Geoscience

Metallogenic Chronology and Geochemistry of the Zhuanshanzi Gold Deposit, Inner Mongolia

OUYANG Xin, GU Xuexiang, ZHANG Yongmei, LIU Li, LIU Tao, WANG Wendong

2020, 34(04): 635-652. DOI: 10.19657/j.geoscience.1000-8527.2020.04.01

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Zhuanshanzi gold deposit is a medium-high temperature magmatic hydrothermal deposit in the combination of North China Plate and Xingmeng orogenic belt, while its depositionl age and tectonic setting are still in debate. In this study, an integrated analysis of isotope chronology and geochemistry for the Zhuanshanzi gold deposit was carried out. The results give a concordant age of (264±3.8) Ma for the ore-hosted rhyolite, (237±1.4) Ma for the ore-hosted porphyritic granite, (219±4.7) Ma for the ore-intercalated diorite. The ore-forming age is limited to 237-219 Ma. The Zhuanshanzi gold deposit is generated in Triassic.The petrogeochemical analysis shows that the (264±3.8) Ma rhyolite is characterized by high content of K₂O and low concentration of MgO, with enrichment in Rb, Th, U, K, Pb, Nd, Hf, Gd and depletion in Ba, Nb, Ce, Sr, P, Ti.The (237±1.4) Ma porphyritic granite is characterized by low concentration of P₂O₅, with enrichment in Rb, Th, U, K, Pb and depletion in Ba, Nb, Ta, Ce, Sr, P, Zr, Ti. The (219±4.7) Ma diorite is characterized by high contents of K₂O, MgO and low concentration of P₂O₅, with enrichment in Rb, K, Sr and depletion in Nb, Ta, P, Ti. In a word, the (264±3.8) Ma rhyolite is formed from the magma that remelting of the lower crust due to the upwelling of the lithospheric during the period that the paleo-Asian ocean subduction to North China Plate. The (237±1.4) Ma porphyritic granite and the (219.7±4.7) Ma diorite generated in the extensional environment after the collision between North China Plate and Siberian Plate.

The Construction and Application of 3D Geological Model of F₇ Structural Belt in Xiangyangping Uranium Deposit, Middle Segment of Miao’ershan Rock Mass, North of Guangxi

LIU Zijie, ZOU Mingliang, SUN Yuanqiang, LI Jie, CHEN Qi, ZHU Pengfei, ZHANG Tao

2020, 34(04): 653-662. DOI: 10.19657/j.geoscience.1000-8527.2020.04.02

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In recent years, 3D geological modeling technology has been widely researched and applied in three-dimensional spatial structure analysis and metallogenic prediction along with the development of computer technology. F₇ structural belt is an important ore-bearing structure of the Xiangyangping uranium deposit in north of Guangxi, and several rich ore bodies have been exposed. In this paper, GOCAD is used as the modeling platform and F₇ structural belt is the key research object. The three-dimensional geological model of the working area is established by using the digital geosciences information such as geology, drilling and geophysical prospecting of the Xiangyangping uranium deposit, which visually displays the rock mass, structure and ore body of F₇ structural belt. The model shows that the boundary of rock mass is complex and variable and the ore body is strictly controlled by the fault zone. At the same time, on the basis of 3D model, the semi-quantitative simulation of geological alteration information is realized through geostatistical method by the digitization of geological catalogue data. The results show that the deep overlapped sites with strong hematitization, silicification, potash feldspathization and hydromicazation are likely to have rich ore bodies. Through the comprehensive study and analysis of the 3D geological model in the working area, it is concluded that the deep south of F₇₁₀ structure has a good metallogenic potentiality, which provides a scientific basis for the next prospecting direction of F₇ structural belt in Xiangyangping uranium deposit.

