KASHAGAN OIL FIELD
Kashagan is a supergiant field located in the northern area of the Caspian Sea in Kazakhstan,see figure 1.0. It is mainly a carbonate reservoir located 4.5km below sea level, (Panfili et al., 2012). It is ice locked during winter and its saturated with over pressured light oil (initial reservoir pressure of 783 bar) with a relatively low permeability, (Z. Malik, et al., 2005). The reservoir contains 44 API gravity oil with associated sour gas of 15% hydrogen sulphide (H2S).
Figure 1.0 North Caspian region and location of the Kashagan field. The dotted line shows the Pre-Caspian Basin contour, (Ronchi, P et al., 2010).
The Kashagan oil field was discovered in 2000 by a consortium of oil companies. The current Caspian PSA companies are affiliates of; ENI, ExxonMobil, Shell and Total, each with a share of 18.52%, Conoco-Philips 9.26%, Inpex and Kazamunaigaz 8.33% each, see figure 1.01, (Z. Malik, et al., 2005).
Figure 1.01 is a chart displaying the owners of Kashagan.
The field is operated by AGIP Kco (an ENI company), (Z. Malik, et al., 2005). Kashagan oil field, believed to be the fifth largest ever discovered in the world and the largest discovered in the past 30 years has estimated total reserves of as high as 50 billion barrels of oil with up to 15-20 million which are estimated to be recoverable and 25 TCF of natural gas, (Babali, 2009). The Caprock consist of shale and salt formations of late Permian periods. The reservoir is represented mainly by limestone which has been split by environmental deposition, (D.Ybray, et al., 2011). Commercial production only began in 2013 and it was discontinued due to toxic leaks from a pipe. It has been identified as the main supplier for the Kazakstan-China pipelines. The cost of developing the field was estimated to be $116 billion as at 2012 which made it the most expensive energy project in the world, (Industry, I. 2018). This course work is aimed at reviewing the geology and exploration his history of the field.
2.0 LITERATURE REVIEW
2.1 GEOLOGY OF KASHAGAN
Kashagan is an isolated, pre-salt carbonate platform in the southern margin of the Pre-Caspian tectonic depression. The strike of the structure is oriented north-east to south-west, and although it is a single platform it is geomorphologically subdivided into an eastern (Kashagan East) and a western platform (Kashagan West), connected by a narrow “neck” (See figure 2.0). The platform dips to the south-west with the top reservoir ranging from 3800 m to 4300 m (C. Albertini, et al., 2013).
Figure 2.0 showing the eastern and western side of kashagan and the neck connecting them, (Rasoul Sorkhabi, 2018).
The present structural setting of the Kashagan platform is in fact the result of several deposition phases with different growth pattern: starting from Early Visean, after the deposition of a regional shaly tuff interval (generally known as Tula shale and here named “HRZ”) representing the maximum platform backstepping, the Kashagan platform started to grow with a slight prograding trend; then, the widest growth occurred during Late Visean and Serpukhovian. At the end of this intense phase within the Bashkirian, an aggrading phase characterized the last platform growth. This pattern can be observed thanks to the 3D4C seismic acquisition (acquired in a limited area of the Eastern Platform) and is confirmed by a sedimentological reinterpretation of the existing cores. (Panfili et al., 2012). According to the Kashagan structural framework several Environments of Deposition (EoDs) can be distinguished: an interior platform (PI) forming a relatively flat area and the platform margin (RIM) forming a slightly elevated ridge that is up to 200 m higher than the interior; between them, a transition zone (TZ) with intermediate petrophysical characteristics is placed. The flanks of the carbonate platform (slope) are rather steep – up to 20°-25° and are commonly affected by gravitational and syndepositional faults, (Francesconi et al., 2009).
2.2 HISTORICAL BACKGROUND
The Caspian Sea became an area of interest in 1992 when an exploration program was announced by the government of Kazakhstan. The government began to look for investors and they sought the interest of over 30 companies to take part in the exploration. In 1993, the Kazakhstancaspiishelf was formed which was consisted of ENI, BG group, BP/Statoil, Mobil, Royal Dutch Shell and Total SA along with the government of Kazakhstan. The consortium lasted four years until 1997 during which the seismic exploration of the Caspian Sea was undertaken. After the 2D seisimic survey was completed in 1997, the company became Offshore Kazakhstan Operating Company (OKIOC), Philips Petroleum Company and Inpex joined the alliance in 1998, shortly afterwards, the giant Kashagan was discovered in 2000 (“RigLogix – Westwood Global Energy Group”, 2018). Kashagan is a single carbonate platform which is geomorphologically subdivided into an eastern platform (Kashagan East) and a western platform (Kashagan West) connected by a narrow “neck” see figure 2.21. They can be further divided into:
- Kashagan East 1, the first well discovered on the Kashagan field in 2000 which was drilled in 3 meters of waters to a depth of 5200 meters. During the tests the well flowed at a rate of 600 cubic meters of oil per day and 200,000 cubic meters of gas per day on a 32/64inch choke.
