Table of Content

25 April 2021, Volume 35 Issue 02

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Petroleum Geology

Distribution Characteristics and Geochemical Significances of the Compound Specific Sulfur Isotope of Saline Lacustrine Source Rocks from the North Dongpu Depression, Bohai Bay Basin

KE Changwei, LI Sumei, ZHANG Hongan, XU Tianwu, ZHANG Yunxian, ZENG Fangang, ZHANG Shuhai, ZHANG Hanjing

2021, 35(02):  301-314.  DOI: 10.19657/j.geoscience.1000-8527.2021.032

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The researches about compound specific sulfur isotopes of saline lacustrine source rocks are rare. Using the technology of gas chromatography with multicollector inductively coupled plasma mass spectrometry (GC-MC-ICP MS), combined with gas chromatography/mass spectrometry (GC/MS), carbon and oxygen isotopic and trace elements to analyze the source rocks (oil sands)from saline lacustrine northern of Dongpu Depression. The analysis results show that the sulfur isotopic values of individual hydrocarbon in source rocks (oil sands) are small, but the δ³⁴S values of different sulfur compounds are quite different (5.16‰ to 44.73‰). Boundary with δ³⁴S=25.00‰ the source rocks (and oil sands) can be divided into two categories, the first type with δ³⁴S>25.00‰ is mainly Weicheng source rock characterized by the sulfur isotope of individual hydrocarbon decreases with the increase of methyl. And the δ³⁴S<25.00‰ is the second type,which characteristics are opposite to Weicheng source rocks. The sulfur isotopes of dibenzothiophenes have obvious positive correlation with the content of saturated hydrocarbon, tricyclic terpane/pentacyclic terpane, and negative correlation with content of “nonhydrocarbon+asphaltene”. The sulfur isotopes of hydrocarbons have obvious positive correlation with gammacerane/C₃₁hopane and C₃₅/C₃₄hopanes, excellent correlation with contents of minerals and ratios of trace elements, indicating that the isotopes of organic sulfur compounds in the closed evaporation environment becomes heavier, and the values in the open terrestrial input environment becomes lighter. The analysis shows that the sulfur isotopes of individual hydrocarbon are significantly controlled by the original sedimentary environment of source rock. Strong reduction and closed evaporation environment are conducive to the sulfuration of organic matter, which makes the sulfur isotope heavier. The sulfur isotopes are also affected by maturity, while the sulfur isotope tends to become heavier with the increase of maturity. Generally speaking, isotopes of organic sulfur compounds have important significance of indicating sedimentary environment and maturity, and the potential of oil-oil and oil-rock correlation.

Geochemical Characteristics and Source Correlation of Low-mature Oil from the Tengeer Formation (2^nd Member) in the Saihantala Sag, Erlian Basin

LI Jiehao, HOU Dujie, CAO Lanzhu, WU Piao, ZHAO Zhe, MA Xiaoxiao

2021, 35(02):  315-325.  DOI: 10.19657/j.geoscience.1000-8527.2020.100

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The Saihantala Sag is located in the western Tengeer depression of Erlian Basin, and contains low-mature oil in the Erlian Basin. The geochemical characteristics and oil source correlation of the low-mature crude oil and oil sandstone samples from the Lower Cretaceous Tengeer Formation (2^nd member) in the Saihantala Sag were studied, by using organic geochemical and biomarker analyses. The low-mature oil from the Tengeer Formation (2^nd member) is characterized by having relatively low content of saturated hydrocarbon, and relatively high content of non-hydrocarbon and asphaltene. The distribution of n-alkanes is of pre-peak type, and the main peak carbon is C₂₁, C₂₃. The Pr/Ph is low (0.29 to 0.58), Pr/nC₁₇ is relatively low (0.29 to 0.59), Ph/nC₁₈ is relatively high (0.57 to 1.37), and C₂₉ 20S/(20S+20R) of sterane is 0.27 to 0.40, C₂₉ββ/(αα+ββ) of sterane is 0.29 to 0.39, the gammacerane index is mainly 0.31 to 1.34, and the tricyclic terpane content is low, which reflects that the sedimentary environment of crude oil is a brackish-saline reducing one with mainly algae source input. According to the study of oil source correlation, there are two sources of the low-mature oils from the Tengeer Formation (2^nd member) in the Saihantala Sag: (1) from the underlying Tengeer Formation (1^st member) source rock and hydrocarbon generation at the early mature stage in salt-water environment; (2) from the self-generation and self-storage of the lowly-mature Tengeer Formation (2^nd member) source rock in the depression center.

Characteristics and Effect on Productivity of the Sandstone and Fractures in Ultra-deep and Fractured Tight Sandstone Gas Reservoirs:A Case Study of KS2 Gasfield in Kuqa Depression,Tarim Basin

FU Xiaotao, WANG Yimin, SHAO Jianbo, ZHU Songbai, WANG Yong, NIE Yanbo, WANG Bin, DUAN Qiqi

2021, 35(02):  326-337.  DOI: 10.19657/j.geoscience.1000-8527.2020.097

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Tectonic fractures are widespread in the KS2 gasfield and the fracture development characteristics differ with the structure locations. The single-well production in this area is controlled by the features of the sandstone and fractures. In this study, we determined the lithological characteristics, sand combination features and properties such as the degree, filling and occurrence of the fractures in the reservoir through analyzing the cores, slices, SEM and well-log data. Besides, we analyzed the relations between the well production and aforementioned sand and fracture features. The results show that the well production is affected by many factors of the property of the sand and fractures: wells located in the areas controlled by both faults and anticline have higher production, as indicated by routine test or acidification test; fractures with single direction are usually widespread in the anticline-controlled areas, and the wells there need to be tested by artificial hydraulic fracturing to obtain higher yield; fractures are poorly-developed in areas such as limbs and axis of the anticlines, and the wells there usually have low productivity.

