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Kashagan Oil Field - Analysis of Geology, Geophysics and Petroleum System
The Kashagan oil field is the largest oil field discovered in the last 30 years, located in the northern Caspian Sea off Kazakhstan. It contains estimated reserves of 6.4 to 20 billion barrels of oil. However, production has been delayed due to the huge costs involved, environmental concerns with drilling, and reluctance of the Kazakh government to involve foreign oil companies. The field's geology includes carbonate reservoir rocks from the Devonian to Carboniferous periods overlain by Permian salt domes that serve as traps for the oil and gas accumulations.
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Kashagan Oil Field - Analysis of Geology, Geophysics and Petroleum System
- 1. Kashagan Oil Field by:- AkhilPrabhakar Subrat Sundaray Abhinav Singh Sunita Mohapatra Ankit Khaitan Tanmay Biswas
- 2. Figure 1: North CaspianRegion showing Kashagan and adjoining oil fields
- 3. Introduction World’s largestoil field discovered in the past three decades. Discovered in July, 2000 its considered to be the largest field of Kazhakhastan. 6.4 – 20 billion barrels have been estimated as recoverable. The field is situated in the northern part of the Caspian Sea close to the city of Atyrau, offshore in 10 – 22 feet water. The size of the field has been estimated to be a whopping 3,20,000 acres. The top of the reservoir is 4.5km below sea level and the oil column extends for more than one kilometre. But…
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- 5. Reasons for nocommercial production yet… The huge money involved. Every single 1% of take involves 1.5 – 2 billion dollars for the first 10 billion barrels alone! Environmental sensitive drilling involved. The reluctance of Kazakhstan government to allow involvement of foreign oil companies. High production cost which is increasing with the subsequent delay in production.
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- 7. Age of rocks Mostly the rocks of the north Caspian sea range from geological age late Devonian to Pleistocene. The hydrocarbon bearing rocks which are mostly carbonates, (limestone & dolomite) they range from age late Devonian to carboniferous. Seal rocks which are composed of salts, they are of lower Permian age. Other rocks are composed of terrigenous sediments which are from continental source.
- 8. Basement The basementof the Kashagan is a stable platform where the reef building activity takes place. This is an optimum place for reef building because for reef building we require- • A stable platform • Optimum Temperature • Optimum salinity conditions • Clear water so that sunlight can reach the basement. Reservoir Rock The hydrocarbon bearing rocks are mostly carbonates in the form of limestone and dolomite. These are secreted by Corals. The reef is about 75km long and 35km across, with a narrow neck joining two broader platforms (Kashagan East and Kashagan West).
- 9. Salt Domes Thereservoir rocks are overlain by saliferous formation of Kungurian stage of Lower Permian age which forms salt domes and troughs. Its thickness varies correspondingly from 0.5 to 1.0 km. 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 in turn develop open spaces on both sides of intrusion. This place acts as a good trap for oil and gas formation.
- 10. Upper Sediments TheUpper Permian – Triassic sediments are represented by terrigenous and terrigenous-carbonate formations. From these sediments some came from the continental area and some are from the carbonate deposit below. Its thickness varies from 1.5 to 2.5 km. A base of Jurassic occurs at a depth of 2.5 – 3.0 km in the inter-dome troughs and upto 0.2 km at the arches of salt domes. At a number of domes, Jurassic – Cretaceous sediments contain multilayer fields with small reserves (Verblyuzhja, Ka-myshitovoe wells). The sediments of Cenozoic age are represented by terrigenous formation of continental genesis. Its thickness is 0.5 – 0.8 km.
- 11. Structures associated with Kashaganoil field Many structures are found in this oil field such as the dome shaped structure which is found due to the diapiric intrusion of salts. We also find faults developed due to this diapiric intrusion and tectonic movement but they are not so prominent. Hence, this oil field is both structurally and stratigraphically controlled.
- 12. Seismic profile ofthe Kashagan Region.
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- 14. Source Rock Paleogeographicconditions of sedimentation and facies architecture indicate that the principal petroleum source rocks in the North Caspian basin 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 – 3 percent 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. This facies contains type II kerogen and is the principal oil source rock in Paleozoic (and many Mesozoic) basins of the world.
