--> --> Paleozoic source rocks and thermal history of the Southern Denver Basin, Western Mid-Continent region USA

AAPG Rocky Mountain Section Meeting

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Paleozoic source rocks and thermal history of the Southern Denver Basin, Western Mid-Continent region USA

Abstract

Since 2009 drilling in the southern Denver Basin in Elbert and Lincoln counties has resulted in several new productive fields from Pennsylvanian and Mississippian reservoirs. The Denver Basin is an asymmetric foreland basin with a north-south configuration that is bounded by the Rocky Mountains to the west. The northeast-southwest Transcontinental Arch lies across the present basin center and to the south of this feature is the depositional center for Pennsylvanian age strata in the Denver Basin. Several thin highly organic (11% TOC) carbonaceous mudstones are found within the Pennsylvanian age Atoka, Cherokee and Marmaton formations. Pennsylvanian strata across and to the north of the Transcontinental Arch in the Denver Basin is thin with almost all of the carbonaceous mudstones absent. Production from the north side of the basin is predominantly from Permian reservoirs. Prior to 2012 the source of the petroleum found in the prolific Mississippian and Morrowan reservoirs in the southern Denver basin had not been identified. Some authors assumed oil migrated from the deep Anadarko Basin into the Denver Basin. The Las Animas Arch, a northeast trending positive structural feature forming the southeastern border of the Denver Basin made any migration into the basin unlikely. Cherokee and Marmaton formation reservoirs in the basin contain oil derived from marine carbonaceous shales of Desmoinesian age. The Atokan Formation lacustrine and terrestrial carbonaceous mudstones rocks have generated oil. Recent drilling in Elbert County has identified Ordovician source rocks that have not been found elsewhere in the basin to date. The Pennsylvanian basin center adjacent to the southern border identified as the Apishapa Uplift has been identified by some researchers to be the area of petroleum maturation and expulsion but there is no evidence to support this. Many of the Pennsylvanian carbonaceous shales in the basin center and on it southern border are either absent or have lower total organic content, dominated by coarse clastic material and there is a lack of oil shows in reservoir rocks or hydrocarbon accumulations. Oil generation and expulsion based on present data indicates that Elbert and Lincoln counties are the most likely source for hydrocarbons found in the southern Denver Basin. This presentation will discuss the likely thermal history and migration trends for these Paleozoic rocks in the southern Denver Basin.