Origin and Potential of Unconventional Jurassic Oil Reservoirs on the NorthernArabian Plate

摘要

Unconventional oil reservoirs exist in deep, overpressured,rnfractured carbonate source rocks, and in adjacent dolomitisedrncarbonate platforms, in the Jurassic Gotnia Basin on thernnorthern Arabian Plate. These reservoirs are an importantrnpotential future resource for the region once the low riskrnprospect inventory in conventional plays is drilled out, andrnproduction from their reservoirs inevitably reaches a plateau.rnAn estimated 5 – 10 trillion barrels of oil have been generatedrnin the Jurassic source kitchen; recovery of just 0.5% of thisrnvolume would yield substantial oil reserves. A high proportionrnof the generated oil has probably been retained in secondaryrnporosity within the Jurassic Petroleum System, beneath anrnevaporite caprock. Pore pressure gradients approaching 1rnpsi/ft during oil generation (at pore pressures of 10,000 –rn20,000 psi), and localized folding in Late Jurassic, Turonianrnand Late Tertiary time, have created a complex fracturedrnreservoir system within the Jurassic source rocks andrninterbedded carbonates. Sustained production from the sourcernrock reservoirs may only be possible in areas where oil filledrnmicrofractures are able to recharge large connected permeablerntectonic fractures during pressure drawdown. Dolomitisingrnfluids and oil have migrated into carbonate platformsrnunderlying, and flanking, the source kitchen. Fluid inclusionrnstudies indicate that dolomitisation occurred at temperaturesrnof 80 – 140 deg C from pore waters with salinities of 100,000rn– 250,000 ppm, locally creating world class oil reservoirsrn(sealed by tight limestones) seen at outcrop in the ZagrosrnMountains on the deformed eastern margin of the basin.rnSuccessful exploration and development of the Jurassicrnreservoirs will be challenging - requiring pre drill seismicrnimaging of fracture systems and of laterally discontinuousrnporous dolomite units, application of unconventional wells inrnchallenging drilling environments, and careful reservoirrnmanagement during production to achieve commercialrnrecovery factors.