Indian Journal of Geo-Marine Sciences Vol. 43(3), March 2014, pp. 329-336 Evaluation of salt-section and interpretation of wells in the Kashagan field, Southern part of Pre-Caspian depression, Kazakhstan Akhmedzhanov T K, A S Abd Elmaksoud, Baiseit D. K & Igembaev I B Institute of Oil and Gas, Kazakh national technical university named after K.I. Satpayev, Kazakhstan [E-mail: [email protected]] Received 26 June 2012; revised 20 November 2012 Pre-Caspian basin has been formed from Paleozoic to Cenozoic time and the Kashagan field is situated in the southern part of Pre-Caspian depression in the Southern flung of Astrakhan-Aktyubinsk elevation system of basal complex and associated with Tengiz-Kashagan interbasin carbonate platform. Platform is composed of separate carbonate early Carboniferous blocks resting on the common Devonian (late Fransian-Fammenian) carbonate base. Four of them are: Kashagan, Kashagan South-West, Aktote and Kairan are located on the Caspian shelf and Karaton, Tengiz and Korolev on the coastal line of the eastern Caspian. Depositions of Devonian through to Elephantine inclusive were penetrated in Kashagan. Maximum section penetrated during drilling was 5172 m in KE-1. Pre-salt and post-salt units separated by thick Kungurian of Lower Permian salt-bearing section were identified in the well log. Description of the tectonics of the field was based on 3D seismic and drilling data from КЕ-1, КЕ-2, КЕ-3, КЕ-5 for East Kashagan and 2D seismic for West Kashagan where one well KW-1 was drilled. 3D seismic data provided more detailed information about geology of the structure, facies distribution and main layer thicknesses, and allowed a more accurate interpretation of tectonic faulting. Introduction and had cut more than 2000 m (6562 ft) of cores The Kashagan oil field is located in the northern in 15 wells (Fig. 2). The Kashagan buildup is Caspian Sea, western Kazakhstan (Fig. 1). Kashagan approximately 75 km (47 mi) in length and 35 km field is one of the largest oil fields discovered in the (22 mi) in width; it is subdivided into an eastern last 30 years. It is a large, isolated carbonate bank, platform (Kashagan East) and a western platform one of the biggest buildups in an archipelago that (Kashagan West) connected by a narrow neck characterized the Pre-Caspian Basin from the (Figure 2). Top of the structure is a seismic horizon Devonian to the Carboniferous and that now hosts correlatable throughout the Pre-Caspian depression large hydrocarbon reserves (Zempolichal., 2002b; that corresponds to the top of the Carboniferous Zempolich, 2005). Kashagan reservoir has a complex Bashkirian limestone. Platform margin is characterized depositional and stratigraphic framework, which by a narrow rim elevated up to 250 m (820 ft) higher experienced multistage diagenetic and tectonic than the platform interior and best evolution. This supergiant field was discovered in Developed in Kashagan East (Figures 1, 2), the 2000 by the Offshore Kazakhstan International platform flanks are steep, sloping up to 25°, with Operating Company (OKIOC) consortium (Agip, the northern slope being steeper than the southern Shell, Exxon Mobil, Phillips, Inpex, KMG, British slope (Fig. 3). In the southern Pre-Caspian Basin, Gas, BP Amoco, Statoil, and Totalfina). Structure main phase of carbonate deposition ranged from the was successfully tested by the Kashagan East-1 late Visean to Bashkirian. and Kashagan West-1 exploration wells, which Majority of the precipitation occurs during spring penetrated a nearly 400 m(1312 ft)-thick oil column and autumn with the annual level exceeding 250 mm in Carboniferous platform carbonates. The oils at times. Strong winds are typical for the area: 46°API and contains 16%H2S. Field is covered In winter prevailing winds are easterly and by a three dimensional seismic survey shot in northeasterly, in summer westerly and north westerly. 2001–2002. By the end of 2008, Agip Kazakhstan Snow blizzards in winter and sand storms and dry hot North Caspian Operating Company N.V. (AKCO) winds are fairly frequent. Development of the field (operating company from 2001 until 2008) had drilled will take place in very complicated geological 2 exploration, 10 appraisal, and 11 development wells environment with abnormally high reservoir pressure 330 INDIAN J. MAR. SCI., VOL. 42, NO. 3 MARCH 2014 Fig. 1—Map of the area. Geological setting Kashagan field is situated in the southern part of Pre-Caspian depression in the Southern flung of Astrakhan-Aktyubinsk elevation system of basal complex and associated with Tengiz-Kashagan interbasin carbonate platform (Fig. 2). Platform is composed of separate carbonate early Carboniferous blocks resting on the common Devonian (late Fransian-Fammenian) carbonate base. Four of them are: Kashagan, Kashagan South-West, Aktote and Kairan are located on the Caspian shelf and Karaton, Tengiz and Korolev on the coastal line of the eastern Caspian. Depositions of Devonian through to Elephantine inclusive