Shale Gas Accumulation Conditions and Gas-bearing Properties of the Lower Carboniferous Jiusi Formation in Western Guizhou

JIANG Bingren, YANG Tongbao, SHI Fulun, HAN Minghui, FU Wei

2021, 35(02):  338-348.  DOI: 10.19657/j.geoscience.1000-8527.2020.099

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In order to study the shale gas accumulation conditions and gas-bearing characteristics of the Jiusi Formation, we conducted thin-section microscopic identification, X-ray diffraction (XRD) analysis, overburden porosity and permeability, scanning electron microscopy (SEM), low-temperature nitrogen adsorption, organic geochemistry, in-situ gas content, and isothermal adsorption analyses. The results show that the organic-rich Jiusi Formation (Fm.) shale is widely developed (mainly 30 to 100m thick) and has high TOC. It is mainly of type II and in the stage of over-mature dry gas generation. The shale contains mainly quartz and clay mineral, and with high brittleness index (>50%), ultra-low porosity and permeability. The rocks have also various pore types, medium pore size, large specific surface area and total pore volume, and good shale gas accumulation conditions. The fractal dimension of micro-pores is large, with average D₁ and D₂ of 2.804,1 and 2.753,8 respectively, reflecting complex pore structure and strong heterogeneity. The gas and adsorbed gas contents are of 1.5 to 3.0 m³/t and 1.58 to 4.52m³/t, respectively. The results indicate good gas-bearing performance, suggesting that the Jiusi Formation has good shale gas exploration potential.

Sedimentary Filling Evolution and Hydrocarbon Control of High Frequency Sequence in Yurtus Formation, Tarim Basin

JIANG Wei, GAO Zhiqian, HU Zongquan, ZHAO Yongqiang, CHU Chenglin

2021, 35(02):  349-364.  DOI: 10.19657/j.geoscience.1000-8527.2021.033

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The Lower Cambrian Yurtus Formation in Tarim Basin is one of the best marine source rocks found in China. In order to understand the evolution of the sedimentary facies of the Yurtus Formation and its restriction on source rocks, this paper has systematically studied and analyzed the sedimentary evolution of the Yurtus Formation and its restriction on source rocks by combining 9 field profiles and 17 wells. After fine correlation, it is found that Yurtus Formation can be divided into two third-order sequences and five fourth-order sequences, among which the first third-order sequence can be divided into three fourth-order sequences and the second third-order sequence can be divided into two fourth-order sequences. The first third-order sequence occurred in the Early Cambrian, the transgression speed was slow and the influence range was relatively limited, while the transgression of the second third-order sequence was relatively rapid and had a large influence range. The fourth-order sequence sq1 developed in the early stage of the first transgression, with shallow water body, mainly deposited with purple-red dolomite and gray-black sand-clastic dolomite, which mainly played the role of filling and filling; sq2 and sq4 developed during the transition period of transgression and regression, and were two sets of deep-water deposits. The deposits are mainly dark mudstone, siliceous mudstone and siliceous rock, and interbedded mudstone and banded dolomite are developed locally; sq3 and sq5 are shallow water deposits in the process of regression, mainly gray dolomite and algal dolomite, with argillaceous interlayers in some parts. Analysis of deposition environment using Mn, Fe, Ti, Rb, K, Sr, Ba, Cu, U, V, Mo and other elements and their ratios shows that in the period of sq2, the water body is deep, low salinity and warm and humid climate, which is conducive to the development of organisms, and its sulfurized and anoxic environment is extremely conducive to the preservation of organic matter; In the period of sq4, the depth of water body is larger, the salinity is relatively small, and the climate is also relatively warm and humid, so it is also suitable for biological survival. Moreover, it is suitable for the preservation of organic matter in the environment of sulfidation, anoxia and secondary oxidation. High-frequency sea level changes and the evolution of the depositional environment affect the source rock distribution and quality of the Yurtus Formation. The quality of the source rocks deposited at sq2 is better than that of sq4, but its distribution area is less than that of sq4.

Petrographic Classification of Lacustrine Fine-grained Rocks Using a Two-level Index Division Method and A Case Study of Its Application

LI Zengxue, SONG Mingshui, LI Ying, LIU Haiyan, XU Chunhua, WANG Dongdong, KONG Fanfei, HAN Qiaoyu

2021, 35(02):  365-377.  DOI: 10.19657/j.geoscience.1000-8527.2020.089

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In this study, the macro/microscopic characterization of fine-grained rocks was observed, and the types and structural features of lacustrine fine-grained sedimentary laminae were taken as the key indicators by which to explore the genesis of fine-grained lithofacies. In addition, the classification indicators and principle of lacustrine fine-grained lithofacies were proposed. Fine-grained sedimentary laminae are important indicators for the classification of fine-grained lithofacies. Two levels of core indicators are proposed as the basis to classify the lacustrine fine-grained lithofacies and subtypes. The level one indicator is the rock composition type, which includes nine index types, e.g. argillaceous rocks, limestone and fine-grained calcareous rocks. The level two indicator is the fine-grained laminae type, mainly including organic, dolomitic, calcareous, silty, gypsum, and carbonaceous laminae. Based on these two levels, the fine-grained lithofacies are classified into 21 types and 33 subtypes. Subsequently, the characteristics of the sedimentary environment of each lithofacies are analyzed.

