Well Log Interpretation of a Devonian Gas Shale: An Example Analysis

SPE Members Because of the complex mineralogy, calculating accurate values of lithology, porosity and saturation in the Devonian Gas Shale cannot be done with traditional well log interpretation models. Tool response and relative volumes for each of the mineral components must be determined first to then accurately calculate porosity. A technique to analyze well logs in Devonian Shale has been developed by ResTech under a contract with Gas Research Institute. This paper demonstrates the steps and procedures used in this log interpretation methodology. A Devonian Gas Shale from the GRI Comprehensive Study Well Program (CSW No. IA) is used to illustrate the analysis technique. For further information on the development of the model, refer to "Devonian Shale Formation Evaluation Model Based on Logs, New Core Analysis Methods, and Production Tests", Guidry, Luffel, Olszewski, and Scheper, Transactions SPWLA, 1990. The volume of heavies which is primarily pyrite is computed from the Photoelectric (PE) curve using an empirical relationship derived from core data. The kerogen volume can either be computed from the Spectralog Uranium curve or the gamma ray log again using an empirical relationship developed from core data. The kerogen density porosity can be found from the slope of the line connecting the bottom points in a crossplot of density porosity corrected for heavies versus kerogen volume as seen in Figure 1. Likewise a crossplot of neutron porosity versus kerogen volume produces the kerogen neutron porosity from the slope of the line connecting the bottom of the points. After correcting for kerogen and heavies, the neutron-density crossplot results in the clay point being computed as shown in Figure 2. Core data indicates that the quartz to clay ratio in the Devonian Shale should be 44/55. The clay point is computed by extending the zero porosity line from quartz across the data cluster mean using this ratio. Once the clay point is known, the porosity, quartz volume and clay volume can be computed. Water saturation can then be derived utilizing Archie's equation. For very thick Devonian sections, the shale may need to be divided into two or three zones each with a unique clay point because of changes in the data cluster mean on the neutron-density crossplot. In the analysis presented, the available logging curves are gamma ray, uranium yield, photoelectric adsorption cross section, bulk density, neutron porosity, and deep resistivity. The paper also briefly describes alternative solution methods in the event only a few of those measurements are available. A primary objective of the paper is to show oil professionals operating in the Devonian Shale formation or similar gas shales a technique for analyzing well logs to calculate mineral component volumes, porosity, and hydrocarbon saturations. The analysis methodology is demonstrated to be straight forward and user friendly, enabling anyone in an organization to perform the analysis themselves. P. 393^