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2020, 34(02): 214-215.

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Characteristics of Petrology,Geochemistry and U-Pb Dating of Zircon from the Jiangbian Pluton in the Yangla Copper Deposit, Western Yunnan

YANG Fan, YU Can, JIAN Runtang, YAO Zhihua

2020, 34(02): 215-232. DOI: 10.19657/j.geoscience.1000-8527.2020.02.01

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Yangla copper deposit is a large deposit of Jinshajiang-Ailaoshan copper-gold metallogenic belt in Three-river area, which located in the bonding zone of Jinshajiang plate between Zhongza micro-continent fragment and Changdu-Simao block. The results show that the major elements of the Jiangbian granodiorite-rock mass is a typical I-type granite, with high SiO₂, Al and lithophile elements, and deficit of Fe, Mg and high field strength elements.Besides, the samples mainly fall into the transitional regions of volcanic-arc granite and syn-collision granite, which reflects the Jiangbian granodiorite pluton is the products of active plate margin, and the main magmatic fluid compositions are mantle and accompanied with input of melting of the crust. Zircon U-Pb dating by LA-ICP-MS method indicated that the ages of ZKJ1-1-14 and ZKJ2-1-7 granodiorite porphyry were 215 Ma and 208 Ma, and ZKJ1-1-17, ZKJ2-1-6 and ZKJ2-1-8 granodiorite were 221 Ma, 220 Ma and 214 Ma, respectively. Based on the crystallizing age of Lunong pluton, Linong pluton, Jiangbian pluton and Beiwu pluton, this paper hold that the magmatic intrusions tended to freshen from southern mining area to northern mining area. And the magmatism in the Jiangbian area was active for a long period (about 25 Ma). This research corroborated that the Jiangbian rock mass is a island-arc type granite associated subduction-collision and formed in post-collisional extensional environment.

Geochemisty and SHRIMP Zircon U-Pb Chronology of Ore-bearing Rocks from Graphite Deposit in Beizhen County, Liaoning Province

YANG Peiqi, LIU Jingdang, ZHANG Yangfei, LIANG Shuai, LIU Shumei

2020, 34(02): 233-243. DOI: 10.19657/j.geoscience.1000-8527.2020.02.02

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Dutun graphite deposit in Beizhen County of Liaoning Province occurs in the Middle Proterozoic Gaoyuzhuang Formation. This study carried out systematic analyses on geochemical characteristics of ore-bearing rocks and SHRIMP U-Pb dating for detrital zircons to understand the forming age and tectonic setting of graphite deposit in this area. The geochemical results show that the ore-bearing rocks are characterized by high contents of REEs and enrichment of LILE (such as Rb, Ba, K and Sr), suggesting that the source rocks derived mainly from terrigenous clastic materials mixed with minor amount of marine materials and deposited in an anoxic environment. SHRIMP U-Pb dating yields an average surface age of (2535.64±25.59) Ma for detrital zircons, indicating that upper limit of deposition of ore-bearing source rocks was later than the early Early Proterozoic; the surface ages of (1216.75±12.33) to (1675.70±17.46) Ma for recrystallized zircons, representing that low limit of deposition of the ore-bearing sequence was not later than the late Mesoproterozoic; and an average surface age of (212.53 ± 3.69) Ma for authigenic zircons, revealing that late Triassic magmatic-tectonic thermal events played an important ore-controlling role in recrystallization, enrichment and mineralization of crystalline graphite. Therefore, it can be concluded that black organic-rich rock series deposited in the Mesoproterozoic neritic environment was transformed into epimetamorphic graphite deposit through carbonaceous recrystallization, driven by late-stage metamorphism and late Triassic magmatic activity.

