ABSTRACT: Gas/Oil Ratios in Source Rocks

Brian Horsfield, Steve Larter, Stephan Duppenbecker, Steve Daines

Gas and oil are thought to be expelled from organic-rich source rocks as a single phase in high pressure-high temperature kitchen areas. Separation of oil-saturated gas from gas-saturated oil then can occur during secondary migration, with the result that shallow structures may be gas or oil filled. The primary gas to oil ratio (GOR) of the petroleum charge that leaves the source rock is a crucial component for modeling the volumetric behavior of petroleum along its migration route and for matching available trap volumes with the volumes occupied by the gas and oil phases. In the past, we have assessed GOR in the laboratory by measuring light vs. heavy compound abundances using high-temperature pyrolysis-gas chromatography, but bulk compositional differences between oils n pyrolysates complicate simple interpretations. We describe the basic compositional properties of primary petroleum charges from marine and nonmarine source rocks using a combination of field data and the results of low temperature closed system (MSSV) pyrolysis. More importantly, we describe how GOR may be determined not only as a function of organic matter type but also as a function of maturity. Vey low initial GORs (approximately 0.05kg/kg or 235 standard ft³/bbl using bulk quantitation) at low degrees of conversion are superceded by higher, and in some cases, essentially constant GOR over the bulk of the oil generation window. Examples of clastic and carbonate source rocks from northwestern Europe, the Middle East, and the United States are discussed with reference to th interplay of kerogen type, organic richness, maturity, and migration in controlling the GOR of reservoired petroleums.

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