Table of Content

20 December 2019, Volume 25 Issue 6

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Redox Conditions of Paleogene Paleolake and Development Models of High-quality Source Rocks in the Dongpu Sag, Bohai Bay Basin

MA Xiaoxiang，YAO Suping，ZHANG Bolin，ZHANG Yunxian，PENG Jun

2019, 25(6):  801.  DOI: 10.16108/j.issn1006-7493.2019081

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 Based on the analysis of major and trace elements in source rock samples of the Paleogene Shahejie Formation in the Dongpu sag, the paleoredox conditions, paleoproductivity level and limitations of paleo-water body during the formation of high-quality source rock of the Shahejie Formation in the Dongpu sag were studied, and the development model of high-quality source rock of Shahejie Formation was discussed. The results show that the development of high-quality source rocks in the third member of Shahejie Formation in the north of Dongpu sag belongs to the model of“stratified sedimentation of deep-water narrow basin water body”. The water body is mainly brackish-super-brackish water, which has strong limitations and deep salinity stratification, which results in stable anoxic conditions of bottom water and is conducive to the preservation of organic matter. According to the analysis of major and trace elements and the reduction of paleoproductivity, the surface layer of Sha 3 member is rich in oxygen, and a large amount of organic matter is developed. After biological death, organic matter 1flocculates, adsorbs on the surface of ferromanganese oxide at the redox interface, and settles to the bottom, and the bottom water is anoxic to reductive, which is suitable for preserving organic properties. The development of high-quality source rocks in the third member of Shahejie Formation in the south of Dongpu sag belongs to the model of“shallow wide basin and basin anoxic deposition”. The lake water body is fresh water, and its limitation is very weak. It belongs to open water area. Because surface rivers and floods bring a lot of nutrients, surface water body has high paleoproductivity, lacking of oxygen-deficient preservation conditions, and consumes more organic matter in biochemical stage. However, weak reducing water body can also be developed locally, thus forming high-quality hydrocarbon source rocks.

Characteristics and Generation Potential of Paleogene Hydrocarbon Organisms in the Dongpu Sag

ZHANG Yunxian，XU Chang

2019, 25(6):  813.  DOI: 10.16108/j.issn1006-7493.2019087

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The hydrocarbon-generating organisms in the source rocks of the Paleogene Shahejie Formation in the Dongpu sag were studied by optical microscopy and SEM. Results show that such organisms are well developed and various in types, including dinoflagellate, green algae, cyanobacteria and coccolithophores. Major combination of hydrocarbon-generating organisms developed in saline water area of the northern Dongpu Sag include dinoflagellate, cyanobacteria and coccolithophores, all of which can form algal laminae. In contrast, major combination of hydrocarbon-generating orgnisms developed in freshwater area of the southern Dongpu Sag include green algae and higher plants, which are developed in massive mudstone. Thermal simulation experiments were conducted on
two samples taken from saltwater and freshwater areas, respectively. It is showed that cyanobacteria and coccolithophores in saline water area have high hydrocarbon generation potential and low oil generation threshold, and can generate large amount of oil at low maturity stage, and the controlling factor of producing low maturity oil is cyanobacteria. The oil generation threshold of dinoflagellates and green algae is higher, among which dinoflagellates are important hydrocarbon generation organisms because of their large amount of biomass. Algae in the freshwater area have lower hydrocarbon generation potential and higher oil generation threshold than dinoflagellates.

Organic Geochemistry of Organic-rich Cherts in the Middle Permian Gufeng Formation in Chaohu, Lower Yangtze Area

