Influences of burial process on diagenesis and high-quality reservoir development of deep-ultra-deep clastic rocks:A case study of Lower Cretaceous Qingshuihe Formation in southern margin of Junggar Basin,NW China

Taking the Lower Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin as an example,the influences of the burial process in a foreland basin on the diagenesis and the development of high-quality reservoirs of deep and ultra-deep clastic rocks were investigated using thin section,scanning electron microscope,electron probe,stable isotopic composition and fluid inclusion data.The Qingshuihe Formation went through four burial stages of slow shallow burial,tectonic uplift,progressive deep burial and rapid deep burial successively.The stages of slow shallow burial and tectonic uplift not only can alleviate the mechanical compaction of grains,but also can maintain an open diagenetic system in the reservoirs for a long time,which promotes the dissolution of soluble components by meteoric freshwater and inhibits the precipitation of dissolution products in the reservoirs.The late rapid deep burial process contributed to the development of fluid overpressure,which effectively inhibits the destruction of primary pores by compaction and cementation.The fluid overpressure promotes the development of microfractures in the reservoir,which enhances the dissolution effect of organic acids.Based on the quantitative reconstruction of porosity evolution history,it is found that the long-term slow shallow burial and tectonic uplift processes make the greatest contribution to the development of deep-ultra-deep high-quality clastic rock reservoirs,followed by the late rapid deep burial process,and the progressive deep burial process has little contribution.

Action mechanisms of abnormal fluid pressure on physical properties of deep reservoirs: A case study on Jurassic Toutunhe Formation in the southern margin of Junggar Basin, NW China

Considering the action mechanisms of overpressure on physical changes in skeleton particles of deep reservoir rocks and the differences in physical changes of skeleton particles under overpressure and hydrostatic pressure, the sandstone of the Jurassic Toutunhe Formation in the southern margin of Junggar Basin was taken as an example for physical modeling experiment to analyze the action mechanisms of overpressure on the physical properties of deep reservoirs. (1) In the simulated ultra-deep layer with a burial depth of 6000-8000 m, the mechanical compaction under overpressure reduces the remaining primary pores by about a half that under hydrostatic pressure. Overpressure can effectively suppress the mechanical compaction to allow the preservation of intergranular primary pores. (2) The linear contact length ratio under overpressure is always smaller than the linear contact length ratio under hydrostatic pressure at the same depth. In deep reservoirs, the difference between the mechanical compaction degree under overpressure and hydrostatic pressure shows a decreasing trend, the effect of abnormally high pressure to resist the increase of effective stress is weakened, and the degree of mechanical compaction is gradually close to that under hydrostatic pressure. (3) The microfractures in skeleton particles of deep reservoirs under overpressure are thin and long, while the microfractures in skeleton particles of deep reservoirs under hydrostatic pressure are short and wide. This difference is attributed to the probable presence of tension fractures in the rocks containing abnormally high pressure fluid. (4) The microfractures in skeleton particles under overpressure were mainly formed later than that under hydrostatic pressure, and the development degree and length of microfractures both extend deeper. (5) The development stages of microfractures under overpressure are mainly controlled by the development stages of abnormally high pressure and the magnitude of effective stress acting on the skeleton particles. Moreover, the development stages of microfractures in skeleton particles are more than those under hydrostatic pressure in deep reservoir. The multi-stage abnormally high pressure plays an important role in improving the physical properties of deep reservoirs.

Simulation for the Controlling Factors of Structural Deformation in the Southern Margin of the Junggar Basin

According to the differences of structural deformation characteristics, the southern margin of the Junggar basin can be divided into two segments from east to west. Arcnate thrust-and-fold belts that protrude to the north are developed in the eastern segment. There are three rows of en echelon thrust-and-fold belts in the western segment. Thrust and fold structures of basement-involved styles are developed in the first row, and decollement fold structures are formed from the second row to the third row. In order to study the factors controlling the deformation of structures, sand-box experiments have been devised to simulate the evolution of plane and profile deformation. The planar simulation results indicate that the orthogonal compression coming from Bogeda Mountain and the oblique compression with an angle of 75° between the stress and the boundary originating from North Tianshan were responsible for the deformation differences between the eastern part and the western part. The Miquan-Uriimqi fault in the basement is the pre-existing condition for generating fragments from east to west. The profile simulation results show that the main factors controlling the deformation in the eastern part are related to the decollement of Jurassic coal beds alone, while those controlling the deformation in the western segment are related to both the Jurassic coal beds and the Eogene clay beds. The total amount of shortening from the Yaomoshan anticline to the Gumudi anticline in the eastern part is -19.57 km as estimated from the simulation results, and the shortening rate is about 36.46%; that from the Qingshuihe anticline to the Anjihai anticline in the western part is -22.01 km as estimated by the simulation results, with a shortening rate of about 32.48%. These estimated values obtained from the model results are very close to the values calculated by means of the balanced cross section.

Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic-Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt,NW China

The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and granodiorites.An age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The εNd(t) values in the rock units vary from +6.70 to +9.64, and initial87Sr/86Sr ratios range between 0.7035 and0.7042. Initial206Pb/204Pb,207Pb/204Pb and208Pb/204Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)PMvalues between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)PMratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust.Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10 vol%contamination from a granitic melt. This was followed by about 5 vol% assimilation of upper crustal rocks. Thus, the high-Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break-off of a subducted slab.

Lithofacies paleogeography restoration and its significance of Jurassic to Lower Cretaceous in southern margin of Junggar Basin,NW China

In view of the difficulties in the study of lithofacies paleogeography and the low reliability of the distribution range of sedimentary sand bodies in the prototype basin caused by less deep drilling, complex seismic imaging and low degree of exploration in the southern margin of Junggar Basin, NW China. A new method based on the source to sink idea was used to restore lithofacies paleogeography and predict glutenite distribution. In the restoration, apatite fission track age was used to define range and uplift time of macro-provenance;the range of provenance area and the migration process of lake shoreline were restored based on the quantitative relationship between gravel diameter and transportation distance, tectonic shortening and other geological parameters;drilling cores and field outcrop sedimentary structures were analyzed, and a series of maps of lithofacies paleogeographic evolution and distribution range of glutenite bodies were compiled. It is concluded that from Early Jurassic to Early Cretaceous, in the southern margin of Junggar Basin, the provenance area gradually expanded from south to north, the lake basin expanded, shrunk and expanded, and the paleoclimate changed from humid to drought to humid. The western section always had proximal fan delta deposits from the southern ancient Tianshan provenance developed, and in the middle and eastern sections, the provenance areas evolved from far source to near source, mainly river-delta, braided delta, fan delta and other sediments developed. The boundary between provenance areas of the western and middle sections is speculated to be Hongche fault zone. In an angle open to the northwest with the current basin edge line, the restored ancient lake shoreline controlled the heterogeneity of reservoirs in the delta plain belt and delta front belt on its both sides. The ancient lake shoreline, current stratigraphic denudation line and current basin margin line limit the types and scope of favorable reservoirs.This understanding provides an important geological basis for oil and gas exploration in the deep lower source-reservoir assemblage at the southern margin of Junggar Basin.

LA-ICP-MS Zircon U-Pb Age of Longtou Syenite body in South Songxian County, Southern Margin of the North China Craton

Objective Indosinian magmatic rocks mainly locate in west Qinling Orogen, which are, however, extremely rare in east Qingling Orogen （Lu Xinxiang, 2000; Zhang Guowei et al., 2001; Guo Xianqing et al., 2017）. The Zhifang Huangzhuang （ZH） area in south Songxian County is located in the southern margin of the North China Craton （Fig. l a）, which is an important lndosinian alkaline magmatic occurrence including 32 syenite bodies and syenitic dykes in east Qinling Orogen. There are five syenite bodes in the ZH area, i.e., the Lang＇aogou, Mogou, Longtou, Jiaogou and Wusanggou from west to east （Fig. l b）.

Petrogenesis of the Early Cretaceous Laojunshan monzogranite at the southern margin of the North China Craton: Constrains on the transition of the tectonic regime

1 Introduction Voluminous Mesozoic magmatic rocks containing abundant Au-Mo polymetallic mineralization resources are developed in the Xiaoqinling-Xiong’ershan district of the southern margin of the North China Craton(NCC).

THE SEQUENCE OF PALEOENVIRONMENTAL CHANGES SINCE ABOUT 4KA B.P., RECORDED BY NIYA SECTION IN SOUTHERN MARGIN OF TARIM BASIN

Through synthetic researches of multi-index geological records of Niya section, which are of high resolution in southern margin of Tarim Basin, this paper has reconstructed the sequences of paleoclimate in this region during historical times (since about 4000a B.P.). During the last 4000 years, the area has experienced alternations of relative cold-moisture and relative warm-dry periods. Three evident cold-moisture periods and three warm-dry periods are identifing. The study shows that the human activities have an intimate relation with the evolution of paleoclimate in the southern Xinjiang. Paleoclimate has played very important role in influencing human being′s agricultural activities.

