Deep overpressure gas accumulation

Deep overpressure gasbesides near the top overpressure surface, the hydrocarbonin overpressure basin was mainly accumulated in the presaccumulationsure transition zone and normal pressure zone, and anaccumulation in deep overpressure zone may be formedunder the given conditions. This paper discusses playZHANG Qimingconditions under deep overpressure over 2.0 pressureChina Offshore Oil Nanhai West Corporation, Zhanjiang 524057, China factor by comparison of the Kela-2 field in the Kuga(e-mail:zhangqm@cnooc.com.cn)Foreland Basin with Ya2 1-1 structure in the Qiongdong-Abstract The Kela-2 gas field in the Kuqa Depression of nan Basinthe Tarim Basin is one of the greatest fields recently discov- 1 Hydrocarbon occurrence in overpressure basinered in the deep overpressure reservoir in the world. Theplay conditions of Kela-2 field and Ya2l-1 structure featuresThe hydrocarbons in overpressure basin are com-in the Qiongdongnan Basin on the South China Sea shelf has monly distributed in the normal pressure zone and thebeen studied, and a gas accumulation under deep overpres. pressure intermediate zone. It is rarely accumulated in thefavourable conditions for gas ith overpressure occurrence level in exploration in overpressure basins. The results ofccumulation): one is that the Soviet Union are the earliest countries with the highesttime of gas charge matchestime, and the other is a good quality caprock. If the charge exploration proved that the hydrocarbon reserves from thewas early and the overpressure occurrence was late, and a deep overpressure reservoir cover only a small part of thefine caprock of the evaporite bed existed, the accumulation total reserveof gas is favourable; conversely, it is difficult to form a gasFor example, the hydrocarbon exploration of Trinidadl3 was started in 1866 and the first drilling well wasKeywords: overpressure, overpressure basin, deep overpressure near the oil seepage and the mud volcano. Trinidad is thereservoir, gas accumulationearliest country conducting high-pressure bed investigation in the world. After World War. the abnormal highBoth the Trinidad and the Former Soviet Union are pressure played an important role in the exploration andthe earliest countries in the world which carried out the development. By the end of 1995, the total productionexploration in the overpressure basin and the drilling from the six fields in interesting areas was 1310. >104m3works in overpressure bed were started in the middle ofthe igth century. In the 1940s-1950s of the 20th century, pressure intermediate belt with pressure over 16.4 KPa/mthe discoveries of overpressure basin became more and whereas the fields in the overpressure and deep overpress-more and the people started to study the overpressure sure belt had hardly commercial value.mechanism from respective speciality. In the 1960sIn the Former Soviet Union+, the overpressure wasoverpressure prediction techniques for prevention of over 5000 holes were encountered in Timan-Pechoeadrilling accident. After the 1980s scholars started the weWest Siberia, East Siberia, Volga-Ural, Sokhalinskiystudy of overpressure-hydro-carbon relationship. So far, North Caspian. South Caspian and North Caucasus. Thethe overpressure basins over 180 have been discovered in statistical results showed that 90% wells with pressurethe world. From the 1950s to now, the overpressure factor below 1. 8 had commercial value, indicating that thesystems have been discovered in 29 basins in Sichuan hydrocarbon in overpressure basin was distributed in theduring hydrocarbon exploration of overpressure fissure- normal pressure and intermediate beltbecame a geologic leading edge technology. Up to now, 2 Accumulation under deepoverpressurescholars have discussed the hydrocarbon occurrence inoverpressure basin, but rarely studied the play condition fissures existing in tight sandstones and limestone, andunder deep overpressure. China has accumulated a wealth pore sandstones being covered by an intact thick mud-of experiences in deep overpressure from the explorations stones, as well as pore sandstones or pore limestone andin the Ying-Qiong Basin, the South China Sea, the fractured limestone being covered by an evaporite,canSichuan Basin, the Dungar Basin and the Tarim