A Large-scale Tertiary Salt Nappe Complex in the Leading Edge of the Kuqa Foreland Fold-Thrust Belt, the Tarim Basin, Northwest China

The tectono-stratigraphic sequences of the Kuqa foreland fold-thrust belt in the northern Tarim basin, northwest China, can be divided into the Mesozoic sub-salt sequence, the Paleocene-Eocene salt sequence and the Oligocene-Quaternary supra-salt sequence. The salt sequence is composed mainly of light grey halite, gypsum, marl and brown elastics. A variety of salt-related structures have developed in the Kuqa foreland fold belt, in which the most fascinating structures are salt nappe complex. Based on field observation, seismic interpretation and drilling data, a large-scale salt nappe complex has been identified. It trends approximately east-west for over 200 km and occurs along the west Qiulitag Mountains. Its thrusting displacement is over 30 km. The salt nappe complex appears as an arcuate zone projecting southwestwards along the leading edge of the Kuqa foreland fold belt. The major thrust fault is developed along the Paleocene-Eocene salt beds. The allochthonous nappes comprise large north-dipping faulting monoclines which are made up of Paleocene-Pliocene sediments. Geological analysis and cross-section restoration revealed that the salt nappes were mainly formed at the late Himalayan stage (c.a. 1.64 Ma BP) and have been active until the present day. Because of inhomogeneous thrusting, a great difference may exist in thrust displacement, thrust occurrence, superimposition of allochthonous and autochthonous sequences and the development of the salt-related structures, which indicates the segmentation along the salt nappes. Regional compression, gravitational gliding and spreading controlled the formation and evolution of the salt nappe complex in the Kuqa foreland fold belt.

The Uplift of the Longmenshan Thrust Belt and Subsidence of the West Sichuan Foreland Basin

Based on fission track dating of apatite, and measurement of vitrinite reflectance of rock samples from the Longmenshan (Longmen Mountain)area and the West Sichuan foreland basin and computer modelling it is concluded that (l)the Songpan-Garze fold belt has uplifted at least by 3-4 km with an uplift rate of no less than 0.3-0.4 mm/a since 10 Ma B.P.; (2) the Longmenshan thrust nappe belt has uplifted at least by 5-6 km with an uplift rate of more than 0.5- 0.6 mm /a since 10 Ma B.P.; (3) the Longmenshan detachment belt has uplifted by 1 - 2 km at a rate of 0.016-0.032 mm/a since 60 Ma B.P.; (4) the West Sichuan foreland basin has uplifted by 1.7-3 km at a rate of 0.028-0.05 mm/a since 60 Ma B.P.; (5) the uplift rate of the area on the west side of the Beichuan-Yingxiu-Xiaoguanzi fault for the last 10 Ma is 40 times as much as that on its east side; (6) the uplifting of the the Songpan - Garze fold belt and the subsidence of the West Sichuan foreland basin 60 Ma ago exhibit a mirro-image correlation, i.e. the rapid uplifting of the the Songpan-Garze fold belt was corresponding to the rapid subsidence of the basin;the Songpan-Garze fold belt has uplifted at a much greater rate than the West Sichuan foeland basin in the last 60 Ma;and (7) the palaeogeothermal gradient was 25℃ /km in the West Sichuan foreland basin.

Indosinian Foreland Fold-and-Thrust Belt Bordering Yunnan and Guangxi, China

Recent discoveries of ophiolites indicate that there must be a Palaeotethyan geosuture zone bordering China and Vietnam, which separates the Vietbac block from the South China subcontinent. The Indosinian foreland fold-and-thrust belt bordering Yunnan and Guangxi provided further evidence for the palaeotethysides. The oceanic crust was subducted southwestwards while the magmatic arc migrated northeastwards, and the continent-arc collision occurred in the Late Triassic with the thrusting being extended towards the north or northeast. The features of thrust-nappe structure are discussed, which proved the continental margin of the Palaeotethyan ocean there to be a complicated one. A face-to-face collision occurred first along the NW-striking segment and then along the ENE-striking segment accompanied by transpression or oblique thrusting occurring along the NW-striking one.

