Showing papers in "Acta Geologica Sinica-english Edition in 2019"

Unconventional Natural Gas Accumulations in Stacked Deposits: A Discussion of Upper Paleozoic Coal‐Bearing Strata in the East Margin of the Ordos Basin, China

Abstract: The Upper Paleozoic (Carboniferous to Permian) succession in the east margin of the Ordos Basin in the North China Craton has a potential to contain significant hydrocarbon resources, though attention have been mainly attracted for its successful development of coalbed methane (CBM). To improve the previous resource estimates and evaluate the hydrocarbon play possibilities, this study incorporated new discoveries of hydrocarbon units and their stratigraphic relation with source rocks, hydrocarbon migration and trapping configurations. Continuous hydrocarbon accumulation units were identified within the Upper Paleozoic, including the Taiyuan, Shanxi and Xiashihezi formations with great tight gas potential, and the Taiyuan and Shanxi formations also containing shale gas and CBM. Different strata combinations are identified with coal deposition and favour for continuous gas accumulations, including the tidal flat, deltaic and fluvial systems distributed in most of the study areas. Methane was not only generated from the thick coal seams in the Taiyuan and Shanxi formations, but also from shale and dark mudstones. The coal, shale and tight sandstones are proved of remarkable gas content and hydrocarbon indications, and the gas saturation of tight sandstones decreases upward. The stacked deposit combinations vary isochronally in different areas, while the coal seams were developed stably showing good gas sources. Two key stages control the hydrocarbon enrichment, the continuous subsidence from coal forming to Late Triassic and the anomalous paleo-geothermal event happened in Early Cretaceous, as indicated by the fluid inclusions evidence. Extensive areas show good hydrocarbon development potential presently, and more works should be focused on the evaluation and selection of good reservoir combinations.

Progress in Global Gas Hydrate Development and Production as a New Energy Resource

Abstract: Natural gas hydrates have been hailed as a new and promising unconventional alternative energy, especially as fossil fuels approach depletion, energy consumption soars, and fossil fuel prices rise, owing to their extensive distribution, abundance, and high fuel efficiency. Gas hydrate reservoirs are similar to a storage cupboard in the global carbon cycle, containing most of the world’s methane and accounting for a third of Earth’s mobile organic carbon. We investigated gas hydrate stability zone burial depths from the viewpoint of conditions associated with stable existence of gas hydrates, such as temperature, pressure, and heat flow, based on related data collected by the global drilling programs. Hydrate-related areas are estimated using various biological, geochemical and geophysical tools. Based on a series of previous investigations, we cover the history and status of gas hydrate exploration in the USA, Japan, South Korea, India, Germany, the polar areas, and China. Then, we review the current techniques for hydrate exploration in a global scale. Additionally, we briefly review existing techniques for recovering methane from gas hydrates, including thermal stimulation, depressurization, chemical injection, and CH4–CO2 exchange, as well as corresponding global field trials in Russia, Japan, United States, Canada and China. In particular, unlike diagenetic gas hydrates in coarse sandy sediments in Japan and gravel sediments in the United States and Canada, most gas hydrates in the northern South China Sea are non-diagenetic and exist in fine-grained sediments with a vein-like morphology. Therefore, especially in terms of the offshore production test in gas hydrate reservoirs in the Shenhu area in the north slope of the South China Sea, Chinese scientists have proposed two unprecedented techniques that have been verified during the field trials: solid fluidization and formation fluid extraction. Herein, we introduce the two production techniques, as well as the so-called “four-in-one” environmental monitoring system employed during the Shenhu production test. Methane is not currently commercially produced from gas hydrates anywhere in the world; therefore, the objective of field trials is to prove whether existing techniques could be applied as feasible and economic production methods for gas hydrates in deep-water sediments and permafrost zones. Before achieving commercial methane recovery from gas hydrates, it should be necessary to measure the geologic properties of gas hydrate reservoirs to optimize and improve existing production techniques. Herein, we propose horizontal wells, multilateral wells, and cluster wells improved by the vertical and individual wells applied during existing field trials. It is noteworthy that relatively pure gas hydrates occur in seafloor mounds, within near-surface sediments, and in gas migration conduits. Their extensive distribution, high saturation, and easy access mean that these types of gas hydrate may attract considerable attention from academia and industry in the future. Herein, we also review the occurrence and development of concentrated shallow hydrate accumulations and briefly introduce exploration and production techniques. In the closing section, we discuss future research needs, key issues, and major challenges related to gas hydrate exploration and production. We believe this review article provides insight on past, present, and future gas hydrate exploration and production to provide guidelines and stimulate new work into the field of gas hydrates.

Heavy Oils and Oil Sands: Global Distribution and Resource Assessment

Abstract: Global recoverable resources of heavy oil and oil sands have been assessed by CNPC using a geology-based assessment method combined with the traditional volumetric method, spatial interpolation method, parametric-probability method etc. The most favourable areas for exploration have been selected in accordance with a comprehensive scoring system. The results show: (1) For geological resources, CNPC estimate 991.18 billion tonnes of heavy oil and 501.26 billion tonnes of oil sands globally, of which technically recoverable resources of heavy oil and oil sands comprise 126.74 billion tonnes and 64.13 billion tonnes respectively. More than 80% of the resources occur within Tertiary and Cretaceous reservoirs distributed across 69 heavy oil basins and 32 oil sands basins. 99% of recoverable resources of heavy oil and oil sands occur within foreland basins, passive continental-margin basins and cratonic basins. (2) Since residual hydrocarbon resources remain following large-scale hydrocarbon migration and destruction, heavy oil and oil sands are characterized most commonly by late hydrocarbon accumulation, the same basin types and source-reservoir conditions as for conventional hydrocarbon resources, shallow burial depth and stratabound reservoirs. (3) Three accumulation models are recognised, depending on basin type: degradation along slope; destruction by uplift; and migration along faults. (4) In addition to mature exploration regions such as Canada and Venezuela, the Volga-Ural Basin and the Pre-Caspian Basin are less well-explored and have good potential for oil-sand discoveries, and it is predicted that the Middle East will be an important region for heavy oil development.