Spatial Distribution and Source Analysis of Heavy Metal Pollution in Soils Around Recycled Lead Industrial Park

LI Pengfei, LIU Chao, TAO Chunjun, WANG Jing, WU Zheng

2020, 34(04): 663-671. DOI: 10.19657/j.geoscience.1000-8527.2020.04.03

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In this study, the concentration, spatial distribution, occurrence and sources were analyzed in the top 20 cm soil and two 300 cm soil profiles in and around recycled lead industrial park in Anhui. The results suggest that the average content of Cd, Hg, As, Pb, Cr, Cu, Ni and Zn were 300.4 ng/g, 39.45 ng/g, 12.22 mg/kg, 42.5 mg/kg, 75.4 mg/kg, 28.5 mg/kg, 33.5 mg/kg and 74.1 mg/kg respectively. The most polluted heavy metals were Cd, Pb and Hg. Apart from Hg, Cd and Pb in the top soil mainly came from industrial activity. The Cd polluted soil covered an area of 21 km² with 65 cm depth. Pb polluted soil covered an area of 12 km² with 20 cm depth. Mercury in soil was moderately enriched in the top section of 65 cm. As to the occurrence, Cd were mainly bounded to carbonate, and secondly with iron-manganese oxides. Pb was mainly bound to residual and iron-manganese oxides. The content of Cd in carbonate and iron-manganese oxides and Pb in residual and iron-manganese oxides are positively correlated to their total concentration respectively. The soil quality of the study area is relatively good, as only a few samples (Cd: 9.5%; Pb: 5.4%) with the metal concentration are beyond the GB 15618—2018 standards of soil environmental quality and soil pollution risk control standard for agricultural land.

Comparative Study of Different Evaluation Methods of Se-rich Soil in Main Grain Producing Area of Cixi, Zhejiang Province

WANG Baoxin, WEI Jikang, YU Xiaoxia, HU Rongrong

2020, 34(04): 672-679. DOI: 10.19657/j.geoscience.1000-8527.2020.04.04

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A total of 94 samples of rice and soil were collected in functional zone of grain production in Cixi City, Zhejiang Province. Soil samples were tested for total content and speciation of selenium, pH and organic matter, and rice samples were tested for selenium. The characteristics of selenium in soil and rice were analyzed. According to the standard of Se-rich soil and multivariate correlation analysis theory, the evaluation method which based on total selenium in soil was compared with which based on total selenium, physical and chemical indicators of soil. The results indicated that 22.3% of soil samples and 53.2% of rice samples reached Se-rich standard, while selenium in soil and rice showed moderate variation, proving low-level impact of external factors and same geological background. Otherwise, there were significant differences between the two evaluation me-thods, the evaluation simply based on total selenium in soil commonly omited most of the Se-rich areas, and the Se-rich areas targeted by this method actually had lower rate of Se-rich rice. In comparison, the evaluation method based on total selenium in soil, physical and chemical indicators of soil outperformed the another evaluation method due to its better results of both more Se-rich samples and higher rate of Se-rich rice in targeted areas.

The Study on Alteration Information Extraction of GF-5 Hyperspectral Data in Vegetation Coverage Area: A Case Study of the Yushui Copper Deposit in Guangdong Province

LIAN Chenqin, YAO Fojun, CHEN Maohong, MA Kezhong, WANG Hailong

2020, 34(04): 680-686. DOI: 10.19657/j.geoscience.1000-8527.2020.04.05

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GF-5 satellite is China’s first hyperspectral integrated observation satellite. Since the launch of GF-5 satellite in 2018, we did not found any studies reported on alteration information extraction. In this paper, GF-5 hyperspectral data were used as the main data source, and the MNF algorithm, PPI algorithm and SAM technology were used to extract the altered mineral information in the vegetation coverage area of the Yushui copper deposit in Guangdong Province. The altered minerals extracted in this study include hematite, goethite, kaolinite, montmorillonite and muscovite. Based on the known geological and mineral resources, the extracted information was analyzed and verified. The verification results show that the information on the altered minerals extracted from the study area is in good agreement with the geological information. In summary, GF-5 hyperspectral data is feasible for the alteration information extraction in this area, and has important guiding significance for copper exploration. At the same time, this study also provides reference for the future research of alteration information extraction of GF-5 hyperspectral data.

Sedimentary Filling Characteristics of Luonan and Shanyang Basins and Constraints on Mesozoic-Cenozoic Intracontinental Evolution in East Qinling

