23 June 2017, Volume 44 Issue 3
    

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    PETROLEUM EXPLORATION
  • Gas exploration potential of tight carbonate reservoirs: A case study of Ordovician Majiagou Formation in the eastern Yi-Shan slope, Ordos Basin, NW China
    WEI Xinshan, CHEN Hongde, ZHANG Daofeng, DAI Rong, GUO Yanru, CHEN Juanping, REN Junfeng, LIU Na, LUO Shunshe, ZHAO Junxing
    Petroleum Exploration and Development. 2017, 44(3): 319-329. https://doi.org/10.11698/PED.2017.03.01
    Abstract ( ) Download PDF ( ) Knowledge map Save
    On the basis of comprehensive analysis of drilling, gas testing, laboratory analysis and testing data, the characteristics and genesis of tight carbonate reservoirs in Ma51+2 Member of Ordovician Majiagou Formation, eastern Yi-Shaan slope, Ordos Basin were examined, and the potential of natural gas exploration and development were analyzed. The tight carbonate reservoir is defined as the reservoir with a porosity of less than 2% and permeability of less than 0.1×10-3 μm2. The Ma51+2 reservoirs are dominantly gypsum mud dolomite, muddy dolomite and Karst-breccia dolomite and has strong heterogeneity, pore types being mainly composed of fracture-dissolution pores and fracture-intercrystalline pores, and thin reservoir layers are distributed in a large area. The unconformity structure adjustment at the top of the Ordovician caused pore creation and pore filling effects, and the joint effect of dissolution pore increase and pore reduction by filling is the major reason for extensive reservoir densification. The thin tight dolomite reservoirs and the overlying adjacent coal source rock in the Upper Paleozoic formed extensive tight carbonate gas with shallow depth (1900-2500 m) and formed a three-dimensional gas containing pattern combined with the Upper Paleozoic tight sandstone gas. The eastern Yi-shaan slope in the Ordos Basin has great exploration and development potential.
  • Controlling factors of hydrocarbon accumulation in Termit rift superimposed basin, Niger
    ZHOU Lihong, SU Junqing, DONG Xiaowei, SHI Buqing, SUN Zhihua, QIAN Maolu, LOU Da, LIU Aiping
    Petroleum Exploration and Development. 2017, 44(3): 330-339. https://doi.org/10.11698/PED.2017.03.02
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    Based on the tectonic evolution and depositional characteristics of Central-West African Rift System and regional geologic background, the characteristics of oil and gas accumulation in the Termit rift superimposed basin were examined, and the factors controlling oil and gas accumulation there were discussed. The Termit basin is a rift superimposed basin formed by two stages of rifting in the early Cretaceous and Paleogene, where the Yogou Formation marine source rock is widespread. The two sets of reservoirs, the first member of the Paleogene Sokor Formation and the Cretaceous Yogou Formation, dominated by quartz sandstone, have relatively good porosity-permeability characteristics. They can form multiple sets of favorable caprock-reservoir assemblage below the regional caprock of the second member of Paleogene Sokor Formation. It is found that fault pattern and oil and gas migration pattern control the oil and gas accumulation patterns in the basin, including “Y-shaped” hydrocarbon migration and accumulation in the Araga graben, “multi-step” vertical hydrocarbon migration and accumulation in the Dinga faulted terrace, and the “composite” hydrocarbon migration and accumulation in the Fana transfer zone. Hydrocarbon enrichment in this basin was controlled by tectonic evolution of rift basins, that is, fault belts formed during the two stages of rifting controlled the horizontal distribution of oil and gas, while favorable reservoir-caprock assemblages in the sequences formed in the late rifting stage controlled the vertical hydrocarbon enrichment.
