The Mandawa Basin situated in south coastal Tanzania, has been a site for scientific research since the 1900s and a prospect for hydrocarbon exploration from the 1950s. The basin is located approximately 80 km west of the offshore Block 2 where significant accumulations of gas has been discovered.
The Early Cretaceous in the Mandawa Basin is poorly understood in terms of its depositional environment, and no scientific publications are available on the Nalwehe Formation. This study is a reconnaissance in providing the first information regarding the understanding of the depositional environments of the Nalwehe Formation of the Mandawa Basin, Tanzania. Six logged outcrops were studied; two representing the Nalwehe Limestone Member and four consist of the Nalwehe Sandstone Member. Analysis of the depositional environments was based on the given logged outcrops together with petrographical analysis, SEM, XRD, in addition to organic geochemical analysis of selected rock samples.
The Nalwehe Limestone Member as studied at locality NQ and NQ2, displays an increase into a high energy environment, marking a transgression period during the deposition of the Nalwehe Formation. Tectonic fissures found in the Nalwehe Limestone and the sandstone at locality NQ3 indicate tectonic activity during deposition or closely after. A possible break in sedimentation between the Nalwehe Limsedone and Sandstone members has been suggested, seen by a possible palaeokarst surface.
The Nalwehe Sandstone Member was likely deposited during a period of regression as sandstones are overlaying the limestone beds. The Nalwehe Sandstone Member is represented by the prograding sandstones with clastic material mainly being supplied from nearby palaeohighs (likely the Masasi Spur) and deposited in the shallow coastal marine environment. The sandstones have influenced by the actions of tides, waves and storms and most importantly, sea level fluctuations. The sandstones are mostly immature and consist of high amounts of calcite cement, with high IGV values. Moreover, early diagenesis evolution indicates a shallow burial depth. The brief organic geochemical analysis suggests the possibility of real hydrocarbons within the area. Main diagenetic phases are:Fe-oxide staining, smectite grain coatings, smectitic clays, calcite cementation, K-feldspar overgrowth, and later Fe-oxide cements and kaolinite.