YU Shangjiang, LI Wei

2020, 34(04): 687-699. DOI: 10.19657/j.geoscience.1000-8527.2020.025

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The east Qinling Orogen had experienced intracontinental orogenic tectonics since the Late Triassic, which formed a series of intermountain basins. The intracontinental evolution involved mainly intensive uplifting and magmatism. We took the Luonan and Shanyang basins in the east Qinling as a case study to analyze the sedimentary filling characteristics, to determine the phases of deformation on stratigraphic structures, and to constrain the Mesozoic-Cenozoic intracontinental evolution of the east Qinling Orogen. The results show that: (1) In Late Cretaceous,major faults of the Luonan and Shanyang basins were subjected to N-S-directed sinistral transtensional movement. Therefore, the basin basement had been activated via the N-S-trending fault depression. Intrusions in the detrital provenance region had been strongly uplifted, and then denudated and cooled, and the detritus were transported to the nearby basins. Various sedimentary systems (fan delta, lake and alluvial fan)were developed in the basins.(2)During Paleocene to Oligocene, sedimentation in the basins gradually waned. The deep faults had controlled the basinal development, as supported by structural evidence and apatite fission-track calculation results.Transtentional tectonics continued to wane, and we proposed that the Luonan and Shanyang basins may have been affected by non-E-W-directed compression, which suppressed the transtentional movement.

Tectonic Evolution of the Southeastern Gulf of Mexico Basin

WANG Kuo, LUAN Xiwu, RAN Weimin, WEI Xinyuan, ZHANG Hao, SHI Yanfeng, MOHAMMAD Saiful Islam

2020, 34(04): 700-709. DOI: 10.19657/j.geoscience.1000-8527.2020.039

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The southeastern Gulf of Mexico (GoM) basin is located in the collision zone and tectonic junction of the North American and Caribbean plates. It involves two major plates (North American and Caribbean) and two minorplates (Yucatan and Bahamas-Florida). We summarized previous research findings, including Deep-Sea Drilling Program (DSDP) data and the latest two-dimensional (2D) seismic data.We discussed the controlling effect of relative plate movement on the southeastern GoM basin,through identifying the seismic phase features and analyzing the tectonic evolution processes. Our main findings are as follows: (1) The southeastern GoM basin now contains the northern Cuba foreland basin, and the irregular central rifts of the passive continental margin in the northern part of the basin. (2) The region has experienced multiphase tectonism, and the South American and North American plates were separated from each other during the Early to mid-Middle Jurassic. The Cretaceous Caribbean plate thrust over the North American plate.Sea-level rose in the early Late Cretaceous and the Yucatan and Florida platforms uplifted. The northern Caribbean arc collided with the North American plate at the end Cretaceous. (3) In the late Early Cretaceous, many extensional faults were developed due to the extension/rifting between the Yucatan and Bahamas-Florida plates in the North American plate, forming the NW-trending (semi-) graben tectonic patterns.

Paleogeomorphologic Reconstruction, Evolution, and Sedimentary System Control on Wenchang Formation in Enping 12 Sub-sag, Eastern Pearl River Estuary Basin

LUO Ze, YAN Zhenghe, XIE Mingying, LIANG Jie, WU Jing

2020, 34(04): 710-717. DOI: 10.19657/j.geoscience.1000-8527.2020.036

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Lacustrine sedimentation of the Enping 12 sub-sag was developed in the Paleocene Wenchang Formation.The lacustrine-facies sediments were substantially modified by a combination of tectonism and magmatism, which severely hampers source rock identification and hydrocarbon exploration. Based on paleogeomorphologic reconstruction with trend extrapolation, the relation between paleogeomorphologic evolution and sedimentary depression of the Enping 12 sub-sag is revealed. The results indicate that: (1) The Enping 12 sub-sag is a typical half-graben in the Wenchang Formation that was intensely denudated. The main denudation occurred in the braided river delta deposits and shore-shallow lacustrine deposits in the lacustrine basin gentle slope zone. (2) The fan bodies are located in the steep slope zone.Based on this study, well A was drilled and oil-bearing sequence was discovered, which confirms that Enping 12 sub-sag is a potential deep lacustrine hydrocarbon source. It also demonstrates the reliability and importance of paleogeomorphic reconstruction in studying sedimentary infill of modified depressions, and provides insight on estimating hydrocarbon resource potential for strongly modified depressions in the eastern Pearl River Estuary basin.