were penetrated in Kashagan. Maximum section penetrated during drilling was 5172 m in KE-1. Pre-salt and post-salt units separated by thick Kungurian of Lower Permian salt-bearing section were identified in the well log. Fig. 3 showing a cross section of the area. Methods and Materials Fig. 2—Structural-tectonic map of Caspian region. Extensive field exploration and appraisal activities and H2S content. Consortium intends to re-inject continued during 2003 – 2005: produced raw gas into the reservoir to increase the Three new wells were drilled in East Kashagan oil recovery and reduce the production of sulphur (KE-4, KE-6, KE-А-01), another three are being (Barker A. and Croasdale K. R., 2004). All this drilled (KE-А-02, KE-А-03, KE-5-01). Gravimetry imposes very strict environmental constraints related survey was carried out. West Kashagan 3D seismic to every sphere of operational activities both survey was completed. PSTM 3D data processing and offshore and onshore. Development plan envisages interpretation and integrated seismic and gravimetry construction of artificial islands and platforms for data interpretation performed. New well data producers and injectors (vertical and directional). acquired: Core, cuttings, logs, well test results. AKHMEDZHANOV et al.: SALT-SECTION AND INTERPRETATION OF WELLS 331 Fig. 3—Cross section of Caspian Sea and Kashagan field. As a result of these studies new data was obtained biofacies typical of the shallow water platform to extend the knowledge of the geology of the field or low energy near tidal lagoon often associated including the number and boundaries of the facies, with fine textured PST, WST, saturated pelloids, reservoir rocks and reservoir properties. The new clots and chunks are quite frequent. Intensive information allowed to update and to increase the micritization of skeletal grains in this section does reliability and quality of the built previously not allow accurate identification. In the upper part geomodel. Core analysis and well test, reservoir fluid of the section these facies are replaced by grainier physical properties data was accumulated and lithofacies typical for the higher energy open platform reviewed. A vast amount of laboratory studies and palaeoenvironment. The thickness of Turnesian experiments was performed to forecast the behavior deposits is approximately 220 m. of the reservoir fluid during the development of the Visean is represented by Kozhimsky suprahorizon field. All the above allowed to estimate STOOIP and and Oksky subhorizon. Thin carbonate deposits reserves based on the model. Later the estimated oil in (pellet with streaks of algal bodies) are widely spread place and reserves were officially approved and in the Lower Visean as a result of the Seven seas rise registered in the State Book of Balance of RoK. and extension of the transgression on the platform. Stratigraphy Lower Visean deposits (Kozhimsky supra-horizon) are represented by pelloid WST/PST, GST, less often Pre-Salt by BST. The rocks are silicified in places and have Upper Devonian deposits were penetrated by two low porosity. wells: Deposits are composed of recrystallized algal- Tulsky is composed of bioclastic, crinoidal, pelloid very tight partially dolomitized WST/PST. In crinoidal-brachiopodal limestone interbedded with Devonian KE-1 penetrated a fairly thick porous zone streaks of vitric tuff. Facies of bioherms represented which could be the result of the karstification due to by small algal and crinoidal-algal bioherms could the significant drop of the sea level at the end of be found also. Towards the top of the section Devonian. Thickness of Upper Devonian interval facies typical of low energy platform prevail. penetrated by KE-1 is 325 m. Carboniferous system is High radioactive zone (HRZ) represented by represented stratigraphically, though not in full, by interbedded tuffous argillites is identifies. This member Lower and Middle series of: Turnesian, Visean and serves as a regional plug and borderline between Serpukhovian of Lower Carboniferous and Bashkirian Oksky and Tulsky horizons represented by shale of Middle Carboniferous. and marl. Its thickness varies between 25.2 m (KE-4) Turnesian was partially penetrated by KW-1, and 91.4 m (KW-1). KE-3, KE-5, KE-А-01 and fully by KE-1. In this Oksky deposits of Upper Visean are composed section the deposits of Malev, Upinsky, Cherepezky, of limestone WST/PST partially recrystallized and Kizelovsky and Kosvinsky were identified. Generally silicified. Base Upper Visean is characterized by Turnesian part of the section is a zone of Eustatic Sea shale and shale-marl limestone with streaks level elevation. In the lower part of the interval of volcanogenic-clastic deposits (HR marl facies). 332 INDIAN J. MAR. SCI., VOL. 42, NO. 3 MARCH 2014 The thickness of the Oksky supra-horizon varies and argillites with the streaks of limestone, marl across the platform from 216 m (KE-6) to 295.8 m and anhydrite. In the top the streaks of light grey in KE-4.