Abstract
Stable carbon (δ¹³C) and hydrogen (δ²H) isotopes have been used to characterize hydrocarbons for exploration, development and production since the 1960s, and have re-emerged as predictive tools with the development of unconventional "tight" oil and gas plays. Here we report on publicly available Colorado Oil and Gas Conservation Commission (COGCC) and U.S. Geological Survey (USGS) datasets of production gases collected from the Greater Wattenberg Area (GWA) of Colorado.
The δ¹³C and δ²H of natural gas can be used to interpret gas origin (i.e., bacterial versus thermal), hydrocarbon maturity, migration and reservoir compartmentalization, and production allocation. Maturities derived from carbon and hydrogen isotopes can serve as controls in regional maturity maps, which are used to help define areas of oil, wet gas and dry gas production. Stable isotope analysis can be executed pre-completion on samples obtained from mud gas and/or gas desorbing from cuttings and core. These data can help predict fluid type, API gravity, and gas-oil ratio (GOR), all of which can help guide land acquisition and development decisions.
Production gases from the lower to upper Cretaceous Muddy "J" Sand, Codell, Niobrara and Sussex formations are characterized isotopically as early-mature to post-mature oil-associated gases. Progressing from shallow to deeper formations, δ¹³C of methane, ethane, propane and δ²H of methane components all increase, reflecting increasing maturity with depth, and the presence of multiple, discrete petroleum systems (Sherwood et al., 2013). Stable carbon and hydrogen isotope values show a strong correlation to both initial and cumulative GOR for the unconventional Niobrara and Codell intervals of Wattenberg Field. While this relationship does not hold for wells actively producing from the Muddy "J", this could be a result of geologic compartmentalization in this interval due to faulting, natural migration and other factors.
Validation of the correlation between Niobrara and Codell production GOR and stable isotope composition was provided by an independent geochemistry dataset from the USGS. The predicted GOR values were then used to accurately distinguish reservoir fluid classification. These results demonstrate the potential of natural gas stable isotope signatures as a useful and reliable fluid quality prediction tool.
