Source rock potential of the Bajocian-Bathonian succession (Mtumbei Formation) in the Mandawa Basin, SE Tanzania

Abstract

Over the past few decades, Middle Jurassic sediments in the Tanzanian Coastal Basin have gained attention on a regional palaeogeographical scale. These sediments, including thick black shales, were deposited following a widespread marine transgression initiated by the breakup of Gondwana supercontinent. Previous studies indicate that these shales possess good to excellent source potential, making them promising regional source rocks. However, no detailed geochemical studies have assessed them in the Mandawa Basin. In this study, geochemical assessment was caried out on cutting samples from two wells to: (i) constrain organic matter richness, type, quality, and: (ii) reconstruct thermal evolution and depositional conditions of the Bajocian-Bathonian black shales in the Mandawa Basin. Organic matter richness was measured using Total Organic Carbon (TOC) analysis, while the quality and thermal maturity of the organic matter were assessed through programmed pyrolysis. Paleo-redox conditions were determined from Th/U ratios derived from Spectra Gamma Ray Logs data. Geochemical data reveal that organic matter content (TOC) is generally low and varies spatially. The analysed TOC content fluctuates along stratigraphy with values ranging from 0.13 to 3.59 wt% with an average of 0.92 wt%. Whereas, Kerogen yield (S2) and Hydrogen Index (HI) are generally low; S2 and HI ranges from 0 to 1.95 (mg HC/g) with an average of 0.29 (mg HC/g), and 0‒92 (mg HC/g TOC) with an average of 24 (mg HC/g TOC), respectively. Organic matter is mainly composed of types III (gas prone) to IV (inert) that have been subjected to wide range of thermal alteration ranging from marginally mature to over mature. Comparison of TOC, Th/U and sediment composition data (clastics and carbonates) derived from cuttings samples suggests episodic deposition of organic matter under sub-oxic conditions. These were mainly controlled by fluctuation in sea level, tectonics and clastic dilution.