Dedolomitization Origin and Model for the Ordovician Majiagou Formation (5^th Member) in the Southern Ordos Basin

LEI Han, HUANG Wenhui, SUN Qilong, CHE Qingsong

2021, 35(02):  378-387.  DOI: 10.19657/j.geoscience.1000-8527.2021.002

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To constrain the diagenesis and property evolution of the Ordovician carbonate reservoir in southern Ordos basin, it is important to understand the origin and mechanism of dedolomitization there. Based on core and thin-section observations, and geochemical analysis, the dedolomitization mechanism and diagenetic evolution were determined by oxygen isotopes and trace elements. Subsequently, combined with the property data, the influence of different dedolomitization styles on the reservoirs was analyzed. Finally, the dedolomitization model of the study area was constructed. Combined with the carbon-oxygen isotopes and trace element data of the samples, we show that the dolomite with gypsum core in M5₁₊₂ is generally affected by freshwater leaching, yet some of them also underwent dedolomitization caused by freshwater leaching. The Th/U, Th/Cr, Cr/Zr, and Y/Ho ratios indicate that meteoric water may have dissolved granitoids and carried terrestrial argillaceous sediments into the strata, again due to freshwater leaching. Ca²⁺ source for the freshwater-leaching-related dedolomitization was likely from the dissolved evaporite (gypsum) and gypsum cores, whilst the {{\mathrm{CO}}_3}^{2-} may have sourced from calcite decomposition during the supergene dolomitization. And the main fluid migration pathways may have been dissolution fractures and suture zones. This process can be divided into 2 stages: replacement (supergene to shallow-burial stage) and cementation (shallow-burial stage). In this process, freshwater rich in Ca²⁺ and {{\mathrm{CO}}_3}^{2-} may have flowed along the migration pathways, and infiltrated along intra-crystal cracks, resulting in the selective dissolution and dolomite replacement with equal volume of calcite, together with the infilling of intra-crystal pores, cracks and fractures by calcite cement, which is destructive to the reservoir.

Dynamic Evaluation of High-capacity Channel in Heavy Oil Reservoirs with Medium-high Permeability

LIU Xiaotong, XUAN Lingling, ZHU Chunyan, ZHAO Ying, GUO Chang, LI Song, ZHANG Hao, SHI Zhenyun, LUO Wanjing

2021, 35(02):  388-395.  DOI: 10.19657/j.geoscience.1000-8527.2020.088

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During water flooding in medium-high permeability reservoir, due to the long-term scouring effect of water flow, some particles of reservoir rocks are transported. Along with the gradual development of high capacity channel of oil well sand production, ineffective circulation of edge and bottom water and injected water has become significant, and hampers the efficiency of reservoirs. We investigated the high-permeability heavy oil reservoir in block G104-5 of Gaoqianbei district. 10 dynamic and static index parameters were optimized based on the production characteristics of well site oil and water wells, and the index weight was determined with the Analytic Hierarchy Process (AHP). Considering the influence of 22 types of production dynamic adjustment measures of oil and water wells on the evaluation of high capacity channel, a set of high capacity channel dynamic evaluation method was established based on the fuzzy comprehensive evaluation principle, and VBA programming was used for calculation. The high capacity channel dynamic evaluation method was verified with the test data of injection profile and liquid producing profile. Evaluation index of high capacity channel was calculated, and the distribution map of high capacity channel with time was drawn by using the Petrel software. Our results provide an important guidance for understanding the spatial-temporal evolution law of high-capacity channel in medium-high permeability heavy oil reservoirs.

Horizontal Well Potential Digging Technology of Point-bar Sandbody in Ultra-high Water Cut Stage: Evaluation of Flow Field Intensity

GUO Qi, LI Zhen, LI Ping

2021, 35(02):  396-402.  DOI: 10.19657/j.geoscience.1000-8527.2020.06.090

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In view of the difficulties in tapping the remaining oil potential of point-bar sandbody in high water cut reservoirs, the changing regularities of physical properties of high water drive in medium and high permeability reservoirs are determined through laboratory experiments, and the time-varying numerical simulation method is determined. Based on the evaluation of seepage field strength, the reservoirs are divided into three types of seepage zones, and the locations of horizontal wells in ultra-high water cut reservoirs are determined by the modeling method of the internal configuration of point-bar sand body. The results show that the method of horizontal well deployment based on flow field intensity distribution can highlight the heterogeneity of fluid flow in point-bar sandbody during the high water cut period, and the development effect of horizontal well deployment in weak flow field intensity range is better improved than in other areas. The new method is applied to Nm IV-6-3 subzone in block 2 of the Gangdong Oilfield. The flow field of the subzone is evaluated and a horizontal well is deployed. The initial daily oil production was 77.3 tons, and the cumulative oil production is predicted to be 164,000 tons at the end of the 15^th years. This new method provides a reference for well location deployment of point-bar sandbody in high water cut stage.

