Basic Characteristics of the Earth''s Temperature Distribution in Southern China

Sourthern China referred to in this paper embraces all the provinces to the south of latitude 32° N within the territory of China, exclusive of Tibet and Taiwan.The paper is based on the data obtained from temperature measurements of 680 boreholes drilled for petroleum, coal and hydrogeology. The depth of these boreholes ranges between 1,000 and 2,000m in most cases, being generally not less than 400m and reaching 7,000m in maximum.Through the elaboration and analysis of the temperature data, isotherm maps showing the temperatures at depths of 1,000m, 2,000m and 3,000m respectively and a geothermal gradient map of southern China have been compiled. These maps indicate that the earth's temperature distribution in southern China is characterized by a elatively low value in areas of its central part and those to the west of the Anning River valley of the western part(30℃, 40℃ and 70℃ at depths of 1,000m, 2,000m and 3,000m respectively), and a comparatively high value in the southeastern coastal areas and western Yunnan(60℃, 80℃ and over 120℃ respectively), and that the geothermal gradient also increases from less than 1.5℃/100 m in the central part towards both the east and the west to 2.0-3.0℃/100m or even over 4.0℃/100m.The earth's temperature distribution is strictly controlled by the deep structure of the earth's crust and is very closely related to the character of the regional geological structure, The stable central areas remarkably differ in the earth's temperature distribution from the tectonicalty active areas such as the southeastern coastal areas and the Jinggu-Tengchong area in western Yunnan, the former being lower than the latter. Meanwhile, due to the well-developed karst features in southern China, the strong groundwater flow may exert certain cooling effect on the temperature down to a depth of about 1,500m.According to the regularity of the earth's temperature distribution, regional deep structure of the earth's crust, regional geological structure and hydrogeological conditions, the following four models are proposed to explain the mechanism of formation and distribution of the earth's temperature fields in southern China: 1) onduction type; 2) convection type; 3) deep heat-source conduction-convection type; and 4) precipitation infiltration cooling type.     

Geochemical Indications of Possible Gas Hydrates in the Northeastern South China Sea

Gas hydrate, mainly composed of hydrocarbon gas and water, is considered to be a clean energy in the 21st century. Many indicators such as BSRs （Bottom-Simulating Reflections）, which are thought to be related to gas hydrate, are found in the South China Sea （SCS） in recent years. The northeastern part of the SCS is taken as one of the most potentials in the area by many scientists. It is situated in the conjunction of the northern divergent continental margin and the eastern convergent island margin, whose geological settings are much preferable for gas hydrate to occur. Through this study, brightness temperature anomalies recorded by satellite-based thermal infrared remotely sensed images before or within the imminent earthquake, the high content of hydrocarbon gas acid-degassed from subsurface sediment and the high radioactive thermoluminescence value of subsurface sediment were found in the region. Sometimes brightness temperature anomalies alone exist in the surrounding of the Dongsha Islands. The highest content of hydrocarbon gas amounts to 393 μL methane per kilogram sediment and the highest radioactive thermoluminescence value is 31752 unit; their geometric averages are 60.5 μL/kg and 2688.9 unit respectively. What is more inspiring is that there are three sites where the methane contents are up to 243, 268 and 359μL/kg and their radioactive thermoluminescence values are 8430, 9537 and 20826 unit respectively. These three locations are just in the vicinity of one of the highest confident BSRs identified by predecessors. Meanwhile, the anomalies are generally coincident with other results such as headspace gas anomaly in the sediment and chloride anomaly in the interstitial water in the site 1146 of Leg 184. The above-mentioned anomalies are most possibly to indicate the occurrence of gas hydrate in the northeastern SCS.     

