Hydrochemistry and H-O-C-S Isotopic Geochemistry Characteristics of Geothermal Water in Nyemo-Nagqu,Tibet

Nyemo-Nagqu, Tibet, is rich in high-temperature geothermal resources. The geothermal fields in Yangbajain and Yangyi as well as 11 unexplored geothermal fields along the geothermal belt from Nyemo to Nagqu were systematically investigated and the hydrochemistry data were collected from the whole field. Meanwhile, H-O-C-S isotope data were obtained for the new fields, and H-O isotope data for the Yangbajain and Yangyi fields. A comparison of the Nyemo-Nagqu geothermal fields with those in the Yangbajain area shows that the types of high-temperature geothermal water are dominated by Cl-Na and Cl·HCO3-Na, while the types of medium-high-temperature geothermal water are dominated by HCO3-Na. The concentrations of Li, F, SiO2, and HBO2 in the geothermal water are positively correlated with Cl content, indicating possible mixing with magma water. The reservoir temperatures range from 90 to 270°C by geothermometers. Slight drifting of 18O was recorded at the Dongweng and Nyingzhong high-temperature geothermal fields, while more significant drifting was recorded at Gulu. The geothermal water is mainly replenished by atmospheric precipitation. The low tritium contents（〈1 TU） of the geothermal water from Nyingzhong, Gulu, and Luoma indicate that it is mainly replenished by sub-modern（prior to 1952） water, while the high tritium content（8.4 TU） in Yuela implies modern water replenishment. Other geothermal fields are replenished by a mix of sub-modern fresher water. The isotopic data in this study show that the carbon and sulfur in the geothermal water originates mainly from sediment leaching, with some of the carbon and sulfur having a deep origin.

Zircon U-Pb age, Lu-Hf Isotopic Characteristics and Origin of the Banshanping Granitoid Rocks in East Qinling Orogenic Belt

The Banshanping granitoid rocks distribute in the east of the North Qinling orogenic belt.It is a diorite-quartz diorite-granodiorite-granite series,spreading in a NW-SE direction,and intrudes into the Erlangping Group.The SiO2 content ranges from 57.04% to 76.56%,Na2O from 2.05% to 4.65%,K2O from 0.84% to 3.40%.Major element characteristics indicate that Banshanping granitoid rocks have properties of I type granotoids.SREE ranges from 36.51 ppm to 473.25 ppm,and LREE/ HREE ratios lie between 3.95 and 22.18.Negative Eu anomalies are not obvious in most samples,though there are obvious Nb,P and Ti positive anomalies.The zircon LA-ICP-MS ages of Banshangping granitoid rocks are 496.0±8.1 Ma-486.9±9.3 Ma.Hf isotope shows that 176Hf/177Hf ratios range from 0.282721 to 0.282876,εHf（t） values from 8.5 to 14,all positive,and corresponding modal ages （TDM2） range from 559 Ma to 908 Ma.Based on Hf isotope characteristics and existing SmNd and Rb-Sr isotope data,we consider that the Banshanping granitoid rocks originate from mantlederived material,i.e.the igneous rocks that formed in Neoproterozoic,and there may be a certain amount of crust-derived material during the formation of Banshanping granitoid rocks.

Geochemistry, Zircon U–Pb Dating and Hf Isotopic Characteristics of Neoproterozoic Granitoids in the Yaganbuyang Area, Altyn Tagh, NW China

