Sediment Characteristics and Hydrocarbon Composition of Makukwa Seep in Southern Tanzania: Potential Implications for Subterranean Hydrocarbon Accumulation

Abstract

Exploring the interplay between natural seeps and nearby sediment and hydrocarbon properties is key to unlocking subsurface hydrocarbon potential. This study presents findings on the hydrocarbon composition and sediment characteristics of the Makukwa seep in southern Tanzania, providing insight into subterranean hydrocarbon accumulation. Sediment analysis across areas A and B, located approximately 20 m apart, showed a decrease in porosity with depth, indicative of typical sandstone formations. Area A, near the seeps, exhibited porosity ranging from 23% to 36%, whereas Area B, further away, showed porosity ranging from 22% to 38%. This suggests that a lower cretaceous sandstone layer may have overlain the hydrocarbon seep. XRD analysis confirmed quartz as the dominant mineral, with concentrations of 46.1% in area A and 50.1% in area B, underscoring the sediment's sandstone-like nature. Additionally, graphite was higher in area A (29.5%) than in area B, indicating a varying degree of hydrocarbon influence across the sampled locations. ICP-OES analysis revealed elevated trace metal concentrations, particularly iron, which varied from 27.47 ppm to 471.47 ppm in area A, significantly higher than in area B, indicating the geochemical influence of hydrocarbon seepage on sediment composition. GC-MS analysis showed a range of heavy aliphatic hydrocarbons (n-alkanes) from n-C14 to n-C38 within the sediments, indicating a dormant state. Notably, the concentrations in area A were higher, with specific hydrocarbons such as octadecane and eicosane measured at 0.330 µg/mL and 0.789 µg/mL, respectively, compared with the lower concentrations in area B (octadecane at 0.105 µg/mL and eicosane at 0.089 µg/mL). This highlights the impact of proximity to the seep on hydrocarbon distribution. This study offers crucial insights into hydrocarbon dynamics at the Makukwa seep, Tanzania, highlighting the relationship between sediment properties and hydrocarbon accumulation. These findings enhance our understanding of hydrocarbon seepage, guiding future exploration and environmental monitoring efforts.