Basin, the gas in deep overpressure reservoirs be reserved to theXinjiang. The study is an urgent affair for developpresthe first three kinds ofChinas natural gas industry Law et al中国煤化工1- scale gas fields. but themost of hydrocarbonsverpressure basinsCNMH GSuch as the Kela-2 fieldaccumulated near the top overpressure and the reservoirs1)Accumulation of the Kela-2 field. The Kela-2hardly produced hydrocarbon when the pressure gradient field is located in Kuqa Foreland Basin, to the north isis over 19 KPa/Tianshan fold mountain chain. and to the south is theAccording to the authors practical experiencesChinese Science Bulletin Vol 47 Supp. December 2002foreland basin that was formed under lithosphere flexure tribution in space, higher overpressure in the highestcoupling with a new orogeny. One side of the foreland stressed gypsum salt slump stratum and the stratum nearbasin is a fold mountain system and the other sidefault, the apparentstress response in drilling and logging, and the Kelasuold land district. The basin was formed by the settlement structure background analysis, someone believed that notof the lithosphere flexure belt due to the late loading effect the uncompaction but the structure compression and theand has undergone a complex development history. Since charge of high-pressure gas induce the overpressure ofthe Mesozoic, the basin underwent three stages, i.e. a Eogene-Cretaceous reservoirs here. Besides reservoirs, inforeland basin development period in the Triassic,anin the Jurassic source rock of the Yinan structure, the Alextending basin develoFormation and the underlying beds also have high pressure with pressure factor of 1.9--1.98. The source rockEogene and a regeneration foreland basin development with kerogen of type LI, and Ro of 1.0%, reached a constage in Tertiary Miocenedensation gas generation peak, so the occurrence of over-The Kuqa Foreland Basinan overpressure cold pressure in Jurassic and the underlying source rock mainlybasin(reservoir pressure factor of 2.0, geothermal gradi- results from gas generationent of 20/100 m)(fig. 1), the overpressure beds areAs the above, there are mainly two factors causingmainly below 3000 m between the Eogene gypsum salt overpressure in the Kuqa Foreland Basin: one is structurebed and Jurassic, and the pressure configuration looks like compression plus gas charge in the gypsum salt beds ofEogene and the Cretaceous under the gypsum salt beds,a three-floor building model with pressure increasing the other is gas generation in the underlying Jurassic andgradually. The Neogene is a normal pressure zone and the Triassic source rock. The overpressure distribution in-Eogene is a pressure intermediate zone, and the forma- ferred is only local in the upper, but wide in the lowertions between the gypsum salt beds in Eogene and theIn Kela-2 field(tablel, fig. 2), the reservoirs are thelayers below the gypsum salt beds are overpressure beds lower sandstones of the Eogene Kumugeliemu Formationand the sandstones of the Cretaceous Bashijinaike Formawith pressure factor up to 2.0 the overpressure body on tion, with gas layer thickness of 267. 8 m(Kela-2 well )the plane is distributed in the middle part of the basin, the caprocks are the gypsum salt and mudstones of Eowhereas the foot of the Tianshan Mountains range in the gene, and source rocks are the coal measure strata of unnorth with monoclinal strata is a normal pressure zone due derlying Jurassic and Triassic. The features of play in theto the outcropping Mesozoic; the area from the south Kela-2 field are early charge, late overpressure and veryslope to Tabei high, uplifted and thinned in Mesozoicgood caprocks, therefore, the greatest field of deep overCenozoic, is also a normal pressure districtpressure reservoir was formed in the worldThe overpressure of Kuga foreland basin was forme1)A very good play condition----early chain gypsum salt beds of Eogene and the underlying Eogenelate overpressure. The Kuqa Basin has two suites ofto Jurassic, and the overpressure surface is laid in theHuangshanjie formationmudstones with maximum thickness of 560 m. maximumincompatible section, which is about 700-1200 m under TOC of 5.58%, average TOC of 1.1%, the other is Jurashe linear segment of sonic( DT)curve, and the porosityand rock density of incompatible section do not change of 231 ToC of 3. 