  • Analogue modeling of through-going process and development pattern of strike-slip fault zone
    XIAO Yang, WU Guanghui, LEI Yongliang, CHEN Tingting
    Petroleum Exploration and Development. 2017, 44(3): 340-348. https://doi.org/10.11698/PED.2017.03.03
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    Nine sets of sand-box experiments were designed according to actual geologic data to investigate the evolution pattern and development mechanism of simple shear strike-slip fault zone using the fault CT scanning technique. The experiment results show that R (Riedel) shear faults were developed early and more in number, and one set of these faults intersect with the principle displacement zone (PDZ) at lower angle; the P shear faults (being in symmetrical distribution with the R shear faults) and Y shear faults (subparallel to PDZ) were developed later than the R shear faults, and the fault zone was through-going only after the formation of Y shear faults. The through-going process of strike-slip fault zone can be divided into four stages: embryonic stage, R (Riedel) shear fault development stage, P shear fault and Y shear fault development stage, and through-going stage of fault zone. In the process, the faults developed progressively from the basement to the top cover, the faults spread upward in the profile at embryonic stage, and spread outward at first and then converged toward the PDZ on the plane at R shear faults development stage. The modeling demonstrates that the en echelon structure developed in “helicoidally drag” pattern, the length of the en echelon fault grew linearly at two times the growth rate of its depth, and the fault intersection angle with the PDZ decreased in quadratic function with the increase of its depth. The analysis reveals that cover thickness and the maximum principal stress direction are the main factors causing the difference in “helicoidally drag” structure. The modeling provides guidance for the strike-slip fault interpretation and evolution study, and for layering and segmentation of faults in the marine carbonates of the Tarim Basin.
  • Structure characteristics and major controlling factors of platform margin microbial reef reservoirs: A case study of Xiaoerbulak Formation, Lower Cambrian, Aksu area, Tarim Basin, NW China
    BAI Ying, LUO Ping, WANG Shi, ZHOU Chuanmin, ZHAI Xiufen, WANG Shan, YANG Zongyu
    Petroleum Exploration and Development. 2017, 44(3): 349-358. https://doi.org/10.11698/PED.2017.03.04
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    The structure characteristics and major controlling factors of platform margin microbial reef reservoirs in the Lower Cambrian Xiaoerbulak Formation in the Aksu area, Tarim Basin were analyzed based on 5 outcrop sections, 162 thin sections, 12 SEM samples, 52 sets of porosity and permeability data. Macroscopically, small-scale microbial reefs form the platform margin. A single microbial reef has several microbial reef progradation complexes, including reef front, fore reef, reef crest, and back reef, but microscopically, they have different kinds of microstructures. The reservoir spaces in the reefs can be divided into microbial structure reservoir space, including fenestral, frame, moldic, and oversized dissolution pores, and non-microbial structure reservoir space, including microcracks and stylolites. The statistical results of porosity and permeability show that in the Yutixi Section, porosities and permeability of the microbial reef reservoirs are generally below 5% and 1.0×10-3 μm2 respectively, characteristics of extremely low porosity and permeability reservoirs; while the reef reservoirs in the Sugaitebulake Section have a wide porosity range between 3% and 10%, and permeability range between 0.1×10-3 μm2 and 50×10-3 μm2, and strong heterogeneity, are low-medium porosity, low-medium permeability reservoirs. This demonstrates different microbial reefs have big differences in physical properties, and even the reservoir in the same reef has obvious heterogeneity. Paleotopography controls the formation of microbial reefs and sedimentation controls the facies distribution and the primary porosity development. Dissolution, controlled by the former two factors, finally decides evolution of reservoir pores.
