23 February 2014, Volume 41 Issue 1

    

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油气勘探
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  Geological and geochemical characteristics of large gas fields in China

  Dai Jinxing; Yu Cong; Huang Shipeng; Gong Deyu; Wu Wei; Fang Chenchen and Liu Dan

  , 2014, 41(1): 1-13.

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  Based on a comprehensive analysis, the geological and geochemical characteristics of large gas fields in China were summarized in this paper. By the end of 2011, altogether 48 large gas fields had been discovered in China, with their proven reserves accounting for 81.5% of the total proven gas reserves in China. The main geological and geochemical characteristics of large gas fields in China are as follows: (1) Alkane gases are the main components of natural gas, with an average methane content of 88.22%, and the average contents of ethane, propane and butane of 3.31%, 0.97% and 0.49%, respectively; (2) The natural gases in 27 of the 48 large gas fields are coal-derived gases which indicates coal-derived gas is the major genetic type of natural gas in China; (3) Reservoirs are in multiple strata, but natural gas is mostly produced from the Permian and Triassic; Sandstones, carbonates and volcanics are the main reservoirs, of which the sandstones have the most reserves; (4) There are 16 tight gas fields in the 48 large gas fields, suggesting the crucial importance of this type of reservoirs in China; (5) Most of these large gas fields were formed at late or very late stages, suggesting late reservoir formation is favorable for gas preservation; (6) Multiple reservoir types are identified, including structural, lithologic and stratigraphic reservoirs.
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  Conventional and unconventional petroleum “orderly accumulation”: Concept and practical significance

  Zou Caineng; Yang Zhi; Zhang Guosheng; Hou Lianhua; Zhu Rukai; Tao Shizhen; Yuan Xuanjun; Dong Dazhong; Wang Yuman; Guo Qiulin; Wang Lan; Bi Haibin; Li Denghua and Wu Na

  , 2014, 41(1): 14-27.

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  Based on the latest global conventional-unconventional petroleum development situation and the conclusion of petroleum geology theory and technology innovation in recent 10 years, the connotation of conventional and unconventional petroleum “orderly accumulation” is formulated. This concept indicates that, unconventional petroleum occurs in the hydrocarbon supply direction of conventional petroleum, and conventional petroleum may appear in the outer space of unconventional petroleum. Proper evaluation methods and engineering technology are important to push the conventional-unconventional petroleum co-development, and the petroleum finding thought from outer-source into inner-source. Unconventional petroleum evaluation focuses on source rocks characteristics, lithology, physical property, brittleness, oil-gas possibility and stress anisotropy. Taking shale gas for examples, in China, these six properties are TOC>2%, laminated silicious calcareous shale or calcareous silicious shale, porosity 3%-8%, brittle minerals content 50%-80%, gas content 2.3-4.1 m3/t, pressure coefficient 1.0-2.3, natural fractures; in north America, these six properties are TOC>4%, silicious shale or calcareous shale or marl, porosity 4%-9%, brittle minerals content 40%-70%, gas content 2.8-9.9 m3/t, pressure coefficient 1.30-1.85, natural fractures. “Sweet spot area” assessment, “factory-like” operation pattern and other core methods and technologies are discussed. And 8 key elements of unconventional “sweet spot area” are proposed, 3 of them are TOC>2% (for shale oil S1>2 mg/g), higher porosity (for tight oil&gas >10%, shale oil&gas >3%), and microfractures. Multiple wells “factory-like” operation pattern is elaborated, and its implementation needs 4 elements, i.e. batch well spacing, standard design, flow process, and reutilization. Through horizontal well volume fractures in directions, “man-made reservoirs” with large-scale fracture systems can be formed underground.
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  Formation and enrichment mode of Jiaoshiba shale gas field, Sichuan Basin

  Guo Tonglou and Zhang Hanrong

  , 2014, 41(1): 28-36.

