Table of Content

20 August 2019, Volume 33 Issue 04

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Petrology,Mineralogy,Ore Deposits

LA-ICP-MS U-Pb Geochronology of Detrital Zircons from the Longwuhe Formation in Taxiu Area, the Junction Area Between the Qinling and Kunlun Orogenic Belts

LU Zongyue, YU Xinqi, LIU Mannian, ZHANG Zhen, WEI Liyong, ZHAO Changcun, ZHENG Xin, KOU Shaolei, XU An’min, WANG Zhaofan

2019, 33(04):  691-702.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.01

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The junction area between the Qinling and Kunlun orogenic belts has received wide attention for its unique geological compositions and complex orogenic structures. The preliminary analysis has been performed previously on the provenance of sediments in this area but still remained controversy. This paper selects clastic rocks from the Longwuhe Formation and uses LA-ICP-MS U-Pb ages of detrital zircons to restrict their provenance characteristics. Our results show that the zircon U-Pb ages can be divided into four groups, i.e., 2 746-2 084 Ma, 1 094-775 Ma, 492-411 Ma and 372-237 Ma. Among them, the 372-237 Ma group accounts for 56.79% of the total. This largest proportion age group is relatively concentrated with a evident peak, representing a series of tectonic magma events caused by the subduction of the A’nyemaqen ocean. The comprehensive analysis reveals that the sources of clastic sediments in the Longwuhe Formation are complex and diverse, including the northern margin of the Qaidam Basin, the Qilian orogenic belt and the Eastern Kunlun orogenic belt. The Eastern Kunlun orogenic belt is the largest provenance area of the Longwuhe Formation and provides most sedimentary materials.

Geochemistry and Geochronology of Early Cretaceous Granitic Intrusions and Tectonic Setting of Qieqiong Area in Tibet,China

HUANG Zesen, JIANGBA Duoji, DAWA Ciren, TAER Jie

2019, 33(04):  703-714.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.02

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A wide range of Early Cretaceous granitic intrusions were formed in the Qieqiong area in Tibet.To better understand the geological characteristics, intrusion age and petrogenetic environment, we have studied the geology, geochemistry and geochronology of intermediate granite rocks from the area. LA-ICP-MS zircon U-Pb dating shows that the granitoids were emplaced in the Early Cretaceous, from (129.3±0.8) Ma to (130.5±0.9) Ma.Geochemical analyses show that the granitoids are high in SiO₂(73.19%-76.7%), alkali(K₂O+Na₂O=7.81%-8.41%), with A/CNK=0.93 to 1.12, displaying strong peraluminous characteristics. The rocks have low CaO(0.32%-7.31%) and MgO (0.01%-1.76%), and distinct negative Eu anomalies (δEu=0.11-1.25). Primitive-mantle normalized multi-element spider diagram displays apparent enrichments in Rb, Th and Nd, and marked depletions in Nb, Sr, P, Ti and K.These intermediate granitoids show continental collision geochemical affinities. Considering the regional tectonic setting and published information, we infer that the granites at Qieqiong were formed in the a post-collisionalsetting, when crustal thickening and delamination occurred. The magmas were sourced by basaltic magma underplating and the subsequent partial melting of the middle-lower crust.

Geochronology, Geochemistry and Tectonic Significance of Linzizong Group Volcanic Rocks in the Nimu-Jiacha Area, Eastern Lhasa Block,Tibet,China

WEI Tianwei, YANG Feng, KANG Zhiqiang, WEI Naishao, LIU Di, CAO Yan, LI Daixian, CHEN Huan, CHEN Linhua

