Three-Dimensional Modelling of a Multi-Layer Sandstone Reservoir： the Sebei Gas Field, China

Multi-layer sandstone reservoirs occur globally and are currently in international production. The 3D characteristics of these reservoirs are too complicated to be accurately delineated by general structural-facies-reservoir modelling. In view of the special geological features, such as the vertical architecture of sandstone and mudstone interbeds, the lateral stable sedimentation and the strong heterogeneity of reservoir poroperm and fluid distribution, we developed a new three-stage and six-phase procedure for 3D characterization of multi-layer sandstone reservoirs. The procedure comprises two-phase structural modelling, two-phase facies modelling and modelling of two types of reservoir properties. Using this procedure, we established models of the formation structure, sand body structure and microfacies, reservoir facies and properties including porosity, permeability and gas saturation and provided a 3D fine-scale, systematic characterization of the Sebei multi-layer sandstone gas field, China. This new procedure, validated by the Sebei gas field, can be applied to characterize similar multi-layer sandstone reservoirs.

Origin,migration,and accumulation of carbon dioxide in the East Changde Gas Field,Songliao Basin,northeastern China

CO2reservoirs are widely distributed within the Yingcheng Formation in the Songliao Basin, but the extreme horizontal heterogeneity of CO2content causes difficulties in the exploration and exploitation of methane. Former studies have fully covered the lithology, structure, and distribution of the reservoirs high in CO2content, but few are reported about migration and accumulation of CO2. Using the East Changde Gas Field as an example, we studied the accumulation mechanisms of CO2 gas. Two original types of accumulation model are proposed in this study. The fault-controlled accumulation model refers to gas accumulation in the reservoir body that is cut by a basement fault(the West Xu Fault), allowing the hydrocarbon gas generated in the lower formation to migrate into the reservoir body through the fault, which results in a relatively lower CO2content. The volcanic conduit-controlled accumulation model refers to a reservoir body that is not cut by the basement fault, which prevents the hydrocarbon gas from being mixed in and leads to higher CO2contents. This conclusion provides useful theories for prediction of CO2distribution in similar basins and reservoirs.

Control Measures of Soil Erosion in a Typical Construction Project Area ( Dina 2 Gas Field)

To treat the relationship between project construction and ecological environment and effectively prevent new soil erosion during the construction, according to the project layout, soil erosion distribution as well as natural and socio-economic conditions, the control measures of newly increased soil erosion along the project were carried out based on site survey and analysis of relevant information. In addition, adhering to the prin- ciple of partition prevention and treatment, some guiding prevention and control measures of the natural zone passed by the project and soil erosion control district were determined, which provided scientific references and technical support for the rational layout of water and soil conservation and ecological restoration measures in Dina 2 gas field.

Breakdown Electric Field of Hot 30% CF_3I/CO_2 Mixtures at Temperature of 300–3500 K During Arc Extinction Process

We calculated the uniform dielectric breakdown field strength of residual 30% CF3I/CO2 gas mixtures during the arc extinction process over the temperature range 300-3500 K at 0.1 MPa. The limiting reduced field strengths are decided by a balance of electron generation and loss based on chemical reactions estimated by the electron energy distribution function （EEDF）, which employs the Boltzmann equation method with two-term expanding approximation in the steady-state Townsend （SST） condition. During the insulation recovery phase, the hot CF3I/CO2 gas mixtures have maximum dielectric strength at a temperature of about 1500 K. At room temperature 300 K, the electric strength after arc extinction （90.3 Td, 1 Td=10-21 V.m2） is only 38% of the original value before arc （234.9 Td）. The adverse insulation recovery ability of CF3I/CO2 gas mixtures in arc extinction hinders its application in electric circuit breakers and other switchgears as an arc quenching and insulating medium.

Research on 1:2 subharmonic resonance and bifurcation of nonlinear rotor-seal system

The 1：2 subharmonic resonance of the labyrinth seals-rotor system is inves- tigated, where the low-frequency vibration of steam turbines can be caused by the gas exciting force. The empirical parameters of gas exciting force of the Muszynska model are obtained by using the results of computational fluid dynamics （CFD）. Based on the multiple scale method, the 1：2 subharmonic resonance response of the dynamic system is gained by truncating the system with three orders. The transition sets and the local bifurcations diagrams of the dynamics system are presented by employing the singular theory analysis. Meanwhile, the existence conditions of subharmonic resonance non-zero solutions of the dynamic system are obtained, which provides a new theoretical basis in recognizing and protecting the rotor from the subharmonic resonant failure in the turbine machinery.