Geological,Geochemical Characteristics and Genesis of the Pegmatite Type Uranium Deposit in Hongshiquan, Gansu Province

WANG Wei, WANG Shengyun, LIU Tao, LI Tianshi, CHEN Yunjie, MA Li, ZHAO Ruyi, SONG Zhentao

2020, 34(02): 244-253. DOI: 10.19657/j.geoscience.1000-8527.2020.02.03

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Hongshiquan uranium deposit is in the western part of Longshoushan Mountain uranium metallogenic belt in Gansu Province. It is the most typical pegmatite type uranium deposit discovered in China. It has the characteristics of rock mass type mineralization. Uranium mineralization is strictly developed in the interior of pegmatite body and in contact-contaminated zone. A systematic study of the pegmatite of the main ore-bearing rock shows that uranium exists in the form of crystalline uranium ore, pitchblende and uranium black in the Hongshiquan deposit. Initial uranium mineralization was formed in the late stage of Zhongtiaoian (1,735±67) Ma, and part of ore underwent hydrothermal superimposition during Hercynian (356±46) Ma. Crystalline uranium deposits were mainly formed in the early magmatic metallogenic stage, and pitchblende was formed in the late hydrothermal superimposed transformation stage. A “four-in-one” alteration association similar to the Jiling sodium metasomatic uranium deposit was developed, the hydrothermal transformation process is a process of K removal and Na addition.

Ore-forming Fluid Characteristics and Genesis of the Wudaogou Gold Deposit in Dongning County, Heilongjiang Province

FANG Yan, HE Mouchun, DING Zhenju, XU Yiran, WEI Lianxi

2020, 34(02): 254-265. DOI: 10.19657/j.geoscience.1000-8527.2020.02.04

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The Wudaogou gold deposit is located at Dongning County, Heilongjiang Province, south part of Taipingling Au-Cu polymetallic metallogenetic belt. Au mineralization is mainly presenting as vein type in Lower Paleozoic Shuangqiaozi Formation carbonaceous slate, the distribution of the orebodies is controlled by NE-trending ductile shear zones and faults. We have distinguished three paragenetic stages, i.e., early quartz stage, followed by quartz-pyrite stage and quartz-carbonate stage, with gold being introduced mainly in the second stage. The ore-forming fluids of the Wudaogou deposit are characterized by moderate-low temperature, low salinity, belonging to an H₂O-NaCl-CO₂ system. The δ18$\mathrm{O}_{\mathrm{H}_{2} \mathrm{O}}$and δ$\mathrm{D}_{\mathrm{H}_{2} \mathrm{O}}$ values for the gold-bearing quartz indicate that the ore-forming fluids were a mixture of magmatic and meteoric waters. The trace elements of pyrite from gold-bearing ores show a characteristic of hydrothermal origin. And the δ³⁴S values suggest a mixed source for sulfur, i.e., the deep-seated magmatic and sedimentary rocks. The²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of gold-associated pyrite show a broad linear correlation with those of the local magmatic and country-rock, implying that lead and possibly gold relate to these sources. Whereas the lead isotope compositions of ores (especially the value of ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb) are higher than those of the Shuangqiaozi Formation slate and the diorite. This indicates that the lead may be a product of leaching of the hosted-carbonaceous slate, which suggests that the ore metals originated from the Shuangqiaozi Formation. Integrating the data obtained from this study on geology, fluid inclusions, and H-O-S-Pb isotope systematics, we suggest that the Wudaogou gold deposit is an orogenic gold deposit.

Geostress Characteristics of Coal Reservoir at Changzhi Block of Southern Qinshui Basin

DAI Zhaoxia, SUN Beilei, ZENG Fangui

2020, 34(02): 266-272. DOI: 10.19657/j.geoscience.1000-8527.2020.014

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The geostress characteristics of coal reservoir has a close relationship with the generation, migration and enrichment of coalbed gas, and a good understanding of the geostress characteristics is vital for the exploration and development of the gas field. Referring to the logging data at 18 coalbed gas well in Changzhi block of southern Qinshui basin, the distribution of the vertical stress, the minimum and maximum horizontal stresses is evaluated using an Anderson model with residual tectonic stress. The results show that the vertical stress basically presents a symmetric distribution from east to west, and the stresses in the east and west are higher than that in the middle, ranging from 15.2 MPa to 16.5 MPa. The minimum horizontal stress presents a change pattern of high→low→high→low from west to east, and ranges from 10.2 MPa to 19.3 MPa. The maximum horizontal stress presents a similar pattern, and ranges from 13.0 MPa to 34.2 MPa. The results suggest that the influence of the buried depth is bigger than the tectonic influence in the eastern area, whereas the tectonic influence is bigger than that of the buried depth in the western area. This finding indicates that the geostress distribution has a high correlation with the gas output of the coalbed well, and can offer a good data reference for the coalbed gas exploration, development, safe production and etc.