  • Physical simulation and quantitative calculation of increased feldspar dissolution pores in deep reservoirs
    GAO Zhiyong, FENG Jiarui, CUI Jinggang, WANG Xiaoqi, ZHOU Chuanmin, SHI Yuxin
    Petroleum Exploration and Development. 2017, 44(3): 359-369. https://doi.org/10.11698/PED.2017.03.05
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    The physical simulation of diagenesis was conducted for the Cretaceous Bashijiqike Formation deep sandstone reservoir in Kelasu structural belt of Kuqa Depression, Tarim Basin, and the dissolution rate and increased dissolution pores of feldspar matrix grains in such reservoirs were quantitatively calculated in the process from long-term shallow burial in early stage to quick deep burial in late stage. Through the field emission large-area SEM analysis, the dissolution rate and increased dissolution pores of feldspar matrix grains in core samples taken from Dabei and Keshen areas were quantitatively calculated. After the experimental data and the actual core data were cross-checked, the evolution model was established for increased feldspar dissolution pores in deep continental reservoirs with high content of feldspar matrix grains. According to the calculation results, the maximum increased feldspar dissolution pores in Keshen area during the process from long-term shallow burial in early stage to quick deep burial in late stage is by 0.86%-2.05%. The simulated sandstone reservoir with burial depth of more than 7 000 m reveals a larger quantity of increased feldspar dissolution pores, with the absolute error value of 0.23% after calibration. In Dabei area, calcite is the primary contributor to secondary dissolution pores, followed by feldspar. Quantitative calculation shows the maximum increased feldspar dissolution pores in Dabei area to be by 0.62%-1.48%. Similarly, the simulated sandstone reservoir with burial depth of more than 7 000 m reveals a larger quantity of increased feldspar dissolution pores, with the absolute error value of 0.15% after calibration. There are two causes of the experiment errors: One cause is that the simulation experiment uses ideal conditions and the simulation reservoirs are homogeneous; Another one is that deep reservoirs have strong heterogeneity and there are big differences in the dissolution within different areas.
  • Distribution pattern and origin of the columnar pull-down anomalies in Gaoshiti Block of Central Sichuan Basin, SW China
    YANG Ping, DING Bozhao, FAN Chang, ZHU Xinghui, WANG Yajing, YANG Pei
    Petroleum Exploration and Development. 2017, 44(3): 370-379. https://doi.org/10.11698/PED.2017.03.06
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    The targets of this study are located in the Gaoshiti block, Central Sichuan Basin. Reprocessing the seismic data with reverse time migration (RTM) and using high-precision coherence analysis, this paper analyzed the distribution pattern and origin of the pull-down anomalies on the seismic sections. The study found that these anomalies are widely developed in Gaoshiti block, and believed that they are hydrothermal channels formed on the basis of tiny-grabens caused by the pull-apart function of strike-slip faults, and they had endured long-period, multistage hydrothermal reformation. Taking the reservoir of Dengying Formation as the example, integrated analysis of seism and geology proved that the hydrothermal activities had constructively reformed the carbonate reservoirs mainly by dissolution during long period. Therefore, it was proposed that the hydrothermal channels and fracture zones are the favorable area for good reservoirs. The discovery of hydrothermal channels is very meaningful for locating high-production well sites and extending exploration fields.
  • Condensate origin and hydrocarbon accumulation mechanism of the deepwater giant gas field in western South China Sea: A case study of Lingshui 17-2 gas field in Qiongdongnan Basin, South China Sea
    HUANG Heting, HUANG Baojia, HUANG Yiwen, LI Xing, TIAN Hui
    Petroleum Exploration and Development. 2017, 44(3): 380-388. https://doi.org/10.11698/PED.2017.03.07
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    Based on the geochemical characteristics of condensates and gases, in combination with geological background, the origin and formation of condensate and oil-gas accumulation mechanism of the deepwater Lingshui 17-2 gas field in the Qiongdongnan Basin, western South China Sea are discussed. The condensate from Lingshui 17-2 gas field has the features of low density, low wax content and high Pr/Ph value. The condensate is generated at mature stage while the co-existing gas is dominated by high mature coal-derived gas. The oil and gas are derived from the Oligocene Yacheng Formation source rocks. The formation of the condensate is related not only to the source rock, but also to the later “gas-washing”. The light oil and gas which was generated at oil-window stage and accumulated in reservoirs have strong reaction of evaporation fractionation during “gas-washing” by high mature natural gas injected largely at later period. The Yacheng source rocks provided sufficient oil and gas, diapiric fractures formed migration paths for oil and gas, and two stages of hydrocarbons charged and accumulated. The Miocene Huangliu Formation turbidite sandstones lithologic trap and Paleogene structural trap close to source Kitchen in the central canyon have favorable oil-gas exploration potential.
