PRELIMINARY GEOCHEMICAL CHARACTERIZATION OF SOME HYDROCARBON PRODUCING INTERVALS IN KANSAS

Hydrocarbons have been produced from different stratigraphic intervals in Kansas since the 1860s, majorly from the Central Kansas Uplift (CKU) subbasin. However, the geochemical characteristics of these hydrocarbons are still poorly understood. This study aimed to determine the geochemical characteristics of hydrocarbon samples from the Arbuckle Group (ARB), Lansing Kansas City Group (LKC), and the Simpson Group (SMS) producing intervals. As such, seven whole-oil samples (three ARB, three LKC, and one SMS) from the CKU, as well as one SMS from the Forest City basin (FCB), were analyzed using Gas Chromatography (GC).

Based on the GC results, the Carbon Preference Index (CPI) was calculated to be 1.0 for all samples except one ARB sample from the CKU with a CPI of 0.9. The Pristane/Phytane (Pr/Ph) ratio for the samples ranged from 1.3 to 1.6 except for one ARB sample from the CKU that showed 0.9. The naphthenes to alkanes (NP/AK) ratio of samples ranged from 0.01 to 0.24 except for one ARB sample from the CKU that showed 0.50. The evaluated CPI, Pr/Ph, and NP/AK all suggest a shale lithology for the source rock that generated the hydrocarbons from the different intervals. A plot of Pr/n-C₁₇ and Ph/n-C₁₈ ratios showed six samples were deposited at the margin of marine and transitional environments while one ARB sample from the CKU and the SMS sample from the FCB were deposited in a marine environment. The organic matter input for the samples was interpreted to be marine-sourced based on Pr/Ph < 2 and low n-C₂₁ to n-C₃₅ ranging (5 to 29 %) for all the samples. An examination of the gas chromatograms for all the samples showed that the persistently outlying ARB sample and the SMS sample from the FCB were characterized by multimodal distributions and severe stripping of long-chain carbon atoms, thereby suggesting biodegradation.

Hierarchical cluster analysis using calculated parameters (Pr/Ph, Pr/n-C₁₇, Ph/n-C₁₈, n-C₁₈/n-C₁₉, n-C₁₇/n-C₂₉, CPI) and the amounts of n-alkanes, branched alkanes, naphthenes, aromatics, and isoprenoids revealed four distinct groups: Group 1 (three LKC and two ARB from the CKU), Group 2 (one SMS from the CKU), Group 3 (one ARB from the CKU), and Group 4 (one SMS from the FCB). Groups 3 and 4 are interpreted to be due to different degrees of biodegradation while Groups 1 and 2 are interpreted to be due to producing interval distinctions.