Caledonian Period Research Articles

3-D spatial distribution of concealed ore-forming granitoid intrusion and structures determined by the CSAMT survey of Wuxu Sb-Zn-polymetallic ore district, South China

Wuxu ore district is an important lead-zinc-antimony polymetallic ore field within the Danchi metallogenic belt of South China. Although previous geochemical studies have indicated that the deposit type is medium to low-temperature magmatic-hydrothermal, the existing metallogenic models remain controversial due to the thick covering layer and lack of exposure of granitic bodies. To address this issue, this study utilised four CSAMT parallel profiles across the Wuxu anticline to perform two-dimensional inversion, yielding high-resolution subsurface resistivity models. By comparing the resistivity anomalies along the regional strike shown by the profiles and integrating geochemical and ore deposit structural analyses, a detailed geological interpretation was conducted. A significant high-resistivity body beneath the core region of the Wuxu anticline is interpreted to be the Yanshanian period granitic intrusion. There are westward- and eastward-dipping high-conductivity belts existing within the western and eastern wings and appear to sandwich the inferred high-resistivity code of the Wuxu anticline, respectively. They may represent the deep faults extending underground and are the result of decoupling of lithological interface within the anticline and thereby rock fracturing. The high-conductivity feature may be the result of filling with high-conductivity saline fluids, and possible hydrothermal alteration. The regional compression stress during the Caledonian period and the early Yanshanian period promoted the formation of the anticline and rock fracturing of both wings of the anticline. After that, the regional extension background of the late Yanshanian period made the fault zones reactivated, becoming the site and channels for magmatic intrusion, upward transporting of hydrothermal fluid and alteration, causing lead-zinc-antimony polymetallic ore formation. The exposed ore deposits above may also corroborate this view point. In summary, our resistivity model visually depicts the spatial distribution of the granitic intrusion controlling ore formation and major structures such as anticlines, fault zones, and hydrothermal alteration zones in the 3-D space, providing new geophysical constraints for solving the controversy over the metallogenic model for Wuxu ore district.

Hydrocarbon Accumulation and Overpressure Evolution in Deep–Ultradeep Reservoirs in the Case of the Guole Area of the Tarim Basin

Usually, deep oil and gas accumulation is often controlled by strike–slip faults. However, in the Tarim Basin, deep Ordovician oil and gas accumulations are also found in areas far from the fault zone. The process of oil and gas accumulation in deep reservoirs far from strike–slip fault zones is still unclear at present. The source and evolution of Ordovician fluids were analyzed using inclusion geochemical methods and the U–Pb dating technique. The analysis of rare earth elements and carbon–oxygen–strontium isotopes in the reservoirs showed that the reservoirs were weakly modified by diagenetic fluid. The fluid was derived from the fluid formation during the same period as the seawater, and no oxidizing fluid invaded the reservoir. The late oil and gas reservoirs had good sealing properties. The U–Pb dating results combined with homogenization temperature data revealed that the first-stage oil was charged during the Late Caledonian Period, and the second-stage natural gas was charged during the Middle Yanshanian Period. The evolution of the paleo-pressure showed that the charging of natural gas in the Middle Yanshanian was the main reason for the formation of reservoir overpressure. The strike–slip fault zone was basically inactive in the Middle Yanshanian. During this period, the charged natural gas mainly migrated to the reservoir along the unconformity surface and the open strike–slip fault zone in the upper part of the Ordovician reservoir. The source of the fluid shows that the reservoir in the late stage had good sealing properties, and there was no intrusion of exogenous fluid. The overpressure in the reservoir is well preserved at present.

The Geology, Fluid Inclusions, and O‐S Isotopes of the Mibei Gold Deposit, Hunan Province, Southern China

