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
.