Origin and migration model of natural gas in L gas field, eastern slope of Yinggehai Sag, China

Based on the chemical and stable carbon isotopic composition of natural gas and light hydrocarbons, along with regional geological data, the genetic type, origin and migration of natural gases in the L lithologic gas field, the eastern slope of Yinggehai Sag were investigated. The results show that these gases have a considerable variation in chemical composition, with 33.6%–91.5% hydrocarbon, 0.5%-62.2% CO2, and dryness coefficients ranging from 0.94 to 0.99. The alkane gases are characterized by δ13C1 values of -40.71‰--27.40‰,δ13C2 values of –27.27‰– –20.26‰, and the isoparaffin contents accounting for 55%–73% of the total C5–C7 light hydrocarbons. These data indicate that the natural gases belong to the coal-type gas and are mainly derived from the Miocene terrigenous organic-rich source rocks. When the CO2 contents are greater than 10%, the δ13CCO2 values are –9.04‰ to – 0.95‰ and the associated helium has a 3He/4He value of 7.78×10^–8, suggesting that the CO2 here is crustal origin and inorganic and mainly sourced from the thermal decomposition of calcareous mudstone and carbonate in deep strata. The gas migrated in three ways, i.e., migration of gas from the Miocene source rock to the reservoirs nearby;vertical migration of highly mature gas from deeper Meishan and Sanya Formations source rock through concealed faults;and lateral migration along permeable sandbodies. The relatively large pressure difference between the “source” and “reservoir” is the key driving force for the vertical and lateral migration of gas. Short-distance migration and effective “source - reservoir” match control the gas distribution.

Ground State Thermodynamic and Response Properties of Electron Gas in a Strong Magnetic and Electric Field: Exact Analytical Solutions for a Conventional Semiconductor and for Graphene

Consequences of an exceedingly strong electric field (E field) on the ground state energetics and transport properties of a 2D spinless electron gas in a perpendicular magnetic field (a Quantum Hall Effect (QHE) configuration) are investigated to all orders in the fields. For a conventional semiconductor, we find fractional values of the Hall conductivity and some magnetoelectric coefficients for certain values of E and B fields that do not result from interactions or impurities, but are a pure consequence of a strong enough in-plane E field. We also determine analytically the ground state energy, and response properties such as magnetization and polarization as functions of the electromagnetic field in the strong E field limit. In the case of Graphene, we obtain more complex behaviors leading to the possibility of irrational Hall values. The results are also qualitatively discussed in connection to various mechanisms for the QHE-breakdown.

Reservoir forming conditions and key exploration technologies of Lingshui 17-2 giant gas field in deepwater area of Qiongdongnan Basin

On September 15,2014,China National Offshore Oil Co.,Ltd announced that a high production of oil and gas flow of 1.6
106 m3/d was obtained in Well LS17-2-1 in deepwater area in northern South China Sea,which is the first great oil and gas discovery for self-run deepwater exploration in China sea areas,and a strategic breakthrough was made in natural gas exploration in deepwater area of Lingshui sag in Qiongdongnan Basin.Under the combined action of climax of international deepwater exploration,high oil prices,national demands of China,practical needs of exploration,breakthroughs in seismic exploration and testing technologies,innovations in geological cognition and breakthroughs in deepwater operation equipment,Lingshui 17-2 gas field is discovered.Among these factors,the innovation in reservoir forming geological cognition directly promotes the discovery.The quality of seismic data in the early time is poor,so key reservoir forming conditions such as effective source rocks,high quality reservoirs and oil-gas migration pathways are unable to be ascertained;with support of new seismic acquisition and processing technology,some researches show that Lingshui sag is a successive large and deep sag with an area of 5000 km2 and the maximum thickness of Cenozoic stratum of 13 km.In the Early Oligocene,the Lingshui sag was a semi-closed delta-estuarine environment,where the coalmeasure and marine mudstones in Lower Oligocene Yacheng Formation were developed.The Lingshui sag is a sag with high temperature,and the bottom temperature of source rocks in Yacheng Formation can exceed 250C,but the simulation experiment of hydrocarbon generation at high temperature indicates that the main part of this set of source rock is still in the gas-generation window,with resources of nearly 1 trillion cubic meters,so the Lingshui sag is a hydrocarbon-rich generation sag.In the Neogene,the axial canyon channel from the Thu Bon River in Vietnam passed through the Lingshui sag,and five stages of secondary channels were developed in the axial canyon channel,where four types of reservoirs with excellent physical properties including the axial sand,lateral accretion sand,natural levee sand as well as erosion residual sand were developed,and lithologic traps or structural-lithologic traps were formed.The diapiric zone in the southern Lingshui sag connects deep source rocks in Yacheng Formation and shallow sandstones in the channels,and the migration pattern of natural gas is a T-type migration pattern,in other words,the natural gas generated from Yacheng Formation migrates vertically to the interior of the channel sandbody,and then migrates laterally in the channel reservoirs and forms the reservoirs.Innovations of geophysical exploration technologies for complicated geological conditions of deepwater areas are made,such as the detuning comprehensive quantitative amplitude hydrocarbon detection technology,which greatly improves the success rate of deepwater exploration;key technologies of deepwater safety exploratory well testing represented by the platform-dragged riser displacement technology are developed,which greatly reduces the drilling test cost.The above key exploration technologies provide a strong guarantee for the efficient exploration and development of Lingshui gas field.

