--> --> ABSTRACT: Thermal Maturity, Organic Geochemistry, and Burial History of Pennsylvanian Rocks, Cherokee Basin, Southeastern Kansas, by Charles E. Barker, Joseph R. Hatch, Robert H. Goldstein, Anthony W. Walton; #91002 (1990).

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ABSTRACT: Thermal Maturity, Organic Geochemistry, and Burial History of Pennsylvanian Rocks, Cherokee Basin, Southeastern Kansas

Charles E. Barker, Joseph R. Hatch, Robert H. Goldstein, Anthony W. Walton

Vitrinite reflectance (Rm) and Rock-Eval pyrolysis data suggest that Pennsylvanian source rocks in southeastern Kansas have generated oil at least locally. Some of the heating required for oil generation may result from lateral movement of fluids from the Sedgwick and (or) Arkoma basins, or possibly spots of increased crustal heat flow. The presence of hydrothermal fluids is corroborated by fluid inclusion data. In some wells, the warm fluids have increased Rm to as high as 1.4% against the regional background level of 0.5-0.7%, suggesting the rocks are marginally mature to mature with respect to oil generation. Rock-Eval Tmax data generally parallel the Rm profiles confirming that the locally high Rm values are a rea signal.

Organic geochemical evidence from Rock-Eval pyrolysis also indicates oil generation has occurred. Hydrogen index (HI) versus oxygen index plots show that the Pennsylvanian source rocks contain a mixture of type II and III kerogen in marine mudrock and type III kerogen in the mudrock associated with coal-bearing rocks. Total organic carbon (TOC) levels are generally favorable in the source mudrocks with most in the range of 0.3 to 5 wt%. One contraindication for oil generation is that the HI values are somewhat low, generally ranging from 50 to 150 mg hydrocarbon/g TOC. However, S1 peak values exceed 1 mg/g rock in the moderate to high TOC mudrocks, an indication of oil generation.

Present-day geothermal gradients in southeast Kansas range from 35 to 50°C/km and the mean annual surface temperature is 13°C. Lopatin analysis using paleogeothermal gradients of 40°C/km or higher with a late Paleozoic through Mesozoic surface temperature of 20-25°C suggests that oil generation could have occurred at, or just after, peak burial. Merriam indicates pre-Cretaceous erosion removed up to several thousand feet of rock (amount depending on position in the basin). Therefore, maximum temperature resulting from burial occurred in the Permian to earliest Mesozoic. The timing of the entrance of hydrothermal fluids into the rocks is poorly constrained. However, the simplest hypothesis is that the hydrothermal fluid flow would coincide with near-maximum burial.< P>

The importance of the hydrothermal fluids is that they would locally enhance thermal maturity and increase total oil generation, possibly explaining the spotty occurrence of oils that can be geochemically correlated to Cherokee Group sources.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990