11%. The two suites contain a totaleffective source rock thickness over 1000 m with a goodof an uncompaction/ s) Generally, in shallow position of kerogen type. Zhao et al. 16) considered that the Jurassicacoustic curve, the segment in which the dt decreasessystems in Baicheng and Yangxia Depressions entered ag depth was considered as a normal com- thermal maturity period in 55 and 97 Ma respectively.Thepacted section, and in deep position, the surface at the Jurassic was buried over 10000 m with a high thermalan abnormal high-pressure surface. Actually, this is not evolution degree because of the rapid filling of Neogenein Baicheng Depression is 2. 3%-2.6%, reaching a dryMexico Basin was considered as a result of uncompaction gas stage and the reflectance of Jurassic is 1.4%-2.4%in the past, but recently, from the correlation of porosity reaching a wet-dry gas stage. The hydrocarbon maturationand density data and acoustic time-depth curve, the overunder inflection point on curves, thus people consider the is up to 2000.>10%m/km, more than 100.>108m/kmpressure surface emerges in the incompacting segmentmudstone normal compacted curve as a two-segmentwith a gas generation area of 1.x104km. Lei et al. depattern including an exponent function partition and a termined the structure evolution history by the growinglayer. The Kelasu anticline structure was formed in a verydown linear partition, which is similar to the Ying-Qiong late time that favored the gas accumulation In the sediBasin and Yaha district in south of the Kuqa, the mea mentary period (5.3 Ma)of the Miocene Kaucun Forma-segment is a normal pressure zone too. As the aboveaccording to the following aspects, i.e. overpressure dis-中国煤化工CNMHGChinese S序嫂弻 etin Vo.47Supp. December2002YangxiaBaicheng★DwKuqa火LengYHI UpODHIYM7★NKIOo Shawan★YD1ElementHydrocarbonndary boundary2040kmTian mountainsTabei upliftKuqa Sag(normal pressure(abnormal pressure)翻国Regional seal Souce rock Hydrocarbon pool Press directionFig. I. Structure of the Kuqa Foreland Basin and the location of the Kela-2 gas field. Q, Qr. Xiyu F. N2, Tp. Kuche F; N, Tm. Kercuen and Jidike F; E, Tpg. Suwiyi and Kumgeliemu F K, K. Bashiji Naike F, J. T, source rocktion, Kelasu faulted anticline began to develop and the gas structure compression. Therefore, the Kela-2 field of over-charge began due to Tian Mountains orogeny, and the pressure anticline type was formed after experiencing anKuqa Formation formed a shaped structure in the late early charge, and late overpressure procesHimalayan orogeny. Experiencing denudation in Kuqa(2)A special excellent seal. Though the aboveFormation and temperature falls and pressure releases in stated conditions are good, a great overpressure reservoirboth Upper Cretaceous and Eogene reservoirs because of gas field is difficult to form yet without good quality sealhorizontal compression and upper wall being uplifted, the The Kela-2 field is a fault-bend anticline structure in pahigh pressure gas in underlying source rock migrated up sive double tectonic layers on the Kelasu thrust structureto reservoir through the faults; both structure displacement belt, with trap crest elevation of -2020 m, spill pointand great uplifting in the Kelasu structure were encoun- depthf of 455 m. Despite the faults cuttered by 2 Ma, which makes the bench-shaped thrust fault off tl中国煤化工 prock in the anticline topform and overlap each other and then a typical double andCNMHGOO m have been preservedtectonic layer pattern formedheight up to 448 mevents, the reservoir pressure was increased by not only fullness up to 98.5% and pressure factor up to 1.9--2.2high-pressure gas charge from source rock but also a The structure with deep overpressure reservoir can beChinese Science Bulletin Vol 47 Supp. December 2002Table 1 Typical field dataParameterYal3-fault-bend anticline ipassive double tectonictructstratigraphic trap structure& stratigraphic traplayersSize/km56.38545.5490EyEsTop depth of reservoir/m35613550Gas layer thickness/m267.8Porosity(%)Permeability/mdCaprock lithologysalt, gyp. mudstonehigh-pressure mudstone350(overpressure)9793-98.2688.110.50.71-20.55-.79Gas column height /mWater typeCaclNahcoNaHCO,7-10Proved reserves/10%m32506.1Recoverable reserves /10%m1879Pressure factorTemperature gradient/.