AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Evaluating the Gas Shale Potential of the Hilliard/Baxter Formation in the Green River Basin, Wyoming
(1) Shell Canada Ltd, Calgary, AB, Canada.
(2) Shell International E&P Inc., Houston, TX.
(3) Weatherford Laboratories, Houston, TX.
(4) Shell International E&P BV, Rijswijk, Netherlands.
Several Paleozoic and Mesozoic petroleum systems exist in the Green River Basin (GRB). However, the source(s) of the dry gas produced from Upper Cretaceous (Lance) tight gas sands at the Pinedale and Jonah fields is uncertain. If the marine shales in the underlying Upper Cretaceous Hilliard/Baxter Formation generated and expelled a significant amount of that gas, this deeper stratigraphic unit also may be a prospective gas-shale resource. Using locally-calibrated gas-source rock (SR) correlation parameters and other methods to identify the source of different GRB gas families, we performed an integrated HC charge evaluation of the Hilliard/Baxter Formation in the GRB to evaluate its commercial potential. We conclude that the large intrinsic storage capacity of this thick, porous silty shale is significantly greater than the volume of natural gas generated by the Type III kerogen in the Hilliard/Baxter Formation - making it unlikely those shales expelled a large amount of natural gas, or that their matrix porosity exhibits high Sg. Geochemical data obtained on cores and cuttings demonstrate the presence of several good Cretaceous SRs in the GRB that are mature enough to have generated gas: i.e., the oil-prone Mowry Shale, Niobrara Formation, and Lewis Shale; gas-prone coals/carbonaceous shales in the Mesa Verde Group. Hilliard/Baxter shales, however, are much leaner (<1.0 wt% TOC) and principally contain gas-prone kerogen. The composition of crude oil and condensate samples from the GRB indicates that most sweet oil was generated by marine SRs, while condensates were generated by non-marine SRs. The C isotopic composition of natural gas samples collected from different reservoirs, and gas samples generated during the pyrolysis of two SR samples (i.e., Rock Springs coal; Hilliard shale) and a GRB oil sample indicate: (1) most of the gas produced from Dakota and Frontier gas sands, and sandstone beds in the Lewis Shale, formed during the cracking of oil generated by Mowry and Lewis SRs; and (2) coaly SRs generated most of the gas found at the Pinedale and Jonah fields. The composition of mud gas and produced gas samples obtained from two recent deep exploration wells in the GRB supports our interpretation that the Hilliard/Baxter Formation is not an effective SR, and does not exhibit good gas-shale potential. We also identified samples that are mixtures of gas generated by the kerogen in the Mowry Shale, the Hilliard/Baxter Formation, and Mesa Verde coals.