Pseudotachylyte-Induced Weakness of Plate-Boundary Fault: Insight from the Indus-Tsangpo Suture between India and Asia

Abstract: Although the Indus-Tsangpo Suture (ITS) is the most spectacular thrust system of continent-continent collision in the world, fundamental questions about its strength evolution and deformation behavior transition remain unanswered. Here we reported, for the first time, frictional melting-induced pseudotachylytes in the intensively deformed felsic rocks along the ITS zone in southern Tibet. This study reveals that pseudotachylytes induced profound weakness of the boundary fault between Indian and Asian plates. The intrinsically low strength of the foliated microlites crystallized from frictional melt or glass (i.e., pseudotachylyte) at seismogenic depths compared with the surrounding coarse-grained quartzofeldspathic rocks in the brittle and semi-brittle regime is sufficient to explain the localization of shear strain, the development of ductile shear zones embedded in strong wall rocks, and the transition from the strong to weak fault behaviors without invoking the presence of high fluid pressure or low friction coefficient metasomatic materials (e.g., smectite or lizardite) within the faults.

High‐precision Dating and Geological Significance of Chang 7 Tuff Zircon of the Triassic Yanchang Formation, Ordos Basin in Central China

Abstract: The Ordos Basin, as the second largest petroliferous basin of China, contains abundant oil and gas resources, oil shale, and sandstone-type uranium mineral resources. Chang 7 shale is not only the major source rock of the Mesozoic petroliferous system of the Basin, but is also crucial in determining the space-time distribution relationship of the shale section for the effective exploration and development of the Basin’s oil and gas resources. To obtain a highly precise age of the shale development section, we collected tuff samples from the top and bottom profile of the Chang 7 Member, Yishi Village, Yaoqu Town, Tongchuan District, on the southern margin of the Ordos Basin and performed high-precision chemical abrasion (CA)–isotope dilution (ID)–thermal ionization mass spectrometry (TIMS) zircon U-Pb dating on the basis of extensive laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating data. Our results show the precise ages of the top and bottom zircon in the Chang 7 shale to be 241.06 ± 0.12 Ma and 241.558 ± 0.093 Ma, respectively. We first obtained Chang 7 age data with Grade 0.1-Ma precision and then determined the age of the shale development in the Chang 7 Member to be the early-Middle Triassic Ladinian. This result is supported by paleontological evidence. The deposition duration of the Chang 7 shale is 0.5Ma with an average deposition rate of the shale section being 5.3 cm/ka. Our research results provide time scale and basic data for further investigation of the basin–mountain coupling relation of the shale section, the sedimentary environment and volcanic ash and organic-matter-rich shale development relation, and the organism break-out and organic-matter enrichment mechanism.

Hydrogeochemical Characteristics and Groundwater Inrush Source Identification for a Multi‐aquifer System in a Coal Mine

Abstract: Correct identification of water inrush sources is particularly important to prevent and control mine water disasters. Hydrochemical analysis, Fisher discriminant analysis, and geothermal verification analysis were used to identify and verify the water sources of the multi-aquifer groundwater system in Gubei coal mine, Anhui Province, North China. Results show that hydrochemical water types of the Cenozoic top aquifer included HCO3−Na+K−Ca, HCO3−Na+K−Mg and HCO3−Na+K, and this aquifer was easily distinguishable from other aquifers because of its low concentration of Na+K and Cl. The Cenozoic middle and bottom aquifers, the Permian fissure aquifer, and the Taiyuan and Ordovician limestone aquifers were mainly characterized by the Cl−Na+K and SO4−Cl−Na+K or HCO3−Cl−Na+K water types, and their hydrogeochemistries were similar. Therefore, water sources could not be identified via hydrochemical analysis. Fisher model was established based on the hydrogeochemical characteristics, and its discrimination rate was 89.19%. Fisher discrimination results were improved by combining them with the geothermal analysis results, and this combination increased the identification rate to 97.3 % and reasonably explained the reasons behind two water samples misjudgments. The methods described herein are also applicable to other mines with similar geological and hydrogeological conditions in

Transpressive Structures in the Ghadir Shear Belt, Eastern Desert, Egypt: Evidence for Partitioning of Oblique Convergence in the Arabian-Nubian Shield during Gondwana Agglutination

Abstract: Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km-long, NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike-slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated. These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones. The earlier fabric (S1), is locally recognized in low strain areas and SW-ward thrusts. S2 is associated with a shallowly plunging stretching lineation (L2), and defines ~NW-SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ~NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones. Dextral ENEstriking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NWto NNW-striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.