Ore-forming Fluid and Mineral Source of the Hongshi Copper Deposit in the Kalatage Area, East Tianshan, Xinjiang, NW China

The Hongshi copper deposit is located in the middle of the Kalatage ore district in the northern segment of the Dananhu-Tousuquan island-arc belt in East Tianshan, Xinjiang, NW China. This study analyses the fluid inclusions and H, O, and S stable isotopic compositions of the deposit. The fluid-inclusion data indicate that aqueous fluid inclusions were trapped in chalcopyrite-bearing quartz veins in the gangue minerals. The homogenization temperatures range from 108°C to 299°C, and the salinities range from 0.5% to 11.8%, indicating medium to low temperatures and salinities. The trapping pressures range from 34.5 MPa to 56.8 MPa. The δ18OH2O values and δD values of the fluid range from ?6.94‰ to ?5.33‰ and from ?95.31‰ to ?48.20‰, respectively. The H and O isotopic data indicate that the ore-forming fluid derived from a mix of magmatic water and meteoric water and that meteoric water played a significant role. The S isotopic composition of pyrite ranges from 1.9‰ to 5.2‰, with an average value of 3.1‰, and the S isotopic composition of chalcopyrite ranges from ?0.9‰ to 4‰, with an average value of 1.36‰, implying that the S in the ore-forming materials was derived from the mantle. The introduction of meteoric water decreased the temperature, volatile content, and pressure, resulting in immiscibility. These factors may have been the major causes of the mineralization of the Hongshi copper deposit. Based on all the geologic and fluid characteristics, we conclude that the Hongshi copper deposit is an epithermal deposit.     

Formation Mechanism and Controlling Factors of Natural Gas Reservoirs of the Jialingjiang Formation in the East Sichuan Basin

The Lower Triassic Jialingjiang Formation reservoirs are distributed widely in the East Sichuan Basin, which are composed mainly of fractured reservoirs. However, natural gas with high concentration of H2S, ranging from 4% to 7%, was discovered in the Wolonghe Gas pool consisting primarily of porous reservoirs, while the other over 20 fractured gas reservoirs have comparatively low, tiny and even no H2S within natural gases. Researches have proved the H2S of the above reservoirs are all from the TSR origin. Most of the Jialingjiang Formation natural gases are mainly generated from Lower Permian carbonate rocks, the Wolonghe gas pool＇s natural gases are from the Upper Permian Longtan Formation, and the natural gases of the Huangcaoxia and Fuchengzhai gas pools are all from Lower Silurian mudstone. The formation of H2S is controlled by the characteristics and temperature of reservoirs, and is not necessarily related with gas sources. The Jialingjiang Formation in East Sichuan is buried deeply and its reservoir temperature has ever attained the condition of the TSR reaction. Due to poor reservoir potential, most of the gas pools do not have enough room for hydrocarbon reaction except for the Wolonghe gas pool, and thus natural gases with high H2S concentration are difficult to be generated abundantly. The south part of East Sichuan did not generate natural gases with high H2S concentration because the reservoir was buried relatively shallow, and did not suffer high temperature. Hence, while predicting the distribution of H2S, the characteristics and temperature of reservoirs are the necessary factors to be considerd besides the existence of anhydrite.     

Experimental Study on Water‐rock Reactions with CO2 Fluid in a Deep Sandstone Formation under High Temperature and Pressure

