ABSTRACT: A New Approach to Oil-Source Rock Correlation--Extraction of the Oil Phase from Source Rocks

CLAYTON, JERRY L., U.S. Geological Survey, Denver, CO

The normal procedure for oil-source rock correlation is to compare the composition of the oil with that of bitumen obtained from solvent extraction of powdered samples of suspected source rocks. The assumption in this approach is that the bitumen obtained from extraction of powdered source rock samples can be considered "unmigrated oil" or is sufficiently similar in composition to the oil generated and expelled from the source rock to accomplish the correlation. Therefore, the composition of the extract obtained from the powdered source rock sample is expected to be similar to that of the expelled oil except for fractionations accompanying primary migration or secondary effects to either the source rock or expelled oil subsequent to expulsion of the oil. Unsuccessful attempts using st ndard procedures in our laboratory to establish positive correlations between organic-rich black shale of the middle member of the Minnelusa Formation and associated oils led to a new approach using extracts obtained from whole, uncrushed core samples for comparison with the oils. The new approach is based on the rationale that because the black shales are sufficiently thermally mature and organic-rich, an oil phase separate from the bitumen phase has formed and undergone primary migration. Bitumen that can be obtained from extraction of powdered samples is still associated with the parent kerogen and is an intermediate in the oil-generation process (i.e., kerogen-oil) that does not correlate with the expelled oil. Extraction of whole core source-rock samples yields mainly an oil phase t at has formed a continuous, separate phase along microfractures or bedding plane partings; this extraction does not remove from the saturated micropores significant amounts of the bulk bitumen. The oil obtained from this procedure has higher ratios of pristane/phytane, n-alkanes/isoprenoids, and hydrocarbons/nonhydrocarbons (resins + asphaltenes) than the bitumen and correlates with the oils produced from reservoirs in the middle member of the Minnelusa Formation. This new approach is helpful not only for correlation studies, but also helps elucidate the process of oil generation and expulsion. Further experiments are in progress to test the applicability of this method for other petroleum systems.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)