2020 Vol. 26, No. 6

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Deformation characteristics of Early Paleozoic marine shale and their influence on the shale gas preservation in the eastern Sichuan-Wulingshan tectonic belt
LI Chunlin, LI Xiaoshi, WANG Zongxiu, LIANG Mingliang, ZHANG Kaixun, TAN Yuanlong, TAO Tao, GAO Li
2020, 26(6): 819-829. doi: 10.12090/j.issn.1006-6616.2020.26.06.064
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Two sets of marine shale systems (the Lower Cambrian Niutitang Formation and the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations) developed in the Lower Paleozoic of the eastern Sichuan-Wuling tectonic belt are not only significant decollement zones,but also key strata for shale gas exploration and development. In this paper,through detailed field structural analysis (macroscopic scale) and indoor microscopic observation (microscopic scale),it is considered that there are at least two stages of structural deformation developed in shale: top-to-the-NW/SE thrusting (D1) and the faults cutting through the bedding (D2). The deformation and strength characteristics of shale are related to the change of distance from the regional fault zones. The deformation characteristics of shale far away from the fault zone are mainly manifested as nearly vertical microcracks,belonging to the brittle domain; near the fault zone,the deformation characteristics of shale gradually show brittle-to-ductile transition and the development of mylonitization structures; within the fault zone,strongly foliated and mylonitization structures developed in the shale indicating the deformation of shale belongs to the ductile domain. By means of ion-milled backscatter SEM,the pore-structure evolution characteristics of deformed shale were analyzed. The result showed that not only the pore types in shale can be transformed,the size and distribution characteristics of pores can also change with the deformation strength. On this basis,the influence of Lower Paleozoic shale deformation on shale gas preservation in the complex structural area of South China is further discussed. It is suggested that the D1 can change the pore system developed in the shale,which is conducive to shale gas enrichment. The extension or strike-slip shearing (D2) can not only destroy the oil and gas traps formed previously,but also cause oil and gas migration from the high potential zone to the low potential zone along the fault zone,thus leading to oil and gas loss.
Effect of shear on the development and adsorption capacity of organic pores in shale
YU Yuxi, WANG Zongxiu, FENG Xingqiang, ZHANG Kaixun, ZHANG Linyan
2020, 26(6): 830-839. doi: 10.12090/j.issn.1006-6616.2020.26.06.065
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Organic pore is the most important type of storage space for adsorbed gas and free gas in highly mature shale reservoirs. The marine shale formations in South China experienced complex structural deformations and the detachment structures widely developed. To investigate the effect of shear-induced deformation on the microscopic structure and adsorption capacity of organic pores in shale,the lower Cambrian Niutitang shales in the Sancha outcrop in Zhangjiajie were taken as an example. Based on scanning electron microscope observations and statistical parameter analysis of pore images,the development characteristics of organic pores from the detachment belt,the near-by detachment belt and the far-away detachment belt were compared. The methane isothermal adsorption tests were also carried out on the three types of samples. The results show that the pores developed within the organic matters have dominant pore sizes less than 20 nm and the organic matter-mineral related pores have larger pore sizes and mainly developed in the organic matter encapsulating mineral fragments in the detachment belt. The organic pores experienced directional elongation and flattening under shear. The adsorption capacity of shales decreased under shear and the degree of such influence gradually decreased as the distance from the detachment belt increased. The shear has an important controlling effect on shale storage and gas-bearing capacity,which is significant to the understandings on shale gas preservation conditions and accumulation patterns.
Effect of structural deformation on permeability evolution of marine shale reservoirs
LIANG Mingliang, WANG Zongxiu, LI Chunlin, LI Huijun, ZHANG Linyan, FENG Xingqiang, ZHANG Kaixun
2020, 26(6): 840-851. doi: 10.12090/j.issn.1006-6616.2020.26.06.066
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Compared to North America,the geological conditions of marine shale reservoirs in South China are highly diversified and complicated due to the multi-tectonic movement,which transformed the structure of shale seams and resulted in structural deformed shale with unique reservoir properties. To investigate the effect of structural deformation on shale permeability,we experimentally examined the impacts of mineralogy,structural and fabric properties,and effective pressure on permeability evolution using two sets of undeformed shales(U) and deformed shales(D) collected from the Wufeng-Longmaxi Formations in the Upper Yangtze Block. Experimental analysis showed that the permeability between 0.2 and 2.69 millidarcies (mD) of strongly deformed shale samples were three orders of magnitude higher than the permeability (between 1.5×10-4 and 1.7×10-3 mD) of undeformed and slightly deformed shale samples. Meanwhile,strong deformation also reduces the pressure sensitivity of shale permeability when effective pressure increases from 700 PSI to 1200 PSI. These results indicated that accompanied by strong tectonic deformation,the shale permeability improved significantly,ant it would contribute to the development of macropore and micro-crack in the strong deformed shale samples. Moreover,the implications of permeability evolution with structural deformation is presented as that it would conducive to the transport and accumulation of shale gas,and may also lead to the leakage of shale gas in areas with poor seal conditions under the geological conditions of tectonic complex areas in South China.