The South Altyn continental block is an important geological unit of the Altyn Tagh orogenic belt, in which numerous Neoproterozoic granitoids crop out. Granitoids are mainly located in the Paxialayidang-Yaganbuyang area and can provide indispensable information on the dynamics of Rodinia supercontinent aggregation during the Neoproterozoic. Therefore, the study of granitoids can help us understand the formation and evolutionary history of the Altyn Tagh orogenic belt. In this work, we investigated the Yaganbuyang granitic pluton through petrography, geochemistry, zircon U-Pb chronology, and Hf isotope approaches. We obtained the following conclusions： （1） Yaganbuyang granitoids mainly consist of two-mica granite and granodiorite. Geochemical data suggested that these granitoids are peraluminous calc-alkaline or high-K calc-alkaline granite types. Zircon U-Pb data yielded ages of 939~7.1 Ma for granodiorite and ~954 Ma for granitoids, respectively. （2） The ~Hf（t） values of two--mica granite and granodiorite are in the range of-3.93 to ＋5.30 and -8.64 to ＋5.19, respectively. The Hf model ages （TDM2） of two-mica granite and granodiorite range from 1.59-.05 Ga and 1.62-2.35 Ga, respectively, indicating that the parental magma of these materials is derived from ancient crust with a portion of juvenUe crust. （3） Granitoids formed in a collisional orogen setting, which may be a response to Rodinia supercontinent convergence during the Neoproterozoic.

Sr and Nd Isotopic Characteristics and Magma Genesis of Mesozoic Volcanic Rocks Along the Coastal Region of Southeastern China

The Mesozoic volcanic rocks in the coastal region of southeastern China were superimposed on some different basement tectonic elements. The volcanic rocks developed in these different basement tectonic elements have great differences in Sr and Nd isotopic compositions. The rocks in western Zhejiang and northeastern Jiangxi Provinces which belong to the Lower Yangtze subplate have lower initial 87Sr/ 86Sr ratios, but are higher in initial Nd isotopic ratios. The initial 143Nd / 144Nd values of the volcanic rocks developed in the Cathaysian subplate increase clearly from early to late in time, and from the core of the Wuyishan uplift coastwards constantly, but the initial 87Sr/86Sr values tend to decrease. The isotopic characteristics and their spatial variations in Mesozoic volcanic rocks in the study region are, to a great extent, manifestations of the isotopic characteristics in basement metamorphic complexes, and the generation of the Mesozoic acid magma in this region is attributed to the recycling of pre-existing crustal materials.

Geochemical Characteristics of Pb Isotope of High-Pressure Metamorphic Rocks and Foliated Granites from HP Unit of Tongbai-Dabie Orogenic Belt

Whole-rock Pb isotopic compositions of the high-pressure (HP) metamorphic rocks, consisting of two-mica albite gneisses and eclogites, and foliated granites from the HP metamorphic unit of the Tongbai-Dabie orogenic belt are firstly reported in this paper. The results show that the HP metamorphic rocks in different parts of this orogenic belt have similar Pb isotopic compositions. The two- mica albite gneisses have 206 Pb/ 204 Pb=17.657-18.168, 207 Pb/ 204 Pb=15.318-15.573, 208 Pb/ 204 Pb=38.315-38.990, and the eclogites have 206 Pb/ 204 Pb=17.599-18.310, 207 Pb/ 204 Pb=15.465- 15.615 , 208 Pb/ 204 Pb=37.968-39.143. The HP metamorphic rocks are characterized by upper crustal Pb isotopic composition. Although the Pb isotopic composition of the HP metamorphic rocks partly overlaps that of the ultrahigh-pressure (UHP) metamorphic rocks, as a whole, the former is higher than the latter. The high radiogenic Pb isotopic composition for the HP metamorphic rocks confirms that the subducted Yangtze continental crust in the Tongbai-Dabie orogenic belt has the chemical structure of increasing radiogenic Pb isotopic composition from lower crust to upper crust. The foliated granites, intruded in the HP metamorphic rocks post the HP/UHP metamorphism, have 206 Pb/ 204 Pb=17.128-17.434, 207 Pb/ 204 Pb=15.313-15.422 and 208 Pb/ 204 Pb=37.631-38.122, which are obviously different from the Pb isotopic compositions of the HP metamorphic rocks but similar to those of the UHP metamorphic rocks and the foliated garnet-bearing granites in the UHP unit. This shows that the foliated granites from the HP and UHP units have common magma source. Combined with the foliated granites having the geochemical characteristics of A-type granites, it is suggested that the magma for the foliated granites in the UHP and HP unit would be derived from the partial melting of the retrometamorphosed UHP metamorphic rocks exhumed into middle to lower crust, and partial magmas were intruded into the HP unit.

Indication of hydrogen and oxygen stable isotopes on the characteristics and circulation patterns of medium-low temperature geothermal resources in the Guanzhong Basin,China

Guanzhong Basin is a typical medium-low temperature geothermal field mainly controlled by geo-pressure in the west of China.The characteristics of hydrogen and oxygen isotopes were used to analyze the flow and storage modes of geothermal resources in the basin.In this paper,the basin was divided into six geotectonic units,where a total of 121 samples were collected from geothermal wells and surface water bodies for the analysis of hydrogen-oxygen isotopes.Analytical results show that the isotopic signatures of hydrogen and oxygen throughout Guanzhong Basin reveal a trend of gradual increase from the basin edge areas to the basin center.In terms of recharge systems,the area in the south edge belongs to the geothermal system of Qinling Mountain piedmont,while to the north of Weihe fault is the geothermal system of North mountain piedmont,where the atmospheric temperature is about 0.2℃-1.8℃in the recharge areas.The main factors that affect the geothermal waterδ18O drifting include the depth of geothermal reservoir and temperature of geothermal reservoir,lithological characteristics,water-rock interaction,geothermal reservoir environment and residence time.Theδ18O-δD relation shows that the main source is the meteoric water,together with some sedimentary water,but there are no deep magmatic water and mantle water which recharge the geothermal water in the basin.Through examining the distribution pattern of hydrogen-oxygen isotopic signatures,the groundwater circulation model of this basin can be divided into open circulation type,semi-open type,closed type and sedimentary type.This provides some important information for rational exploitation of the geothermal resources.

Characteristics of Authigenic Pyrite and its Sulfur Isotopes Influenced by Methane Seep--Taking the Core A at Site 79 of the Middle Okinawa Trough as an Example

Objective Authigenic pyrite often develops extensively in marine sediments,which is an important product of sulfate reduction in an anoxic environment.It has a specific appearance and complicated sulfur isotopic properties,and acts as important evidence of methane seep in marine sediments.Strong AOM（anaerobic oxidation of methane）activity has developed in the Okinawa Trough.

Leaf stable carbon isotope composition in Picea schrenkiana var. tianschanica in relation to leaf physiological and morphological characteristics along an altitudinal gradient

To understand the effects of leaf physiological and morphological characteristics on δ13C of alpine trees, we examined leaf δ13C value, LA, SD, LNC, LPC, LKC, Chla+b, LDMC, LMA and Narea in one-year-old needles of Picea schrenkiana var. tianschanica at ten points along an altitudinal gradient from 1420 m to 2300 m a.s.l. on the northern slopes of the Tianshan Mountains in northwest China. Our results indicated that all the leaf traits differed significantly among sampling sites along the altitudinal gradient(P<0.001). LA, SD, LPC, LKC increased linearly with increasing elevation, whereas leaf δ13C, LNC, Chla+b, LDMC, LMA and Narea varied non-linearly with changes in altitude. Stepwise multiple regression analyses showed that four controlled physiological and morphological characteristics influenced the variation of δ13C. Among these four controlled factors, LKC was the most profound physiological factor that affected δ13C values, LA was the secondary morphological factor, SD was the third morphological factor, LNC was the last physiological factor. This suggested that leaf δ13C was directly controlled by physiological and morphological adjustments with changing environmental conditions due to the elevation.

Characteristics of Volcanic Rocks in the Shoshonite Province, Eastern China, and Their Metallogenesis

The shoshonite province in eastern China is characterized by extensive distribution (ca. 80000 km2) of Late Jurassic-Early Cretaceous (165-100 Ma) shoshonite series with subordinate high-K calc-alkali series. It was formed in a dominantly tensile stress field. In comparison with their analogues in island arcs and active continental margins in other countries, the volcanic rocks in the shoshonite province have their specific characteristics in petrology, mineralogy and geochemistry as well as related mineralization association, which are the comprehensive reflection of the special composition and structure of the mantle and crust of the province and the special Mesozoic regional tectonic setting.

Magma Origin and Evolution of Tengchong Cenozoic Volcanic Rocks from West Yunnan,China:Evidence from Whole Rock Geochemistry and Nd-Sr-Pb Isotopes

Tengchong Cenozoic volcanics that have record key information on the tectonic evolution and mantle features of the southeast margin of the Tibetan Plateau are of great importance because of its unique eruption history spanning the entire Quaternary period. Magma origin and evolution of Tengchong Cenozoic volcanic rocks were studied on the basis of Nd-Sr-Pb isotope and major and trace element data from different eruptions in the Ma＇anshan area. Different samples within one eruption show relative identical lithologies, chemical and isotopic compositions. However, the geochemical features for the five eruptions are distinct from each other. These volcanic rocks show low Mg# values （〈45）, moderate to high fractionation of LREEs and HREEs, and enrichment of Pb and Ba and depletion of Nb. Tengchong Cenozoic volcanic rocks were derived from an enriched mantle based on Nd-Sr-Pb isotopic studies. And lines of evidence show that crustal contamination should be involved before the eruption of different periods of Tengchong Cenozoic volcanic rocks. Older subducted components may be responsible for adakite recycling at various stages of evolution, which results in the origin of the enriched mantle source magma accounting for the isotopic features of Tengchong Cenozoic volcanic rocks. Segregated primitive magma pulsating injected into magma chamber, fractional crystallized and contaminated with crust component. Finally, magmas with distinct chemical and isotopic compositions for each eruption formed. The extension of the northeast segment of the Yingjiang tectonic belt triggered the pulsating eruption of the Cenozoic volcanics in the Tengchong area.

Research on basic characteristics of ~2H, ^(18)O and ^(14)C in geothermal fluid in Guangdong Province, China

In general, previous geothermal geochemical studies in Guangdong Province mainly involves single method to cover limited aspects and areas. In that way, various methods available cannot actually provide more convincing results of geothermal fluid's circulation system and evolution process from different dimensions, especially in terms of isotope. As a result, more comprehensive researches remain to be done on geochemistry of geothermal fluid, in particular, the space-time law of isotope's evolution pattern as well as recharge cycle. Based on data of environmental isotopes(~2H and ^(18)O) and the isotope of radiometric dating(^(14)C), geothermal geology, characteristics of groundwater flow field and types of goethermal reservior in Guangdong Province are taken into account in this paper, so as to analyze numerical rule and spatial distribution features of isotopes. Thus, corresponding main causes, mechanism and hydrogeological significance can be revealed to further study the potential of geothermal fluid to renew and recharge in the long run, which is conducive to enrich geothermal theories and solve existing hydrogeological problems.

Zircon U-Pb age, Trace Element, and Hf Isotopic Compositions of Nordmarkite in the Lizhuang Rare Earth Element Deposit in the Western Margin of the Yangtze Block

The Mianning-Dechang （MD） rare earth element （REE） belt, located in the northern Kangdian axis （KDA） in the western margin of the Yangtze platform, is one of the most economically significant REE mineral belts in China. REE mineralization is associated with Himalayan carbonatite- alkaline complexes. The Lizhuang nordmarkite occurred in the northern part of the MD REE belt. The majority of zircons from the Lizhuang nordmarkite are characterized by pronounced positive Ce yet slightly negative Eu anomalies and high U/Yb. Moreover, all zircons have stable Hf isotopic compositions with initial 176Hf/177Hf ratios ranging from 0.282739 to 0.282808 and an average value of 0.282773. The negative Lu/Hf and positive till（t） range from -0.98 to -0.94 （average value of -0.96） and from -0.56 to 1.89 （the majority is positive, with an average of 0.66）, respectively. These characteristics indicate that the rock is derived from an enriched mantle and subducted material. LA-ICP-MS analysis of the zircons from the intrusion yields a weighted mean 2~6pb/23SU age of 28.57~0.61Ma. During this period, the tectonic activity in the KDA is not plate subduction but an intraplate tectonic exhibiting fold-thrust and strike-slip behaviors in the western marginal zone of the Yangtze platform （WMYB）. We suggest the possibility of an existing eastward old slab subduction under WMYB combined with a regional tectonic evolution. The Lizhuang nordmarkite may be derived from an enriched mantle beneath the western part of the Yangtze craton, which originated from the remelting of the Tethys subducting slab, because of the Himalayan strike-slip that formed a special type of REE deposit called strike-slip-type REE deposits.

Geochemical characteristics of abiogenic alkane gases

Recently abiogenic alkanes have been found in various locations in the world and other celestial bodies. The chemical composition of abiogenic alkane gases varies widely. The content of methane is low and nearly no C2+ is found in the abiogenic alkane gases from fluid inclusions in volcanic rocks or hot springs in China. In the unsedimented submarine hydrothermal vent system C1/C2+ ratios are much greater than those for the thermogenic gases, mostly >800 and in some cases up to 8,000. In the Songliao Basin, China, C1/C2+ of some abiogenic gases are often less than 150. Abiogenic alkane gases which have been found in nature often have carbon isotopic reversal among C1–C4 alkanes (δ13C1>δ13C2>δ13C3>δ13C4), whereas both regular and reversed hydrogen isotope distribution pattern among C1–C4 alkanes have been reported. The δ13C of abiogenic methane is mainly greater than ?30‰ though laboratory synthesized methane can have δ13C as low as ?57‰, and its δD1 values vary widely and overlap with biogenic gases. High 3He/4He ratios often indicate the addition of mantle-derived helium and are related to abiogenic gases. However, some biogenic gases can also have high 3He/4He ratios up to 8. The CH4/3He end-member is often lower than 106 for abiogenic alkane gases while greater than 1013 for biogenic gases, and the values between these two end-members often reflect the mixing of biogenic and abiogenic gases.

Acidolysis hydrocarbon characteristics and significance of sediment samples from the ODP drilling legs of gas hydrate

To study on the significance and basis of acidolysis index to China marine gas hydrate exploring, since 2006, 111 samples derived from Leg 164 and 204 of the Ocean Drilling Program （ODP） were analyzed in the experiment center of China Petroleum Exploration Research Institute to obtain data on acidolysis hydrocarbon index and methane carbon isotopes by the gas chromatography （GC） of PE AutoSystem XL and isotope mass spectrometer （IRMS） of Finnigan MAT25 I. Through these, we study the reliability of the acidolysis method and characterize the gas hydrate potential. The results show that the acidolysis hydrocarbon index has a stable correspondence with the Gas Hydrate Stability Zone （GHSZ） in the ODE and that there are clear abnormal signs in shallow samples that might reliably reflect the existence of authigenic carbonate caused by hydrocarbon migration from bottom hydrate. We therefore propose that the ability to characterize the acidolysis hydrocarbon is crucial to submarine gas hydrate exploration in China.

Grain-size Characteristics and Climate Variability in TMS5e Sequence of Tumen Section in Southern Tengger Desert,Northwestern China

The TMS5e sequence from the Tumen section,at the southern edge of Tengger Desert in the northwestern China,is synchronous with Marine Isotope Stage 5e(MIS5e).It consists of 16 layers of aeolian dune sands,11 layers of lacustrine loess-like facies,and 5 layers of lacustrine facies.The results of grain-size analysis shows that the palaeo-mobile dune sands,palaeo-fixed to semi-fixed dune sands and loess-like sandy loams are mainly composed of sands,ranging from 70% to 96%;their silt contents ranged from 4% to 20%,and their clay contents ranged from 1% to 5%;the climate under which the aeolian dune sands were deposited is similar to that under which modern mobile dune sands form,which is caused by the dominance of the cold,dry East Asian winter monsoon.In contrast,the lacustrine loess-like facies and lacustrine facies had a lower sand contents than those of the three aeolian dune sands,but have higher silt and clay contents,most of their sand content ranged from 30% to 60%,their silt contents ranged from 35% to 55%,and their clay contents ranged from 6% to 20%.The lacustrine loess-like facies and lacustrine facies formed under the influence of the warm,humid East Asian summer monsoon based on their similarity with modern sediments.The grain-size indicator Mz(mean grain diameter) and the SC/D value in the TMS5e sequence indicate climatic insta-bility at the southern edge of the Tengger Desert during MIS5e,with at least 14 fluctuations between a warm,humid climate and a cold,dry climate,divided into five stages:TMS5e5(139 kyr to 129.3 kyr B.P.),TMS5e4(129.3 kyr to 124 kyr B.P.),TMS5e3(124 kyr to 119.5 kyr B.P.),TMS5e2(119.5 kyr to 116.5 kyr B.P.),and TMS5e1(116.5 kyr to 113.7 kyr B.P.).These correspond roughly to MIS5e5,MIS5e4,MIS5e3,MIS5e2,and MIS5e1,respectively,in the GRIP ice core data.

瓯江流域下游温州平原地下水化学演化特征、控制因素及对人体健康的影响

【研究目的】滨海地带地下水化学演化特征及其控制因素研究对沿海城市地下水资源可持续利用具有重要意义。【研究方法】在野外调查取样和历史资料综合分析的基础上,运用水化学图解、离子比例关系、多元统计分析及环境同位素方法,系统分析了温州平原地下水化学演化特征,探讨了影响地下水化学演化的主要控制因素。【研究结果】(1)温州平原潜水以淡水为主,HCO_(3)^(-)、Na^(+)、Ca^(2+)占主导地位,承压水以微咸水和咸水为主,Cl^(-)、Na^(+)占绝对优势;(2)从山前到海积平原,研究区潜水由低矿化度的HCO3型水向较高矿化度的Cl型水转变,主要受天然水化学作用控制,人类活动使局部地区地下水化学出现异常;(3)十年尺度上,潜水主要组分含量存在一定程度下降,水化学类型向Cl^(-)比重减少、HCO_(3)^(-)比重增加的方向演化;(4)水岩相互作用、海水作用、氧化还原环境等自然因素和工农业生产、生活排污等人为因素是温州平原地下水化学演化的主要控制因素。【结论】地下水健康风险评价结果显示,区内地下水存在一定的潜在非致癌风险,潜水的非致癌风险小于承压水,饮水途径摄入是危害人体的主要途径,相同环境下儿童的非致癌风险高于成人。因此,有必要对存在健康风险的地下水进行长期监测,加强这类地区的地下水资源管理和污染防治。

松辽盆地天然气中稀有气体地球化学特征及其地质意义

稀有气体地球化学已运用于地球深部物质及天体陨石研究,随着测量技术的进步,在天然气研究中也开始得到越来越多的应用。使用目前最先进的稀有气体质谱仪,对中国东部松辽盆地中深部天然气的稀有气体全组分丰度及同位素进行了系统的分析,明确了断陷盆地稀有气体的组成特征。研究表明,天然气中稀有气体的丰度从轻稀有气体到重稀有气体具有逐渐减小的反“厂”字形趋势。稀有气体的同位素比值^(3)He/^(4)He为2.64×10^(-6)、^(20)Ne/^(22)Ne为9.94、21Ne/^(22)Ne为0.02922、^(40)Ar/^(36)Ar为743.7,均高于大气值,^(80)Kr、^(84)Kr、^(86)Kr、^(131-136)Xe均表现出相对大气过剩的特征,显示天然气中有幔源气体的混入。综合稀有气体的组成特征,说明松辽盆地中深部天然气均为壳幔混源的无机成因,而且不同构造区块、不同类别天然气的幔源组分贡献存在一定差异。对比稀有气体组成与天然气类型发现,轻稀有气体可以较好地区分天然气的类型,而重稀有气体的组成在各种天然气中没有明显差别。稀有气体同位素除了可以示踪天然气来源外,还可以应用在判别天然气成因、区分天然气类型、表征大地构造等方面。

赣南铁山垅钨矿田花岗斑岩地球化学特征、锆石U-Pb年龄及Hf同位素特征

赣南铁山垅钨矿田位于南岭钨锡多金属成矿带东段,已探获黑钨矿资源量超10万吨。铁山垅复式岩体包括主体似斑状黑云母花岗岩和补体细粒二云母花岗岩两部分,花岗斑岩呈脉状分布。矿田内花岗岩具有相似的地球化学特征,都属过铝质高钾钙碱性花岗岩类,表现出高硅、富铝、富碱、高钾、富成矿元素(W、Sn、Cu、Mo)和亏损Ba、Sr、Ti、P、REE、Eu,稀土配分曲线呈典型的“海鸥式”分布和M型四分组效应等特征。利用LA-ICP-MS锆石U-Pb定年方法获得花岗斑岩^(206)Pb/^(238)U年龄为146.7±0.5 Ma(MSDW=0.5),成岩时代属晚侏罗世。锆石的n(^(176)Lu)/n(^(177)Hf)=0.000973~0.001989,f_(Lu/Hf)=-0.97~-0.94,ε_(Hf)(t)=-17.9~-10.3,二阶段模式年龄(TDM2)为1.86~2.33 Ga,显示原岩为古元古代地壳。综合分析认为,铁山垅矿田岩浆活动可划分为170~155 Ma、155~150 Ma、150~145 Ma三个阶段,钨锡矿成矿主要集中在第二阶段,且第三阶段花岗斑岩与铜多金属矿成矿关系密切,推测铜岭矿区深部具有较大的找矿前景。

海相沉积岩中钙质结核成因综述

目的由自生碳酸盐矿物集合体构成的钙质结核是深时海相地层中较为常见的沉积构造,因其特殊的生长条件,已成为研究沉积环境、古气候和沉积物孔隙流体演化的重要载体。为深入了解不同钙质结核的成因,突出其地质意义,方法本文介绍了不同成因类型钙质结核的宏观形态、微观组构和同位素等特征,并对其成岩时限、生长模式和涉及的生物地球化学过程进行了综述。结果钙质结核可形成于准同生沉积阶段、早期成岩阶段和晚期成岩阶段,包括同心圈层状生长和弥漫性生长两种生长模式。钙质结核的形成机制主要包括3种:(1)在厌氧环境中,沉积物中的有机质为硫酸盐还原菌提供电子供体,发生氧化,生成碳酸氢根,导致孔隙水中碱度升高和钙质结核的形成;(2)在硫酸盐还原菌和甲烷氧化古菌共同作用下,冷泉流体中的甲烷发生厌氧氧化作用,孔隙水中过饱和的碳酸盐发生沉淀,形成钙质结核;(3)深埋藏环境下,孔隙水中的流体超压形成的晚期成岩构造,以发育“牛肉”和“锥中锥”为特征。结论目前对参与钙质结核形成的微生物过程已有了较为深入的认识,但仍需更多高分辨率原位分析,以揭示不同的生物地球化学过程(如有机质矿化作用、硫酸盐还原与甲烷厌氧氧化耦合作用等)如何控制结核生长,并对其成因机制作进一步探究。

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文中以地震、钻井、岩心、气体组分及同位素等资料为基础,分析了川西地区不同构造中三叠统雷口坡组的气藏特征,划分了天然气来源,明确了油气成藏模式。研究表明:根据天然气组分特征差异,可将川西地区雷口坡组气藏划分为烷烃体积分数略低、H2S和CO_(2)等非烃体积分数略高的山前隐伏构造带,以及烷烃体积分数略高、H2S和CO_(2)等非烃体积分数略低的山前坳陷带;根据天然气烷烃同位素特征差异,山前隐伏构造带可划分为δ13C2偏轻、同位素倒序分布的石羊镇、鸭子河构造和δ13C2偏重、同位素正序分布的金马构造;山前坳陷带可划分为δ13C2偏轻、碳同位素值正序分布的新场构造和δ13C2偏重、碳同位素值正序分布的永兴、马井构造。基于天然气运移条件和成藏模式分析认为:石羊镇、鸭子河构造天然气来源于龙潭组和雷口坡组烃源岩;金马、马井构造天然气来源于龙潭组、雷口坡组和马鞍塘组烃源岩;新场构造天然气来源于雷口坡组烃源岩,顶部部分层段由马一段石灰岩供烃;永兴构造天然气来源于马二段页岩。油气成藏模式和断层发育规模是控制川西地区不同构造雷口坡组气藏特征及天然气来源差异的主要因素。