2019, 33(04):  715-726.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.03

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Lhasa block is featured by extensive Mesozoic-Cenozoic magmatism, which is important for understanding the Neo-Tethyan evolution and the tectonic history of the Tibetan plateau. In this paper, LA-ICP-MS zircon U-Pb dating and whole-rock geochemical analyses of the Bima Formation volcanic rocks in the Nimu-Jiacha area (eastern Lhasa block) were carried out. Zircon U-Pb ages of the Bima Formation volcanic rock are about 55.2-57.6 Ma, which are significantly younger than the published Pima volcanic rock age of 177-195 Ma, and are coeval with the Linzizong Group volcanic rocks (30.4-70.0 Ma). Accordingly, the Bima Formation that was originally thought to be in the area should be redefined as Nianbo Formation of the Linzizong Group. The main volcanic rocks in the study area are basalt, basaltic andesite, andesite and dacite, belonging to the middle-to high-K calc-alkaline series. The volcanic rocks are enriched in LILEs (e.g., Rb, Th, U) and show clear negative anomalies of HFSEs (e.g., Nb, Ta, Zr, Ti), with typical island arc geochemical affinities. The REE patterns reveal LREE enrichments, and no or moderately-negative Eu anomaly. In addition, the Nianbo Formation volcanic rocks in the study area are younger than the Nianbo Formation in the middle-western Lhasa block. Combined with published information, it is indicated that the India-Asia collision progressed from west to east.

Geophysical and Geochemical Characteristics of Cryptoexplosive Breccias at Tanshang, Daixian, Shanxi Province and Their Geological Significance

HOU Yanming, WANG Yunzhang, XU Yongjing, ZHANG Xuemei

2019, 33(04):  727-737.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.04

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Cryptoexplosive breccia type gold deposit is an important and economic deposit. We conducted a detailed geological investigation of the Tanshang Au-polymetallic deposit in Daixian County, Shanxi Province, and analyzed its geophysical and geochemical characteristics. Combining petrographic studies of the cryptoexplosive breccias, it is revealed that the Au-polymetallic ores were formed by the local enrichment of sulfides in the quartz porphyry, which is in the Tanshang intrusive complex, the endo-/exo-contact zones of the intrusion and xenoliths from the Sijizhuang Formation. The ore-forming materials were mainly from the post-magmatic hydrothermal altered Tanshang intrusion, while the ore host is the pyritized cryptoexplosive breccia. The faults and its secondary structures may have provided the fluid conduits and ore depositional sites for the mineralization. The ore-forming material and fluids were mainly from the post magmatic hydrothermal alteration of Yanshanian intermediate-acidic (sub-) hypabyssal rocks in this area.

Formation and Mineralogy of Porphyritic Biotite Hornblende Diorite in Qicun Pluton, Southern Hebei Province

GAO Xinyu, ZHANG Juquan, WANG Yiqi, SUN Tianqi, ZHAO Gaichao, WANG Linxuan, LI Yaqi, XU Cui

2019, 33(04):  738-750.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.05

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The Qicun pluton is a representative Mesozoic high-Mg diorite complex in the southern Taihang Mountains. It is mainly composed of porphyritic biotite hornblende diorite, hornblende diorite, gabbro-diorite and monzonite. In this study, formation and mineralogy of porphyritic biotite hornblende diorite in Qicun is studied in detail to reveal its petrogenesis and geological significance. We found that the hornblende belongs mainly to magnesiohornblende and magnesiohastingsite, and the biotite is magnesiobiotite. The core of the plagioclase is mainly andesine and labradorite, and the rim is mostly oligoclase. Hornblende thermobarometry shows that the hornblende phenocrysts were formed at 930-970 ℃ and 300-340 MPa, corresponding to crustal depth of ca. 10-15 km, with ${f_{O}}_{2}$ of ΔNNO +0.1 to +2.3 and magma water content of 4.8%-5.5%. Hornblende, associated with biotite that represents the final magma solidification, was formed at 684-760 ℃ and 42-61 MPa, with magma water contents decreased to 3.1%-4.3%, and slightly elevated ${f_{O}}_{2}$(ΔNNO +0.7 to +2.4). Additionally, results of biotite thermometry are in good agreement with those of the hornblende thermobarometry, supporting that the estimation of magma-forming conditions are reliable. Consequently, we suggested that the high-Mg magma in Qichun area with high oxygen fugacity was temporarily stored in the middle crust, and underwent hornblende-dominated fractionation. The volatile-rich magma was then rapidly emplaced and formed the high-Mg diorite and the associated iron skarn deposits.

