Spectral Gamma-Ray (K-U-Th) Response of Upper Cretaceous Woodbine Formation and Eagle Ford Group Mudstones in the USGS Gulf Coast #1 West Woodway Borehole Near Waco, Texas

Abstract

As part of the assessment of undiscovered, technically recoverable hydrocarbon resources of the Cretaceous mudstones in central Texas, the U.S. Geological Survey (USGS) cored and logged a borehole in the Woodbine Formation and Eagle Ford Group near Waco, Texas. The borehole yielded about 600 feet of core and geophysical logs, including a spectral gamma-ray (SGR) log. Mudstones units described from the borehole are, in ascending order, the Pepper Shale Member of the Woodbine Formation, and the Bluebonnet, Cloice, and Bouldin Members of the Lake Waco Formation and the South Bosque Formation, all in the Eagle Ford Group.

Differences in relative proportions of potassium (K), uranium (U), and thorium (Th) derived from SGR logs provide information about sediment provenance, climate variation, and redox conditions in sedimentary environments. In the context of this study, we evaluated the geology of mudstones by normalizing K, U, and Th to total SGR and plotted the values on a ternary diagram.

Thorium dominates the SGR signal in the Pepper Shale. Consequently, the Pepper is the only mudstone in the borehole with a mean SGR signature near the Th pole of the triangle. The proportion of normalized Th in the Pepper Shale is suggestive of a terrestrial provenance. Normalized U in the Pepper Shale, an indicator of degree of chemical reduction and potential for preservation of organic matter, is low relative to other mudstone units in the core. Correspondingly, based on geochemistry, the Pepper Shale has the lowest total organic carbon (TOC). Moreover, abundance and ratios of trace elements indicate the Pepper Shale was deposited in an oxic environment. These multiple lines of evidence show that the depositional setting for the Pepper Shale was the least chemically reducing among units present in the core.

Uranium strongly dominates the SGR signal of the Cloice Member of the Lake Waco Formation. The normalized Cloice data cluster near the U pole of the ternary diagram. This grouping indicates chemically reducing conditions in the depositional setting, suggestive of maximum marine flooding. Based on geochemistry, the Cloice Member has the highest TOC in the core. Trace elements indicate the Cloice was the most anoxic-euxinic during deposition. Collectively, these data indicate that the depositional setting for the Cloice Member was the most chemically reducing in the core. The Cloice, and equivalent strata, offer the most potential for regional hydrocarbon prospectivity.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018