AbstractThe Mibei gold deposit, located in the southwestern part of the Xuefengshan uplift zone, the middle section of the Jiangnan orogenic belt in southern China, has estimated gold resources of approximately seven tons. This deposit is primarily a quartz vein‐type gold deposit, with ore bodies occurring mainly within Neoproterozoic metasediments. The main metallic minerals in the ore are pyrite, chalcopyrite, and arsenopyrite. In this study, the petrography and microthermometry of ore‐forming fluid inclusions, oxygen isotopes of gold‐bearing quartz, and sulfur isotopes of gold‐bearing sulfides and arsenopyrite were analyzed. Three types of fluid inclusions were identified: type Ia three‐phase inclusions comprising vapor and two phases of liquids (VCO2 + LCO2 + LH2O), type Ib two‐phase liquids (LCO2 + LH2O), type II two‐phase vapor‐rich inclusions (V/V + L > 50%), and type III pure liquid inclusions. Type I inclusions were heated uniformly to the liquid phase, type II inclusions were heated uniformly to the gas phase, and type III inclusions were heated without change. In general, the temperature range of homogenization to liquid phase of fluid inclusions in the Mibei gold deposit is 204–227°C. The salinity of the inclusion ranges from 4.6 to 12.2 wt% NaCl equiv. The δ 18OSMOW of gold‐bearing quartz varies from 16.9‰ to 17.5‰. The δ 18OH2O of gold‐bearing quartz are varied from 6.5‰ to 7.5‰. The δ34S values of gold‐bearing pyrite range from 1.7‰ to 6.8‰. The δ34S values of gold‐bearing arsenopyrite range from 5.6‰ to 5.9‰. The δ34S values of pyrite from wall rocks slate range from 6.4‰ to 11.6‰. This evidence implies that the ore‐forming fluids of the Mibei gold deposit originated from magmatic‐hydrothermal processes, mixing with minor S from the surrounding metasediments. Combined with the evolution of the Jiangnan orogenic belt, due to the magmatic and tectonic activities of the Xuefengshan uplift during the Caledonian period, the fault seal mechanism controlled the ore‐forming process. Overall, the Mibei gold deposit is more akin to a magmatic‐hydrothermal gold deposit.

Residual carbonate karst reservoir reconstructed by karst planation: A case study of Ordovician paleokarst reservoir characterization in Ordos Basin, North China

Based on the global typical karst characteristics reported in recent decades and the latest petroleum and gas exploration results, it is found that the characteristics of the karst reservoirs in the Ordos Basin are dramatically different from those of traditional karst reservoirs, indicating that the main contribution to the formation of the karst reservoirs in the region may not have been karst dissolution. The previous view of the formation mechanism of karst reservoirs suggests that we should think about the formation and development processes of Ordovician karst and the reservoir pores in the Ordos Basin from another perspective. In this investigation of reservoir formation, karstology, geography, sedimentology, and reservoir geology were all utilized. On the basis of the initial understanding of the control of large-scale landforms on the sediments in sedimentary basins, this study focuses more on the dynamic spatial-temporal relationships between the evolutions of the landforms, karsts, and reservoirs. By studying the petrological characteristics, pore filling, karst zonation, and planation model of the weathering crust reservoir in the study area, it has been determined that the primary reservoir space was formed in advance rather than in the supergene dissolution period. A gypsum-containing dolomite flat served as the material foundation for reservoir construction and later karst planation transformation. The penecontemporaneous dissolution dominated the spatial development of the early dolomite reservoir. The traditional karst cycle is actually caused by the erosion of the bottom of the karst floor on the double-layer leveling surface. The karst planation during the Caledonian period played a discontinuous and destructive filling role throughout the entire reservoir zone by changing the diagenetic environment, the main contribution of which was communication between the pores. Therefore, this weathered crust reservoir is actually a residual reservoir controlled by karst planation rather than a traditional karst reservoir. The “sweet spots” are primarily dispersed in the upper portion of the karst slope and the top of the karst residual hillock in the gypsum-containing dolomitic flat environment.

Characteristics and Controlling Role in Hydrocarbon Accumulation of Strike-Slip Faults in the Maigaiti Slope

The Maigaiti Slope is a significant oil-gas-bearing field in the Tarim Basin. Based on 3D and 2D seismic data, systematic interpretation, stage sorting and genetic analysis of strike-slip faults in the Maigaiti Slope were carried out for the first time. The relationship between strike-slip faults and hydrocarbon accumulation was studied in combination with the fine dissection of hydrocarbon reservoirs. The study suggested that: (1) Staging and segmentation characteristics of strike-slip faults are evidently presented in the Maigaiti Slope. According to active periods, strike-slip faults can be divided into early Caledonian period, late Caledonian period, Hercynian period, and Himalayan period. According to plane distribution characteristics, strike-slip faults can be divided into the west Maigaiti Segment, mid-Maigaiti Segment, Madong Segment, and Bachu Segment. The main active periods and plane distribution of strike-slip faults in different sections are remarkably different. This analysis suggests that it is the response to multi-period and multi-directional tectonic movements, which are primarily dominated by the migration and evolution of the Hetian paleo-uplifts. (2) The coupling relationship between the active period of strike-slip faults and the trap forming period is the key to hydrocarbon accumulation in the Carboniferous–Ordovician, which determines the petroleum properties and enrichment horizon of the Cambrian post-salt system; medium-heavy oil is in the Caledonian period, light oil in the Indosinian period, and dry gas in the Himalayan period.

