The Denver Basin is an asymmetric foreland basin with a north-south configuration that is adjacent to the ancestral and present day Rocky Mountains. The depositional center for Pennsylvanian strata for Denver Basin lies to the south of the Transcontinental Arch that crosses through the center of the basin. The Pennsylvanian Desmoinesian and Atoka age strata in the southern part of the basin contain several (28+) thin highly organic carbonaceous mudstones, two to three feet on average that are laterally extensive. North of the Transcontinental Arch in the Denver Basin the Pennsylvanian strata is very thin with only one or two of the carbonaceous mudstones present. The identity of the source of the petroleum found in the prolific Mississippian and Morrowan reservoirs has historically not been identified. The literature has assumed oil migrated from the deep Anadarko Basin into the Denver Basin, The Las Animas Arch, a northeast trending positive structural feature and the basin’s eastern tectonic boundary is a major impediment for fluid movement. The Cherokee and Marmaton formation reservoirs contain oil that is derived from marine carbonaceous shales and limestones present in the Cherokee strata. The TOC content of the Cherokee Formation mudstones average 12% and can be as high as 27%. The Atokan carbonaceous mudstones have generated oil in some areas (average 11% TOC) and are lacustrine or terrestrial in origin. Recent drilling has identified Ordovician source rocks on the western side of the basin that have not been found elsewhere in the basin to date. The Pennsylvanian basin center, located at the very edge of the southern end of the basin, has been proposed in the literature to be the area of petroleum maturation and expulsion but there is no evidence for this. Many of the Pennsylvanian carbonaceous mudstones in the basin depocenter have lower organics, contain coarse clastic material and there is a lack of oil shows or accumulations. Recent data presented here that suggests that oils in Paleozoic reservoirs were thermally altered and expelled from Atoka and Cherokee carbonaceous mudstones in the area of the present day basin center on the western side of the basin.