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  The Silurian Longmaxi shale gas play in the Jiaoshiba structure in the southeast margin of the Sichuan Basin is studied to discuss the key control factors of shale gas enrichment in complex tectonic and high evolution zone. The Jiaoshiba structure is a faulted anticline which experienced multiphase tectonic movement. The Longmaxi Formation has high thermal evolution degree with Ro more than 2.2%, and has a 35-45 meters thick high-quality shale (TOC > 2%) in its lower part. The reservoir is overpressure with a pressure coefficient of 1.55, and the shale gas production and pressure are stable. Structure type, evolution and geochemical analysis show that there are several stages of hydrocarbon generation, migration and accumulation in the Longmaxi Formation. The joint action of two groups (two stages) of fault systems and the detachment surface at the bottom of the Longmaxi Formation controls the development of reticular cracks and overpressure preservation, and it is the key to the shale gas accumulation and high yield. The closed box system in the Longmaxi Formation ensures the gas reservoir dynamic balance. The model of high yield and enrichment of the Jiaoshiba shale gas play is “ladder migration, anticline accumulation, fault–slip plane controlling fractures, and box shape reservoir”. Like in conventional gas plays, good preservation and tectonic conditions are also required to form high yield shale plays in areas which have complex structures, experienced multi-stage tectonic movement, and have high evolution shale.
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  Re-discussion on the reservoir formation by pressure-suck filling

  Zhang Shanwen

  , 2014, 41(1): 37-44.

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  The dominant water consumption reaction in sandstones was figured out by determining variables in the mineral alteration in sandstone reservoirs, on this basis, the conditions of major water consumption reaction and its correspondence with oil-generating window were analyzed to reveal the petroleum geological significance of water consumption reaction. The kaolinitization of feldspar plays a dominant role in the water consumption reaction during diagenetic stage, which corresponds to the hydrocarbon-generating window, and the acid hydrocarbon-bearing fluid produced by source rocks facilitates the kaolinitization reaction. Water consumption causes the pressure drop and porosity increase in the reservoir formation in the compartment sandstone, at the same time, huge pressure difference between source rocks and reservoirs comes about because strong pressure built up in the source rocks owing to hydrocarbon generation, and the oil and gas are pushed and sucked into the reservoir sands. The universality of the kaolinitization water consumption suggests this “push-suck” mode is very important for reservoir formation. This mode of the Jiyang depression can be used to explain the origin of similar reservoirs across the world, and guide exploration for oil and gas.
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  Evolution modes of interbedded weathering crust karst: A case study of the 1st and 2nd members of Ordovician Yingshan Formation in EPCC block, Tazhong, Tarim Basin

  Feng Renwei; Ouyang Cheng; Pang Yanjun; Li Zhuzheng; Luo Xuan; Zhang Qin; Li Shiyin; Zhou Yu and Cheng Liang

  , 2014, 41(1): 45-54.

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  The evolution of interbedded weathering crust karst on the top of the Ordovician Yingshan Formation were described based on the geological background, oil and gas show while drilling, core, log imaging and seismic reflection features in the Tazhong area. The study shows that: Filling features of dissolved caves near unconformities depend on the hydrodynamic strength, which, together with the vertical lithofacies, determines the development degree of the interbedded karst. Karst units are classified into several first order units, namely highland, sub-high, slope, and sub-sag, and some secondary units such as dissolved peak, cliff, platform, and dissolved mound, stretching like a strip from northwest to southeast on the whole. The surface weathering zone developed only in the north of the EPCC block, the vertical vadose zone has different thickness in south and north, while the seasonal variation zone changes to horizontal vadose zone from south to north. Sedimentary microfacies affects karst development, but it is not the deciding factor of karst cycle classification, the main factors resulting in the karst difference between south and north are structural uplift, climate, and ancient hydrological conditions. Three cycles and eight secondary cycles are identified in the interbedded weathering karst in Member Ying1-Ying2 in the EPCC block, among them, the third cycle, including the fifth to the eighth secondary cycles, remains, and the fifth and eighth secondary cycles show obvious “different facies” features from south to north.
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  Application of seismic sedimentology to prediction of beach and bar sandbodies in gentle slope of lacustrine basin: A case study of the Lower Cretaceous in Chepaizi area, Junggar Basin, NW China

  Zhao Dongna; Zhu Xiaomin; Dong Yanlei; Wu Dong and Zhu Mao

  , 2014, 41(1): 55-61.