Qiongdongnan Basin has a tectonic geological background of high temperature and high pressure in a deep reservoir setting,with mantle-derived CO2.A water-rock reaction device was used under high temperature and high pressure conditions,in conjunction with scanning electron microscope(SEM)observations,to carry out an experimental study of the diagenetic reaction between sandstone at depth and CO2-rich fluid,which is of great significance for revealing the dissolution of deep clastic rock reservoirs and the developmental mechanism of secondary pores,promoting deep oil and gas exploration.In this study,the experimental scheme of the water-rock reaction system was designed according to the parameters of the diagenetic background of the deep sandstone reservoir in the Qiongdongnan Basin.Three groups of single mineral samples were prepared in this experiment,including K-feldspar samples,albite samples and calcite samples.Using CO2 as a reaction solution,a series of diagenetic reaction simulation experiments were carried out in a semi-closed high temperature and high pressure simulation system.A field emission scanning electron microscope(SEM)was used to observe the microscopic appearance of the mineral samples after the water-rock reaction,the characteristics of dissolution under high temperature and high pressure,as well as the development of secondary pores.The experimental results showed that the CO2-rich fluid has an obvious dissolution effect on K-feldspar,albite and calcite under high temperature and high pressure.For the three minerals,the main temperature and pressure window for dissolution ranged from 150℃to 300℃and 45 MPa to 60 MPa.Scanning electron microscope observations revealed that the dissolution effect of K-feldspar is most obvious under conditions of 150℃and 45 MPa,in contrast to conditions of200℃and 50 MPa for albite and calcite.Through the comparative analysis of experimental conditions and procedures,a coupling effect occurred between the temperature and pressure change and the dissolution strength and calcite.Under high temperature and high pressure,pressure changed the solubility of CO2,furthermore,the dissolution effect and strength of the sandstone components were also affected.The experiment revealed that high temperature and high pressure conditions with CO2-rich fluid has a significant dissolution effect on aluminosilicate minerals and is conducive to the formation of secondary pores and effective reservoirs.Going forward with the above understanding has important implications for the promotion of deep oil and gas exploration.     

内陆盐湖石盐流体包裹体均一温度指示意义的现代过程研究

Halite precipitation with water and air temperature was observed in detail,and homogenization temperature of fluid inclusions in halite formed in ancient and modem Chaka Salt Lake was studied.Halite precipitates mainly in August every year and largely precipitates between 13 and 15pm at one day when water temperatures reach 20℃but can seldom reach 30℃.Homogenization temperatures of fluid inclusions in halite formed in Chaka Salt Lake range from 14℃to 38℃with an average of 23.7℃.The number of inclusions appears an obvious peak value at homogenization temperatures between 18～25℃,which probably represent the water temperature in which halite mainly precipitates when water temperatures reach 20℃.Therefore,homogenization temperatures of fluid inclusions in halite formed in Chaka Salt Lake can well reflect the water temperature.     

内陆盐湖石盐流体包裹体均—温度指示意义的现代过程研究

Halite precipitation with water and air temperature was observed in detail, and homogenization temperature of fluid inclusions in halite formed in ancient and modem Chaka Salt Lake was studied. Halite precipitates mainly in August every year and largely precipitates between 13 and 15pm at one day when water temperatures reach 20℃ but can seldom reach 30℃. Homogenization temperatures of fluid inclusions in halite formed in Chaka Salt Lake range from 14℃ to 38℃ with an average of 23.7℃. The number of inclusions appears an obvious peak value at homogenization temperatures between 18 - 25℃, which probably represent the water temperature in which halite mainly precipitates when water temperatures reach 20℃. Therefore, homogenization temperatures of fluid inclusions in halite formed in Chaka Salt Lake can wall reflect the water temperature.     

The ＂CO2-rich gas vents＂ of Mt. Amiata volcano （Tuscany, Central Italy）： Geochemistry,genetic mechanism and hazard evaluation

Mt. Amiata （Southern Tuscany, Central Italy） is an extinct Quaternary volcano located in an area still marked by high heat-flow that is caused by deep seated （6-10 km） hot masses related to Pliocene magmatic activity. The anomalous geothermal gradient gives rise, within the Mesozoic limestone formation （Tuscan series）, to geothermal systems that fed the Ca-SO4 thermal springs characterizing this area. Besides of thermal fluids, several cold, dry CO2-rich gas emissions seep out on the NE flank of the volcano. These gas vents mostly consist of large sub-circular craters at variable depth and diameter （5-15 m and 10-50 m, respectively）, and represent a serious hazard for the local population, as testified by the several asphyxia casualties that have been repeatedly occurred within these morphological depressions. In this work, the chemical and isotopic compositions of the Mt. Amiata ＂CO2-rich gas vents＂ and the estimation of both the CO2 flux from the soil and the CO2 distribution in air of their surroundings, has been carried out in order to： （1） assess the origin of gases, （2） recognize the mechanism of formation for these gas emissions and their relationship with local tectonics, and （3） to evaluate the CO2 hazard in the high flux emanations. The chemical composition of the gases is largely dominated by CO2 （up to 98 % by vol） and shows relatively high concentrations of N2, CH4 and H2S （up to 1.1%, 0.9% and 3.9 % by vol, respectively）. These features, coupled with the carbon and nitrogen isotopic signatures, suggest that the origin of the main gas compounds may be related to the contribution of deep （i.e., thermometamorphic processes on carbonate formations for CO2） and shallow （i.e. thermal decomposition of organic material for CH4, N2 and H2S） sources.     

