In the Upper Cretaceous Niobrara Formation shale oil play, considerable oil fractions are produced directly from C14+ bitumen in place (e.g., extractable organic matter). This paper provides new insights on the geochemistry and producibility of C14+ bitumen by integrating the full process of generation, retention, conversion, expulsion, and production, as a whole, in the Niobrara shale oil system.
Hydrocarbons are found not to be generated primarily from kerogen but rather from the secondary conversion of polar compounds. Maturation is essentially the underlying driving force that determines the geochemistry of C14+ bitumen in source rocks. However, the in situ cracking of C14+ bitumen does not occur in chalk reservoirs at the maturity stage investigated (430°C–465°C for the reading temperature at the maximum rate of petroleum generation by Rock-Eval 2 pyrolysis on a solvent-extracted sample). Instead of maturation, the geochemistry of C14+ bitumen present in reservoirs is principally determined by migration and associated fractionation. The intraformational migration has fractionated the generated oil into a higher saturated fluid; therefore, C14+ bitumen accumulated in chalk reservoirs is more producible than that retained in source rocks. In comparison, the bulk fractionation effect associated with production is not as pronounced as that of expulsion. Therefore, the bulk compositions of chalk bitumen strongly resemble those of crude oils. Among the least mobile nitrogen-, sulfur-, and oxygen-containing compounds, N1 species are nevertheless partitioned into crude oils during production, and the molecular weight exhibits a less apparent effect on their producibility than chemical affinity (i.e., aromaticity and aliphaticity). Accordingly, a relative enrichment of C5+ heavier carbazoles into crude oils rather than the lighter C1–5 carbazoles was documented.