Abstract

Low-permeability sandstones of the Frontier Formation produce gas from reservoirs along the south-plunging Moxa Arch in the Green River Basin, southwestern Wyoming. Petrographic examination of 247 Frontier thin sections indicates that the main causes of low porosity and permeability in these sublitharenites and litharenites are compaction and cementation by quartz, calcite, and clay minerals. Major differences in burial history between the northern and southern ends of the arch have resulted in an average of 5% quartz cement in Frontier sandstones at the north end, compared with 13% at the south end.

Burial histories of four wells along the north-south trend of the Moxa Arch and one well off the arch in the deep Green River Basin were reconstructed using stratigraphic data from well logs. Uplift and erosion events incorporated into the burial-history curves occurred during the Late Cretaceous, at the end of the Cretaceous, and from the Oligocene to the present. Modern geothermal gradients of 1.7 to 2.0°F/100ft (3.1 to 3.7°C/100 m) were used to model thermal maturity. Second Frontier sandstone on the La Barge Platform at the northern end of the arch (T28N, R113W) has a calculated Time-Temperature Index (TTI) value of 30, compared with a TTI of 334 in Henry field at the southern end of the arch (T13N, R113W). Second Frontier sandstone in the deep basin well (T22N, R106W) has a TTI value of 3079.

In the five wells for which TTI was calculated, an excellent correlation (r = 0.95) exists between thermal maturity of the sandstone (log TTI) and average volume of quartz cement. Frontier sandstones at the southern end of the Moxa Arch and in the basin were buried to greater depths and exposed to higher temperatures than those at the northern end, so they experienced more stylolitization and intergranular pressure solution of chert, thereby liberating more silica for quartz cement. However, TTI is not a good predictor of reservoir quality because porosity and permeability in Frontier sandstones are controlled by many other variables in addition to quartz cement volume.