The Denver-Julesburg (Dj) basin has multiple oils and gas producing unconventional reservoirs but the oil-source-reservoir correlation of hydrocarbon from these reservoirs are not well fingerprinted through detailed geochemistry dataset. It is important to determine the origin of hydrocarbons to estimate the hydrocarbon phase, GOR and production prediction. Many of these reservoir parameters vary based on the type of source rock and nature of its expulsion in varying PVT conditions. This study focuses on the detailed geochemistry from source rock, extracted oil, mud gas, and production gas and oil to determine the origin of the hydrocarbon stored in different Cretaceous intervals from Denver basin and their production equivalent phases.
Geochemistry data were generated from cored rocks, cuttings, mud gas, extracted oils and compared with the produced gas and oils from the Denver basin. This article includes source rock analysis through Rock-Eval pyrolysis on cored and cuttings rocks, Leco-TOC, gas composition and compound specific isotopes via GC-IRMS, thermal extract gas chromatography (TEGC), high resolution gas chromatography, Gas Chromatography-Mass Spectrometry (GCMS) biomarker analysis on MPLC (medium pressure liquid chromatography) separated saturates and aromatics, bulk carbon isotope analysis on extracted oil and produced oil (Peters et al., 2005; Rahman et al., 2016; Rahman et al., 2017).
Clayton and Swetland (1980) concluded that all the Cretaceous oils are compositionally similar throughout the basin. But the extracted oils from cored rock and cuttings and associated gas and oil data from several intervals from this study clearly depict there are significant differences in oils found in these Cretaceous reservoirs. Geochemistry data from source rock suggests that most of the organic matter in different Cretaceous source rocks are of Type II kerogen. However, the source rock differs in chemistry because of depositional environment associated with marine shale vs carbonate. It is evident from the pyrolysis, mud gas, and extracted oil chemistry data from the Denver basin that there are distinct differences in origin of oil and gas in these reservoirs.
The major highpoints of this study are as follows: 1) the novel organic geochemistry data should be used to better characterize any basin for conventional and unconventional exploration and development; 2) this approach helps to model better petroleum systems, basin evaluation, and overall understanding of the quality of petroleum, expulsion histories, migration pathways and type of petroleum stored in rocks.