High-resolution Stratigraphy of Upper Carboniferous-Middle Permian in Southern North China and Its Northern Surroundings

LI Yang, RUAN Zhuang, LI Zhongming, YU Bingsong, SONG Nannan, DONG Guoguo, ZHANG Dong

2020, 34(04): 718-731. DOI: 10.19657/j.geoscience.1000-8527.2020.021

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Permo-Carboniferous source rocks in southern North China are widely developed and become an important target for shale-gas exploration. However, regional study on the Carboniferous-Permian sequences is still inadequate due to the complex structure and unclear stratigraphic correlation. Based on high-resolution stratigraphy, sedimentology, core-logging, and seismic data from twelve wells, we first establish the stratigraphic sequence of the region. The results show that the Taiyuan, Shanxi and Xiashihezi formations belong to an incomplete super-long cycle, which could be divided into five long-term cycles and 14 medium-term cycles. The medium-term base-level circle structure patterns can be divided into three types:A and B, incomplete symmetrical type dominated by (A) ascending hemicycle and (B) descending semicircular gyrus; C, nearly half ascending and half descending with the same type of symmetry. A-type patterns are mostly developed in the nearshore delta sedimentary environment, whilst B-type patterns are mostly developed in the intertidal to subtidal/lagoon environment. Meanwhile, C-type patterns are common in shallow-water (deltaic or tidal) environments. Geostatistical analysis of the stratigraphic sequences (from north to south and from east to west) has found no regional discontinuities or unconformities.Due to local late-tectonic influence, the seesaw-style uplift may have generated local discontinuities, which formed the pseudo-unconformity between the upper and lower parts of the sequence.

Paleogene-Neogene Pollen and Climate Change in Dahonggou Region of Qaidam Basin

LU Jingfang, ZHANG Kexin, SONG Bowen, XU Yadong, ZHANG Jianyu, HUANG Wei, ZHANG Daolai

2020, 34(04): 732-744. DOI: 10.19657/j.geoscience.1000-8527.2020.027

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Pollen samples were obtained from the northern margin of Qaidam Basin, where Paleocene to Pliocene lacustrine-facies sequences are exposed. Fourteen palynological assemblages have been identified by pollen identification and analysis, which shed light on the vegetation and climate change history of the basin. The pollen analysis results have recognized the Eocene Climatic Optimum, and revealed a vegetation evolution of northern subtropical ever green broad leaf forest→warm, temperate deciduous broad-leaved coniferous forest→temperate mixed forest→coniferous forest→arid mountain desert. The paleoclimate can be inferred from the vegetation correlation with the global climate trends, which matches the global climatic events. Cold to warm and dry to humid fluctuations can also be distinguished on the basis of the palynofloras recovered from the basin. A cold and dry period during 11-9 Ma, different from the Middle Miocene Climatic Optimum (MMCO),may have been caused by tectonic movements in the Qaidam Basin.

Discovery of Devonian Bryozoa in Longjiang, Guangxi and Its Geological Significance

LI Yuewei, WANG Chengshan, LI Guobiao, XU Xing, MA Wen’en, LI Xingpeng, LÜ Changliang, LÜ Beibei, ZHANG Wenyuan, HAN Dandan

2020, 34(04): 745-756. DOI: 10.19657/j.geoscience.1000-8527.2020.040

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Devonian stratigraphy is well-developed and widely distributed in Guangxi, which provides a good target for studying the Devonian stratigraphic sequence and paleontology. Bryozoans were first discovered from the Devonian in Longjiang area (Longlin, Guangxi),including 21 species of 13 genera:Cystiramus sp., Eostenoporahuishuiensis, Fenestella? sp., Fistuliporacollina, Fistuliramusparallelus, F.mishanensis, F.parallelus grandoporus, F.pusillus, F.sp., Heloporahastifera, H. ninglangensis, Leptotrypa adhaerens, L. cf. adhaerens, L. lamellata, Meekopora wangyouensis, Orthoporasinensis, Petalotrypa tumida, P. cf.tumida, Semicoscimum cf. dushanensis, Rhombopora sp., and Streblotrypa sp.Based on the sedimentary environment and ecological characteristics of bryozoans from the Devonian Longjiang, the local sedimentary evolution is described as follows: (1) In Early Devonian, the sedimentary environment belonged to sand flat subfacies(tidal flat facies) during the earliest Devonian (D₁l), then evolved into mixed-flat subfacies (shore intertidal zone with alternating mud flat and sand flat) (D₁h)→platform-concave (alcove) subfacies of a half-limited-open-platform facies (D₁d) →low-energy subtidal zone (D_1-2s¹)→high-energy subtidal zone-tidal-flat facies (D_1-2s²). (2)In Middle Devonian, with the seawater deepened, the area transited into low-energy subtidal sedimentary environment (D_1-2s²)→platform shelf subfacies of the platform basin (D_1-2d). (3)In Late Devonian, basin subfacies(platform-basin facies)occurred with seawater deepening during the early stage (D₃l),and the same subfacies maintained till the late stage (D₃w)in spite of the slightly shallowing of seawater.