Geological Factors Controlling the Accumulation and High Yield of Marine-Facies Shale Gas: Case Study of the Wufeng-Longmaxi Formation in the Dingshan Area of Southeast Sichuan, China

Abstract: The main geological factors controlling the accumulation and yield of marine-facies shale gas reservoirs are the focus of the current shale gas exploration and development research. In this study, the Wufeng-Longmaxi Formation in the Dingshan area of southeast Sichuan was investigated. Shale cores underwent laboratory testing, which included the evaluation of total organic carbon (TOC), vitrinite reflectance (Ro), whole-rock X-ray diffraction (XRD), pore permeability, and imaging through field emission scanning electron microscopy (FE-SEM). Based on the results of natural gamma ray spectrum logging, conventional logging, imaging logging, and seismic coherence properties, the exploration and development potential of shale gas in the Dingshan area have been discussed comprehensively. The results showed that (1) layer No. 4 (WF2-LM4) of the Wufeng-Longmaxi Formation has a Th/U ratio < 2 and a Th/K ratio of 3.5-12. Graptolites and pyrite are relatively abundant in the shale core, indicating sub-high-energy and low-energy marine-facies anoxic reducing environments. (2) The organic matter is mainly I-type kerogen with a small amount of II1-type kerogen. There is a good correlation among TOC, Ro, gas content, and brittle minerals; the fracturing property (brittleness) is 57.3%. Organic and inorganic pores are moderately developed. A higher pressure coefficient is correlated with the increase in porosity and the decrease in permeability. (3) The DY1 well of the shale gas reservoir was affected by natural defects and important late-stage double destructive effects, and it is poorly preserved. The DY2 well is located far from the Qiyueshan Fault. Large faults are absent, and upward fractures in the Longmaxi Formation are poorly developed. The well is affected by low tectonic deformation intensity, and it is well preserved. (4) The Dingshan area is located at the junction of the two sedimentary centers of Jiaoshiba and Changning. The thickness of the high-quality shale interval (WF2-LM4) is relatively small, which may be an important reason for the unstable production of shale gas thus far. Based on the systematic analysis of the geological factors controlling high-yield shale gas enrichment in the Dingshan area, and the comparative analysis with the surrounding typical exploration areas, the geological understanding of marine shale gas enrichment in southern China has been improved. Therefore, this study can provide a useful reference for shale gas exploration and further development.

A Quantitative Evaluation of Shale Gas Content in Different Occurrence States of the Longmaxi Formation: A New Insight from Well JY‐A in the Fuling Shale Gas Field, Sichuan Basin

Abstract: Comprehensive quantitative evaluation of shale gas content and the controlling factors in different occurrence states is of great significance for accurately assessing gas-bearing capacity and providing effective well-production strategies. A total of 122 core samples from well JY-A in the Fuling shale gas field were studied to reveal the characteristics of S1l shale,15 of which were selected to further predict the shale gas content in different occurrence states, which are dependent on geological factors in the thermal evolution process. Geological parameters were researched by a number of laboratory programs, and the factors influential in controlling shale gas content were extracted by both PCA and GRA methods and prediction models were confirmed by the BE method using SPSS software. Results reveal that the adsorbed gas content is mainly controlled by TOC, Ro, SSA, PD and pyrite content, and the free gas content is mainly controlled by S2, quartz content, gas saturation and formation pressure for S1l in well JY-A. Three methods, including the on-site gas desorption method, the empirical formula method, and the multiple regression analysis method were used in combination to evaluate the shale gas capacity of well JY-A, all of which show that the overall shale gas content of well JY-A is in the range of 2.0–5.0 m/t and that the free gas ratio is about 50%, lower than that of well JY-1. Cause analysis further confirms the tectonics and preservation conditions of S1l in the geological processes, especially the influence of eastern boundary faults on well JY-A, as the fundamental reasons for the differences in shale gas enrichment in the Jiaoshiba area.

Pore Structure Heterogeneity of the Xiamaling Formation Shale Gas Reservoir in the Yanshan Area of China: Evaluation of Geological Controlling Factors

Abstract: Micro-heterogeneity is an integral parameter of the pore structure of shale gas reservoir and it forms an essential basis for setting and adjusting development parameters. In this study, scanning electron microscopy, high-pressure mercury intrusion and low-temperature nitrogen adsorption experiments were used to qualitatively and quantitatively characterize the pore structure of black shale from the third member of the Xiamaling Formation in the Yanshan area. The pore heterogeneity was studied using fractal theory, and the controlling factors of pore development and heterogeneity were evaluated in combination with geochemical parameters, mineral composition, and geological evolution history. The results show that the pore structure of the reservoir was intricate and complicated. Moreover, various types of micro-nano scale pores such as dissolution pores, intergranular pores, interlayer pores, and micro-cracks are well developed in member 3 of the Xiamaling Formation. The average porosity was found to be 6.30%, and the mean value of the average pore size was 4.78 nm. Micropores and transition pores provided most of the storage space. Pore development was significantly affected by the region and was mainly related to the total organic carbon content, vitrinite reflectance and mineral composition. The fractal dimension, which characterizes the heterogeneity, is 2.66 on average, indicating that the pore structure is highly heterogeneous. Fractal dimension is positively correlated with maturity and clay mineral content, while it is negatively correlated with brittle mineral content and average pore size. These results indicate that pore heterogeneity is closely related to thermal history and material composition. Combined with the geological background of this area, it was found that the pore heterogeneity was mainly controlled by the Jurassic magmatism. The more intense the magma intrusion, the stronger the pore heterogeneity. The pore structure and its heterogeneity characteristics present today are a general reflection of the superimposed geological processes of sedimentary-diagenetic-late transformation. The influence of magmatic intrusion on the reservoir is the main geological factor that should be considered for detailed evaluation of the Xiamaling Formation shale gas reservoir in the Yanshan area.