Mineral Resource Spatial Association Analysis and Prediction:A Case Study in Western China

LIU Guo, WANG Yizhe, XUE Tao, WU Chenyao, XUE Bo, TANG Tiantian, LIU Shiming

2019, 33(04):  751-758.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.06

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Based on the similarity-analogy theory of concealed orebody prediction, a method of spatial location correlation analysis and prediction is designed and developed. By analyzing the published spatial distribution data of mineral resources, correlation of known mineral occurrences is performed to analyze and predict the concealed orebodies. Taking some areas of Western China as an example, the Apriori algorithm is used to analyze the relationship between the spatial location of known mineral occurrences, and the relationship between the symbiosis and mineral association in that area. Consequently, prediction is made on the locations of concealed orebodies. Effectiveness and feasibility of this method are proven by comparing with the existing results. Visualization tools, such as GoogleEarth, are used to show the predicted results, and to make a better comparative study. This project has newly designed and developed a method based on spatial location correlation analysis and prediction.

Geochemistry

Geochemical Characteristics of Stream Sediments and Prospecting Direction in the Wuheshalu Area, Wuqia County, Xinjiang

LIU Kunfeng, FENG Changrong, ZHAI Liming, XU Lei, ZHANG Jiasheng, WANG Shaohua, KOU Xin

2019, 33(04):  759-771.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.07

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The Wuheshalu area is located in the west margin of Tarim basin, Wuqia County, Xinjiang. More than 20 metal deposits (mineralized spots) have been found in Wuheshalu and the surrounding area, which have a good prospecting direction for copper, lead and zinc. Through 1∶50,000 geochemical survey of stream sediments in the Wuheshalu area, we have analyzed the geochemical characteristics of 15 elements such as Cu, Pb, Zn, Ag. Based on the differentiation, enrichment and superposition degree, we conclude that there is good potential for the concentration and mineralization of elements of Cu, Pb, Zn, Ag, Sb, Sn, Sr, and W,and these elements are the main ore-forming elements in the study area. According to the anomaly characteristics and metallogenic geological conditions, fifteen geochemical complex anomalies, in addition to three prospective areas are delineated, namely the Wuheshalu Cu-Sr-Pb-Ag polymetallic ore prospective area, the Xiaoerbulake-tao Cu polymetallic ore prospective area and the Aketao Cu-Sr polymetallic ore prospective area,respectively.This paper points out the direction for the prospecting work in the next stage.

Anomalies and Prospecting Directions of Regional Geochemistry Survey in the Lanping Basin of Yunnan Province, China

XU Shengchao, XIAO Gaoqiang, GONG Qingjie, LIU Ningqiang, YANG Tianyi, DAO Yan, XIANG Longzhou, LI Zhong

2019, 33(04):  772-782.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.08

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The Lanping basin located in the middle section of the Sanjiang range in southwest China is a famous polymetallic mineralization area in which dominant economic metals are copper, lead, zinc, and silver. The regional geochemistry survey with the scale of 1∶250 000 has been completed by Yunnan Geological Survey Institute from 2012 to 2016 in this area. The sampling medium is stream sediment with the size of -10 to +60 meshes, and the averaged sampling density is ca. 1.02 per square kilometer. Based on the analytical results of the regional geochemistry survey, the enrichment coefficients of 29 trace elements on average concentration in the study area are calculated relative to their average background values in the Southwest Sanjiang ore zone and discussed focusing on the main economic metals firstly. The results indicate that it is not suitable to determine economic metals only depending on the enrichment coefficients. Then geochemical anomalies of 29 trace elements are determined and classified in the Lanping basin with an area of 8 800 km² on the method of seven levels’ classification to eliminate the elemental enrichment effects during weathering. The results indicate that the determined anomaly areas are consistent with the known metal deposits spatially and metal deposits with larger scales are commonly located in the anomaly areas with higher levels. The known metal deposits in this area are commonly located in the anomaly areas with levels higher than 3 on the seven levels’ classification method. According to this criterion, two favorable areas are determined for future prospecting for polymetallic deposits. One is the Laojunshan area in the northeast of the basin, and the other is the south section of Baiyangchang area in central-south of the Lanping basin of Yunnan province, China.