Analysis of heterogeneous characteristics in a geothermal area with low permeability and high temperature

An analytical methodology for reservoir characterization was applied in the central and southwestern zones of Los Humeros geothermal field(LHGF). This study involves analysis of temperature, pressure,enthalpy and permeability in wells and their distribution along the area. The wells located in the central western side of the geothermal field are productive, whereas those located at the central-eastern side are non-productive. Through temperature profiles, determined at steady state in the analyzed wells, it was observed that at bottom conditions(approximately 2300 m depth), temperatures vary between 280 and360℃. The temperatures are higher at the eastern side of central zone of LHGF. A review of transient pressure tests, laboratory measurements of core samples, and correlation of circulation losses during drilling suggest that permeability of the formation is low. The enthalpy behavior in productive wells shows a tendency of increase in the steam fraction. It was found that productivity behavior has inverse relation with permeability of rock formation. Further, it is observed that an imbalance exists between exploitation and recharge. It is concluded from the results that the wells located at central-eastern area have low permeability and high temperature, which indicates possibility of heat storage.     

The Impact of Aqueous Medium on Gas Yields and Kinetic Behaviors of Hydrogen Isotope Fractionation during Organic Matter Thermal Degradation

In order to recognize the impact of aqueous medium on gas yields and the kinetic behaviors of hydrogen isotope fractionation during organic matter thermal degradation, the gold tube apparatus was used to conduct thermal simulation experiments by mixing the nC18 with the water of different properties and proportions. The yields of natural gas components, the relation among hydrogen isotope composition of each component and the experimental temperatures vs. heating rates have been obtained, and the results indicate that under the higher temperature conditions, the hydrous experiment has obvious impact on gas yields, such as when more water is added, higher amounts of hydrocarbon gas and H2 are yielded, and the existence of water obviously prolongs the temperature interval with the existence of heavy hydrocarbon gas. It also shows that the hydrogen isotope of hydrocarbon gas generated by the hydrous experiment is obviously lighter than that generated by the anhydrous experiment, and with the increasing amount of added water, the δD value of hydrocarbon gas gradually decreases. Compared with gas yields, the variation of δD value is more sensitive to aqueous medium in the thermal simulation experiment. However, compared with the amount of the added water, the aqueous medium property has smaller impact on the gas yields, which still shows the inherit effect on hydrogen isotope composition of aqueous medium. Through the model simulation and the isotope fractionation behavior analysis, it is validated that the hydrogen isotope fractionation process can be well described by the chemical kinetic model. The difference of reaction fraction of normal methane and D-containing methane is large, corresponding to the same activation energy. The content of normal methane is obviously higher in the part with lower activation energy, while the content of D-containing methane is higher in the part with higher activation energy. Therefore, it will result in larger hydrogen isotope fractionation amplitude, and the δD values will be more sensitive to the variation of maturity. Meanwhile, the average activation energy of methane generation from nC18 in the hydrous experiment is higher than that in the anhydrous experiment, and the greater amount of added water, the larger the average activation energy of methane generation reaction. This has laid foundation for its exploratory application in the study of gas reservoir forming history and the gas-source correlation, which indicates the research and application prospects in this orientation.     