New progress and breakthrough difficulties on shale gas geological survey in the lower Yangtze area
ZHENG Hongjun, ZHOU Daorong, YIN Qichun, XIONG Qiangqing, WANG Zhongpeng, FANG Chaogang, TENG Long, SHAO Wei, WANG Yuanjun
2020, 26(6): 852-871. doi: 10.12090/j.issn.1006-6616.2020.26.06.067
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Shale gas breakthroughs are in urgent need in the construction of clean energy system in the Yangtze River Economic Belt. The previous geological and geophysical data in the lower Yangtze area were fully integrated,and a 1:250000 shale gas geological survey was systematically carried out in the lower Yangtze area. Ten shale gas wells were drilled in the "two belts and one zone",which includes the depression belt along the Yangtze River in the southern regions of Jiangsu and Anhui,the Jiangxi Pingle-Zhejiang Qiantang depression belt,and the southern North China zone. The geological accumulation conditions of organic-rich shales in the Permian,Silurian,Ordovician and Cambrian were basically identified. Among those formations,"three types of gas and three types of oil",including shale gas,tight sandstone gas,coal bed gas,shale oil,fractured sandstone oil and fractured limestone oil,were discovered from nine wells. The great discoveries deepened our understanding of Permian and Silurian hydrocarbon accumulations. The hydrocarbon-bearing properties of the Triassic,Ordovician and Cambrian were explored. Meanwhile,the new zones and layers were established and the potential for hydrocarbon and shale gas exploration were revealed in the lower Yangtze and southern North China in the Meso-Palaeozoic. In the research,ten shale gas prospecting areas were delineated. The preliminary evaluation of the prospective resources of the main target layers in nine shale gas prospecting areas was completed and the problems of resource prospects were solved. At last the bottlenecks in the lower Yangtze shale gas exploration were discussed and the direction of breakthrough was put forward.
The characteristics and geological significance of highdensity methane-bearing inclusions in the Permian Gufeng Formation shale, southern Anhui
DONG Min, WANG Zongxiu, DONG Hui, ZHANG Linyan, SONG Wei, GUO Yingchun, LI Xiaoshi, FENG Xingqiang, MA Licheng, JI Changjun
2020, 26(6): 872-880. doi: 10.12090/j.issn.1006-6616.2020.26.06.068
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Samples of quartz veins were collected from the Permian Gufeng Formation siliceous shale fractures along the Changqiao profile,Jingxian County in southern Anhui. Inclusion composition analysis,thermodynamic temperature measurement,and laser Raman spectroscopy were carried out on the samples,and high-density methane inclusions were detected. The measured displacements of laser Raman scattering peak v1 of the inclusions vary from 2910.6~2911.2 cm-1,and densities of the methane inclusions were calculated as 0.2295~0.2618 g/cm3,indicating characteristics of high-density methane inclusions. The two-phase vapor-liquid brine inclusions coexisting with the methane inclusions have homogenization temperatures ranging from 216.8 ℃ to 242.3 ℃. The combination of methane inclusion composition by laser Raman spectroscopy with the simulation of high-density methane inclusions using the equation of state of supercritical methane system shows that the trapping pressure at 216.8 ℃ was 76~95 MPa. By calculating the density of the high-density methane inclusion and the capture pressure,it reveals the evidence for high-pressure methane yield in the Permian Gufeng Formation highly evolved siliceous shale in geological history,and provides scientific grounds for the exploration and evaluation of Permian shale gas in southern Anhui.