Genetic Types, Sedimentation Models and Distribution Regularities of Sandbodies of Subaqueous Fan and Slopping Subaqueous Fan in Chengdao Low-uplift Slope Area

XIE Zongkui, WANG Youjie, JIANG Xiaolan, YANG Bin, ZHANG Zaizhen, NIU Mingchao, LUO Yang, LIU Wenfang

2020, 34(04): 757-768. DOI: 10.19657/j.geoscience.1000-8527.2020.04.12

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The main reservoir type of Dongying Formation in the eastern slope of Chengdao low-uplift belongs to deep-water sediment. Based on core-logging and seismic data, we study the distribution and connection of sandbodies from the developmental background, sedimentation process and response characteristics of the reservoirs. It is revealed that the deep-water reservoir of Dongying Formation is composed of sandy debris flow, argillaceous debris flow, turbidity current, and other gravity flow deposits that were modified by traction underflow. In the various sequence development stages, slope/slump subaqueous fans were developed. During the low systems tract sedimentation, floods carried terrestrial debris from weathering, denudation and collapse on slopes, and formed debris flow with point source supply. The debris flow was accelerated along the slope, and migrated laterally in an inclined, wide, flat, and long-distance underwater trough. Meanwhile, pure sandstone was precipitated by lake-water dilution and sand-mud differentiation, forming braided channels of sandy debris flow. Sandbodies are superimposed in many layers and show a mosaic reservoir structure. During the high systems tract sedimentation, unstable prograding fan delta at sedimentary slope break may have collapsed, forming block flows with line source supply. The block flows seeped into lake water and moved bumpily. Channels dominated by sandy debris flow, turbidity tongues dominated by turbidity and turbidity sheet sand modified by traction underflow were developed by dissociation, gravity variation and density stratification of the block flow in lake water. Sheet sandbodies are isolated from mudstone, forming pie-like reservoir structure. The sandbodies of slope subaqueous fans are distributed regularly, and are large and continuous, whereas those of slump subaqueous fans are distributed randomly, non-fragmented, in large number and with varying thicknesses.

Color Origin of the Lower Triassic Liujiagou and Heshanggou Formations Red Beds in the Ordos Basin

TAN Cong, YU Bingsong, YUAN Xuanjun, LIU Ce, WANG Tongshan, ZHU Xi

2020, 34(04): 769-783. DOI: 10.19657/j.geoscience.1000-8527.2020.04.13

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The Lower Triassic Liujiagou and Heshanggou formations (Fms.) of the Ordos Basin in China contain large-scale red sandstone-mudstone deposition. Due to their lack of fossils and marker beds, study of these strata is difficult and little research attention has been paid to them. In this study, the red beds of the Liujiagou and Heshanggou formations were studied by means of drill-core and outcrop observation, optical microscopy, scanning electron microscope (SEM), X-ray diffractometer (XRD) and whole-rock major-and trace-element analyses. In addition, comparison with other strata of different colors from around the red beds was performed. The XRD and SEM analyses show that the dominant clay minerals in the red beds are smectite±illite. Trace amounts of fine disseminated hematite on the smectite flakes of pore infill have given the red color to the rocks. Sedimentary facies and elemental geochemistry reveal that the red beds are the products of a hot arid climate and a strongly oxidizing environment. These red beds are of sedimentary origin. By comparing the red beds with the rocks of different colors, it is shown that the different colors of sedimentary rocks are due mainly to their mineral contents, which reflect the basinal tectonic evolution and changes in the climate and sedimentary environment.

Provenance and Tectonic Setting of Chang 10-Chang 8 Member in the Southern Ordos Basin

