Geochemical,mineralogical,and petrological analyses for the interpretation of the sedimentary environment of the Middle-Late Ordovician Majiagou Formation(northern China)as a tool for more effective gas exploration

Core samples from the deeply buried Ordovician Majiagou Formation below the Huainan Coalfield(E China) have been investigated for their carbonate types,major and trace elements(including rare earth elements) and C and O isotopes,The objective was to get a better insight into the possible occurrences of gas(and possibly oil) derived from Carboniferous coals.It was found that the carbonates are dolomites with strongly varying amounts of CaO and MgO.The low concentrations of SiO_(2),Al_(2)O_(3),and Fe_(2)O_(3)indicate deposition in a normal marine environment with little terrigenous input,The Na_(2)O/K_(2)O,Fe/Mn and Sr/Ba ratios,as well as the Ga values indicate mainly a marine salinity and a hot and humid climate.The slight depletion of Ce and Eu,the depletion of heavy rare earth elements(HREE) and the enrichment of light rare earth elements(LREE) indicate deposition in a reducing environment.It thus appears that the Majiagou Formation below the Huainan Coalfield closely resembles that in the eastern part of the Ordos Basin,where several gas reservoirs are present,so that the Majiagou Formation under the Huainan Coalfield represents a promising target for hydrocarbon exploration.

An Updated Review of the Middle-Late Jurassic Yanliao Biota: Chronology,Taphonomy,Paleontology and Paleoecology

The northeastern Chinese Yanliao Biota （sometimes called the Daohugou Biota） comprises numerous, frequently spectacular fossils of non-marine organisms, occurring in Middle-Upper Jurassic strata in western Liaoning, northern Hebei, and southeastern Inner Mongolia. The biota lasted for about 10 million years, divided into two phases： the Bathonian-Callovian Daohugou phase （about 168-164 million years ago） and the Oxfordian Linglongta phase （164-159 million years ago）. The Yanliao fossils are often taphonomically exceptional （many vertebrate skeletons, for example, are complete and accompanied by preserved integumentary features）, and not only are taxonomically diverse but also include the oldest known representatives of many groups of plants, invertebrates, and vertebrates. These fossils have provided significant new information regarding the origins and early evolution of such clades as fleas, birds, and mammals, in addition to the evolution of some major biological structures such as feathers, and have demonstrated the existence of a complex terrestrial ecosystem in northeast China around the time of the Middle-Late Jurassic boundary.

Field Efficacy Trial and Optimum Control Period of Rice False Smut(Ustilaginoidea virens) in Single Cropping Middle-late Rice

[Objective]The paper was to study optimum chemical agents and control periods against rice false smut( Ustilaginoidea virens) in single cropping middle-late rice. [Method]Taking chemical agents( triadimefon,armure,Jinggangmycin) as main treatments and spraying periods( within 7 d before initial heading stage,within 7 d before initial heading stage + initial heading stage,initial heading stage) as assisted treatments,the field efficacy trial and optimum control period of U. virens in single cropping middle-late rice were studied using Huanghuazhan and Jinnongsimiao as experimental materials in 2013. [Result] Three chemical agents,triadimefon,armure,Jinggangmycin,extremely reduced diseased panicle rate and disease index of U. virens,but there was no significant difference among three chemical agents. Spraying period did not have significant impact on diseased panicle rate and disease index of U. virens,whereas spraying within7 d before initial heading stage and during initial heading stage had relatively good control effect. Yield increased significantly after application of three chemical agents,and armure led to greater increase. The reason for yield increase was that seed setting rate was significantly increased,and the number of filled grains per panicle was significantly increased. Although spraying period influenced yield,the difference was not significant. [Conclusion] Triadimefon,armure and Jinggangmycin had good control effects against U. virens in single cropping middle-late rice,of which armure had better control effects. Spraying chemical agents increased seed setting rate,and further increased the number of filled grains per panicle and yield. Spraying within 7 d before initial heading stage and during initial heading stage had relatively better control effect against U. virens.

