, Volume 40 Issue 6
    

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    油气田开发
  • Techniques for high-efficient development of offshore fluvial oilfields
    Guo Taixian; Yang Qinghong; Huang Kai and Zhang Jianmin
    , 2013, 40(6): 2168-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    The geological reservoir conditions of fluvial oilfields offshore in Bohai are complex, with thin layer reservoirs, significant horizontal changes, poor connectivity, so these oilfields face the challenges of production well deployment, fine reservoir description, optimum resource allocation, and so on. To realize the efficient development of fluvial oilfields, this research has formed the technique of reservoir fine description and quantitative characterization in a sparse well network, the technique of regional sedimentation research and single river reservoir recognition, the well network optimization technique of single sand body, the rolling tapping potential technique in the fluvial oilfield. It is verified by 11 oilfields development of Bohai: the techniques are effective in the development of proved reserves resources, and 5 000×104 ton discovered reserves in the evaluation of rolling development are newly added. Thanks to the technology of reservoir fine description in forecasting reservoirs, the drilling rate is up to 98%; by the technique of fluvial single river reservoir recognition, the recovery ratio increases by 3% on average; the fluvial oilfield rolling development technique can bring a rapid transformation to the resources around the oilfield and achieve efficient development of oil and gas resources.
    • 油气勘探
  • Formation, distribution and potential of deep hydrocarbon resources in China
    Sun Longde; Zou Caineng; Zhu Rukai; Zhang Yunhui; Zhang Shuichang; Zhang Baoming; Zhu Guangyou and Gao Zhiyong
    , 2013, 40(6): 2191-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    The onshore exploration realm has been expanded to (ultra-) deep oil and gas in China. New cognition and significant breakthroughs have been made in generation and preservation conditions, reservoirs formation mechanism, exploration potential, petroleum resources assessment, and exploration engineering technology of deep oil and gas. Deep oil and gas reservoirs include clastics, carbonates, volcanics, etc. The temperature of deep oil can be up to 295 ℃. A context of long time of shallow burial and rapid deep burial in late stage helps preserve the pores of deep clastic rocks, and dissolution and fracturing improve their reservoir properties. Affected by faulting, karst hydrothermal process, dolomitization and early oil and gas injection, carbonate rocks have good reservoir properties even at the depth of over 8 000 m. Controlled by tectonism, volcanism, diagenesis and diagenetic reconstruction during supergene and burial stages, deep volcanic reservoirs develop primary and secondary weathering types of reservoirs. Deep oil and gas resources in China are distributed within three main practical areas of carbonate, clastic and volcanic reservoirs. Dominated by gas, some practical areas were formed including the Tarim Basin, Ordos Basin, Sichuan Basin, Junggar Basin, Songliao Basin, Santanghu Basin and Bohai Bay Basin. Deep oil and gas in China have entered an age of breakthrough and discovery. Relevant engineering technologies, such as ultra deep well drilling and ultra high temperature drilling fluid techniques, have the ability to find (ultra-) deep oil and gas.
  • Formation and accumulation of tight sandy conglomerate gas: A case from the Lower Cretaceous Yingcheng Formation of Xujiaweizi fault depression, Songliao Basin
    Feng Zihui; Yin Changhai; Lu Jiamin and Zhu Yingkang
    , 2013, 40(6): 2192-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    Based on oil and gas drilling data, the main control factors and accumulation rules of tight sandy conglomerate gas in the Lower Cretaceous Yingcheng Formation of the Xujiaweizi fault depression in the Songliao Basin are discussed by combining geology, geophysics and geochemistry. Tight sandy conglomerate gas of the Yingcheng Formation is controlled by the reservoir properties and hydrocarbon accumulation happened after the formation was compacted tight. The gas reservoir mainly produces dry gas, and various reservoirs, different in productivity, have a porosity lower limit of 2.5% and a permeability lower limit of 0.1×10-3 μm2. Structural setting has little effect on tight gas distribution. Gas-bearing “sweet spots” are controlled macroscopically by gas source rock development conditions, reservoir sedimentary facies belts and sites rich in faults. Reservoirs in delta-front subfacies are fairly good in physical properties. The strip zone where strike-slip faults connect sandy conglomerate reservoirs and underlying gas source rocks is the favorable area for tight sandy conglomerate gas exploration.
