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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. Unconventional systems include coalbed methane, shale gas, and basin-center or deep gas. Conventional resources are buoyancy-driven deposits, occurring as discrete accumulations in structural or stratigraphic traps, whereas unconventional resources are regionally pervasive and largely independent of such traps.
The origin of natural gas in these accumulations may be from thermogenic sources or from a mix of thermogenic-biogenic sources. The natural gases and the associated formation waters of the Uinta Basin are analyzed for composition and stable isotopes. Our results have provided overwhelming evidence to support regional-scale groundwater migration. By analyzing gas geochemistry, gas genesis and migration are elucidated with evidence of thermogenic and mixed thermogenic/biogenic sources. The major findings to date 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 17 wells sampled by us in 2006. 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 type with significant Na excess and completely avoid the carbonate dissolution line, suggesting evaporite dissolution. For the 2006 samples, Cl/Br versus Na/Br plots near the seawater evaporation and NaCl dissolution line, indicating a similar origin of basinal brines despite the varying formation age and location. Compared to the GMWL, ?D versus ?18O displays 18O enrichment, a likely result of mixing of evaporatively concentrated seawater with fresh meteoric water. Significantly, TDS (mg/l) variation suggests regional topography-driven migration and upwelling of deep NaCl brines towards the Green River. Due to temperature and salinity variations, variable-density effect is apparent along the transverse flow direction (i.e., cyclic flow pattern). At the regional scale, fluid migration crosses multiple formations indicating significant hydraulic communication rather than compartmentalization.
Gas Geochemistry: From literature, industry database, and the 2006 sampling campaign, natural gas composition and isotope data are compiled. Overall, 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 both chemistry and isotopes, the gases can be divided into two major groups: (1) Associated gases of the Green River Formation (Altamont-Bluebell, Redwash oil fields) characterized by methane ?13C of -60 to -45 %0, methane ?D of -280 to -225 %0 and a gas dryness index (log10[C1/(C2+C3)]) ranging from 0.6 to 2.1. 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, Wasatch, and Green River Formations (basin-center Natural Buttes and fields sampled in 2006) characterized by methane ?13C of -42 to -34 %0, methane ?D of -200 to -165 %0, CO2 ?13C of -5.7 to -12.5%0, and a dryness index from 0.8 to 1.5. ?13C of DIC in co-produced waters (2006 samples only) ranges from -6.8 to 15.9%0. 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 fluid displacement from hydrocarbon generation. .
Future Work: Gases and waters sampled in 2007 will be analyzed for composition and isotopes. Results will augment the existing databases to allow a more complete interpretation of regional fluid flow and gas migration. To understand the complex interplay between variable-density flow, salinity transport, and gas migration, three-dimensional mathematical modeling will be conducted.