- Kashagan West 1 was the second discovery well. Discovered in 2001, the tests showed that the well flowed at a rate of 3,400 barrels of oil per day, while the oil gravity measured between 42 and 45 API degrees. (“Kashagan Offshore Oil Field Project – Offshore Technology | Oil and Gas News and Market Analysis”, 2018)
- Kashagan East 2 was discovered in late 2001, Kashagan East 2 was drilled to a depth of 4142 meters and flowed at a rate of 7,400 barrels of oil per day. (“Kashagan Offshore Oil Field Project – Offshore Technology | Oil and Gas News and Market Analysis”, 2018)
2.3 RESERVOIR ROCKS AND SOURCE ROCKS
2.31 Reservoir Rocks
The hydrocarbon bearing rocks in Kashagan are mostly carbonates from limestone and dolomite secreted by corals. The reservoir rocks are overlain by saliferous formation of Kungurian stage of lower Permian age which forms salt domes and troughs, see figure 2.31.
Figure 2.31 North–south seismic line through the Kashagan platform showing the regional framework, (Ronchi, P et al., 2010).
This deposit plays the most important role in the formation of oil pool in the Caspian region due to accumulation of salts which tries to uplift the overlying deposit and it in turn develop open spaces on both sides of intrusion. These spaces act as a trap for oil and gas formation, (“Kashagan Oil Field – Analysis of Geology, Geophysics and Petroleum SY”, 2018).
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Reservoir properties of carbonate rocks strongly depend on diagenetic changes, primarily on leaching. Vuggy porosity related to leaching is better developed in reef reservoirs, especially in reef-core carbonates. Porosities averaging 10–14 percent are characteristic of porous limestones and dolomites which mainly describes the field . Most of the porosity in Kashagan is related to vugs, whereas the primary pore space does not exceed 2–3 percent. Permeability of carbonates is mainly controlled by fracturing and was observed to vary widely from a few to hundreds of millidarcies.
2.32 Source Rocks
Paleogeographic conditions of sedimentation and facies architecture indicate that the principal petroleum source rocks in Kashagan are basinal black-shale facies contemporaneous with upper Paleozoic carbonate platform deposits on the basin margins. Total organic carbon (TOC) content varies from as low as 1.3 – 3percent in Lower Permian basinal facies of the west basin margin to as high as 10 percent in Lower Permian black shale on flanks of the Karachaganak reef. Although data are few, high TOC and silica contents in basinal shales of all margins and characteristically high X-ray readings on gamma logs are typical of the deep-water anoxic black-shale facies. These facies contain type II kerogen and is the principal oil source rock in Paleozoic (and many Mesozoic) basins of the world, (“Kashagan Oil Field – Analysis of Geology, Geophysics and Petroleum SY”, 2018).
2.4 STATIGRAPHY OF KASHAGAN
The internal architecture of the Kashagan consists of six third-order depositional sequences from the base upward: upper Visean I, upper Visean II, upper Visean III, lower Serpukhovian, upper Serpukhovian, and lower Bashkirian (see figure 2.41), covering an interval of about 20 m.y. This third-order sequence-stratigraphic framework was first defined based on seismic interpretation and regional seismic-stratigraphic correlation and was then confirmed by the data acquired by the first exploration wells drilled on the structure and later by the appraisal and development wells (Ronchi, P et al., 2010).
Figure 2.41. Schematic cross section of the Kashagan reservoir unit 1 and stratigraphic scheme with the third-order sequences. HRZ = high-radioactivity zone, (Ronchi, P et al., 2010).
