Multimineral petrophysics of thermally immature Eagle Ford Group and Cretaceous mudstones, U.S. Geological Survey Gulf Coast 1 research wellbore in central Texas

Abstract

Traditional petrophysical methods to evaluate organic richness and mineralogy using gamma ray and resistivity log responses are not diagnostic in source rocks. This study presents a deterministic, non-proprietary method to quantify formation variability in total organic carbon (TOC) and three key mudrock mineralogical components of non-hydrocarbon bearing source rock strata of the Eagle Ford Group by developing a set of log-derived multimineral models calibrated with FTIR core data from the research borehole USGS Gulf Coast #1 West Woodway. This study determined bulk density response is a reliable indicator of organic content in these thermally immature, water-bearing source rocks.Multimineral findings indicate a high degree of laminae-scale mineralogical heterogeneity exists due to thinly interbedded carbonate cements amid clay-rich mudstone layers. The lower part of the Eagle Ford Group contains the highest average TOC content (4.7 wt%) and the highest average carbonate volume (64.1 vol%), making it the optimal target in thermally mature areas for source rock potential and hydraulic fracture placement. In contrast, the uppermost portion of the Eagle Ford Group contains the highest average volume of clay minerals (42.6 vol%), which increases the potential for wellbore stability issues. Petrophysical characterization reveals porosity is approximately 30% in this relatively uncompacted formation. In this thermally immature source rock, water saturation is nearly 100% and no free hydrocarbons were observed on the resistivity logs. No evidence of borehole ellipticity was observed on the three-arm caliper log, and horizontal stresses are presumed to be directionally uniform in the vicinity of this near-surface wellbore. This shallow wellbore has a temperature gradient of 1.87 ºF/100 ft (16.3 °C/km) and is likely influenced by Earth surface heating.