SHRIMP Zircon U-Pb Age of the Sidingheishan Mafic-Ultramafic Intrusion in the Southern Margin of the Central Asian Orogenic Belt,NW China and its Petrogenesis implication

Objective The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the North Tianshan Mountains. This work used zircon U-Pb age data, bulk rock major and trace elements, Sr-Nd-Pb isotope data to assess mantle source characteristics and crustal assimilation of the parental magma of the Sidingheishan intrusion. We have also discussed the tectonic evolution of the southern margin of the Central Asian Orogenic belt in the Late Paleozoic.

Permo–Triassic and Liassic Tethyan Oceanic Tracts within the Pontide Belt Along the Southern Margin of Eurasia, Northern Anatolia

The Pontide belt in northern Turkey includes three major tectonic terranes,the Strandja Massif（Sj M）,and the Istanbul（ISZ）and Sakarya Zones（SZ）(Fig.1).We present new age and geochemical data from ophiolites and ophiolitic mélanges within the Sakarya Zone and show that these mafic–ultramafic rocks are the remnants of Tethyan oceanic lithosphere formed in different tectonic settings.The main ophiolite occurrences investigated in this study along the Karakaya Suture（KS）are associated with the latest Triassic Cimmeride orogeny,and in the Küre–Yusufeli ophiolite belt are part of the Alpide orogeny.The Karakaya Suture Zone ophiolites in northern west Turkey are comprised mainly of the Denizgoren（?anakkale）ophiolite,Bo?azk?y（Bursa）,Geyve（Sakarya）,Almac?k（Düzce）and?ele（Bolu）metaophiolites.The Denizg?ren ophiolite largely contains upper mantle peridotites,which are equivalents of the Permo–Triassic Lesvos peridotites and mélange units farther SW in the northern Aegean Sea.The Bo?azk?y ophiolite includes serpentinite and metagabbro,and the Almac?k and Geyve ophiolites display an almost complete Penrose–type sequence consisting of serpentinizeduppermantleperidotites,cumulate ultramafic–mafic rocks,isotropic gabbros,dolerite and plagiogranite dikes,and extrusive rocks.U–Pb zircon dating of plagiogranite dikes from?ele has revealed an igneous age of 260 Ma,and 255,235,227 Ma from Almac?k（Bozkurt et al.,2012a,b）.Consistent with the previouslypublished Permo–Triassic age,we obtained a 268.4±6.3 Ma U–Pb zircon age from a plagiogranite dike within the Almacik ophiolite to the west.This KS ophiolite belt containing the?ele,Almac?k,Geyve ophiolites within the SZ extends westward into the Armutlu Peninsula and then into the Biga Peninsula（i.e.Denizg?ren ophiolite）and most likely connects with the remnants of the Triassic Meliata–Meliac ocean basin（Stampfli and Borel,2002）in the Balkan Peninsula.The KS ophiolites also continue eastward within the Pontide Belt into the Elekda?ophiolite（eastern Kastamonu）and then to the Refahiye ophiolite in NE Anatolia.Triassic granites in the SZ represent a magmatic arc that formed as a result of the northward subduction of the Izmir–Ankara–Erzincan oceanic lithosphere existing during the late Paleozoic through Cretaceous（Sarifakioglu et al.,2014）beneath the Pontides.We obtained a U–Pb zircon age of 231±2 Ma from a metagranitic intrusion into the Variscan basement of the SZ in the Kastamonu region of the central Pontides.This metagranite is enriched in LILE（Rb:63 ppm;Ba:65 ppm;Sr:200 ppm）and depleted in HFSE（Y:12.58 ppm;Yb:1.26 ppm;Ti O2:0.2 wt.%;Nb:7.6 ppm;Hf:3.9 ppm）,characterizing it as subduction–related calc-alkaline pluton.Lead（3.9 ppm）,U（1.6 ppm）and Ce（59 ppm）contents are interpreted as evidence for contamination by continental crust.The Küre basin to the north opened during the late Triassic to Liassic,following a backarc rifting episode in the central Pontides.Metabasic dike intrusions in the Devrekani metamorphic massif represent the first magmatic stage of this backarc rifting event.Whole-rock 40Ar-39Ar dating ofthe metabasic dikes has yielded cooling ages of 160.5±1.2 Ma. We infer that this age was reset due to thermal heating during the emplacement of the Middle Jurassic granitoids as the Küre oceanic basin was closing. The Küre ophiolite contains upper mantle peridotites and oceanic crustal rocks composed mainly of pillow–massive–breccia basalts, dacitic and rhyolitic lavas–tuffs, diabase dyke swarms, massive gabbros and a limited extent mafic cumulates. We obtained 182.6±1.9 Ma as a whole-rock 40Ar-39 Ar age from a pillow basalt and a U–Pb zircon age of 171±1 Ma from the granitic intrusion cross-cutting the peridotites. The easternmost representatives of the Küre ophiolite occur in the Yusufeli（Artvin） area in the eastern part of the Pontide belt. Here, oceanic crustal rocks are tectonically related to metamorphic rocks of the Variscan basement of the SZ. The ophiolitic crustal rocks contain isotropic gabbro and mafic and felsic dikes. Serpentinized upper mantle peridotites are scarce. Pillow lava basalts are overlain by a thick metasandstone–metashale association with locally foliated meta–lava and some manganiferous chert and mudstone interlayers. We obtained a U–Pb zircon age of 172.5±1.4 Ma from the granitic intrusion cross-cutting the Yusufeli ophiolite and of 181.9±0.9 Ma from a felsic dike（plagiogranite） in the Yusufeli ophiolite. The Middle Jurassic granites are related to the closure of the Küre-Yusufeli marginal ocean basin. The Küre and Yusufeli ophiolites have been previously interpreted as the remnants of the Paleotethys or the Intra-Pontide Ocean. However, we posit that these ophiolites represent amarginal, short-lived（;0 Ma） ocean basin, which opened during the late Triassic through Liassic, and then closed in Dogger. This oceanic lithosphere is similar to the Evros ophiolite in the northeastern Greece in terms of its ages and geochemical characteristics.