Factors Affecting Mobility of Lead in the Soils of the Dongting Lake Plain, China

DAI Gaole, HOU Qingye, YANG Zhongfang, JIANG Hua, YUAN Jiaxin, TANG Zhimin

2019, 33(04):  783-793.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.09

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In order to systematically explore the mobility characteristics of Pb in the soils of the Dongting lake plain and its influence factors, 110 soil samples had been collected in Changde, Yiyang and Zhuzhou city. This study discussed the relationships between mobility of Pb and organic matter, pH, cation exchange capacity (CEC) and other parameters in soils. Based on their correlations, we established prediction models for the mobility of Pb. The results showed that Pb content, pH, organic matter content, CEC and weathering degree were the main factors affecting the mobile fraction and coefficient of Pb. They affected the mobility of Pb by changing the buffer capacity of the soil. Soil Cd content and Zn/Pb ratio were also significantly correlated with the mobile fraction of Pb. The interactions between Cd and Pb were different in different concentration gradients. The mobile fraction of Pb was lower when the soil had a higher Zn/Pb ratio. The multivariate stepwise regression analysis was adopted to establish prediction models for mobile fraction and mobility coefficient of Pb. The presented models had achieved good predicting effectiveness,when mobile fraction of Pb in soils was predicted by soil pH, Pb content, CEC and Na₂O/K₂O ratio and mobility coefficient of Pb was predicted by pH, CEC, SiO₂/Al₂O₃ ratio.

Geochemical Characteristics of Nutrient Elements in Soils of the Wohushan Farm in the Wudalianchi Area, Heilongjiang Province

WANG Shikuan, WANG He, WANG Zhe, HOU Tuo, LI Xiaowei, FENG Haiyan

2019, 33(04):  794-801.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.10

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In order to investigate the concentration and spatial distribution, as well as geochemical characteristics of nutrition elements in the volcanic rock area of the Wohushan farm in Wudalianchi, Heilongjiang pro-vince, a total of 41 topsoil samples were collected and the concentration of essential nutrient elements and physic-chemical indexes, such as nitrogen, phosphorus, potassium,organic matter as well as pH were measured. The abundance and deficiency level of nutrient elements in soils were evaluated according to the relevant standards. The results show that: (1) the type of soil in the study area belongs to acidic soil, and the soil nutrient elements of the study area are of high quality, except that boron element content is slightly deficient or lacking; (2) the spatial distribution of soil nutrient elements, is characterized by a higher value in the north of the Wohushan farm and a lower level near the Yuejin reservoir; (3) the soil within the study area were deve-loped under the moderate chemical weathering and warm humid conditions. The nutrients were released continuously into the soils in the process of soil forming, and organic matters had good adsorption capacity for soil nutrients. This study provides first-hand data for the rational use of nutrients in the local agricultural production and the planning of agricultural planting.

Petroleum Geology

Characteristics of Pre-Cenozoic Basement in Bohai Sea and its Significance in Oil and Gas Exploration

WU Qingxun, GAO Kunshun, WU Haoming, GUO Ying, CHEN Xinlu

2019, 33(04):  802-810.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.11

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Based on the Bohai drilling and 3D seismic data, lithology, stratigraphy and structural features of the Pre-Cenozoic basement in the Bohai sea have been integrated. The Bohai sea basement is divided into three structural layers: Pre-Cambrian, Paleozoic and Mesozoic. Distribution characteristics of the residual layers in each structural layer are discussed, and the basement structure is divided into three types: single, double and three level. Tectonic evolution of the basement comprises six stages: vertical rise and fall, strong extrusion, local extension, strong extension and inversion, overall uplift, stretch and strike-slip compound transformation. Combining the rock sheet and material data, the development regularity of the basement reservoir is explored. Precambrian metamorphic rocks of the single-layer type, carbonate or volcanic rocks at the top of the double-layer type, and Mesozoic volcanic rocks at the top of the three-layer type are favorable exploration targets. Buried hill around the hydrocarbon rich sag has a good exploration potential under the coupling of lithology and oil-supply window.

