Thermal Maturation and Petroleum Source Rocks in Forest City and Salina Basins, Mid-Continent, U.S.A.
K. David Newell, W. Lynn Watney, Joseph R. Hatch, Gu Xiaozhong
Shales in the Middle Ordovician Simpson Group are probably the source rocks for a geochemically distinct group of low pristane and low phytane oils produced along the axis of the Forest City basin, a shallow cratonic Paleozoic basin. These oils, termed Ordovician-type oils, occur in some fields in the southern portion of the adjacent Salina basin. Maturation modeling by time-temperature index (TTI) calculations indicate that maturation of both basins was minimal during the early Paleozoic. The rate of maturation significantly increased during the Pennsylvanian because of rapid regional subsidence in response to the downwarping of the nearby Anadarko basin. When estimated thicknesses of eroded Pennsylvanian, Permian, and Cretaceous strata are considered, both basin remain elatively shallow, with maximum basement burial probably not exceeding 2 km.
Despite this shallow burial, marginal maturity can be demonstrated for Simpson shales in the deepest parts of both basins by assuming an average geothermal gradient of 30°C/km. Available vitrinite and pyrolysis data are sparse but corroborate the TTI modeling. TTI calculations also indicate Pennsylvanian shales in both basins should be immature. In addition, several Pennsylvanian shales contain reworked or oxidized organic matter. Devonian-Mississippian Chattanooga Shale should be immature in the Forest City basin but may be marginally mature in the southern Salina basin.
According to maturation modeling and regional structure mapping, the axes of both basins should contain Simpson rocks in the early stages of oil generation. The probability of finding commercial accumulations of Ordovician-type oil along the northwest-southeast trending axis of the Salina basin will decrease in a northwestward direction because of (1) westward thinning of the Simpson Group, and (2) lesser maturation due to lower geothermal gradients and shallower paleoburial depths. The optimum localities for finding fields of Ordovician-type oil in the southern Salina basin will be in down-plunge closures on anticlines that have drainage areas near the basin axis.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.