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  The exploration of beach and bar sandstone reservoirs is a tricky issue. In this study, under the guidance of seismic sedimentary theory, the 90° phase shift technique, stratal slicing, and quantitative seismic sedimentology were used to identify and characterize the distribution of beach and bar sandbodies and reflect the evolutionary history in the Chepaizi area, Junggar Basin. Four third-order sequences in the Lower-Cretaceous and relevant systems tracts in the study area were studied using seismic sedimentology, and with the assistance of the comprehensive analysis of core, lithology and well-logging data, stratal slices were interpreted and calibrated, and the distribution and evolution of beach bar sandbodies were predicted. Based on the detailed and extensive analyses on sedimentary characteristics and seismic sedimentary quantitative results, this study established two typical beach and bar sedimentary models in the Chepaizi area, namely normal bedrock-beach bar and fan delta-beach bar. In early sedimentary period of SQⅠ, beach and bar sandbodies were reformed by coastal, wave and onshore currents, and developed around the lateral margin of the fan delta front. During sedimentary periods of SQⅡ to SQ Ⅳ, the northern provenance stopped supplying sediments, and the wave and coastal currents eroded the bedrock of the western Chepaizi uplift and transported sediments to the study area, forming beach and bar sandbodies parallel to or obliquely intersecting with coastal line.
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  Basement geology along Southwest Indian Margin

  Gireesh R and Pandey D K

  , 2014, 41(1): 62-66.

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  Results from detailed analyses of multi-channel seismic reflection data in association with gravity anomalies and lithology data from drilled holes are used to find out the basement structure features of the Laccadive Ridge and Laccadive basin in southwest India. Morphology and seismic interpretation results show the basement in the study area is faulted block horst/graben pattern, and there are basement highs parallel to the coastal line (SN), while there is no sediments on the margin highs, thick sediments deposited in areas around the margin highs, with local thickness up to 2-3 s (TWT), and these sedimentary layers usually pinch out suddenly at the lower wall of steep normal faults. There deposited a thin layer of sediments on Laccadive Ridge which has several ridge peaks (distributed along ridge extension direction) that outcrop seafloor sometimes. The basement ridge peaks are usually several thousand meters wide, and show obvious high abnormal gravity. The basement of ocean ridge is highly faulted, and the center of the ocean ridge has evolved into lows like rift.
油气田开发
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  Experiments on water flooding in fractured-vuggy cells in fractured-vuggy reservoirs

  Wang Jing; Liu Huiqing; Ning Zhengfu; Zhang Hongling and Hong Cheng

  , 2014, 41(1): 67-73.

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  Based on the theory of scaling criteria, a physical experimental model of fractured-vuggy cell which meets the geometric similarity, kinematic similarity, dynamic similarity and characteristic parameters similarity was designed. The factors affecting fluid flow and water flooding characteristics, and the EOR features of different displacing modes were studied by experiments. The experimental results show that: Oil and water displacement effect caused by the density difference between oil and water is the main mechanism of water flooding; Injection rate has no effect on the remaining oil and water-cut; Fracture-vuggy connection is the dominant factor controlling water-free recovery, ultimate limit of remaining oil and water-cut variation in the fractured-vuggy cell; Oil-water viscosity ratio has an important effect on oil production rate, oil replacement ratio and economic limit remaining oil; When the filling degree is low, it has no effect on remaining oil, but when the filling degree is high, the remaining oil contains both “attic oil” and the oil existing in porous media; Foam flooding after water flooding enhances oil recovery by starting the attic oil at the top of vugs, and the remaining oil is in the center of the vugs after foam flooding; Polymer flooding after water flooding enhances the economic limit recovery by increasing the oil production rate and oil draining ratio, but it has no effect on the limit remaining oil of water flooding.
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  Transient pressure analysis of multiple-fractured horizontal wells in boxed reservoirs

  Wang Xiaodong; Luo Wanjing; Hou Xiaochun and Wang Junlei

  , 2014, 41(1): 74-78.