Hydrodynamic Evolution and Hydrocarbon Accumulation in the Dabashan Foreland Thrust Belt,China

There are two plays in the Dabashan foreland tectonic belt： the upper and the lower plays. The lower play experienced one sedimentary hydrodynamic stage, two burial hydrodynamic stages, two tectonic hydrodynamic stages and two infiltration hydrodynamic stages from the Sinian to the Cenozoic, while the upper play had one sedimentary hydrodynamic stage, one burial hydrodynamic stage, two tectonic hydrodynamic stages and one infiltration hydrodynamic stage from the Permian to the Cenozoic. Extensive flows of both sedimentary water, including hydrocarbons, and deep mantle fluid occurred in the Chengkou faults during collision orogeny in the Middle-Late Triassic Indosinian orogeny, and fluid flow was complicated during intracontinental orogeny in the Middle-Late Jurassic. In addition to these movements, infiltration and movement of meteoric water took place in the Chengkou faults, whereas in the covering-strata decollement tectonic belt, extensive sedimentary water flow （including hydrocarbons） occurred mainly in the Zhenba and Pingba faults. During the stage of rapid uplift and exhumation from the Cretaceous to the Cenozoic, the fluid flow was characterized mainly by infiltration of meteoric water and gravity-induced flow caused by altitude difference, whereas sedimentary water flow caused by tectonic processes was relatively less significant. Sedimentary water flow was more significant to the lower play in hydrocarbon migration and accumulation during collision orogeny in the Middle-Late Triassic Indosinian orogeny, but its influence is relatively slight on the upper play. On one hand, hydrodynamics during intracontinental orogeny in the Middle-Late Jurassic adjusted, reformed or oven destroyed oil reservoirs in the lower play; on the other hand, it drove large amounts of hydrocarbons to migrate laterally and vertically and is favorable for hydrocarbon accumulation. Infiltration hydrodynamics mainly adjusted and destroyed oil reservoirs from the Cretaceous to the Cenozoic.

Efectiveness and petroleum geological signifcance of tectonic fractures in the ultra‑deep zone of the Kuqa foreland thrust belt:a case study of the Cretaceous Bashijiqike Formation in the Keshen gas feld

The buried depth of the gas-producing reservoir in the Kuqa foreland thrust belt of the Tarim Basin exceeds 6000 m.The average matrix porosity of the reservoir is 5.5%,and the average matrix permeability is 0.128×10^(−3)μm^(2).In order to reveal the characteristics and efectiveness of ultra-deep fractures and their efects on reservoir properties and natural gas production,outcrops,cores,thin section,image logs and production testing data are used to investigate the efectiveness of tectonic fractures in ultra-deep reservoirs in the Kuqa foreland thrust zone,and the corresponding geological signifcance for oil and gas exploration and development are discussed.Tectonic fractures in the thrust belt include EW-trending high-angle tensile fractures and NS-trending vertical shear fractures.The former has a relatively high flling rate,while the latter is mostly unflled.Micro-fractures are usually grain-piercing-through cracks with width of 10-100 microns.In the planar view,the efective fractures are concentrated in the high part and wing zones of the long axis of the anticline,and along the vertical direction,they are mainly found in the tensile fracture zone above the neutral plane.The adjustment fracture zone has the strongest vertical extension abilities and high efectiveness,followed by the nearly EW longitudinal tensile fracture zone,and the netted fracture zone with multiple dip angles.The efectiveness of fracture is mainly controlled by fracture aperture and flling degrees.Efective fractures can increase reservoir permeability by 1-2 orders of magnitude.The higher part of the anticline is associated with high tectonic fracture permeability,which control enrichment and high production of natural gas.The netted vertical open fractures efectively communicate with pores and throats of the reservoir matrix,which forms an apparent-homogenous to medium-heterogeneous body that is seen with high production of natural gas sustained for a long term.

Three-dimensional discrete element numerical simulation of Paleogene salt structures in the western Kuqa foreland thrust belt

Taking the Paleogene salt strata in the west of Kuqa foreland thrust belt as study object, the deformation features of salt structure in the compression direction and perpendicular to the compression direction were examined to find out the control factors and formation mechanisms of the salt structures. By using the three-dimensional discrete element numerical simulation method, the formation mechanisms of typical salt structures of western Kuqa foreland thrust belt in Keshen and Dabei work areas were comprehensively analyzed. The simulation results show that the salt deformation in Keshen and Dabei work areas is of forward spread type, with deformation concentrated in the piedmont zone;the salt deformation is affected by the early uplift near the compression end, pre-existing basement faults, synsedimentary process and the initial salt depocenter;in the direction perpendicular to the compression direction, salt rocks near the compression end have strong lateral mobility with the velocity component moving towards the middle part, and the closer to the middle, the larger the velocity will be, so that salt rocks will aggregate towards the middle and deform intensely, forming complex folds and separation of salt structures from salt source, and local outcrop with thrust faults. Compared with 2 D simulation, 3 D simulation can analyze salt structures in the principal stress direction and direction perpendicular to the principal stress, give us a full view of the formation mechanisms of salt structures, and guide the exploration of oil and gas reservoirs related to salt structures.

