U.S. GEOLOGICAL SURVEY Gas:Oil Ratios for Source Rocks Containing Type-I,-II,-US, and-HI Kerogens as Determined by

This report is preliminary and has not been reviewed for conformity with U. S. Geo'ogical Survey editorial standards (or with the North American Stratigraphic Code). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U. S. Government. INTRODUCTION Predicting the gas:oil ratio (GOR) of a petroleum play or prospect is important in evaluating the economics of an exploration venture and in assessing petroleum resources. Estimates of the GOR of prospects in well-drilled areas can typically be determined with analogs from neighboring production in the same play. In poorly drilled areas, GOR of prospects may be estimated by using analogs from more distant well-drilled plays that have similar geological settings or by models based on oil and gas generation from potential source rocks (e.g., Mackenzie and Quigley, 1988; and Kuo and Michael, 1994). GORs have also been determined by hydrous pyrolysis of potential source rocks in an exploration play or petroleum system (e.g., Noble and others, 1991; and Lillis and others, 1999). Unlike open-or closed-system anhydrous pyrolysis, hydrous pyrolysis generates an expelled oil that is physically and chemically similar to natural crude oil (Lewan, 1993 a; 1997). As a result, the quantity of this expelled oil and the gas generated allows one to calculate GORs of a particular source rock at hydrous-pyrolysis temperatures representing different stages of oil generation. The objectives of this report are to present GORs from hydrous pyrolysis experiments conducted on immature source rocks and report how they are affected by kerogen type and thermal maturity during petroleum generation. The source rocks used in this study contain the major kerogen types, including Type-I,-II,-US, and-III. Thermally immature samples were used to provide a complete understanding of changes in GOR from incipient petroleum generation through peak petroleum generation. METHODS Source Rock Samples: Experimental data were available on five immature source rocks, which are described in Table 1. The major kerogen types in these source rocks include Type-I,-II,-US, and-III. Collaborators that collected and pyrolyzed aliquots of these samples are also referenced in Table 1. With respect to vitrinite reflectance (%Ro), all of these sample have initial random mean %Ro values equal to or less than 0.5 or atomic H/C and O/C ratios indicative of pre-catagenesis (i.e., diagenesis) according to Tissot and Hydrous Pyrolysis Experiments: Complete details of hydrous pyrolysis experiments are given by Lewan (1993a). Briefly, the experiments consist of …