Introduction World’s largest oil field discovered in the past three decades. Discovered in July, 2000 its considered to be the largest field ofKazhakhastan. 6.4 – 20 billion barrels have been estimated as recoverable. The field is situated in the northern part of the Caspian Sea close to thecity of Atyrau, offshore in 10 – 22 feet water. The size of the field has been estimated to be a whopping 3,20,000acres. The top of the reservoir is 4.5km below sea level and the oil columnextends for more than one kilometre.But…
Reasons for no commercialproduction yet… The huge money involved. Every single 1% of take involves 1.5 – 2 billion dollars for the first 10billion barrels alone! Environmental sensitive drilling involved. The reluctance of Kazakhstan government to allow involvement offoreign oil companies. High production cost which is increasing with the subsequent delay inproduction.
Age of rocks Mostly the rocks of the north Caspian sea range from geologicalage late Devonian to Pleistocene. The hydrocarbon bearing rocks which are mostly carbonates,(limestone & dolomite) they range from age late Devonian tocarboniferous. Seal rocks which are composed of salts, they are of lower Permianage. Other rocks are composed of terrigenous sediments which are fromcontinental source.
Basement The basement of the Kashagan is a stable platform where the reefbuilding activity takes place. This is an optimum place for reef building because for reef buildingwe 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 oflimestone and dolomite. These are secreted by Corals. The reef is about 75km long and 35km across, with a narrow neckjoining two broader platforms (Kashagan East and Kashagan West).
Salt Domes The reservoir rocks are overlain by saliferous formation ofKungurian stage of Lower Permian age which forms salt domesand troughs. Its thickness varies correspondingly from 0.5 to 1.0 km. This deposit plays the most important role in the formation of oilpool in the Caspian region due to accumulation of salts which triesto uplift the overlying deposit and in turn develop open spaces onboth sides of intrusion. This place acts as a good trap for oil and gas formation.
Upper Sediments The Upper Permian – Triassic sediments are represented byterrigenous and terrigenous-carbonate formations. From these sediments some came from the continental area andsome are from the carbonate deposit below. Its thickness varies from 1.5 to 2.5 km. A base of Jurassic occurs at adepth of 2.5 – 3.0 km in the inter-dome troughs and upto 0.2 km atthe arches of salt domes. At a number of domes, Jurassic – Cretaceous sediments containmultilayer fields with small reserves (Verblyuzhja, Ka-myshitovoewells). The sediments of Cenozoic age are represented by terrigenousformation of continental genesis. Its thickness is 0.5 – 0.8 km.
Structures associated withKashagan oil field Many structures are found in this oil field such as the dome shapedstructure which is found due to the diapiric intrusion of salts. We also find faults developed due to this diapiric intrusion andtectonic movement but they are not so prominent. Hence, this oil field is both structurally and stratigraphicallycontrolled.
Source Rock Paleogeographic conditions of sedimentation and faciesarchitecture indicate that the principal petroleum source rocks in theNorth Caspian basin are basinal black-shale faciescontemporaneous with upper Paleozoic carbonate platformdeposits 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 toas high as 10 percent in Lower Permian black shale on flanks of theKarachaganak reef. Although data are few, high TOC and silica contents in basinalshales of all margins and characteristically high X-ray readings ongamma logs are typical of the deep-water anoxic black-shale facies. This facies contains type II kerogen and is the principal oil sourcerock in Paleozoic (and many Mesozoic) basins of the world.
Events chart of North Caspian Paleozoic Total Petroleum System
Mysterious presence of sourcerocks in suprasalt sequence. Some investigators believe that oil pools in salt dome-related trapswere generated from these strata. Although some source rocks of inferior quality may be presentamong Triassic strata, these rocks could have reached maturity onlyin some deepest depressions between salt domes and thus are ofonly local significance. Recent geologic and geochemical data show that suprasalt oils weregenerated from subsalt source rocks and migrated upward fromdepressions between domes where the salt has been completely oralmost completely with-drawn .