Molecular Structure of Kerogen in the Longmaxi Shale: Insights from Solid State NMR, FT-IR, XRD and HRTEM

Abstract: Kerogen plays an important role in shale gas adsorption, desorption and diffusion. Therefore, it is necessary to characterize the molecular structure of kerogen. In this study, four kerogen samples were isolated from the organic-rich shale of the Longmaxi Formation. Raman spectroscopy was used to determine the maturity of these kerogen samples. Highresolution transmission electron microscopy (HRTEM), C nuclear magnetic resonance (C NMR) , X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy were conducted to characterize the molecular structure of the shale samples. The results demonstrate that VReqv of these kerogen samples vary from 2.3% to 2.8%, suggesting that all the kerogen samples are in the dry gas window. The macromolecular carbon skeleton of the Longmaxi Formation kerogen is mainly aromatic (fa’=0.56). In addition, the aromatic structural units are mainly composed of naphthalene (23%), anthracene (23%) and phenanthrene (29%). However, the aliphatic structure of the kerogen macromolecules is relatively low (fal+fal=0.08), which is presumed to be distributed in the form of methyl and short aliphatic chains at the edge of the aromatic units. The oxygen-containing functional groups in the macromolecules are mainly present in the form of carbonyl groups (fa=0.23) and hydroxyl groups or ether groups (fal=0.13). The crystallite structural parameters of kerogen, including the stacking height (Lc=22.84 Å), average lateral size (La=29.29 Å) and interlayer spacing (d002=3.43 Å), are close to the aromatic structural parameters of anthracite or overmature kerogen. High-resolution transmission electron microscopy reveals that the aromatic structure is well oriented, and more than 65% of the diffractive aromatic layers are concentrated in the main direction. Due to the continuous deep burial, the longer aliphatic chains and oxygen-containing functional groups in the kerogen are substantially depleted. However, the ductility and stacking degree of the aromatic structure increases during thermal evolution. This study provides quantitative information on the molecular structure of kerogen samples based on multiple research methods, which may contribute to an improved understanding of the organic pores in black shale.

Genetic Types and Metallogenic Model for the Polymetallic Copper–Gold Deposits in the Tongling Ore District, Anhui Province, Eastern China

Abstract: The Tongling ore district is one of the most economically important ore areas in the Middle–Lower Yangtze River Metallogenic Belt, eastern China. It contains hundreds of polymetallic copper–gold deposits and occurrences. Those deposits are mainly clustered (from west to east) within the Tongguanshan, Shizishan, Xinqiao, Fenghuangshan, and Shatanjiao orefields. Until recently, the majority of these deposits were thought to be skarnor porphyry–skarn-type deposits; however there have been recent discoveries of numerous vein-type Au, Ag, and Pb-Zn deposits that do not fall into either of these categories. This indicates that there is some uncertainty over this classification. Here, we present the results of several systematic geological studies of representative deposits in the Tongling ore district. From investigation of the ore-controlling structures, lithology of the host rock, mineral assemblages, and the characteristics of the mineralization and alteration within these deposits, three genetic types of deposits (skarn-, porphyry-, and vein-type deposits) have been identified. The spatial and temporal relationships between the orebodies and Yanshanian intrusions combined with the sources of the ore-forming fluids and metals, as well as the geodynamic setting of this ore district, indicate that all three deposit types are genetically related each other and constitute a magmatic–hydrothermal system. This study outlines a model that relates the polymetallic copper–gold porphyry-, skarn-, and vein-type deposits within the Tongling ore district. This model provides a theoretical basis to guide exploration for deep-seated and concealed porphyry-type Cu (–Mo, –Au) deposits as well as shallow vein-type Au, Ag, and Pb–Zn deposits in this area and elsewhere.

Rock Mass Characteristics in Beishan, A Preselected Area for China's High-Level Radioactive Waste Disposal

Abstract: Geological and hydrological characteristics, joint geometric features, rock physical and mechanical properties and rock mass quality are studied in the Beishan area, preselected for China’s high-level radioactive waste (HLW) disposal engineering. A comprehensive survey method is developed to study joint geometric features in the outcrop and samples from borehole BS06 into the Xinchang rock mass were tested. The optimal joint sets are determined by rose diagrams and equal-area lower hemisphere plots of joint poles. Results show that: 1) the distribution of joint occurrence obeys a normal distribution, while the distribution of joint spacing obeys a negative exponential distribution; 2) concentric circular and tangent circular sampling windows are applied to study the trace length and the trace midpoint density. Results indicate that tangent circular sampling window is more stable and reasonable; 3) Beishan granite shows high density, low porosity and high strength based on many laboratory tests and the physical properties and mechanical properties are closely related; and 4) a synthesis index, Joint Structure Rating (JSR), is applied to evaluate the quality of rock mass. Through the research results of rock mass characteristics, the Xinchang rock mass in the Beishan preselected area has the favorable conditions for China’s HLW disposal repository site.

New Evidence for a Cretaceous Age for a Mesozoic Nonmarine Bivalve Assemblage from Paekto‐dong, Sinuiju City, The Democratic People's Republic of Korea

Abstract: The Sinuiju Formation in Paekto-dong, Sinuiju City in the Democratic People’s Republic of Korea has yielded Mesozoic nonmarine bivalve fossils, which is the first occurrence of such in the DPRK. Based on these fossil specimens, a new Cretaceous bivalve assemblage, the Arguniella yanshanensis-Sphaerium anderssoni Assemblage is erected. This assemblage includes Arguniella yanshanensis, A. lingyuanensis and Sphaerium anderssoni and can be compared with the Jehol Biota. The age of the Sinuiju Formation is also clarified and on the basis of the bivalves and the presence of a Eosestheria–Ephemeropsis–Lycoptera (E–E–L) assemblage, the formation is not Upper Jurassic, but Lower Cretaceous in

Fluid Characteristics and Evolution of the Zhawulong Granitic Pegmatite Lithium Deposit in the Ganzi‐Songpan Region, Southwestern China