Mineral Characteristics and the Mineralization of Leptynite-Type Nb–Ta Ore Deposit in the Western Qilian Orogenic Belt

A large Nb–Ta ore deposit was found in the Yushishan leptynite in the west Qilian Orogenic Belt (QOB). Based on a field geological survey and using a Mineral Liberation Analyser (MLA, including scanning electron microscopy (SEM) and energy-dispersive spectrometer (EDS)) methods, eight Nb minerals (fergusonite, polycrase, columbite, Nb-rutile, aeschynite, pyrochlore, microlite, and ilmenorutile) were found to occur in the leptynite. This accounted for approximately 69% of Nb, with fergusonite, polycrase, and columbite being the dominant phases. The other 17.90% Nb as a minor element was dispersed in titanium magnetite–maghemite, and another 13.00% Nb was dispersed in gangue minerals. Nb minerals are formed mainly by two metallogenesis stages. The first stage is magmatic genesis to form four Nb minerals, euhedral-subhedral fergusonite, polycrase, pyrochlore, and microlite, which are crystallized within or between primary minerals, such as quartz and feldspar. Late alteration phenomena are locally observed. The second stage is the hydrothermal genesis of columbite, anhedral fergusonite, Nb-rutile, and aeschynite, which are dispersed in the fissures of the wall rocks as irregular veins and lump assemblages. Meanwhile, they are closely associated with metasomatic chlorite, albite, and secondary quartz. Furthermore, direct metasomatism among different Nb minerals is also found at the local scale. The Nb percentage of these two Nb mineral mineralization types is approximately equal, which reflects two main mineralizing periods. The first stage of mineralization occurred in the Neoproterozoic Era (834–790 Ma). Magmatism of this period produced early niobium and formed fergusonite, polycrase, pyrochlore, microlite, and zircon. The initial enrichment of Nb, Ta, and other rare metals occurred during this stage. The second stage of mineralization occurred in the Caledonian period (490–455 Ma). Large-scale and intense tectonic–magmatic thermal events occurred in the western part of the QOB due to the plate subduction and convergence (510–450 Ma). Hydrothermal activity in this period formed columbite, fergusonite, Nb-rutile, and aeschynite. Moreover, rare metal elements in the Nb-bearing rocks activated and migrated at short distances, forming in situ Nb–Ta-rich ore deposits.

Characteristics, formation mechanism and evolution model of Ordovician carbonate fault-controlled reservoirs in the Shunnan area of the Shuntuogole lower uplift, Tarim Basin, China

The Ordovician carbonate fault-controlled reservoir in the Shunnan area is substantially different from the typical karst reservoir closely related to an unconformity. The development of fault-controlled reservoirs is controlled by the deep and large strike-slip fault in the Middle Caledonian period. The characteristics, diagenesis types, diagenetic fluid evolution characteristics, fluid-rock interactions, and reservoir response of fault-controlled reservoirs were investigated systematically by analyzing 3D seismic, logging, and drilling core data. Production data, reservoir petrology, and chemical filling geochemistry were also used. The formation and evolution of fault-controlled reservoirs are affected by the coupling of the structure, lithofacies, and fluid. The dolomitization in the quasi-syngenetic period in the early stage of Episode I in the Middle Caledonian provided the material basis for the development of fault-controlled reservoirs. The Mg-rich high-salinity thermal fluid in the late Caledonian and the Early Hercynian provided a constructive transformation for the development of fault-controlled reservoirs. In contrast, the transformation of reservoirs by the Si-rich magmatic thermal fluid in the late Hercynian was mainly destructive, and the action range of the deep thermal fluid was strongly controlled by strike-slip fault zones. The releasing bend and superimposed parts of different structural styles in the fault zone have high activity intensity. These are areas where dissolution and transformation of the fluid occur preferentially. Dolomite is often more conducive to the development of fault fracture zones and promotes the dissolution and transformation of the deep thermal fluid. Therefore, the strike-slip fault cutting through the basement causes the hydrothermal fluid to enter the fault, dissolving carbonate minerals and forming a hydrothermal-modified fault-controlled reservoir composed of fractures, dissolution pores, and dolomite intercrystalline pores. The results provide a new approach and direction for the study of different types of deep and ultra-deep carbonate reservoirs.