Stratigraphic Division of Buried-hill and Residual Strata Distribution in Eastern Section of Dongying Depression Southern Slope

YANG Huaiyu

2019, 33(04):  811-819.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.12

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The degree of buried-hill exploration is low in the eastern section of the Dongying depression southern slope, as impacted by the complex structure of residual sedimentary sequences and development model. The stratigraphic division of deep sedimentary formations in the study area have been reviewed by means of field lithostratigraphic correlation, paleontological division and unconformity logging integrated stratigraphic division. Based on that, tectonic reconstruction has been applied to evaluate the control of the multiphase structural activities on the erosion of the sedimentary sequences. Distribution characteristics of residual sequences are clarified. The results show that Mesozoic residual sequences in the study area are only distributed in the area north of Wanggu Well 2, whereas the southern Caoqiao-Wangjiagang contains mainly Upper Paleozoic sequences. The compressional Indosinian tectonics and the extensional Yanshanian rifting may have created the NW-trending seesaw-type pattern for the buried-hill residual sequences. Our results offer a theoretical guidance on the complex buried-hill exploration in the Dongying depression.

Diagenetic Evolution and Exploration Prospect of Oolitic Bank in Cambrian Zhangxia Formation at Xiaweidian Area, Beijing

GUO Qiheng, JIN Zhenkui, ZHU Xiaoer, CHANG Rui, JIANG Mengya, WANG Jinyi

2019, 33(04):  820-830.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.13

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Diagenesis controls the development of the storage space of oolitic banks, and their different diagenetic sequences determine the physical properties of the final reservoir. The Cambrian Zhangxia Formation at Xiaweidian occurs mainly in the oolitic bank facies in the carbonate platforms. In this study, based on outcrop descriptions, thin section observations and XRD, we discuss the major diagenetic types and sequence, and exploration prospect of the Zhangxia Formation. We conclude that: (1) Major diagenetic processes that affect the reservoir quality of oolitic bank include compaction, pressure dissolution, cementation and dolomitization. Silicification occurs mainly as silicide around the ooids; (2) Dolomitization of ooids mainly occurs in the syngenetic diagenetic stage, dolomite intercrystal pores are the main reservoir space of the local oolitic bank. The eogenetic and telogenetic diagenetic stage is mainly based on the compaction and pressure dissolution of the oolitic bank, resulting in a sharp porosity drop; (3) Migration and storage complex system, composed of the stylolite and dolomite intercrystal pores, is a possible storage space in the local oolitic bank. In the provision that the hydrocarbon supplying ability of the Proterozoic source rocks is confirmed, the Zhangxia Formation represents a new potential sequence for exploration, and warrants further investigation.

Depositional Models and Differences of Lacustrine Carbonate Rocks: Comparison Between Qikou Sag and Biyang Sag

YANG Youxing, GAO Yongjin, ZHANG Junfeng, ZHOU Xingui, ZHANG Jinhu, BAI Zhongkai, HAN Miao

2019, 33(04):  831-840.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.14

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Based on core logging and spore and pollen analyses, comparison between the depositional models of the Qikou and Biyang sags was made and the major controlling factors were determined. The results show that the two sags contain clear differences in rock types, locations, scales and facies. The sedimentary area of lacustrine facies is over 2000 km², which is distributed in the southwestern gentle slope area. It is featured by its large distribution area, small thickness, diverse rock and sedimentary facies and long deposition cycle. Carbonate rocks in the Biyang sag are mainly distributed in the central part of the depression, and are featured by small distribution area, large thickness, single sedimentary facies and short deposition cycle. The ancient landforms, openness of water body and climate conditions are likely the major causes for these differences. Factors such as salinity, temperature, biological groups and lacustrine levels also contributed substantially to these differences.

