Porosity Versus Depth: A Case Study from Neem Field of Muglad Basin, Sudan, Africa

Trivedi, K.B., Mustafa Eltayeb, Kamil Idris, Greater Nile Petroleum Operating Company, Khartoum, Sudan

Sudanese rift-related system consists of four NW-SE trending rift basins, covering an 1000 km. wide area. Out of four the most important one is the Muglad basin extending along its length upto 800Km., and through the South Sudan Shear (SSS) it is connected with the Anza rift in the North Kenya.

Sedimentary sequences ranging in age from Early Cretaceous to Tertiary and recent are encountered in drilled wells, where Cretaceous makes the most of basin in fills.

. The major reservoir targets are Bentiu (Aptian –Cenomanian), Darfur group, Amal and Tendi S.St. The Sandstones of Abugabra Fm. also serves as a reservoir which qualifies it as a target but constrained by deeper depth of occurrence.

A concept of Economic Floor Porosity (EFP) is used in the area under study and the tar­gets of wells are normally set above EFP level, which is set at 15% for concession area, cal­culated by linear equation based on compaction of sediments due to overburden.

Authors have studied more then 15 wells in producing Neem field and surrounding area and found that though normally porosity decreases with the depth but it does not follow an inverse relationship with depth always. They brought out role of lithofacies, ductile miner­als, pore fluid and its chemistry,unconformity surfaces, as a appreciable moderator of this hitherto accepted concept, and it is found that porosity are facies dependent and only con­sidering depth as moderator might mislead and a commercial target might be missed in Abugabra and Bentiu resrvoir. .