GENG Zi’ao，WEI Hengye

2019, 25(6):  823.  DOI: 10.16108/j.issn1006-7493.2019082

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The black cherts in the middle Permian Gufeng Formation in Chaohu area in Lower Yangtze, South China are rich in organic matter. This formation is the potential source rocks of petroleum and shale gas exploration in China. The study on the organic matter accumulation of the Gufeng Formation is important for the petroleum exploration. Here we analyze total organic carbon (TOC) content and biomarkers characteristics of organic matter in the Gufeng cherts of middle Permian in the Pingdingshan, Chaohu area, so as to address the organic matter accumulation mechanism. Our results show that the organic matter thermal mature proxy C29 ααα 20S/(20S+ 20R) ratios range from 0.37 to 0.46, and C31 22S/(S+R) ratios range from 0.53 to 0.59. These suggest that the thermal mature level is the early stage of“oil window”. The n-alkane compounds are mainly composed of short-chain n-alkanes and peak at C18. Hopane contents are
higher than the terpane. C27 ααα 20R sterane is higher than the C28 and C29 ααα 20R. These suggest that the organic matter of the Gufeng Formation originates from phytoplankton algae and bacteria with a small amount of terrestrial high plant. The Pr/Ph ratios of the organic matter in the Gufeng Formation is less than 0.6, and the average gamacerane index is 0.2. These suggest that the redox conditions of paleo-water column during the deposition of the Gufeng Formation were moderate stratification and anoxia. The organic-rich Gufeng Formation is probably related to this stratification and anoxia during warm climatic conditions. Associated with the suddenly cooling, enhanced upwelling increased greatly the primary productivity greatly and yielded more large-scale organic matter accumulation.

Reconstructing Redox Conditions of Upper Cretaceous Source Rocks in the Houjingou Section of the Songliao Basin

SONG Li，CHEN Ruiqian，SHANG Fei

2019, 25(6):  838.  DOI: 10.16108/j.issn1006-7493.2019077

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The Songliao Basin is a large-scale continental lacustrine basin in China. The Upper Cretaceous Nenjiang Formation developed high-quality source rocks, which were mainly formed in the first (K2n1 ) and second members of the Nenjiang Formation (K2n2 ). However, the redox environment has been debated over years. The Houjingou section, in the typical area of Songliao Basin, is selected as the study site. By measuring and analyzing organic carbon and nitrogen isotopes, trace elements and other geochemical indicators of the source rocks in this area, it is found that there are obvious differences between K2n1 and K2n2 . The higher TOC value, the negative deviation of δ13Corg value and the smaller value of δ15N in K2n2 than those in K2n1 indicate that the anoxic environment formed in K2n2 after marine transgression. Combined with trace element ratios, the paleo-productivity parameters such as Ba/Al and Ba/Ca reflect the higher productivity of K2n2, which is consistent with the change of TOC; the redox parameters such as Mo/Al and V/Cr indicate the water environment is more reductive in K2n2 deposition, which is consistent with the conclusion that the δ15N value is small indicating the formation of anoxic environment in K2n2. The redox environment of the water body during the deposition period is reconstructed. The research results can not only predict the distribution of high-quality source rocks, but also provide a reference for the study of sedimentary environment of other lacustrine shales in China, and provide a new basis for oil and gas exploration.

The Control Factors and Geological Implications of the Nanopore Morphology of the Lower Paleozoic Black Shales in the Sichuan Basin, China

JIAO Kun，XIE Guoliang，PEI Wenming，LIU Shugen，LIU Xiangjun，KANG Yili，DENG Bin，PANG Qian，LIU Wenping，LUO Chao

2019, 25(6):  847.  DOI: 10.16108/j.issn1006-7493.2019080

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The nanopore characteristics especially the pore morphology of Longmaxi and Qiongzhusi black shales of typical wells in Sichuan Basin and its periphery such as Well A to D were analyzed by Field emission scanning electron microscopy (FESEM), Pores and Cracks Analysis System (PCAS)—a SEM image processing software, and low-pressure N2 adsorption was used as a secondary tool. The form factor was the major parameter to characterize the pore shapes in this study. The results show that the nanopore morphology of black shales are controlled by the nanopore type (occurrence location of nanopores), maceral type, burial depth, thermal maturity of organic matters and the pore sizes. Here are the details: (1) The ratios, pore sizes, form factors of interparticle pores, intraparticle pores and organic matter pores are quite different, which shows that the evolution of the three types of nanopores have different paths. (2) The pore shapes of nanopores in solid bitumen are more regular than those in other macerals. (3) The compaction related to the burial depth can compress the pores vertically, to reduce the pore sizes and change the pore shapes from round or elliptical-shaped to slit-shaped. (4) The nanopores shapes of organic matter pores generally tend to be more regular with the increase of thermal maturity, although the trend may be disrupted by the compaction and the merging of nanopores. (5) The nanopores that have smaller sizes tend to have more regular pore shapes. The preliminary study shows that the morphology of nanopores in solid bitumen may indicute the pressure environment of main reservoir space in the high-over mature shale gas reservoirs. However, the pattern of the pore morphology evolution
in solid bitumen，and the possibility of using form factor of nanopores in solid bitumen to evaluate the overpressure and shale gas preservation condition in potential gas shales still need further research.

