Publications

The Wiriagar field was discovered in 1981. Oil is trapped in the Upper Miocene Kais Limestone formation which is developed in reefal, lagoonal and shoreface facies.The trap at Wiriagar developed by a combination of structural, stratigraphic and diagenetic processes. The reefs probably developed on local highs which were produced by Late Oligocene folding. Subsequent east-west directed compression in the Pliocene created the structural trap.In addition, trapping of the oil has been facilitated by the presence of low-porosity tight zones, which protected the oil accumulation from fresh-water flushing. The tight zones, underwent a different diagenetic hisiory compared to the overlying porous reservoir. Reservoir description and zonation resulted from the recognition of primary depositional facies, diagenetic histories and petrophysical analysis of the intervals. The field oil-water contact was established by comparison of log analysis data, hydrocarbon shows in cores and RFT analysis.In the KBSA contract area, after 18 exploration wells drilled to date to test Kais objectives, oil has been found only at Wiriagar. The accumulation is not controlled simply by depositional or diagenetic facies, but is explained by a unique set of circumstance5 which allowed oil to migrate from deep, mature, Pre-Tertiary source rocks into the Kais reservoir.The most likely source of the oil is the Jurassic Kembelangan formation, although more than one source ha5 been suggested by the results of fluid inclusion studies and geochemical analysis.Migration probably occurred along Pliocene wrench faults, in an area where the Cretaceous Jass formation is relatively thin. The Jass is regarded as a regional seal for hydrocarbons potentially trapped in Jurassic or Triassic reservoirs elsewhere in the KBSA area.Future exploration in the area will be directed towards the relatively untested Pre-Tertiary Play.