Breakdown voltage analysis of Al_(0.25)Ga_(0.75)N/GaN high electron mobility transistors with partial silicon doping in the AlGaN layer

In this paper,two-dimensional electron gas（2DEG） regions in AlGaN/GaN high electron mobility transistors（HEMTs） are realized by doping partial silicon into the AlGaN layer for the first time.A new electric field peak is introduced along the interface between the AlGaN and GaN buffer by the electric field modulation effect due to partial silicon positive charge.The high electric field near the gate for the complete silicon doping structure is effectively decreased,which makes the surface electric field uniform.The high electric field peak near the drain results from the potential difference between the surface and the depletion regions.Simulated breakdown curves that are the same as the test results are obtained for the first time by introducing an acceptor-like trap into the N-type GaN buffer.The proposed structure with partial silicon doping is better than the structure with complete silicon doping and conventional structures with the electric field plate near the drain.The breakdown voltage is improved from 296 V for the conventional structure to 400 V for the proposed one resulting from the uniform surface electric field.

Origin of abnormal high pressure and its relationship with hydrocarbon accumulation in the Dina 2 Gas Field, Kuqa Depression

Based on distribution of formation pressure by indirect estimation and formation testing,this study investigates origin of abnormal high pressure in the Dina 2 Gas Field in the Kuqa Depression in combination with the latest research findings.Contribution of major overpressure mechanisms to this gas field is estimated,and generation of the abnormal high pressure as well as its relationship with natural gas accumulation is explored.Disequilibrium compaction,tectonic stress,and overpressure transfer are the major overpressure mechanisms.Overpressure transfer resulted from vertical opening of faults and folding is the most important cause for the overpressure.Gas accumulation and abnormal high pressure generation in the reservoirs of the Dina 2 Gas Field show synchroneity.During the early oil-gas charge in the Kangcun stage,the reservoirs were generally normal pressure systems.In the Kuqa deposition stage,rapid deposition caused disequilibrium compaction and led to generation of excess pressure(approximately 5-10 MPa)in the reservoirs.During the Kuqa Formation denudation stage to the Quaternary,reservoir overpressure was greatly increased to approximately 40-50 MPa as a result of vertical pressure transfer by episodic fault activation,lateral overpressure transfer by folding and horizontal tectonic stress due to intense tectonic compression.The last stage was the major period of ultra-high pressure generation and gas accumulation in the Dina 2 Gas Field.