Heterogeneity and differential hydrocarbon accumulation model of deep reservoirs in foreland thrust belts: A case study of deep Cretaceous Qingshuihe Formation clastic reservoirs in southern Junggar Basin, NW China

Using the data of drilling, logging, core, experiments and production, the heterogeneity and differential hydrocarbon accumulation model of deep reservoirs in Cretaceous Qingshuihe Formation(K1q) in the western section of the foreland thrust belt in southern Junggar Basin are investigated. The target reservoirs are characterized by superimposition of conglomerates, sandy conglomerates and sandstones, with high content of plastic clasts. The reservoir space is mainly composed of intergranular pores. The reservoirs are overall tight, and the sandy conglomerate has the best physical properties. The coupling of short deep burial period with low paleotemperature gradient and formation overpressure led to the relatively weak diagenetic strength of the reservoirs. Specifically, the sandy conglomerates show relatively low carbonate cementation, low compaction rate and high dissolution porosity. The special stress-strain mechanism of the anticline makes the reservoirs at the top of the anticline turning point more reformed by fractures than those at the limbs, and the formation overpressure makes the fractures in open state. Moreover, the sandy conglomerates have the highest oil saturation. Typical anticline reservoirs are developed in deep part of the thrust belt, but characterized by "big trap with small reservoir". Significantly, the sandy conglomerates at the top of anticline turning point have better quality, lower in-situ stress and higher structural position than those at the limbs,with the internal hydrocarbons most enriched, making them high-yield oil/gas layers. The exponential decline of fractures makes hydrocarbon accumulation difficult in the reservoirs at the limbs. Nonetheless, plane hydrocarbon distribution is more extensive at the gentle limb than the steep limb.

SEGMENTATION OF FORELAND BELTS AND RELATIONSHIP WITH BASEMENT DEFORMATION IN THE SOUTHWEST TARIM BASIN

In the foreland belts of Pamir\|West Kunlun (PWK) and Southern Tienshan (ST), the thin\|skinned thrusting systems (tsts) from mountains thrusting to the Southwest Tarim Basin (STB) and the back\|thrusting systems (bts) from basin thrusting to the adjacent mountains were developed. The different active structural systems can be segmented: Kazike\|Arte deformed bts and tsts, Yigeziya tsts, Qimugen arcurate bts, Qipan thrusting and faulted\|folded system, Kekeya tsts, Keliyang—Yuliqun bts, Sangzhu\|Duwa tsts and Cele arcurate thr usting system along the PWK foreland belts, and Kalajun—Kalpin tsts, Atushi\|Bapanshuimo bts along the foreland belts of ST.Through the study of aerial magnetic anomalies and many seismic profiles, we found that all bts and their triangle zones were formed over the NE—SW basement depressions of STB (thickness of sedimentary covers >10km). We also found that all tsts were formed over the basement uplift belts of STB (thickness of sedimentary covers <10km, 6～10km in general).In the basement depression of the basin, the bts and their triangle zones could be transformed to tsts due to continuous compression. The largest displacement of the tsts or bts is located in the convex part but the shortest displacement is located in the conjunction area of two segments. The arcurate tsts and bts display different displacements along foreland belts.

An alternative interpretation for the map expression of "abrupt" changes in lateral stratigraphic level near transverse zones in fold-thrust belts

The map expression of ＂abrupt＂ changes in lateral stratigraphic level of a thrust fault has been traditionally interpreted to be a result of the presence of （1） a lateral （or oblique） thrust-ramp, or （2） a frontal ramp with displacement gradient, and/or （3） a combination of these geometries. These geometries have been used to interpret the structures near transverse zones in fold-thrust belts （FTB）. This contribution outlines an alternative explanation that can result in the same map pattern by lateral variations in stratigraphy along the strike of a low angle thrust fault. We describe the natural example of the Leamington transverse zone, which marks the southern margin of the Pennsylvanian-Permian Oquirrh basin with genetically related lateral stratigraphic variations in the North American Sevier FTB. Thus, the observed map pattern at this zone is closely related to lateral stratigraphic variations along the strike of a horizontal fault. Even though the present-day erosional level shows the map pattern that could be interpreted as a lateral ramp, the observed structures along the Leamington zone most likely share the effects of the presence of a lateral （or oblique） ramp, lateral stratigraphic variations along the fault trace, and the displacement gradient.