LI Yijia, RUAN Zhuang, LIU Shuai, CHANG Qiuhong, LAI Wei, YANG Zhihui

2020, 34(04): 784-799. DOI: 10.19657/j.geoscience.1000-8527.2020.04.14

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During the Middle-Late Triassic, all tectonic blocks surrounding the Ordos Basin were in different stages of tectonic evolution. Especially in the southern margin of the basin, the detrital source is still controversial. Provenance of the Yanchang Formation in the basin is still under much debate, especially the Chang 10 to Chang 8 section. In this study, the sandstone detrital component and geochemical statistics in the bottom of Yanchang Formation (Chang 10-Chang 8) are analyzed to unravel their provenance. The results show that the Chang 10-Chang 8 section have the characteristics of shallow-water, intermediate-proximal source deposition with weak oxidation. The major materials of the Chang 10-Chang 8 section may have formed in an island arc. Moreover, there are recycled-orogenic materials in Chang 8 in the western part of the southern margin. The Chang 10-Chang 8 member was likely originated from the Alxa block and Yinshan paleo-continent in the southern part of the basin, the Chang 10-Chang 9 member may have originated mainly from the Alxa block in the western part of the southern margin, whilst the Chang 8 member was likely a mixture of clastic materials from the Qilian and Qinling regions. This multi-source and varying material composition may have closely related to the orogenic activities around the Middle Triassic basin, indicating that this area is an ideal target to revel the source to sink system. Our findings can assist the reconstruction of the basin depositional environment, and the prediction of favorable reservoir sandbodies, which provides the theoretical basis for future oil and gas exploration in the study area.

Geochemical Characteristics of Source Rocks of M53 Well and Chang-8 Member Oil-source in Pingliangbei Exploration Area, Ordos Basin

ZHAO Yande, DENG Xiuqing, QI Yalin, SHAO Xiaozhou, YANG Bin, LU Xinchuan, LUO Anxian, XIE Xiankui

2020, 34(04): 800-811. DOI: 10.19657/j.geoscience.1000-8527.2020.04.15

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Based on organic petrology and geochemistry, the dark mudstones of the Chang-7 Member (M53 well, Triassic Yanchang Formation) at Pingliangbei on the southwestern Ordos Basin margin was analyzed. Molecular geochemical characteristics and fine oil source comparison of Chang-8 Member oil (M53 well) and its surrounding was studied. We newly reveal the geological and geochemical characteristics, hydrocarbon generation potential and oil source of the dark Chang-7 Member mudstone in the Pingliangbei area. The results show that the Chang-7 Member mudstone in Pingliangbei area has medium-high organic contents, of I-and II₁-type and mature, and was formed in freshwater-brackish water environment. The source rocks have relatively good hydrocarbon generation capacity, and share much geological/geochemical similarities with the high-quality source rocks inside the basin. The Chang-8 Member crude oil has similar characteristics to the Chang-7 Member source rocks, in terms of maternal properties, sedimentary environment and maturity, and both have good genetic relationship. The Chang-8 Member crude oil in the Pingliangbei area may have come from the Chang-7 Member source rocks at/around M53 well. Our results expand the range of source rocks and broaden the field and scope of Mesozoic oil and gas exploration at Pingliangbei in the southwestern margin of the Ordos Basin.

Application of Diamondoids in Geochemical Research of Deep Oil and Gas

YAN Jifa, MA Anlai, LI Xianqing, CONG Gangshi, HE Yukai, ZHANG Yachao

2020, 34(04): 812-820. DOI: 10.19657/j.geoscience.1000-8527.2020.04.16

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In recent years, oil and gas exploration in China has expanded to ultra-deep formations, and oil and gas maturity and reservoir preservation have become the focus of research. Because of their unique cage molecular structure, strong thermal stability and biodegradability, diamondoids have broad application potential in deep oil and gas exploration. In this paper, laboratory sample pretreatment and detection methods of diamondoids are described, and their genesis and geological evolution are summarized. New advancement in maturity, oil-cracking, biodegradation, thermochemical sulfate reduction (TSR), evaporative fractionation, and other identification methods with diamondoids are reviewed. Applications of diamondoids in the petroleum geochemistry are limited due to its unknown origin. It is proposed that comparative studies on the evolution process of diamondoids in different sedimentary environments, different types of source rocks and crude oils should be performed, in order to study the methyl-diamantane baseline of crude oils in different sedimentary basins. This can supplement and improve the applications of diamondoids, and can greatly enhance our understanding in deep hydrocarbon accumulation processes and guide deep hydrocarbon exploration.