A Preliminary Study on Fluid Inclusions and Mineralization of Xitieshan Sedimentary-Exhalative (SEDEX) Lead-Zinc Deposit

The Xitieshan lead-zinc deposit is located at the northern margin of the Qaidam Basin, Qinghai Province, China, and had developed a complete marine sedimentary-exhalative system. Our preliminary study of ore-forming fluids shows that fluid inclusions in quartz from altered stockwork rocks that represent the pipe facies have a wide range of temperature and salinity. The intense fluid activities are characteristics of the pipe facies of the exhalative system. Fluid inclusions in carbonates near the unstratified ore bodies hosted in the thick-bedded marble which represents vent-proximal facies are large in size and have moderate to high temperatures. They represent unerupted sub-seafloor fluid activity. Fluids in altered stockwork rocks and carbonates have similar H2O-NaCl-CO2 system, both belonging to the sedimentary-exhalative system. The fluids migrate from the pipe facies to the unstratified ore bodies. Boiling of the fluids causes the separation of CO2 vapor and liquid H2O. When the fluids migrate into the unconsolidated thick-bedded marble, the escape of CO2, decreasing temperature and pressure as well as some involvement of seawater into the fluids result in the unmixing of fluids with high and low salinity and deposition of ore-forming materials. The two unmixed fluids were trapped in unconsolidated carbonates and the ore-forming materials were deposited in the unconsolidated carbonates to form the sedimentary-exhalative type unstratified ore bodies. The ore-forming temperature of unstratified ore bodies is up to high temperature indicating that there is a huge ore-forming potential in its deep.     

Natural Gas Types,Distribution Controlling Factors,and Future Exploration in the Western Qaidam Basin

The Paleogene and Neogene oil and gas in the western Qaidam basin have a regular distribution in three concentric zones from the edge to the center of the basin.Natural gas mainly occurs in the inner zone,and the gas-oil ratio of the northern area of the basin is significantly higher than that of the southern area.Large amounts of carbon isotope data of natural gas,plotted in Xshaped and comprehensive identification diagrams for the southern area and northern area,respectively,were used to identify the types of natural gas.The large-scale distribution of natural gas is highly consistent with the Ro values of major source rocks,but is poorly correlated with the type of organic matter.This indicates that the main controlling factor of natural gas distribution is organic matter maturity,and the kerogen types act as the basis for the formation of different types of natural gas.Paleouplifts and squeezed anticlines near hydrocarbon generation depression centers,which are major natural gas-rich regions,control the migration directions of natural gas,while hydrocarbon migration pathways and fault systems connecting gas sources are the most important factors for natural gas reservoir formation in the inner basin.Therefore,favorable zones for natural gas distribution can be predicted on the basis of the distribution of thermal evolution and the gas generation intensity of major source rocks as well as the structural map.The Shizigou-YoushashanYingdong-Dawusi,Youquanzi –Kaitemilike- Youdunzi,and Xiaoliangshan – Nanyishan- Dafengshan structural belts are favorable zones for natural gas accumulation.This study has important theoretical and practical significance for future natural gas exploration.     

A Preliminary Study on Fluid Inclusions and Mineralization of Xitieshan Sedimentary-Exhalative (SEDEX) Lead-Zinc Deposit

The Xitieshan lead-zinc deposit is located at the northern margin of the Qaidam Basin, Qinghai Province, China, and had developed a complete marine sedimentary-exhalative system. Our preliminary study of ore-forming fluids shows that fluid inclusions in quartz from altered stockwork rocks that represent the pipe facies have a wide range of temperature and salinity. The intense fluid activities are characteristics of the pipe facies of the exhalative system. Fluid inclusions in carbonates near the unstratified ore bodies hosted in the thick-bedded marble which represents vent-proximal facies are large in size and have moderate to high temperatures. They represent unerupted sub-seafloor fluid activity. Fluids in altered stockwork rocks and carbonates have similar H2O-NaCl-CO2 system, both belonging to the sedimentary-exhalative system. The fluids migrate from the pipe facies to the unstratified ore bodies. Boiling of the fluids causes the separation of CO2 vapor and liquid H2O. When the fluids migrate into the unconsolidated thick-bedded marble, the escape of CO2, decreasing temperature and pressure as well as some involvement of seawater into the fluids result in the unmixing of fluids with high and low salinity and deposition of ore-forming materials. The two unmixed fluids were trapped in unconsolidated carbonates and the ore-forming materials were deposited in the unconsolidated carbonates to form the sedimentary-exhalative type unstratified ore bodies. The ore-forming temperature of unstratified ore bodies is up to high temperature indicating that there is a huge ore-forming potential in its deep.     

