The Powder River Basin is a prolific petroleum province, situated in northeastern Wyoming and southeastern Montana. The geology and structure of the region are well established but the geochemistry of the oils that occur in the numerous Cretaceous reservoirs are not well understood. The intent of this paper is to characterize the chemistry of Cretaceous Powder River Basin oils and source rocks, thereby establishing oil families and identifying their sources. This study reports whole oil bulk molecular compositions, saturate and aromatic fractions, carbon isotope compositions, and biomarker ratios for 124 oils from 13 Cretaceous reservoirs. Source rock data include Leco total organic carbon (TOC), Rock-Eval pyrolysis, and vitrinite reflectance for the Mowry and Niobrara formations.
Cretaceous oils observed in this study are derived from organic-rich marine clastic source rocks and are observed to be chemically homogenous with only minor variations in their molecular composition and biomarker chemistries. Geochemical data including bulk molecular compositions, isoprenoid distributions, and biomarker ratios suggest two major families of oils are produced from Cretaceous reservoirs in the Powder River Basin. Cretaceous oils produced from reservoirs below the Mowry are interpreted to be Mowry sourced and similarly, oils produced from reservoirs above the Niobrara are interpreted to be Niobrara sourced. Oils from reservoirs that occur between Mowry and Niobrara contain either Niobrara or Mowry derived oils. Vitrinite reflectance data and its equivalent from Tmax (Jarvie et al., 2001), Hydrogen Index (HI) (Olson, 2008), and triaromatic sterane ratio (TAS) (Mackenzie et al., 1981; Peters et al., 2005) show excellent agreement with one another. These data suggest that the highest maturity oils and source rocks occur near the basin axis; and become less mature with increasing distance from the basin's center. Leco-TOC and pyrolysis data show both Mowry and Niobrara have significant oil source potential and contain mainly type II kerogen with lesser amounts of Type III.
Hydrocarbons have been produced from various Cretaceous reservoirs in the Powder River Basin (PRB) for more than 50 years (Momper and Williams, 1984). The Powder River Basin (Figure 1) developed during the Laramide orogeny that began in the very late Cretaceous time and ended in the Eocene (Clayton and Ryder, 1984). The Laramide structure (Momper and Williams, 1984) may have controlled the direction of oil migration into different Cretaceous reservoirs from two major Cretaceous source rocks namely: Lower Cretaceous Mowry Shale and Upper Cretaceous Niobrara Formation (Figure 2). The Cretaceous Niobrara and Mowry Shale in the Powder River Basin (PRB) and their equivalents rocks have the most potential to be source rocks for hydrocarbon generation and production in the Rocky mountain region or throughout the Western Interior Seaway (Barlow, 1986; Davis et al., 1989; Longman et al., 1998; Landon et al., 2001; Martinsen, 2003). These two formations in the PRB are emerging petroleum resource plays.