  • Cenozoic volcanic rocks in the Pearl River Mouth and Southeast Hainan Basins of South China Sea and their implications for petroleum geology
    Zhang Bin; Wang Pujun; Zhang Gongcheng; Sun Xiaomeng; Lu Baoliang and Ni Weiyi
    , 2013, 40(6): 2193-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    Drilling, gravity, magnetic observation and seismic facies are combined to identify the Cenozoic volcanic rocks in the Pearl River Mouth Basin and the Southeast Hainan Basin in the northern South China Sea, and to analyze how their lithology, distribution characteristics and volcanic activity influence on hydrocarbon accumulation. As a result, more than 70 volcanic bodies are ascertained. They are divided into two types: (1) laminated lava and pyroclastic rocks in the Cenozoic sedimentary sequence; and (2) conoid, mounded or wedged basalt with rigid basement under the Cenozoic sedimentary sequence. In the early Cenozoic (E1-E2), the acidic, neutral and basic volcanic rocks developed, while in the mid-late Cenozoic (E3-Q), single basic rocks existed intensively. The volcanic lithofacies mainly include the lower and middle effusive subfacies and the pyroclastic flow explosive subfacies. Spatially, from the Southeast Hainan Basin to the Pearl River Mouth Basin, the Cenozoic volcanic activity gradually enhances; from shallow water area to the central basin in the South China Sea, the scale of Cenozoic volcanic rocks increases gradually, and the age tends to be younger. These activities improve the formation temperature, reduce the hydrocarbon generation threshold, and promote the hydrocarbon accumulation. Volcanic rocks in Wenchang Formation (E2w) to Zhujiang Formation (N1z) provide favorable conditions for hydrocarbon accumulation, so they are important targets in the deep exploration.
  • Development and reservoir significance of mud mounds in Sinian Dengying Formation, Sichuan Basin
    Li Ling; Tan Xiucheng; Zeng Wei; Zhou Tao; Yang Yu; Hong Haitao; Luo Bing and Bian Lizeng
    , 2013, 40(6): 2194-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    Based on core and outcrop profile observation, the characteristics and reservoir significance in oil and gas accumulation of mud mounds in the Sinian Dengying Formation of the Sichuan Basin were studied. Small-scale mud mounds in platform are widespread in the Sinian Dengying Formation, Sichuan Basin, mostly less than 2 m thick per cycle, but high in occurrence frequency, especially in Member 2 and Member 4 of the Sinian Dengying Formation. Particles constituting the mud mounds are thrombolte, algal stromatolite, oncolite and bindstone, related to cyanobacteria biocoenosis buildup, which bind and capture lime mud for the mud mound construction, and the relevant rock types include algal bound clotted dolomitite, algal bound frame-dolomitite, and undulating-cylindrical algal stromatolite associated with clotted dolomitite. Multiphase mud mounds with positive relief have abundant frame structures, zebra structures and stromatactis. Two kinds of depositional models are mound base-core-flat and mound base-core-cap, the former is more common. The mud mounds are closely related to reservoirs. Different sizes of pores developed in the mud mound core provide space for oil and gas accumulation, so the mound-shoal complex is the main exploration target of the Sinian Dengying Formation.
  • Prediction of weathering paleokarst reservoirs by combining paleokarst landform with unconformity: A case study of Sinian Dengying Formation in Leshan-Longnüsi paleo-uplift, Sichuan Basin
    Tang Jiguang; Hu Wangshui; Li Wei and Zhang Guangya
    , 2013, 40(6): 2195-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    Based on the research of unconformity type and distribution, and restoration of structural feature of paleo-uplift in the late sedimentary period of Sinian Dengying Formation, the evolution of paleokarst landform and karstic hydrodynamic field during depositional hiatus between the Cambrian and Sinian of Leshan-Longnüsi paleo-uplift is revealed to predict weathering paleokarst reservoir of Dengying Formation dynamically and effectively. The restoration of ancient structures shows that salient structures were developed continuously in the west of Weiyuan-Renshou, forming concurrent erosional-overlap unconformity, but in the east of Weiyuan- Renshou line, the palaeostructure high shifted from east of Zizhong-Yingshan to the west, forming inversion erosional-overlap unconformity. On this basis, the ancient karstic hydrodynamic field change caused by the change in paleogeomorphology was analyzed to find out the modes and strength of dissolution for different paleokarst landforms. The study indicates that areas along Hechuan-Dazu, Peng’an-Guang’an-Anyue-Neijiang and north of Renshou experienced strong dissolution, developing multiple sets of karst layers; Dazu-Tongnan-Hechuan area underwent fairly strong dissolution, resulting in fairly developed karst reservoirs; the two ends of the area west of Weiyuan-Lezhi- Xichong-Yilong also experienced certain degree of dissolution, with fairly developed karst reservoirs, whereas the middle of this area experienced weak dissolution, resulting in poor development of karst reservoirs.