An Unique Glomerophyric Diorite Porphyry from the Southern Margin of North China Craton:Geochronology,Geochemical and Quantitative Textural Analysis Constraints

1 Introduction The Wulong glomerophyric diorite porphyry has an extremely peculiar texture with plagioclase phenocrysts clustered as flower-like glomerocrysts(Figs.1a&b),which is never discovered elsewhere of the world.The

Positioning the Southern Margin of Asia Prior to Its Collision with India: Paleomagnetic Constraints from Late Cretaceous Dykes in Gangdise Belt

The pre-collisional southern margin of Asia can be restored using paleomagnetic data from late Cretaceous rocks from the Lhasa terrane.However,the available data are based either on the red beds or on the intercalated thin layers of lava flows,both of which had been involved in strongly folding.Recent studies show clear evidence for the possibility of serious overprint hence the data could not be reliably used for tectonic interpretation.We report paleomagnetic data from diorite dykes and the grano-diorite country rock in the Gandise belt near the city of Lhasa.U–Pb isotopic dating indicates the intrusive rocks have an age of;2–86 Ma.Fifteen sites yield acceptable Ch RM directions which pass a reversal test.SEM and light microscope observations show primaryintergrowth relationship between magnetite and other minerals within the thin sections.AMS measurement defines a primary magma flow fabric for the intruded dykes and the country rocks.All the characteristics support that the Ch RMs are primary.The paleomagnetic pole calculated from the remanence of the dykes and the country rocks yields a paleolatitude of;4°N which provide a reliable constraint for the southern margin of Asia near Lhasa.Furthermore,the recorded declination shows significant counterclockwise rotation of;0°for the sampling location relative to the north.In consideration of the strike and tectonic setting of the dykes,the strike of the southern margin of Asia is restored which is compatible with the hypothesis of a quasi-linear margin of Eurasia prior to its collision with India.

