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ABSTRACT: Oil-Source Correlations Between the Mississippian Heath Shales and the Reservoired Oils in the Pennsylvanian Tyler Sands, Montana

Gary A. Cole, Richard J. Drozd, John A. Daniel

The Mississippi Heath Formation exposed in Fergus County, central Montana, is comprised predominantly of nearshore, marine, black, calcareous shales and carbonates with minor anhydrite and coal beds. The black shales and limestones have been considered as sources for shale oil via Fischer Assay and pyrolysis analysis. These shales are potential

source units for the oils reservoired in the overlying Pennsylvanian Tyler Formation sands located 50 mi (80 km) to the east of the Fergus County Heath sediments studied.

Heath Formation rocks from core holes were selectively sampled in 2-ft increments and analyzed for their source rock characteristics. Analyses include percent total organic carbon (%TOC), Rock-Eval pyrolysis, pyrolysis-gas chromatography, and characterization of the total soluble extracts using carbon isotopes and gas chromatography-mass spectrometry. Results indicated that the Heath was an excellent potential source unit that contained oil-prone, organic-rich (maximum of 17.6% TOC), calcareous, black shale intervals. The Heath and Tyler formations also contained intervals dominated by gas-prone, organic-rich shales of terrestrial origin.

Three oils from the Tyler Formation sands in Musselshell and Rosebud counties were characterized by similar methods as the extracts. The oils were normally mature, moderate API gravity, moderate sulfur, low asphaltene crudes. Oil to source correlations between the Heath shale extracts and the oils indicated the Heath was an excellent candidate source rock for the Tyler reservoired oils. Conclusions were based on excellent matches between the carbon isotopes of the oils and the kerogen-kerogen pyrolyzates, and from the biomarkers.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990