Geothermal Resources

Characteristics and Major Controlling Factors of Geothermal Field of North Cangxian Uplift

SU Yuchi, MAO Xiaoping, ZHANG Fei, MAO Ke, LU Pengyu

2021, 35(02):  403-411.  DOI: 10.19657/j.geoscience.1000-8527.2021.031

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The northern part of Cangxian uplift is rich in geothermal resources, but the formation mechanism and major controlling factors of geothermal fields are still controversial. Based on the compilation of regional geological and well-temperature data, and integrated with forward modeling of heat conduction, we systematically analyze the horizontal and vertical temperature gradient characteristics in the study area, and the distribution patterns of geothermal field under different geological conditions. We then forward-model two measured geothermal profiles to analyze the major controlling factors of local thermal field distribution. The results show that the horizontal geothermal gradient is relatively high in the salient area, but relatively low in the depression area, which corresponds well with the bedrock undulation. The vertical geothermal gradient can be divided into two sections: the shallow sedimentary caprock is higher and of mainly heat conduction, whilst the lower thermal reservoir is lower in the Cambrian, Ordovician and Jixian reservoirs, and mainly of thermal convection. Meanwhile, the local thermal field distribution is mainly controlled by the bedrock burial depth, and (in some places) is also affected by the upwelling of underground hot-water along faults. This study provides an important basis for future selection of geothermal development sites.

Geochemistry

Characteristics and Genesis of Soil Geochemical Baselines in Western Yunnan Province

WANG Qiaolin, SONG Yuntao, LÜ Xupeng, PENG Min, ZHOU Yalong, HAN Wei, WANG Chengwen

2021, 35(02):  412-424.  DOI: 10.19657/j.geoscience.1000-8527.2021.02.11

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Based on the geochemical survey data of land quality in western Yunnan Province, the median with robust characteristics was used to represent the geochemical baselines of 52 elements in deep soil. The distribution and enrichment and dilution features of elements in different parent materials and land use types were compared and analyzed. The causes of geochemical baselines were further analyzed from the perspective of element combination characteristics by factor analysis mechanism. The results showed that the baselines of iron group elements, chalcophile elements, mineralizer and halogen elements, rare, scattered, rare earth metal elements, radioactive elements, tungsten molybdenum group elements and rock-forming elements in the study area were higher than those of national geochemical baselines; the baseline values of alkaline earth metals were significantly lower, of which CaO and Na₂O only with 13% and 7% of the national level; soil parent material was the main control factor of soil element baseline values. The high baselines of TFe₂O₃, V, Co and Cr were mainly controlled by basic parent rocks, W, Sn and Pb were mainly related to the high temperature magmatic mineralization, the life elements including C, N, S, P and Se were related to land use types and supergene geochemical cycle. The results showed that the types of parent rock, weathering leaching, mineralization, bioaccumulation and physicochemical adsorption of clay jointly affect the characteristics of soil baselines, among which the weathering of parent rocks had an important influence. The results provide foundational geochemical information for regional resource and environment assessment.

Selenium-rich Characteristics and Risk Assessment of Heavy Metals in Soil and Crop in A Typical Black Shale Area of the Central Part of Zhejiang Province, China

CHENG Xiaomeng, WU Chao, SUN Binbin, HE Ling, ZENG Daoming

2021, 35(02):  425-433.  DOI: 10.19657/j.geoscience.1000-8527.2021.02.12

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Selenium (Se)-rich soil is a valuable resource for the production of Se-rich agricultural products. Soil derived from black shales weathering is usually of high concentration of Se and heavy metals. To guide the similar safe use of Se-rich soil, a black shales area in the central part of Zhejiang Province was selected to identify possible risks of heavy metals in crop exceeding standards, through the investigation on the concentration and bioavailability of heavy metals and Se in soil and crop. The results showed that the Se-rich and Se-sufficient soil in the study area accounted for 38% and 62% coverage; the Se-rich rice and lotus seeds were 85.71% and 100%, respectively. 26% of soil samples is of cadmium (Cd) content over the soil pollution screening value and less the soil risk control value. The proportions of Cd active and potential active states were of 57% and 40%, respectively. Up to 31% of rice samples exceeds national cadmium limit. The risk assessment based on the total concentration of heavy metals and sequential extraction shows that the Cd risk areas for the development and utilization of Se-rich soil are mainly located in Xipishangang and Yaotangbian area,and the cultivation of agricultural products should be prohibited. Other Se-rich hilly areas overlying black shales should be protected and rationally developed and utilized as low risk of Cd in crops and low bioavailability of heavy metals in soil, although cadmium concentration in soil is high.

