Characterization of Hydrocarbon Molecular Signatures across Sedimentary Subbasins and Stratigraphic Units: Insights into Source Rock Attributes in Kansas

Abstract

Hydrocarbons constitute an important component of Kansas’s energy sector, but the geochemical characteristics and source rocks of these hydrocarbons are not well understood. This study analyzes 18 hydrocarbon samples from five subbasins and six stratigraphic intervals within the state of Kansas using gas chromatography (GC) and gas chromatography–mass spectrometry (GC-MS) to determine the lithology, sources of organic matter, paleodepositional environment, and thermal maturity of their source rock(s). Analytical results and figures, such as Pr/Ph ratios (≥1), CPI (≥1), low naphthenes/alkanes, low C29/C30 hopane ratios (<1), and DBT/PHEN vs Pr/Ph plots, suggest a shale lithology for the source rocks. C19/C23 tricyclic terpane ratios of 0.02–0.32 and sterane/hopane ratios of 0.70–1.08 are observed in hydrocarbons from the Central Kansas Uplift, Hugoton Embayment, Salina Basin, and Sedgwick Basin. In contrast, hydrocarbons from the Forest City Basin display higher C19/C23 ratios (1.12–2.08) and lower sterane/hopane ratios (0.11–0.13). These geochemical patterns confirm that the hydrocarbons from the former subbasins are derived primarily from marine organic matter, whereas those from the Forest City Basin reflect a dominantly terrigenous source. Paleoenvironmental proxies, including Pr/Ph values (0.98–2.25), C35 homohopane index, C32/C31 hopane ratios (0.47–0.68), and crossplots of Pr/n-C17 vs Ph/n-C18, and Gam/C30 hopane vs Pr/Ph align with deposition under marginal marine, suboxic conditions with little evidence for water stratification. Thermal maturity proxies (e.g., CPI, OEP, moretane/hopane ratios, C29 sterane and C32 homohopane isomerization, and Ts/Tm) indicate that the organic matter responsible for these hydrocarbons is within the oil generation window. Overall, the results demonstrate that hydrocarbons from the Forest City Basin were derived from a distinct, terrigenous-rich source rock, whereas the hydrocarbons from the other four subbasins share a genetically related, marine-influenced source. This highlights multiple source rocks for the hydrocarbons produced in Kansas. These findings provide a framework for future studies to consider both local and external sourcing as part of an integrated petroleum system model for Kansas.