Accumulation conditions and key technologies for exploration and development in Sebei gas field in Qaidam Basin,NW China

The Sebei gas field,located in the northern slope of the Sanhu depression in the eastern Qaidam Basin,is the largest inland biogas producing area in China.It is a typical Quaternary biogenetic gas reservoir composed of multiple layers of unconsolidated sandstone.Through systematic study of gas generating mechanism of source rock,reservoir-caprock combination and trap evolution,the unique biogas accumulation model characterized by“continuous hydrocarbon generation and dynamic accumulation”is established.Its uniqueness is reflected in the biogas generation mechanism under dual effect of lowtemperature thermodynamics and microbial degradation,source-reservoir one body,self generation and self storage,dynamic source-reservoir-caprock assemblages and anticline trap formation mechanism of multi-factor mutual promotion.This understanding reveals the intrinsic factors for formation of largescale biogas field under frigid climate,high salinity and low organic matter abundance.A number of exploration and development technologies have been worked out during the exploration and development of the Sebei gas field.Among them,the low-amplitude structure identification technique,multiparameter gas-bearing detection technique,sand control and comprehensive water control technique for unconsolidated sandstone are well applied in discovering the gas field,increasing reserve,and increasing and maintaining gas production.The above understandings and key technologies have great reference significance and demonstration effect for biogas exploration and development in China and the world.

Effect of Plastic Deformation and H_2S on Dynamic Fracture Toughness of High Strength Casing Steel

The effects of plastic deformation and H2 S on fracture toughness of high strength casing steel（C110 steel） were investigated. The studied casing specimens are as follows： original casing, plastic deformation（PD） casing and PD casing after being immersed in NACE A solution saturated with H2S（PD＋H2S）. Instrumented impact method was employed to evaluate the impact behaviors of the specimens, meanwhile, dynamic fracture toughness（JId） was calculated by using Rice model and Schindler model. The experimental results show that dynamic fracture toughness of the casing decreases after plastic deformation. Compared with that of the original casing and PD casing, the dynamic fracture toughness decreases further when the PD casing immersed in H2 S, moreover, there are ridge-shaped feature and many secondary cracks present on the fracture surface of the specimens. Impact fracture mechanism of the casing is proposed as follows： the plastic deformation results in the increase of defect density of materials where the atomic hydrogen can accumulate in reversible or irreversible traps and even recombine to form molecular hydrogen, subsequently, the casing material toughness decreases greatly.

柴达木盆地涩北地区第四系泥岩型生物气储层孔隙有效性评价

通过低场核磁共振实验分析了柴达木盆地涩北地区第四系泥岩型生物气储层在饱和水状态及渐变烘干温度状态下的T_(2)谱,明确了孔隙流体的核磁响应特征,以评价孔隙的有效性。研究结果表明:(1)核磁共振实验是以饱和水状态T_(2)谱为基础,采用正态分布函数拟合构建了离心束缚水T_(2)谱,确定了可动流体和毛管束缚流体T_(2)截止值,用于划分出流体类型并开展了孔隙有效性评价。(2)研究区岩样饱和水状态T_(2)谱谱峰呈左小右大的形态,右峰幅度值远大于左峰,占T_(2)谱幅度值90%以上;随着烘干温度的升高,T_(2)谱幅度减小且左移趋势明显;束缚水T_(2)谱形态近似于正态分布,起始位置与饱和水状态的T_(2)谱基本重合。(3)研究区可动流体T_(2)截止值T_(2C1)平均为3.3 ms,毛管束缚流体T_(2)截止值T_(2C2)平均为1.8 ms;孔隙流体包括可动水、毛管束缚水和黏土束缚水,黏土束缚水T_(2)小于T_(2C2),毛管束缚水T_(2)大于T_(2C2)且小于T_(2C1),可动流体T_(2)大于T_(2C1);毛管束缚水含量最高,黏土束缚水其次,两者占总孔隙流体的84.43%~95.06%,可动水含量低。(4)研究区储层有效孔隙占总孔隙的54.99%,主要为毛管束缚孔,黏土束缚孔为无效孔隙;黏土含量越高,有效孔隙度越小。

Gas accumulation mechanism in Denglouku Formation of Changling fault depression, southern Songliao Basin, China