Genetic Types of the tp12cx Strike-Slip Fault Segments and Their Role in Controlling Reservoirs in the Tarim Basin

The change of motion mode of multistage active strike-slip faults controls the segmentary types of strike-slip faults, which is seldom studied.Based on high-precision 3D seismic data and the principle of structural analysis, this paper defines the structural evolution characteristics of the tp12cx strike-slip fault in the key structural period and identifies the fault segmentation types. Combined with the statistical results of drilling production data and fault width, and fault width, it is demonstrated that different fault segments display various reservoir architecture and hydrocarbon potential. The tp12cx strike-slip fault experienced two phases of tectonic activity controlling reservoir development: the middle Caledonian and the late Caledonian to early Hercynian. During the middle Caledonian period, a left-lateral and left-step strike-slip fault was formed. The overlapping segments of the left steps were transtension zones, and the rest were pure strike-slip segments. From the late Caledonian to the early Hercynian, the movement mode changed from left-lateral to right-lateral, and the arrangement of left steps remained unchanged, forming right-lateral and left-step strike-slip faults. That is, as a weak zone, the transtension zones of all the preexisting overlapping segments took the lead in moving into many pure strike-slip segments and maintained the transtensional property. During the right-lateral slipping process of all the original pure strike-slip segments along the fault, they were blocked and squeezed by the surrounding rocks on both sides, forming a series of “positive” flower-shaped fault anticlines, which became overlapping segments, and the fault property became transpressional. Under the continuous action of the right-lateral slipping, a regional right-lateral and right-step strike slip fault formed. The interiors of the right-step-arranged faults were composed of the left-step arranged faults. Among them, the right-step overlapping segments were weakly step overlapping segments were weakly transtensional, and the larger the fault width of the internal left step pure strike slip and overlapping segments, the stronger the dissolution. The deformation of the right-step pure strike-slip segments was weak and basically maintained the characteristics of the previous stage. According to the evolution and superposition of pure strike-slipped and overlapped segments and the changes in fault properties, four types of strike-slip fault segments and corresponding reservoir models are divided. Type I: left-step pure strike-slip segment + left-step transpressional segment + right-step transtensional segment; Type II: left-step transtensional segment + left-step pure strike-slip segment + right-step transtensional segment; Type III: left-step pure strike-slip segment + left-step transpressional segment; and Type IV: left-step transtensional segment + left-step pure strike-slip segment. The fault width and oil production of type II and type IV with transtensional properties are much larger than those of type I and type III with transpressional properties.

Petrology, mineralogy and geochemistry of Ordovician rocks in the southwest of Tarim Basin, implications for genetic mechanism and evolution model of the hydrothermal reformed-paleokarst carbonate reservoir

Paleokarst carbonate reservoir plays an important role in the exploration of oil and gas from shallow to ultra-deep strata. Multistage karstification is superimposed to form the present ultra-deep karst-hydrothermal reformed carbonate reservoirs in the Yubei area of the Tarim Basin. Based on petrology identification, stable carbon and oxygen isotope analysis, inductively coupled plasma mass spectrometry (ICP-MS), and Laser Ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we investigated paleokarst periods, chemical property of hydrothermal fluids, and the evolution model and formation mechanism of paleokarst-hydrothermal reformed Ordovician carbonate reservoirs. The results show that the Ordovician strata experienced five periods of karstification from the middle Caledonian period to the late Himalayan period. The Caledonian quasi-contemporaneous karst and the Hercynian epigenetic karstification provided the most fundamental pore – fracture framework for carbonate reservoirs, and the later multistage deep burial karstification re-transformed the established reservoir space by hydrothermal dissolution under thermochemical sulfate reduction (TSR) reaction. It is proposed that the thermal convection between the hydrothermal fluids from the upper mantle and carbonate rocks formed two kinds of hydrothermal fluids in Ordovician strata. The first kind of hydrothermal fluid is performed with a SiO2-enriched fluid under the acidic environment based on TSR, forming siliceous cements and hydrothermal dissolution pores in the Lower Ordovician strata. The second kind of hydrothermal fluids appeared as an alkaline CO32—enriched fluid with the reversible reaction between HCO3− and CaCO3, forming hydrothermal calcite and dolomite near the migration channel in Upper Ordovician carbonate rocks. The periodic changes of in situ trace element concentrations of multistage quartz fillings indicated that paleokarst reservoirs are controlled by thermal convection under episodic tectonic activity. As a summary, we established the migration pattern of hydrothermal fluids in the thin section scale and the evolution model of the hydrothermal reformed paleokarst reservoirs in basin scale.

