How Interpreting Log Data can Improve the Accuracy of Resource Assessment for Shale Plays

Abstract

The recent developments of shale gas in the US have greatly encouraged exploration for the shale gas resource in Western Australia. The Merlinleigh Sub-basin of the Southern Carnarvon Basin in Western Australia has been found to contain high quality gas-prone source rocks from existing geochemical data. This paper demonstrates how log data can be used in the absence of geochemical data to increase the accuracy of resource assessments. An assessment of the Merlinleigh Sub-basin was used to calibrate and test the reliability and accuracy of the log method. A major parameter for resource estimation is the thickness of a source rock layer. This is usually determined by geochemical analysis and identifying high TOC intervals from cores. Undeveloped shale gas basins however, are limited in geochemical data whereas most of the drilled wells have wireline log data. Passey's (et al. 1990) ΔlogR method encompasses a multitude of algorithms that predicts TOC from well logs and precisely determines the thickness of a layer. The method is first calibrated with known TOC from cores before applying to wells without geochemical data. Prior studies identified three formation layers in the Merlinleigh Sub-basin; Byro Group, Wooramel Group and Gneudna Formation; potentially containing high quality source rocks. After applying Passey's delta log R method to determine thicknesses, each layer was ranked according to rock characterisation factors such as TOC, generation potential, vitrinite reflectance, porosity and permeability. The Monte Carlo method by Crystal Ball software was then selected to estimate the probabilistic resources of all layers. The production ability of each formation can also be estimated using standard open-hole log data for estimating the Ultimate Recovery in shale gas resources. In comparison, the TOC calculated from Passey's method produced results consistent with the known geochemical data, thereby validating both sets of data. The Wooramel Group was identified to contain the best quality source rocks, followed by the Byro Group, and lastly, the Gneudna Formation. According to P50 estimations from Crystal Ball, the Byro Group, Wooramel Group and Gneudna Formation contained resources of 51.6tcf, 40.7tcf and 1.4tcf respectively. The Ultimate Recovery for the above was found to be 4.69tcf, 3.71tcf and 0.13tcf respectively. Due to the low estimations for the Gneudna Formation, this paper will focus on the Byro and Wooramel Groups. This paper explores a unique way of using Passey's method for undeveloped resources having limited log data but no geochemical data. The log data was also used to estimate the Ultimate Recovery based on the production ability of each well. The exploration potential of the Carnarvon Basin in Western Australia for unconventional gas has been identified and is the first geochemical assessment for the Merlinleigh Sub-basin.