The Shishugou Fauna of the Middle-Late Jurassic Transition Period in the Junggar Basin of Western China

The Middle-Late Jurassic transition period is a critical period for the evolution of terrestrial vertebrates,but the global fossil record from this time is relatively poor.The Shishugou Fauna of this period has recently produced significant fossil remains of dinosaurs and other vertebrate groups,some representing the earliest known members of several dinosaurian groups and other vertebrate groups and some representing the best-known specimens of their group.These discoveries are significant for our understanding of the origin and evolution of several vertebrate lineages.Radiometric dating indicates that the fauna is aged approximately 159-164 Ma.Comparisons with other similarly-aged terrestrial faunas such as Shaximiao and Yanliao show both taxonomic similarities and differences between these faunas and indicate that the Junggar deposits might have preserved the most complete vertebrate fossil record for a Middle-Late Jurassic Laurasian terrestrial fauna.

Percutaneous microwave ablation and transcatheter arterial chemoembolization for serum tumor markers and prognostics of middle-late primary hepatic carcinoma

BACKGROUND Primary hepatic carcinoma(PHC)has an insidious onset and is usually diagnosed in the middle and late stages.Although transcatheter arterial chemoembolization(TACE)is the preferred option for treating middle-and advanced-stage PHC,it has limited efficacy in killing tumor cells and poor long-term efficacy.TACE plus percutaneous microwave coagulation therapy(PMCT)is more effective than interventional therapy alone and can improve survival time.However,there are few reports on the effects of TACE and PMCT on serum marker levels and the prognosis of patients with advanced PHC.AIM To investigate the effect of PMCT+TACE on serum tumor markers and the prognosis of middle-late PHC.METHODS This retrospective study included 150 patients with middle-late PHC admitted to Zhongshan People’s Hospital between March 2018 and February 2021.Patients were divided into a single group(treated with TACE,n=75)and a combined group(treated with TACE+PMCT,n=75).Before and after treatment,the clinical efficacy and serum tumor marker levels[carbohydrate antigen 19-9(CA19-9),alpha-fetoprotein(AFP),and carcinoembryonic antigen(CEA)]of both groups were observed.The 1-year survival rates and prognostic factors of the two groups were analyzed.RESULTS The combined group had 21 and 35 cases of complete remission(CR)and partial remission(PR),respectively.The single group had 13 and 25 cases of CR and PR,decreased,with the decrease in the combined group being more significant(P<0.05).The 1-year survival rate of the combined group(80.00%)was higher than that of the single group(60.00%)(P<0.05).The average survival time within 1 year in the combined group was 299.38±61.13 d,longer than that in the single group(214.41±72.97 d,P<0.05).COX analysis revealed that tumor diameter,tumor number,and the treatment method were prognostic factors for patients with middle-late PHC(P<0.05).CONCLUSION TACE+PMCT is effective in treating patients with mid-late PHC.It reduces the levels of tumor markers,prolongs survival,and improves prognosis.

Characteristics and impacts on favorable reservoirs of carbonate ramp microfacies: A case study of the Middle-Lower Ordovician in Gucheng area,Tarim Basin,NW China

Based on the latest drilling core, thin section, 3D seismic, well logging data as well as exploration results, the sequence stratigraphy and sedimentary microfacies of the Middle-Lower Ordovician carbonates in Gucheng area, and their controlling effects on the development of reservoir were examined by the theories and methods of fine carbonate sedimentology.The results show that the Middle-Lower Ordovician in Gucheng area is a set of typical carbonate ramp deposits, which can be divided into 10 microfacies in 4 subfacies as follows: back ramp, inner shallow ramp, outer shallow ramp, and deep ramp. The back ramp subfacies consists of muddy-dolomitic flat and dolomitic lagoon microfacies;it is dominated by lamellar micrite dolomite tight in lithology. The inner shallow ramp subfacies includes dolomitic shoal and dolomitic flat in shoal top and dolomitic flat between shoals microfacies;it is mainly composed of crystal dolomite with metasomatic residual structure, as well as abundant karst vugs and intercrystalline pores. The crystal dolomite has an average porosity of 4.36%. The outer ramp subfacies includes medium-high and low energy grain shoal and inter-shoal microfacies;it is dominated by sandy limestone,oolitic limestone, and micrite limestone with few pores. The deep ramp subfacies is dominated by low-energy argillaceous deposits, with local presences of storm shoal microfacies. The Lower-Middle Ordovician has six third-order sequences from bottom to top, among which sequence(SQ3) represents the third member of Yingying Formation. The main reservoir has three fourth-order high-frequency sequences. Apparently, the favorable reservoir in the study area is jointly controlled by sedimentary microfacies and high frequency sequence in the carbonate ramp. The former controls the primary pore structure, and the latter controls the intensities of karstification exposure and dolomitization in the penecontemporaneous period. The dolomitic shoals and top dolomitic flats of different stages, superimposed and connected into favorable reservoirs of considerable scale like "platform margin", are favorable exploration facies in the carbonate ramp.

