Online Journal for E&P Geoscientists

Maulana, Asep I.^*1; Sitorus, Septian L.²; Wydiabhakti, Tety B.²
(1) Department of Theoretical Bases of Petroleum Research and Prospecting, Gubkin Russian State University of Oil and Gas, Moscow, Russian Federation. (2) Department of Exploration Geophysics and Computer System, Gubkin Russian State University of Oil and Gas, Moscow, Russian Federation.

Fractured basement is one of the most important reservoirs of Zeit Bay field, contains nearly one-third of oil in place of the field, as well as significant volumes of gas. It is well established fact that the reservoir porosity, permeability, and hydrodynamic behaviour in crystalline basement are attributed to natural fractures present in it.

The fractured granite reservoirs of the study area are flanked by porous and permeable reservoir sands and carbonates. The permeability of the granite horst blocks and the fact that they are in hydraulic communication with these flanking sand and carbonates makes them ideal drainage systems. Porosity system studies of these granite reservoirs indicate the development of secondary porosity along fractures and a pervasive leaching of feldspars.

Statistical analysis of fracture data in the Zeit Bay Field indicates that four parameters are necessary to asses when evaluate a particularly interesting variation fractal or frequency relationship between the computed fracture aperture and orientation. First, fractal fracture 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 and orientation in each layer. 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, and 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 mylonitization of rock. The interaction of all parameters showed that density of fracturing and width of fracture aperture, and also fracture orientation were the main criteria and significantly influence for initial production in the field.