Geochemistry of organic-rich shales in the Miocene Miadol Formation, Melut Rift Basin, South Sudan: implications for their organic matter input and oil/gas generation potentials.

Organic-rich shale samples from the Miocene Miadol Formation were collected from seven exploration wells in the Melut Rift Basin, South Sudan and analyzed using organic geochemical analyses coupled with microscopic examination. We report the geochemical and petrological characteristics of 23 cuttings shale samples to provide information on the quantity, quality, and thermal maturity, thereby contributing to organic matter characteristics and their potential for oil and gas generation. The geochemical results indicate that the analyzed shale samples have moderate-to-high organic matter content with total organic carbon (TOC) content between 0.57 and 2.88 wt. %. The organic matter in the Miadol shale samples is primarily of type III kerogen, with a small amount of type II kerogen, as implied by the broad range of hydrogen index (HI) values (56 and 548 mg HC/g TOC), thereby exhibiting the ability to generate gas with some oil. The kerogen is consistent with the distributions of normal alkanes and isoprenoids in the aliphatic hydrocarbon. The n-alkane and isoprenoid distributions, with their ratios and parameters, suggest that the examined Miadol shale samples contain a mixture of organic matter and were deposited under suboxic to relatively anoxic environmental conditions. The analyzed Miadol shale samples at relatively shallow burial depths (470–820 m) are still immature as implied from their VR values of less than 0.50% VRo (0.28–0.39% VRo). Therefore, these Miocene organic-rich shales in the deeper burial depths may be promising source rocks for primarily gas and limited oil production in the Melut Rift Basin where they may reach high thermal maturity levels of oil- and gas-generation windows. [ABSTRACT FROM AUTHOR]