Dynamics of Low Resistivity Pay Acacus Formation; North Africa Formation Testing Experience and Challenges Available to Purchase

The Silurian formation is a main source rock as well as a major hydrocarbon reservoir in North Africa; it is under active exploration in Algeria and Libya, and is the main hydrocarbon producing reservoir in Tunisia. In Algeria, the Silurian A2, A1, A0 and M1 are the key targets in the Berkine, while the F6 is a target in Illize. Owing to diagenetic processes such as chloritisation, siderite cementation and quartz overgrowth, the porosity to permeability relationship is weak and is rarely used for permeability prediction. Layers are not laterally extensive and often contain different fluids. Conventional resistivity saturation cannot differentiate wet layers and hydrocarbon sands due to lack of resistivity contrast caused by conductive minerals. Downhole fluid analysis (DFA) is used to help reduce the saturation uncertainties; however this is only possible for mobility above 1 mD/cP. In this paper, the flow potential (mobility) of 36 wells with a total of 2000 was evaluated. 32' of the data points show mobility greater than 1 mD/cP, and all other points remain unevaluated as downhole fluid analysis was not possible. To overcome these challenges 3D radial probe was used for the first time in North Africa on an exploration well referred in this paper as Well A. It was possible to evaluate intervals with mobility as low as 0.04 mD/cP positively identifying reservoir fluid. These tests include 11 stations with mobility between 0.04 and 0.15 mD/cP. The data distribution from these 36 wells in the Acacus formation indicate that 30.4' of all the point to have mobility between 0.1 and 1mD/cP. The 3D radial probe made the evaluation of these low mobility points possible, improving well-A evaluation to 62.2' compared to 32' success with standard probe. Through this work, the uncertainties of saturation evaluation were reduced, and the perforation of wet intervals was prevented.