Characterization of Sand Body Architecture by Well and Seismic Data and Remaining Oil Distribution: A Case Study in the NmⅢ12-1 Area, Minghuazhen Formation, Jidong Oilfield

QIAO Hui, WANG Zhizhang, LI Haiming, ZHANG Lijiao, ZHOU Yi, LIU Chang

2019, 33(04):  841-852.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.15

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Well pattern distribution is not uniform in the NmⅢ12-1 area (Minghuazhen Formation) of the Jidong oilfield, but the quality of seismic data is good. Therefore, integration of well and seismic data method was adopted to study the sand body architecture, and conceptual models of architecture boundaries were established. The accuracy of single channel boundary and point bar sand body identification were effectively improved by combining seismic and well log data. The point bar and its internal configuration were characterized by the “mode-fitting” method, and the horizontal well and dynamic data were used to verify the results. Consequently, 3D reservoir models were established to predict the remaining oil distribution. In the middle and later stages of oilfield development, the remaining oil distribution is mainly affected by the micro-structures, reservoir architecture and well network. The positive micro-structures (e.g. micro-fault nose structure and micro-anticline), is the main site with relatively rich remaining oil. Point bar sand bodies generally have positive rhythm. Under the gravity influence, the lower part of the reservoir is substantially flooded, thus the remaining oil is concentrated mainly in the upper part. Since the abandoned channel and laterally-accreted shale beds often have certain shielding effect on the fluid flow, there are commonly plenty of remaining oil.

Influence of Thermal Action on Composition and Distribution of Steranes in Marine Crude Oil, Tarim Basin

YI Chuanjun, ZHANG Min, TENG Li

2019, 33(04):  853-862.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.16

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Based on systematic geochemical analysis, a suite of 26 marine crude oil samples and 1 simulation sample from the Tarim Basin (western China) were analyzed to investigate the influence of thermal action on the formation and distribution of steranes in crude oil. The results reveal distinct changes in the composition and distribution of steranes in different thermal evolution stages. Thermal cracking of crude oil results in decreasing sterane contents in the highly-mature stage. Meanwhile, the transient sterane content increase in the over-mature stage is likely related to the broken-down of hydrocarbons (with high molecular mass) in the crude oil. With increasing thermal maturity, the C₂₇/C₂₉ ratios of regular steranes shows an initial decrease and then an increase, which is likely controlled by the different generation and cracking rates of C₂₇ and C₂₉ regular steranes. Therefore, the C₂₇/C₂₉ ratios of regular steranes cannot be used to evaluate the sediment source in mature to highly-mature stage. In addition, the isomerization parameters of C₂₉ steranes in crude oil exhibit a significant maturity reduction or “reversal” in the highly-mature stage, which can be used to determine the thermal evolution stage of crude oil. Meanwhile, the value of diasteranes/regular steranes shows good positive correlation with maturity, and can be used as an effective maturity evaluation parameter of mature to highly-mature crude oil.

Comparative Analysis of Hydrocarbon Generation Kinetics of Dark Shale and Black Shale of Chang 7 in Southern Ordos Basin

QI Yulin, ZHANG Zhihuan, XIA Dongling, ZHANG Huimin, HUANG Caixia, ZHENG Duo, JIN Xiao, CAO Yongle, ZHU Lei

2019, 33(04):  863-871.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.17

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Previous studies on the hydrocarbon generation potential of Chang 7 hydrocarbon source rocks in the southern Ordos Basin were mainly focused on analyzing static geochemical parameters. No systematic evaluation of hydrocarbon generation potential has ever been carried out, which limited our understanding of the potential of Chang 7 hydrocarbon source rocks and the shale oil resource distribution. For this reason, rock samples were collected from two types of Chang 7 source rocks for thermal simulation experiments, and comparative analysis was performed on the hydrocarbon generation products, yield characteristics and hydrocarbon generation kinetics. We suggest that: (1)Distribution of activation energy of black shale is more concentrated than that of dark shale, and the peak frequency of activation energy and frequency factor of black shale are lower than those of the dark shale. This indicates that the black shale has the characteristics of short-term rapid hydrocarbon generation. (2)Under geological conditions, R_o values of the black shale during the main hydrocarbon generation period (0.70%~0.87%) are lower than those of the dark shale (1.06%~1.72%). (3) In a closed system, R_o is in the range of 0.9%~1.3%. The total hydrocarbon production rate and the amount of oil produced are both higher in the black shale than in the dark shale. Comprehensive analysis of geochemical data of the dark and black shales suggest that the former is more favorable source rock for shale oil resources in the study area.