  • Neogene differential structural inversion and hydrocarbon accumulation in the Yitong Basin, East China
    Tang Daqing; Chen Honghan; Jiang Tao; Zhang Hui; Qiu Yuchao and Chen Li
    , 2013, 40(6): 2197-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    Based on the latest seismic and drilling data, this paper analyzes the differential transformation features, evolution/ deformation rules and kinetic background of the Neogene inversion structures in the Yitong basin, East China aiming to find the relation of differential structure inversion and hydrocarbon accumulation. At the northwestern edge of the basin, inversion structures are dominated by right-lateral strike-slip thrusts, which can be divided into three types: high-swelling overturn thrust, moderate-swelling echelon thrust and low-swelling rupture thrust - all of them formed a fault-fold belt 2 to 4 km wide along the northwestern, providing good conditions for hydrocarbon accumulation. At the southeastern edge, inversion structures are mainly composed of right-lateral compresso-shear and uplifting tilts, and also arch-tension faults at the tops of Jianshan, Wanchang, Soudeng and other structural belts, which destructed the pre-existed reservoirs. These structures originated as a result of the compression and uplifting of the northeastern China continent which occurred due to the back-arc spreading after the westward subduction of the Pacific plate, and also the long-distance compression caused by the collision between the Indian plate and the Eurasian plate. In addition, their origin is related to the position and intensity of uplifts at the edges of the Yitong basin.
  • Control factors on tight sandstone reservoirs below source rocks in the Rangzijing slope zone of southern Songliao Basin, East China
    Li Dan; Dong Chunmei; Lin Chengyan; Ren Lihua; Jiang Tao and Tang Zhenxing
    , 2013, 40(6): 2198-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    Based on the analysis of reservoir characteristics, this paper studies on the control factors on quality of the tight sandstone reservoirs in Fuyu oil layer below source rocks in Rangzijing slope zone in the Southern Songliao Basin from 5 aspects, i.e. base level cycle, deposition, diagenesis, tectonic activity and CO2 infilling, so as to identify the sweet spots in the reservoirs. The following results are obtained: (1) base level cycle and deposition condition control the relation of source rocks and reservoirs, stacking pattern of reservoirs and original property of reservoirs; (2) compaction is the main factor making the reservoirs tight, and cementation exacerbates the tightness; (3) the secondary pores produced by organic acids migrating downward into reservoirs improve the reservoir quality to some extent; (4) tectonic activity plays a positive role in improvement of reservoir quality in three levels, and (5) CO2 infilling can improve the percolation of local reservoirs, but it is adverse for the formation of effective reservoirs. In the study area, the formation of high-quality tight sandstone reservoirs is controlled by the coupling of base level cycle, main sedimentary facies, dominant corrosion zone and tectonic uplifting area.
  • A new kinetic model for authigenic quartz formation under overpressure
    Meng Yuanlin; Xu Cheng; Xie Hongyu; Tian Weizhi; Tong Chuanxin; Liu Jinghuan; Gao Yuting and Wang Youchun
    , 2013, 40(6): 2199-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    To model the change of authigenic quartz content precisely in geological history, a new kinetic model combining time, temperature and pressure was established considering the retardation of overpressure to the formation of authigenic quartz on the basis of Walderhaug’s kinetic model including time and temperature. The model was tested to be practically applicable in the Yinggehai Basin. The Cenozoic quartzarenite in the central depression of the Yinggehai Basin was affected by overpressure during the formation of authigenic quartz. The overpressure inhibiting index was determined based on the measured pressure and authigenic quartz content. The model with overpressure inhibiting index can simulate the evolution of authigenic quarzt much better than Walderhaug’s model. Overpressure sandstones with a pressure coefficient between 1.7 to 2.0 can cause the total amount of authigenic quartz to drop by 2.72% to 4.72%, which would result in high-quality reservoirs in deep overpressured formation.
