GEOCHEMICAL ASSESSMENT OF THE PETROLEUM SOURCE ROCK POTENTIAL IN LOWER SHIRE BASIN (MALAWI)

One of the essential steps in hydrocarbon exploration and exploitation is to understand the source rock evolution and the related petroleum system. Understanding how and where hydrocarbons were originated, help to comprehend, and predict where they can be discovered. Geochemical source rock evaluation provides valuable information ready to be used for upgrading areas under investigation, concentrating exploration activities in particular places and reducing risks.

This study involved use of geochemical parameter (TOC, HI, OI, PP, GP, S2, S1 etc.) which was delivered from rock eval pyrolysis process to classify organic matter richness, kerogen types and their thermal maturity of hydrocarbons present in the collected rock samples. Geochemical analysis of the surface cretaceous sedimentary rocks was undertaken to understand oil and gas potentially of the petroleum source rocks in the lower shire river, based on the case study on the two basins namely, Lengwe and Mwabvi.

Shire basin is located within middle east Africa, and it is made up of perma-Triassic and cretaceous sedimentary rocks and intruded by a variety of alkaline rocks of Jurassic to cretaceous age. It is located within eastern Africa rifting valley that was developed and is associated with the rifting process.

Geochemical methodology approach has been used in assessment of petroleum source rocks whereby data collected from eval rock pyrolysis process has been used. Excellent organic matter richness has been measured in each rock sample as indicated by high TOC values which is of greater than 2wt%. Very poor and fair petroleum potential has measured for the samples indicated by very low S1 and S2. Many samples have shown to be in post-maturity window, two samples are in mature and one in immature window hence giving hope and encourage for some more exploration to be conducted in the area. Thus, a considerably amount of petroleum that has already been generated from them, type IV kerogen have been reported for all samples, indicating inactive source rocks.

This study involved use of geochemical parameter (TOC, HI, OI, PP, GP, S2, S1 etc.) which was delivered from rock eval pyrolysis process to classify organic matter richness, kerogen types and their thermal maturity of hydrocarbons present in the collected rock samples. Geochemical analysis of the surface cretaceous sedimentary rocks was undertaken to understand oil and gas potentially of the petroleum source rocks in the lower shire river, based on the case study on the two basins namely, Lengwe and Mwabvi. Shire basin is located within middle east Africa, and it is made up of perma-Triassic and cretaceous sedimentary rocks and intruded by a variety of alkaline rocks of Jurassic to cretaceous age. Many samples have shown to be in post-maturity window, two samples are in mature and one in immature window hence giving hope and encourage for some more exploration to be conducted in the area.

The Lower Shire Basin occupies the southern end of the Malawi Rift. The basin developed during the Karoo times and re-activated during the Cenozoic time due to the rifting associated with the EARS (Casting, 1990). It is situated near the southern end of Malawi between latitudes 15°30'S and 16°45'S and longitudes 34°15'E and 35°15'E and bordered by the Malawi-Mozambique boundary on its southern edge.

This area has been chosen because the previous researchers on the composition of water in lower shire basin recommended that it is associated with high level of organic matter since it has highly deep depth enabling water to flow at very slow rate which allows the accumulation of the organic matter. It all must also be noted that this area is also associated with coal which also formed from organic remains under enormous pressure in sedimentary sequence. This makes this area to have high probability of the petroleum source rocks.

Malawi Rift lies within the Mozambique belt (ca 700-400 my BP) in the southern part of Africa. It is underlain by para- and orthogneiss crystalline rocks (Carter, G.S., Bennet, J.D, 1973). These metamorphic rocks are overlain by Permo-Triassic and Cretaceous sedimentary rocks and intruded by a variety of alkaline rocks of Jurassic to Cretaceous age. The topmost units are younger sediments that are found in the major drainage basins of the country associated with Cenozoic to recent deposition and rifting processes.

Detailed geological accounts of the study area are documented in various publications of the Geological Survey of Malawi covering the study area (Habgood, 1965). The study area comprises Proterozoic metamorphic rocks of both igneous and sedimentary parentage and assigned to the Basement Complex. These are uncomfortably overlain by Karoo volcano-sedimentary rocks and later by Cretaceous and Quaternary sequences. The Lengwe and Mwabvi domain are one of the most well-known Karoo domains within the study area.

District: Chikwawa

Coordinates: -16.2153, 34.6804