The third-order sequences are separated by major subaerial unconformities marked by paleo- karst and pedogenetic alteration of variable intensity which display a fine-layered internal architecture clearly recognizable from logs and cores (Claps et al., 2009). Particularly in the Visean III and II sequences, a transgressive systems tract (TST) and a highstand systems tract (HST) are recognized. The Serpukhovian can be subdivided into the sequences Serpukhovian 1 and Serpukhovian 2, which include an interval termed Serpukhovian karst. The Bashkirian sequence shows a very thin transgressive interval a few meters thick. Based on 3D and drilling data the following seismic facies zones were identified in Kashagan: Platform Interior, Transition Zone, Rim, Outer Shelf and Slope, see figure 2.42
The main horizon, yielding important petrographic and geochemical evidence of subaerial exposure is identified at the top Serpukhovian, which can be correlated in all the wells and corresponds to a global sea level fall. The interval underlying this major exposure was subject to deep diagenetic modifications and dissolution as observed in Kashagan cores. The biotic assemblages of this interval include Serpukhovian affinity algae, (Koninckopora sp., Calcifolium sp.) and benthic foraminifera (Endothyranopsis spp., Omphalotis spp., Endostaffella spp., Eostaffella gr. proikensis, Eostaffella gr. ikensis, Lituotubella magna, Cribrostomum sp.), (Ronchi, P et al., 2010). From the logging of about 2000 m (6562 ft) of cores, a scheme of 24 reservoir lithofacies were defined based on depositional attributes (texture, grain types, size and sorting, skeletal components, sedimentary structures, and early diagenetic features). These lithofacies were then clustered into six facies groups based on the interpretation of paleowater depth, hydrodynamic energy, and depositional environment (Claps et al., 2009; Ronchi, P et al., 2010) (see figure 2.43). These include:
• Group S includes lithofacies yielding a variety of textures and components all related to shal- low to very shallow water-depth conditions and variable hydrodynamic energy. Oolitic grain- stone, oolitically coated skeletal grainstone and packstone, and fenestral peloidal wackestone and packstone belong to this group.
• Group M comprises grainstone, packstone, and wackestone lithofacies containing dominant open-shelf biota with diversified algae and foraminifera in association with subordinate crinoids, encrusting forms, brachiopods, bivalves, and coral fragments. This association and the common bur- rowing suggest a moderate water-depth setting.
• Group D includes poorly sorted skeletal grain-stone to wackestone dominated by relatively deep open-shelf biota, such as crinoids and brachiopods, but also some algae, suggesting a setting still in the photic depth range.
• Group B consists of variable types of biohermal associations, characterized by boundstone and associated rudstone and floatstone dominated by colonial corals, or sponges, algae (Donezzella or Paleoberesellids), and brachiopods, as well as by clotted algal or peloidal and microbial or peloidal bind stone. The interpreted water depth is shallow for the coral and sponge bioherms, moderate for the algal and microbial bioherms, and deep subtidal for the brachiopod-dominated bioherms.
• Group R includes resedimented breccia and conglomerate deposited in the outer shelf or in the interior channels of the platform margin.
• Group T includes green tuffaceous argillite associated with subaerial exposure surfaces. The argillite forms thin layers or fills dissolution pockets but may also show evidence of reworking in the post exposure marine transgression. The relative abundances of different facies groups and their association integrated with seismic geometries were the basis for a conceptual framework of environments of deposition, (Claps et al., 2009; Ronchi, P et al., 2010).
Figure 2.43. Main depositional facies types: (1) ooid grainstone, facies group S; (2) algal and skeletal fragments grainstone, facies group M; (3) crinoid- and brachiopod-rich bioclastic wackestone, facies group M; (4) crinoid wackestone, facies group D; (5) coral boundstone, facies group B; (6) argillite bed at cycle top, facies T, (Ronchi, P et al., 2010).
- Panfili, P., Cominelli, A., Calabrese, M., Albertini, C., Savitsky, A., & Leoni, G. (2012). Advanced Upscaling for Kashagan Reservoir Modeling. SPE Reservoir Evaluation & Engineering, 15(02), 150-164. doi: 10.2118/146508-pa
- IPTC 10636 The Supergiant Kashagan Field: Making a Sweet Development Out of Sour Crude Z. Malik, M. Charfeddine, and S. Moore, Shell Kazakhstan Development, and L. Francia and P. Denby, Agip KCO 2005
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- Industry, I. (2018). Kazakhstan’s Kashagan tagged world’s most expensive energy project. Retrieved from https://en.tengrinews.kz/industry_infrastructure/Kazakhstans-Kashagan-tagged-worlds-most-expensive-energy-14913/
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- Ronchi, P., Ortenzi, A., Borromeo, O., Claps, M., & Zempolich, W. (2010). Depositional setting and diagenetic processes and their impact on the reservoir quality in the late Visean–Bashkirian Kashagan carbonate platform (Pre-Caspian Basin, Kazakhstan). AAPG Bulletin, 94(9), 1313-1348. doi: 10.1306/01051009130
- Claps, M., W. G. Zempolich, F. Casaglia, and P. Ronchi, 2009, Sedimentology and stratigraphy of the Kashagan buildup, Carboniferous, Pricaspian Basin, Kazakhstan: AAPG An- nual Convention Abstracts, v. 18, p. 44.
- Kashagan Oil Field – Analysis of Geology, Geophysics and Petroleum Sy…. (2018). Retrieved from https://www.slideshare.net/akhilprabhakar/kashagan-oil-field-analysis-of-geology-geophysics-and-petroleum-system
- Rasoul Sorkhabi,P.(2018).KashaganComesOnstream.Retrievedfromhttps://www.geoexpro.com/articles/2013/12/kashagan-comes-onstream
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