Petrographic criteria for recognition of lacustrine and fluvial sandstone, P.R. Spring oil-impregnated sandstone area, southeast Uinta Basin, Utah

Reserve estimates indicate about 3.7 billion barrels of oil in place in the P. R. Spring area, most of which is in lacustrine sandstone of the Garden Gulch and Parachute Creek members of the Green River Formation (Eocene). Fluvial sandstone bodies in the Wasatch Formation (Paleocene-Eocene) produce gas in the south and southeast Uinta Basin, Utah. Elsewhere in the Uinta Basin, however, free oil and oil-impregnated sandstone bodies are dominantly in lacustrine facies of the Green River Formation. For good economic exploration of oil and gas in the Tertiary of the Uinta Basin it is desirable to understand the petrography of lacustrine and fluvial sandstone and to be able to distinguish them. This paper presents information on the petrography of lacustrine and fluvial sandstone of the upper Wasatch Formation and the lower Green River Formation in the P. R. Spring area, southeast Uinta Basin, Utah. Eighty-five percent of the lacustrine and fluvial sandstone is arkose. On the basis of modal analyses, the lacustrine sandstone contains significantly more quartz and authigenic carbonate cement and less plagioclase, matrix (terrigenous material less than 1/16 mm) and coarse mica than does fluvial sandstone. Some evidence indicates that lacustrine sandstone contains less potassium feldspar and rock fragments than fluvial sandstone. Plots of quartz versus feldspar, authigenic carbonate versus matrix, quartz versus plagioclase, quartz versus matrix and feldspar versus matrix are especially useful in differentiating lacustrine and fluvial sandstone; allochems (intraclasts, oolites or fossils) and minor analcime also characterize lacustrine sandstone. Detrital grains of the lacustrine and fluvial sandstone suggest heterogeneous sources. Plutonic rocks (granite, pegmatite and quartz vein) of the Uncompahgre uplift on the south and southeast probably were the main sources; weathered and eroded clastic and carbonate rocks possibly supplied almost as much sediment as the plutonic sources in some sandstone. The lacustrine sandstone is mineralogically and texturally more mature than is the fluvial sandstone. This increased maturity reflects the energy applied to the detritus after transport from its source areas. The present differences are independent of tectonism; mineral changes that took place during weathering in the source areas and during transportation of the grains also do not contribute greatly to the variations between the two types of sandstone. Significant differences are essentially a function of the environments of deposition. Minor red pigment (generally less than 1 percent) in both the lacustrine and fluvial sandstone is essentially post-depositional in origin. The fluvial sandstone contains more red pigment than the lacustrine sandstone.