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  A new analytical solution of fluid unsteady flow was presented for wells with finite-conductivity vertical fractures by determining impact functions of fracture conductivity. Based on the new solution, the transient pressure distribution of a fractured horizontal well with multiple finite-conductivity vertical fractures was obtained using the principle of superposition. Calculation results show the pressure transient behavior of fractured horizontal wells under the condition of constant bottom rate. In the ideal fracture arrangement, flow patterns occur in the order of early bilinear flow → early linear flow → middle radial flow → middle linear flow → late pseudo steady state. Uneven length of fractures with equal conductivity and uniform arrangement may affect the transform of early linear flows from earlier bilinear flows and the premature birth of middle radial flows. Nonuniform arrangement of fractures with even length and equal conductivity may influence middle radial flows. Different conductivity of fractures with equal length and uniform arrangement shortens the period of bilinear flows and lengthens the linear flows. Increase in the number of fractures with even length, equal conductivity and uniform arrangement accelerates pressure drops in the reservoir, shortens the period of middle radial flows and lengthens the middle linear flows. Increase in length of all fractures shortens the period of middle radial flows and may result in absence of radial flows. It is demonstrated that the horizontal well with multiple vertical fractures may improve fluid flow patterns.
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  Performance and gas breakthrough during CO2 immiscible flooding in ultra-low permeability reservoirs

  Gao Yuncong; Zhao Mifu; Wang Jianbo and Zong Chang

  , 2014, 41(1): 79-85.

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  The performance data, water/gas injection profile monitoring, tracer monitoring, and composition of crude oil and formation water are analyzed in the CO2 immiscible flood pilot area of Yaoyingtai Oilfield, Jilin Province, to examine the well production characteristics and gas breakthrough law of CO2 immiscible flooding. In the pilot, the characteristics of CO2 extraction, viscosity reducing, oil volume distension and effective formation opening increment are combined, reflecting obvious effects of recovery enhancement. For the ultra-low permeability and strong heterogeneity reservoir, monolayer breakthrough and plane fingering may take place in either water flooding or gas drive. The alterative water/gas injection can increase the sweep volume effectively and help to mitigate gas breakthrough. The increment of oil production is less when the injection shifts from water to gas than when the injection shifts from gas to water. So, the adjustment of water injection half-cycle plays a dominant role in improving gas drive effect. In the Yaoyingtai Oilfield, gas show is mainly observed along fractures, along the high permeability bands, and in low-permeability zones. According to the established standard of gas breakthrough in oil well, the key to improving the immiscible flooding effect is to keep the oil well to see gas but avoid gas channeling and also to slow down the gas/oil ratio ascending. Once gas breakthrough occurs in a well, it is necessary and effective to shut down the well, adjust the bottom hole flowing pressure in production well and implement the alternative water/gas injection.
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  Influences of sandstone superimposed structure and architecture on waterflooding mechanisms: A case study of Kumkol Oilfield in the South Turgay Basin, Kazakhstan

  Zhao Lun; Wang Jincai; Chen Li; Chen Xi; Fan Zifei; Zhang Xiangzhong and Tian Zhongyuan

  , 2014, 41(1): 86-94.

