Geochemical and well logs evaluation of the Triassic source rocks of the Mandawa basin, SE Tanzania: Implication on richness and hydrocarbon generation potential

Abstract

Selected core and cutting samples from the Triassic succession of the Mbuo claystone and the Nondwa shale were collected from the Mandawa-7, Mbuo-1 and Mbate-1 wells, onshore Mandawa basin, to understand the hydrocarbon generation potential. Earlier studies show that Mbuo claystone and Nondwa shale source rocks have fair to good generation potential based on percent total organic carbon (TOC) with kerogen type II/III, but no detailed studies include the potential for hydrocarbon generation from Triassic source rocks in the Mandawa basin. Detailed parameters like TOC, level of organic maturity (LOM), net to gross (NTG) ratio and integrated geochemical and well log analysis approaches are applied to understand the generation potential of the Triassic source rocks. The results show that TOC derived from well log analysis ranges from 0.01 to 11 wt percent (wt%), clearly including fair to good average TOC values with less than 0.5 NTG for both the Mbuo claystone and the Nondwa shales using a marginal cut-off value of 0.5 wt% TOC. The S1 and S2 values range from 0.1 to 1.7mgHC/g Rock and 1.2–17.2mgHC/g Rock respectively. The cross plot between TOC and S1 and S2 values from Rock Eval shows that these source rocks have low potential for hydrocarbon generation, and that S1 and S2 are independent to the thermal maturity. Integrating results obtained from various studies shows that both Mbuo claystone and Nondwa shales in the Mbate-1 and Mandawa-7 wells have poor hydrocarbon generation potential in their respective organic-rich intervals.