Analysis of Sedimentary Environment and Sequence Stratigraphy of Middle-Late Permian Sediments in Coastal Fars, Iran (Zireh Gas Field, Well ZH-A)

The current paper is focused on the facies, sedimentary environment and depositional cycles study of the Middle-Late Permian sediments in the South Fars Zireh Gas Field, the subsurface section of well ZH-A. Four hundred thin-sections obtained from cores and cuttings were examined under standard petrographic microscope. For this study research, Corel Draw X6, Corel Photo-Paint, DN2 Microscopy Image Processing System, Scope Photo, Autodesk Map, Geocalc and Auto Cad 2014 were utilized. Based on microfacies analysis and significant founa and flora, fourteen major facies in four facies associations comprising tidal flat (A), lagoon (B), shoal (C) and open marine (D) identified in the well ZH-A. This formation was deposited in inner part of a homoclinal ramp. Based on depositional cycles, seven main 3rd order sequences were revealed in the Well ZH-A. The ooid grainstone facies with interparticle and oomoldic porosity has high reservoir potential. The diagenetic processes like dolomitization and dissolution have significant effect in the reservoir quality. Based on research results, a major framework can be weaved and used to correlate reservoir layering.

Ordovician reservoirs in Fuxian area:Gas accumulation patterns and their implications for the exploration of lower Paleozoic carbonates in the southern Ordos Basin

In recent years,the Fuxian area in the southeastern Ordos Basin has undergone significant exploration,with industrial gas flow tested in wells drilled into the Ordovician marine carbonates.Despite this,the gas accumulation patterns of this area are not fully understood,posing challenges for further exploration.Our analysis of geological conditions indicates that the Ordovician Majiagou Formation in this area hosts two gas plays:one found in weathering crusts and the other found in interior of the formation.We investigated various typical gas reservoirs in the area,focusing on differentiating the geological conditions and factors controlling gas accumulation in the weathering-crust and interior gas reservoirs.The results suggest three primary gas accumulation patterns in the Majiagou Formation in the Fuxian area:(1)upper gas accumulation in weathering crusts,present in the high parts of landforms such as residual paleo-hills or buried paleo-platform(Pattern I);(2)the stereoscopic pattern with gas accumulation in both weathering crusts and strata interior,arising in high parts of landforms such as residual paleo-hills or buried paleo-platforms(Pattern II);(3)lower gas accumulation in strata interior,occurring in the upper reaches and on both sides of paleo-trenches(Pattern III).This study will serve as a geological basis for future exploration deployment in the Fuxian area.

Tectono-sedimentary evolution and oil-gas geological significance of first to third member of Ordovician Majiagou Formation in Ordos Basin,NW China

Based on logging,core,thin section and geochemical analysis,the tectonic-lithofacies paleogeographic pattern of first member to third member of Ordovician Majiagou Formation(O_(1)m_(1)–O_(1)m_(3)for short)in Ordos Basin is reconstructed,and the tectono-sedimentary evolution characteristics and oil-gas geological significance are discussed.The results are obtained in four aspects.First,a set of marginal argillaceous dolomites with high gamma ray value developed steadily and diachronously at the bottom of Majiagou Formation,which distributed over the Huaiyuan Movement unconformity,with δ^(13)C values positive drift characteristics comparable to global transgression of the Early Ordovician Floplian.Second,the global sea level rose and the ancient land was submerged into the underwater uplift in O_(1)m_(1)to O_(1)m_(2),and the central uplift was deposited for the first time in the Ordovician,forming a tectonic pattern of“one uplift and two depressions”.Subsequently,the subduction and extrusion outside the basin and the differentiation of uplift and depression in the basin of O_(1)m_(3)resulted in the activation of the Wushenqi–Jingbian bulge.Third,the evolution of the tectonic pattern had a significant impact on the sedimentary paleoenvironment.The O_(1)m_(1)overlaps westward,and saline lagoon is formed in eastern depression and influenced by the transgression.The transgression continued in O_(1)m_(2)and resulted in communication with the wide sea,and the large-scale grain shoal developed around eastern depression,and the late dry shrinkage formed a small scale evaporite lagoon in upper part.Under the influence of highland sealing in O_(1)m_(3),the water body gradually differentiated into dolomitic gypsum and saline lagoons to the east,and the grain shoal spread along the highs around sag.Fourth,the source rocks developed diachronously at the bottom of Majiagou Formation form a favorable source-reservoir assemblage with the shoal facies reservoir distributed around the slope of O_(1)m_(2)–O_(1)m_(3),and they have certain exploration potential for natural gas.