The Changling gas field is occurs in tight sandstone reservoirs of the Lower Cretaceous Denglouku Formation in the Changling fault depression,southern Songliao Basin,China,which constitutes a new gas-producing area in the depression. Using information on the source-reservoir-cap rock assemblage of the Denglouku Formation,fault activity,and single well burial history of well CS1,together with data on reservoir fluid inclusion and laser Raman spectroscopy,we described the formation of the Changling gas field and determine that this fault depression did not possess suitable conditions for hydrocarbon generation. Coal-derived methane generated from underlying hydrocarbon source rock accumulated in the Lower Cretaceous Yingcheng Formation. At the end of the Late Cretaceous Qingshankou Stage,underwater volcanic eruptions occurred in the northern part of the Changling gas field near Qian'an,resulting in the reactivation of deep faults. Mantle-sourced inorganic CO2 migrated along faults to hydrocarbon gas reservoirs in volcanic rocks of the Yingcheng Formation; Meanwhile,displaced methane( hydrocarbon gas) migrated upward to sand reservoirs of the Denglouku Formation.The methane accumulated and formed secondary gas reservoirs,Therefore fault activity was the main factor controlling the generation of gas reservoirs in the Denglouku Formation. The main accumulation period of the Yingcheng hydrocarbon gas reservoirs was 82 Ma. Whereas gas reservoir formation in the overlying Denglongku Formation was 79 Ma,slightly later than the time of formation of the Yingcheng gas reservoir in CS1 well area.At 79 Ma,the burial depth of the Denglouku Formation was 1 800--2 000 m,the diagenesis is relatively weak and the physical properties of the reservoir are relatively favorable for accumulation. This period is not only at gas generation peak time of three sets of source rock but also at the reactivation of deep faults during the formation of fault-bound depressions,thereby providing favorable conditions for the migration and accumulation of methane.

Geochemical characteristics and formation process of natural gas in Kela 2 gas field

On the basis of a large amount of natural gascomponents and the carbon isotope as well as some otheranalysis data in Kela 2 gas field, the geochemical character-istics, source, origin, and formation process of natural gashave been discussed. The components of gas in the field tendto be 'dry', and the drying coefficient is close to 1.0. Thecarbon isotope tends to be heavier, for instance, the averageof δ13C1 is -27.36‰ and that ofδ13C2 is -18.5‰. Compre-hensive analysis shows that humic natural gas in the Kuqapetroleum system comes mainly from Triassic and Jurassicsource rocks, and the contribution of Jurassic source rocks tothe pool maybe is more than that of Triassic rocks. The maincause that the gas tends to be dry and bears heavier isotopecomposition lies in the fact that Kela 2 natural gas is the ac-cumulation of late production of humic source rocks, and itis affected by the abnormal high pressure as well. Consider-ing the hydrocarbon generating and structural history, wecan regard the gas pool formation processes as twice fillingand twice adjusting （destroying）, that is, the filling anddestroying process in the early Himalayan movement and thefilling and adjusting in the late Himalayan movement.

Migration and accumulation of natural gas in Kela-2 gas field

With the guidance of petroleum system theory,the dynamic filling history of natural gas in the Kela-2 gasfield is analyzed by using a large suite of oil and gas geo-chemistry evidence in combination with the tectonic evolu-tion history and reservoir evolution history. It concludes thatthe Kela-2 gas field was formed by capturing the gas gener-ated during the main gas generation period, while the latekerogen cracking gas contributed a little to the gas field. Itsuggests that the gas generated during the main gas genera-tion accumulated in the early-formed wide-gentle anticline,which is the necessary condition for natural gas to re-migrateand enrich late to form the large-scale gas reservoir. Thenewest research shows that the filling history of gas in theDabei-1, Yinan-2, Tuziluoke and Dina-2 gas fields was re-lated with the natural gas accumulation in the early wide-gentle anticline as well as late re-migration and enrichmentof natural gas.

Thermodynamic conditions of framework grain dissolution of clastic rocks and its application in Kela 2 gas field