Strontium Isotope Geochemistry of Ordovician Paleokarst Reservoirs in Huanghua Depression

ZHANG Jinning, ZHOU Jiansheng, FU Lixin, LI Hongjun, LOU Da

2020, 34(04): 821-827. DOI: 10.19657/j.geoscience.1000-8527.2020.04.17

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To study the strontium (Sr) isotopic characteristics of paleokarst reservoirs, the origin of Sr isotope in Ordovician karst reservoirs in Huanghua depression was discussed via isotopic experiments and comparison with the global Sr isotope background. Isotopic differences between the southern and northern parts of the Huanghua Depression were analyzed. The results show that the ⁸⁷Sr/⁸⁶Sr variation range of the Ordovician strata in the Huanghua depression is wider than that of the global Ordovician sedimentation, but their ⁸⁷Sr/⁸⁶Sr variation trends are similar. The Ordovician primitive carbonate strata have undergone late paleokarstification and fluid transformation, and the paleo-continental ⁸⁷Sr-rich fluids were used for various types of karst and filling. The dolomite ⁸⁷Sr/⁸⁶Sr values are distinctly higher than those of the limestone in the same horizon, suggesting that the dolomite captures ⁸⁷Sr more readily than the limestone. Duration of supergene karstification in the northern Huanghua depression is much longer than that in the southern part. The karstification intensity of the former is also higher, and its ⁸⁷Sr-rich fluid displacement stronger, which probably generates the higher ⁸⁷Sr/⁸⁶Sr values in the northern part of Huanghua depression.

Pore Structure Characteristics of Longmaxi Formation Shale of Well LD-1 in Laifeng, Hubei

HUANG Yuqi, ZHANG Peng, ZHANG Jinchuan, YANG Junwei

2020, 34(04): 828-836. DOI: 10.19657/j.geoscience.1000-8527.2020.04.035

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Pore structural analysis of the Longmaxi Formation shale in Laifeng (Hubei) was conducted based on geochemistry, petrophysics, and combined experiments on high-pressure mercury injection and low temperature N₂ adsorption-desorption. The Longmaxi Formation shale is over-matured with 0.41% to 2.35% TOC (total organic carbon) and 3.09% to 3.42% R_o (vitrinite reflectance). Mineral contents include mainly quartz (29.9% to 49.3%) and clay minerals (17.8% to 49.6%). Major pore types include inorganic inter-/intra-granular pores, micro-cracks and organic pores. The combined experiments show that the pore size is mainly micro-/meso-porous, pore throat is small and complex, and that the pore morphology is mainly fine-necked wide-body ink bottle shape. Based on the FHH model, the low relative pressure (P/P₀≤0.5) fractal dimension D₁ (2.73 to 2.76, average 2.74) and the high relative pressure (P/P₀>0.5) D₂ (2.80 to 2.89, average 2.85) are obtained. By establishing the relationship between fractal dimension and other pore structure parameters, it is concluded that fractal dimension can characterize the pore size, pore complexity and reservoir property, which can be an effective pore structure characterization parameter. Our results suggest that the Longmaxi Formation shale has high gas adsorption capacity, but the pore structure is complex and the connectivity is poor, yielding high requirements for reservoir transformation measures. Further research shows that the fractal dimension is positively correlated with the contents of organic matter, clay minerals and pyrite, but negatively correlated with the quartz content. Since the organic matter of Longmaxi shale in the study area is over-mature, its adsorption capacity is reduced, and the control of organic matter abundance on the shale adsorption capacity is unobvious.

Critical Controlling Factors of Enrichment and High-yield of Land Shale Oil

LI Hao, LU Jianlin, WANG Baohua, SONG Zhenxiang, LI Zheng

2020, 34(04): 837-848. DOI: 10.19657/j.geoscience.1000-8527.2020.04.19

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Clarifying the critical controlling factors of enrichment for high-yield continental shale oil is a key to prospecting target evaluation. Studies on shale reservoir indicate that in-situ accumulation is difficult to form high-yielding shale reservoir, which is characterized by short-distance migration. Therefore, mobility is critical for the enrichment of high-yield shale oil, and represents an important evaluation factor for shale oil block target selection. According to reservoir seepage mechanics and material balance calculation, it is proposed that shale oil abundance is the foundation, and fluidity and permeability are the key to high-yield and short-distance transport, which is important for oil enrichment. Research shows that residual oil abundance, maturity, crude oil viscosity (including gas to oil ratio), thin interlayers or cracks, and abnormal pressure are key factors for high-yield shale oil enrichment. The abundance of retained oil is the resource basis. The maturity and viscosity of the crude oil affect the fluidity of the shale oil. The degree of interlayer and fracture development determines the permeability of the shale formation, whilst the abnormal pressure is the dynamic condition. The inner hydrocarbon generation sag does not form high-yield shale reservoir easily, whereas high-in-sag and near-sag slope belt represent favorable structural belt. In general, the medium-high maturity (R_o,0.9% to 1.1%) organic-rich shale development area is prospective for shale oil enrichment. The above-mentioned high-yield controlling factors can be combined to understand the inner slope, inner slope proximal and deep area, and the relatively low-medium area (such as the low-profile anticline at the center of depression) are generally favorable sites for shale oil enrichment.