Deformation patterns and hydrocarbon potential related to intracratonic strike-slip fault systems in the east of Central Uplift Belt in the Tarim Basin

Strike-slip faults, proven to be closely linked to hydrocarbon migration and accumulation, are widespread in the Tarim Basin Craton. Deformation patterns in the eastern part of the Central Uplift Belt of the Tarim Basin are analyzed in detail to understand the deformation model and genetic mechanism of strike-slip faults, how their patterns impact reservoir formation and influence oil and gas potential. Regional tectonic events are taken into consideration to identify the primary factors that controlled the development of strike-slip faults in the region. The Tazhong Uplift in the eastern part of Central Uplift Belt is believed to be a complex anticlinal uplift formed by compression-and-torsion acting on the Cambrian – Middle Ordovician platform during the middle–late Caledonian period. Multi-stage development of this north-trending fault–fold belt and the NE-trending strike-slip faults make it structurally complex. The NE-trending Paleozoic strike-slip faults were formed jointly by the differential thrusting of the Tazhong Uplift along Fault No. I and the NE-trending shear sliding of the basal fault blocks during the middle–late Caledonian – early Hercynian. Based on the distribution of effective source rocks, the tectonic setting during the period critical to hydrocarbon accumulation, and the distribution of conductive faults, the northern slope of the Tazhong Uplift— especially its west-central part where the NE-trending faults are developed is not only located close to the northern hydrocarbon source rock area but also possesses conditions suitable for the development of carbonate reservoirs. In addition, the NE-trending faults provide passage for initial development of the reservoir and the subsequent migration of oil and gas from the Aman area to the uplifted zone, making the Tazhong Uplift an extremely favorable area for oil and gas accumulation with substantial potential for exploration.

Quantitative prediction of multi-period tectonic fractures based on integrated geological-geophysical and geomechanics data in deep carbonate reservoirs of Halahatang oilfield in northern Tarim Basin

Multi-period tectonic fractures are common in deep carbonate reservoirs. It's difficult to identify and has the characteristics of stochastic spatial distribution. The quantitative prediction of fractures is critical for unravelling their spatial distribution and for understanding tectonic evolution and stress field distribution. Focusing on the Halahatang oilfield in northern Tarim Basin, outcrop surveys, drilling cores, thin sections, well logs, and seismic data were comprehensively used to better determine the development characteristics, formation time, and spatial distribution of multi-period tectonic fractures based on finite element tectonic stress field simulation. The strike-slip fault system was characterised based on post-stack seismic structural interpretation. By simulating the tectonic stress field in three different tectonic movement periods, taking the fracture density as the evaluation parameter, the spatial distribution characteristics of multi-period fractures were quantified. The numerical simulation results indicated that the development of fractures is controlled by the strike-slip fault, and the Middle Caledonian period is the main formation period of these fractures. This method can accurately predict multi-period complex fractures and can also be applied to other deep carbonate reservoirs.

Investigation on Characteristics of Natural Fractures within Carbonate Formations and Mechanical Features of Related Tectonic Movement in a Block of Shunbei Oil Field

Investigation has been performed on natural fractures within carbonate formation in a block in Shunbei Oilfield. Classification has been done over natural fractures in the block. Patterns of distribution of natural fractures within the carbonate reservoir formation have been identified. Principal results include: 1) seismic data and sedimentation information along with history of tectonic movement have been used to analyze natural fractures. 2) It is found that natural fractures withinT74-T76 carbonate reservoir formation are generated by tectonic movement of middle Caledonian I and II periods, and are mainly through fractures/faults with strike-slip characteristics. 3) 3D finite element model was built for this block with seismic data. Numerical calculations of 3D Continuum Damage Mechanics were performed. Results of numerical forward modeling has given out the 3D distribution of natural fractures which are represented by the localization band of damage variables under loading of tectonic movement of middle Caledonian period.