Abstract: The Zhawulong granitic pegmatite lithium deposit is located in the Ganzi-Songpan orogenic belt. Fluid inclusions in spodumene and coexisting quartz were studied to understand the cooling path and evolution of fluid within albite–spodumene pegmatite. There are three distinguishable types of fluid inclusions: crystal-rich, CO2–NaCl–H2O, and NaCl–H2O. At more than 500°C and 350~480 MPa, crystal-rich fluid inclusions were captured during the pegmatitic magma-hydrothermal transition stage, characterized by a dense hydrous alkali borosilicate fluid with a carbonate component. Between 412°C and 278°C, CO2–NaCl–H2Ofluid inclusions developed in spodumene (I) and quartz (II) with a low salinity (3.3–11.9 wt%NaCl equivalent) and a high volatile content, which represent the boundary between the transition stage and the hydrothermal stage. The subsequentNaCl–H2Ofluid inclusions from the hydrothermal stage, between 189°C and 302°C, have a low salinity (1.1–13.9 wt%NaCl equivalent). The various types of fluid inclusions reveal the P–T conditions of pegmatite formation, which marks the transition process from magmatic to hydrothermal. The oreforming fluids from the Zhawulong deposit have many of the same characteristics as those from the Jiajika lithium deposit. The ore-forming fluid provided not only materials for crystallization of rare metal minerals, such as spodumene and beryl, but also the ideal conditions forthe growth of ore minerals. Therefore, this area has favorable conditions for lithium enrichment and excellent prospecting potential.

Geochemistry, Paleoenvironment and Mechanism of Organic‐Matter Enrichment in the Lower Silurian Longmaxi Formation Shale in the Sichuan Basin, China

Abstract: To investigate the mechanism of the organic-matter enrichment in the Lower Longmaxi Formation shale, the geochemistry and total organic carbon (TOC) of the Longmaxi Formation black shales in the Jiaoshiba, Zhaotong, and Weiyuan areas of the Sichuan Basin were analyzed. Paleoproductivity proxy parameters (Babio, Siex, and Ni/Al), clastic influx proxies (TiO2 and Ti/Al), redox indices (V/Cr, Ni/Co, V/[V+Ni], and U/Th), and hydrothermal indicators (Fe, Mn, and Y concentrations; Fe/Ti ratio and a Ni-Zn-Co diagram) were employed to decipher the paleoenvironment of the Lower Longmaxi Formation shales. TiO2 and Ti/Al indicated low terrigenous detrital influx in all three areas. However, Babio, Siex, and Ni/Al indicated high productivity in the Jiaoshiba area. V/Cr, Ni/Co, and U/Th indicated higher oxygen content with larger fluctuations in the Zhaotong and Weiyuan areas. Fe, Mn, and Y concentrations and the Fe/Ti ratio implied greater active hydrothermal activity in the Weiyuan area. These heterogeneities were considered to be closely related to the paleoenvironment and paleogeography, and the large basement faults that developed during the Chuanzhong paleo-uplift could have provided vents for deep-hydrothermal-fluid upwelling. The redox indices (V/Cr, Ni/Co, and U/Th) and a paleoproductivity proxy (Ni/Al) displayed a significant correlation with the TOC, suggesting that both excellent preservation conditions and high paleoproductivity were the controlling factors for the enrichment of organic matter in the Longmaxi Formation shale. There was no obvious correlation between the clastic influx proxy (Ti/Al) and the TOC due to the extremely low supply of terrigenous debris. The hydrothermal indicator (Fe/Ti) was negatively correlated with the TOC in the Weiyuan area, indicating that hydrothermal activity may have played a negative role in the accumulation of organic matter. This study suggests that the enrichment of organic matter in the Longmaxi Formation marine shale varied according to the paleogeography and sedimentary environment.

Quantitative Prediction of Fracture Distribution of the Longmaxi Formation in the Dingshan Area, China using FEM Numerical Simulation

Abstract: Fracture prediction is a technical problem in the field of petroleum exploration and production worldwide. Although there are many ways to predict the distribution of cracks underground, there are also problems. Therefore, according to the characteristics of the shale reservoir and the fracture development characteristics of the Longmaxi formation in Dingshan area. Definite the connection of the finite-element method (FEM) numerical simulation of the tectonic stress field and the fracture distribution prediction from the angle of geologic genesis. It was applied using the three-dimensional FEM combined with rock mechanical parameters from the acoustic emission. The paleotectonic stress field of the crack formation period was simulated for the Longmaxi formation in the Dingshan area. According to the rock breaking criterion, the splitting factor in the study area was calculated. The coefficient of fracture development was selected as the quantitative prediction classification criteria for the cracks. The results show that a higher coefficient of fracture development indicates a greater degree of fracture development. On the basis of the fracture development coefficient classification, a favorable area was identified for the development of fracture prediction in the study area. According to the prediction results, the south of the Dingshan area and the DY3 well of the central region are favorable zones for fracture development.