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  Taking the Kumkol oilfield of high water cut in the South Turgay Basin, Kazakstan as an example, this article built the architecture models of meandering river sandstones, braided river sandstones and delta sandstones, and analyzed the influence of the superimposed structure characteristics between different types of sandstones and internal architecture features on warterflooding mechanisms and effects. The meandering river sandstones are mainly characterized by lateral accretion shale beddings inside, with simple overlay-relationship in sand bodies. In meandering river sandstones, injected water almost sweeps through the bottom of sand bodies and the bottom part of the sandstones is previously waterflooded, and the thickness ratio of strong and moderate waterflooded zone is about 51.8%. Braided river sandstones are characterized by nearly horizontal interlayers inside, with low heterogeneity and uniform waterflooding sweep vertically, and the thickness ratio of strong and moderate waterflooded zone is as high as 81.7%. The superimposed structure and architecture of delta sandstone is relatively complicated; different types of sandstones overlay each other frequently and shale interbeds are developed in sandstones. The injected water is obstructed not only by impermeable or low-permeable layers between sand bodies but also by shale interbeds in sandstones, with low sweep range of injected water, and the thickness ratio of strong and moderate waterflooded zone is only 32.6%. The braided river sandstone has low rising velocity of water cut and high no-water recovery degree, and its waterflooding development effect is the best. The waterflood sweep of delta sandstones is poor, with fast rising velocity of water cut and low no-water recovery degree, and its waterflooding development effect is the worst. The waterflooding development effect of meandering river sandstones is moderate among these three types of sandstones.
石油工程
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  Drilling fluid loss model and loss dynamic behavior in fractured formations

  Jia Lichun; Chen Mian; Hou Bing; Sun Zhen and Jin Yan

  , 2014, 41(1): 95-101.

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  In view of the lost circulation in fractured formations, a two-dimensional transient model for describing a power-law drilling fluid loss in an arbitrarily-oriented, compressible, permeable, rough-walled fracture was introduced. In this model, the mechanical fracture aperture and fracture tortuosity were considered to investigate the effect of fracture roughness on fluid loss dynamics. The governing equation of power-law fluid loss model was given and solved to analyze the fluid loss dynamics in fractured formations. The results show that the shear thinning behavior of power-law drilling fluid can result in high fluid loss rate at the initial stage of loss event; the fluid loss rate decreases as the fracture tortuosity increases, meanwhile, the effect of fracture tortuosity on fluid loss rate will decrease as the fracture aperture becomes larger; the larger the initial fracture aperture, fracture dip, fracture dimensions or fracture length, the higher the fluid loss rate will be; the fluid loss rate of rectangular fractures is much lower than that of square fractures; the higher the total leak-off coefficient, the higher the fluid loss rate will be; the fluid loss rate is the highest when the wellbore intersects the fracture at the center location; the fluid loss rate increases sharply as the differential pressure increases; the larger the normal stiffness of the fracture, the lower the mud loss rate will be.
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  Calculation model for bore-hole collapse volume of a horizontal openhole in multiple-weak-plane formation

  Liu Zhiyuan; Chen Mian; Jin Yan; Yang Xiangtong; Lu Yunhu and Xiong Qiquan

  , 2014, 41(1): 102-107.

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  In view of borehole collapse in testing of horizontal wells, a calculation model for borehole collapse volume of horizontal open-hole in multiple-weak-plane formation was set up to analyze the factors affecting collapse volume. It was compared with the single-weak-plane model by field application. After analyzing the influences of fracture occurrence, well-bore azimuth, in-situ stress anisotropy, number of weak planes and fluid density on well-bore collapse volume, this study reveals: When the fracture dip is small, the volume of well-bore collapse is big and the well-bore stability is poor; When the fracture dip is big, the collapse volume varies greatly with the change of well-bore azimuth and fracture trend; The maximum collapse volume appears with different well-bore azimuth if fracture trend is different; The greater the in-situ stress difference is, the greater the well-bore collapse volume will be; The collapse volume decreases with the increase of borehole fluid density, and is more sensitive to the change of fluid density when fluid density is near the equivalent density of pore pressure. Field application results show that, compared with the single-weak-plane model, the multiple-weak- plane model gets big collapse volume in most cases so it can assess the collapse risk more reliably.
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  Identification and ranging of reservoir boundary in horizontal well geo-steering

  Li Lin; Teng Xinmiao; Sheng Limin and Zhang Mingbao

  , 2014, 41(1): 108-111.