Depth of volcanic basalt degassing forecasted from CO2 fluid inclusions

Fluid inclusions have recorded the history of degassing in basalt. Some fluid inclusions in olivine and pyroxene phenocrysts of basalt were analyzed by micro-thermometry and Raman spectroscopy in this paper. The experimental results showed that many inclusions are present almost in a pure CO2 system. The densities of some CO2 inclusions were computed in terms of Raman spectroscopic characteristics of CO2 Fermi resonance at room temperature. Their densities change over a wide range, but mainly between 0.044 g/cm3 and 0.289 g/cm3. Their micro-thermometric measurements showed that the CO2 inclusions examined reached homogenization between 1145.5℃ and 1265℃ . The mean value of homogenization temperatures of CO2 inclusions in basalts is near 1210℃. The trap pressures (depths) of inclusions were computed with the equation of state and computer program. Distribution of the trap depths makes it know that the degassing of magma can happen over a wide pressure (depth) range, but mainly at the depth of 0.48 km to 3.85 km. This implicates that basalt magma experienced intensive degassing and the CO2 gas reservoir from the basalt magma also may be formed in this range of depths. The results of this study showed that the depth of basalt magma degassing can be forecasted from CO2 fluid inclusions, and it is meaningful for understanding the process of magma degassing and constraining the inorganogenic CO2 gas reservoir.     

Analysis of groundwater level trend in Jakham River Basin of Southern Rajasthan

Groundwater accounts for about half of the water use for irrigation in India.The fluctuation pattern of the groundwater level is examined by observing rainfall replenishment and monitoring wells.The southern part of Rajasthan has experienced abrupt changes in rainfall and has been highly dependent on groundwater over decades.This study presents the impact of over-dependence on groundwater usage for irrigation and other purposes,spatially and temporally.Hence,the objective of this study is to examine the groundwater level trend by using statistical analysis and geospatial technique.Rainfall factor was also studied in groundwater level fluctuation during 2009-2019.To analyze the influence of each well during recharge or withdrawal of groundwater,thiessien polygonswere generated from them.In the Jakham River basin,75 wells have been identified for water level trend study using the Mann-Kendall statistical test.The statistics of trend analysis show that 15%wells are experiencing water level decline in pre-monsoon,while very low percentage of wells have such trend during post-monsoon season.The average rate of water level decline is 0.245 m/a in pre-monsoon and 0.05 m/a in post-monsoon.The aquifer recharge potential is also decreasing by year.it is expected that such type of studies will help the policy makers to adopt advanced management practices to ensure sustainable groundwater resource management.     

Kinetic Model of Gaseous Alkanes Formed from Coal in a Confined System and Its Application to Gas Filling History in Kuqa Depression, Tarim Basin, Northwest China