The Late Carboniferous–Early Permian Ocean-Continent Transition in the West Junggar, Central Asian Orogenic Belt: Constraints from Columnar Jointed Rhyolite

Abstract: The West Junggar of the western Central Asian Orogenic Belt is one of the typical regions in the term of ocean subduction, contraction and continental growth in the Late Paleozoic. However, it is still controversial on the exact time of ocean-continent transition so far. This study investigates rhyolites with columnar joint in the West Junggar for the first time. Based on zircon U-Pb dating, we determined that the ages of the newly-discovered rhyolites are between 303.6 and 294.5 Ma, belonging to Late Carboniferous–Early Permian, which is the oldest rhyolite with columnar joint preserved in the world at present. Geochemical results show that the characteristics of the major element compositions include a high content of SiO2 (75.78–79.20 wt%) and a moderate content of Al2O3 (12.21–13.19 wt%). The total alkali content (K2O + Na2O) is 6.14–8.05 wt%, among which K2O is 2.09–4.72 wt% and the rate of K2O/Na2O is 0.38–3.05. Over-based minerals such as Ne, Lc, and Ac do not appear. The contents of TiO2 (0.09–0.24 wt%), CaO (0.15–0.99 wt%) and MgO (0.06–0.18 wt%) are low. A/CNK=0.91–1.68, A/NK=1.06–1.76, and as such, these are associated with the quasi-aluminum-weak peraluminous high potassium calc-alkaline and some calc-alkaline magma series. These rhyolites show a significant negative Eu anomaly with relative enrichment of LREE and LILE (Rb, Ba, Th, U, K) and depletion of Sr, HREE and HFSE (Nb, Ta, Ti, P). These rhyolites also have the characteristics of an A2-type granite, similar to the Miaoergou batholith, which indicates they both were affected by post-orogenic extension. Combining petrological, zircon U-Pb dating and geochemical characteristics of the rhyolites, we conclude that the specific time of ocean-continent transition of the West Junggar is the Late Carboniferous–Early Permian.

Microbial Geochemical Characteristics of the Coalbed Methane in the Shizhuangnan Block of Qinshui Basin, North China and their Geological Implications

Abstract: Methanogens and sulfate reducing bacteria were detected by the 16SrRNA sequencing of coalbed methane (CBM) co-produced water in the south of the Qinshui Basin, which is indicative of the presence of secondary biological gas in the south of this basin, in contradiction to the previous understanding of thermogenic gas. This work systematically collected water samples from the CBM wells in the Shizhuangnan Block and analyzed the microbial geochemical characteristics from the aspects of water ions, hydrogen and oxygen isotopes, dissolved inorganic carbon and microbial diversity. It is shown that the Shizhuangnan Block has a nearly SN-trending monoclinic structure, and the elevation of coal seam decreases gradually from the east to west. Because of the water blocking effect of Sitou fault in the west, the precipitation flowed from the east to west, and gradually transited to stagnant flow area. The concentration variation of some ions such as Na, K, Ca, Mg, Cl, HCO3 and total dissolved solids (TDS) suggest the variation of redox condition in the coal reservoir water. The 16SrDNA sequencing analysis of the collected water samples detected the presence of methanogens and sulfate reduction bacteria. The presence of methane production zone and sulfate methane transition zone (SMTZ) was identified. The effect of methanogens in the methane production zone leads to an increase in the methane concentration, resulting in a high gas content in the study area. In the SMTZ, most methane is consumed by anaerobic oxidation due to high sulfate concentrations.

REE Tetrad Effect as a Powerful Indicator of Formation Conditions of Karst Bauxites: A Case Study of the Shahindezh Deposit, NW Iran

Abstract: Study of the concentration of major, trace, and rare earth elements (REE) in the Shahindezh karst bauxite deposit, northwestern Iran clarifies the relationship of the tetrad effect with geochemical parameters in the bauxite ores. The existence of irregular curves in the chondrite-normalized REE patterns as well as non-CHARAC behavior of geochemically isovalent pairs (Y/Ho) are related to the tetrad effect. The meaningful positive correlation between the sizes of the calculated T3 tetrad effect and some geochemical factors such as Y/Ho, ∑REE, La/Y, (La/Yb)N, and (LREE/HREE)N as well as some major oxides-based parameters like Al2O3 + LOI/SiO2 + Fe2O3, Al2O3/Fe2O3, Al2O3 + LOI, IOL, and SiO2 + Fe2O3 indicate that the studied bauxite horizon was likely deposited by different (acidic and/or alkalic) solutions at different stages. The lower part of the studied horizon with a thickness of ~4.7 m displays alkali characteristics whereas the upper parts of the horizon with a thickness of ~5.3 m are characterized by more acidic conditions. These results are fully supported by the co-occurrence of convex-concave tetrad effect curves in the chondritenormalized REE patterns. Therefore, the tetrad effect phenomenon used in this study has proved to be a good and reliable geochemical proxy to assess the conditions of the depositional environment in the Shahindezh bauxite ores.

A Model of Hydrothermal Dolomite Reservoir Facies in Precambrian Dolomite, Central Sichuan Basin, SW China and its Geochemical Characteristics

Abstract: Hydrothermal mineral assemblages and related hydrothermally enhanced fracturing are common in the Precambrian Dengying Formation of Central Sichuan Basin. Petrographic and geochemical analyses of core samples show that the hydrothermal dolomite reservoirs of Dengying Formation consist of four main types of pores in the reservoir facies. These include: 1) hydrothermal dissolution vug (or pore), 2) intercrystalline pore, 3) residual inter-breccia vug (or pore), and 4) enlarged dissolved-fracture. There are three different fabrics dolomite in hydrothermal dolomite reservoirs, namely, saddle dolomite, fine-medium dolomite and micritic dolomite. Micritic dolomite is the original lithology of host rock. Saddle dolomite with curved or irregular crystal faces was directly crystallized from hydrothermal fluids (average temperature 192°C). Fine-medium dolomites are the products of recrystallization of micritic dolomite, resulting in abnormal geochemical characteristics, such as slight depletion of δO, significant enrichment of Mn-Fe and Sr/Sr, and positive Eu anomaly. A model for the distribution of various hydrothermal dolomite reservoir facies is proposed here, which incorporates three fundamental geological controls: 1) extensional tectonics and tectono-hydrothermal events (i.e., the Xingkai Taphrogenesis of Late Sinian-Early Cambrian, and Emei Taphrogenesis of Late Permian), 2) hydrothermal fluid storage in clastic rocks with large thickness (e.g., Nanhua System of Chengjiang Formation and part of Doushantuo Formation), and 3) confining bed for hydrothermal fluids (such as, the shale in Qiongzhusi Formation). The supply of hydrothermal fluid is critical. Large basement-rooted faults and associated grid-like fracture system may function as the channels for upward migration of hydrothermal fluid flow. The intersection of the above-mentioned faults (including the conversion fault), especially transtensional sags above negative flower structures on wrench faults can serve as a key target for future hydrocarbon exploration.