Hydrocarbon Accumulation Characteristics and Prospectivity Prediction on Medium-Shallow Sequences in Rift Basins: Case Study on the 2^nd Member of Shahejie Formation in Bonan Sag

LIU Peng, DU Pengwei

2020, 34(04): 849-857. DOI: 10.19657/j.geoscience.1000-8527.2020.04.20

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The medium-shallow traps of rift basin often covered source rocks, and the reservoir has the characteristics of multi-sourced hydrocarbons. To deepen the understanding of hydrocarbon accumulation characteristics and delineate favorable exploration targets on medium-shallow sequences in rift basins, we took the 2^nd member of the Shahejie Formation in the Bonan sag as an example. Based on data of drilling, logging, seismic and other analyses, we demonstrate that three source rock suites (Es₄^s, Es₃, Es₁) in the Bonan sag may have provided the oil and gas for hydrocarbon accumulation in Es₂, during the deposition of Upper Guantao Formation and Upper Minghuazhen Formation. Fault system is likely a key condition for hydrocarbon accumulation, which connects oil sources and provides a vertical channel for oil and gas migration. Physical properties of the reservoir determines the oil-bearing properties, which in turn determines the degree of oil endowment. Oil and gas migration and accumulation regularity of “two-stage reservoir-forming and multi-sourced hydrocarbon” exists in the 2^nd member of Shahejie Formation, and the single-sourced oil and mixed-source oil reservoirs were formed in different types of traps with the different hydrocarbon migration and accumulation regularities. The model of “multi-sourced hydrocarbon” and its major controlling factors indicate the favorable exploration direction of Es₂ in Bonan sag, The favorable exploration direction is likely the central beach-bar sandstone, the southern fan delta-front edge and the southwestern multi-type reservoir development zone. The focus of oil and gas exploration in each area is likely different, and so is the risk of exploration.

Distribution Characteristics of Cenozoic Volcanic Rocks and Its Geothermal Exploration Potential in Bohai Bay Basin

MAO Xiang, LUO Lu, WANG Xinwei, GUO Dianbin

2020, 34(04): 858-864. DOI: 10.19657/j.geoscience.1000-8527.2020.04.21

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The Bohai Bay Basin (BBB) is located in the Beijing-Tianjin-Hebei region, which is one of the most important areas of developing hydrothermal energy in China. According to the classification of basin types, the BBB belongs to a Mesozoic-Cenozoic superimposed fault basin. The extension of lithospheric controls the distribution of sags in the basin, and meanwhile leads to volcanic activities that affects the distribution of igneous rocks. The Cenozoic volcanic activities were closely related to rifting and depression in different regions of the BBB. In the Paleogene, the BBB entered the initial stage of rifting, and the tectonic volcanic activities were intense. The volcanic rocks erupted along large faults in the basin and were widely distributed. In the Neogene, the basin entered a depression stage, and the intensity of tectonic volcanic activities were obviously weakened. Since the Quaternary, volcanic activities in the BBB were weak and volcanic rocks were sporadic distribution. Volcanic activities of Paleogene to pre-Quaternary in the BBB can be divided into five stages, in which the volcanic apparatus and volcanic rocks formed by Quaternary volcanic activities are mainly distributed in the areas of vicinity of Cangzhou City and the coastal areas of Bohai Sea. The calculation results of radioactive heat generation rate of the latest basalt samples of volcanic apparatus found in Dashan and Xiaoshan are 1.62 to 2.07 μW/m³ and 1.87 μW/m³, which are higher than the average heat generation rate of Cenozoic rocks (0.80 μW/m³) in BBB, and also higher than that of basalt volcanic rocks of Cenozoic in other parts of China. The heat generation rate shows that the volcanic rocks formed during the Quaternary volcanic active period may be used as additional heat sources, forming geothermal anomaly areas and even potential areas for exploration of modern volcanic hot-dry rock resources.

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