O'HEARN,TERRY, and STEVE ELLIOTT* Karachaganak Integrated Organisation (KIO), London

Abstract: Development of Karachaganak Field, Kazakhstan

The giant Karachaganak field was discovered in Kazakhstan in 1979 by Uralskneftegasgeologia; and, was first produced by KarachaganakGazprom in 1984. Partners Agip, BG,Texaco and Lukoil have a 40-year PSA for development of the complex retrograde gas-condensate reservoir with a 1650 m hydrocarbon column including a 200 m oil rim. 236 wells have been drilled on the field of which 181 were drilled for production. An ongoing workover program has restored declining production to levels (12 M m3/d raw gas and 11.4 K tonnes/day unstabilised condensate from 33 wells) not seen since the break-up of the Soviet Union. Current development scenarios call for a combination of gas re-cycling with horizontal and vertical wells to maximise liquids production.

The heterogeneous reef and platform carbonate reservoirs range in age from Late Devonian (Famennian) to Early Permian (Artinskian). From Late Devonian to Lower Carboniferous, the Karachaganak massif (15 km x 30 km) developed as an isolated platform along the northern margin of the Pricaspian basin. The platform features bioherms (microbial and algal boundstones) along the margins with an internal lagoon. Permian pinnacle-like bioherms (Tubiphytes, bryozoa, phylloid algae, Palaeoaplysina framestones and microbial boundstones) and bioherm slope facies overlie an erosional unconformity at the top of the Carboniferous.

The limestone and dolomite reservoirs are generally low porosity and low permeability. While dolomitisation can enhance reservoir quality locally, there is not obvious correlation of dolomite content with reservoir quality. Initial reservoir quality is impacted adversely by early diagenetic, marine calcite cementation and late diagenetic, anhydrite precipitation. The three producing objects (Permian gas-condensate, Carboniferous gas-condensate, Carboniferous oil) have average core porosities and permeabilities of 10.6% & 8.35 mD, 10.4 % & 15.10, and 9.5% & 10.53 mD, respectively. Maximum core porosity is 34.2% and maximum core permeability is 2198 mD.

In 1992 Agip and BG geoscientists with little conventional core data available produced a layer cake model by correlating "porosity packets" interpreted from wireline log data within the context of the existing isolated platform model. Seventeen layers were defined: five in the Early Permian and twelve in the Early Carboniferous and Late Devonian. The current reservoir characterisation project will result in refinement of the existing reservoir model, allowing simulation to better match static reserves and historical production. The current work involves interpretation of lithofacies, diagenesis and porosity types and distribution from available conventional core samples (approximately 800), as well as newly acquired rotary-drilled sidewall cores. Core interpretations will be integrated with wireline, petrophysical, PLT and DST data to define flow units and fluid flow barriers and the results will be integrated with a 3D seismic interpretation to produce a 3D stochastic model.

3D seismic data will be acquired during 1999 using double sets of heavy vibrators in conventional flip-flop configuration which will provide 840 sq. km. seismic coverage. The selected bin size is 25m x 25m with a theoretical full fold coverage of 4000 %. The maximum offset will be 5870m. Additional VSP data will be acquired to improve well ties and to facilitate reservoir characterisation. The objectives of the survey are to understand the internal structure.and heterogeneity of the reservoirs, to delineate the margins of the field especially where overlain by the flanks of salt domes, and to appraise the potential of Middle Devonian horizons at depths to 7000m.

A reserves re-determination has recently been undertaken in co-operation with the Republic of Kazakhstan (State Reserves Commission) experts and has been submitted to the Kazakhstan authorities for approval prior to submitting a Technical and Economic evaluation and then a Development Plan later this year.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England