Magnetic Fabric Studies of Xiong'er Volcanic Rocks in Southern Margin of the North China Craton and its Implications

The Paleoproterozoic Xiong’er Group is composed of mafic to felsic volcanic rocks and minor sedimentary rocks,distributed along the southern margin of the North China craton(NCC).It is a key marker for regional

Mineralogical Study of Almandine-Hercynite-Muscovite-Ilmenite Hornblendite Dykes from the Southern Margin of the Gondwana Graben at Richughuta in Palamau District of the Eastern Indian Shield

Introduction The rectangular block of Proterozoic formation lying between north of the Singhbhum Mobile Belt(SMB,2.3-2.4 Ga,Saha 1994),Neogene sediments of the Bengal basin and the Quaternary-Recent alluvium of the Ganga

Gold Distribution in Tellurium-Rich Pyrite and Tellurides from the Xiaoqinling Gold District,Southern Margin of the North China Craton

Gold is commonly associated with arsenic in As-rich pyrite or arsenopyrite in a variety types of gold deposit,such as sediment-hosted gold deposits,epithermal Au-Ag deposits,Au-rich VMS deposits,and mesothermal lode gold deposits(Ciobanu and Cook,2002;Pals et al.

Mesozoic Deformation and Its Geological Significance in the Southern Margin of the South China Sea

The pre-Eocene history of the region around the present South China Sea is not well known. New multi-channel seismic profiles provide valuable insights into the probable Mesozoic history of this region. Detailed structural and stratigraphic interpretations of the multi-channel seismic profiles, calibrated with relevant drilling and dredging data, show major Mesozoic structural features. A structural restoration was done to remove the Cenozoic tectonic influence and calculate the Mesozoic tectonic compression ratios. The results indicate that two groups of compressive stress with diametrically opposite orientations, S(S)E– N(N)W and N(N)W–S(S)E, were active during the Mesozoic. The compression ratio values gradually decrease from north to south and from west to east in each stress orientation. The phenomena may be related to the opening of the proto-South China Sea(then located in south of the Nansha block) and the rate at which the Nansha block drifted northward in the late Jurassic to late Cretaceous. The Nansha block drifted northward until it collided and sutured with the southern China margin. The opening of the present South China Sea may be related to this suture zone, which was a tectonic zone of weakness.

Major breakthrough of Well Gaotan 1 and exploration prospects of lower assemblage in southern margin of Junggar Basin, NW China

Well Gaotan 1 was tested a high yield oil and gas flow of more than 1 000 m^3 a day in the Cretaceous Qingshuihe Formation,marking a major breakthrough in the lower assemblage of the southern margin of Junggar Basin. The lower assemblage in the southern margin of the Junggar Basin has favorable geological conditions for forming large Petroleum fields, including:(1) Multiple sets of source rocks, of which the Jurassic and Permian are the main source rocks, with a large source kitchen.(2) Multiple sets of effective reservoirs,namely Cretaceous Qingshuihe Formation, Jurassic Toutunhe Formation and the Khalza Formation etc.(3) Regional thick mudstone caprock of Cretaceous Tugulu Group, generally with abnormally high pressure and good sealing ability.(4) Giant structural traps and litho-stratigraphic traps are developed. The northern slope also has the conditions for large-scale litho-stratigraphic traps.(5) Static elements such as source rocks, reservoirs and caprocks are well matched, and the dynamic evolution is suitable for large oil and gas accumulation. The lower assemblage of the southern margin of the Junggar Basin has three favorable exploration directions, the Sikeshu Sag in the west part, the large structures in the middle and eastern part, and the northern slope.

Late Mesozoic magmatism and tectonic evolution in the Southern margin of the North China Craton

Late Mesozoic granitic magmatism(158–112 Ma) are widespread in the southern margin of the North China Craton(NCC), contemporary with many world-class Mo-Au-Ag-Pb-Zn polymetallic deposits. There are abrupt changes in the elements and isotopic compositions of these granites at about 127 Ma. The early stage(158–128 Ma) granites show slightly or no negative Eu anomalies, large ion lithophile elements enriched and heavy REE depleted(such as Y and Yb), belonging to typical I-type granite. The late stage(126–112 Ma) granites are characterized by A-type and/or highly fractionated I-type granite, with higher contents of SiO2, K2 O, Y, Yb and Rb/Sr ratio and lower contents of Sr, δEu value and Sr/Y ratio than that of the early-stage granites.Moreover, the whole rock Nd and Hf isotopic compositions of the granites younger than 127 Ma show more depleted than those of the older one. The two stages of Late Mesozoic granites were derived from a source region of the ancient basement of the southern margin of the NCC incorporated the mantle material. The late stage(126–112 Ma) granites contain more fractions of mantle material with depleted isotopic composition than the early ones. The granites record evidence for a strong crust-mantle interaction. They formed in an intracontinental extensional setting which was related to lithospheric thinning and asthenospheric upwelling in this region, which was possibly caused by westward subduction of the Paleo-Pacific plate. 127 Ma is an critical period of the transformation of the tectonic regime.