- 15. Events chart ofNorth Caspian Paleozoic Total Petroleum System
- 16. Mysterious presence ofsource rocks in suprasalt sequence. Some investigators believe that oil pools in salt dome-related traps were generated from these strata. Although some source rocks of inferior quality may be present among Triassic strata, these rocks could have reached maturity only in some deepest depressions between salt domes and thus are of only local significance. Recent geologic and geochemical data show that suprasalt oils were generated from subsalt source rocks and migrated upward from depressions between domes where the salt has been completely or almost completely with-drawn .
- 17. Source rocks…(continued) Caspianbasin, the top of subsalt rocks occurs in the oil window or in the upper part of the gas window. The geothermal gradient in the basin is relatively low apparently because of the cooling effect of the thick Kungurian salt sequence. Geothermal gradients in salt domes and adjacent depressions are different with the rocks beneath the salt domes being much cooler. Qualitatively, it can be stated that maturation in deep parts of the basin started before deposition of the salt. Most oil generated at this stage probably was lost because of the absence of a regional seal (local seals among mostly carbonate rocks are uncommon and easily breached). This loss of early-generated hydrocarbons is demonstrated by heavy, paleo-biodegraded oils found in a number of fields at depths reaching 5.5 km
- 18. Principal stage ofhydrocarbon generation The principal stage of hydrocarbon generation and formation of fields, especially in marginal, shallower areas of the basin, probably was in Late Permian–Triassic time when the Kungurian salt seal was in place and thick orogenic molasse clastics were deposited. Significant hydrocarbon generation in later times could have occurred only locally in depressions adjacent to growing salt domes.
- 19. Reservoir Rocks Thereare mainly two types of reservoir rocks in the North Caspian Basin; Carbonates and Clastics. Kashagan field consists of carbonate reservoir in the form of limestone and Dolomites. 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 reefal vuggy, porous limestones and dolomites. Most of the porosity in this field 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.
- 20. Seal Rocks TheLower Permian ‘Kungurian’ evaporite sequence is the principal regional seal for subsalt reservoirs of the North Caspian basin. It covers the entire basin area except for a narrow zone along the east and south margins. Where the seal is absent, hydrocarbons migrate from subsalt source rocks vertically into Suprasalt reservoirs. Thus the salt formation divides the sedimentary succession into two well-defined hydrodynamic systems - Subsalt system (over- pressured and high salinity) and Suprasalt system (hydrostatic pressure and low salt content) Both subsalt and suprasalt systems constitute a single total petro- leum system (TPS) because they were charged by hydrocarbons from the same subsalt source rocks; however, the upper system was designated as a separate assessment unit within the TPS.
- 21. Traps The LowerPermian ‘Kungurian’ evaporite sequence is the principal regional seal for subsalt reservoirs of the North Caspian basin. Various morphological types of reefs are present, but atolls and pinnacle reefs contain the largest hydrocarbon accumulations. Several subsalt fields on the east basin margin are in structural anticlinal traps. However, the discontinuous character of clastic reservoir rocks and large variability in flows from adjacent wells suggest that hydrodynamic connection between the wells is poor or absent and that many of these pools are actually in stratigraphic traps. In the suprasalt section, all productive traps are related to salt tectonics and are morphologically variable. Among them, anticlinal uplifts with a salt core and traps sealed updip by faults and by walls of salt domes are the most common types.
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- 23. Geophysics The fieldis now moving toward the production phase. In order to optimize the planning of the future drilling activity, it is necessary to better understand the fracture network that is difficult to see using conventional 3D surface seismic data. The effect of fracturing on the seismic velocities, Vp and Vs, in low- porosity limestones from the Kashagan oil field in Kazakhstan is investigated. Laboratory experiments have shown that sonic velocity of carbonates is mainly controlled by porosity and pore types
- 24. Moduli M, Kand G are the uniaxial P-wave, bulk, and shear drained- frame moduli, respectively. Seismic-wave velocities are decomposed into their frame and pore-space moduli (Murphy et al., 1993).
- 25. Geophysics (cont.) Forinvestigations, a heuristic model for fractured limestones has been developed. The approach was to: • Treat intact and fully fragmented limestone as distinct end-member rock-fabric elements, • develop a model for intact limestone, • develop a model for fully fractured and fragmented limestone, • combine the intact- and fragmented-limestone rock-fabric elements using a springs-in-series approach and a simple linear scaling factor, and • compare the heuristic model to well-log data.