Geochemical Characteristics of the Sinian Doushantuo Formation Source Rocks of the Sichuan Basin: Implications for the Organic Matter Accumulation

WANG Wenzhi，XIAO Wenyao，HE Yuan，ZUO Zhaoxi，DENG Sisi，CAO Jian

2019, 25(6):  860.  DOI: 10.16108/j.issn1006-7493.2019086

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Deep ancient strata system is getting more and more attention in the field of hydrocarbon exploration, among which the Sinian Doushantuo Formation of the Sichuan Basin is considered as one of the most potential source rock. During the past decades, few work aimed at this formation had been taken. To deepen understanding of the Doushantuo Formation as a set of potential source rock, taking well-sampled Minyuexiang Section of Chengkou County of northeastern of Sichuan Basin as a case, the geochemical characteristics and organic matter accumulation mechanism of Doushantuo shale are systematically discussed by means of comprehensive analysis of petrology, major, trace and rare earth elements. The results show that the organic-rich black shale developed in the Doushantuo Formation was mainly deposited in suboxic-anoxic environment, and significant positive Eu anomalies indicate that the Doushantuo Formation wasinfluenced by hydrothermal activity during the sedimentation process. The organic matter accumulation is strongly controlled by the anoxic sedimentary environment. Due to the low sedimentation rate, the organic matter gradually enriched with the increase of deposition rate from bottom to top. In addition, hydrothermal activity led to the elevated accumulation and burial of organic matter. Further, the Doushantuo Formation has certain oil and gas potential, but its heterogeneity is developed, which may be related to the eustatic change during the glacial-interglacial period.

Kinetics Characteristics of Hydrocarbon Generation of Paleogene Shales in the Naner Sag, South Yellow Sea Basin

SU Peng，XU Jianyong，WANGHang，ZHU Xiangfeng，SHEN Chuanbo，SONG Yu，HU Shouzhi

2019, 25(6):  871.  DOI: 10.16108/j.issn1006-7493.2019079

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Hydrocarbon resource potential in the South Yellow Sea Basin has always been a concern for offshore exploration in China, but no breakthrough has been made yet. The kinetics of hydrocarbon generation is very important for the evaluation of hydrocarbon generation potential of source rocks, and the researches of hydrocarbon generation kinetics of Paleogene shale is very weak in the study area. Hence, based on the conventional geochemical analysis and statistics of source rocks in the Southern Depression, South Yellow Sea Basin, low-mature samples in the Naner Sag were selected for experimental analysis such as PY-GC, SRA, MSSV, and then they were used for kinetic simulations. The results demonstrated that the fourth members of Funing Formation in Naner Sag has entered the early stage of hydrocarbon generation, and its predicted petroleum type was paraffinic-naphthenic-aromatic (PNA) low wax oils. The distribution range of the activation energy is 43-60 kcal/mol, with the main peak at 51kcal/mol accounting for 72% of the total. The frequency factor of the bulk petroleum generation is A=9.66×1012 and the temperature of the hydrocarbon generation window ranges from 128 to 158℃. The hydrocarbon generated from the whole evolution process existed as two-phase fluid in the Funing Formation at the end of Oligocene (23-30 Ma), and in the rest of the time period as a single liquid phase. And the conversion rate (TR) of these source rocks was still between 50% and 60%. The hydrocarbon generation kinetics of the Paleogene shale of Naner Sag in South Yellow Sea Basin obtained in the study is very important for the estimation of hydrocarbon resource potential and provides theory basis for the further petroleum exploration and exploitation decision in this region.

