Biomarker Characterization of Tertiary Sediments, Orange Basin, South Africa—Implications for Thermal Maturity, Petroleum Source Potential and Depositional Conditions

Tertiary sediments of KH-1 well Orange basin were examined by integrating organic geochemistry and microscopic observation. This approach assessed the characteristic attributes of organic facies including preservation settings and their importance to source rock potential. The examined samples have an average TOC value of 0.95% and this indicates a fair source rock potential. It has a good yield EOM (1119.46 ppm) with 32.20–39.39 wt.% hydrocarbon fractions. The relatively higher amount of phytane and low Pr/Ph ratio between 0.43 and 0.50, CPI (0.86–0.88), Pr/nC17 versus Ph/nC18 and C31R/C30 versus Pr/Ph low acyclic biomarker ratios reveal oxygen deficient marine environment as such good preservation of organic matter. Also, Pr/nC17 less than 0.5 depicts source rocks derived from open water conditions. The highly reducing marine conditions are ascertained by the absence of C19 tricyclic terpane, averagely high C25/C26 (1.32) tricyclic terpanes, low gammacerane index, averagely high C35/C34 tricyclic terpane ratio of 0.94. Sterane distribution confirmed the presence of planktonic land plant matter. Low concentration of ol/C30H signifies terrigenous inputs. Oleanane presence validates the sediments Tertiary age. The mean vitrinite reflectance values (0.852–0.862) show the samples are at the peak of oil generation. Both microscopic and geochemical maturity parameters show that the examined source rocks have a mature organic matter even though not buried deeper down for commercial hydrocarbon generation.