Middle Jurassic (Bajocian–Callovian) Source Rock Evaluation Across the Tanzanian Coastal Basin: A Regional Synthesis

Abstract

ABSTRACT The breakup of Gondwana is considered one of the key tectonic events in Tanzania's geological history. These events were accompanied by the deposition of key elements, including the deposition of black shales in the Middle Jurassic that could serve as regional source rocks. Previous local reports suggested poor to good petroleum source potential for these shales. However, no attempts have been made to assess the satial variability of these source intervals across the Tanzanian coastal belt. Here, we present a spatial variability of these source rock intervals, focusing on five sub‐basins within the Tanzania coastal basin. Using rock pyrolysis and gamma ray log data from five boreholes, we aim to assess total organic carbon (TOC) content, determine the type and quality of organic matter (OM), and reconstruct the OM preservation conditions and its syn‐ and post‐diagenetic alteration. Results show that overall TOC content ranges from 0.05 to 3.6 wt% with an average of 1.2 wt%. S 2 and hydrogen index (HI) values also vary, ranging from 0.01 to 4.5 mg HC/g with an average of 1 mg HC/g and from 7 to 191 mg HC/g TOC with an average of 55 mg HC/g TOC, respectively. The OM primarily consists of a mixture of Type III–IV kerogen, with varying degrees of thermal maturation ( T max ). Samples from Well‐1 (Tanga Basin) and a few from Well‐4 (Mandawa Basin) exhibit fair to good source potential compared to samples from other wells. The variation in source potential is attributed to local micro paleoclimate, physiography, and degree of thermal maturation. This work demonstrates the necessity for a regional evaluation of source intervals to identify areas where enriched source rocks may be present.