Numerical Taxonomy and Bayes Discriminant Analysis on 42 Fossil Species in Dicksoniaceae from China

Abstract: As the basal group of Polypodiales, the specific taxonomy of Dicksoniaceae is still being debated. As a quantitative analysis method, numerical taxonomy has been applied to the taxonomic study of many plant families and genera in recent years due to its simplicity and high accuracy. However, the numerical analysis of the Dicksoniaceae fossils has not been reported at present. In the present study, the pinnule morphological data of 42 Mesozoic fossil species of the Dicksoniaceae were analyzed using cluster analysis, principal component analysis and correlation analysis. The results revealed that 42 taxonomic units could be divided into six representative groups, which are consistent with the traditional taxonomy. After screening, an identification key on 28 fossil species of four genera with a definite taxonomic position was established. According to the quantitative analysis, a Bayes discriminant model was established for the selected species. Lastly, the model was tested using the morphological data of the fossil pinnules in Dicksoniaceae from the Yaojie Formation, suggesting that the discriminant model is accurate to a certain extent. As a result, the numerical taxonomy can be applied to the classification of the Dicksoniaceae fossils.

Meso-Cenozoic Tectonothermal History of Permian Strata, Southwestern Weibei Uplift: Insights from Thermochronology and Geothermometry

Abstract: This study provides an integrated interpretation of the Mesozoic Cenozoic tectonothermal evolution of Permian strata in the Qishan area of the southwestern Weibei Uplift, Ordos Basin. Apatite fission-track and apatite/zircon (U-Th)/He thermochronometry, bitumen reflectance, thermal conductivity of rocks, paleotemperature recovery and basin modeling were used to recover the Meso-Cenozoic tectonothermal history of Permian Strata. The Triassic AFT data has a pooled age of ~180 ± 7 Ma (one age peak) with P(χ) = 86%. The average corrected apatite (U-Th)/He age of two Permian sandstones is ~168±4 Ma and a zircon (U-Th)/He age from Cambrian strata is ~231±14 Ma. Bitumen reflectance and maximum paleotemperature of two Ordovician mudstones are 1.81%, 1.57% and ~210°C, ~196°C respectively. After undergoing a rapid subsidence and increasing temperature in the Triassic by a thermal abnormal, the Permian experienced four stages of cooling-uplift history after the time when the maximum paleotemperature reached in the late Jurassic: (1) A cooling stage ( ~163 Ma ~140 Ma ) with temperatures ranging from ~132 C to ~53 C and cooling rate of ~3 C/Ma, erosion thickness of ~1900 m and an uplift rate of ~82 m/Ma; (2) A cooling stage ( ~140 Ma ~52 Ma ) with temperatures ranging from ~53 C to ~47 C and cooling rate less than ~0.1 C/Ma, erosion thickness of ~300 m and an uplift rate of ~3 m/Ma; (3) (~52 Ma ~8 Ma) with ~47 C to ~43 C and ~0.1 C/Ma, erosion thickness of ~500 m and an uplift rate of ~11 m/Ma; (3) (~8 Ma – present ) with ~43 C to ~20 C and ~3 C/Ma, erosion thickness of ~650 m and an uplift rate of ~81 m/Ma. Tectonothermal evolution history of the Qishan area in the Triassic was controlled by the interaction of the Qinling Orogeny and the Weibei Uplift, and the exposed Permian strata had the earliest uplift-cooling time compared to other parts within the Weibei Uplift. The early Eocene at ~52 Ma and late Miocene at ~8 Ma, as two significant turning points after which both the rate of uplift and the rate of temperature changed rapidly, were two key time of uplift-cooling history for the exposed Permian in the Qishan area.

Fracability Evaluation of Shale of the Wufeng-Longmaxi Formation in the Changning Area, Sichuan Basin

Abstract: The fracturing technology for shale gas reservoir is the key to the development of shale gas industrialization. It makes much sense to study the mechanical properties and deformation characteristics of shale, due to its close relationship with the fracability of shale gas reservoir. This paper took marine shale in the Changning area, southern Sichuan Basin of China as the research object. Based on field profile and hand specimen observation, we analyzed the development of natural fractures and collected samples from Wufeng Formation and Longmaxi Formation. Combining with the indoor experiment, we investigated the macroscopic and microscopic structural features and the remarkable heterogeneity of shale samples. Then we illustrated the mechanics and deformation characteristics of shale, through uniaxial compression test and direct shear test. The shale has two types of fracture modes, which depend on the angular relation between loading direction and the bedding plane. Besides, the Wufeng shale has a higher value of brittleness index than the Longmaxi shale, which was calculated using two methods, mechanical parameters and mineral composition. Given the above results, we proposed a fracability evaluation model for shale gas reservoir using the analytic hierarchy process. Four influence factors, brittleness index, fracture toughness, natural fractures and cohesive force, are considered. Finally, under the control of normalized value and weight coefficient of each influence factor, the calculations results indicate that the fracability index of the Wufeng Formation is higher than that of the Longmaxi Formation in Changning area, southern Sichuan Basin.