Source rocks…(continued) Caspian basin, the top of subsalt rocks occurs in the oil window or inthe upper part of the gas window. The geothermal gradient in the basin is relatively low apparentlybecause of the cooling effect of the thick Kungurian salt sequence. Geothermal gradients in salt domes and adjacent depressions aredifferent with the rocks beneath the salt domes being much cooler. Qualitatively, it can be stated that maturation in deep parts of thebasin started before deposition of the salt. Most oil generated at this stage probably was lost because of theabsence of a regional seal (local seals among mostly carbonaterocks are uncommon and easily breached). This loss of early-generated hydrocarbons is demonstrated byheavy, paleo-biodegraded oils found in a number of fields at depthsreaching 5.5 km
Principal stage of hydrocarbongeneration The principal stage of hydrocarbon generation and formation offields, especially in marginal, shallower areas of thebasin, probably was in Late Permian–Triassic time when theKungurian salt seal was in place and thick orogenic molasseclastics were deposited. Significant hydrocarbon generation in later times could haveoccurred only locally in depressions adjacent to growing saltdomes.
Reservoir Rocks There are mainly two types of reservoir rocks in the North CaspianBasin; Carbonates and Clastics. Kashagan field consists ofcarbonate reservoir in the form of limestone and Dolomites. Reservoir properties of carbonate rocks strongly depend ondiagenetic changes, primarily on leaching. Vuggy porosity related to leaching is better developed in reefreservoirs, especially in reef-core carbonates. Porosities averaging 10–14 percent are characteristic of reefalvuggy, porous limestones and dolomites. Most of the porosity in this field is related to vugs, whereas theprimary pore space does not exceed 2–3 percent. Permeability of carbonates is mainly controlled by fracturing and wasobserved to vary widely from a few to hundreds of millidarcies.
Seal Rocks The Lower Permian ‘Kungurian’ evaporite sequence is the principalregional seal for subsalt reservoirs of the North Caspian basin. It covers the entire basin area except for a narrow zone along theeast and south margins. Where the seal is absent, hydrocarbons migrate from subsalt sourcerocks vertically into Suprasalt reservoirs. Thus the salt formation divides the sedimentary succession into twowell-defined hydrodynamic systems - Subsalt system (over-pressured and high salinity) and Suprasalt system (hydrostaticpressure and low salt content) Both subsalt and suprasalt systems constitute a single total petro-leum system (TPS) because they were charged by hydrocarbonsfrom the same subsalt source rocks; however, the upper system wasdesignated as a separate assessment unit within the TPS.
Traps The Lower Permian ‘Kungurian’ evaporite sequence is the principalregional seal for subsalt reservoirs of the North Caspian basin. Various morphological types of reefs are present, but atolls andpinnacle reefs contain the largest hydrocarbon accumulations. Several subsalt fields on the east basin margin are in structuralanticlinal traps. However, the discontinuous character of clastic reservoir rocks andlarge variability in flows from adjacent wells suggest thathydrodynamic connection between the wells is poor or absent andthat many of these pools are actually in stratigraphic traps. In the suprasalt section, all productive traps are related to salttectonics and are morphologically variable. Among them, anticlinaluplifts with a salt core and traps sealed updip by faults and by wallsof salt domes are the most common types.
Geophysics The field is now moving toward the production phase. In order to optimize the planning of the future drilling activity, it isnecessary to better understand the fracture network that is difficult tosee 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 isinvestigated. Laboratory experiments have shown that sonic velocity ofcarbonates is mainly controlled by porosity and pore types
Moduli M, K and G are the uniaxial P-wave, bulk, and shear drained-frame moduli, respectively.Seismic-wave velocities are decomposed into their frame and pore-spacemoduli (Murphy et al., 1993).
Geophysics (cont.) For investigations, a heuristic model for fractured limestones hasbeen developed. The approach was to:• Treat intact and fully fragmented limestone as distinct end-memberrock-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 elementsusing a springs-in-series approach and a simple linear scalingfactor, and• compare the heuristic model to well-log data.
Exponents m and n control the rate that moduli K and G decrease withincreasing porosity.They incorporate the effects of pore geometry and fabric configuration inlimestones.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 variedtogether.For Intact limestone:
Incorporating fragmented limestone into the framework
Results (Geophysics): For any porosity, increasing the ( m,n) values causes P-wave velocity todecrease. The (m,n) = (2.00,1.88) curve tracks the upper limit of the datasets. These values are used to model intact limestone and explore the effectsof fracturing.