Distribution of Soil Selenium of the Cultivated Land and Its Controlling Factors in Zhenyuan of Guizhou Province

YANG Zhizhong, ZHOU Wenlong, LUO Yongjun, PU Qinglong, LING Hudong, SONG Xiaojun

2021, 35(02):  434-442.  DOI: 10.19657/j.geoscience.1000-8527.2021.02.13

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The distribution characteristics and controlling factors of soil selenium in the surface layer of cultiva-ted land were analyzed based on the investigation of cultivated land quality in Zhenyuan County. The results showed that the average soil Se content of cultivated land is (0.64±0.56) mg /kg, Se content of the Se-rich cultivated land is (0.72±0.42) mg /kg. The area of Se-rich cultivated land accounted for 82.71% of the total cultivated land area in Zhenyuan County. The content of soil selenium in cultivated land is affected by pH value, elevation, landform, parent rock, soil type and land use type. Also, the results showed that there are relatively high selenium contents in the acid cultivated land, the cultivated land below 800 m above sea level, the cultivated land in furrow and valley, the cultivated land in carbonaceous strata, the coarse-grained soil and red soil, the soil in tea garden and paddy field. It is considered that the parent rock is the most important source of soil selenium, which controls the content of selenium in soil, especially the carbonaceous siliceous rock series. In addition, soil organic matter and pH can change the physical and chemical properties of soil and affect the migration and accumulation of Se, and then affect the content of Se in soil.

Temporal and Spatial Changes of Soil Carbon Pool and Its Influencing Factors in the Sanjiang Plain

LIU Guodong, DAI Huimin, YANG Ze, XU Jiang, ZHANG Yihe, WEI Minghui

2021, 35(02):  443-454.  DOI: 10.19657/j.geoscience.1000-8527.2021.02.14

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Based on the soil geochemical survey data of the Sanjiang Plain in northeast China, the surface soil organic carbon density and carbon reserves were calculated. By comparing the data of Multi-Purpose Regional Geochemical Survey with the second national soil census, this paper analyzed soil pool storage changes and main influencing factors. The results show that in the study area, the surface soil carbon is dominated by organic carbon, and there is significantly difference in the contents of organic carbon density and total carbon density in different soil types. Compared with other soil types in the area, marsh soil and peat soil have high organic carbon density, with an average content of over 8 kg/m². The current distribution area of soil organic carbon density of >8.4 kg/m² is significantly lower than that in 1980s.The current reserve of soil organic carbon in the surface layer (0-20 cm) of the study area is about 348 Tg, and the soil organic carbon has decreased by 293.07 Tg in 30 years, which is 45.7% lower than that in the base period. About 1.46 Tg was loss with the migration of water flow to the outside of the basin. The soil organic carbon density of cultivated land transformed from swampy wetlands was reduced by more than 2.8 kg/m²on average. The authors believe that the main reason for the decrease of soil organic carbon in the study area is the rapid change of land use. The land use changes from wetland to dry farmland led to the largest loss of organic carbon, with a loss of 63.47%.Other types of land use changes resulted in soil organic carbon loss between 40% and 50%.The decrease of soil organic carbon pool caused by temperature rise in the study area over the past 30 years was about 7.4 Tg, accounting for 2.52% of the total loss.

Application of Soil Geochemical Survey in the Exploration of Lead-Zinc Polymetallic Ore in Waziping,Mianxian County,Shaanxi Province

CHEN Bingjin, ZHENG Cuiyong, ZHAO Liangliang, YUAN Bo, WANG Zhaofei, ZHANG Yi, GUO Bin, YU Hongbo

2021, 35(02):  455-465.  DOI: 10.19657/j.geoscience.1000-8527.2021.02.15

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The Mianlueyang triangle area is an important polymetallic ore concentration area in the south of Shaanxi Province. The Waziping survey area is located in the eastern part of the Mianluekang structural melange metallogenic belt, silver-lead-zinc has high background, rich mineral sources and favorable geological conditions. On the basis of 1:25,000 soil geochemical survey in the whole region, 12 elements of Au, Ag, Cu, Pb, Zn, Co, As, W, Ni, Cr, P and Mn were studied by means of multivariate statistical method. The geochemical characteristic parameters, element correlation, element distribution and single element anomaly characteristics were analyzed. In this paper, the law of element combination,enrichment dilution and the corresponding relationship between each element and its geological background are summarized. Combined with the information of metallogenic geological structure background and known ore spots in the area, 14 comprehensive anomalies are delineated, and three prospecting targets are optimized. Through the verification of typical anomalies, the channel exploration project is used to expose them, 6 alteration zones and 13 ore bodies are found in this area, which shows that this method has a good prospecting application value in this area.

Petrology

Zircon U-Pb Age of Granitoids from the Mamu Pb-Zn Skarn Mineralized Area in Western Gangdese and Its Geological Significance