    • 油气田开发
  • Numerical simulation of fire flooding for heavy oil reservoirs after steam injection:A case study on Block H1 of Xinjiang Oilfield, NW China
    Xi Changfeng; Guan Wenlong; Jiang Youwei; Liang Jinzhong; Zhou You; Wu Jian; Wang Xiaochun; Cheng Hongjie; Huang Jihong and Wang Bosheng
    , 2013, 40(6): 2200-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    With the 3a pilot area in Block H1 of Xinjiang Oilfield as an example, a basic model for accurate numerical simulation of fire flooding is established combining with physical simulation and field performance, after oil components are divided reasonably, high temperature combustion formula is defined. This model is used to trace and simulate the fire flooding in Block H1. The main results are as follows: (1) secondary water generated after steam injection is mostly displaced by flue gas during initial combustion phase, and only a small fraction works for limited wet combustion; (2) the flooded reservoirs, from the gas injection well to the production well, are divided into burnt zone, fire wall, coking zone, high temperature condensed zone (added newly), oil bank, and residual oil zone; and (3) the fire flooding for reservoir after steam injection, unlike original reservoir, experiences a process of oil bank building and migration featuring “pit filled and bank built”, with bank shape related to the performance of adjacent production wells. Moreover, the field experiment indicates that the numerical simulation of fire flooding contributes to the prediction and management of flooding performance.
  • Causes of hydrodynamic pressure distribution: A case of the 4th Pay in Zubair Formation, Rumaila Oilfield, Iraq
    Liu Zhuo; Tian Changbing; Zhang Weimin; Ma Ji and Luo Hong
    , 2013, 40(6): 2201-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    This paper summarizes the genetic causes of hydrodynamic pressure distribution based on study of theoretical mechanisms and reservoir cases and determines the causes of the 4th Pay in the Zubair Formation of the Rumaila Oilfield using exclusive method and numerical simulation. These causes can be classified into three types as the hydrodynamic aquifer type, the reservoir compartmentalization type and the changing fluid density type. Among them, the hydrodynamic aquifer type may result from depletion of hydrodynamic pools or adjacent oilfields. The aquifer pressure in the 4th Pay reservoir decreases from north to south, without tilted oil-water contact. Considering the geological settings of the 4th Pay reservoir, its hydrodynamic pressure distribution is hydrodynamic aquifer. By numerical simulation of the pressure distribution, fluids contacts geometry and adjusting time, the hydrodynamic aquifer in the 4th Pay is due to the depletion of adjacent oilfields. Combining regional geological setting and investigation of production history of adjacent oilfields, the hydrodynamic pressure distribution phenomenon in the 4th Pay reservoir is affected by the depletion of the Raudhatain oilfield in the south.
  • Experimental study on gelling property and plugging effect of inorganic gel system (OMGL)
    Niu Liwei; Lu Xiangguo; Xiong Chunming; Tang Xiaofen; Wu Xingcai; Jia Xu and Zhang Song
    , 2013, 40(6): 2202-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    By means of instrumental test and theoretical analysis, the gelling degree, extreme gelation concentration and plugging performance of inorganic gel (OMGL) were studied and its appearance on surface of rock skeleton was observed. Results show that inorganic gel is formed by reaction between OMGL system and Ca2+. The higher the concentration of OMGL and Ca2+, the higher the gelling degree. When the concentration of OMGL system is 2 to 3 times that of Ca2+, the inorganic gelling degree is the most suitable. The gelling degree is not sensitive to temperature. Under the same core permeability, residual resistance factor increases with the increase of concentration of OMGL system. When the concentration of OMGL system remains constant, residual resistance factor decreases with the increase of core permeability. There exists a reasonable matching relation between core permeability and concentration ratio of OMGL system and Ca2+. Plugging effect of cores can be speculated from Ca2+ concentration. The inorganic gel coats on rock surface by absorption in the form of ionic bond of “-Si-O-Si-” instead of depositing in the channels.
