A multiple paramenter approach to log analysis: a petrophysical study
Standard Archie water saturation and formation porosity are the two key parameters determined from wireline logs that are used in the evaluation of a subsurface reservoir as a potential hydrocarbon producer. They are measures of reservoir content but not reservoir performance, and by themselves do not provide an actual indication of the hydrocarbon productivity of a reservoir. The Desmoinsian Prue Sands, in the granite wash facies of the deep Anadarko Basin in southwestern Oklahoma, are established gas reservoirs that have proven difficult to evaluate petrophysically in order to distinguish hydrocarbon productive and nonproductive reservoirs. This study is intended to develop a different approach to log analysis in the texturally and mineralogically variable Prue Sands to improve the reliability of log analysis as an indicator of Prue hydrocarbon productivity. A suite of petrophysical parameters from a basic log package was used to examine reservoir characteristics in addition to Archie water saturation and formation porosity. These include the resistivity of the connate water in the reservoirs, reservoir quality, hydrocarbon movability and alternate methods to calculate water saturation values for the reservoir. Bulk volume water values were used to assess the relationship of water saturation of the reservoir to irreducible water saturation and to the grain size distribution that comprises the reservoir framework. Selected parameters can indicate reservoir characteristics that suggest the presence of excess conductivity associated with reservoir shaliness, a condition that is otherwise difficult to determine in the Prue granite wash. Logs from a series of wells with a range of established Prue production characteristics, from excellent to poor, were examined using the suite of petrophysical parameters included in this study. Results from the analysis of this suite of parameters were grouped into ranges of values representing the range of hydrocarbon production from the wells analyzed. The groups of values were ranked with a series of integer scores from 5 to 1 to represent the range of hydrocarbon productivity. Outlier scores of 0 and -1 represented conditions outside the normally expected ranges. A petrophysical analysis program was developed on a personal computer using the spreadsheet program Microsoft Excel to compute the multiple parameters used in the petrophysical analysis of the Prue. Results are extracted from the program and a series of charts generated in Excel are used to display and interpret the results. This study has demonstrated that production-calibrated ranking of multiple petrophysical parameters can be used to evaluate the hydrocarbon productivity of the Prue.