Determination of groundwater recharge regime and flowpath in the Lower Heihe River basin in an arid area of Northwest China by using environmental tracers: Implications for vegetation degradation in the Ejina Oasis

Environmental tracers (CFCs, stable isotopes ¹⁸O, ²H, and ³H) and major ions were employed to study river infiltration and groundwater recharge in the aquifer system in the basin of the Lower Heihe River, Northwest China. Three groups of waters have been recognized: (1) young groundwater, connected to the river, with large variation of CFC apparent ages ranging from <10 a to 40 a, and δ¹⁸O and δ²H values which are similar to the river water; (2) regional background water, unaffected by the river, having CFC apparent ages >40 a, and being depleted in ¹⁸O and ²H compared with the river water; and (3) groundwater in Gurinai, a grassland located about 100 km from the river, in which the predominant discharge is from the Badain Jaran desert, with CFC apparent ages ranging from 25 to >50 a and being enriched in ¹⁸O and ²H compared to the river water. The groundwater along the river contains CFCs and ³H down to depths of about 120 m, and the shallow groundwater exhibits CFC apparent ages in a wide range which are not dependent on the well depth. Groundwaters along the river show a similar trend of enrichment in ¹⁸O and ²H as the river water whereas groundwaters in depression cones are depleted in heavier isotopes, and have low CFC and ³H concentrations. The CFC apparent age of the groundwater increases with increasing distance downstream, indicating that the dominant part of the groundwater is from infiltration of river water in the upper reaches. Modifications of groundwater recharge are reflected in variations of stable isotope compositions, as well as CFC and ³H concentrations in the groundwater that was recharged from the river over the last decades. Despite recharging from river water, groundwater abstraction has induced a water balance deficit. The riparian ecosystem in the Ejina Oasis is constrained by both decreased river flow and increased groundwater abstraction. The vegetation degradation in the Ejina Oasis is controlled not only by natural aridification but also worsened by heavy groundwater abstraction and decreased river flow.     

Geochemistry of organic matter and elements of black shale during weathering in Northern Guizhou,Southwestern China: Their mobilization and inter-connection

Different from previous studies on effect of weathering upon geochemical variation along a single weathered profile, this paper provides a new methodology validated by comparing a weathered outcrop samples and their stratigraphic counterpart un-weathered core samples in a nearby shallow borehole. This outcrop and borehole penetrated the Ordovician-Silurian Wufeng–Longmaxi shales, located in the same anticline structure in the northern part of Guizhou Province, Southern China. The mineral composition, major, trace and rare earth elements (REEs) composition and Rock-Eval parameters of outcrop and core samples were analyzed and compared. Organic matter (OM) was observed in the microscope and extracted for elements analysis. The results show that short-term weathering still has significant influence on OM, mineral and elemental composition of black shales. The elements composition shows the outcrop profile was moderately weathered. The REEs compositions do not alter much during weathering process and the REEs composition and their relative ratios still are valid for rock origin determination. The OM, mainly composed by graptolite and bitumen, even entering the highly-over thermal maturity, is still sensitive to the weathering with a systematic loss 30–50% of TOC along the outcrop profile, which suggests that the OM consumption is predominantly controlled by weathering duration and the distance from the weathering surface. In turn, OM has significant influence on the trace elements transportation behavior during weathering. Some trace elements associated with the OM such as V, Cr, Th, U, Ni and Co, change significantly in their absolute concentration during weathering, but their relative ratios do not necessarily change too much and might be still reliable proxies for paleo-environmental determination. The mobility of shale minerals during weathering is in the following order: plagioclase?>?potassium feldspar and dolomite >pyrite and OM. Short-term weathering can also result in considerable transportation of elements and significant variation of minerals content in black shale, which may pose potentially high environmental and engineering risk in the regions rich in black shale.     

Water resource development and protection in loess areas of the world: a summary to the thematic issue of water in loess

Loess is mainly distributed in arid and semiarid areas of the world, and water is essential and precious in the loess areas. Water is also an important factor inducing various geohazards and soil erosion in the loess areas. The thematic issue of Environmental Earth Sciences was edited to reflect the latest research on water and water-related themes in the loess areas of the world, especially on the Chinese Loess Plateau. This editorial introduced the background and initiative of editing this thematic issue, and it also introduced briefly the water environment projects currently underway on the Chinese Loess Plateau such as the grain for green project, the huge land creation project, and the sponge city project. The papers included in the thematic issue were also introduced to give readers a general understanding of their main research points. Finally, some suggestions to boost further water environment research in the loess areas were put forward. All the papers included in the thematic issue will scientifically support the sustainable socio-economic development in the loess areas of the world.     

北祁连西段肃南地区下志留统肮脏沟组河流相的发现及其大地构造意义

北祁连地区位于中国中央造山带中段,具有典型造山带的特征.区内志留系肮脏沟组沉积环境存在较大争议,以致对其沉积期古地理格局的认识就存在很大差异.本研究选取志留系肮脏沟组发育齐全的肃南地区老虎沟剖面为研究对象,通过野外剖面实测、室内薄片鉴定和碎屑岩粒度分析等方法对老虎沟剖面肮脏沟组沉积充填序列、沉积构造和沉积相类型进行了细...     

