The stable carbon (d13C) and hydrogen (d2H) isotopic composition of hydrocarbons has been used in petroleum exploration and development since the 1960s. With the development of tight oil and gas plays over the last decade, analysis of stable isotopes has re-emerged as a powerful interpretive and predictive technology. In particular, isotopic analysis of mud and production gases is useful in the analysis of hydrocarbon maturity, migration and reservoir compartmentalization. Here, we report on a publicly available Colorado Oil and Gas Conservation Commission (COGCC) database of discrete and co-mingled production gases from the Lower to Upper Cretaceous Muddy " J" Sand, Codell, Niobrara and Sussex formations of the Wattenberg Field of Colorado. Production gases are characterized as early-mature to post-mature, oil-associated gases, ranging from −55 to −40 ‰ in d13C methane and 5–45 % C2+ concentration. Going down-section from the Sussex to J Sand formation, d13C of methane, ethane and propane components all increase, reflecting increasing maturity with depth, and the presence of multiple, discrete source rock/petroleum systems. Using a previously established calibration developed for the Upper Cretaceous Uinta Basin in Utah, we convert d13C ethane and propane values to a Vitrinite Reflectance Equivalent (VREiso) maturity scale. Mapped VREiso coincides with the regional thermal gradient. These results indicate that production gases from the Codell through Sussex have generated and accumulated in situ, with no apparent migration from distant source areas. Highest maturities, up to 1.5 VREiso, are centered over the geothermal hotspot, and indicate an area of gas-prone hydrocarbon production. Lower maturities (VREiso < 1.2) are located around the margins of the field, and indicate the liquids-rich " sweet-spot". By contrast, some J Sand formation gases located along the Longmont wrench fault zone exhibit maturity values as high as 2.1 VREiso, implying vertical migration of these gases from a deeper source. These results highlight gas stable isotope signatures as a useful and reliable hydrocarbon phase prediction and migration analysis tool.
The Wattenberg Field is located in the north-south trending Denver Basin of Colorado (Fig. 1). The first well was drilled in 1970. The field actively produces unconventional oil and gas from low permeability (" tight") reservoirs of the Lower to Upper Cretaceous Muddy (" J") Sandstone, Codell Sandstone Member of the Carlisle Shale, Niobrara Formation and Sussex Sandstone Member of the Pierre Shale (Higley and Cox, 2007; Fig. 2). Known source rocks include the Graneros, Huntsman, Mowry and Skull Creek shales, and Smokey Hill Shale of the Niobrara Formation (Higley and Cox, 2007; Thul, 2012). Production of gas and liquids from multiple unconventional reservoirs in the Wattenberg provides an ideal test area for analysis of maturity and migration using stable isotopes.