Tectonic Characteristics of the East Sichuan Basin and Regional Tectonic Evolution Characteristics of Regional Tectonic Belt

Most sedimentary basins in China have undergone multi-stage tectonic evolution. This paper takes the eastern Sichuan region as the research object and relies on modern geological research methods to systematically study the basin’s dynamic characteristics and tectonic evolution process in the eastern Sichuan region. The results show that the tectonic squeezing of SW and NW from the Daba Mountain orogenic belt and the Jiangnan-Xuefeng Mountain orogenic belt, respectively, make eastern Sichuan a complex structural interference zone that is both combined and compounded. The eastern Sichuan NE and NW structural combinations The type clearly records the structural transformation effect of the two. The history of tectonic evolution since the Middle Paleozoic in the eastern Sichuan Basin is divided into five phases: the deposition of source rocks of the Wufeng Formation-Longmaxi Formation in the Caledonian Period and the parallel unconformity contact with the overlying strata caused by the end of Silurian tectonic compression; Haixi The tectonic transformation of the Early-Indochina period is mainly manifested by the inheritance of the Caledonian period, and the interaction between the Devonian-Early Carboniferous strata and the early Indosinian marine facies and terrestrial sediments is lacking, and the fold deformation in the region is weak. In the early period, a series of NE-directional faults formed mainly in the eastern Sichuan region during the NNW-SSE compression. In the late Yanshan-Early Himalayan period, the NNW-SSE ~ NWW-SEE compression became stronger than in the early Yanshan period, and the eastern NE NE-boundary faults formed. In the late Himalayan period, NE-SW was squeezed. The Wujiang fault system at the border of the southeast of Sichuan was mainly formed during this period, and the southern strata were obviously transformed by this period.

Tectono-thermal evolution of Cambrian–Ordovician source rocks and implications for hydrocarbon generation in the eastern Tarim Basin, NW China

The tectono-thermal evolution is an important control on petroliferous basin development; however, this evolution is difficult to reconstruct in sedimentary basins subjected to multiple tectono-thermal events. The Tarim Basin is the largest petroliferous basin in northwestern China and hosts a complex oil–gas system comprising several sets of source, reservoir, and cap rocks that have experienced multiple tectonic, thermal, and sedimentary events. Large oil and gas reserves have been discovered in the central and northern Tarim Basin during the last 30 years, but none in the eastern part of the basin. This is generally attributed to the high thermal maturity reached by source rocks during the Late Ordovician, and subsequent hydrocarbon loss. Therefore, reconstructing the Cambrian–Ordovician tectono-thermal evolution is critical in facilitating successful oil and gas exploration in this region. We compiled equivalent vitrinite reflectance data (n = 178) of source rocks from seven wells in the Tarim Basin, which for Cambrian–Ordovician source rocks are >1.3%, beyond the hydrocarbon generation threshold and in the main gas and dry gas generation stages. The Cambrian–Ordovician source rocks experienced rapid burial and heating during the Caledonian period, entered the hydrocarbon generation stage during the Middle–Late Ordovician, and reached peak hydrocarbon generation at the end-Caledonian period. They might have experienced secondary hydrocarbon generation after the Jurassic. Homogenization temperatures of fluid inclusions (n = 616) from wells GC4, LX1, MD1, and YD2 as well as the modeled thermal evolution reveal a main hydrocarbon generation event during the Caledonian period and a secondary event in the Yanshanian and Himalayan period in other wells (LX1, MD1, and YD2). Hercynian and Indosinian uplift and exhumation in the eastern Tarim Basin controlled the main structures, and faulting created favorable conditions for secondary hydrocarbon generation and migration at this time. Since the Yanshanian and Himalayan, the Tarim Basin has experienced further sedimentation and heating, and the tectono-thermal evolution might have been related to the formation of the Tibetan Plateau. In summary, our reconstruction of the tectono-thermal evolution of Cambrian–Ordovician strata and the hydrocarbon generation history of the Tarim Basin suggests that the eastern Tarim Basin may also have hydrocarbon generation potential and can guide future oil and gas exploration.

Characteristics of the Lower Cambrian Yuertus Formation source rocks in the Tarim Basin, NW China

In this paper, the distribution of source rocks of the lower Cambrian Yuertus Formation in the Tarim Basin is analyzed, a series of organic geochemical characteristics are tested, the characteristics of the source rocks are analyzed, and the thermal evolution and hydrocarbon generation history of the source rocks are simulated. The results show that the distribution of the source rocks is obviously controlled by sedimentary facies; the source rocks are sapropelic organic matter with high abundance and have reached the stage of mature-overmature thermal evolution; those in the eastern depression area have reached the peak of oil generation in the middle Caledonian period, the peak of gas generation in the middle Hercynian period and the end of hydrocarbon generation in the late Hercynian period. In the western uplift area, the source rocks entered the peak of oil generation during late Hercynian period, then entered the stage of gas generation in the early Indosinian period, and then stopped hydrocarbon generation due to uplift and denudation.