Target window spatial distribution prediction based on network fracability: A case study of shale gas reservoirs in the Changning Block, southern Sichuan Basin
SHEN Cheng, ZHAO Jinzhou, XIE Jun, FAN Yu, SONG Yi
2020, 26(6): 881-891. doi: 10.12090/j.issn.1006-6616.2020.26.06.069
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Target window optimization has significance for marine shale gas reservoir stimulation. Taking the marine shale gas reservoir in the Longmaxi Formation of the Changning Block in the southern Sichuan Basin as the research object,argon ion polishing scanning electron microscope,Brazilian disk experiment and fracturing operation statistics were carried out and the influence of mineral size,cracking toughness and crustal stress on hydraulic fracture penetration,propagation and diversion were analyzed. In view of the inadequate fracturing-quality effect of the current methods on the prediction of target window in shale gas reservoir,a new target window prediction method based on the network fracability theory,illustrated by the morphology of logging curves,was established,using brittle-ductile index calculated by sonic,density,gamma and mineral content logging data,and stress difference index calculated by the optimal stress-stress difference coupling model derived from the mechanics theory and experiment in a fast track. The results show that the Ⅰ-type target window,showing a considerable overlap between the curves of brittle-ductile index and stress difference index with "double-trough",is the most favorable for development,followed by the successive cross-overlap as the Ⅱ-type and the discontinuous cross as the Ⅲ-type. The application in the Changning Block shows that most of target windows in each well have the "double-trough" shape,but the corresponding layer of each target window is different. The target windows from west to east have changed from the 1 layer to the 2 layer and then back to the 1 layer. The high drilling-encounter rate of various target windows in different well-controlled zone positively controlled the stimulation reservoir volume,test productivity and cumulative productivity.
Simulation of oil-gas accumulation of clastic rocks and potential areas in the Tahe Oilfield
FENG Xingqiang, SONG Haiming
2020, 26(6): 892-900. doi: 10.12090/j.issn.1006-6616.2020.26.06.070
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To systematically study the oil-gas migration and accumulation of clastic rocks in the Tahe Oilfield,we carried out the simulation of oil-gas migration and accumulation using the comprehensive method of flow routing and percolation on the basis of the geologic character in the study area. The simulation of oil-gas migration and accumulation based on the fault-sandstone migration pathway system were carried out. Based on the study of fault classification and stages,the criterion of fault opening was formed by combining with the actual data at first,and then the model of fault migration framework was established. Using compaction curves and the relationship between porosity and permeability,we built the sandstone migration framework by facies-controlled modeling technology. The simulated results are in good agreement with that of the discovered hydrocarbon reservoirs. It indicates that the method is valid. We pointed out the potential areas by the simulated result as well. The works in this paper have significance and practical value for the exploration of clastic rocks in the study area.
Classification of sedimentary basins and distribution patterns of oil and gas resources in the Arctic
ZHANG Kaixun, HAN Shuqin, MENG Qiuhan, HU Jingjing, YAN Chunfeng
2020, 26(6): 901-910. doi: 10.12090/j.issn.1006-6616.2020.26.06.071
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The Arctic has great potential for oil and gas resources, and the exploration and development of oil and gas are still in the early stage. It is an important strategic region for the future petroleum exploration. On the basis of up to date data from IHS and USGS as well as investigation results of other studies, this paper aims at documenting the basin types and the distribution patterns of oil and gas resources in the Arctic. Our results show that there are at least 35 sedimentary basins in the Arctic, which consist of five rift, sixteen passive margin, five intra-cratonic, five forland and four oceanic basins. Rift basin is the type with the most abundant oil and gas resources in the Arctic, followed by the forland, passive margin and cratonic basin types. The proved and probable oil and gas reserves in rift basins amount to 44.112 billion tons of oil equivalent and accounts for 74.6% of the total reserves in the Arctic. Most of the oil and gas reserves in the Arctic are reservoired in the clastic reservoirs in the Cretaceous, Jurassic and Permian with a minor proportion in the Carboniferous and Devonian carbonates. The age and lithogogy of main reservoirs are different in different countries and petroliferous basins. Hydrocarbons in the Arctic were mainly derived from Jurassic and Cretaceous mudstones and shales, followed by Triassic and Devonian mudstones and shales. The trap types are dominated by combination trap, followed by structural trap. The investigation results of this study will establish a sound foundation for long-term oil and gas exploration programs of Chinese oil companies in the Arctic.
Distribution characteristics and genetic analysis of the abnormal fluid pressure in the middle Permian in the northern fault-fold zone of the western Sichuan Depression
LIU Wen, QIU Nansheng, MA Yinsheng, XU Qiuchen
2020, 26(6): 911-922. doi: 10.12090/j.issn.1006-6616.2020.26.06.072
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The temperature and pressure field is the key point for studying on the mechanism of hydrocarbon accumulation in the petroliferous basin, and the reconstruction of paleo-pressure particularly plays an important role in the study of hydrocarbon migration and accumulation, and even diagenesis in reservoirs and hydrocarbon generation for source rocks. In this paper, with pressure distribution as constraint, the paleo-pressures in the middle Permian of the northwestern Sichuan Basin were reconstructed by adopting basin modeling. In the northwest fault-fold belt of the Sichuan Basin two stages of overpressure developed, in the middle Permian, especially from the late Jurassic to the end of early Cretaceous.The overpressure accumulated rapidly in reservoirs under the effect of compressive stress which varied greatly in different structural units. The weak overpressure developed near the orogenic belt, which recovered to normal pressure after the late Cretaceous; but the strong overpressure developed in the fold and foredeep zone far from the orogenic belt even after the structure uplift, which was mainly controlled by the external compressive stress from northwest.