Formation mechanism of condensates, waxy and heavy oils in the southern margin of Junggar Basin, NW China

It is a challenge to determine the source and genetic relationship of condensate, waxy and heavy oils in one given complicated petroliferous area, where developed multiple sets of source rocks with different maturity and various chemical features.The central part of southern margin of Junggar Basin, NW China is such an example where there are condensates, light oils, normal density oils, heavy crude oils and natural gases. The formation mechanism of condensates has been seriously debated for long time;however, no study has integrated it with genetic types of waxy and heavy oils. Taking the central part of southern margin of Junggar Basin as a case, this study employs geological and geochemical methods to determine the formation mechanism of condensates,waxy and heavy oils in a complicated petroliferous area, and reveals the causes and geochemical processes of the co-occurrence of different types of crude oils in this region. Based on detailed geochemical analyses of more than 40 normal crude oils, light oils,condensates and heavy oils, it is found that the condensates are dominated by low carbon number n-alkanes and enriched in light naphthenics and aromatic hydrocarbons. Heptane values of these condensates range from 19% to 21%, isoheptane values from1.9 to 2.1, and toluene/n-heptane ratios from 1.5 to 2.0. The distribution of n-alkanes in the condensates presents a mirror image with high density waxy crude oils and heavy oils. Combined with the oil and gas-source correlations of the crude oils, condensates and natural gas, it is found that the condensates are product of evaporative fractionation and/or phase-controlled fractionation of reservoir crude oils which were derived from mature Cretaceous lacustrine source rocks in the relatively early stage. The waxy oils are the intermediate products of evaporative fractionation and/or phase-controlled fractionation of reservoir crude oils, while the heavy oils are in-situ residuals. Therefore, evaporative fractionation and/or phase-controlled fractionation would account for the formation of the condensate, light oil, waxy oil and heavy oil in the central part of southern margin of Junggar Basin, resulting in a great change of the content in terms of light alkanes, naphthenics and aromatics in condensates, followed by great uncertainties of toluene/n-heptane ratios due to migration and re-accumulation. The results suggest that the origin of the condensate cannot be simply concluded by its ratios of toluene/n-heptane and n-heptane/methylcyclohexane on the Thompson's cross-plot, it should be comprehensively determined by the aspects of geological background, thermal history of source rocks and petroleum generation,physical and chemical features of various crude oils and natural gas, vertical and lateral distribution of various crude oils in the study area.

Properties of Continental Margin and its Hydrocarbon Exploration Significance in Cambrian in the Southern Ordos Kratogen of North China

It is important to determine the properties of the tectonics in Cambrian period for the sake of prospecting deep hydrocarbon in the near future in the southern Ordos Kratogen of North China. Authors chose the marginal areas of the southern Ordos basin as the object of research, avoided the effects of both the Qinling Orogenic Belts （QOB） and Weihe River Graben （WRG） whose geological structures are too complicated. By surveying typical Cambrian outcrops and profdes in the basin edges and based on the cores of 57 wells which penetrated the Cambrian in the basin, combined with the seismic profiles, the field gammaray measuement results and the carbon isotope analysis, Authors conclude that the southern margin of the Ordos Kratogen during Cambrian was a passive continental margin which resulted from sea-floor spreading of the Ancient Qinling Ocean. Epicontinental sea carbonate sediments formed in the south Ordos continental margin during Cambrian, and were predominant as tidal flat and o61itic shoal. Both transgression-regression process and the change in palaeostructure have the obvious cyclicity. Using the junction between the late Nangao age of Qiandong epoch and the early Duyun age of Qiandong epoch as a boundary, each had a full transgression cycle at the upper and lower stages. The early cycle is characterized by high energy clastic littoral facies while the late cycle is characterized by carbonate ramp on which clear water and muddy water developed alternately changing to carbonate platform last. During the early stages, An aulacogen was formed in the middle section of the southern margin. The southern Ordos margin was uplifted and denudated by the Huaiyuan Movement which occurred from the late Furongian age to the middle Flolan age and the history of the passive continental margin ended and entering into a new tectonic cycle. The unconformity surface caused by the Huaiyuan Movement, along with its neighborhood areas where dissolved pores and cavities are developed, may be another important district for good hydrocarbon reservoirs （excluding the unconformity surface on the top of the Ordovician in the Ordos basin）.