(--(100 m)3.95-,42Top depth of deep overpressure bed/m33004957(well No 4)pressure rise--gypsum-salt and pressure seal(1. The Failure reason of drilling in Ya2l-1 struc-The Ya21-1 structure is located on Ya21-1 structurbelt of the Qiongdongnan Basin that is different fromKuqa Basin. Qiongdongnan is an extending basin(fig. 3)formed under a stretching of lithosphere. This kind ofbasin has a double tectonic layers and the lower structurelayer, Eogene is a rift, and the upper structure layer, Neogene is evolved into a passive basin after an experience ofbreak unconformity and the following deep thermal decay.The Neogene is marine facies and gradually developsfrom a neritic facies into a bathyal facies. The QiongFig. 2. Sketch of Kela-2 gas pools. Modified after hydrocarbon explo- dongnan Basin is an overpressure (pressure factor up toration in the Tarim Basin. 20012.2)thermal(temp. gradient 4.(m)basinThe overpressure of the Qiongdongnan Basin occurssealed because the gypsum salt and gypsum-mudstone from the bottom of the Pliocene Yinggehai Formation towith high plasticity are best caprocks with displacement the Oligocene Yacheng Formation and pressure of thepressure up to 60 KPalll and the gypsum salt caprock itself lower beds is not clearly known. The overpressure surfaceis a overpressure bed with the same pressure factor as laterally cross different strata, the Formation that overreservoir, therefore, the double preservative strata formedpressure surface crossed is more new if the surface posia special excellent seal. The gas pool can be preserved to tion is closer to the basin center, contrarily much olderthe present because of anticline trap and overpressure Thegypsum salt sealwith中国煤化工 two-floor building modelIn sum, the play model of Kela-2 field is: structure FornThe Pliocene yinggehaiC N MH Gottom of Yinggehai Forthrust being uplifted and then overlying bed denudation- mation, Miocene Huangliu Formation and Meishan Forreservoir temperature drop and pressure fall.-high pres- mation are overpressure; the underlying Miocene Sanyasure gas charge and structure compression-stimulating Formation sandstone is normal pressure, and the oligoChinese S序嫂 letin Vo.47Supp. December2002LingshuHainan IslandBaodao h. trendBaodao DLinshui DLedong d204060kmod hCDP77746382481000014000Fig. 3. Structure of Qiongdongnan Basin. The numbers in the circle represent the code of the faults. (a) Structure style and Y13-1, Y19Y26-1, Y21-1 Wells location of Qiongdongnan Basin. H, Horst; D, Depression; Qr, Quaternary; Tp, Pliocene; Tm, Miocene: ToOligocene: Te, Eocene; F, Formation.(b) Structure cross section along the seismic line 97D 1060 in Qiongdongnan Basin. T20, Thebottom of Qr: T20--T40, Te. Yinggehai F: T40--T60, Tm Meshan Sanya F. T60--T70, To. Lingshui F. T70--T80, To Yacheng F.T80--T100, Te Wenchang F.cene Lingshui Formation(the hydrocarbon-bearing target) coal-bearing clastic rocks as source rocks with relativelyand the Yacheng Formation consist of a lower overpres- good diagenesis, and mainly secondary porosity. Accordsure system with pressure factor up to 2. 2. On the plane, ing to the rock slice data, the pore was filled with muchthe overpressure bodies are mainly distributed in the south asphalt, which shows that Eogene overpressure wasof Yacheng Uplift and Lingshui Upliftformed by the oil cracking to gasThe rapid subsidence and sedimentation, behaving assedimentation rate up to 600--1000 m/Ma and poor di中国煤化工 aperature gradient is uagenesis with mainly original sandstone pore, resulted in 4: vCNMHGrmal basin with a presentNeogene overpressure system in Qiongdongnan Basin heat flow of 82 mw.om-2since 5.0 ma. the oligocene yacheng formation and theThe Ya13-1 structure lies to the northwest end of theLingshui Formation in the lower overpressure system are Yal3-1 structure belt with trend NW-ES. with the southChinese Science Bulletin Vol 47 Supp. December 2002estern flank cut by fault No I and adjacent to the Ying- 1-3 well are dry layers, water layers or gas-bearing watergehai Basin, facing Yanan Depression on the east. Oligo- layers from the logging and test data. According to DST3cene Lingshui [l is a main target of gas, with trap size of between 4630 and 4629. 