Wadi Fatima Thin-Skinned Foreland FAT Belt: A Post Amalgamation Marine Basin in the Arabian Shield

Wadi Fatima fold-thrust (FAT) belt is a distinctive foreland FAT belt in the Arabian-Nubian Shield (ANS) involving unmetamorphosed to slightly metamorphosed sedimentary sequence of Fatima Group, deposited over a metamorphic/igneous basement, comprising ortho-amphibolites, orthoand para-schists (with chaotic unmappable blocks of marbles, pyroxenites and metagabbros), older granite (773 ± 16 Ma) and younger granite. The basement exhibits structural fabrics, such as attenuated tight isoclinal folds, sheared-out hinges, NE-SW penetrative foliation and subhorizontal stretched and mineral lineations, related to an oldest prominent dextral shearing phase affected the main Wadi Fatima during the Neoproterozoic. In Wadi Fatima FAT belt, the style of deformation encompasses flexural-slip folding forming mesoscopicand map-scales NE to ENE plunging overturned antiforms and synforms, and a thrust duplex system bounded by floor thrust and sole thrust (basal detachment) dipping gently towards the hinterland (SE to SSE direction) and rises stratigraphically upwards towards the foreland. Such style is affiliated to thin-skinned deformation. Several lines of evidence, such as geometry of interacting outcropand map-scale folds and thrusts, patterns of thrust displacement variations and indications for hinge migration during fold growth, strongly suggest that folding and thrusting in Wadi Fatima FAT belt are geometrically and kinematically linked and that thrusting initiated as a consequence of folding (fold-first kinematics). Thrusts frequently show flat-ramp-flat geometry, and every so often give an impression that they are formed during two main sub-stages;an older sub-stage during which bedding sub-parallel thrusts were formed, and a younger sub-stage which generated younger ramps oblique to bedding. Thrust ramps with SE to SSE dipping regularly show sequential decrease in dip or inclination (due to piggy-back imbrication) into their transport direction which is proposed to be towards NW to NNW. Evidence indicating this transport direction of Wadi Fatima FAT belt embrace NW to NNW oriented stretching lineations recorded along thrust planes, NW to NNW folding vergence, and diminishing of the intensity of deformation and thrust stacking and imbrication from SE to NW;i.e. from hinterland to foreland. The tectonic transport vector is congruent with the mean orientation of slickenline striae formed by layer-parallel slipping along folded bedding planes. The mean orientation of slickenline lineations, after their host beds were rotated to horizontal about their strikes, is found to be N25°W - S25°E. Two tectonic models are proposed to unravel the structural history of the study area and to illustrate the tectonic evolution of Wadi Fatima FAT belt which represents one of interesting foreland FAT belts recorded worldwide. In the first model, the area was evolved from dextral shearing during the early convergence and amalgamation between East and West Gondwana, to emplacement of the older granite during a period of crustal cessation and relaxation, NNW SSE extension and extrusion of dyke swarms, emplacement of younger granite, deposition of Fatima Group over an ancient peneplain, layer parallel shortening, folding and fold tightening and overturning, thrusting, NE-SW (to NNE-SSW) shortening, and eventually NE tilting accompanied with Red Sea rifting (?). The second model suggests the presence of basement ramps (pre-existing normal faults), with NW to NNW dipping, have a strong effect on overlying Fatima Group which was evolved throughout gravitational, soft-sediment slumping and deformation.