    • 石油工程
  • Current status and trend of separated layer water flooding in China
    Liu He; Pei Xiaohan; Luo Kai; Sun Fuchao; Zheng Lichen and Yang Qinghai
    , 2013, 40(6): 2203-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    Separated layer water injection is one of the crucial means to keep long-term high and stable production of oilfield. “Bridge eccentricity + steel cable measuring and adjustment” has come into being as the principal water flooding technology, which, with improving adaptability, can meet the need of conventional water injection wells. As the water flooding fields enter high water-cut period, sand control for water injectors, separated layer flowrate test at low injection rate, high pressure and high temperature water injection, separated layer water injection for high-inclination wells and separated layer water injection monitoring and control will be the developing trend, directing water flooding towards automatization, intelligence and integration to meet the requirements of water injection under different reservoir and well type conditions for the purpose of ensuring efficient oil field development. Currently periodic progress have been made in downhole wireless communication, flowrate monitoring, nozzle dynamic adjustment, etc, forming the theoretical basis for automatic water flooding.
  • Analyses and countermeasures of deepwater drilling riser grounding accidents under typhoon conditions
    Liu Xiuquan; Chen Guoming; Chang Yuanjiang; Liu Kang; Zhang Lei and Xu Liangbin
    , 2013, 40(6): 2204-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    For deepwater drilling riser system facing grounding risk in the South China Sea, a mechanical analysis model of deepwater drilling riser grounding accident was established. Focusing on drilling riser grounding accidents of a deepwater floating drilling platform in the South China Sea, mechanical characteristics and grounding accidents of riser were analyzed. And the riser retrieving window was determined. The results showed that the shape of the riser system after riser grounding is catenary. The maximums of bending moment and von Mises are located at lower flex joint. Platform moon pool, tensioner and lower flex joint are damaged due to severe bending deformation in the upper and lower drilling riser system after riser grounding accidents. The riser may fracture if drilling platform continues to move toward shallow water. Drilling platform displacement and riser lift height must be within the riser operation window during riser retrieving, or else the riser may fracture and the LMRP may suffer further damage. Residual riser and LMRP can be retrieved in a fixed position when the drilling riser system is separated from seabed.
  • Selection of deepwater floating oil platform based on grey correlation
    Mao Dongfeng; Duan Menglan; Li Xinzhong; Su Junwei and Wang Yingying
    , 2013, 40(6): 2205-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    To select a suitable floating oil platform for a given deepwater oilfield, a method based on grey-correlation theory was proposed and verified. By analyzing factors that may affect selection of deepwater platform, thirteen main factors were chosen. This article also summed up the statistics on 65 deepwater floating platforms (including tension leg platform, deep-draft column type platform, semi-submersible platform, and floating storage mining and unloading device) performing well in the world. With a deepwater oilfield to be developed as a sample, the application of the method was illustrated: Considering the factor series of the given oilfield to be developed as referent series, the relevance between referent series and factor series of each platform in statistics was calculated with grey-correlation analysis, and the optimal platform was identified as the one that had the highest relevance. Two examples proved the feasibility and reliability of the method.
  • Application of rotary geosteering drilling in deep and thin reservoirs of Tarim Basin
    Zhang Chengguang; Wu Qianli; Wang Xiaoliang and Lü Ning
    , 2013, 40(6): 2206-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    Geological features, such as deep and thin reservoirs and unstable structure margin, occur in an oilfield of the Tarim Basin. To deal with these problems while drilling, a rotary geosteering drilling technique was introduced. The effect of its application was analyzed with an example, the shortcomings were summarized, and suggestions were given for other oilfileds. The feasibility and necessity of using rotary geosteering drilling were presented according to the engineering challenges in the oilfield, then the theory and main tools of it were introduced. Focusing on a double-step horizontal well in field application, the features and effects of this technique were elaborated. With rotary geosteering drilling in this well, the average rate of penetration was high and oil reservoir encountering rate was up to 82% in thin layers less than 1 m. Also, it avoided effortless weight on bit in sliding drilling method and made the wellbore more smooth and clean. The problems met were concluded, for example, the tools build-up rate was affected by several factors, measurement while drilling (MWD) signal was interfered, and the tools working performance was unstable. Suggestions were put forward for applying the rotary geosteering drilling technique in other oilfields.