LIU Chang, YANG Zhusen, XU Peiyan, ZHAO Xiaoyan, XIA Wenjie, YANG Xiaoxu

2021, 35(02):  466-476.  DOI: 10.19657/j.geoscience.1000-8527.2021.038

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The Mamu area in Gakyi, Tibet,China is a Pb-Zn skarn mineralized zone discovered in recent years. The area is located on the southern margin of the western central Gangdese metallogenic belt (or the central Lhasa terrane). Mineralization occurred in the contact zone between granite porphyry and limestone. Metallic minerals include galena, sphalerite, magnetite, specularite, and pyrite, whilst non-metallic ones include garnet, diopside, rhodonite, actinolite, epidote, quartz, feldspar, and calcite. Alteration styles include silicic, argillic, marl, and chlorite. The mineralization is hosted in the Jiega Formation (Fm.) limestone. To obtain geochronological constraints on the magmatism and mineralization, the granite porphyry is LA-ICP-MS zircon U-Pb dated to be (117.6±0.4) Ma, which shows that ore-causative porphyry was emplaced in the Early Cretaceous. The age of crystal lithic tuff is (119.7±0.5) Ma, and the tuff should thus belong to the Zenong Group (J₃K₁Z) instead of the Dianzhong Formation (E₁d) in the 1∶250,000 Yagra Geologic Map Sheet. The study area experienced intense medium-felsic volcanism and plutonism in the Early Cretaceous. Combined with previous studies, the deep geodynamic background was mainly controlled by the south-dipping subduction of the Bangong-Nujiang Suture Zone. As an Early Cretaceous Pb-Zn mineralization event, the Mamu Pb-Zn prospect is of great significance to supplement the study of regional Pb-Zn mineralization and to improve our understanding on multi-phase Pb-Zn metallogenic events.

LA-ICP-MS Zircon U-Pb Dating, Geochemical Characteristics and Tectonic Significance of Neoproterozoic Granite in the Quruqtagh Area,Northern Margin of Tarim Craton,NW China

CHEN Bangxue, XU Shengli, ZHOU Nengwu, BAI Quanjin, LI Chao, ZHANG Hongshen

2021, 35(02):  477-491.  DOI: 10.19657/j.geoscience.1000-8527.2020.056

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In this study, we conducted LA-ICP-MS zircon U-Pb dating on the monzogranite from the Quruqtagh area, which yielded a concordant age of (832.3±3.3) Ma (MSWD=2.8,n=24).The monzogranite is metaluminous,high-K calc-alkaline,and I-type. As for trace elements, the samples are enriched in large-ion lithophile elements (LILEs, e.g., Ba, K, Sr and U) and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, Ti and P). The total rare earth element (REE)contents are relatively low (29.88×10^-6 to 63.57×10^-6), and the samples exhibit weakly positive Eu anomaly (δEu=0.87 - 1.39). The overall trace element distribution pattern resembles that of the lower crust. Comprehensive analysis of magmatic lithogeochemistry and regional geological background shows that the monzonitic granite was likely formed in an island arc environment.Combined with the regional tectonic evolution, we considered that the oceanic crust in the region began to subduct at around 832 Ma. This may have made the crust thicker, and the heating from the upwelling mantle plume may have partially melted the ancient crust materials, forming the Neoproterozoic granites.

Geochronology,Geochemistry and Tectonic Significance of Songkarsu Granites in Karamali,Eastern Junggar

YANG Shuo, LIU Ge, JIN Liuyuan, ZHENG Haifeng

2021, 35(02):  492-503.  DOI: 10.19657/j.geoscience.1000-8527.2020.074

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The Eastern Junggar region was magmatic active and intrusive rocks were extensively developed.The Songkarsu pluton is located in the eastern margin of Junggar and the southern side of the Karamaili tectonic belt. The small pluton is composed mainly of monzogranite and syenite granite. Zircon U-Pb dating yielded a formation age of (410.2±2.2) Ma(MSWD=0.30),representing the first Early Devonian granite reported in the southern margin of the Karamaili belt. The rocks are alkaline granitic,and are featured by high Si,low Ti,high Al,alkali-rich,Ca-poor,and weakly peraluminous. The granites belong to A2-type,and are (primitive mantle-normalized) enriched in large ion lithophile elements (LILEs, e.g., Rb,Th,U, and K) with obvious negative Sr,P and Ti anomalies. Based on regional geological data analysis, we considered that the Songkarsu pluton was formed under continuous expansion and growth after the initial opening of the Karamaili ocean basin.The Karamaili area was under extensional tectonic regime during the Early Devonian.

Geochronological and Geochemical Characteristics of Tuanbaoshan Granite Pluton in Qinghai and Geological Implications

ZHANG Baotao, HU Zhaoguo, JIANG Xiaoping, WANG Xiangwei, SUN Luwei, ZHANG Yang, WANG Xiaoyu, ZHAO Xiaobo, GUO Wei

2021, 35(02):  504-513.  DOI: 10.19657/j.geoscience.1000-8527.2020.069

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The Tuanbaoshan granite pluton is located in the eastern South Qilian, and is emplaced into the Paleoproterozoic epimetamorphic rocks. Samples from the granite pluton are LA-ICP-MS zircon U-Pb dated and geochemically analyzed to constrain its emplacement age, petrogenesis and tectonic setting. The zircon U-Pb dating yielded weighted mean age of (776±8) Ma. The rocks contain relatively contents of SiO₂, Al₂O₃ and K₂O, and relatively low contents of FeO^T, MgO and CaO, with K₂O/Na₂O>1. The Tuanbaoshan granites are enriched in Rb, Th, and depleted in Ba and Zr, with chondrite-normalized LREE-enriched patterns. Based on these characteristics, we suggest that: the Tuanbaoshan granites are calc-alkalic highly-differentiated S-type, and were emplaced in a Late Proterozoic continent-continent collision setting. Source rocks of the Tuanbaoshan granites may have been argillaceous sandstone or graywacke, and the pluton may have subsequently experienced two magmatic events during the end of Jinning movement and the Mid Caledonian movement.