Numerical simulation of temporal and spatial evolution of fold-thrust belts in the northern and northeastern margins of the Qinghai-Tibetan Plateau

Fold-thrust belts are common structural styles under the background of long-term regional tectonic shortening.The northern and northeastern margins of the Qinghai-Tibetan Plateau are located on the edge of the growth and expansion of the Qinghai-Tibetan Plateau.Since nearly 10 Ma,some significant and typical fold thrust belt have been formed.The spatial-temporal evolution of these fold-thrust belts and the characteristics of surface deformations are significant issues in geodynamics.In this paper,we use the elastoplastic finite element model with considering the contact nonlinearity to study the spatialtemporal evolution of the fold-thrust belts in the northern and northeastern margins of the Qinghai-Tibetan Plateau,with particular attention to the details of the relationship between the depth and the shallow,the spatialtemporal order,and the characteristics of the surface deformation,etc.,in order to make a relatively complete mechanical interpretation of the spatial-temporal evolution of the foldthrust belts in the northern and northeastern margins of the Qinghai-Tibetan Plateau from the perspective of geodynamics.

Geological structures and potential petroleum exploration areas in the southwestern Sichuan fold-thrust belt, SW China

Based on the latest geological,seismic,drilling and outcrop data,we studied the geological structure,tectonic evolution history and deformation process of the southwestern Sichuan fold-thrust belt to find out the potential hydrocarbon exploration areas in deep layers.During key tectonic periods,the southwestern Sichuan fold-thrust belt developed some characteristic strata and structural deformation features,including the Pre-Sinian multi-row N-S strike rifts,step-shaped platform-margin structures of Sinian Dengying Formation,the western paleo-uplift in the early stage of Late Paleozoic,the Late Paleozoic–Middle Triassic carbonate platform,foreland slope and forebulge during Late Triassic to Cretaceous,and Cenozoic multi-strike rejuvenated fold-thrusting structures.The fold-thrust belt vertically shows a double-layer structural deformation controlled by the salt layer in the Middle Triassic Leikoupo Formation and the base detachment layer at present.The upper deformation layer develops the NE-SW strike thrusts propagating toward basin in long distance,while the deeper deformation layer had near north-south strike basement-involved folds,which deformed the detachment and thrusting structures formed earlier in the upper layer,with the deformation strength high in south part and weak in north part.The southern part of the fold-thrust belt is characterized by basement-involved fold-thrusts formed late,while the central-northern part is dominated by thin-skin thrusts in the shallow layer.The Wuzhongshan anticlinal belt near piedmont is characterized by over-thrust structure above the salt detachment,where the upper over-thrusting nappe consists of a complicated fold core and front limb of a fault-bend fold,while the deep layer has stable subtle in-situ structures.Favorable exploration strata and areas have been identified both in the upper and deeper deformation layers separated by regional salt detachment,wherein multiple anticlinal structures are targets for exploration.Other potential exploration strata and areas in southwestern Sichuan fold-thrust belt include the deep Sinian and Permian in the Wuzhongshan structure,pre-Sinian rifting sequences and related structures,platform-margin belt of Sinian Dengying Formation,and Indosinian paleo-uplift in the east of the Longquanshan structure.

Tectonic evolution of the Dabashan orocline, central China: Insights from the superposed folds in the eastern Dabashan foreland

The Dabashan orocline is situated in the northwestern margin of the central Yangtze block,central China.Previous studies have defined the orthogonal superposed folds growing in its central-western segment thereby confirming its two-stage tectonic evolution history.Geological mapping has revealed that more types of superposed folds have developed in the eastern segment of the orocline,which probably provides more clues for probing the structure and tectonic history of the Dabashan orocline.In this paper,based on geological mapping,structural measurements and analyses of deformation,we have identified three groups of folds with different trends （e.g.NW-,NE-and nearly E-trending folds） and three types of structural patterns of superposed folds in the eastern Dabashan foreland （e.g.syn-axial,oblique,and conjunctional superposed folds）.In combination with geochronological data,we propose that the synaxial superposed folds are due to two stages of ～N-S shortening in the west and north of the Shennongjia massif,and that oblique superposed folds have been resulted from the superposition of the NW-and NE-trending folds onto the early ～ E-W folds in the east of the Shennongjia massif in the late Jurassic to early Cretaceous.The conjunctional folds are composed of the NW-and NE-trending folds,corresponding to the regional-scale dual-orocline in the eastern Sichuan as a result of the southwestward expansion of the Dabashan foreland during late Jurassic to early Cretaceous,coeval with the northwestward propagation of the Xuefengshan foreland.Integration of the structure and geochronology of the belt shows that the Dabashan orocline is a combined deformation belt primarily experiencing a twostage tectonic evolution history in Mesozoic,initiation of the Dabashan orocline as a foreland basin along the front of the Qinling orogen in late Triassic to early Jurassic due to collisional orogeny,and the final formation of the Dabashan orocline owing to the southwestward propagation of the Qinling orogen during late Jurassic to early Cretaceous intra-continental orogeny.Our studies provide some evidences for understanding the structure and deformation of the Dabashan orocline.

