--> ABSTRACT: Characterization of Fractured Basement Reservoirs (FBR), by A. I. Younes and N. F. Hurley; #91021 (2010)

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Characterization of Fractured Basement Reservoirs (FBR)

YOUNES, AMGAD I., and NEIL F. HURLEY

Characterization of basement reservoirs differs from those in sedimentary rocks because of the lack of stratification and the poor porosity and permeability of the former. Analysis of FMI logs from the Ashrafi field, Gulf of Suez, Egypt, and outcrop data from the nearby Gebel El Zeit fault block show that four parameters are necessary to assess when evaluating FBR. First, fracture density, which commonly includes two orthogonal fracture sets, is significantly enhanced if sheet fractures are present. The frequency of the sheet fractures gradually decreases from the top of the basement, thus, deeply eroded basement is a poorer reservoir than these covered with pre-rift sediments. Second, layering of the basement rocks compartmentalizes the reservoir because of the different fracture distribution in each layer and/or the sharp sealing magmatic contacts. In the Ashrafi field, two granitic layers were identified and correlated with analogs in outcrop. The top layer is dominated by the less conductive, north-south fracture set, whereas the lower layer is dominated by a more conductive, northwest-oriented, rift-parallel set. Third, dikes show the highest fracture density in the field and are associated with the highest effective porosity returns. Dikes terminating at a layer are potential fluid conduits, and either enhance the recovery or draw water into the well depending on their position relative to the oil-water contact. Fourth, shallow faults (< 10 km) are accompanied by fault-related fractures that gradually increases in frequency towards the fault, whereas deep faults are sealing because of the high shear zones and mylotinization of rock. The interaction of all parameters significantly influence the production of the field.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.