Conodont Biostratigraphy of the Middle Cambrian through Lowermost Ordovician in Hunan, South China

Since 1985, samples with a total weight of more than 14,000 kg, mainly from three key sections in western and northwestern Hunan, South China, have been processed for conodonts. In strata older than the late Late Cambrian paraconodonts have proved useful for stratigraphic subdivision and correlation. Thirteen conodont zones are proposed in the Middle Cambrian through lowermost Ordovician. The correlation between these zones and those of North China, western U. S.A., western Newfoundland, Canada, and Iran is discussed. In ascending order, these 13 zones are as follows: The Gapparodus bisulcatus-Westergaardodina brevidens Zone, Shandongodus priscus-Hunanognathus tricuspidatus Zone, Westergaardodina quadrata Zone, Westergaardodina matsushitai-W. grandidens Zone, Westergaardodina lui-W. am Zone, Westergaardodina cf. calix-Prooneotodus rotundatus Zone, Proconodontus tenuiserratus Zone, Proconodontus Zone, Eoconodontus Zone, Cordylodus proavus Zone, Cordylodus intermedius Zone, Cordylodus lindstromi Zone, and Cordylodus angulatus Zone (lower part). The Westergaardodina lui-W. ani and Westergaardodina cf. calix-Prooneotodus rotundatus Zones replace the Westergaardodina proligula and Westergaardodina cf. behrae-Prooneotodus rotundatus Zones, respectively, in the lowermost Upper Cambrian. Two new species (Westergaardodina Iui and Westergaardodina ani) and one conditionally identified species (Westergaardodina cf. calix) are described.

Hydrocarbon Accumulation Conditions of Ordovician Carbonate in Tarim Basin

Based on comprehensive analysis of reservoir-forming conditions, the diversity of reservoir and the difference of multistage hydrocarbon charge are the key factors for the carbonate hydrocarbon accumulation of the Ordovician in the Tarim Basin. Undergone four major deposition-tectonic cycles, the Ordovician carbonate formed a stable structural framework with huge uplifts, in which are developed reservoirs of the reef-bank type and unconformity type, and resulted in multistage hydrocarbon charge and accumulation during the Caledonian, Late Hercynian and Late Himalayan. With low matrix porosity and permeability of the Ordovician carbonate, the secondary solution pores and caverns serve as the main reservoir space. The polyphase tectonic movements formed unconformity reservoirs widely distributed around the paleo-uplifts; and the reef-bank reservoir is controlled by two kinds of sedimentary facies belts, namely the steep slope and gentle slope. The unconventional carbonate pool is characterized by extensive distribution, no obvious edge water or bottom water, complicated oil/gas/water relations and severe heterogeneity controlled by reservoirs. The low porosity and low permeability reservoir together with multi-period hydrocarbon accumulation resulted in the difference and complex of the distribution and production of oil/gas/water. The distribution of hydrocarbon is controlled by the temporal-spatial relation between revolution of source rocks and paleo-uplifts. The heterogenetic carbonate reservoir and late-stage gas charge are the main factors making the oil/ gas phase complicated. The slope areas of the paleo-uplifts formed in the Paleozoic are the main carbonate exploration directions based on comprehensive evaluation. The Ordovician of the northern slope of the Tazhong uplift, Lunnan and its periphery areas are practical exploration fields. The Yengimahalla-Hanikatam and Markit slopes are the important replacement targets for carbonate exploration. Gucheng, Tadong, the deep layers of Cambrian dolomite in the Lunnan and Tazhong-Bachu areas are favorable directions for research and risk exploration.

