Palynological, palynofacies, thermal maturity and burial modelling analyses of the Cretaceous to Cenozoic sediments from a series of Tanzanian onshore and offshore boreholes

Abstract

Tanzania is one of the East African countries where rich hydrocarbon accumulations are starting to be discovered in both onshore and offshore sedimentary basins, although there is currently little research that has been published in relevant research areas of geology, micropalaeontology or hydrocarbon prospectivity. The hydrocarbon reserves discovered so far are mainly gas, and these have been found in Cretaceous and Cenozoic reservoirs that provide signatures of multi-source rock origins that demonstrate promising future prospects. This thesis presents a series of studies focussing on improving our understanding of the (bio)stratigraphy and hydrocarbon source rock potential of Cretaceous-Cenozoic sediments of Tanzania. The samples studied have unfortunately yielded impoverished organic-walled microfossil (palynomorph) assemblages, and thus the biostratigraphic analysis has formed a smaller component of this project than originally intended. The main aspect of the research undertaken has thus focussed on analysing organic matter composition (palynofacies analysis) and quality (organic richness, kerogen typing), in order to provide interpretations of the palaeoenvironmental conditions prevailing at the time of sediment deposition and hydrocarbon source potential. The hydrocarbon potential has also been examined by undertaking assessments of the thermal maturity of the host sediments using vitrinite reflectance measurements in order to construct thermal histories and burial models for the several of the study locations. This study examines three hundred Cretaceous-Palaeogene cutting samples from three boreholes drilled on the continental shelf (the shallow offshore basin sites Msufi # 1, Mtiki # 1 and Mkongo # 3), one hundred Cretaceous cutting samples from one well from the deep offshore basin (Mvule # 1), and twenty-nine Cretaceous core samples from shallow boreholes from Tanzania Drilling Project (TDP) sites 21, 24, 36 & 37. Biostratigraphic results are limited, with the three shallow offshore basin wells proving largely barren of palynomorphs. The samples from the deep offshore basin (Mvule #1) produced the highest species richness, and although lacking many age-diagnostic taxa, palynological analysis has permitted the recognition of three dinocyst-based bioevents, using first downhole occurrences of taxa, as is appropriate for borehole studies. The four TDP boreholes yielded very restricted palynological assemblages, but a series of downhole dinocyst bioevents have been recognised in the core material, and although not defined using definitively age-diagnositic taxa, the events have been dated by correlation with existing calcareous microfossil biozonations. The sedimentary organic matter (palynofacies) analyses of the onshore and shallow offshore basin sites indicates the sediments studied were deposited in oxic open shelf conditions in close proximity to land, due to the high supply of terrestrial input diluting other palynomaceral components. Certain stratigraphic intervals were subject to periodic lowering of the oxygen conditions. The samples from the deep offshore basin well (Mvule #1) indicates a more marine-influenced setting. Overall, all samples are dominated by Type III kerogens, which are indicative of gas-prone source rocks. The organic richness (total organic carbon: TOC) results range from 0.1% to 3.4% TOC, demonstrating good source potential. In particular, the TOC values for the shallow offshore basin wells are as follows: for Msufi # 1, TOC ranges from 0.3-0.8%; for Mtiki #, 0.1-3.0%; for Mkongo # 3, 0.4-1.1%, and for the deep offshore well Mvule # 1, the TOC values range from 1.0-3.4%. Thermal maturity of the sediments was assessed using vitrinite reflectivity (Rv %), and values range from 0.3-3.4% Rv, indicating mature to post-mature maturity levels, and thus that the sediments studied have reached values appropriate for hydrocarbon generation. Computer modelling of the burial histories of the successions studied reveals that they have entered the window for early liquid hydrocarbon generation and reached the stages for wet-dry gas generation.

More information

Identifiers

ORCID for Ian Harding: orcid.org/0000-0003-4281-0581

Catalogue record

Export record