Characteristics of Condensate Oil Formation and Evolution from Continental Source Rocks Based on Simulation Experiments

ZHANG Jiayu，HU Guang，SHU Yuchuan，TANG Hong

2019, 25(6):  879.  DOI: 10.16108/j.issn1006-7493.2019078

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In order to investigate the generation and evolution of the condensate oil from the continental source rocks during deep burial, the high temperature and pressure simulation was conducted using samples from Shu 13, Manan 603 in the western sag and Tao 10 in the eastern sag of the Liaohe Basin. Results show that the maximum yield of condensate oil generated from type-I kerogen (source rocks from Shu 13) was 2.32 mg/g TOC, which occurred at the vitrinite reflectance (VR) of 1.76% Ro and the main components were hydrocarbons with low molecular mass. Those may indicate that such condensate oil mainly generated from secondary cracking of the liquid hydrocarbon. The maximum yield of condensate oil in type-II kerogen (source rocks from Manan 603) was 2.28 mg/g TOC, and appeared at VR of 2.42% Ro, which was obviously later than that of type-I kerogen. These condensate oil products were mainly composed of hydrocarbons with higher molecular mass and part of hydrocarbons with low molecular mass. The yields of condensate oil of type-III kerogen (source rocks from Tao 10) were the lowest and the maximum yield was only 0.92 mg/g TOC. Those results may indicate that the potential of condensate oil derived from type-III organic matter was poor.

Characteristics of LA-ICP-MS Trace Elements in Pyrite: Implications for the Genesis of Xitieshan Deposit in Qinghai Province

YAO Xizhu，YANG Tao，ZHU Zhiyong，DANG Ben，FENG Zhijie，WU Changzhi

2019, 25(6):  888.  DOI: 10.16108/j.issn1006-7493.2019028

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The Xitieshan Pb-Zn deposit is located in the northern margin of Qaidam basin. Three type of ore bodies, that is, stratiform, veined and irregular were identified from the deposit in schist, marble and their transition zone. The Tangjianshan group, host rock of the deposit, is greenschist metamorphosed mafic to felsic volcano-sedimentary formation which deposited from late Ordovician shallow marine. There are many disputes about genetic types of the deposit, such as sediment-hosted Zn-Pb deposit (SEDEX), volcanic-associated massive sulphide deposit (VMS) and metamorphic overprinting exhalative sedimentary deposit. According to macro-and micro-observation on ore fabric, three main types of pyrite from stratiform ore bodies had been identified, that is, colloidal pyrite (Py-I), anhedral pyrite (Py-II) and subhedral-euhedral pyrite (Py-III), which could represent primary depositional pyrite, low level of recrystallized pyrite and high level of recrystallized pyrite, respectively. In this contribution, laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) was used to measure minor and trace element concentration for the above three types of pyrite, and obvious differences had been recognized. Metallogenic elements, such as Cu, Pb, Zn, Co, Ni and so on, are enriched in Py-I, Co/Ni> 1, and yield smooth time-space resolved signal of LA-ICP-MS analysis. Among them, Ni, Co and As might be situated in lattice of pyrite as solid solution, while Cu, Pb, Zn and other elements might exist in lattice（defect）of pyrite in form of nanometer mineral inclusions. Metallogenic elements in Py-II are variable and obviously lower than which in Py-I, and yield lots of pumps in time-space resolved signal of LA-ICP-MS analysis, indicating their existence of micrometer mineral inclusions in pyrite lattice. Metallogenic elements in Py-III are extremely low and generally below detection limit, except for some spot with minor amount of trace elements. Based on their occurrence and fabric characteristics, we assume that the Py-I was formed in the process of the submarine-exhalative sedimentation with volcanic activities, while Py-II and Py-III were produced from Py-I by later diagenetic and metamorphic
recrystallization in different degrees. Combined with recent studies of petrographic, zircon U-Pb geochronology, geochemistry and tectonic background of their host rocks, it is strongly suggested that the Xitieshan deposit should be a VMS, which had be undergone significant metamorphism of orebodies and reactivation of metallogenic elements during regional orogenic process after mineralization.

