Unconventional hydrocarbon resources in Tanzania: insight from geology and previous exploration of conventional resources

Abstract

Unconventional oil and gas exploration has been developing rapidly in recent decades, while Tanzania has been paid very little consideration. This review aimed to infer these unconventional hydrocarbon resources in some basins, mostly the intracratonic Karoo basins and one coastal basin (Ruvu basin), from a piece of the very limited information available from the previous exploration of conventional resources. The presence of thick dark carbonaceous shales of Makarawe formation at Ruvu basin with TOC% 2.4–4.0% and a thickness between 107 and 698 m capped with non-porous oncolitic limestone and the gas shows encountered at an interval of 107 m of Makarawe shales at Makarawe-1 well makes Ruvu basin as important play for unconventional shale gas. The intracratonic Karoo basins have a thick deposit of Karoo sediments interbedded with coal deposits. Coal bedded methane has been encountered in a deep coal exploration borehole at Songwe-Kiwira coalfield. Geochemical analysis of outcrop samples from Namwele, Mbamba Bay, Njuga, and Mhukuru coalfields shows that the interbeds of coal-carbonaceous shales/mudstone have the rich organic matter of type III or both II and III and trace of oil on samples from Ketewaka and Ngaka coalfield make these sub-basins to be considered as potential plays for shale oil/gas. The presence of probable shale gas resources ranging from 50 to 200 trillion cubic feet at the Selous basin increases Karoo basins’ potentiality in exploring unconventional gas resources. In addition to the discovered conventional gas resource in offshore Tanzania, any unconventional resources will impact the country’s economic growth.