Strike-slip faults and their significance for hydrocarbon accumulation in Gaoshiti–Moxi area, Sichuan Basin, SW China

Abstract Based on the three dimensional seismic data and drilling data in Gaoshiti–Moxi area, the structural characteristics and evolution process of strike-slip faults in the study area and their significance for gas accumulation were examined using the method of fault structure analysis. The results show that: (1) Transtensional strike-slip faults are developed in the Paleozoic in study area. On the profile, there are three types of structural styles: steep and erect, flower structures, “Y” and reversed “Y” type faults. On the plane, strike-slip faults developed in the Cambrian extending linearly and along the nearly EW, NW and NE direction. The faults are composed of multiple secondary faults in en-echelon arrangement; faults in the Permian are mainly along nearly EW and NW direction, and the Permian faults are distributed in the middle east part of the study area and they are more in the north than in the south. (2) The nearly EW and NW trending faults are dextral strike-slip faults, and the NE trending faults are sinistral strike-slip faults. The strike-slip strength of the nearly EW trending faults are the strongest and the maximum horizontal displacement is 550 m, while the strike-slip strength of the NE trending faults is the weakest. The Cambrian faults had stronger activities than the Permian faults. (3) The transtensional strike-slip faults were active in two periods, the early Caledonian period and the late Hercynian period, and were the products of oblique extension of pre-existing weak zones in Xingkai and Emei taphrogenesis, with certain inheritance in the main faults. (4) The strike-slip faults and surrounding fractures enhance the porosity and permeability of the reservoir in Cambrian Longwangmiao Formation and control the distribution of karst reservoirs in the Permian Qixia Formation–Maokou Formation, bringing about the situation of multiple gases bearing series in this area.

Eogenetic Karst Control of Carbonate Reservoirs during a Transient Exposure: A Case Study of the Ordovician Yingshan Formation in the Northern Slope of the Tazhong Uplift, Tarim Basin, China

The Tazhong area of the Tarim Basin contains abundant oil and gas resources in Ordovician carbonate rocks, especially in the karst pores and caves of the Yingshan Formation. Research has indicated that the Yingshan Formation underwent a 7–11 Ma exposure during the middle Caledonian Period, resulting in large-scale karst pores and caves. However, the continental freshwater karst model cannot adequately explain the origin and distribution of karst pores and caves. In order to develop a more accurate karst model to guide petroleum exploration in the region, we analyzed the karst morphology, cave development statistics, and paleokarst environments. Karst reservoir characteristics were analyzed on the basis of the following analysis: (1) Karst morphological analyses based on core description and formation micro-imager (FMI) log analyses. The results showed that alveolar-like and Swiss cheese-like solution pores, spongy dissolution zones, pit cenotes, and small continuous karst caves developed in the Yingshan Formation. (2) The statistical analysis of pore and cave characteristics indicated that most of the karst pores and caves developed within 50 m below the unconformity where the average height of these features ranged from 0.1 to 3.0 m and their widths ranged up to 100 m. These pores and caves were commonly filled with gravel, clay, and calcite. Horizontal well and seismic attribute analysis indicated that these pores and caves were distributed over a large area. In plain view, the karst pore-cave system is comprised of cross-linked anastomosing networks of horizontal cave passages. And (3) Cathode luminescence and electron microprobe analyses suggested that clay filling within karst caves was freshwater related, while calcite filling was of seawater origin. Cements within solution pores showed three phases of luminescence, suggesting an alternating freshwater and seawater environment. Based on these characteristics, the karsts of the Yingshan Formation in the Tazhong area are interpreted to be similar to the eogenetic karsts in the Yucatan Peninsula of modern Mexico. Accordingly, this study indicates that the pore-caves of the Yingshan Formation can be subdivided into three sections. Further, the development and filling of these pore-cave sections are interpreted to have formed by eogenetic mixed-water karstification during three phases of relatively stable sea level in a coastal margin environment.