Geochemical Characteristics and Depositional Environment Implications of Sedimentary Rocks in the Chang 7 Member of Yanchang Formation in the Ordos Basin

MA Huanhuan, LIU Chiyang, ZHANG Long, ZHANG Dongdong, WANG Wenqing, ZHAO Yan, GAO Ming, QUAN Xiaoyuan

2019, 33(04):  872-882.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.18

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Geochemical characteristics of sediments can reflect the depositional environment. In order to study its paleoclimate, redox conditions, and paleosalinity in the deposition period of the Yanchang Formation, geochemical (major, trace and rare earth elements (REEs)) characteristics of 15 sandstone and shale samples from the Chang 7 Member of the Yanchang Formation in the southeastern Ordos Basin were studied. The Al₂O₃/SiO₂ and K₂O/Na₂O values of fine-grained sedimentary rocks in the Chang 7 Member indicate similar compositional maturity and high clay contents. The average ∑REE value (132.65×10^-6) is significantly lower than that of the NASC and the UCC. The rocks also display relatively high LREE contents, weakly positive Eu and no Ce anomalies. The “C value”, Sr/Cu, Rb/Sr, Mg/Ca indices suggest that the entire deposition period of the Yanchang Formation was in a warm humid climate. Samples in the study area are rich in certain trace elements, such as U, Sc and Cs (average 20.11×10 ^-6, 9.66×10^-6 and 6.08×10^-6, respectively). This result and the values of V, δU, Th/U, V/Cr, V/Sc and V/(V+Ni) altogether indicate that the deposition period of the Yanchang Formation was dominated by a reducing, strongly stratified subsurface water layer. The values of Li, Sr, Ni, Ga, Sr/Ba, CaO/(CaO+Fe) and (MgO/Al₂O₃)×100 indicate that the water body of the deposition comprised mainly freshwater-brackish water.

Geochemistry Character of Longmaxi Formation Shale Gas in Western Hunan and Hubei Areas

ZHANG Peng, CHEN Shijing, LIU Ziyi, HUANG Yuqi, YANG Junwei, LIU Hongyang

2019, 33(04):  883-889.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.19

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The Longmaxi Formation organic-rich shale, featured by high total organic carbon (TOC) and high thermal maturity, is located in western Hunan and Hubei region of the middle Yangtze plate. Organic matter in the region are type Ⅱ₁ to type Ⅱ₂ kerogen and are dominated by the former. In the region, brittle minerals (dominant) and clay minerals are abundant in the organic-rich shale. The CH₄ content of the gas samples is high (90.34% to 99.64%), and the gas also contains minor non-hydrocarbon components, as revealed by the experiment under 20℃, 65℃, and 90℃. CH₄ carbon isotopes of the hydrocarbon gases are of -41.9% to -30.8‰, whilst those of C₂H₆ and δ ²H_CH4 are of -42.3% to -36.2‰ and -193.4% to -156.0‰, respectively. With increasing maturity, carbon isotopes of CH₄ and C₂H₆, and hydrogen isotope of CH₄ all become heavier. Carbon isotopes of the hydrocarbon gases show clear reversals, i.e.δ¹³C₁>δ¹³C₂. Accurate calculation of the free gas and adsorbed gas in the shale gas is of substantial guiding significance for the future exploration and development. Therefore, our work establish the calculation formula of the proportion of free gas and adsorbed gas according to the gaseous isotope characteristics of the three temperature stages of the experiment and the gas bearing characteristics measured at each stage. The free gas volume and the amount of adsorbed gas contained in the total gas content could thus be determined.

