45596-AC2
An Integrated Hydrogeochemical and Modeling Study of a Hydrocarbon-Rich Intermontane Basin (Uinta Basin, Utah)
Introduction Tertiary intermontane basins in the Rocky Mountain Region have hosted more giant gas fields in the last 10 years than any other major US onshore province. Gas is the fastest growing component of world energy consumption so identifying reserves from unconventional or continuous resources has gained momentum. The origin of natural gas in basins may be from thermogenic or a mix of thermogenic-biogenic sources. In this study, the natural gases and the associated formation waters of the Uinta Basin are analyzed for composition and stable isotopes. Our results have provided evidence to support regional-scale migration of both saline formation waters and hydrocarbon gases. The gases are interpreted to be originating from both thermogenic and mixed thermogenic/biogenic sources. The major findings are summarized below. Water Geochemistry From USGS, Utah Geological Survey, and Utah Department of Water Resources, an extensive water chemistry database is assembled. A screening analysis is conducted to remove (1) data points with charge balances > 5%; (2) all hydrocarbon test well data; (3) data with incomplete spatial information. A final record of 844 wells has the complete information (latitude, longitude, elevation, sampling depth) for each well, augmented by 56 wells sampled by us in 2006 and 2007. We observe that the formation water total dissolved solid (TDS) ranges from the very fresh to a maximum of ~80 g/L. The salinity does not show an increasing trend with depth, e.g., the Wasatch Formation is often fresher than the overlying and interbedded Green River Formation. The former was deposited in fluvial settings while the later contains evaporites, i.e., halite, trona (Na3HCO3CO3), nahcolite (NaHCO3). Most formation waters are Na-Cl-HCO3-SO4 type with significant Na excess at both shallow and deep depths. The Ca+Mg versus HCO3 plot completely avoids the carbonate dissolution line, suggesting the dissolution of sodium-carbonate evaporative salts, although locally within the Green River Formation we found evidence of calcite dissolution. To investigate the origin of solutes in the formation waters and the geochemical reactions that may have occurred, a subset of the water samples we collected was analyzed for stable isotopes. The hydrogen and oxygen stable isotope compositions of formation suggest that the Uinta Basin formation waters are a mix of evaporatively concentrated seawater with ancient, evolved meteoric water that has experienced water-rock interactions. This interpretation is consistent with that of the Br systematics. TDS (mg/l) variation suggests regional topography-driven migration and upwelling of deep saline brines towards the Green River which drains much of the basin. Due to temperature and salinity variations, variable-density effect is apparent along the transverse flow direction. At the regional scale, fluid migration crosses multiple formations indicating significant hydraulic communication rather than compartmentalization among the major hydrostratigraphic units. Gas Geochemistry From literature, industry database, and our own sampling, natural gas composition and isotope data are compiled. Gases in the Uinta Basin consist of methane and C2+ hydrocarbons (C2+ by volume ranges from 2 ~ 23%) with minor amount of Nitrogen and CO2 (< 2%) and trace amount of He, H2, O2 (0.001~0.04%). Based on chemistry and isotopes, the gases can be divided into two groups: (1) Associated gases of the Green River Formation (Altamont-Bluebell, Redwash oil fields) characterized by methane carbon isotope values of -60 to -45 permil, methane hydrogen isotope values of -280 to -225 permil and a gas dryness index [C1/(C1-C5)]) from 0.76 to 0.98. These gases are interpreted to be thermogenic (reservoir depth >9000 ft) and mixed thermogenic/biogenic (depth < 9000 ft); the source rock type is Type-I kerogen of the Green River Formation. (2) Non-associated gases of the Mesaverde and Wasatch Formations in the Nature Buttes Gas Field and southeastern basin characterized by methane carbon isotope values of -42 to -34 permil, methane hydrogen isotope values of -200 to -165 permil, CO2 carbon isotope values of -1.1 to -12.5 permil, and a dryness index from 0.8 to 0.9. The carbon stable isotope value of DIC in co-produced waters ranges from -6.8 to 15.9 permil. These gases are interpreted to be thermogenic, originating from the Type-III kerogen of the deeply buried Mesaverde Group. In the southeastern basin, gas migrates from basin center towards southeast, opposite of the regional flow gradient, given credence to the hypothesis of regional-scale fluid displacement from hydrocarbon generation. The Uinta gas signature is further compared to that of the midcontinent basins. Again, source rock, migration, and whether biogenesis plays a role distinguish the Uinta gases into two groups, whereas a subgroup exists for the Altamont-Bluebell field. Future Work A dynamic and coupled system of regional-scale fluid flow, hydrocarbon generation/migration, and salinity transport exists in the Uinta Basin. To understand their complex interplay, three-dimensional mathematical modeling will be conducted in future work.
