Geochemistry & Stratigraphy of the Mesozoic & Cenozoic Sedimentary Rocks Encountered in the Mandawa Basin, South Eastern Tanzania

The Mandawa Basin in south eastern Tanzania has long been regarded as the most hydrocarbon-prospective onshore sedimentary basin in the country. However, despite containing the ingredients necessary for a working petroleum system, no commercial hydrocarbon accumulations have been discovered. One of the keys to understanding the Mandawa Basin petroleum system is understanding the stratigraphy of the Mesozoic and Cenozoic sedimentary rocks encountered within. Whilst a wide range of stratigraphic techniques have been applied to these rocks, comparatively little attention has been paid to elemental chemostratigraphy and how it could contribute to the understanding of the evolution of the Mandawa Basin. In this thesis, 1993 rock samples have been subjected to inorganic geochemical analysis. The dataset has been employed to produce a chemostratigraphic framework for the Mandawa Basin that comprises 10 chemostratigraphic sequences and 22 higher resolution chemostratigraphic packages. The index elements employed model variations in the abundance of quartz, feldspars, clay minerals, heavy minerals, and authigenic minerals. In many cases, the mineralogical interpretations drawn from the inorganic geochemical data are corroborated by published mineralogical datasets and by supplementary heavy mineral and pyrolysis data acquired in this thesis. The geochemical and mineralogical data illustrate changes in depositional environment, climate, and sediment provenance throughout the Mandawa Basin sedimentary record, with key geochemical boundaries often coinciding with documented tectonostratigraphic and palaeoclimatic events that occurred in East Africa during the Mesozoic and Cenozoic Eras. The chemostratigraphic framework is tied into the existing lithostratigraphy of the Mandawa Basin and every formation has a unique geochemical signature. Based on the geochemical data, revisions are proposed to the lithostratigraphy, which adjust the time limits and thicknesses of all formations to varying degrees. Of the more substantial adjustments in the subsurface is the recognition of a thicker interval of evaporitic sedimentary rocks deposited during the Pliensbachian and Toarcian, necessitating the definition of a thicker Nondwa Formation with two members, and a thinner Mbuo Formation. The deposition of the laterally contiguous Kihuluhulu / Kiturika / Makonde Formations is restricted in time to the Aptian, meaning that the succeeding Kilwa Group spans a greater interval of time than previously interpreted. The subsurface chemostratigraphic framework is correlated with confidence to outcrop exposures sampled across the Mandawa Basin. Again, whilst there is a great deal of consistency between the Mandawa Basin ?chemo map? proposed here and previous lithostratigraphic maps, revisions to the outcrop geology are proposed. Of the more substantial adjustments to the outcrop stratigraphy, is the recognition that Cretaceous rocks probably cover a greater area of the Central Mandawa Sub Basin than originally thought (possibly onlapping the Masasi Spur) and the revelation that the Upper Kipatimu Member is likely Early Cretaceous rather than Late Jurassic in age and coeval to the Kihuluhulu Formation. The end Aptian marine transgression (Mavuji - Kilwa Group boundary) is clearly defined geochemically and is mapped in both the northern and central sub basins with ease. Enrichment factors of the redox-sensitive elements Mo, U, Ni, and Cu have been calculated from the extensive Mesozoic and Cenozoic claystone dataset to identify anoxic / euxinic environments in the Mandawa Basin in which commercially viable source rocks could be preserved. The results highlight that euxinic environments exist but are restricted in time to the Early and Middle Jurassic Epochs (Pliensbachian to Aalenian Stages) only. Selected claystone samples with high enrichment factor values have been subjected to pyrolysis to determine TOC content and kerogen type. The results revealed that source rocks with Type III kerogens derived from terrestrial plants are the most dominant in the Mandawa Basin and that rocks with Type II kerogens are subordinate. No source rocks with Type I kerogens have been identified in the data acquired for, or reviewed in, this thesis. The results indicate that gas is the likely hydrocarbon product of the pre-salt source rocks, whereas gas and oil are possible from intra-salt and post salt source rocks