Jurassic evaporites and diapirism along the southern Tanzanian continental margin: Implications for petroleum exploration
Citation:FREITAS DA SILVA VUCINIC, MARIA ISABEL, Jurassic evaporites and diapirism along the southern Tanzanian continental margin: Implications for petroleum exploration, Trinity College Dublin.School of Natural Sciences, 2020
PhD Thesis_Maria Isabel Freitas da Silva Vucinic.pdf (PDF) 15.54Mb
One of the key issues for petroleum exploration along the entire Tanzanian margin is the extent of evaporite deposition and its diapirism. This leads to scenarios for the configuration and relative movement of conjugate fragments across East Africa. In Mandawa Basin, the separation from Africa provided restricted conditions for salt deposition in the Lower Jurassic, with the rise to continued deposition and sedimentation of anoxic marine shale and locally evaporites trigged by the evolution of the salt minibasin approximately in the Middle Jurassic. In this study, the methodological approach of combining geological and geophysical techniques has been addressed to characterise the types of salt, the lithostratigraphy, and seismic sequences of the geological framework by variations through time in stratigraphy (source rocks) and structural geometry (salt diapirs). The provided legacy containing 2D seismic lines, geophysical well logs, well data were used as input data to re-access the onshore Mandawa salt basin. The workflow involved in importing the dataset, performed a quality check of the 2D seismic lines seismic attribute analysis horizon and fault interpretation, structural modelling, and depth conversion from the final 3D geocellular model from time to depth. The identification of the lithological units has been carried out which helped to identify salt structures according to change in signal responses, lateral continuity, bedding sequences, and thickness surfaces. A well correlation in this thesis was performed resulting in seven zones defined in between the markers where four of them specifically may correlate to previous stratigraphic units proposed firstly by Shell in 1990 for the onshore Mandawa Basin: Mbuo Formation, Nondwa Formation, and Mtumbei Limestone Formation. This study observed different types of salt of marine evaporite diapirism present in Mandawa Basin: halite, anhydrite and gypsum. Results in this study further suggest the existence of sequences of halite in most of the wells, such as Mbuo-1, Mihambia-1, East Lika-1, Mita Gamma-1, Mbate-1 and Mandawa-7. Another evaporite type as anhydrite salt has been identified interbedded with claystones and sandstones sediments in Milhambia-1 and East-Lika-1 wells. The salt in Mandawa Basin has been interpreted in this work as mostly autochthonous building up from the Lower Jurassic, below the present-day shelf. In addition to the findings here, the appearance of an allochthonous detached salt sheet structure extending into the Lower Cretaceous strata in central Mandawa Basin suggests that the salt pillow originates from Upper Jurassic and with deposition during the Cretaceous. The use of 2D restoration technique revealed that the salt layer and the overlapping layers had tabular geometry prior to extension followed by a rise of a diapir by reactive bearing to the formation of a graben associated with the extension of sedimentary overload. Sequence stratigraphy analysis has also been carried out during this research according to the identification of reliable reflectors in conjunction with the well tops (markers) in the wells although poor quality in some areas was visible. The 3D geocellular model shows the extensional block during the Lower Jurassic. A stratigraphic pinch out setting formed during restricted conditions triggered by the evolution of the mini-basin occurred approximately in the Middle Jurassic followed by a major Upper Jurassic transgression. A lack of wells of the onshore Ruvuma Basin accessed in the present work poised ambitious to generate a detailed structural framework and a final model. Yet, in this thesis, a lithostratigraphy analysis was carried out making possible the characterisation of three horizons and two respective markers in a tentative equivalence of lithology and nomenclature from the Mandawa Basin with the Ruvuma Basin. Faults played a major control on sedimentation causing lateral changes. Continental clastic sandstones, marine shale siltstones interbedded with successions of non-deposition sandstones suggested transitional conditions during the Jurassic that widespread to three unconformity periods in the Lower Cretaceous. This work also presents a tentative well correlation based on well logs from one well in the Selous Basin. Five markers and respective seismic horizons have been named as the geological ages respectively with other studies.
Publisher:Trinity College Dublin. School of Natural Sciences. Discipline of Geology
Type of material:Thesis
Availability:Full text available