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Shaly Sand Analysis as an Indicator of Hydrocarbon Production Potential in the Olmos Formation, Las Tiendas Trend, South Texas
K. T. Barrow (1), G. B. Asquith (2), G. L. Causey (2*)
The sandstones of the Olmos Formation in the downdip Las Tiendas trend of Webb County, Texas originated on a storm-dominated shelf seaward of a major deltaic complex. Stratigraphically trapped gas is produced along a northwest-southeast trend 40 mi long and up to 8 mi wide. The reservoir comprises thin-bedded sandstones separated by non-productive thin siltstones and stacked into two to six, 20- to 50-ft-thick productive intervals. Porosity and permeability sufficient for gas production are preserved in the bases of individual storm-event sandstones below the depth to which bioturbation has completely homogenized sand with overlying silt and clay.
Due to the presence of chlorite and illite-smectite mixed-layer clays in the Olmos sandstones, shaly sand analysis was required to determine accurate hydrocarbon pore-feet thicknesses. Because of the diversity of logging suites, the same types of porosity logs were not available from all wells and effective porosities had to be calculated in different wells using a variety of porosity-log types. In addition effective water saturations had to be calculated by both the Fertl and the Automatic Compensation methods. A volume of clay cutoff of less than 30% and an effective porosity cutoff of 10% were used to determine net pay intervals. Hydrocarbon pore-feet thicknesses were determined using shale-corrected effective porosities and water saturations.
Cross plots of hydrocarbon pore-feet thickness versus cumulative gas production have a correlation coefficient of +0.79. The cross plots also reveal that wells that do not contain net pay in both major productive intervals, the Olmos A2 and Olmos B2 zones, and that have hydrocarbon pore-feet thicknesses less than 1.5 ft, typically have cumulative productions of less than 0.5 Bcf. Notwithstanding the diverse logging suites that were run on wells in the Las Tiendas trend, hydrocarbon pore-feet thicknesses can be related to well performance using detailed shaly sand analysis.
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