  • General method of calculating annular laminar pressure drop of drilling fluids with different rheological models
    Peng Qi; Fan Honghai; Zhou Haobo; Li Chaowei; Chen Xuyue; Wang Echuan and Ye Zhi
    , 2013, 40(6): 2207-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    Traditional annular laminar flow analysis method applied in drilling engineering has poor accuracy and does not apply to complicated rheological models. A general method of calculating annular laminar pressure drop of drilling fluids with different rheological models was proposed, compared with traditional method, and verified using experiment data in published articles. Based on annular slot flow model, the pipe flow equation was popularized in annular and an annular flow equation was established, and the relationship between annular flow and shear stress at pipe wall was established by combining annular flow equation and fluid’s rheological equation. If annular flow is given, shear stress at pipe wall can be got, and then the annular pressure drop is also available. Compared with traditional methods, the general method is adapt to any rheological model of annular laminar flow, and has good universality, simple modeling process and high accuracy. The calculation results based on experiment data in published articles show: no matter the flow rate is high or low, the results of the general method match the measured results well, which makes up the error of traditional method.
    • 综合研究
  • Patterns of inter-layers in the alluvial fan reservoirs: A case study on Triassic Lower Karamay Formation, Yizhong Area, Karamay Oilfield, NW China
    Yin Senlin; Wu Shenghe; Feng Wenjie; Li Junfei and Yin Hang
    , 2013, 40(6): 2208-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    This paper discusses the overall distribution characteristics of alluvial fan in the Triassic Lower Karamay Formation, Yizhong Area, the Karamay Oilfield, with core, outcrop, logging and seismic data. On this basis, the shapes, sizes and stacking patterns of inter-layers in different levels and sub-facies are analyzed using the hierarchy interface method. In the study area, the Lower Karamay Formation contains (from bottom to top) upper fan, mid fan and lower fan deposits, with size-reducing grains, as a complete retrograding sedimentary sequence. The inter-layers in the alluvial fan reservoir can be divided into two types: lithological interlayer (fine sediments) and physical interlayer (argillaceous sandstone/conglomerate). Origin, pattern and size of inter-layers vary significantly in different subfacies zones. In particular, the inter-layers in upper fan are “scattered generally and interlaced locally”, presenting as partially discontinuous screen in a high-permeable network; the inter-layers in mid fan are relatively stable inter-layers and lateral inter-layers, with a higher capacity to shade or screen the seepage; the inter-layers in lower fan present as a layered cake, with reservoir bodies mosaic-like along the source and lenticular in perpendicularity to the source that are separated by thick interlayer, showing the highest capacity of screen.
    • 学术讨论
  • Evolution of overmature marine shale porosity and implication to the free gas volume
    Wang Feiyu; Guan Jing; Feng Weiping and Bao Linyan
    , 2013, 40(6): 2209-0.
    Abstract ( ) Download PDF ( ) Knowledge map Save
    The porosities of the Lower Cambrian shale and Upper Ordovician-Lower Silurian marine shale from the Sichuan Basin were analyzed using He-Hg porosimetry and field-emission scanning electron microscopy, to discuss the relationship of porosity to the organic matter maturity, total organic carbon (TOC) and free gas volume. Overmature (Ro greater than 2%) shale samples with a TOC less than 5% display a positive correlation between TOC and porosity, but samples with a TOC greater than 5% display a limited increase in porosity with increasing TOC. The porosity of shale decreases generally with increasing maturity in the overmature stage. The free gas in porosity is the key factor for successful development of shale gas, and free gas volumes are controlled by porosity and gas saturation. Higher TOC shale has higher porosity and gas saturation. The Lower Silurian Longmaxi Formation shale has a higher TOC, Ro ranging from 2.0% to 2.3%, its porosity and gas saturation are significantly higher than the Lower Cambrian shale. The high free gas volume results in high production in the Lower Silurian Longmaxi Formation shale pay.
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