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Abstract: Jurassic Carbonate Reservoirs of the Amu Darya Basin, Uzbekistan and Turkmenistan

V. S. Shein, N. K. Fortunatova, J. E. Neilson, M. D. Simmons

The Amu Darya basin is a world class hydrocarbon province. Current reserves estimates are 220 TCF of gas and 800 MMbbl of oil and condensate, 50% of which is reservoired in Late jurassic carbonates. Exploration opportunities still exist in large parts of the basin which are relatively undrilled.

Within the 100-600 m thick carbonate sequence, reservoir facies include reefs, shelf grainstones and turbidite fans. The major seal are Kimmeridgian-Tithonian evaporites which are up to 1600 m thick in the basin centre. Stratigraphic trapping is common and often enhanced by structural modifications. The reservoirs are in communication with a major gas-prone Early-Middle Jurassic source rock. Oil-prone source rocks are thought to occur in basinal sediments which are coeval with the Late Jurassic reservoirs.

Carbonate sedimentation commenced during the Late jurassic with the development of a ramp complex. This evolved into a rimmed shelf with barrier and pinnacle reefs. Several cycles of relative sea-level change (largely eustatic?) influence the carbonate ramp/shelf systems and effect the distribution of reservoir facies. Numerous empirical observations by VNIGNI scientists on carbonate successions have enabled them to develop mathematically calculated indices for facies and reservoir prediction, which have been applied successfully in the Amu Darya basin.

Reservoir quality in the limestones is strongly controlled by primary facies. Reefs and shelf grainstones display the best reservoir characteristics. Whilst many facies have good total porosity, it is only the reef and grainstone belts where connected porosity (with pore throats greater than 10um) becomes effective. Burial cements are rare. Freshwater solution and cementation has often improved or preserved primary porosity.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France