Petroleum maturation modelling, Abu Gabra-Sharaf area, Muglad Basin, Sudan

Structural and maturation studies in the northwest of the Muglad Basin, Sudan showed a thick sedimentary section up to 9000 m in the deep sub-basins and an average of about 6000 m in the Sharaf-Abu Gabra Ridge. The isopach map of the Lower Cretaceous Abu Gabra-Sharaf formations, which contain source rocks, showed a thickness of sedimentary section between 2000 and 5000 m.

The burial history showed high subsidence rates during the first rifting phase with relatively low subsidence rates during the following two phases in this part of the basin. Significant erosion of the sedimentary section occurred in the ridge area during Miocene and Cenomanian-Albian times. The geothermal gradients in the modelled wells range between 18 and 27.5 degreesC/km corresponding to the relatively wide range of present day heat flow of between 37 and 63 mW/m(2), averaging 50 mW/m(2). Heat flow history curves consistent with vitrinite reflectance data were adopted with elevated high heat flow (75 mW/m(2)) during the first rifting phase followed by minor peaks (average 60 mW/m(2)) during the second and the third rifting phases. Application of a kinetic vitrinite maturation model resulted in a present day oil window between 2000 and 4500 m.

The source rock of the Abu Gabra (AG) and Sharaf (SH) formations, which has an average total organic carbon of 1.0% and hydrogen index (HI) of 280, was modelled for hydrocarbon generation. The section was divided into four layers; AG-1 (Albian), AG-2 (Aptian), SH-1 (Barremian) and SH-2 (Neocomian). Conditions for hydrocarbon generation are present throughout the Sharaf formation and in AG-2. The generation model suggests an average generation amount of 4 mgHC/g rock in the lower three modelled layers with a timing range between 120 Ma and the present. Total calculated oil volume is about 370 US billion bbls and the expected volume expelled and migrated (40%) is 150 US billion bbls. The total gas expelled and migrated (90%) is 1600 US trillion cu ft. Retention of hydrocarbons may be adversely affected by long-continued faulting and lack of good seals. (C) 2002 Published by Elsevier Science Ltd.