Logging characterization of Carboniferous fracturedvuggy karst reservoirs in the eastern Qaidam Basin
PENG Bo, ZHANG Hao, YANG Shenghao, WANG Shuai, YANG Yuanyuan, SHI Hui, HU Junjie, FANG Xinxin
2020, 26(6): 923-931. doi: 10.12090/j.issn.1006-6616.2020.26.06.073
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Since the Carboniferous reservoirs are characterized by low porosity and permeability in the Qaidam Basin, the later diagenetic transformation is particularly important for oil and gas exploration. Karstification of different scales, such as pores, fractures, and dissolution residual mounds, have been found in the Carboniferous of many wells in the eastern Qaidam Basin. In addition, some well data shows obvious gas logging abnormalities in this section. Therefore, the characteristics and the logging response of fractures and caves are the necessary study issues for Carboniferous reservoirs in this area. Basing on petrology and the logging data, and the logging response of paleokarst fractures, the study shows that the Carboniferous karst fractures and caves can be divided into two types: structural faults related-type and weathering crust-type. The later one commonly develops in the first and second sections of the Keluke Formation, the Zhabusagaxiu Formation, and the boundary between the Huaitoutala Formation and the overlying formation. The horizontal comparison of the characteristics of the two types of karst fractures and caves shows that the value of natural potential is negative, the natural gamma is medium and low (20.5~35.0 API), the acoustic jet difference is abnormally increased, and the neutron (13.9%) and density (2.50 g/cm3) have mirror image feature, resistivity (215.2 Ω·m~1100.0 Ω·m) is lower than that of surrounding rock (>1100.0 Ω·m). The mature weathering crust-type karst reservoir could be classified into five structural units according to the logging curves, the overlying layer, paleosol layer, cave layer, fractured-vuggy layer, and tight layer. In general, the weathering crust-type karst reservoir was more dominant in the study area, with larger alteration intensity. However, these two types of reservoirs were also associated with each other in many cases. Except for their different alternation mechanisms, they shared similar timing in terms of their responses to tectonism, which was speculated to be from the Indosinian period to the Early Middle Jurassic and even the Early Cretaceous.
Characteristics and origin of crude oils in the Wulanhua sag
FENG Weiping, WANG Feiyu, WANG Zongxiu, SHI Yulei, JIANG Tao, WANG Yufan, ZHOU Hongfeng
2020, 26(6): 932-940. doi: 10.12090/j.issn.1006-6616.2020.26.06.074
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The Wulanhua sag is a newly discovered oil-rich sag in the Erlian basin. The physical and geochemical properties of crude oils in the Wulanhua sag were systematically analyzed to reveal the characteristics and origins. The physical properties of crude oils suggest that the API gravities range from 20.2° to 40.0°, and most are normal crude oil. The biomarker parameters show considerable variation among the oils from different tectonic units, which can be separated into two crude oil families. Crude oil family-I is dominated by oils from the Tumuer tectonic unit, and characterized by low Pr/Ph and C21/C23 TT, and relatively high gammacerane/C31 hopane and regular sterane/C30 hopane. Crude oil family-I is mostly derived from the algal organic matter at low maturity levels. Crude oil family-Ⅱ consists of oils from the Saiwusu and Hongjing tectonic units, which is characterized by high Pr/Ph and C21/C23 TT, and relatively low gammacerane/C31 hopane and regular sterane/C30 hopane. Crude oil family-Ⅱ is mainly derived from the mixed algal and terrigenous organic matter at higher maturity levels. Carbon isotopic compositions of crude oil and individual n-alkanes indicate that these two crude oil families should be the products of the same set of source rocks at different maturity levels, and likely derived from the K1ba source rock in the southern sub-sag. The factors controlling differences between the two crude oil families include the heterogeneous source rock and its maturation process.