Zircon U-Pb Geochronology, Geochemistry and Petrogenesis of Granites from the Wuduoshan Pluton, the North Qinling Terrane

ZHOU Shu，ZHANG He，CHEN Fukun

2019, 25(6):  901.  DOI: 10.16108/j.issn1006-7493.2019030

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The Wuduoshan pluton, located in the eastern segment of the North Qinling terrane, is a large-scale composite granitic intrusion formed via multi-stage emplacement. Zircon U-Pb dating yields crystallization ages of 441-430 Ma for representative samples of biotite granite and monzogranite, and inherited zircons have been identified in some samples with ages ranged from Proterozoic to Neoproterozoic. Granites from the Wuduoshan pluton exhibit high silica, low magnesium, alkaline-rich, and weakly to strongly peraluminous signatures. On the primitive mantle-normalized trace element spidergrams, they show enrichment of Rb, Ba, K and Pb and depletion of Nb, Ta, P and Ti. Chondrite-normalized REE distribution patterns exhibit distinct fractionation between LREE and HREE, and with δEu values of 0.36-1.45. 87Sr/ 86Sr(i) and εNd(t) values for granites from the Wuduoshan pluton are 0.70304-0.71290 and -4.6- -1.9 respectively, with two-stage Nd model ages of 1.34-1.58 Ga. These granites were likely formed via a partial melting process by using REE discrimination diagram. Isotopic compositions indicate that they were originated from a source characterized by mixing of metasedimentary rocks from the North Qinling terrane and mantle-derived materials. Among them, one part of granites were formed in lower crust, and another part were generated in middle-upper crust. The mantle-derived materials in source region may be intermediate-basic plutons that intruded into the crust during an early stage. Combined with crustal evolution of the North Qinling terrane during Early Paleozoic, we suggest that the granites from the Wuduoshan pluton likely reflect the collapse process of the thickened North Qinling crust under a post-collisional setting during Early Silurian.

Mineralogical Behavior andMetallogenic Process of Rare and Rare EarthMetals in Shishisi NYF-Type Pegmatite, Lin’an, Zhejiang Province

WANGWU Mengyu，RAO Can，DONG Chuanwan，LIN Xiaoqing，QIN Lixi，LYU Qing

2019, 25(6):  914.  DOI: 10.16108/j.issn1006-7493.2019027

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Shishisi pegmatite, a typical Nb-Y-F (NYF) type pegmatite with different kinds of rare and rare earth minerals, occurs in the northwest of Heqiao granite pluton, Lin’an county, Zhejiang Province. Based on field and microscopic observations, in combination with back-scattered electron (BSE) imaging and quantitative electron-microprobe technology (EMPA), the rare and rare earth minerals from Shishisi NYF-type pegmatite are systematically analyzed for the enrichment, migration, crystallization, and mineralization processes of rare and rare earth metal elements in this paper. The results show that (1) many rare and rare earth metal minerals in the Shishisi pegmatite are recognized including Nb-Ta oxide minerals (columbite, tapiolite, microlite etc.), Y-bearing minerals (fergusonite
and polycrase), W-bearing minerals (wolframite, scheelite and Nb-W-bearing minerals), Ce-bearing minerals (monazite, fluocerite and bastnaesite) and Th-bearing minerals. (2) The WO3 content ranges from 8.30 to 70.51 wt.% in Nb-W-bearing minerals, suggesting a series of polysomic minerals between columbite and wolframite. (3) LA-ICP-MS U-Pb dating of columbite-(Fe) from the Shishisi pegmatite yields an age of 133±2 Ma, indicating its genetic relationship with Heqiao granitic pluton. (4) The rare and rare earth elements mineralization processes in Shishisi NYF-type pegmatite were closely related to the crystallization and evolution of magma. During the magmatic stage, zircon, thorite and monazite were crystallized; polycrase, columbite, fergusonite and fluocerite were formed
in the magmatic-hydrothermal stage; wolframite and wolframoixiolite were crystallized in the hydrothermal stage. In the meantime, the early monazite and fluocerite were altered to secondary Ce-bearing minerals by late hydrothermal fluids.

