ABSTRACT

The Excello Shale is one of the best exposed examples of Pennsylvanian cyclothemic, organic-rich shales in the Midcontinent of the United States. This study describes thermal maturation, hydrocarbon generation potential, and organic facies of the Excello black shale and relates to stratification, paleocirculation, and paleogeography of the study area during the glacial-related paleoclimatic cyclic anoxic episodes. The establishment of a density-stratified water column, or pycnocline, was the most important factor in the development of the anaerobic sediments.

The total organic carbon (TOC) content of the Excello black shale ranges from 1 to 17 wt.% with an average of about 10 wt.%. The distribution pattern of TOC, lack of bioturbation, and organic facies types indicate that the Excello black shale was deposited in a highly productive epeiric sea with bottom-water anoxia. Thin laminations and fine particle size indicate very slow sedimentation rate in quiet bottom water conditions, and gentle undulations of strata suggest a generally flat bottom topography. The dominance of clay-sized materials and the deficiency of silt-sized materials within the Excello black shale suggest that organic-rich muds were deposited very slowly, far from sources of sediment in a sediment-starved shallow basin. Besides marine organic matter (60-80 vol.%), abundant terrigenous vegetation provided structured kerogen (20-40 vol.%) that was transported into the Excello epeiric sea and accumulated at a rapid rate. Based on paleogeograhic setting, the Excello Shale was deposited under conditions of high rainfall and a warm paleoclimate along the tropical belt (4°-8°N paleo-latitude).

Vitrinite reflectance (Ro %=0.51-0.63), Rock-Eval, Soxhlet extraction, and elemental analysis of kerogen indicate that the outcrop samples are relatively immature, but that cores from west-central and northwestern Oklahoma are mature (Ro %=0.61-1.44) with respect to hydrocarbon generation. Vitrinite reflectance studies also suggest that much of the terrigenous kerogen in the cores is recycled, and shows multimodal distribution.