Facies Architecture Model of the Shimentan Formation Pyroclastic Rocks in the Block‐T Units, Xihu Sag, East China Sea Basin, and its Exploration Significance

Abstract: A gas field consisting of volcanic reservoir rocks was discovered in the block-T units of the Xihu Sag, East China Sea Basin. The lithology of the volcanic rocks is dominated by tuff and reworked tuff. The lithofacies are dominated by base surge deposits of explosive facies. As the architecture model of volcanic facies is still uncertain, it has restricted the exploration and development of mineral resources in this area. Using core and cuttings data, the lithology, lithofacies, geochemistry as well as grain size characteristics of volcanic rocks were analyzed. Based on these analyses, the volcanic rocks in the well section are divided into three eruptive stages. The transport direction of each volcanic eruption is analyzed using crystal fragment size analysis. The facies architecture of the block-T units was established based on the reconstruction results of paleo-geomorphology. The results show that the drilling reveals proximal facies (PF) and distal facies (DF) of the volcanic edifices. However, the crater-near crater facies (CNCF) are not revealed. Compared with the reservoirs of the Songliao Basin, it is shown that the volcanic rocks in the Xihu Sag have good exploration potential; a favorable target area is the CNCF near the contemporaneous fault.

Limitations of Lattice Boltzmann Modeling of Micro‐Flows in Complex Nanopores

Abstract: The multiscale transport mechanism of methane in unconventional reservoirs is dominated by slip and transition flows resulting from the ultra-low permeability of micro/nano-scale pores, which requires consideration of the microscale and rarefaction effects. Traditional continuum-based computational fluid dynamics (CFD) becomes problematic when modeling micro-gaseous flow in these multiscale pore networks because of its disadvantages in the treatment of cases with a complicated boundary. As an alternative, the lattice Boltzmann method (LBM), a special discrete form of the Boltzmann equation, has been widely applied to model the multi-scale and multi-mechanism flows in unconventional reservoirs, considering its mesoscopic nature and advantages in simulating gas flows in complex porous media. Consequently, numerous LBM models and slip boundary schemes have been proposed and reported in the literature. This study investigates the predominately reported LBM models and kinetic boundary schemes. The results of these LBM models systematically compare to existing experimental results, analytical solutions of Navier-Stokes, solutions of the Boltzmann equation, direct simulation of Monte Carlo (DSMC) and information-preservation DSMC (IP_DSMC) results, as well as the numerical results of the linearized Boltzmann equation by the discrete velocity method (DVM). The results point out the challenges and limitations of existing multiple-relaxation-times LBM models in predicting micro-gaseous flow in unconventional reservoirs.

Origin of Tafoni in the Late Cretaceous Aeolian Sandstones, Danxiashan UNESCO Global Geopark, South China

Abstract: Tafoni occur mainly in granular rocks around the world, but their origin remains controversial. In this study, the roles of lithology, micro-climate, and organism in tafoni development are investigated in the Danxiashan UNESCO Global Geopark of South China. The Jinshiyan Temple and Luyitang sites along the Jinjiang River are chosen to conduct tafoni morphometry, bedrock petrography, and micro-climate analyses. The research methods used in this study include field observations and measurements, Voronoi diagrams, polarizing microscopic and scanning electron microscopic (SEM) image analysis, and meteorological detection. The tafoni are mostly 2-10 cm in length and elliptical in shape. The Late Cretaceous Jinshiyan sandstones are characterized by a grain-to-grain contact fabric and moderate sorting with a high proportion of soluble grains and cements. The Voronoi diagrams developed through the ArcGIS software are largely consistent with the tafoni openings. Moreover, owing to exposure to solar insolation, the external temperature values are higher than the internal ones, while the external relative humidity values are lower than the internal ones. Therefore, the permeable Jinshiyan sandstones are fundamental for tafoni development, while the abundant moisture from the Jinjiang River and frequent rains is favorable for salt weathering in tandem with biological activities. At last, a five-stage development model is proposed for the tafoni progression in the study area.

A Volumetric Model for Evaluating Tight Sandstone Gas Reserves in the Permian Sulige Gas Field, Ordos Basin, Central China

Abstract: Accurately measuring and evaluating reserves is critical for ensuring successful unconventional oil and gas production, but the methods and information for comprehensive and quantitative reservoir evaluation are limited. This paper proposes a volumetric model to evaluate the tight gas reservoir. The reserve is determined by four major parameters, which are controlled by reservoir characteristics and sedimentation. The reservoir cutoffs, net pay, gas-bearing area and compression factor Z, were analyzed. The techniques included well logging, seismic analysis, core analysis and gas testing, also the thin section observation and SEM analysis were used to analyze the pore evolution and pore-throat structure. The porosity and permeability cutoffs were determined by distribution function curve, empirical statistics and intersection plot. Effective thickness and gas-bearing area were determined based on the cutoffs, gas testing and sand body distribution, and the compression factor Z was obtained by gas component. The results demonstrate that the reservoir in the Sulige gas field is characterized by ultralow porosity and permeability, and the cutoffs of porosity and permeability are 5% and 0.15 ×10 -3 μm 2 , respectively. The effective thickness and gas-bearing area are mainly affected by the sedimentary facies, sand body types and distribution. The gas component is dominated by methane which accounts for more than 90%, and the compression factor Z of H8(P2h8) and S1(P1s1) are 0.98 and 0.985, respectively. The distributary channels stacked and overlapped, forming a wide and thick sand body with good developed intergranular and intercrystalline pores. The upper part of channel sand with good porosity and permeability can be the sweet spot for gas exploration. The complete set of calculation systems proposed for tight gas reserve calculation proved to be effective based on application and feedback. This model provides a new concept and consideration for reserve prediction and calculation in other areas.