Diagenesis and Reservoir Quality of Sandstones from Offshore Kipatimu Formation of Songo Songo Gas Field with Comparison to the Onshore Kipatimu, Mitole and Nalwehe Formations of Mandawa Basin
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The potential of a sandstone to serve as a reservoir for producible hydrocarbons is closely related to its diagenetic history, which in turn is dependent upon its composition. The Mandawa Basin is probably the most prospective onshore basin for hydrocarbons in Tanzania. Different exploration data show the presence of hydrocarbons, both at the surface (oil seeps) and in the subsurface. Despite of these occurrences, no economic accumulations have so far been discovered.Core logging, petrography and X-ray diffraction methods were used to analyse both Onshore Formations from Mandawa Basin (Onshore Kipatimu, Mitole and Nalwehe Formation) and offshore formation from Songo Songo gas field (offshore Kipatimu Formation). Compositionally quartz is the main detrital grains, then feldspars (both K- feldspar and plagioclase) and lithic fragments. Mandawa Basin sanstones contain non-ferroan calcite, pyrite, quartz overgrowth and diagenetic clay minerals mainly kaolinite and illite as cements, while the Songo Songo gas field has locally abundant non ferroan calcite, local quartz overgrowths, pyrite, glauconite and authigenic clay minerals, especially kaolinite and chlorite.Generally sandstones studied range from subarkose to arkosic compositions, where Onshore Kipatimu and Nalwehe formations consist of subarkosic sandstones while Mitole Formation is arkosic sandstones, and the Offshore Kipatimu formation contains subarkose to arkosic sandstones. In the Offshore Kipatimu Formation porosity ranges from approximately 5%- 25%, while the Onshore formations range from 5.5% to 21.5% with average porosity of 14.9% and 11.3% respectively. Cementation is the main factor that affects the reservoir properties of the Mandawa Basin especially by diagenetic clay minerals. Diagenetic clay minerals such as kaolinite and illite lower porosity of the sandstones by filling the pores and reduce permeability by blocking pores throats. The abundant clay content is due to early diagenetic processes, as the Mandawa Basin formations has experienced shallow maximum burial depth compared to Songo Songo.