Pore and Fracture Structure of Coal Reservoir Constrained by Coal Metamorphism

FAN Qizhang, CAI Yidong, BEI Jinhan, WANG Weihao, ZHANG Xueying

2020, 34(02): 273-280. DOI: 10.19657/j.geoscience.1000-8527.2019.016

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To study the effects of coal metamorphism on the structure of pores and fractures, mercury intrusion porosimetry, low temperature nitrogen adsorption/desorption, vitrinite reflectance test, scanning electron microscopic (SEM) analysis, and fluorescence microscopy observation, were conducted to investigate the influence of coal metamorphic degree on porosity, Brunauer-Emmett-Teller (BET) specific surface area, Barrett-Joyner-Halenda (BJH) total pore volume and fracture structure and density. The results indicate that with increasing coal metamorphism, the coal porosity increases first and then decreases, and finally increases slightly. Micropore volume (<10 nm) shows a U-shaped change with the increase of vitrinite reflectance, and small pores show an inverted U-shaped trend. The BET specific surface area, BJH total pore volume, and fracture density (fracture type B, C, and D) of medium- to high-grade metamorphosed coal have the same U-shaped trend. However, for type B, C, and D facture density of medium- to high-grade metamorphosed coal, the minimum values occur near R_o,m=1.1%, and the fracture density reduction of type D fractures is highest when the fracture density decreases. Integrated with the results of vitrinite reflectance, the pore and fracture development are closely related to the four abrupt changes of the coal metamorphism.

Syncline-confined-water Model of Coalbed Methane Enrichment Area in Liuhuanggou Mining Area, Southern Junggar Coalfield

GAO Beidou, WANG Haichao, TIAN Jijun, HAN Xu, FENG Shuo, WANG Di, ZHANG Zhendong

2020, 34(02): 281-288. DOI: 10.19657/j.geoscience.1000-8527.2019.017

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Through systematic collection and analysis of the latest exploration data and existing coal mine production data, controlling factors on the coal rank, structure, hydrology, and caprock enrichment of coalbed methane in Liuhuanggou mining area, Southern Junggar coalfield are studied. The results show that the multiphase tectonic activity might cause the formation of the Akede syncline and the reverse fault on its northern limb. The delta plain swamp controls the development of thick coal seams, and the fan delta, fluvial and lacustrine facies control the lower caprock layer, and the burning-zone weak runoff groundwater dynamic system blocks the coalbed methane. These factors jointly control the enrichment model of the syncline-confined coalbed methane in Liuhuanggou area. It is considered that the Akede synclinal axis, featured by its deep burial depth, high-salinity groundwater and closed caprock conditions, is the most favorable enrichment zone for coalbed methane.

Mechanism and Numerical Simulation of Liucun Coal Mining Subsidence of Yangquan Mining Area in Shanxi