  • A fracture evaluation by acoustic logging technology in oil-based mud: A case from tight sandstone reservoirs in Keshen area of Kuqa depression, Tarim Basin, NW China
    TANG Jun, ZHANG Chengguang, XIN Yi
    Petroleum Exploration and Development. 2017, 44(3): 389-397. https://doi.org/10.11698/PED.2017.03.08
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    To solve the problem of poor fracture identifying effect on electrical logging in oil-based mud, the application of acoustic logging to the quantitative characterisation of fractures is expanded from three aspects, namely, Stoneley waves, longitudinal and transverse waves and cross dipole acoustic waves, and a fracture logging evaluation model closely related to production capacity is established considering the radial extension characteristics of fractures. The Stoneley reflection coefficient is used to determine fractures locations to help detect fractures during normal micro-resistivity imaging logging. Based on the experiment on the relationship between fracture width and acoustic attenuation coefficient, empirical formulae for calculating fracture width have been established by primary wave and shear wave energy information considering the effect of porosity. The new parameters, including spectrum correlation coefficient and energy difference from cross dipole array acoustic logging data, can be used for fractures evaluation. The more developed the fractures are, the greater the energy difference becomes, and the smaller the spectrum correlation coefficient is, the higher the production is. The fracture effective evaluation parameters can be separated into two components, specified as the degree of fracture vertical opening and radial extension. Combining the conventional logging and array acoustic logging (including cross dipole array acoustic logging), a fracture radial extension evaluation model is presented closely related to productivity.
    • OIL AND GAS FIELD DEVELOPMENT
  • Geological characteristics and development strategies for Cambrian Longwangmiao Formation gas reservoir in Anyue gas field, Sichuan Basin, SW China
    LI Xizhe, GUO Zhenhua, WAN Yujin, LIU Xiaohua, ZHANG Manlang, XIE Wuren, SU Yunhe, HU Yong, FENG Jianwei, YANG Bingxiu, MA Shiyu, GAO Shusheng
    Petroleum Exploration and Development. 2017, 44(3): 398-406. https://doi.org/10.11698/PED.2017.03.09
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    Seismic, drilling, logging and production performance data were analyzed to study the key geological factors such as reservoir properties and gas/water distribution, which influence the development of the Lower Cambrian Longwangmiao reservoirs in the Moxi block, Anyue gas field, Sichuan Basin, and the development strategies are established. The results indicate that: (1) Four stages of grain shoals developed longitudinally and two main shoals with one trench formed laterally in the development area; (2) Three types of reservoirs are identified, which are reservoir with millimeter sized dissolved vugs, reservoir with solution pores, and reservoir with inter-particle/inter-crystal pores; (3) Low matrix porosity but mid-to-high permeability affected by developed high-dip fractures; and (4) Three gas/water contacts (GWC) can be defined and step down from west to east, with a unique gas/water contact of -4 385 m in the target developing area. Based on geologic characterization, reservoir simulation and case study of similar gas reservoirs, the development strategies are made: to select the main grain shoal area as the prior production area; to locate wells on the structural high to delay edge water breakthrough and prolong stable production; to drill horizontal wells enhancing single well deliverability; to keep reasonable production of wells for improving the production effect.