Figures 4 and 5, respectively, show the effect of fragmented-limestone moduli ( Kfrag , Gfrag) and fraction ( χfrag) on the P-wavevelocity of saturated intact-limestones. For any porosity, decreasing Kfrag= G frag and/or increasing χfragcauses P-wave velocity to decrease. To model the effects of fluidsaturation, we use Kfrag = Gfrag = 5 GPa and χfrag = 0.1.
Figure 6: Effect of fluid modulus ( Kf) on P -wave velocity in a combinedmodel. The curve colors are gray, black, blue, green, and red for emptyframe, bitumen, brine, oil, and gas, respectively. Gray dots are well-log data fromKashagan.
Moreover, Figure 6 shows that there is a very small effect of fluidsubstitution into intact limestone, especially in the porosity range ofKashagan-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 (solidcurves)• Fluids affect fractured limestones much more (dashed curves)• Brine (blue) and bitumen (black) stiffen fractured limestone• Oil (green) and gas (red) affect fractured limestone lessThe results mentioned above were also confirmed with FiniteDifference Modeling on Kashagan field, looking at the seismic responseon fractures and fluid content.
Hence, the elastic framework of intact limestones is very stiff. With allelse parameters remaining equal, porosity is the only factor thatcontrols the seismic-wave velocities in intact limestones. Seismic responses from fluid substitution in intact limestones are verysmall, because the intact-limestone frame is so stiff. If the intact limestone becomes fractured, the limestone frameworkcomes less stiff. Seismic P- and S -wave velocities decreaseaccordingly. The seismic response from fluid substitution in fractured limestones isvaried. In porous and fractured limestones, the largest seismicresponse is at the lowest porosity. This response diminishes as porosityincreases. The primary response is a stiffening of the rock frame by fluids, causingP-wave velocities to increase. This stiffening is greatest for bitumenand then brine. It is much less important for gas and live oil.
The area covering the Kashagan Contract has changed hands severaltimes since independence of Kazakhstan. Interest in the Caspian Sea first began in 1992 when an explorationprogram was begun by the Kazakhstan government. They sought theinterest of over 30 companies to partake in the exploration. In 1993 the Kazakhstancaspiishelf (KCS) was formed which consistedof Eni, BP Group, BP/Statoil, Mobil, Shell and Total, along with the Kazakhgovernment. This consortium lasted 4 years until 1997 when the seismic explorationof the Caspian Sea was undertaken. Upon completion of an initial 2D seismic survey in 1997, KCS becamethe Offshore Kazakhstan International Operating Company (OKIOC).
In 1998 Phillips Petroleum and Inpex bought into the consortium. Theconsortium changed again slightly when it was decided that onecompany was to operate the field instead of the joint operatorship asagreed before. Eni was named the new Operator in 2001. In 2001 BP/Statoil also chose to sell their stake in the project with theremaining partners buying their share. With Eni as operator, the projectunderwent another change in name to Agip Kazakhstan North CaspianOperating Company (Agip KCO). In 2003, BG Group attempted to sell their stake in the project to twoChinese companies CNOOC and Sinopec. However, the deal did not gothrough due to the partners exercising their pre-emption privileges. Eventually, the Kazakhstan Government bought half of BGs stake inthe contract with the other half shared out among the five Westernpartners in the consortium that had exercised their pre-emption rights. Thesale was worth approximately $1.2 billion.
On 27 August 2007, Kazakhstan government suspended work at theKashagan development for at least three months due to environmentalviolations. On 27 September 2007, Kazakhstan parliament approved the lawenabling Kazakhstan government to alter or cancel contracts withforeign oil companies if their actions were threatening the nationalinterests. In October 2008, Agip KCO handed a US$31 million letter of intent forFEED work on phase two to a joint venture of AkerSolutions,WorleyParsons and CB&I. WorleyParsons and Aker Solutions areengaged also in the phase one, carrying out engineering services,fabrication and hook-up. The budget for the development of Kashagan oilfield on KazakhstansCaspian Sea shelf in 2010 was reduced by $ 3 billion.