Temporal Distribution of Diagnostic Biofabrics in the Lower and Middle Ordovician in North China:Clues to the Geobiology of the Great Ordovician Biodiversification Event

The temporal distribution of the diagnostic biofabrics in the Lower and Middle Ordovician in North China distinctly illustrates that the sedimentary systems on the paleoplate have been changed markedly as consequences of the Great Ordovician Biodiversification Event （GOBE）. The pre-GOBE sedimentary systems deposited in Tremadoc display widespread microbialite and flat-pebble conglomerates, and a less extent of bioturbation. Through the transitional period of early Floian, the sedimentary systems in the rest of the Early and Mid- Ordovician change to GOBE type and are characterized by intensive bioturbation and vanishing flat-pebble conglomerates and subtidal microbial sediments. The irreversible changes in sedimentary systems in North China are linked to the GOBE, which conduced the increase in infaunal tiering, the expansion of infaunal ecospace, and the appearance of new burrowers related to the development of the Paleozoic Evolutionary Fauna during the Ordovician biodiversification. Thus, changes in sedimentary systems during the pivotal period of the GOBE were consequences of a steep diversification of benthic faunas rather than the GOBE＇s environmental background.

Burial Dissolution of Ordovician Granule Limestone in the Tahe Oilfield of the Tarim Basin,NW China,and Its Geological Significance

With a comprehensive study on the petrology, geology and geochemistry of some Ordovician granule limestone samples in the Tahe Oiifieid of the Tarim Basin, two stages of burial dissolution were put forward as an in-source dissolution and out-source dissolution based on macro-microcosmic petrology and geochemistry features. The main differences in the two stages are in the origin and moving pass of acid fluids. Geochemical evidence indicates that burial dissolution fluids might be ingredients of organic acids, CO2 and H2S associated with organic matter maturation and hydrocarbon decomposition, and the in-source fluid came from organic matter in the granule limestone itself, but the out-source was mainly from other argillaceous carbonate rocks far away. So, the forming of a burial dissolution reservoir resulted from both in-source and the out-source dissolutions. The granule limestone firstly formed unattached pinholes under in-source dissolution in situ, and afterwards suffered wider dissolution with out-source fluids moving along unconformities, seams, faults and associate fissures. The second stage was much more important, and the mineral composition in the stratum and heat convection of the fluid were also important in forming favorable reservoirs.

Depositional architecture of the late Ordovician drowned carbonate platform margin and its responses to sea-level fluctuation in the northern slope of the Tazhong region, Tarim Basin

The Tazhong Uplift of the late Ordovician is a drowned rimmed carbonate platform. The carbonate rock of the late Ordovician Lianglitage Formation in the northern slope of the Tazhong region is one of the significant petroliferous intervals. Based on petrofacies, depositional cycles, natural gammaray spectrometry and carbon/oxygen isotope data from the Lianglitage Formation, one 2nd-order, three 3rd-order and several 4th-order sequences have been recognized, and the late Ordovician relative sealevel fluctuation curve has been established. The sequences O3 1-1 and O3 1-2 on the platform are composed of highstand and transgressive systems tracts, but lack the lowstand systems tract. The sequence O3 1-3 is a drowning sequence. The sequence O3 1-1 overlapped the eroded slope and pinched out to the northwest and landward. The highstand systems tract in the sequence O3 1-2 consists of low-angle sigmoid and high-angle shingled progradation configuration. Major sedimentary facies of the Lianglitage Formation include reef and shoal in the platform margin and lagoon, which can be subdivided into coral-sponge-stromatoporoid reef complex, sand shoal, lime mud mound, and intershoal sea. Reefs, sand shoals and their complex are potential reservoir facies. The reefs and sand shoals in the sequence O3 1-1 developed in the upper of its highstand systems tract. In the sequence O3 1-2, the highstand systems tract with an internal prograding configuration is a response to the lateral shifting of the complex of reef and sand shoal. The transgressive systems tract, in particular the sand shoals, developed widely on the slope of the platform margin and interior. The reefs in the sequence O3 1-3 migrated towards high positions and formed retrograding reefs in the western platform and low relief in the platform interior. Basinward lateral migration of the reefs and pure carbonate rock both characterize highstand systems tract and show that the rise of the relative sea-level was very slow. Shingled prograding stacking pattern of the 4th-order sequences and reefs grow horizontally, which represents the late stage of highstand systems tract and implies relative sealevel stillstand. Reefs migrating towards high land and impure carbonate rock both indicate transgressive systems tract and suggest that the relative sea-level rose fast. Erosional truncation and epidiagenetic karstification represent a falling relative sea-level. The relative sea-level fluctuation and antecedent palaeotopography control the development and distribution of reef complexes and unconformity karst zones. Currently, the composite zone of epidiagenetic karstic intervals and high-energy complexes of reefs and sand shoals with prograding configuration is an important oil and gas reservoir in the northern slope of the Tazhong carbonate platform.

