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Chemostratigraphy of the Woodbine and Eagle Ford Groups, Brazos Basin, Texas

Abstract

Despite the success of unconventional resource plays, much has yet to be understood about the controls of organic matter (OM) richness and source rock quality in mudstone-rich formations. Elemental analysis and chemostratigraphy are powerful forensic tools for determining variations in depositional conditions in shale units that otherwise appear homogenous. In the study presented here, a chemostratigraphic framework is established for the Cenomanian-Turonian Woodbine and Eagle Ford Groups from 10 cored wells throughout the Brazos Basin, which lies geographically between the San Marcos Arch and East Texas Basin. A total of 12,282 XRF, 623 XRD, and 708 TOC measurements, derived from core, were integrated to define five distinct chemofacies, which vary systematically in mineralogy and organic matter content, within five regionally correlative chemozones. Several key elements —Ca, Si, Mo, Mn, and Ni— are correlated to depositional conditions and used in a hierarchical cluster analysis to characterize five chemofacies throughout the Woodbine and Eagle Ford Groups: (1) argillaceous, OM-poor; (2) transitional, OM-poor; (3) transitional, OM-moderate; (4) calcareous, OM-rich; and (5) calcareous, OM-moderate. Characterizations of OM-richness and mineralogy are established by correlations between key element, TOC, and XRD measurements and supported by thin section petrography. Five temporally and chemically-distinct packages, chemozones EB 1 through EB 5, coincide with major sequence stratigraphic units (sequences and system tracts) as defined by outcrop studies along the western margin of the basin. Chemozone EB1 is equated to the Woodbine Group, dominated by argillaceous, OM-poor chemofacies deposition, and contains fair-to-poor source rock. Chemozone EB 2 is equated to the Lower Eagle Ford Formation, dominated by calcareous, OM-rich chemofacies deposition, and contains excellent source rock. Chemozones EB3, EB4, and EB5 are all assigned to the Upper Eagle Ford Formation. Chemozone EB 3 is dominated by transitional, OM-moderate deposition and contains fair-to-good source rock. Chemozones EB 4 and EB 5 are dominated argillaceous- and transitional-OM poor chemofacies deposition containing poor source rock. Key elements associated with OM enrichment were identified and spatial changes in these elements were quantified by mapping their average values in each chemozone across the basin. This chemostratigraphic framework quantifies major shifts in environment (e.g. OM productivity, redox conditions, and detrital input) during Eagle Ford and Woodbine Group deposition and allows for a better assessment of their source-rock potential.