Structural Characteristics and Chronological Constraints on the Yemaquan-Lebaquan Ductile Shear Zone in Mazongshan Area, Beishan

LI Zhi，ZHU Wenbin，WU Hailin

2019, 25(6):  932.  DOI: 10.16108/j.issn1006-7493.2019024

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The Yemaquan-Lebaquan ductile Shear Zone, as the result of microcontinent collision in the Beishan Orogen, is of great geological significance in the Early Paleozoic collision-orogenic process of the Beishan orogenic belt. Previous studies mainly focused on the tectonic metamorphic core complex and the ophiolite in Lebaquan area. In order to better constrain its active age, the structural profile and microstructural characteristics of the Lepaquan-Yemaquan ductile shear zone were studied. It shows that the shear zone thrust from NWW to SEE during the earlier period, which then transformed into nearly E-W dextral strike-slip during the late stage. The mylonite zone was intruded by the late undeformed granite veins, which have a zircon U-Pb age of 416 Ma. Integrated with the previous studies, the time ductile deformationwas constrained between 423-416 Ma.

Research on Petroleum Geological Conditions and Exploration Directions in Mandong Area of Qaidam Basin

WEI Hengfei，GUAN Ping，WANG Haifeng，WANG Peng

2019, 25(6):  943.  DOI: 10.16108/j.issn1006-7493.2019054

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On the basis of core observations and systematic sampling in the Mandong area of Qaidam Basin, and using rock pyrolysis, casting thin sections, sub-ion polishing scanning electron microscopy and other techniques, this paper studies petroleum geological conditions of reservoir formation and prospecting in the Mandong area of Qaidam Basin through typical reservoir anatomy. The results show that well-developed mature source rocks occur in Lower Gangchaigou Formation, Upper Gangchaigou Formation, and Lower Youshashan Formation in the Huangguamao-Kaitemilike-Youdunzi, and are the favorable reservoir-forming areas. Due to the long-term shore-shallow lake environment and lack of terrestrial debris supply in the Mangdong area, the reservoir is mainly composed of beach-bar sand bodies and stucco flat of the shore-shallow lake. Overall, the reservoir property is poor and most are tight reservoirs. The oil and gas reservoir in the Mangdong area is mainly structural reservoir, which is characterized by“upper oil and lower gas”and near-source accumulation. Also, it is mainly transported and accumulated vertically through faults. Based on the analysis of petroleum geological conditions and drilling results in the Mangdong area, deep natural gas is considered one of the most important targets for oil and gas exploration in this area. In addition, lithologic lens traps such as beach dams and plaster flats are the types that have great exploration prospects in this area.

Distribution Pattern of Present-day Formation Temperature in the Qiongdongnan Basin: Implications for Hydrocarbon Generation and Preservation

GAN Jun，WU Di，ZHANG Yingchao，LIU Shaowen，GUO Minggang，LI Xianglan，ZHU Jitian，XIONG Xiaofeng，WANG Hui，YIN Hongwei

2019, 25(6):  952.  DOI: 10.16108/j.issn1006-7493.2019053

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Temperature is a key factor that controls the generation and perseveration of hydrocarbon, and accurate determination of deep formation temperature is of great significance for hydrocarbon resource assessment of sedimentary basins. The Qiongdongnan Basin, located at the northern margin of the South China Sea, is the ongoing target for deepwater hydrocarbon exploration in China, so understanding the spatial pattern of the deep formation temperature for this basin is fundamental for deepwater exploration in this area.On the basis of measured borehole temperatures and estimation of deep temperature with systematic dataset of thermal properties, the distribution patterns of the deep temperatures for the T30-T70 were presented here. The results demonstrated that the dominative range of measured reservoir temperatures of the basin is 90 to 150℃ , higher than the recently proposed golden temperature zone (60 to 120℃), which is partially due to the high thermal background of the Qiongdongnan Basin. In addition, the spatial patterns of the deep temperatures for the T30-T70 formations are characterized by high in the western basin and lower in the eastern basin, and the high temperature anomaly is found in the Yanan Depression, as a result of differential burial depth, thermal conductivity and basal heat flow associated with regional crustal extension within the basin. The results would provide robust geothermal constraints on the hydrocarbon exploration and drilling safe design for this basin.