Comparison and exploration of hydrocarbon expulsion patterns of different types of source rocks
HU Jinjie, TANG Youjun, HE Daxiang, FU Ning, LI Meijun
2020, 26(6): 941-951. doi: 10.12090/j.issn.1006-6616.2020.26.06.075
Abstract (426) HTML (204) PDF (6436KB)(38)
Hydrocarbon expulsion from source rocks is a very important part in the accurate prediction of oil and gas resources in petroliferous basins. In order to obtain the hydrocarbon expulsion efficiency and mechanism, the source rocks of different organic matter types were selected for the gold tube simulation experiments, product change characteristics and hydrocarbon expulsion efficiency of different types of source rocks in different evolution stages are summarized. The results show that, the type has a great influence on the proportion of light and heavy hydrocarbons in total oil and residual oil, but has little on that in discharged oil; the discharged oil is dominated by heavy hydrocarbons in the immature-mature stage and light hydrocarbons in the high-mature-overmature stage. The type has obvious influence on the amount of oil generation and discharge. The better the type of source rock is, the higher the amount of oil generation and discharge is. The oil generation and discharge amount of typeⅠsource rocks is the highest and that of typeⅢsource rocks is the lowest. The better the type of source rock is, the higher the oil discharge efficiency is. The low oil discharge efficiency of typeⅢsource rocks is related to the high generation of gaseous hydrocarbon and the high content of vitrinite in macerals.
Organic geochemistry and geological significance of oil seepage from the Devonian of Luquan area, Yunnan Province
MA Zhongliang, TAN Jingqiang, ZHAO Han, ZHENG Jiaxi, ZHAO Zhongxi
2020, 26(6): 952-960. doi: 10.12090/j.issn.1006-6616.2020.26.06.076
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With the successful exploration and development of marine conventional natural gas and shale gas in the Sichuan Basin, the oil and gas explorations in new areas and new strata are gradually put on the agenda. Devonian shales in Yunnan are widely distributed in local areas with high abundance of organic matter; however, little drilling work and research have been done there so that the oil and gas enrichment is unknown. Through the field investigation, liquid oil seepages were found in the Huaning Formation of Middle Devonian in Luquan area of Yunnan Province. The organic geochemical analysis shows that oil seepage had a good relationship with marl in the third section of the Huaning Formation of Middle Devonian. The marls in the third section of the Huaning Formation of the Middle Devonian were generally deposited in a strong reduction-reduction environment and were in their mature stage. Hydrocarbon generation materials were mainly algaes and other lower aquatic organisms, and higher plants did contribute too. They had strong hydrocarbon generation ability, equivalent to that of type Ⅱ1 source rock. The Devonian marls in the western margin of the Kangdian ancient land once produced oil and gas, and the crude oil produced were mainly saturated hydrocarbon and aromatic hydrocarbon, which has certain potential for shale oil and gas exploration and conventional oil and gas exploration under good preservation conditions.
Discussion on the depositional timing of the Zhabusagaxiu formation in the eastern Qaidam Basin, China
MA Licheng, JIANG Wan, XIAO Zhouxuan, LI Zongxing, PENG Bo, HU Junjie, DONG Min
2020, 26(6): 961-972. doi: 10.12090/j.issn.1006-6616.2020.26.06.077
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The Carboniferous hydrocarbon exploration in the Qaidam Basin, Qinghai province in northern China, is the focus in the investigation of newly developed area and new series of strata. The Carboniferous sedimentary strata are fully exposed in the eastern part of the Qaidam Basin, and it has been assumed that there are no sedimentary strata of Permian. Nowadays, it is still controversial whether the Zhabusagaxiu formation belongs to the Carboniferous or Permian. In this paper, detrital zircons from sandstones of the Zhabusagaxiu formation were collected and the zircon U-Pb LA-ICP-MS isotopic dating was carried out. The harmonious ages of zircons were obtained, ranging from 2488±34 Ma to 288±3 Ma. The weighted average age of the youngest group is 288.0±2.0 Ma (MSWD=0.112, n=21). This means that the deposition of sandstones in the Zhabusagaxiu formation happened after 288 Ma, later than the international Carboniferous-Permian boundary (299 Ma). Based on the field investigation and previous research, there are abundant fossil families of Fusulina in the Zhabusagaxiu formation, and especially the appearance of pseudoschwagerina or spyroschwagerina is a crucial sign of entering the Permian. Consequently, combined with the zircon geochronology of classic rocks and typical fossil characteristics in the strata, it is suggested that the Zhabusagaxiu formation should belong to the Early Permian, indicating the existence of Permian stratigraphic deposition in the eastern Qaidam Basin. This recognition provides a geological basis for deepening the sedimentary formation and structural evolution from the late Paleozoic to Mesozoic in the Qaidam Basin.
2020, 26(6): 973-976.
Abstract (58) PDF (1805KB)(6)