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The Devil in the Small Details: Pore-Level Reservoir Characterization of the Cotton Valley-Taylor Tight Gas Sand, Rusk County, Texas

Charles L. Vavra
North Star Geological Services, LLC, Lucas, TX

Detailed reservoir characterization can be critical in unconventional and tight reservoirs, where an understanding of the controls and distribution of reservoir quality can make the difference between economic success or failure. Pore-level characterization, including assessment of pore geometry and its impact on reservoir quality and petrophysical response, is a critical component of detailed reservoir characterization. The Cotton Valley - Taylor interval in Rusk County consists of tightly cemented, very fine- to fine-grained quartzose sandstone, mudstone, siltstone and carbonate. Matrix porosity and permeability are quite low: the mean (geometric) stressed helium permeability is 0.005 mD, and mean helium porosity is 6.5 percent. Three porous rock types were distinguished based on pore geometry and cementing mineralogy: Rock Type A consists of quartz-cemented sandstone with reduced primary macropores interconnected by slot-like pore throats. Rock Type B consists of quartz-cemented sandstone with virtually isolated secondary pores (molds). Rock Type C consists of microporous clay-cemented sandstone. Although helium porosity and air permeability of the three porous rock types are similar, variations in pore geometry result in differences in capillary pressure and relative permeability between the rock types. Consequently, the rock types vary significantly in ultimate reservoir quality (bulk volume gas and effective permeability to gas at connate water saturation). Rock Type A has the highest reservoir quality and is highly productive. Rock Type B has intermediate reservoir quality. Although Rock Type C makes up over 3/4 of the reservoir volume, it contributes very little to productivity. The understanding of pore-level controls on reservoir quality and petrophysics provided a critical key to the economic exploitation of the reservoir.