  • Influences of delta sandstone architecture on waterflooding sweep characteristics: A case study of layer J-Ⅱ of Kumkol South oilfield in South Turgay Basin, Kazakstan
    ZHAO Lun, WANG Jincai, CHEN Li, ZHANG Xiangzhong, CAO Haili, WANG Chenggang, ZHAO Liangdong
    Petroleum Exploration and Development. 2017, 44(3): 407-414. https://doi.org/10.11698/PED.2017.03.10
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    Taking layer J-Ⅱ of Upper Jurassic of Kumkol South oilfield in South Turgay Basin, Kazakstan as an example, the article studied the architecture characteristics and heterogeneity of delta sandstones, and determined the influence of sandstone architecture on waterflooding sweep characteristics using fine characterization of sandstones, comprehensive interpretation of waterflooded layers, statistical analysis of production and injection profiles, reservoir numerical simulation methods. The result shows that the delta sandstones have a complex architecture of mosaic structure with mudstone baffles and mud-interbeds, and the sandstone architecture controls the sweeping characteristics of waterflood: due to impermeable or low permeable boundaries between single sand bodies, and lateral and vertical blocking of underwater distributaries bay mudstone, sands in delta have very complicated architecture, and the injected water flows priorly into underwater distributaries channel sand and mouth bar sand with better physical properties, making these two types of sands suffer stronger warterflooding, and have higher recovery degree and better waterflooding sweep effect in plane. Inside single sand bodies, on the one hand, underwater distributaries channel sand and mouth bar sand have high vertical producing degree and good waterflooding sweep effect because mud interbeds in the sands have vertical blocking and shunt effect on injected water; on the other hand, these two types of sands have inhomogeneous waterflood sweeping because of large permeability difference and high development of dominant water channels. In contrast, made up of thin homogeneous fine sand with poor physical properties, lateral sheet sand often has even water-flooding and poor waterflooding sweep effect.
  • A numerical simulation model for multi-scale flow in tight oil reservoirs
    FANG Wenchao, JIANG Hanqiao, LI Junjian, WANG Qing, KILLOUGH John, LI Linkai, PENG Yongcan, YANG Hanxu
    Petroleum Exploration and Development. 2017, 44(3): 415-421. https://doi.org/10.11698/PED.2017.03.11
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    A discrete fracture model for multi-scale flow in large-scale fractured tight oil reservoirs is proposed considering the compressibility of reservoir rock and fluid, and the non-linear flow in the tight matrix. Validation of the model is performed, followed by the field application of the model. The two-point flux-approximation scheme is adopted in the model to calculate conductivity, and small grids at the fracture intersections are eliminated by the “star-delta” transformation method to improve the computational stability. The fully implicit discretization scheme is performed on the temporal domain. Automatic differentiation technique which can improve model establishment efficiency and computational accuracy is applied in the model to solve the numerical model. The model is validated with the simulation results of Eclipse and the historical production data of a long fractured horizontal well in a tight oil reservoir in Xinjiang oilfield. Simulation results of a field-scale reservoir show that the model proposed can simulate reservoirs with large-scale complex fracture systems; well productivity is positively correlated with the scale of the stimulated reservoir volume, and the difference in planar fracture density and fracture connectivity are proved to be the key factors that lead to the heterogeneous distribution of remaining oil in tight oil reservoirs.
    • PETROLEUM ENGINEERING
  • Applicable conditions and analytical corrections of plane strain assumption in the simulation of hydraulic fracturing
    ZHAO Jinzhou, PENG Yu, LI Yongming, TIAN Zhisheng
    Petroleum Exploration and Development. 2017, 44(3): 429-436. https://doi.org/10.11698/PED.2017.03.13
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    Plane stain assumption is widely used in the simulation of the distribution of stress and displacement around hydraulic fractures. According to the comparison of the solutions of elliptical fracture and plane stain fracture, the applicable conditions of plane strain assumption were discussed and the correction method was put forward. On the basis of the past research, a classical stress and displacement solution around a flat elliptical fracture was deduced, fulfilled and corrected. In comparison with the plain strain fracture solution, simulating results show that if taking plane strain assumption into consideration in the fracture height profile, the difference between the elliptical fracture solution and plane strain solution is negligible when the ratio between fracture length and fracture height is larger than 10, and the above critical value could be relaxed to 5 in some simulations which normal stress perpendicular to fracture face plays a decisive role. The plane strain fracture solution is accurate enough when the above critical value is satisfied, the correction of the plane strain fracture solution is needed when the length-height ratio is small. The correction charts of the additional initial stress of the horizontal well with an open-hole and the width of a single fracture were drawn under different length-height ratios. The fracture width of pseudo three dimension propagation models is easier to be modified by the correction charts.