Simulation of the Subsidence and Deposition Processes of the Foreland Basin in the Southwestern Margin of Ordos

Characteristics of deformation in the thrust belt and sequence stratigraphic framework in the foreland basin, structural features of the basin margin, and the episodic thrusting are studied in this paper by combining the eastern Qilian thrust belt and the Late Triassic foreland basin on the southwestern margin of Ordos. On this basis, a geological model and a mechanical model of coupling mechanism were established for the pair of thrust belt and foreland basin, and the subsidence and deposition process of the foreland basin were simulated on given parameters.

Dynamic Settings and Interactions between Basin Subsidence and Orogeny in Zhoukou Depression and Dabie Orogenic Belt

This paper presents a study of the geo dynamic setting and the relation between orogenic uplift and basin subsidence in the inland Zhoukou depression and Dabie orogenic belt. Since the Mesozoic the evolution of Zhoukou depression can be divided into three stages: (1) foreland basin, (2) transitional stage, (3) fault depression. Formation and variations of basin were not only related to the orogenesis, but also consistent with the orogenic uplift.

Application Effect of CEMP in the Foreland Basin on the Western Edge of Ordos

Foreland basins on the western edge of Ordos are characterized by typical loess geomorphologic features. Its surface is severely cut, and its underground geological structures are complicated.Making use of the advantage of CEMP in exploration in frontland as well as successful multi-year exploration experiences in West China, we first employed the CEMP method to conduct exploration on the western edge of the Ordos basin and its surrounding areas. The practices proved that rich and reliable geological results have been achieved with the CEMP method in complicated areas covered with loess. This can provide important evidence for researching the contact relationship among basins and mountains, structural feature of target layers and predict favourable areas.

Influence of Salt Beds on the Segmentation of Structure and Hydrocarbon Accumulation in Qiulitag Structural Belt,Tarim Basin,China

Seismic information and balanced profile technology were used to reveal the influence of the salt bed in segmentation of structure and hydrocarbon accumulation in Qiulitag structural belt in Tarim basin. From west to east, the shortening of strata above the salt beds gradually decreases, while, the shortening below the salt beds gradually increases, which shows that the segmentation of structure integrated the seismic profile. There is great difference of the deformation of strata below and above the salt beds between the west segment and the east segment. The analysis of the distribution of oil/gas fields and the hydrocarbon properties indicates the similar segmentation to the structure segmentation. The salt beds in relatively shallow layers change the stress condition from basement of Kuqa foreland basin, which leads to the segmentation of Qiulitag structural belt. Because the salt beds in the west segment came into being earlier than those in the east segment, the west segment captures hydrocarbon from two sets of source rock, while the east segment can only capture hydrocarbons from one set of source rock. So, the salt beds play an important role in the segmentation of structure and hydrocarbon accumulation.

Structural geology and favorable exploration prospect belts in northwestern Sichuan Basin, SW China

The northwestern Sichuan region has experienced multi-stage tectonic evolution including marine cratonic basin from the Sinian to the Middle Triassic and intra-continental basin from the Late Triassic to the Cenozoic. Several regional tectonic activities caused complicated stratigraphic distribution and structural deformations in the deep-buried layers. During the key tectonic periods, some characteristic sedimentary and deformation structures were formed, including the step-shaped marginal carbonate platform of Dengying Formation, the western paleo-high at the end of Silurian, and the passive continental margin of the Late Paleozoic–Middle Triassic. The Meso-Cenozoic intra-continental compressional tectonic processes since the Late Triassic controlled the formation of complex thrusting structures surrounding and inside the northwestern basin. The northern Longmenshan fold-thrust belt has a footwall in-situ thrust structure,which is controlled by two sets of detachments in the Lower Triassic and Lower Cambrian and presents as a multi-level deformation structure with the shallow folds, the middle thin-skin thrusts and the deeper basement-involved folds. The thrust belt in front of the Micangshan Mountain shows a double-layer deformation controlled by the Lower Triassic salt detachment, which is composed by the upper monocline and deep-buried imbricate thrust structures. The interior of the basin is characterized by several rows of large-scale basement-involved folds with NEE strike direction. From the perspective of structural geology, the favorable exploration reservoirs and belts in northwestern Sichuan have obvious zoning characteristics. The favorable exploration layers of Dengying Formation of Upper Sinian are mainly distributed in the eastern and northern areas of the northwestern Sichuan Basin, in which the Jiulongshan structural belt, Zitong syncline and Yanting slope are the most favorable. The Lower Paleozoic was transformed by Caledonian paleo-uplift and late Cenozoic folding, and the midwest area such as the Zitong syncline is a potential area for hydrocarbon exploration. The favorable part of the Upper Paleozoic is mainly distributed in the northern Longmenshan belt and its frontal area, where the deep-buried thin-skin thrust structures in the footwall are the key exploration targets.