Feldspar and clastic debris are the most important constituent framework grains of sedimentary clastic rocks and their chemical dissolution plays an essential role in the formation and evolution of the secondary pore in the reservoir rocks. On the basis of thermodynamic phase equilibrium, this study investigates the chemical equilibrium relationships between fluid and various plagioclase and K-feldspar in diagenesis of the sediments, particularly, the impact of temperature and fluid compositions (pH, activity of K+, Na+, Ca2+ and so on) on precipitation and dissolution equilibria of feldspars. Feldspar is extremely easily dissolved in the acid pore water with a low salinity when temperature decreases. The dissolution of anorthite end-member of plagioclase is related to the Ca content of the mineral and the fluid, higher Ca either in the mineral or in the fluid, easier dissolution of the feldspar. Moreover, the dissolution of albite end-member of plagioclase is related to Na of both the mineral and fluid, lower Na out of both the mineral and fluid, easier dissolution of the mineral. Similarly, lower-K fluid is more powerful to dissolve K-feldspar than the higher. The anorthite component of plagioclase is most easily dissolved in ground water-rock system, the albite is the secondary, and K-feldspar is the most stable. Selective dissolution of plagioclase occurs in diagenesis because of the plagioclase solid solution that is mainly composed of albite and anorthite end-members, Ca-rich part of which is preferentially dissolved by the pore water, in contrast to the Na-rich parts. Based on investigation of reservoir quality, secondary pore, dissolution structures of feldspar, clay minerals and ground water chemistry of the Kela 2 gas field of Kuqa Depression in the Tarim basin of Western China, we discovered that the secondary pore is very well developed in the highest quality section of the reservoir, and the plagioclase of which was obviously selectively dissolved, in contrast to the overgrowth of K-feldspar. Chemistry of the ground water out of the highest quality section of the reservoir is consistent with the complete dissolution field of anorthite, partial dissolution field of albite component of plagioclase, and the precipitation of K-feldspar on the temperature-logK phase equilibrium diagram drawn from this study. The thermodynamic modeling calculation gave us an insight into formation of the secondary pore within the reservoir, and the dissolution of plagioclase an important cause of the secondary pore. It is assumed that the solvent to dissolve the plagioclase frame grains originated from the origin-enriched compacted fluid acids out of the mudstone buried beneath the top of geopressured zones in the Kuqa Depression.

Geological analysis and physical modeling of structural pumping in high effective formation of Kela 2 gas field

In a 3-D closed geological body, in case “structural expanding” inside is induced by stress, it can produce the pressure difference between the expanding cell and surrounding rock, then generate a pumping force directed toward the cell and accelerate the directional flow of fluid in the strata. The structural style and conditions of gas reservoir-formation in the Kuqa depression are favorable to the structural pumping. According to similarity principle, a physical modeling of structure formation and gas filling process of the Kela 2 gas field has justified the occurrence of structural pumping and its important role in gas-reservoir formation with high efficiency under the compressive and well-sealed circumstance. Therefore, authors propose that structural pumping is an important mechanism of gas reservoir-formation with high efficiency in the Kuqa depression.

Control factors and porosity evolution of high-quality sandstone reservoirs of Kela-2 gas field in Kuqa Depression

By using the integrated methods includingsandbodies modelling of the outcrops, sedimentary facies ofthe cores and well logs of the drilled wells, and the reservoircorrelation of interwells, it is thought that the sandstone res-ervoirs of Kela-2 gas field are a suit of high-quality naturalgas reservoirs with great thickness, extensive and continuousdistribution, high porosity and permeability, and a few bar-rier beds. Sedimentary facies and microfacies are the mainfactors controlling the reservoir distribution and interiorheterogeneity. Based on a great deal of data of rock’s thinsections, porosity, permeability, and the parameters of capil-lary pressure, the reservoir diagenesis, controls, mechanismand evolution of pores have been studied. It is consideredthat compaction in the early stage and diagenetic dolomiteand calcite cements have effect on the decline of reservoirsproperty. Now compaction is chiefly middle to weak. Thebetter reservoirs have no or a few calcite cements. In theearly of deep buried stage, there are still mainly remainderprimary intergranular pores. The authigenic kaolinite ofreservoirs is the production of the dissolution of feldsparsand lithic fragments. The dissolution results in the partlymodified and broadened secondary intergranular pores. Inthe late of deep buried stage, structure fissures and over-pressure were in favor of improving and preservation ofthese pores.

Petroleum geological characteristics of Kela-2 gas field

The Kela-2 gas field is located in the center ofKelasu structural belt in Kuqa Depression. This trap is oneof a series of traps in the folded belts which are distributed ina string of pearls in the dual structure. The primary gas-bearing layers are sandstone of Lower Cretaceous K1bswhile the secondary layers are dolomite member and gluten-ite member of Lower Tertiary E1-2km and sandstone ofLower Cretaceous K1b. The main component of natural gasis methane whose content is higher than 97%. It is charac-terized by dry gas whose source rock is Jurassic coal meas-ure. The Kela-2 structural trap was formed during the Xiyuperiod and then became a reservoir in the late time. The res-ervoir formed late and the thick seal rock of Lower Tertiarygipsmantle are the avail reason why the giant Kela-2 gasfield has been well kept. The abnormal high pressure of theKela-2 gas field results from the strong structural compres-sion in the northern part during the Xiyu period.