Organic Geochemistry and Sedimentary Paleoenvironment of Chang 7 Source Rocks in Yanchi-Dingbian Area, Ordos Basin

XI Shengli, GANG Wenzhe, YANG Qingyu, CHEN Guo, LIU Yazhou, WANG Ning, LIU Lan

2019, 33(04):  890-901.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.20

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Analysis of the Chang 7 source rocks in Yanchi-Dingbian area (Ordos Basin) indicates that the total organic carbon, hydrocarbon potential and chloroform extracted bitumen “A” are on average 3.22 %, 12.8 mg/g and 0.55 %, respectively. Based on these results, the source rocks can be classified into the fair to good categories, suggesting a high hydrocarbon potential. Organic petrology, rock-eval and biomarker evidence suggest that the main types of organic matter are Ⅱ₁ and Ⅱ₂, and that the organic matter source is dominated by algae or bacteria with minor high plant. The R_o and T_max of the source rocks are of 0.42% to 0.76% and 440 ℃ to 460 ℃, respectively. The normal distribution, steranes and hopanes isomerization parameters indicate that the organic matter varies from lowly-mature to mature. In addition, the parameters of steranes, hopanes, tricyclic terpene, gammacerane, β-carotane and Pr/Ph indicate that the sedimentary environment of Chang 7 was lacustrine-brackish, and weakly oxidized or weakly reduced.

Organic Matter Pores Developments of Lower Silurian Longmaxi Shale in Southeast Chongqing: Based on Focused Ion Beam Helium Ion Microscopy (FIB-HIM)

WANG Pengfei, JIANG Zhenxue, JIN Can, LÜ Peng, LI Xin, ZHANG Kun, WANG Kai, HUANG Pu

2019, 33(04):  902-910.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.21

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Organic matter pores in shale are critical to the occurrence of hydrocarbon gases. The focused ion beam helium ion microscope (FIB-HIM) technique was used to determine the organic matter pore development features of the Lower Silurian Longmaxi Formation (Fm.) shale in southeastern Chongqing. The sub-nanometer resolution of FIB-HIM can effectively identify pores with 0-20 nm diameter. The results show a large amount of organic matter pores inside the pyrobitumen of the Longmaxi Fm. shale. The organic matter pore diameter is large with good connectivity. A large number of smaller pores are nested in the larger pores, which increases the connectivity of the pore system. The specific surface area and pore connectivity are beneficial to the occurrence and effective seepage of hydrocarbon gases in the pores. Development characteristics of the organic matter pores in solid kerogen of the Longmaxi Fm. shale are considerably different from those of the pyrobitumen. The organic matter pores developed in the solid kerogen are small in both size and number, and with poor connectivity that the pores are mostly isolated inside the solid kerogen.

Feasibility Study on Applying CO₂-flooding Micro-visualization Technology in Ultra-low Permeability Reservoirs: A Case Study in Ordos Basin

LI Ming, ZHU Yushuang, LI Wenhong, YUAN Guowei, DU Chaofeng

2019, 33(04):  911-918.  DOI: 10.19657/j.geoscience.1000-8527.2019.04.22

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CO₂-flooding is an important way to improve oil displacement efficiency in extra-/ultra-low permeability reservoirs. However, the microscopic mechanism of CO₂-flooding and oil displacement effect evaluation are hampered by the lack of micro-visualization technology. In this study, we suggested that micro-visualization experiments of CO₂-flooding in extra-/ultra-low permeability reservoirs under high temperature and pressure can satisfactorily reveal the CO₂-flooding mechanism.Ultra-low permeability reservoirs in the Ordos Basin are taken as examples, and micro-visualization experiments of CO₂-flooding were newly and successfully carried out by using the high temperature, high pressure and anti-riot real sandstone model. Dynamic phenomena of the different CO₂ phases (in crude oil displacement) in the complex pore throat of the ultra-low permeability reservoirs were simultaneously observed under the microscope, and the occurrence and seepage of CO₂ in the different pore throat structures were obtained. The pore throat structure, especially the uniformity of pore throat size distribution (critical for the CO₂-flooding efficiency), directly determines whether the CO₂ can enter the reservoir and the subsequent seepage path. The micro-visualization experiment of CO₂-flooding can not only effectively reveal the microscopic mechanism of CO₂-flooding and evaluate the oil displacement effect, but can also provide guidance for later-stage reservoir analyses.