  • Gas kick during carbonate reservoirs drilling and its risk assessment
    GUO Yanli, SUN Baojiang, GAO Yonghai, LI Hao, WU Changfu
    Petroleum Exploration and Development. 2017, 44(3): 437-444. https://doi.org/10.11698/PED.2017.03.14
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    The gas kick characteristics during carbonate reservoir drilling were analyzed taking carbonate reservoirs in Shunnan area of Tazhong as an example, a quantitative evaluation method for gas kick risk was established, and then a case was simulated. Looking into reservoir space characteristics and gas invasion mechanisms of carbonate reservoirs in Shunnan area based on drilling geologic data, it is found that the reservoirs are rich in fractures, pores and caves, and the gas invades into wellbore through gas-liquid replacement and differential pressure. By fully considering the gas invasion mechanism, gas migration law and wellbore temperature-pressure field, and introducing the gas volume fraction density function, a quantitative evaluation method for gas kick risk and a method for ranking well control risk were established. The case study shows that the wellhead back pressure method can be used to tell the mode of gas invasion; pore-fracture-cave and acid gas characteristics in carbonate reservoirs have stronger effect on the gas volume fraction in wellbore, and may cause hidden and severe gas kick; the effects of well depth, well diameter, drilling fluid density, drilling fluid displacement, drilling fluid viscosity and drilling rate on gas kick risks weaken in that order.
  • Experimental investigation on fracture initiation and non-planar propagation of hydraulic fractures in coal seams
    TAN Peng, JIN Yan, HOU Bing, HAN Ke, ZHOU Yingcao, MENG Shangzhi
    Petroleum Exploration and Development. 2017, 44(3): 445-451. https://doi.org/10.11698/PED.2017.03.15
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    True tri-axial test system was deployed for fracturing simulation of coal outcrops to investigate the initiation and propagation of hydraulic fractures in vertical and directional wells. The influences of in-situ stress and cleats on non-planar propagation of hydraulic fractures in directional wells under different relative azimuths were analyzed. The test results show that the general propagation pattern of hydraulic fractures is jointly controlled by azimuth, cleats and in-situ stress. As the relative azimuth increases, the hydraulic fractures become more complicated in geometry and subject to increasing pumping pressure and propagation pressure. If the hydraulic fractures are initiated along a direction skewed with wellbore, the effect of cleats would alter the extension path and appear distortion of hydraulic fractures, inducing more complicated fracture geometry near the wellbore, with many fractures at the initiation point. Compared with vertical wells where I-shaped or X-shaped hydraulic fractures are formed, directional wells often have twisty propagation of dominant fractures near the wellbore and presence of multi-level fractures, which impede the further extension of hydraulic fractures in coal seams.
  • Plugging performance of sepiolite drilling mud with calcium carbonate
    ETTEHADI Ali, ALTUN Gursat
    Petroleum Exploration and Development. 2017, 44(3): 452-461. https://doi.org/10.11698/PED.2017.03.16
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  • A new type of anti-heave semi-submersible drilling platform
    CHEN Bo, YU Zhiyong, LYU Yong, LI Xiaojian, LI Chunfang
    Petroleum Exploration and Development. 2017, 44(3): 462-468. https://doi.org/10.11698/PED.2017.03.18
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    A new type of semi-submersible drilling platform is designed. The numerical simulation software is used to analyze the heave response of the new platform in the frequency domain and time domain, and the semi-submersible drilling platforms with double-floating four-column structure and heave plate structure are compared with the new platform. This paper introduces the design principles of the new platform and the theoretical basis, mathematical model and boundary conditions during correlation analysis. The numerical simulation results show that the maximum and mean values of the heave response of the new platform are significantly reduced in the frequency domain analysis compared with the double-floating four-column and the heave-plate structure platform; and the new platform has a significant increase in the natural heaving period of the new platform, which can effectively prevent the occurrence of resonance. In the mooring time domain coupling analysis, the surge, sway and roll response of the new platform is small, and the heave response is greatly reduced. In the spectral analysis, the new platform has a smaller peak response and better wave frequency characteristics. The new platform has excellent anti-heave performance, reasonable structure and feasibility, and can provide reference for the design and selection of new generation semi-submersible drilling platform.
