--> ABSTRACT: Influence of Reservoir Heterogeneity on Gas Resource Potential for Geologically Based Infill Drilling, Brooks and I-92 Reservoirs, Frio Formation, South Texas, by Mary L. W. Jackson and William A. Ambrose; #91029 (2010)

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Influence of Reservoir Heterogeneity on Gas Resource Potential for Geologically Based Infill Drilling, Brooks and I-92 Reservoirs, Frio Formation, South Texas

Mary L. W. Jackson, William A. Ambrose

Gas resource potential for strategic infill drilling or recompletion in a reservoir can be calculated by subtracting gas volumes derived using the material balance (pressure decline) method from volumes derived using a volumetric method. This resource potential represents remaining gas that is not in communication with existing wells. Frio reservoirs in mature, nonassociated gas plays located downdip from the Vicksburg fault zone are characterized by multiple, vertically stacked sandstones. The Brooks reservoir, in La Gloria field, lies in a fluvial-dominated system that contains dip-elongate channel sandstone belts 1-2 mi wide. Within these belts are six or more vertically stacked channel-fill, point-bar, and splay deposits. Depositional environments were interpreted fro SP logs. Individual sandstones are separated vertically by thin mudstone layers and pinch out laterally into flood-plain deposits.

In contrast, the I-92 reservoir, in Julian North field, is located within deltaic facies. The reservoir contains single, thin sandstone bodies that are laterally continuous along strike and represent wave-reworked channel-mouth bar, prodelta, barrier-core, and tidal-inlet deposits.

Gas in place for these reservoirs was calculated volumetrically by planimetering net-sandstone-thickness maps for two stratigraphic sub-units in the Brooks reservoir and for one unit in the I-92 reservoir. These volumes were compared with gas-in-place estimates made from commonly used, reservoir-wide pressure-decline plots. Gas resources calculated for infill drilling in the geologically heterogeneous Brooks reservoir are 23% of postcycling gas in place, or 40 bcf at 92% recovery. Strategically placed infill recompletions in poorly drained reservoir compartments are the key to recovering this gas. The target gas volume for the I-92 reservoir was much smaller, indicating that existing wells may adequately drain this relatively homogeneous reservoir.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.