- 26. Exponents m andn control the rate that moduli K and G decrease with increasing porosity. They incorporate the effects of pore geometry and fabric configuration in limestones. The endpoint properties at φ equals 0 and 1 are benchmarks values. Exponents m and n are set to achieve these benchmarks; they must be varied together. For Intact limestone:
- 27. Incorporating fragmented limestoneinto the framework
- 28. Results (Geophysics): Forany porosity, increasing the ( m,n) values causes P-wave velocity to decrease. The (m,n) = (2.00,1.88) curve tracks the upper limit of the data sets. These values are used to model intact limestone and explore the effects of fracturing.
- 31. Figures 4and 5, respectively, show the effect of fragmented- limestone moduli ( Kfrag , Gfrag) and fraction ( χfrag) on the P-wave velocity of saturated intact-limestones. For any porosity, decreasing Kfrag= G frag and/or increasing χfrag causes P-wave velocity to decrease. To model the effects of fluid saturation, we use Kfrag = Gfrag = 5 GPa and χfrag = 0.1.
- 32. Figure 6: Effectof fluid modulus ( Kf) on P -wave velocity in a combined model. The curve colors are gray, black, blue, green, and red for empty frame, bitumen, brine, oil, and gas, respectively. Gray dots are well-log data from Kashagan.
- 33. Moreover, Figure6 shows that there is a very small effect of fluid substitution into intact limestone, especially in the porosity range of Kashagan-East limestones. The main important results are: • Porosity and fluid moduli control the properties of intact limestone • Fluids have small effects on the velocities of intact limestones (solid curves) • Fluids affect fractured limestones much more (dashed curves) • Brine (blue) and bitumen (black) stiffen fractured limestone • Oil (green) and gas (red) affect fractured limestone less The results mentioned above were also confirmed with Finite Difference Modeling on Kashagan field, looking at the seismic response on fractures and fluid content.
- 34. Hence, theelastic framework of intact limestones is very stiff. With all else parameters remaining equal, porosity is the only factor that controls the seismic-wave velocities in intact limestones. Seismic responses from fluid substitution in intact limestones are very small, because the intact-limestone frame is so stiff. If the intact limestone becomes fractured, the limestone framework comes less stiff. Seismic P- and S -wave velocities decrease accordingly. The seismic response from fluid substitution in fractured limestones is varied. In porous and fractured limestones, the largest seismic response is at the lowest porosity. This response diminishes as porosity increases. The primary response is a stiffening of the rock frame by fluids, causing P-wave velocities to increase. This stiffening is greatest for bitumen and then brine. It is much less important for gas and live oil.
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- 36. The areacovering the Kashagan Contract has changed hands several times since independence of Kazakhstan. Interest in the Caspian Sea first began in 1992 when an exploration program was begun by the Kazakhstan government. They sought the interest of over 30 companies to partake in the exploration. In 1993 the Kazakhstancaspiishelf (KCS) was formed which consisted of Eni, BP Group, BP/Statoil, Mobil, Shell and Total, along with the Kazakh government. This consortium lasted 4 years until 1997 when the seismic exploration of the Caspian Sea was undertaken. Upon completion of an initial 2D seismic survey in 1997, KCS became the Offshore Kazakhstan International Operating Company (OKIOC).
- 37. In 1998Phillips Petroleum and Inpex bought into the consortium. The consortium changed again slightly when it was decided that one company was to operate the field instead of the joint operatorship as agreed before. Eni was named the new Operator in 2001. In 2001 BP/Statoil also chose to sell their stake in the project with the remaining partners buying their share. With Eni as operator, the project underwent another change in name to Agip Kazakhstan North Caspian Operating Company (Agip KCO). In 2003, BG Group attempted to sell their stake in the project to two Chinese companies CNOOC and Sinopec. However, the deal did not go through due to the partners exercising their pre-emption privileges. Eventually, the Kazakhstan Government bought half of BG's stake in the contract with the other half shared out among the five Western partners in the consortium that had exercised their pre-emption rights. The sale was worth approximately $1.2 billion.
- 38. On 27August 2007, Kazakhstan government suspended work at the Kashagan development for at least three months due to environmental violations. On 27 September 2007, Kazakhstan parliament approved the law enabling Kazakhstan government to alter or cancel contracts with foreign oil companies if their actions were threatening the national interests. In October 2008, Agip KCO handed a US$31 million letter of intent for FEED work on phase two to a joint venture of AkerSolutions, WorleyParsons and CB&I. WorleyParsons and Aker Solutions are engaged also in the phase one, carrying out engineering services, fabrication and hook-up. The budget for the development of Kashagan oilfield on Kazakhstan's Caspian Sea shelf in 2010 was reduced by $ 3 billion.
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