ZHANG Chao, ZHANG Yufei, SUN Yingjie, YAO Yahui

2020, 34(02): 289-296. DOI: 10.19657/j.geoscience.1000-8527.2019.018

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In this paper, we investigated the single seam coal mining subsidence, and introduced in detail the geo-environment background, developing features and deformation characteristics of the Liucun coal mining subsidence of Yangquan mining area in Shanxi. Based on the geological characteristics of the rocks, 12 groups were divided and their formation mechanism was analyzed by key stratum and composite key stratum theory. Subsequently, the Hoek-Brown rock mechanical parameters were obtained, followed by inverse modeling by Flac5.0 Extrusion. Numerical simulation reflects the development and distribution of mining-induced fissures on the surface at different stages, and calculates the final magnitude of subsidence. The natural recovery period of fissures in overburden and bedrocks was estimated to be 2 and 3 months, respectively, and the fissures finally present a “θ” shape in the plane. The final magnitudes of No.1 and No.2 coal mining subsidence centers reach 4.5 m and 4 m, respectively, with the subsidence area being 1.85 times of the workface area. The results are highly consistent with the survey monitoring data, and can reflect the development process of the surface deformation and deep overburden collapse accurately, which is beneficial to subsidence mechanism analysis. The rock mechanical parameters and the simulation methods are applicable to accurate prediction on mining subsidence in Yangquan mining area.

Geothermal Resources Evaluation of Karst Thermal Reservoir in Taiyuan Basin

WANG Tinghao, WANG Xinwei, ZHANG Xuan, MAO Xiang, WANG Di, WU Minghui

2020, 34(02): 297-308. DOI: 10.19657/j.geoscience.1000-8527.2020.02.09

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Taiyuan basin is a representative low-moderate temperature geothermal field in China, with well-developed fault structure and abundant geothermal resources. The overall evaluation of geothermal resources in Taiyuan basin is of great significance to its large-scale development and utilization. Based on the structural evolution analysis, seismic and electrical profiles interpretations and the latest well logging interpretations, the basin is divided into eight geothermal fields as evaluation units with the “thermal energy storage method” based on the secondary structural units. Carbonate karst thermal reservoirs of Cambrian-Ordovician in eight evaluation units have been carefully evaluated. The results show that the carbonate karst thermal reservoir in the basin is characterized by stable thermal reservoir and caprock, shallow burial depth, multiple reservoir sections and large reserves. The cap rocks of karst thermal reservoir result in burial depths of 400 to 2,000 m. From top to bottom, six major aquifer sections of Ordovician Fengfeng Formation, Upper Majiagou Formation, Lower Majiagou Formation, Liangjiashan Formation, Cambrian Fengshan Formation and Changshan Formation are deposited successively. The average geothermal gradient is 3.0 to 4.0 ℃/100 m, and the geothermal water temperatures range is 30 to 80 ℃. Based on that and the parameters of geothermal reservoir area, thickness, temperature, porosity, heat capacity and density, the total exploitable geothermal reserves in the geothermal field is calculated to be 13.84×10⁸ GJ, which is equivalent to 4,721.9×10⁴ t of standard coal. Distribution regularity and amount of geothermal resources in the Taiyuan basin is thus preliminarily clarified.

Sedimentary Model and Identification Marks of Modern Lacustrine Beach-bar on Southeast Coast of Qinghai Lake

YUAN Kun, JIN Zhenkui, PENG Biao, ZHU Xiao’er, TIAN Tian, LI Rui

2020, 34(02): 309-320. DOI: 10.19657/j.geoscience.1000-8527.2020.02.10

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Qinghai Lake is a favorable target for lacustrine beach-bar research. In this study, the sedimentary sequence and characteristics, formation mechanism, sedimentary model and identification marks of beach-bars are systematically examined through field work and micro-analysis on modern beach-bars in Qinghai Lake. There are three lithofacies developed in the beach-bar on the southeast coast of Qinghai Lake, i.e.massive pebbly sandstone, massive conglomerate, and normal-graded sandy conglomerate. Combining lithofacies and hydrodynamic observation on modern lake shore, the wave zone of study area is divided into the wave symmetric, breaker and swash zones. The beach-bar sedimentary model of Qinghai Lake is established, and four identification marks of beach-bar (c.f.mouth-bar) are proposed: (1) beach-bar deposits always change sharply to the underlying semi-deep lacustrine mud deposits with a clear discontinuous surface, whereas in mouth-bar deposits the transition is gradual; (2) beach-bars show typical “ABC” vertical sequence, and the sediments coarsen upwards then return to fine grain; (3) swash cross-bedding and wash-over deposits (tilted to land) can be found in beach-bars, whereas mouth-bars are featured by trough cross-bedding; (4) beach-bar deposits are always shore parallel, whereas mouth-bar deposits are usually shore perpendicular.