Zircon U-Pb Dating, Geochemical Characteristics, and Tectonic Significance of the Chaganobo Monzogranite in the Northern Margin of Alxa Block, Inner Mongolia

LI Jie, CONG Diange, GU Yanchun, TIAN Ying

2021, 35(02):  514-522.  DOI: 10.19657/j.geoscience.1000-8527.2020.045

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The Chaganobo monzogranite is located in Alxa of Inner Mongolia. The intrusion is the main component of the large Norgong intrusive complex in the Norgong-Langshan tectonic belt, located in the northern margin of Alxa block. The monzogranite is LA-ICP-MS zircon U-Pb dated to be late Early Permian (279±1 Ma). The monzogranite has SiO₂=72.88%-75.44%, total alkali (K₂O+Na₂O)=8.16%, K₂O/Na₂O=0.87-1.41, Al₂O₃=12.38%-13.98%, aluminum saturation index (ASI=A/CNK)=0.88-1.02, falling into the alkaline field in the AR-SiO₂ diagram. These geochemical characteristics show that the Chaganobo monzogranite is high Si-K metaluminous granite. The right-inclining chondrite-normalized REE patterns indicate that they are enriched in LREEs (LREE/HREE=4.70-14.66) and show moderately to strongly negative Eu anomalies (δEu=0.18-0.87, average 0.37). The primitive mantle normalized trace element diagrams are characterized by obvious depletions of HFSEs, such as Nb, Ta, Ti, and P. The rocks contain very low average Sr (94.44×10^-6) and high Yb (3.10×10^-6). Geochemical characteristics show that the Chaganobo monzogranite formed in the post-collision setting, where extensional-related underplating may have started the magma intrusion event. Based on the regional data, we deduce that the northwestern margin of the north China plate (represented by the Alxa block) and the southern margin of the Tarim plate may have entered the post-collisional stage in the Early Permian.

Zircon U-Pb Geochronology and Significance of Granites from Zhuozishan Region, Wuhai, Inner Mongolia

HE Zeyu, SHEN Junfeng, ZHANG Shanming, LIU Jun, DU Baisong

2021, 35(02):  523-534.  DOI: 10.19657/j.geoscience.1000-8527.2020.050

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The Zhuozishan granites in Wuhai (Inner Mongolia)are located at the intersection of the western margin of Ordos basin and the eastern margin of Alxa block, and comprise mainly syenogranite and monzogranite.LA-ICP-MS zircon U-Pb dating indicates that the syenogranite and monzogranite were emplaced in (2,039±79) Ma and (1,866±120) Ma, respectively, which are the products of Paleoproterozoic magmatsm.According to geochemical characteristics, the syenogranite and monzogranite are calc-alkaline, and both are rich in Si, Al and Ka, but relatively poor in Ti, Na and Ca. In addition, the granites are peraluminous (A/CNK=1.19-1.32).There is significant total REE difference between the monzogranite (17.36×10^-6-11.70×10^-6) and syenogranite(241.05×10^-6).Meanwhile, our data show that LREEs are obviously enriched and HREEs are relatively depleted. The former shows obvious negative Eu anomaly (Eu/Eu^*=0.36), and the latter shows obvious positive Eu anomaly (Eu/Eu^*=3.39-4.89), indicating that the fractionation degree of syenogranite is higher.In terms of trace elements, syenogranite is characterized by enrichments in Th, U, Pb, and large ion lithophile elements (LILEs, e.g., Rb, K),but depleted in Ba, Nb, and Ti. Monzogranite is characterized by enrichments in LILEs and depletions in high field strength elements (HFSEs, e.g., Th, Nb, Ta, and Ti).Considering the regional geological background, these rocks were likely formed by partial melting of argillaceous rocks in different stages of the same magmatic phase, representing the products of the same collision stage. This indicates that there were two magmatic events in the Late Paleoproterozoic Zhuozishan.Considering the contact characteristics of strata and the mineralization distribution characteristics, we infered that the final orogenic event between the Alxa and Ordos blocks was completed by the end Paleoproterozoic.

Genesis of Eocene Porphyry in Masjed Daghi, Iran: Evidence from Spectrographic, U-Pb Dating and Geochemical Characteristics

KOU Guanyu, ZHOU Ye, ZHENG Yuanchuan, YU Jiaxing

2021, 35(02):  535-551.  DOI: 10.19657/j.geoscience.1000-8527.2020.105

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The Ahar-Julfa/Arasbaran metallogenic belt is located in the Urmieh-Dokhtar magmatic Arc of the central part of the Tethyan metallogenic belt. The Masjed Daghi porphyry deposit in Iran is located in the northwestern part of the Ahar-Julfa metallogenic belt, in which the porphyry deposits have not been sufficiently studied. This paper is based on the Eocene porphyry from the Masjed Daghi porphyry deposit, with the aim to deepen the understanding on the metallogenic background and magmatic environment of the Ahar-Julfa metallogenic belt. We used infrared spectroscopy, raman spectroscopy, electron probe microscopic analyzer (EPMA), whole-rock major- and trace-element analyses, and zircon trace element and U-Pb-Lu-Hf isotope analyses to clarify the formation setting, oxygen fugacity, water content and other characteristics of the ore-forming magma. Zircon U-Pb dating of the porphyry yielded Eocene (54.1±1.5 Ma) age and positive ε_Hf(t) ratios (+4.5 to +13.5), with T_DM ages of 260 to 841 Ma. The rocks have high K₂O (4.9%) and Sr (120×10^-6-122×10^-6) but low Y (5×10^-6) contents. In addition, the porphyries have high zircon Ce⁴⁺/Ce³⁺ ratios (152-543), and their plagioclase has high Al content. Such evidence indicates that the Eocene Masjed Daghi porphyries have water-rich and high-O fugacity features, and were generated from mafic magma upwelling during subduction.

