Devonian strata of the Thirtyone Formation at Dollarhide field, Andrews County, Texas, originally contained 144 million bbl of oil in place, with 18.6 million bbl produced by primary recovery, 39.3 million bbl projected to be produced by secondary recovery (waterflood), and 27.4 million bbl projected to be produced by tertiary recovery (CO2 flood). Devonian strata contain five main lithologies (from bottom to top): (1) carbonate-chert mudstone deposited in a basinal environment; (2) burrowed chert-dolomite (10–35 ft or 3–11 m thick), which accumulated as siliceous spiculitic wackestone and carbonate mud in a low-energy slope environment; (3) laminated tripolitic chert (0–65 ft or 0–20 m thick) deposited as a sponge spicule sand in submarine channels and fans on the slope; (4) bioclastic limestone (47–92 ft or 14–28 m thick), which accumulated as crinoidal grainstones in moderate to high-energy, outer shelf environments; and (5) upper dolomite (50–90 ft or 15–27 m thick) with dolomitized peloidal grainstones deposited in high-energy, shallow subtidal to intertidal environments, and thin chert beds deposited in supratidal environments. Deposition of the 180-ft-thick (55 m) reservoir section occurred during a basinward progradation of shelf and slope facies.
Porosity within Devonian strata is controlled mainly by depositional facies. Porosity occurs in two stratigraphic zones separated by the tight bioclastic limestone. The lower porosity zone (laminated tripolitic chert and part of the burrowed chert-dolomite; 0–100 ft or 0–30 m thick) is relatively homogeneous (not stratified), with high porosity (commonly 25–35%), moderate permeability (5–20 md), and short fractures causing elevated permeabilities near the bottom. The lower reservoir is thick in fan-shaped and elongate areas where sponge spicules were deposited on submarine fans and in submarine channels. The upper porosity zone (0–90 ft or 0–27 m thick) is within the upper dolomite and is heterogeneous, with stratified porosity that bifurcates, coalesces, and pinches out in several different intervals. The relative homogeneity and moderate permeability of the lower reservoir should help achieve a uniform sweep during CO2, flooding. Higher permeabilities near the base of the lower reservoir should minimize CO2, override, thereby improving sweep efficiency. The thinner, more heterogeneous, less continuous, more stratified, more permeable pay in the upper reservoir will result in more rapid and less uniform CO2 sweep.