Fine Description of Deep-water Gravity Flow Deposits and Its Significance for Hydrocarbon Development in Middle Member 3 of Shahejie Formation in the Shi 112 Wellblock in Dongying Depression

XIN Bo, LIU Jingyan, CHEN Xinkai, MA Xiao, ZHONG Zhao

2020, 34(02): 321-332. DOI: 10.19657/j.geoscience.1000-8527.2020.02.11

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As an important and special type of sedimentary system, deep-water gravity flow deposits have been widely studied since its discovery, and have been given increasingly more attention in the oil and gas exploration industry. Deep-water gravity flow system is developed in the Middle Member 3 of the Paleogene Shahejie Formation in the Dongying depression. We finely described the types and characteristics of deep-water gravity flow deposits, and the development and evolution in different sequences in the Shi 112 block (Haojia Oilfield), according to the seismic and core logging data. The gravity flow deposition pattern in this area is summarized, which provides an important reference for reservoir prediction in the developed wellblock. This study shows that the local deep-water gravity flow system is developed in the highstand system tract of third-order sequence, which comprises four fourth-order sequences ($\mathrm{SQQ}_{1-4}^{3}$) from bottom to top. The deep-water gravity flow deposits developed three major sedimentary types: slump, debris flow and turbidity current deposits. Furthermore, 12 types of lithofacies are identified. The gravity flow scale was relatively small in the $\mathrm{SQQ}_{3}^{2}$ period, but expanded in the $\mathrm{SQQ}_{3}^{3}$ period with the Dongying delta progressing towards the lake basin center. Slump deposits developed mainly at the deltaic front or the root of pro-delta slope, while sandy debris flow deposits were developed at the front of the slump deposits. Debris flow may have transformed to turbidites gradually during its forward transportation process. The turbidity current and debris flow in the deltaic front and pro-delta represent favorable sites for reservoir exploration in the developed wellblock.

Discovery of the Soft-sediment Deformation Structure in the Middle-Upper Triassic Sequence of the Middle Segment of Southern North China Plate Margin and Its Geological Significance

QUAN Xiaoyuan, LIU Chiyang, ZHANG Long, ZHANG Dongdong, WANG Jianqiang, MA Huanhuan, FENG Qi, ZHAI Guanghui

2020, 34(02): 333-344. DOI: 10.19657/j.geoscience.1000-8527.2020.02.12

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The Middle-Upper Triassic sequences in western Henan are mainly exposed at Jiyuan, Yima, Yichuan and Nanzhao. The Qinling orogenic movement controls the properties of the Middle-Upper Triassic basins in the region and the development of Indosinian sedimentary sequences. Outcrop and profile geological characteristics of the study area show, a series of typical soft-sediment deformation structures in the Middle-Upper Triassic sequences, including liquefied diapir, pillow structure, pillar, liquefied convolute, ball-and-pillow, flame-and-load structures, and liquefied breccias. These soft-sediment deformations structures show obvious reproducibility in vertical direction and lateral continuous. It is indicated that the local stratigraphy was affected by frequent and intense seismic activities during this period. By identifying these soft-sediment deformation features and determining its formation mechanism, it is revealed that the triggering factor is the earthquakes related to the Qinling orogeny. The existence of seismite in the local stratigraphy is likely a response to the Qinling orogenic movement.