Characteristics,origin and controlling effects on hydrocarbon accumulation of overpressure in foreland thrust belt of southern margin of Junggar Basin,NW China

Aiming at the differential distribution of overpressure in vertical and lateral directions in the foreland thrust belt in the southern margin of Junggar Basin,the study on overpressure origin identification and overpressure evolution simulation is carried out.Based on the measured formation pressure,drilling fluid density and well logging data,overpressure origin identification and overpressure evolution simulation techniques are used to analyze the vertical and lateral distribution patterns of overpressure,genetic mechanisms of overpressure in different structural belts and causes of the differential distribution of overpressure,and the controlling effects of overpressure development and evolution on the formation and distribution of oil and gas reservoirs.The research shows that overpressure occurs in multiple formations vertically in the southern Junggar foreland thrust belt,the deeper the formation,the bigger the scale of the overpressure is.Laterally,overpressure is least developed in the mountain front belt,most developed in the fold anticline belt,and relatively developed in the slope belt.The differential distribution of overpressure is mainly controlled by the differences in disequilibrium compaction and tectonic compression strengths of different belts.The vertical overpressure transmission caused by faults connecting the deep overpressured system has an important contribution to the further increase of the overpressure strength in this area.The controlling effect of overpressure development and evolution on hydrocarbon accumulation and distribution shows in the following aspects:When the strong overpressure was formed before reservoir becoming tight overpressure maintains the physical properties of deep reservoirs to some extent,expanding the exploration depth of deep reservoirs;reservoirs below the overpressured mudstone cap rocks of the Paleogene Anjihaihe Formation and Lower Cretaceous Tugulu Group are main sites for oil and gas accumulation;under the background of overall overpressure,both overpressure strength too high or too low are not conducive to hydrocarbon enrichment and preservation,and the pressure coefficient between 1.6 and 2.1 is the best.

Geochemical signatures of potassium metasomatism in anthracite from the Himalayan fold‑thrust belts of Sikkim,India

The present study focuses on the inorganic geochemical features of the bituminous coal samples from the Raniganj and the Jharia Basins,as well as the anthracite samples from the Himalayan fold-thrust belts of Sikkim,India.The SiO_(2)content(48.05 wt%to 65.09 wt%and 35.92 wt%to 50.11 wt%in the bituminous and anthracite samples,respectively)and the ratio of Al_(2)O_(3)/TiO_(2)(6.97 to 17.03 in the bituminous coal samples and 10.34 to 20.07 in the anthracite samples)reveal the intermediate igneous source rock composition of the minerals.The ratio of the K_(2)O/Al_(2)O_(3)in the ash yield of the bituminous coal samples(0.03 to 0.09)may suggest the presence of kaolinite mixed with montmorillonite,while its range in the ash yield of the anthracite samples(0.16 to 0.27)may imply the presence of illite mixed with kaolinite.The chemical index of alteration values may suggest the moderate to strong chemical weathering of the source rock under sub-humid to humid climatic conditions.The plot of the bituminous coal samples in the A–CN–K diagram depicts the traditional weathering trend of parent rocks,but the anthracite samples plot near the illite feld and are a bit ofset from the weathering trend.This may imply the plausible infuences of the potassium-metasomatism at post coalifcation stages,which is further supported by high K_(2)O/Na_(2)O ratio(29.88–80.13).The Fourier transform infrared spectra further reveal the hydroxyl stretching intensity of illite in the anthracite samples substantiating the efect of the epigenetic potassium-metasomatism.The decrease in total kaolinite intensity/compound intensity of quartz and feldspar may provide additional evidence towards this epigenetic event.