Based on the pyrolysis products for the Jurassic low-mature coal under programmed temperature, and chemical and carbon isotopic compositions of natural gas from the Kuqa Depression, the genetic origin of natural gas was determined, and then a gas filling model was established, in combination with the geological background of the Kuqa Depression. The active energy of CH4, C2H6 and C3H8 was gotten after the data of pyrolysis gas products under different heating rates (2°C/h and 20°C/h) were fitted by the Gas Oil Ratio (GOR) Isotope Model soft. When the frequency factor (Af) was chosen as 1×1014, the active energy of CH4, C2H6 and C3H8 was 58?kcal/mol, 57?kcal/mol and 54?kcal/mol, respectively. The distributive ranges of the δ13C1,? δ13C2 and δ13C3 values for the pyrolysis gas products are –35.9‰ to –30.7‰, ?26.2‰ to ?21.3‰ and ?26.4‰ to ?22.7‰, respectively. All of the natural gases from the Kuqa Depression are dominated by hydrocarbon gases, with the high gas dryness (C1/C1?4) at the middle and northern parts of the depression and the low values at both east and west sides and the southern part. The carbon isotopes of methane and its homologs as a typical coal-type gas are enriched in 13C, and the distributive range of the δ13C1,? δ13C2 and ?δ13C3 value is ?32‰ to ?38‰, ?22‰ to ?24‰ and ?20‰ to ?22‰, respectively, with the carbon isotopes of gaseous alkanes being less negative with the carbon number. With the ethane being enriched in 13C the increasing tendency of the geological reserve of natural gas in the Kuqa Depression is observed. This observed change is consistent with the results of pyrolysate gas yield of coal as a potential gas source in the Kuqa Depression, suggesting natural gas was thermally derived from the humic organic matters and the carbon isotopes of gaseous alkanes would coarsely predict the geological reserve of gas in the Kuqa Depression. Through the simulation of kinetic processes of gas generation for the Jurassic coal in the Kuqa Depression, the gas in the Kela 2 gas field would get the threshold of gas expulsion after 27?Ma, be expelled out of source rocks as “pulse action”, and then filled in the gas reservoir. The peak gas-filling history took place during the past 2 Ma.     

Activity of Silica-Rich Hydrothermal Fluid and Its Impact on Deep Dolomite Reservoirs in the Sichuan Basin, Southern China

Well-developed dissolution pores occur in the dolomites of the Sinian Dengying Formation, which is an important oil and gas reservoir layer in the Sichuan Basin and adjacent areas in southern China. The pores are often filled with quartz, and some dolomites have been metasomatically altered to siliceous chert. Few studies have documented the characteristics, source or origin of silica-rich fluids and their effects on the dolomite reservoir. The peak homogenisation temperatures(T_h) of fluid inclusions in pore-filling quartz are between 150℃ and 190℃, with an average of 173.7℃. Gases in the inclusions are mainly composed of CO_2, CH_4 and N_2. Compared with host dolomite, pore-filling quartz and metasomatic chert contain higher amounts of Cr, Co, Mo, W and Fe, with average concentrations of 461.58, 3.99, 5.05, 31.43 and 6666.83 ppm in quartz and 308.98, 0.99, 1.04, 13.81 and 4703.50 ppm in chert, respectively. Strontium levels are lower than that in the host dolomite, with average concentrations in quartz and chert of 4.81 and 11.06 ppm, respectively. Rare earth element compositions in quartz and chert display positive Eu anomalies with a maximum δEu of 5.72. The δD_(SMOW) values of hydrogen isotopes in water from quartz inclusions vary from-85.1‰ to-53.1‰ with an average of-64.3‰, whereas the δ~(18)O_(SMOW) values range from 7.2‰ to 8.5‰ with an average of 8.2‰. The average ~(87)Sr/~(86)Sr ratios in quartz and chert are 0.711586 and 0.709917, respectively, which are higher than that in the host dolomite. The fluid inclusions, elemental and isotopic compositions demonstrate that the formation of quartz and chert was related to silica-rich hydrothermal fluid and that the fluid was the deep circulation of meteoric water along basement faults. Interactions with silica-rich hydrothermal fluids resulted in densification of dolomite reservoirs in the Dengying Formation through quartz precipitation and siliceous metasomatism. However, it increased the resistance of the host dolomite to compaction, improving the ability to maintain reservoir spaces during deep burial. Evidence for silica-rich hydrothermal activity is common in the Yangtze Platform and Tarim Basin and its influence on deep dolomite reservoirs should be thoroughly considered.     