5 m, water output is 760.3 m/d,49.9 km, closure of 460 m, crest burial depth of 3500 m, gas output is 1.025.>10-m /d containing mainly methanewhich was a draping anticline structure developed for a component, and the gas-water ratio is 13.45 m/m, whichlong time, with structure traps from the basement to Low- shows that gas in the reservoir is a kind of high pressureer Miocene Sanya Formation and the Middle miocene water soluble gas. The measured reservoir temperature isMeishan Formation; the top of Lingshui [I reservoir has 200.6 C, and reservoir pressure is 104.73 MPa. The theobeen denuded, inducing some thick and good propertyretic methane saturation in water is 24 m/m3 under thesandstones in flank with high production ability. Resersame temperature and pressure, which is bigger than theof Yal3-1 is 8.5.> gasls, and annual output is 3.5 measured saturation 11 m/m3. So the gas is unsaturatedX10%m' gas transmitting to Hong Kong and Hainan There was no free gas, so no gas pool can formProvince. The discovery of Yal3-1 field is heartening2) Fractured mudstone cover over overpressure reFollowing the discovery, both Ya19-1 and Ya26-1 struc- servoir and hydrocarbon escape. On the statistical basistures in the south of the Ya13-1 structure belt in deep the hydraulic fracturing fault of overburden cover willverpressure, as well as Ya21-1 structure on the Ya21-1 emerge when reservoir pressure is equal to 70%-98%structure belt in the south of Yanan Depression were geostatic pressure. The reservoir pressure factor in Ya21-1drilled respectively The reservoirs in the three structures structure is 2.2 and the overlying lithostatic pressure facare under deep overpressure and the drillings were failed. tor is 2.3, thus the cover has been fractured. The aboveThe Ya21-1 structure had a highest exploration degree n a21-1-1 well target in structure top, 8-m gas layer withthe three structures and similar to Ya13-1. the reservoir is normal pressure, occurs in Miocene Sanya FormationOligocene Lingshui LI sandstones and the caprocks are that the gasresult of the fluid in underlying deepOligocene lil,Mudstones and Miocene mud- overpressure reservoir migrating through the coverstones, and the source rock is underlying the Oligocene3) Unfavourable gas accumulation-early overpressYacheng Formation coal-bearing strata. The failure rea- sure, late charge. From the pressure study, the overpressons of Ya2l-1 exploration in comparison with Kela-2field are as followssure of Ledong Depression formed in 16.5 Ma.After1)Not enough gas charge. The Ya2 1-1(table 1, figstructure uplifting and pressure release for 10 Ma, theoverpressure returned again in 5. 5 Ma and the and to be4)structure lies to the south of Yanan hydrocarbon gen- high-pressure fluid began to flow and to be expelled fromeration depression and on Ya2l secondary structure beltThe drilling of four wells was failed. The targets of Ya2140005000中国煤化工Lingshui Gas dissolved SandsFormation in waferCNMHGFig. 4. Cross-section W-E of Y21-I structure. Modified after 1994 annual report of Nanhai west of CNOOC. 1, 2 and 3 are the codes of exploratedChinese S序嫂弻 etin Vo.47Supp. December2002south to north up to the present. The Ya2l-lstructure is a overpressure stratum must have a very good caprock tod vpass area"of high-pressure fluid always keeping a form a play. So the caprock's fracture pressure is a thresh-early period, the overpressure fluid entered the Oligocene pressure basi overpressure gas accumulates in an over-ep overpressure state. The other data prove that in the old as the dereservoir in the Ya21-l structure. The first, the homoge(1, Match of overpressure formation stage and gasneous temperature of reservoir fluid inclusion is appar- charge stage. The second key condition is a match ofently less than the drilling temperature, which shows that charge with overpressure. In the situation of early chargethe thermal fluid effect in structure was early. The second, and late pressure rise(including various factors causinthe dry sandstone porosity is 4%6% less than the high overpressure), if the caprock is good, an overpressure gapressure water-bearing sandstone at the same depth, accumulation can be preserved as Kela-2 field and northshowing that the high-pressure fluid has entered early the western Sichuan pool For the shallow overpressure