of the beds on outcrops and within Gigapan photomosaics, hand-held spectral gamma-ray scintillometer profiles, and examination of polished hand samples and thin sections were used to correlate Eagle Ford Group strata across Lozier Canyon and Antonio Creek. The results add value by increasing the understanding of local horizontal heterogeneities and the depositional environments of Eagle Ford Group strata and potentially influencing how and where wells are drilled and completed.

Five distinct lithostratigraphic units, termed A-E from the base up, and their subunits, are laterally continuous over several miles in terms of thickness, lithology, and spectral gamma ray response. However, there are notable differences in thickness and sedimentary structures in units A and B. Unit A has the largest difference in thickness (7%), suggesting higher accommodation in the southeast part of the study area. Moreover, sedimentary structures and bed morphology of skeletal packstone beds in unit B, the primary target of horizontal wells in the subsurface, vary over a 4-mi interval from discontinuous lenses to laterally continuous stacked beds. Simulated wireline logs obtained from outcrop exposures suggest that spectral gamma ray data is superior to total gamma ray data in correctly identifying the most desirable sub units for completion. Geochemical data and trace fossil abundance suggest primarily anoxic bottom water conditions during deposition of the Lower Eagle Ford Formation and oxic conditions during deposition of the Upper Eagle Ford Formation. Widespread zones of deformed bedding within the Eagle Ford Group strata typify certain units and were likely caused by paleoseismicity. Laterally extensive bedding plane exposures in Antonio Creek provide three-dimensional views of macrofossils and the bedform morphology that were previously only described from two-dimensional outcrops. Sedimentary structures suggest that units A, C, D, and E were deposited above storm wave base; and deposition of unit B was episodically above storm wave base.