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  To make quantitative measurement of the distance between the drilling tool and the reservoir boundary in horizontal well drilling, this paper presented the method of “layer-detection ranging”, established the physical and mathematic models and designed the circuit supporting the method. Moreover, the simulation experiment was made to verify the method’s feasibility. With the method, the reservoir boundary was detected and the distance between it and the drilling tool was measured through transmitting directional high-frequency electromagnetic waves and receiving reflection waves, in process of measurement while drilling (MWD) in the horizontal well drilling. Two “layer-detection ranging” circuits, i.e. high-voltage narrow pulse and single frequency modulating pulse, were designed. Based on the circuit theory, a “layer-detection ranging” system was established and three media were used for simulation experiment. The results show that: more details of the reservoir are acquired from received waveforms using the high-voltage narrow pulse circuit, and the received waveforms are more easily identified and discriminated using the single frequency modulating pulse circuit; under the conditions of different media and distances, the relative error between the true distance and the distance measured with the “layer-detection ranging” method is less than 5%, suggesting that the method is reliable.
综合研究
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  Classification and characterization of barrier-intercalation in sandy braided river reservoirs: Taking Hegli Oilfield of Muglad Basin in Sudan as an example

  Sun Tianjian; Mu Longxin and Zhao Guoliang

  , 2014, 41(1): 112-120.

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  Based on comprehensive analysis of core data, well logs and outcrops, the barrier-intercalations in the sandy braided river of Hegli Oilfield, Muglad Basin, Sudan were classified and analyzed. The barrier-intercalations in single wells were identified using grey theory, which, in combination of the modern braided river deposit measurements, was used to establish the quantitative calculation formulas for different barrier-intercalations in different scale. Three hierarchies of the barrier-intercalations are developed in the sandy braided river reservoirs in the study area, including barriers between the two single layers, intercalations between the two single sandbodies and intercalations in a single braided river bar. The barriers are 700–1 500 m wide and 1 000–2 000 m long, in continuous sheet shape. The intercalations of abandoned channel are 170–350 m wide; the intercalations of chute are 60–100 m wide and 900–1 500 m long; and the intercalations of braided river bars are 100–400 m wide and 300–800 m long. Based on the constraint of barrier- intercalation scale, 3-D geological models which accurately delineate different types of barrier-intercalations were built by facies-control and stochastic modeling.
学术讨论
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  Controlling effects of paleo-climate and paleo-ocean on formation of carbonate reservoirs

  Zhang Jing; Zhang Baomin and Shan Xiuqin

  , 2014, 41(1): 121-128.

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  The controlling effects of paleo-climate and paleo-ocean on the formation of marine carbonate reservoirs were studied taking key carbonate strata in the Tarim, Sichuan and Ordos Basins as examples. Dry heat/aridity and moist heat/moisture are the two distinctive types of paleo-climate background as far as carbonate sedimentation and reservoir formation are concerned. The geochemistry of the Phanerozoic ocean experienced cyclic variation, with calcite sea and aragonite sea occurring alternately. Carbonate exploration practice in the Tarim, Sichuan and Ordos Basins shows that the features of carbonate reservoir are evidently controlled by paleo-climate and paleo-ocean environment. The aragonite sea under dry heat/arid climate is most favorable for the formation of porous dolomite reservoirs. The calcite sea under dry heat/arid climate is favorable for the formation of porous dolomite reservoirs with evaporite moldic pores. The calcite sea and aragonite sea under moist heat/moist climate may only lead to the formation of cavernous carbonate reservoir. Paleo-climate and paleo-ocean environment exert great influence on carbonate sedimentation and its diagenesis, and ultimately determine the porosity features and types of carbonate reservoirs.