Paleoecology of Early Ordovician Reefs in the Yichang Area,Hubei:a Correlation of Organic Reefs Between Early Ordovician and Jurassic

The Early Ordovician System is composed mainly of a series of carbonate platform deposits interbedded with shale and is especially characterized by a large number of organic reefs or buildups that occur widely in the research area.The reefs have different thicknesses ranging from 0.5 m to 11.5 m and lengths varying from 1 m to 130 m.The reef-building organisms include Archaeoscyphia, Recepthaculitids,Batostoma,Cyanobacteria and Pulchrilamina.Through the research of characteristics of the reef-bearing strata of the Early Ordovician in the Yichang area,four sorts of biofacies are recognized,which are(1) shelly biofacies:containing Tritoechia-Pelmatozans community and Tritoechia-Pomatotrema community;(2) reef biofacies:including the Batostoma,Calathium-Archaeoscyphia, Pelmatozoa-Batostoma,Archeoscyphia and Calathium-Cyanobacteria communities; (3) standing-water biofacies:including the Acanthograptus-Dendrogptus and Yichangopora communities;and(4) allochthonous biofacies:containing Nanorthis-Psilocephlina taphocoense community.The analysis of sea-level changes indicates that there are four cycles of sea-level changes during the period when reef-bearing strata were formed in this area,and the development of reefs is obviously controlled by the velocity of sea-level changes and the growth of accommodation space.The authors hold that reefs were mostly formed in the high sea level periods.Because of the development of several subordinate cycles during the sea-level rising,the reefs are characterized by great quantity, wide distribution,thin thickness and small scale,which are similar to that of Juassic reefs in northern Tibet.The research on the evolution of communities shows that succession and replacement are the main forms.The former is favorable to the development of reefs and the latter indicates the disappearance of reefs.

Chitinozoans from the Fenxiang Formation (Early Ordovician) of Yichang,Hubei Province,China

Chitinozoans collected from upper Tremadocian to lower Floian strata of Chenjiahe section, Yichang, western Hubei, China comprise six species belonging respectively to the genera Euconochitina including a new species, Euconochitina fenxiangensis, Lagenochitina and Bursachitina, together with Desmochitina sp. and Eremochitina sp. The chitinozoan succession across the interval is correlated with relevant conodont and chitinozoan biozones and two new regional chitinozoan biozones, the Lagenochitina destombesi Biozone and the Euconochitina symmetrica Biozone are proposed based on their stratigraphic ranges in the Fenxiang to Honghuayuan formations in the Chenjiahe section.

Biostratigraphic and Chemostratigraphic Correlation for the Base of the Middle Ordovician between Yichang and Western Zhejiang Areas,South China

The base of the Middle Ordovician （i.e. Dapingian Stage） has been defined at the first appearance datum （FAD） of conodont Baltoniodus？ triangularis at Huanghuachang, Yichang, China,but the precise correlation of the boundary to regions of other facies remain to be resolved. Herein we review the biostratigraphy and chemostratigraphy of the Huanghuachang Global Standard Stratotype-Section and Point （GSSP） section, and present our latest stratigraphic work on the nearby Chenjiahe s ection in Yichang, and the Hengtang Quarry section, Jiangshan, Zhejiang, which is regarded as representative of slope facies. The conodont and graptolite biostratigraphy as well as chemostratigraphy of the Chenjiahe section indicate that the base of the Middle Ordovician also falls within the graptolite Azygograptus suecicus Zone, and coincides with a high or maximum δ13C value within a minor positive carbon isotope excursion, suggesting that the base boundary can be readily recognized across the entire Yangtze Gorges area. The integrated graptolite and conodont biostratigraphy and chemostratigraphy of the Hengtang section, Jiangshan, indicates that the basal boundary probably falls within the graptolite lsograptus caduceus imitatus Zone that overlies the Azygograptus suecicus Zone, and coincides with a remarkable drop of δ13C. This difference indicates that a multi-disciplinary approach is critical to identify the base boundary in those regions where the Baltoniodus ？ triangularis is absent.