tightpermeable sandstone. So the compaction was restricted. sandstone(mainly fissure plus pore)gas pool in JurassicThe generation and expelling of hydrocarbon was late, of north western Sichuan, the overpressure transferredmainly in 2 Ma, so the gas charge into overpressure reser- with gas from the underlying high-pressure source rock upvoir and accumulation here is difficult. So the hydrocar- to the overlying Jurassic tight sandstonebons from the depression center will pass through theFrom the physical opinion, geotherm and geopresoverpressure structures, enter the normal pressure reser- sure belong to energy, and the motion direction is fromvoir of Ya13-1 and accumulate, lastly, the great offshore high to low potential zone and not reverse. The fluids, oilfield was formedgas and water, are al ways migrating from a high to a lowpotential zone. Lastly, the hydrocarbon will accumulate ina suitable place. Therefore, for hydrocarbon exploration inGenerally, besides common play conditions(sourceoverpressure basins we should pay more attention to therock, reservoir,caprock, trap preservation, migration and lower pressure transition belt and the normal pressure beltaccumulation), the key play conditions of an overpressure (which may be rich in hydrocarbon)than to the overpresgas field are as follows(1 Caprock. Due to high activity, natural gas can Refwater and oil, or flowing freely. At the same time, the gas1. Zhang Qiming, Dong Weiliang, Overpressure system of hydrocarmolecular diffusion ability is high due to its small size andbon bearing basin in China, Acta Petroleli Sinica (in Chinese).low density. Moreover, the pressure and temperature in000,21(6):1crease with burial depthincreasing, which furtherenhancesLaw, B. E, Spencer, C. W, Abnormal pressure in hydrocarbonthe diffusion and migration ability, and all demand a highenviroments. APPG Memoir. 1998. 70: 1quality caprock. So long as the condition is suitable, the3. Heppard, P D, Cander, H. S, Eggertson, E. B, Abnormal pres-shallow uncemented water-bearing clay band in Quater-sure and the occrrence of hydrocarbons in offshore eastern trininary can seal a normal pressure reservoir of 10 m gas,dacl west indies, in Abnormal Pressures in Hydrocarbon Envi-ronments: AAPG Memoir (eds. Law, B. E, Uimishek, G. Fthe Sebei gas field sealed by Quaternary uncemented claySlavin,V.L),1998,70:215-246caprock is just a case history, its buried depth is 526 mlavin, V. L, Abnormally high formation pres-and reserves is about 1000_>10m, and another examplesures in petroleum regions of Russia and other countries of theis Ledong 22-l gas field sealed by Quaternary unce-C L.S., in Abnormal Pressures in Hydrocarbon Environmentsmented clay caprock, with reserves of 500.>10m gasAAPG Memoir (eds. Law, B. E, Uimishek, G. F. Slavin, V.L.)and burial depth of 392 m. Those show that the unce-1998,70:115-123mented clay layer is a good seal. That is because clay5. Hunt, J. M, Relation of shale porosities gas generation and com-permeability greatly drops after pore being filled withpaction to deep overpressures in the U.S. gulf coast, in Abnormalwater due to high clay plasticity. The overpressure aPressures in Hydrocarbon Environments: AAPG Memoir(edsaw, B E, Uimishek, G. F. Slavin V. L).1998. 70: 87-104.cumulation requires higher demand on the overlying 6. Zhao, J M Qins Hengfei, Analysis on the pool-forming of natu-caprock because of the increasing burial depth, tempera-ral gas accumulation in the Kuche Depression, Experimentalture and pressure. When an underl ying stratum pore fluidPetroleum Geology (in Chinese), 1999, 21(4): 307pressure reaches 70%0-90% of the overlying stratum 7. Lei Ganglin, Zhang Guowei, Liu Zhihong, The growth formationpressure, the hydraulic fracturing fault occurs, and thef Kuche foreland overthrust belt and its application in oil-gas ex-hydrocarbons together with other fluids will be expelled中国煤化工1,2220from the bed and migrate up. If an extending basin lacks8Qiming et al., Oil and Gas Disevaporite and the pressure factor is more than 2, the deepCNMHGn Chinese), Beijing: Geologicaloverpressure reservoir formation will be very difficultPublishing house. 2000. 184-196unless it is a small lens gas pool. Therefore, the deep(Received August 2, 2002)Chinese Science Bulletin Vol 47 Supp. December 2002