Unconventional oil reservoirs exist in deep, overpressured, fractured carbonate source rocks, and in adjacent dolomitised carbonate platforms, in the Jurassic Gotnia Basin on the northern Arabian Plate.These reservoirs are an important potential future resource for the region once the low risk prospect inventory in conventional plays is drilled out, and production from their reservoirs inevitably reaches a plateau.An estimated 5 - 10 trillion barrels of oil have been generated in the Jurassic source kitchen; recovery of just 0.5% of this volume would yield substantial oil reserves. A high proportion of the generated oil has probably been retained in secondary porosity within the Jurassic Petroleum System, beneath an evaporite caprock. Pore pressure gradients approaching 1 psi/ft during oil generation (at pore pressures of 10,000 - 20,000 psi), and localized folding in Late Jurassic, Turonian and Late Tertiary time, have created a complex fractured reservoir system within the Jurassic source rocks and interbedded carbonates.Sustained production from the source rock reservoirs may only be possible in areas where oil filled microfractures are able to recharge large connected permeable tectonic fractures during pressure drawdown. Dolomitising fluids and oil have migrated into carbonate platforms underlying, and flanking, the source kitchen.Fluid inclusion studies indicate that dolomitisation occurred at temperatures of 80 - 140 deg C from pore waters with salinities of 100,000 - 250,000 ppm, locally creating world class oil reservoirs (sealed by tight limestones) seen at outcrop in the Zagros Mountains on the deformed eastern margin of the basin. Successful exploration and development of the Jurassic reservoirs will be challenging - requiring pre drill seismic imaging of fracture systems and of laterally discontinuous porous dolomite units, application of unconventional wells in challenging drilling environments, and careful reservoir management during production to achieve commercial recovery factors.
Introduction
The Late Jurassic Gotnia Basin of the northern Arabian Plate underlies a prolific petroleum province (comprising the Mesopotamian and Zagros Foreland Basin containing giant oil fields producing from Cretaceous and Tertiary carbonate and sandstone reservoirs. Figure 1 illustrates the extent of the Gotnia Basin; it extends for 2200 km from NE Iraq to the NW Gulf and covers an area of about 700,000 sq km.The margins of the Gotnia Basin are poorly defined, and only a small proportion of exploration wells penetrate the Jurassic section.The eastern part of the basin was buried beneath the Mid Miocene - Pliocene Foreland Basin of the Zagros and strongly deformed during Late Miocene - Recent compression.
Jurassic basinal source rocks outcrop in the Zagros in NE Iraq and SW Iran; these outcrops were first described by field geologists of the Anglo Persian, Anglo Iranian and Iraq Petroleum Companies in the late 1920's - 1950's.Local people refer to the source rocks as "coal" - locally using them for fuel. Equivalent rich source rocks and overlying thick evaporites have been drilled in Kuwait and Iraq.
Exploration wells located in the margin of the Jurassic carbonate platform on the western side of the basin have encountered porous dolomite and tightly cemented limestones.A similar association of porous dolomites and tight limestones has recently been described in outcrops in the carbonate platform margin on the eastern side of the basin in the Zagros Mountains of Iran.
The purpose of this paper is to draw attention to the regional hydrocarbon potential of unconventional Jurassic fractured source rock and dolomite reservoirs in the Gotnia Basin.