Main progress and problems in research on Ordovician hydrocarbon accumulation in the Tarim Basin

The Tarim Basin is the largest petroliferous basin in the northwest of China, and is composed of a Paleozoic marine craton basin and a Meso-Cenozoic continental foreland basin. It is of great significance in exploration of Ordovician. In over 50 years of exploration, oil and gas totaling over 1.6 billion tonnes oil-equivalent has been discovered in the Ordovician carbonate formation. The accumulation mechanisms and distribution rules are quite complicated because of the burial depth more than 3,500 m, multi-source, and multi-stage accumulation, adjustment, reconstruction and re-enrichment in Ordovician. In this paper, we summarized four major advances in the hydrocarbon accumulation mechanisms of Ordovician carbonate reservoirs. First, oil came from Cambrian and Ordovician source rocks separately and as a mixture, while natural gas was mainly cracked gas generated from the Cambrian-Lower Ordovician crude oil. Second, most hydrocarbon migrated along unconformities and faults, with different directions in different regions. Third, hydrocarbon migration and accumulation had four periods： Caledonian, early Hercynian, late Hercynian and Himalayan, and the latter two were the most important for oil and gas exploration. Fourth, hydrocarbon accumulation and evolution can be generally divided into four stages： Caledonian （the period of hydrocarbon accumulation）, early Hercynian （the period of destruction）, late Hercynian （the period of hydrocarbon reconstruction and re-accumulation）, and Himalayan （the period of hydrocarbon adjustment and re-accumulation）. Source rocks （S）, combinations of reservoir-seal （C）, paleo-uplifts （M）, structure balance belt （B） matched in the same time （T） control the hydrocarbon accumulation and distribution in the Ordovician formations. Reservoir adjustment and reconstruction can be classified into two modes of physical adjustment and variation of chemical compositions and five mechanisms. These mechanisms are occurrence displacement, biodegradation, multi-source mixing, high-temperature cracking and late gas invasion. Late hydrocarbon accumulation effects controlled the distribution of current hydrocarbon. The T-BCMS model is a basic geological model to help understanding the control of reservoirs. At present, the main problems of hydrocarbon accumulation focus on two aspects, dynamic mechanisms of hydrocarbon accumulation and the quantitative models of oil-bearing in traps, which need further systemic research.

Hydro-geochemistry evolution in Ordovician limestone water induced by mountainous coal mining: A case study from North China

This research aimed to investigate the changing mechanism of hydro-geochemistry in Ordovician limestone karst(OL) water induced by mountainous coal mining activities. Thus, the hydrogeochemistry evolution of OL water over 40 years within a typical northern mountainous coal mine named Fengfeng Mine was studied by using Piper diagram, Gibbs scheme, ions correlation and Principal component analysis(PCA) methods. Results showed that, except for HCO3^-, the ions of Ca^2+, Mg^2+, SO4^2-,Na^++K^+, Cl^- and total dissolution solids(TDS) values all increased by years as mining continues. Different hydro-geochemical characteristics in different periods can reflect different water-rock interactions.Accordingly, sulfates dissolution gradually took place of carbonates in water-rock interaction. Especially,OL water-rock interactions in different periods were all affected by rock weathering and evaporationconcentration together. At last, evaporationconcentration co-effect dominated the hydrogeochemistry evolution slowly, along with significant cations exchange over years.

Ordovician Basement Hydrocarbon Reservoirs in the Tarim Basin, China

Ordovician marine carbonate basement traps are widely developed in the paleo-highs and paleo-slopes in the Tarim Basin. Reservoirs are mainly altered pore-cavity-fissure reservoirs. Oil sources are marine carbonate rocks of the Lower Paleozoic. Thus, the paleo-highs and paleo-slopes have good reservoiring conditions and they are the main areas to explore giant and large-scale oil reservoirs. The main factors for their reservoiring are: (1) Effective combination of fenestral pore-cavity-fracture reservoirs, resulting from multi-stage, multi-cyclic karstification (paleo-hypergene and deep buried) and fracturing, with effective overlying seals, especially mudstone and gypsum mudstone in the Carboniferous Bachu Formation, is essential to hydrocarbon reservoiring and high and stable production; (2) Long-term inherited large rises and multi-stage fracture systems confine the development range of karst reservoirs and control hydrocarbon migration, accumulation and reservoiring; (3) Long-term multi-source hydrocarbon supply, early reservoiring alteration and late charging adjustment are important reservoiring mechanisms and determine the resource structure and oil and gas properties. Favorable areas for exploration of Ordovician carbonate basement hydrocarbon reservoirs in the Tarim Basin are the Akekule rise, Katahe uplift, Hetianhe paleo-high and Yakela faulted rise.