Hydrocarbon maturity and migration analysis using production gas stable isotopic signatures in the Wattenberg field, Denver Basin, Colorado, USA
Sherwood, OA; Travers, PD; Dolan, MP
The stable carbon (13C) and hydrogen (2H) 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 in13C methane and 5-45 % C2+ concentration. Going down-section from the Sussex to J Sand formation, 13C 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 13C 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. © Copyright 2013, Unconventional Resources Technology Conference (URTeC)