Genetic Mechanism of Dolomite in Middle Permian Maokou Formation:Case Study of Central and Eastern Sichuan Basin

LI Ting, ZHU Dancheng, LI Haiping, YANG Minglei, LI Tao, LI Pingping, ZOU Huayao

2020, 34(02): 345-355. DOI: 10.19657/j.geoscience.1000-8527.2020.02.13

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Dolomite reservoir is important in the Maokou Formation, and its genetic mechanism is a hot topic of research. Therefore, outcrop and well core observation, thin-section identification and geochemical analysis are conducted to unravel the dolomite genetic mechanism of the Maokou Formation (Fm.). The results show that the Maokou Fm. dolomite can be divided into five types: (1) layered powdered, (2) layered fine-medium crystalline, (3) scattered in limestone, (4) lentoid medium-coarse-grained and (5) saddle cement, each with different genetic origins. The layered powdered dolomite has similar geochemical characteristics to original limestone. Its original structure is well preserved whereas gas-liquid inclusions are not developed, and appears dark red in cathodoluminescence (CL) images, suggesting that the layered powdered dolomite was formed in an early burial environment. The layered fine-medium crystalline dolomite is generally developed under the layered powdered dolomite and remaining grain apparition, whilst the δ¹⁸O value (-7.31‰) is more negative than that of the limestone (-3.77‰). Its homogenization temperature (116.9 ℃) is higher than the normal burial temperature (100 ℃), and it appears as partially bright red in CL images, suggesting that the layered fine-medium crystal dolomite was formed by hydrothermal recrystallization of layered powdered dolomite. Genesis of the scattered dolomite in limestone was likely related to the mineral stabilization processes. Both the lentoid medium-coarse dolomite and the saddle dolomite cement, in which hydrothermal minerals (quartz, pyrobitumen, pyrite) are discovered, show a curved crystal edge and undulating extinction under the microscope. Both of them have markedly negative δ¹⁸O value (-10‰), high homogenization temperature (120 ℃) and positive Eu anomaly. It appears bright red in CL images, which indicates that these two kinds of dolomite are typically hydrothermal.

Control of Eogenetic Karst Reservoir by Shoals in Middle Permian Maokou Formation, Southern Sichuan Basin

YANG Minglei, ZHU Dancheng, LI Tao, LI Haiping, LI Ting, ZOU Huayao

2020, 34(02): 356-369. DOI: 10.19657/j.geoscience.1000-8527.2020.02.14

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Shoals control the eogenetic karst reservoir of Maokou Formation in southern Sichuan Basin. However, research on the carbonate deposition and diagenetic characteristics in different sedimentary environments, especially on the influence of pore differential evolution after shallow burial on karstification, is inadequate. In this paper, based on outcrop, drill-core, petrographic thin-section and cathodoluminescence data, eight rock types are identified from the Maokou Formation in the study area. Three types of sedimentary environments are found to have developed in shoals (including high-/low-energy shoals), intershoal sea and open subtidal sedimentary environments. Bioclastic rocks in high-energy shoals have high porosity and permeability after their shallow burial, which are conducive to karstification influence and show large-scale layered caves and spotted zones of dissolution-filling. Porosity and permeability of bioclastic packstone in low-energy shoal are slightly lower than those of bioclastic rocks, and the scale of dissolution fractures and caves are smaller. Bioclastic wacky and marls in the intershoal sea and open subtidal environments are compacted before their shallow burial, and karst is not developed. The distribution of vertical shoal is controlled by the sea-level change, and high stand system tract of sequences (P₂m² and P₂m³) are developed. The planar distribution of shoals is controlled by the sedimentary paleogeomorphologic changes formed by the basement uplift, and has a large thickness in the Luzhou paleo-uplift area. The layers of cave drilling anomalies and the locations of high-yield wells are consistent with those of the shoals, which indicates that the shoals exert a controlling effect on the eogenetic karst reservoirs.

