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Application of Natural Gas Composition to Modeling Communication within and Filling of Large Tight-Gas Sand Reservoirs, Jonah Field, Wyoming

Ko, Ting-Wei 1; Harris, Nicholas B.1; Cavanagh, Andrew 2
1 Colorado School of Mines, Golden, CO.
2 Permedia Research Group Inc, Ottawa, ON, Canada.

Jonah Field, located in the northwestern part of the Greater Green River Basin, Wyoming, produces large volumes of gas from the Upper Cretaceous Lance Formation, which consists of somewhat isolated fluvial channel sands encased in finer grained overbank sediments. The MPath modeling program, based on invasion-percolation (IP) theory, is used to model the gas migration and filling processes in the Jonah Field. Three scenarios of gas migration within the tight-gas-sand reservoir are tested: 1) gas filling the reservoir by hydraulic fracturing of the overbank shales; 2) gas filling the reservoir by upward diffusion through the shales; 3) gas filling the reservoir by vertical migration along the major faults or fracture systems, then lateral migration away from these faults and fractures. In order to model the gas isotopic compositional fractionation in these models, two types of possible source rocks are used - Type III kerogen with average initial total organic carbon (TOC) values ranging from 1.00 to 1.25%; Type II kerogen with average TOC ranged from 1.00% to 1.40 %. Two major bounding faults, the west and south faults, which entrap the gas in the wedge-like Jonah Field, were modeled in this study for major trapping mechanism. Conceptual models suggest that the hydraulic fracturing and diffusion models will produce vertical hydrogen and carbon isotopic fractionations while the faults/fractures-conduit model will produce lateral isotopic fractionation within the reservoirs. The diffusion model will result in a higher degree of vertical fractionation than the hydraulic fracturing model does. We present quantitative tests of these qualitative models using the MPath program, developed by Permedia. Future work will focus on the comparison of modeling results and the real data, trying to unravel the detailed mechanism of the filling processes in the large tight-gas-sand reservoirs in the Rocky Mountain regions.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009