Palynology and organic geochemistry analysis of borehole cores from the Maniamba Basin, northern Mozambique: Potential for hydrocarbon

Abstract

The Maniamba Basin in northern Mozambique, southern Africa, is one of the well known coal bearing basins within the country. Their geological formations are contemporaneous with the Karoo Supergroup with an almost complete record of sediment deposition occurring during the Late Carboniferous to Early Jurassic. The basins are linked to regions of old crustal sutures where high-strain structures formed during the Pan-African Orogeny. Since studies of the organic facies are scarce, the main objective of this thesis is to determine the potential for hydrocarbon generation together with a good understanding of the biostratigraphy of organic-rich rock sequences intercepted by four boreholes. JOG16N-5, JOG16N-6, JOG16-7, and JOG16N-8 together with four outcrops at Luiga, Luchai, Michunwa and Nhamago, in the Maniamba Basin. The multiproxy analysis includes palynology, palynofacies, elemental analysis, XRD, vitrinite reflectance, TOC, Rock Eval pyrolysis and biomarkers. Two hundred thirty-seven rocks were subsampled from borehole cores, together with twenty samples from outcrop were analyzed for the type, content, quality, thermal maturity, age of deposition, palaeoenvironments for hydrocarbon potential of the Basin. The lithostratigraphic units are organically rich in regard to TOC, thermally matured into gas and oil windows, however more gas with minor oil is generated given the preservation of the organic matter in a proximal - distal environment under oxic conditions (type III kerogen, mixed type II/III and type IV kerogen). Over-maturation occurrence is linked to kimberlite emplacement that is associated with the first or second graben faulting stages in the Basin. The Maniamba Basin organic-rich rocks differ geochemically from oil fluids in the Rovuma and Mozambique basins that has been assumed to be the same since the latter fluids are suggested to be sourced from marine shales. Biostratigraphically, a Guadalupian age was inferred for outcrop strata of the Luchai and Nhamago sites being coeval with the K4 Formation and Changhsingian-Induan for Luiga site suggesting correlation with the K5 Formation. No age was assigned to the Michunwa outcrop strata due to their poor preservation. Additionally, a Wuchiapingian age was suggested for the K4 Formation, whereas a Changhsingian-Induan age is inferred for the K5 formations in the Maniamba Basin contradicting previous findings. In the K5 Formation for the first time the Permo-Triassic transition is recorded reinforcing correlations in Central Gondwana. In its turn, in the K4 Formation, intra Gondwanan provincialism is observed with the northern Karoo palynoevents differing from their southern Karoo counterpart. The revised coal biostratigraphy could potentially spark interest for coal bed methane in the world seeking for cleaner energy. The Basin palaeoenvironment is characterised by oscillating paleoredox conditions that could have been influenced by sediment infilling during the Permian-Triassic boundary to possibly the end of the thermal uplift caused by the jurassic Karoo magmatism. Yet, during the middle-late Permian to Early Triassic this Basin was made up of meandering and braided rivers together with coal-bearing beds that comprise floodplains, crevasse splay and fluvial channel lithofacies associations. This is a representative of a typical fluvial/lacustrine settings marked by the co-existence of sub environments and hypautocthonous sedimentation. In conclusion, this multiproxy study has demonstrated that the integration of approaches has given a deeper understanding of the evolutionary history of the Maniamba Basin.