Organic-Rich Source Rocks of Pennsylvanian Age in the Paradox Basin of Utah and Colorado

The Paradox basin of southeast Utah and southwest Colorado contains a thick sequence of rocks of Pennsylvan- ian age which include numerous organic-rich black shales. The richest of these shales are associated with a series of evaporitecarbonafe cycles and form the important petroleum source rocks of this basin. Recently the entire Pennsyl- vanian interval was sampled in two Department of Energy core holes. One of these core holes, the Gibson Dome No. 1 (GD1) in sec. 21, T. 30 S., R. 21 E., San Juan County, Utah, provided most of the samples used in this report. The Paradox black shales consist of clay-sized quartz and feldspar, various clay minerals, and a high percentage of calcite and dolomite. The organic carbon content in these rocks is highly variable ranging from about 0.5 to 13.0 weight percent. Shales in the evaporitecarbonate cycles appear to grade laterally into a prodelta facies, which is part of the Silver- ton fan delta, located on the southeast margin of the basin. Although the black shales were deposited in an euxinic- evaporitic environment, pyrolysis and extraction data suggests they contain a mixture of type I1 and 111 kerogen. The somewhat Unexpected abundance of vitrinite in the macerals of these shales suggests a large contribution of terres- trial material associated with fluviatile influxes from the Silverton delta. Vitrinite reflectance values (RO) in the shales are strongly suppressed and are <0.50%in some samples that have generated as much as 16,ooO ppm hydrocarbons. One of the important source shales, known as the "Gothic," underlies many of the producing carbonate reservoirs in the southern part of the basin. At the GP1 location the "Gothic" is 32 ft (9.7 m) thick and has generated at least 4,970 bbls of oil per acre. This shale thickens to over 160 ft (48.7 m) near the Silverton fan delta complex where it may intertongue with porous delta front and distributary channel sandstone reservoirs. This intertonguing relationship sug- gests a potential petroleum exploration target; however. the quality and quantity of kerogen in the shale may decrease in this part of the basin.