Tectonic Characteristic and Origin of Tazhong Uplift, Tarim Basin

This paper presents the analysis on geological characteristics of Tazhong Uplift, such as the configuration and nature of the fault structures, and the aeromagnetic anomaly, etc. The study result shows that Tazhong Uplift presents several large-scale fault belts that mainly extend in three directions: NW, approximate (approx.) EW and NNE. The NW fault belts lie in the north and northwest of the research area; the approx. EW fault belts rest in the south and southeast of the research area, whereas the NNE fault belts are mainly strike-slip faults and cut the NW fault belts into several structural segments. It is concluded that the late Caledonian and the late Hercynian period were the two major forming periods of Tazhong Uplift. In the late Caledonian period, the NW thrust faults developed in Tazhong Uplift under the tectonic compression stress from the southwest Foreland Basin. In the late Hercynian period, the subduction of Altyn Orogenic Belt influenced the central Tarim and generated the approx. EW thrust faults in the central and southern Tazhong Uplift. This subduction also forced the early-formed fault belts into an arc-shape in the central Tazhong Uplift and caused NNE strike-slip faults in the northern Tazhong Uplift. Laterally the tectonic stress decreased from the south to the north. From regional perspective, the fault belts diverge to the west and converge to the east.

Interpretation of gravity data for fault distribution near the Mongolia–Hinggan metallogenic belt in the eastern China-Mongolia frontier area

The Central Asian Orogenic Belt (CAOB) is a giant suture zone produced by the reduction of the Paleo- Asian Ocean between the Siberian, North China, and Tarim cratons. The CAOB formed in three main stages, including continental accretion, late collision, and intracontinental orogeny. Strong crust-mantle interaction also occurred during these stages. The eastern China-Mongolia frontier area is an important part of the CAOB. Since the Caledonian period, this region has frequently experienced polycyclic tectonic reformation and intense magmatic activities. All of these geological activities lay the foundation for abundant metal resources. In the past, several large metal ore deposits have been founded there. However, the deep structure of different tectonic units and the fault distribution in the eastern China-Mongolia border frontier area are still not clear owing to the complex geological environment. Existing works in that region are insufficient for an in-depth understanding of the metallogenic deposits. The work discussed in this paper was carried out in the eastern China-Mongolia frontier area with measured gravity data along a profile and gravity data extracted from the WGM2012 earth’s gravity model for detailed fault distribution. In this study, empirical mode decomposition (EMD) and tilt angle analysis (TDR) were utilized for processing the gridded gravity data. The measured gravity data were inverted with a 2D inversion algorithm for density distribution along the survey line. The inversion result was used to verify the existence of known faults and describe their deeper extensions. Meanwhile, new faults were also identified along the survey line and then marked on the gridded data to get their horizontal distribution. These results provide significant information for the in-depth understanding of the tectonic units in the study area.

Indo–Chinese Magmatic Activity in the Duobaoshan District of Heilongjiang Province and Its Geological Significance

AbstractThe Duobaoshan ore concentration area, located in Nenjiang County of Heilongjiang Province, is an important porphyry Cu‐Mo ore concentration area in China, which is characterized by complex magmatic activities and multi‐phase overprinting metallogenesis. On the basis of field geological observation, systematic sampling, in‐lab analysis and the metallogenic regularity in the Xiang'an‐Mongolian metallogenic belt, this work carried out high‐precision dating and geochemical analysis on the Yuejin, 173‐kilometer and Wolihedingzi rock bodies. These rock bodies are renamed monzonitic granite and their consistent age (238 Ma) show that they were formed not in Variscan but in Indosinian. Therefore, it is inferred that the ore spots formed in the potassium silicate and sericite alteration zones of the rock mass also belong to Indosinian. In addition, we collected granodiorite from the Tongshan mining pit, and its zircon age is 223.1±2.8 Ma and the Cu content of the sample is high. The Tongshan mineralization is inferred to undergo the superimposition of Indosinian diagenetic mineralization. The age of the granodiorite porphyry related to copper‐molybdenum mineralization in the Xiaoduobaoshan area is 222.1±5.5 Ma, and the earlier age of granodiorite is 471.8±7.4 Ma, indicating that the initial magmatic activities belong to the Duobaoshan porphyry system in the Caledonian period. The geochemical characteristics of the Indosinian rock samples show continental arc features, with reference to tectonic‐magmatic activities of the whole Daxing'anling area. We consider that the magmatic activities and mineralization of the Indosinian period are affected by the southward subduction of Okhotsk Ocean since Late Permian. By combining the mineralization rules of Daxinganling area and the structural systems of Duobaoshan ore concentration area, we divide two rock‐mineralization belts in this area including the Yuejin‐Duobaoshan‐Tongshan belt and 173‐kilometer‐Xiaoduobaoshan‐Wolihedingzi belt, which are distributed nearly parallel along the NW‐trending fractures and show similar geotectonic settings and the timing of the magmatic activities. It is favorable for discovering porphyry Cu‐Mo deposits in these two metallogenic belts, especially in the Yuejin, 173‐kilometer and Wolihedingzi areas where less research work has been made.