--> Abstract: Material-Balance Approach to Petroleum Systems: New Albany Shale/Chesterian of the Illinois Basin, by M. D. Lewan, M. E. Henry, D. K. Higley, and J. K. Pitman; #90937 (1998).

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Abstract: Material-Balance Approach to Petroleum Systems: New Albany Shale/Chesterian of the Illinois Basin

LEWAN, M. D., M. E. HENRY, D. K. HIGLEY, and J. K. PITMAN; U.S. Geological Survey, Box 25046, MS 977, Denver Federal Center, Denver, CO 80225

More than 75 percent of the oil and gas wells drilled in the world occur in the United States, which represents only about 6 percent of the world's land area. Assuming that expulsion, migration, and trapping mechanisms responsible for petroleum in the United States are representative of those in the rest of the world, a quantitative understanding of domestic petroleum systems can provide better evaluations of hydrocarbon potential in international petroleum systems. One method of developing such a quantitative understanding is through a material-balance approach. In its simplest form, the amount of undiscovered petroleum is the difference between the amounts of petroleum expelled from a source rock and the sum of petroleum discovered in-place and lost as a result of secondary migration within or leakage from a petroleum system. The enormous quantities of undiscovered petroleum determined by past attempts using this approach trivialized its utility because of gross exaggerations of source-rock expulsion efficiencies. Results from this study indicate that the quantities of expelled oil determined by various laboratory pyrolysis techniques can be in excess by a factor of two to four. This exaggeration of petroleum expulsion is further amplified by the application of the approach to an entire basin with no regard to regional variations.

The New Albany Shale/Chesterian of the Illinois basin, with its extensive drilling history and well-established geological frame work, has served as an excellent petroleum system to calibrate and constrain quantities of petroleum expelled and lost in a material-balance approach. Organic geochemical correlations indicate that more than 99 % of the discovered petroleum was derived from the New Albany Shale, and 60 % of this petroleum occurs in Chesterian reservoirs. Previous problems with scarcity and suppression of vitrinite in the New Albany Shale required a more detail delineation of thermal maturity that more specifically relates to oil generation and expulsion. More than 500 subsurface samples were collected and analyzed by Rock-Eval pyrolysis. The resulting hydrogen indices (HI) were calibrated against atomic H/C ratios of isolated kerogens to determine stages of petroleum generation and were mapped as HI contours (Figure 1). Original organic richness of the source rocks within the generative basin was determined through a relationship observed between source rock thickness and TOC of immature New Albany Shale sections bordering the basin. The generative basin was divided into seven oil-migration catchments on the basis of buoyancy-drive using a surface-flow model on top of the New Albany Shale (Figure 2). Amounts of petroleum expelled from the New Albany Shale, undiscovered and discovered in place, and lost during migration and erosion were evaluated for each catchment. Based on these defined catchments and realistic expulsion efficiencies, the amount of petroleum undiscovered within and lost from each catchment was determined in hundreds of millions to billions of barrels, as opposed to the hundreds of billions of barrels previously assessed for the entire basin.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah