Geoscience

Genesis of the Chalcopyritebearing Silicalite in the Tongyugou Copper Ore District, Qinghai Province and Its Geological Prospecting Significance

LUO Fan，XUE Chunji，ZHAO Xiaobo，HUANG Yongsen，WANG Xiongfei， DAI Zhijie，WU

2016, 30(4): 723-738.

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The Tongyugou area, located in the conjunction between the East Kunlun and West Qinlin orogenic belts, is one of the most important copper deposits in the Ela mountain metallogenic belt. Further resource prospecting at the area faces great challenges, and the silicalite might be one of the clues. The silicalite in the Tongyugou copper ore district occurs as bedding and banded structure, and is mainly hosted in conformity within LowerMiddle Triassic rocks. It is mainly made up of microfine crystalline quartz grain with less sericite, chalcopyrite and pyrrhotine, however, no biological related texture has been observed. The Al2O3TiO2 and FeMn(Cu+Ni+Co)×10, P2O5Y diagrams and SiO isotope signature suggests that the silicalite was formed by hydrothermal sedimentary processes. Its Si/Al and U/Th values and rare earth element geochemical characteristics also imply that the hydrothermal sedimentary system has been affected by some terrigenous material. Based on the examination in Al2O3/(Al2O3+Fe2O3)Fe2O3/TiO2, Ti/VV/Y and (La/Ce)NAl2O3/(Al2O3+Fe2O3) discrimination diagrams, the silicalite is proved to be formed at continental margin setting. Accompanying with the northward subduction of the Anyemaqen ocean in the EarlyMiddle Triassic, the Tongyugou area appears to be a backarc basin. In the basin, exhalative sedimentation occurred and also resulted in a lot of oreforming minerals to form protore beds. The silicalite is an important recorder to preserve the information of the mineralization event. Also, this study suggests that the Cuozama and Tadong may be important targets for the exploration of VMS type deposits.

Study on Formation Mechanism of Epigenetic Altered Zone in Zhiluo Formation, Northeastern Ordos Basin, North China

LI Xide，YI Chao，GAO Hewei，CHEN Xinlu，ZHANG Kang，WANG Mingtai

2016, 30(4): 739-747.

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The area of northeastern Ordos Basin, which is rich in uranium resource, is one of the important metallogenic belts for sandstonetype uranium deposits in China. Previous study shows that uranium mineralization here is closely related to paleo interlayer oxidation. On the basic study of material composition and geochemical index of each epigenetic altered sandstone and original sandstone in paleo interlayer oxidation zone in Zhiluo Formation, it can be concluded that the main differences of material composition between green sandstone and gray sandstone are the content of clay minerals, calcites and pyrites. Moreover, the sandstone has its own characteristic of epigenetic alteration in different altered type. Finally, the formation mechanism of paleo interlayer oxidation has been discussed in this paper through the research of each epigenetic altered sandstone features which provides an important theoretical basis for deepening the uranium mineralization mechanism and establishing regional uranium metallogenic model in the study area.

Zircon UPb Age, Geochemical Characteristics and Geological Significance of Shanzixingshan Basalt in Shuanghu, Northern Tibet

MA Long1, LI Zhongxiong1, LI Yong1, LIU Han1, WU Chengshu2, WEI Hongwei1, DENG Q

2016, 30(4): 748-759.

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This paper reports the isotopic geochronology and geochemistry of Shangzixingshan basalt in Shuanghu, Northern Tibet for the first time. Zircon LAICPMS UPb dating yields a mean age of (2358±27) Ma, indicating that these lava should form in the Ladinian period. The geochemical characteristics of the basalt suggest that the SiO2 contents range from 4284% to 5222%, TiO2 contents range from 161% to 269%, FeOT/MgO ratios range from 152 to 194, which can be attributed to tholeiitic basalt of subalkaline series. For Shanzixingshan basalt, the ∑REEs range from 11409×10-6 to 20847×10-6, (La/Yb)N ratios range from 490 to 650, and relatively enriched in LREE. The trace element patterns for Shanzixingshan basalt are very similar to those of the OIBtype basalts. Petrogenesis research shows that crystallization differentiation played important roles in the magma evolution. The incompatible element ratios (La/Nb, La/Ta, Zr/Ba) and Ti content indicate that the basaltic magma may be generated by the interaction of the asthenosphere and lithosphere mantle. The high Zr content and Zr/Y ratio show the characteristics of intra plate basalts in the extensional setting. Combined with regional geological available data, we suggest that the Middle Triassic basaltic magma was originated from the partial melting of the upwelling asthenosphere due to the slab window opening, which probably caused by slab breakoff in the process of the SouthQiangtang Block and the NorthQiangtang Block collision, whereas the exhumation of Late Triassic metaluminousperaluminous magmatic rocks and highpressure metamorphic rocks is resulted from extensional tectonics in response to orogenic collapse.

Characteristics of Diorite from Jiling Pluton and Its Relationship with Uranium Metallogenic in Longshou Mountains, Gansu Province

NIE Li, ZHAO Ruyi，CHEN Xu, FENG Bo, WANG Gang, LI Ying

2016, 30(4): 760-769.

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Jiling pluton is Caledonian magmatic rock in Longshoushan uranium ore belt. Diorite is one of the main rock types of Jiling pluton, which emplaced in the Early Caledonian. Diorite is mainly distributed in Xinshuijing and Jiling area;the early diorite is graydark gray mediumgrained; while the later one is dark gray finegrained.This investigation obtains (4856±14) Ma as the age of the mediumgrained diorite by LAICPMS zircon UPb dating. The research shows that the diorite plays an important role in later uranium metallogeny. The altered diorites provide the Ubearing hydrothermal fluids for unloading space. There are albite, chlorite, calcite, and hematite in altered diorite. With increasing of U content, SiO2 content is a bit increasing while Na2O increasing obviously; the K2O,FeO and P2O5 contents are obviously decreasing; and CO2 content increases at first and decreases when U content is much higher, but it is slightly increasing overall. By comprehensively studying and analyzing, the diorite is regarded as a favorable host rock to sodium metasomatic uranium, and the contact belt of diorite and granite provides a physicochemically sharply changing setting as a physicochemical barrier for the uranium enrichment.

The Metallogenic Regularities and Metallogenic Prognosis for the Xinping Gold Deposit in Pingnan County, Guangxi

ZHOU Yequan，PANG Baocheng，LI Yuanqiang，L Jiawen，LAI Xin

2016, 30(4): 770-780.

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The Xinping gold deposit, located in the eastern part of Dayaoshan metallogenic belt in southeastern Guangxi, is a typical altered rock type gold deposit. Controlled by EW and NE secondary faults, the ore bodies mainly occur in fracture zones which formed in Cambrian strata, or in contacting belt between strata and graniteporphyries, or in graniteporphyries. The gold mineralization has close relationship with silicification, pyritization and sericitization. Based on geological characteristics of the deposit, metallogenic regularities and the geochemical anomalies, we put forward six prospecting targets in two prospective areas.

Oreforming Fluids and Genesis of Yindongpo Gold Deposit, Henan Province

ZENG Wei, DUAN Ming, WAN Duo, SIMA Xianzhang, AO Cong1,REN Aiqin,

2016, 30(4): 781-791.

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Yindongpo gold deposit which is located in the middle part of Weishancheng AuAg ore belt in Tongbai County is a super large gold deposit and associated silver,lead and zinc.Petrographic,microthermometric and laser Raman spectroscopic studies were carried out on fluid inclusions in quartz from gold ores.The results show that three types of primary fluid inclusions occur in gold ores:vaporliquid two phase inclusions,CO2bearing three phase inclusions and vaporrich inclusions. Oreforming fluid is NaClH2OCO2 system with a little N2,CH4,H2S and H2.The fluid immiscibility is the main factor leading to the precipitation of ore materials.The homogenization temperatures and salinities of three types of inclusions range from 1692 ℃ to 3992 ℃ and 18% to 122%. The salinity of CO2bearing three phase inclusions is lower than that of vaporliquid two phase inclusions.The oreforming pressure of major mineralization phase is mainly 62-1263 MPa, corresponding to the depth of 52 km.The δ34S value of pyrite in the ore varies from 16‰ to 33‰，which is lower than δ34S value of pyrite in wall rock which varies from 33‰ to 62‰. It indicates that the sulfur in the ore may be derived from the mixing of mantle sulfur and wall rocks.

Characteristics and Prospecting Significance of the Thermoluminesence and Cell Parameters of Quartz from the Wangershan Gold Deposit, Jiaodong Peninsula, China

ZHANG Long，LI Shengrong，ZHU Suizhou，LIU Ran，LI Wentao，SONG Yingxin

2016, 30(4): 792-801.

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Gold mineralization of the Wangershan deposit can be divided into 4 stages including pyritequartz stage, quartzpyrite stage, polymetallic sulfide stage and quartzcarbonate stage. The thermoluminesence and cell parameters of quartz from the 4 stages were analyzed in this paper.The characteristics of the thermoluminesence curves show obvious single peak at middle temperatures for the quartz from the early and later stages and asymmetric twin peaks with strongest emitting intensity and complexity for the quartz from the main mineralization stage. It is an important prospecting criteria for gold in this area. At the depth of the veinⅢ, the thermoluminesence curves of the quartz appear to have shoulderlike and twin peaks, implying a good mineralization prospect downwards. The cell parameters of the quartz show a regular variation in time and space. In the main mineralization stage, the quartz cell parameters a0, c0 and v0 are relatively large with (Δa0/a0)/(Δc0/c0) ranging from 165 to 440, suggesting that trace elements are combined into quartz by gap filling and replacement with interstitial mode as the major way. The change of halfpeak bandwidth of the thermoluminesence is as the same as the cell parameters for quartz; therefore,the halfpeak bandwidth can also be taken as a criteria for goldrich and goldpoor quartz grains. According to the spatial variation tendency of the cell parameters and halfpeak bandwidth of quartz, the authors infer that the unmined region at large depth is presumably profitable for further gold mineralization prospect.

Regional Geochemical Distribution Characteristics of Rare Earth Elements in the Dashizhai Area, Inner Mongolia

YANG Fan , SONG Yuntao, ZHANG Shunyao, HAO Zhihong, GUO Zhijuan,

2016, 30(4): 802-810.

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Based on the regional geochemical survey, stream sediment sampling and analysis, the REE distribution characteristics of the stream sediments from the Dashizhai area in Inner Mongolia were evaluated systematically. Results show that ΣREE, ΣLREE/ΣHREE, δCe, δEu, (La/Sm)N, (La/Tb)N, (La/Lu)N, (Ce/Yb)N and (Gd/Lu)N of stream sediments have significant differences from the biotite granite in the central part of the study area and strata around the biotite granite. The REE distribution is closely related to the biotite granite; light rare earth elements are mainly distributed in the periphery of the biotite granite, and heavy rare earth elements are mainly distributed in the top of that biotite granite. The REE concentration of stream sediments from regional geochemical survey can reasonably describe the REE distribution, and can reflect the geological characteristics of the survey area, thus can provide references for the interpretation of regional geochemical anomalies.

Determination of the Element Anomaly Threshold and the Enclosed Abnormal Area by Using ContentArea Fractal Method Based on MAPGIS：Exampled by Yangmingshan Area of Hunan Province

CUI Yuliang, WANG Genhou, LANG Xinxin, ZENG Zhifang

2016, 30(4): 811-817.

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The determination of thresholds of chemical elements is one of the important problems in geochemical exploration. At present, there is not a scientific way to confirm the thresholds.The traditional way to determine the thresholds of chemical elements is based on the assumption of normal distribution of the element contents or their logarithms. In fact,the space distribution of geochemical element content is extremely complex; and the studies showed that the geochemical landscape may be a lowdimensional chaotic attractor system. The background values and anomalies of the elements have their own exponential relationship, which lead to a multifractal distribution, thus obtaining the anomaly threshold of element through the distribution of fractal elements. In this paper, the method of dividing area of geochemical exploration sample analysis data in Yangmingshan in Hunan is adopted. The threshold is determined through mixed application of fractal and traditional methods according to the distribution of data，drawing out the Sn anomaly map. It obtained better performance than by using the traditional method only, and the abnormal area enclosed by the element anomaly thresholds which is determined by the new method highly conformed to the known ore spots.

Effect of Natural Organic Matter on Removal of Cr(Ⅵ) from Water by Nanoscale ZeroValent Iron

WANG Enze, WEI Zhiwei, CAO Xi, YAN Zelong, CHEN Jiawei

2016, 30(4): 818-824.

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The application of nanoscale zerovalent iron (nanoFe0) in contaminated groundwater is fast developed for environmental remediation in recent years. However, the reactivity of nanoFe0 is impacted by different environmental factors. Among them, the ubiquitous natural organic matter (NOM) is an important issue. Herein, humic acid (HA) is taken as a model of NOM and batch experiments were conducted on the effect of HA on the removal of Cr(Ⅵ) from water by nanoFe0. The results showed that the presence of HA could significantly mitigate the reaction rate and removal efficiency of Cr(Ⅵ), which is due to the adsorption of HA onto nanoFe0 and the decrease of active sites on the surface of nanoFe0. In addition, the dissolved oxygen could enhance the removal of Cr(Ⅵ) by nanoFe0. The interaction of HAnanoFe0Cr(Ⅵ) could provide theoretical and practical value for further revealing the mechanism of remediation systems.

Hydrogeochemical Characteristics and Formation of Groundwater in Tu Ha Basin, Xinjiang

JIANG Wanjun，ZHAO Dan，WANG Guangcai，GUO Yonghai，LIU Chenglong，

2016, 30(4): 825-833.

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Due to the serious shortage of water resources in TuHa Basin, Xinjiang, groundwater has become the main water resources; therefore, studying hydrogeochemical characteristics of the groundwater is very important for the development and protection of regional groundwater. By using the methods of Piper diagram, stable hydrogen and oxygen isotopes, Gibbs graph and ion correlation analysis, the hydrochemical characteristics and evolution were analyzed in the paper. The results showed that the recharge source of groundwater mainly came from the atmospheric precipitation and the evaporation was the main discharge way in TuHa Basin. The groundwater chemical compositions were mainly formed by lixiviation and evaporation. Hydrochemical type gradually evolved from the Cl·SO4Na to ClNa along the groundwater flow path. The evolution of the groundwater chemical components had high consistency and obvious enrichment phenomenon in the study region.

Soil Moisture Retrieval from Remote Sensing and Its Impact on Evapotranspiration in Qaidam Basin

WANG Kailin,JIN Xiaomei,GUO Renhong,WEI Shanrong, LIANG Hua

2016, 30(4): 834-841.

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Soil moisture is the core content and link of the groundwatersoil wateratmospheric water circulation system. Evapotranspiration(ET) is the important driving force of this system. The study of distribution of soil moisture and its effect on ET from regional scale has great significance on ecoenvironmental protection in arid areas. Based on MODIS and GLDAS data, the monthly soil moisture from June to September in 2014 was estimated in plain area in Qaidam Basin after downscale treatment of 0-10 cm GLDAS surface soil moisture data using Apparent Thermal Inertia(ATI) method. The retrieval results were validated by combination of field measurement and NDVI. Furthermore, the mean daily ET was estimated in 9 drainage basins of plain using the Surface Energy Balance System(SEBS), and the relationship between soil moisture and ET was also analyzed in this study. The result indicated that the correlation between ATI soil moisture and 0-10 cm downscaling GLDAS data was good and the determination coefficient R2 is more than 07. The determination coefficient R2 between retrieval result of soil water content and field measurement is 0791, and the R2 between soil water content and NDVI is 0954. Therefore, it is reliable to retrieve soil moisture in Qaidam Basin based on downscale treatment of GLDAS data using ATI method. The daily ET is positively correlated with soil moisture and the determination coefficient R2 is more than 096. The impact of soil moisture on ET is much more than other impact factors.

Highresolution Sequence Stratigraphy and Lithofacies Palegeographic Analysis of Qigu Formation in Fudong Slope Area

U Jingwei，LIU Xu，LIU Ni，XUE Cheng，WEN Huaguo，XU Wenli

2016, 30(4): 842-851.

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The Upper Jurassic Qigu Formation is the main exploration layer in the Fudong slope area in the east of Junggar Basin, which is thought to be the favorable replace objects of lithologic and stratigraphic reservoirs．Guided by highresolution sequence stratigraphic theory, one longterm base level circle，three midterm base level circles and eight shortterm base level circles are divided from Qigu Formation of Fudong slope area in east of Junggar basin through outcrops, drilling cores, seismic and logging dataMidterm base level circles have three basic types of sequence structure, and sandbodies positioned in initial stage of midterm base level circles are favorable for reservoir development, while shortterm base level circles have five basic types of sequence structure. Lithofacies palegeography is depicted in the framework, and the result shows that evolution of lithofacies palegeography is mainly affected by tectonic movement, and distribution range of delta front subfacies becomes larger, while delta plain becomes smaller, and the favorable sedimentary microfacies for reservoir development is distributary channel.

Sedimentary Composition of Reef Succession Since Cenozoic in Xisha Areas: An Example from the Xike1 Well

ZHAO Xinwei, XU Hong, SUN Zhipeng

2016, 30(4): 852-862.

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Reefs are essentially in place calcareous deposits created by sessile organisms. Xisha areas developed thick strata of reef that are formed since the Miocene. Using microscopic and macroscopic observation for the Xike1 well that is fullcoring, combing with paleontology and core test results, we found that Miocene and Quaternary were the main reefbuilding stages in the Xike1 well, then formed two sets of reef succession that is respectively masked by the coralline algae and the coral as the main reefbuilding organism, in which accessory reef organisms are benthic foraminifera. However, Pliocene developed a set of bankfacies deposits. The rock type of this reef succession principally include framestone, boundstone and bafflestone, in which framestones are dominated. Nonreef facies rocks included marlstone, grain limestone and bioclastic limestone. Dolomite formation developed extensively in the Late Miocene to Early Pliocene, which was more caused by the penecontemporaneous dolomitization and affected by hydrothermal activity. Deposition analysis of the reef successions can provide the basic data for oil and gas exploration and distribution of reef reservoir in the South China Sea.

Paleogene Calcareous Algae and Sedimentary Environment in Tüna Area of Yadong in Southern Tibet

NIU Xiaolu, LI Guobiao, WANG Tianyang

2016, 30(4): 863-870.

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Continuous Paleogene marine deposits are outcropped in southern Tibet. Yadong area in southern Tibet was once located in the Tethyan residence sea basin and developed the youngest wellexposed marine sediments (the highest marine layer, HML). The age of the HML stands for the final closing age of the residual basin. The study on HML in this area can provide direct information about the late evolutionary history and the closing age of eastern Tethys. Based on the detailed analysis on the calcareous algae fossils from the Gulupu section in Tüna of Yadong area, eleven species of seven genus of Corallinaceae, four species of four genus of Dasycladaceae, one species of one genus of Codiaceae have been identified and three assemblages have been recognized from the Zongpu Formation and Zhepure Formation, which are ranked from bottom to up as follows: Lithoporella melobesioidesLithophyllum carpathicum assemblage, Lithoporella melobesioidesOvulites margaritula assemblage, Distichoplex biserialisJania denotata assemblage. Based on the study of the characteristics of calcareous algae fossil assemblages, it is inferred that during Late Paleocene to Eocene Tüna region should be located in a shallow shelf environment and the depth of sea water might gradually decrease to be less than 30 m until Late Eocene.

Rearranged Hopanes Compositions in Pyrolysis Experiment of Crude Oil and Geochemical Significance

CHEN Julin, ZHANG Min

2016, 30(4): 871-879.

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In order to study the influence of thermal evolution on the formation and distribution of rearranged hopanes as well as geochemical significance, pyrolysis experiment was conducted on crude oil from the TZ62 well in Tarim Basin. The analytical results showed that thermal maturity played an important role on the formation and distribution of rearranged hopanes. At the experimental temperature of 400 ℃ to 500 ℃,with the increase of degree of thermal evolution, the thermal cracking of microcrystalline wax and the thermal degradation of the asphaltene in crude oil have made a main contribution to the formation of rearranged hopanes. Peak oil cracking was corresponding to the experimental temperature at 500 ℃ to 550 ℃,and hopanoids concentrations decreased obviously. Furthermore, 17αhopane had a faster rate of thermal cracking than rearranged hopanes during peak oil cracking stage. Higher degree of thermal evolution played an important role in methyl rearrangement during the experimental temperature increasing from 550 ℃ to 600 ℃. Rearranged hopanes parameters (17αdiahopane/17αhopane, Ts/Tm) were nearly unchanged when the experimental temperature ranges from 400 ℃ to 500 ℃. Rearranged hopanes parameters increased gradually while experimental temperature ranging from 500 ℃ to 550 ℃(corresponding peak oil cracking). However, the parameters then shows an apparent reversal at the temperature of 550 ℃ to 600 ℃. The reversal in these parameters was mainly the result that 17αhopane had faster rate of hydrocarbon generation than rearranged hopanes. Rearranged hopanes parameters can be used as effective maturity parameters at the stage of the late oil window to peak oil cracking.

Tight Sand Reservoir Features of He 8 Member in Eastern Ordos Basin: A Case Study of Qingjian District in Zizhou Gasfield

GUO Zhi, JI Guang, WANG Guoting, PENG Yanxia

2016, 30(4): 880-889.

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Qingjian district is one of the most promising succeed blocks for stable production of Zizhou gasfield in eastern Ordos Basin. Because the study area is large with only a few drilling wells, the degree of geological knowledge is relatively low and the study area is still in the early stage of development evaluation. Taking He 8 reservoir as the research object, based on a series of fine geological research work such as sedimentary analysis and reservoir evaluation in a comprehensive comparison with Sulige tight sand gasfield, effective sand bodies thickness, scale, development frequency and distribution regularities were defined. Through analysis, it is clear that effective sand bodies scattered in space with poor continuity, have a good corresponding relationship with the distributions of the favorable sedimentary facies such as channel bars. The effective sand bodies are mainly concentrated in western sand belt in plane, and are more developed in the second layers of upper and lower submembers of He 8 member vertically. Combining geological information and production testing data, “continuous effective thickness” was chosen as the main parameter for reservoir evaluation, and reservoirs were classified into 4 types: good, medium, poor and dry layer. Then, enriched area was optimized, and the study area was divided into 3 types of zones according to different development potentials. It is recommended to deploy vertical wells in enriched zone and not to apply horizontal wells under the current technical and economic situation. This study provides a reliable geological basis for the preparation of gasfield development plan, and it is also of a certain reference significance to the geological research in the gasfields with similar conditions.

Determination of Hydrocarbon Charging History by Diagenetic Sequence and Fluid Inclusions: A Case Study of the Angdaercuo Area in the Qiangtang Basin

LI Junwei, LI Qi, XIA Xianghua, GE Dongsheng, SHI Yalan, WANG Xing

2016, 30(4): 890-895.

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This paper systematically analyzed the fluid inclusion samples from outcrop of the Middle Jurassic Buqu Formation in Angdaercuo area of Qiangtang Basin using fluorescent indicator, microscopic temperature and salinity measures. Two stages of fluid inclusions were identified in calcite veins and microfractures in calcite. Aqueous inclusions indicate that homogenization temperatures are mainly 64 to 198 ℃, therein, 80 to 130 ℃ and 130 to 180 ℃ represent two large scale hydrocarbon accumulation phases: the early hydrocarbon accumulation is dominated by oil and gas; the late hydrocarbon accumulation is dominated by oil. Combined with the burial history of Qiangtang Basin, the results of this study suggest that the hydrocarbon accumulation occurred during 100 to 156 Ma (Late Jurassic to Early Cretaceous).

Characteristics and Controlling Factors of Micropore Structures of the 5th Shale Member of the Xujiahe Formation of Upper Triassic in Sichuan Basin

PENG Jianlong，ZHANG Jinchuan，WEI Pengfei，LONG Shuai，HUANG Xiao，

2016, 30(4): 896-905.

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To deeply study the micropore characteristics of the 5th shale member of the Xujiahe Formation of Upper Triassic in Sichuan Basin, the microscopic pore structures of the shale reservoir were systematically studied by using Argoion polishing Scanning Electron Microscope(SEM), low temperature nitrogen adsorptiondesorption technology and related geochemical experiments. Meanwhile, the influences on the development of nanopore were discussed. The studies revealed that the micropores in shale can be classified into organic pores, inorganic pores (intergranular pores, intragranular, intercrystalline pores and dissolution pores) and the microcrack can be classified into structure microcrack, pressure crack form hydrocarbon generating and expelling, diagenetic constricted crack. Whats more, the pore structures can be mainly divided into three kinds, such as cylindrical pore with both ends open, Slit shaped holes between the parallel planes and taper pipe bore. The micropore diameters are largely ranged from 1 to 80 nm and the peak is distributed in 2 to 8 nm. The pore volume is mainly composed of mesopores (2 to 50 nm) with the proportion of 6021% (or mainly composed of the pores of clay mineral with the proportion of 4462%). In addition, organic richness and the content of clay minerals are believed to be the main controlling factors of nanopore development.

Factors Controlling the Low Radioactivity of Oil Shale in the 7th Section of Dameigou Formation of Jurassic in Northern Qaidam Basin

GUO Wang1, ZHANG Yunpeng1, LI Yonghong2, JIANG Ting1, YANG Haixing3, DANG Hongli

2016, 30(4): 905-913.

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The factors controlling the low radioactivity of oil shales in the 7th Section of Dameigou Formation of Jurassic in Northern Qaidam Basin were primarily discussed.Taking oil shales and dark shales in typical wells in Yuqia area as the research objects,the constituent, watermedium condition, radioactivitysource supply and their relation with radioactivity were analyzed in detail combined with logging data, oilyield test and the analysis of organic geochemistry, Xray diffraction and element geochemistry.The results show that, compared with dark shales, oil shales are generally characterized by the following attributes, lower GR, higher oil yield and organic matter content,stronger reducingsedimentary water, lower claymineral content and weaker source weathering.Therefore, it can be inferred that the enrichment of organic matters and the condition of anoxic water medium hardly influence the radioactivity of oil shales, however, the limited input of terrestrial clay minerals and radioactive substances, caused by the semiarid climate, may dominate the low radioactivity of oil shales in the study area.

Permeability Variation Models and Case Studies for Undersaturated Coalbed Methane Reservoirs

Lv Yumin, LIU Yinghong, TANG Dazhen, LI Zhiping

2016, 30(4): 914-921.

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Coal reservoir permeability is under very complicated change due to the combination of the effective stress effect and matrix shrinkage effect during the depletion of reservoir pressure. Nowadays a large number of models have been established to describe permeability variation with the depletion of reservoir pressure. However, no attempt has been made to draw enough attention to the difference of the effect of various factors on permeability variation in different production stages of unsaturated coalbed methane (CBM) reservoirs. This paper summaries the existing and common permeability models, and determines the relationship among various effects (effective stress effect, matrix shrinkage effect and Klinkenberg effect) and desorption characteristics of the recovery of unsaturated CBM reservoirs, then establishes two improved models to describe quantificationally permeability variation by improving the existing models, and finally discusses influence factors on permeability variation with a case study of Hancheng CBM field in Ordos Basin. Results show that the permeability variation during the recovery of unsaturated CBM reservoirs can be divided into two stages: the first one is that the permeability variation is only affected by the effective stress effect, and the second is that the permeability variation is affected by the combination of effective stress effect, matrix shrinkage effect and Klinkenberg effect. In the second stage, matrix shrinkage effect and Klinkenberg effect play much more significant role than effective stress effect, leading to the increase of permeability with depletion of reservoir pressure. A comparative study of the improved models indicates that the improved SD model is more sensitive to various parameters than the improved PM model and the improved models, and can describe dynamic permeability variation more exactly than the original ones. Thus, the case study, by improved SD model, on Hancheng CBM field shows that the increase in coal reservoir permeability is determined by gas saturation, and the damage in coal reservoir permeability is controlled by the difference between the initial reservoir pressure and critical desorbed pressure, and the descending order of the average increment is 3#, 11# and 5#.

Formation Behaviors of Methane Hydrate in Porous Media below Freezing Point

ZHANG Yu, LI Xiaosen, CHEN Zhaoyang, YAN Kefeng

2016, 30(4): 922-928.

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The hydrate formation in porous media below freezing point is a complicated multiphase transition process, including water, ice, hydrate and bound water. In order to study the formation behaviors of methane hydrate in porous media below freezing point, such as water conversion, gas consumption and stable conditions, the formation experiments of methane hydrate in porous media were carried out. The silica gels with the mean pore diameters of 1295 nm, 1796 nm and 3320 nm were used as the porous media for the experiments. The experimental results indicate that the water conversion in the porous media after the hydrate formation increases with the increase of the initial formation pressure, the decrease of the bath temperature and the increase of the mean pore diameter. The particle size of the porous media reduces the formation rate of the hydrate but has a slight effect on the final gas consumption. At the same experimental temperature, the system pressure after the hydrate formation, the final gas consumption and the final water conversion increase with the increase of the initial formation pressure. The differences of final system pressure and water conversion in the different initial pressure experiments increase with the formation temperature. Due to the effect of the capillary force and the bound water in porous media, the water conversion of the hydrate formation significantly becomes lower. At the experimental conditions in this study, the highest water conversion is up to 32.39%.

Threedimension Experimental Investigation of Hydrate Dissociation by the Combined Method with Dual Horizontal Wells

FENG Jingchun, LI Xiaosen, WANG Yi, ZHANG Yu, LI Gang

2016, 30(4): 929-936.

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Depressurization in conjunction with warm water injection with dual horizontal wells was employed to investigate the gas production performance of hydrate dissociation in a cubic threedimension simulator. Based on the process of pressure and temperature changing with time, the experiment can be divided into four sections,i.e. the free gas release stage, the quiescence stage, the depressurization stage and the injection stage. The experimental results showed that hydrates formed a second time in the free gas release stage and the quiescence stage. In the injection stage, the hydrate was dissociated under the synergistic effect of depressurization and thermal stimulation. The change of the hydrate/gas/water saturation indicates that the hydrate can be dissociated completely. In addition, the energy ratio in this study is higher than that in the previous study by the vertical well with the Huff and Puff method.

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