Source Rock Potential and Thermal Maturity of Lower Cretaceous Strata: Monkman Pass Area, British Columbia

Abstract

Shale and coals from 1.2 km of core through most of the preserved Lower Cretaceous in western Canada were analyzed for source rock potential and maturation level. Although much of the core lies within the oil window, none of the formations are notably oil prone. Oxidizing bottom conditions and the large predominance of terrestrial organic matter (OM) in the relatively nearshore depositional setting of the marine sediments have led to the dominance of terrestrial OM throughout this sequence of rocks, including the marine shales. As such, mostly gas will be, or has been, generated. The nonmarine Minnes Group and Gething and Gates Formations average 2.7, 6.3, and 5.2% total organic carbon (TOC), respectively, with fair to very good gas source potential. The nonmarine Boulder Creek Formation contains 1.1% TOC and has poor source rock potential due to the presence of several thick paleosols. The marine Moosebar, Hulcross, and basal Shaftesbury Formations contain 1.5, 1.7, and 1.5% average TOC contents, respectively, and have poor to fair source rock potential. These marine sediments, which may have generated limited amounts of oil, show a progressive decrease of hydrogen index (HI) with depth, due to thermal degradation with increased burial. HI for nonmarine sediments remains constant with depth. The marine sediments contain mostly type III kerogen with lesser amounts of thermally degraded type II kerogen. Vitrinite reflectance increases systematically with depth from 0.94% to 1.5% Romax. Tmax values correspond well with vitr nite reflectance, although Tmax values for marine sediments increase more slowly with increasing maturation than nonmarine sediments.