Application of the geochemical prospecting to improve oilfield structure models

Background： Complicated structure of surveyed area has minor efficiency of seismic surveys-the main method for petroleum and gas prospecting and interpretation of subsurface structure of the Earth. This paper gives an example that in certain cases geochemical surveys can be used as additional tool to establish suitable geological model of the trap. This paper presents geochemical studies in the southeastern part of European territory of Russia-Nagumanovskoe oilfield. Complicated geological structure causes difficulties to create suitable geological model of the trap itself. There are two models of geological structure for the Nagumanovskoe oilfield： Variant A： Researchers assume the presence of gas condensate accumulation and perspective for exploration traps. Variant B： Researchers assume presence of 2 local structures. Method and results： Geochemical data came from over than 2000 shallow probes （2 meters, 6.6 feet） of soil gas samples collected every 200 meters over more than 7 different long profiles. The adsorbed soil gases were analyzed for hydrocarbones. All samples were decomposed into geochemical populations. According to geochemical data the Nagumanovskoe oilfield was well defined on all geochemical profiles. Examination and comparison of hydrocarbon ratio plots and magnitude graphs for each of the two models indicate that the more anomalous magnitude sites agree with the disposition of structures according to model B. The geochemical oil/gas data exhibit clearly defined compositional sub-populations which match the composition of the underlying reservoirs, difference in standard deviation of hydrocarbon values at background and Cretaceous structure （model B） leads to the deduction that variant B is more suitable geological structure model of the Nagumanovskoe oilfield.     

Reserve and Pressure Change of Paleo-oil Reservoir in Puguang Area,Sichuan Basin

The Puguang (普光) gas field is the largest gas field found in marine carbonates in China.The Feixiangnan (飞仙关) and Changxing (长兴) reservoirs are two such reservoirs that had been buried to a depth of about 7 000 m and experienced maximum temperature of up to 220 ℃ before uplift to the present-day depth of 5 000-5 500 m,with present-day thermal maturity between 2.0% and 3.0% equivalent vitrinite reflectance (Ro).Bitumen staining is ubiquitous throughout the Feixianguan and Changxing formations,with the greatest concentrations in zones with the highest porosity and permeability,suggesting that the solid bitumen is the result of in-situ cracking of oil.According to the distribution of bitumen in the core,the paleo-oil boundary can be approximately determined.The paleo-oil resource is calculated to be about (0.61-0.92)×109 t (average 0.76×109 t),and the cracked gas volume is about (380.80-595.80)×109 m3 (average 488.30×109 m3); at least 58.74% of cracked gas is preserved in Puguang gas field.The study area experienced not only the cracking of oil but also thermochemical sulfate reduction,resulting in large quantities of nonhydrocarbon gas,with about 15.2% H2S and 8.3% CO2,together with the structural reconfiguration.During the whole process,the great change of volume and pressure compels the PVTsim modeling software to simulate various factors,such as the cracking of oil,the thermochemical sulfate reduction (TSR) and the tectonic uplift in both isolated and open geological conditions,respectively.The results show that although any one of these factors may induce greater pressure changes in an isolated system than in a closed system,the oil cracking and C3+involving TSR lead to overpressure during the early stage of gas reservoir.Therefore,the tectonic uplift and the methane-dominated TSR,as well as the semi-open system contribute to the reducing pressure resulting in the current normal formation pressure.     

Report of LA-ICP-MS zircon U-Pb dating (272 Ma) of gabbros in Yagan area,northern Alxa,Inner Mongolia: Insights into the tectonic evolution of the southern Paleo-Asian Ocean

1.Objective The northern margin of the Alxa Block in Inner Mongolia is located in the middle part of the southern margin of the Central Asian Orogenic Belt.It serves as a key position that connects the eastern and western tectonic units of the Central Asian Orogenic Belt and is an ideal site for the research on the final closing process of the Paleo-Asian Ocean.