Characterization of Core Extracted from the Olmos Formation (Cretaceous), Southwest Texas

Introduction

This preliminary report presents data from ongoing activities of the U.S. Geological Survey (USGS), Energy Resources Program, Gulf Coast Framework Studies Project (GCFSP). The purpose of the GCFSP is to conduct geologic framework studies that will aid in the evaluation of all fossil fuel energy commodities (coal, gas, and oil) in the Gulf Coast Region. The project will: (1) define the petroleum systems of the region; (2) conduct specific geologic framework studies and petroleum system analyses on selected priority intervals; (3) study the coal-bearing intervals to evaluate coal distribution and quality, and coal-bed methane, and source rock potential; (4) work in cooperation with the National Oil and Gas Assessment (NOGA) Project to conduct a focused assessment of the Gulf Region; and (5) contribute to the next phase of the National Coal Resource Assessment.

The USGS, through a Cooperative Research and Development Agreement (CRADA) with The Exploration Company (TXCO) of San Antonio, has drilled a series of wells to assess the coalbed methane potential in the South Texas portion of the Gulf Coast Region. This report presents lithologic descriptions, photographs, and x-ray prints of cored intervals from the Olmos Formation (Cretaceous) from two wells, 1-117CR and 1-613CR, located in Maverick County. The cored interval for 1- 117CR (from 1090.0 feet to 1212.5 feet below the surface) is 122.5 feet in length. For the 1-613CR well, the cored interval (from 1390.0 feet to 1525.7 feet below the surface) is 135.7 feet in length. The depths and lithology thicknesses for 1-117CR and 1-613CR were corrected based on comparison with borehole e-logs from the well locations. Due to depth correction of intervals for 1-613CR, information on cardboard or wood block labeled markers used in the core photographs to denote portions of the cores that were removed or missing, at the time of photography, do not reflect the corrected depth intervals.

X-ray radiographs were taken of the coal and carbonaceous shale portions of the cores to determine internal structural properties such as, partings, cleats, and burrows. For conversion to digital format the x-ray prints were scanned and, as a result, they appear as reversed images in this report. Additionally, in the scanned x-ray prints, carbonaceous shales or clays appear white and coals are black.

Initial analyses of sample data from cores 1-117CR and 1-613CR show average as-received ash yield values of 61.09 percent and 58.94 percent, respectively. Less than 50 percent ash yield occurs for five of the samples from 1-117CR and four of the samples from 1-613CR. Based on these initial results, intervals described as coal in 1-117CR and 1-613CR, in some cases, would be classified as carbonaceous shales and not coal (Wood and others, 1981).