| Abstrakt: |
The Miocene sandstones in the Tanga Basin of northern coastal Tanzania have been poorly studied. Reports on their porosity, shaliness, water saturation, and permeability are unavailable in the public domain. This study conducts a petrophysical analysis of geophysical wireline logs, including density, spontaneous potential, gamma ray, and resistivity logs from an exploration well X to characterize the Miocene sandstones of the study area and assess their petroleum prospectivity for the first time. Results show that the Miocene sediment of the Tanga Basin may be subdivided into three parts characterized by reservoir zones with varying porosity, water saturation, and shaliness. The identified reservoir zones show water saturation below 70% and shaliness under 25%, while total porosity ranges between 15 and 30%. Despite the reported low shaliness, these reservoirs record different clay distributions: diagenetic, laminated, and structural clays. Overall, diagenetic clay distribution is dominant over laminated and structural clays. The revealed clay distributions are interpreted to have resulted from an interplay of depositional processes, environment, and tectonic activities. The lowermost section of the Miocene stratigraphy appears to be more prospective due to thicker sand layers with low water saturation. The low porosity in this section is linked to diagenetic clays; laminated and diagenetic clays have decreased permeability of the studied sedimentary fill. The dominance of clay-filled pores suggests limited Miocene reservoir potential in the Tanga Basin. This research sets a benchmark for further studies in the area, including special core analysis to accurately determine the clay distribution modes and their effects on the pore-perm properties. Our method may be applied in other basins with limited core data for preliminary reservoir characterization. |
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