Calculation and Evaluation Method of Reservoir Parameters in Multiple Oil Reservoirs Based on Flow Unit

MA Fengchun, LIU Jincheng, WU Yanxiong, LIANG Xiaoyu, JIANG Yinghai, WANG Lin, ZHOU Yan, XIANG Yiwei

2020, 34(02): 370-377. DOI: 10.19657/j.geoscience.1000-8527.2020.02.15

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For the long oil-bearing section and high heterogeneity reservoir in Yingdong area, the conventional permeability model cannot meet the requirement of reservoir evaluation accuracy. Integrating the data of formation testing and MDT, the logging response features of oil, gas, water and dry layer are analyzed. We attempt to characterize the permeability fractions with the flow-band index, which characterizes the stratigraphy according to “unitized classification evaluation”. Meanwhile, we have established the calculation model of permeability flow-band index under electrical parameters. Evaluation results of actual production data in the study area show that the absolute error (of flow zone exponential permeability model) is basically controlled under half order of magnitude, and the relative errors are below 25%. The results improve significantly the prediction accuracy of heterogeneous reservoir permeability, and satisfy the actual reservoir conditions to meet the needs of reservoir description and production. This has an important guiding significance for heterogeneity studies and for enhancing interlayer water absorption contradiction of complex reservoirs.

Geological Characteristics of Hydrocarbon Pool in Yingxiongling Structural Zone, Qaidam Basin

WU Kunyu, LIAO Chun, LI Xiang, ZHANG Chengjuan, ZHANG Qiang, LI Guimei, ZHANG Jing, TAN Xiucheng

2020, 34(02): 378-389. DOI: 10.19657/j.geoscience.1000-8527.2020.02.16

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To understand the geological features and controlling factors on hydrocarbon accumulation in the Yingxiongling area, we systemically studied the structures, reservoir, and the oil & gas origin and accumulation evolution model. Our results showed that the structural styles of the Yingxiongling structural zone change significantly from west to east. The middle-western Yingxiongling area has a vertical ‘double-layer’ structure. The structural style in eastern Yingxiongling was simpler than that in middle-western Yingxiongling. The main pore type of carbonate reservoirs in middle-western Yingxiongling is inter-crystal pore, together with local development of dissolution pores and fracture aggregates. Pore space developed in clastic reservoirs in eastern Yingxiongling is mainly interparticle pore, with some interparticle dissolution pores and fractures. The upper member of the Xiaganchaigou Formation is the main source rock. The composition and isotopic factors of the natural gas indicated that the CH₄ in middle-western Yingxiongling was originated as crude oil associated gas, and the CH₄ in eastern Yingxiongling was associated gas of condensate oil. Due to the existence of halite seal layers, abnormally high-pressure hydrocarbon pools were formed in the deeper parts of middle-western Yingxiongling. In contrast, halite seal layers are absent in eastern Yingxiongling, and the hydrocarbon pool there is characterized by episodic accumulation from the deeper to the shallower parts.

Genetic Mechanism of Low-Resistivity Neogene Zhujiang Formation in Wenchang X-2 Oilfield of Pearl River Estuary Basin

HU Xiangyang, LIANG Yunan, WU Feng, LIAO Mingguang, ZHANG Hengrong, YANG Dong, YANG Yi, DAI Jin, ZHONG Huaming, WU Yixiong

2020, 34(02): 390-398. DOI: 10.19657/j.geoscience.1000-8527.2020.02.17

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A set of low-resistivity oil reservoirs is developed in the Zhujiang Formation of Wenchang X-2 Oilfield in the Pearl River Estuary Basin. Resistivity of the oil reservoirs is very close to that of the water layers, which creates challenges in identifying reservoir fluids and calculating water saturation. At present, the genetic mechanism of these low resistivity oil reservoirs is yet to be clear. Based on mercury injection, nuclear magnetic resonance, casting thin section, CT scan, X-ray diffraction and other experimental analyses, and integrated with geological and well-log data and electrical conductivity numerical simulation technology, the genetic mechanism is analyzed from both macro- and micro-scale perspectives. The results show that if the banded shale content is above 20%, the rocks (with the shale) have a good continuity and can provide good conductive network. The resistivity of the oil reservoirs is below 1.6 Ω·m, and the resistivity differences between oil reservoirs and water layers are not distinct. Low-amplitude structures and shallow-sea sedimentary environment are the major macroscopic causes of the low-resistivity oil reservoirs. Complex pore structure, fine rock granularity, high irreducible water saturation, high banded-shale content are the major microscopic causes of low-resistivity oil reservoirs.

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