Zircon U-Pb Age and Its Geological Significance of the Anjiacha Formation in Xicheng Pb-Zn Orefield, Western Qinling

HAO Decheng, DING Zhenju, GAO Zhaokui, HAN Yaoquan, ZHOU Hong

2021, 35(02):  552-567.  DOI: 10.19657/j.geoscience.1000-8527.2020.053

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The Qinling-type Pb-Zn deposit in the Xicheng Pb-Zn orefield (west Qinling orogen) is hosted in Middle Devonian carbonate and fine-clastic sequences, and was formed or transformed by hydrothermal sedimentation. The Anjiacha Formation near the Changba-Lijiagou super-large Pb-Zn deposit is affected by regional metamorphism and contact metamorphism, and high-grade biotite-quartz schist is widely distributed. Detrital zircon U-Pb dating of the biotite-quartz schist yielded a maximum age of (401±6) Ma. Combined with lithogeochemical evidence, it is considered that the sedimentary structural environment was mainly active continental margin, which was likely the product of Early-Middle Devonian fault basin in the northern margin of Yangtze plate. The detrital provenance may have been the Yangtze Craton and north Qinling region.

Paleochronology, Geochemical Characteristics, and Geological Significance of the Upper Permian Linxi Formation in the Northern Songliao Basin

ZHANG Haihua, LI Xiaohai, ZHANG Jian, ZHENG Yuejuan, CHEN Shuwang, ZHANG Dejun, SU Fei, BIAN Xiongfei, SUN Lei

2021, 35(02):  568-578.  DOI: 10.19657/j.geoscience.1000-8527.2021.040

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The paleontological fossils, major and trace elements (including rare earth elements (REEs)) of the Upper Permian Linxi Formation (Fm.) clastic rocks in well Heifudi 1 in the northern Songliao Basin were analyzed, in order to explore the age, geochemical characteristics, sedimentary environment, and tectonic background of the Linxi Formation. The Protohaploxypinus fertilis, Piceaepollenites opimus, and Alisporites communis palynological fossil assemblages obtained in this study indicate that the stratigraphic age is Late Permian. The average content of major elements is SiO₂=60.41%, CaO=3.65%, Al₂O₃=16.36%, K₂O=2.69%, Na₂O=3.25%, K₂O/Na₂O=0.40-7.01, and A1₂O₃/(CaO+Na₂O)=0.92-6.89. The total REE (ΣREE) contents are of 63.20×10^-6-208.05×10^-6(average 147.56×10^-6), with mainly negative Eu anomalies (δEu) of 0.56-1.34 (average 0.73) and weakly-negative Ce anomalies (δCe=0.90-0.98). The clastic rocks are characterized by LREE enrichments and HREE depletions. The trace element contents are characterized by depletions in Nb, Ta, and Sr, and enrichments in Rb, Ba, La, Sm, Pb, and Nd. Geochemical features of the clastic rocks and the discrimination diagrams of La/Th-Hf and TiO₂- Ni show that the Linxi Formation has diverse sediment sources, mainly from upper-crustal felsic rocks and minor from mafic rocks. The primitive-mantle normalized multi-element and chondrite-normalized REE diagrams, K₂O/Na₂O-SiO₂ plot, together with tectonic discrimination functions (F₁, F₂) and geochemical parameters indicate that the northern Songliao basin provenance source region was largely in an active continental margin and continental island arc setting, whilst the sedimentary facies was transitioned from marine-terrestrial to terrestrial.

Altered Rock Characteristics and Gold Prospecting of Gangcha-Kemo Area in Gansu Province

WANG Shuhao, SHEN Junfeng, PENG Zidong, XU Liwei, NIU Gang, LIU Haiming, LI Jinchun, DU Baisong, LIU Jiajun

2021, 35(02):  579-588.  DOI: 10.19657/j.geoscience.1000-8527.2020.070

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The Gangcha-Kemo gold deposit in the Xiahe-Hezuo region is located on the northern margin of the west Qinling orogen. The deposit is of epithermal type with possible occurrence of deeper porphyry mineralization system. We studied the rock alteration features of the gold deposit with the short-wave infrared spectral (SWIR) mineral analysis technique. The results indicate that local alteration minerals include mainly muscovite, illite, montmorillonite, kaolinite, dickite, chlorite, epidote and quartz. Phyllic alteration is developed mainly around the orebody. The central phyllic zone, transitional argillic zone, and the peripheral propylitic zone are developed successively away from the Xiajiamengoukou area. Besides, illite crystallinity has shown obvious decreasing trend outward from Xiajiamengoukou. This shows that Xiajiamengoukou likely represents the hydrothermal center.