    • COMPREHENSIVE RESEARCH
  • Genesis and reservoir significance of multi-scale natural fractures in Kuqa foreland thrust belt, Tarim Basin, NW China
    LIU Chun, ZHANG Ronghu, ZHANG Huiliang, WANG Junpeng, MO Tao, WANG Ke, ZHOU Lu
    Petroleum Exploration and Development. 2017, 44(3): 469-478. https://doi.org/10.11698/PED.2017.03.17
    Abstract ( ) Download PDF ( ) Knowledge map Save
    Natural fractures in the deep Cretaceous Bashijiqike Formation sandstone reservoirs of Kuqa foreland thrust belt, NW China, are classified according to fracture aperture based on the data of outcrops, cores, thin sections and imaging logging, using industrial CT scanning, laser scanning confocal microscope (LSCM), cathodoluminescence (CL), electron probe, and scanning electron microscopy (SEM). The types, characteristics, genesis ages and formation sequence as well as reservoir significance of such natural fractures are examined. Four categories of fractures are classified. Category I (aperture>100 μm) is macro structural fractures, which cut single sand body to form the dominant migration pathway, helping to increase the reservoir permeability. Category II (aperture=10-100 μm) is associated micro structural fractures, which cut matrix grains to connect large matrix pores and improve the seepage performance. Category III (aperture=1-10 μm) is micro digenetic fractures at grain edges, which connect medium and small pores to improve the pore network connectivity and the gas migration and charging efficiency. Category IV (aperture<1 μm) is nano-scale matrix fissures, which connect intragranular micro pores to expand the reservoir space, thereby increasing the reserves scale. Category I and Category II fractures were developed in three stages (early, middle and late). The early- and middle-stage fractures, predominantly half-filled-filled fractures, were formed before early Pliocene when extensive oil and gas charging had not occurred. The late-stage opened fractures were formed after the Late Pliocene, they were at the same time as or later slightly than extensive oil and gas charge. The fracture network has low contribution to porosity, but it can improve the permeability by 2-3 orders of magnitudes in the parallel direction of fractures.
    • ACADEMIC DISCUSSION
  • Lower limits and grading evaluation criteria of tight oil source rocks of southern Songliao Basin, NE China
    LU Shuangfang, HUANG Wenbiao, LI Wenhao, XUE Haitao, XIAO Dianshi, LI Jijun, WANG Min,WANG Weiming, CHEN Fangwen, ZHANG Jun, DENG Shouwei, TANG Zhenxing
    Petroleum Exploration and Development. 2017, 44(3): 479-486. https://doi.org/10.11698/PED.2017.03.19
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    With southern Songliao Basin as the target area, the hydrocarbon expulsion intensity of source rocks was quantitatively evaluated based on the material balance method. Under the “Overpressure” module of PetroMod software, the overpressure history of the source rocks was evaluated. According to the relationships among hydrocarbon expulsion intensity, residual organic carbon content and overpressure within source rocks and related inflection points, the lower limits of tight oil source rocks were determined: hydrocarbon expulsion amount of per unit mass rock 2 mg/g, residual hydrocarbon content 0.8%, and overpressure 1 MPa. The source rocks with hydrocarbon expulsion > 8 mg/g, a residual organic carbon content 2.0%, and overpressure 7 MPa were defined as the limits of excellent source rocks. As a result, the tight oil source rocks can be divided into three types, excellent source rocks (typeⅠ), inefficient source rocks (typeⅡ) and invalid source rocks (type Ⅲ). The evaluation has been made for favorable areas distribution range of tight oil in Southern Songliao Basin according to the lower limits and grading evaluation criteria of tight oil source rocks. The result shows that the excellent source rocks have an obvious control on the distribution of tight oil, areas with excellent source rocks and nearby formations are favorable for the accumulation of tight oil.
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