黔北地区奥陶纪—志留纪之交黑色页岩地球化学特征及意义——以贵州道真道页1井为例

奥陶纪—志留纪之交是华南被动大陆边缘盆地向前陆盆地演化的重要转折时期,形成广泛分布的五峰组—龙马溪组黑色页岩。为进一步探讨该套黑色页岩的形成背景,以黔北地区道页1井为例,开展沉积序列和地球化学研究。研究结果表明:五峰组及龙马溪组底部主要为硅质碳质页岩,夹多层斑脱岩,U/Th为0.2～2.90,V/Cr为1.18～14.34,Ni/Co为2.31～11.59,TOC为0.68%～5.91%,平均3.76%,稀土配分曲线为平坦型,δEu以弱正异常为主、个别为弱负异常;龙马溪上部及新滩组主要由泥岩及钙质泥岩组成,U/Th为0.16～0.23,V/Cr为0.88～1.79,Ni/Co为1.77～3.91,TOC为0.1%～0.84%,δEu以明显负异常为主,表现出低斜率右倾型稀土配分曲线。以上特征表明五峰组及龙马溪组底部形成于间歇性缺氧的还原环境,龙马溪组上部及新滩组为富氧环境。结合斑脱岩的分布特征综合分析认为,五峰组及龙马溪组底部页岩是汇聚背景下前陆盆地早期沉积的产物,强烈火山作用给大陆边缘海带来营养元素,刺激初级生产力的增加,造成水体迅速缺氧,可能是造成同期海底缺氧的主要诱因。     

南海北部天然气水合物富集特征及定量评价

为寻找有资源前景的高富集天然气水合物及水合物储层的精细刻画方法,利用南海6次钻探发现的高饱和度水合物层的测井、岩心和三维地震数据,分析水合物富集层测井与地震异常特征.发现:（1）不同饱和度的孔隙与裂隙充填型水合物层的测井和地震异常不同,裂隙充填型水合物层具有各向异性;（2）受高通量流体运移的影响,在粉砂沉积物的水合物稳定带底界附近能形成中等饱和度的水合物层,识别标志为稳定带内极性与海底一致的强振幅反射,而非BSR和振幅空白;（3）裂隙充填型中等饱和度水合物层在地震剖面上表现为地层上拱和弱-中等强度振幅反射.储层-疏导-气源的耦合控制着水合物的富集特征和分布,断层与流体运移控制着细粒粉砂质沉积物中水合物的富集与厚度.基于饱和度岩相的统计学反演,能识别3 m非水合物和低饱和度水合物层及空间分布.      

川滇黔邻区龙马溪组页岩气富集主控因素与勘探方向

中国的页岩气资源评价必须立足于中国主要陆块的地质条件与构造演化特点之上,对于不同层系、不同地区的页岩气选区评价,也必须抓住关键因素。川滇黔邻区的五峰组-龙马溪组富有机质页岩发育,但保存条件较渝东南地区更为复杂。印支期以来,该区经历了多期次构造应力场转换,断裂、褶皱发育,因此,建立针对性的页岩气选区评价参数体系是该区页岩气勘探走出困局的当务之急。本文在分析该区已有页岩气钻井成效的基础上,立足于扬子海相层系页岩气勘探应是基于保存条件的"连续型甜点区"的选区评价基本思路,结合前人研究成果,系统梳理出一套评价指标体系。该体系涵盖5大因素(区域盖层、直接盖层、底板条件、页岩层自身条件、构造改造强度)、4个等级、17个选区评价参数。另外,根据已有相关钻井数据,总结提出了5个页岩气层评价表征参数(含气量、孔隙度、电阻率、氮气组分含量和压力系数)。根据该指标体系,对川滇黔邻区的主要构造单元进行了梳理分析,优选出5个有利区,提出了各有利区的首选可能突破点。本研究对复杂构造区页岩气有利区带优选具有一定的指导意义。     

准噶尔盆地腹部侏罗系地层超压成因机制——以东道海子北凹陷为例

准噶尔盆地腹部地区侏罗系地层超压普遍发育,超压层段与声波时差、电阻率两者之间具有较好的响应关系,尤以电阻率响应最为显著,超压层段声波时差表现为异常高值,电阻率为异常低值的特征。利用准噶尔盆地腹部地区大量测井、测试及相关地质资料综合论述超压成因,指出泥岩和砂岩中发育的超压并非由压实不均衡、构造挤压等作用导致,而主要由生烃作用造成的。研究表明有以下几方面主要证据：1）超压泥岩密度不具有异常低值特征;2）超压段泥岩密度与岩石颗粒垂直有效应力之间不存在线性关系;3）超压砂岩不存在异常高地温梯度现象;4）超压顶界面深度范围为3 800～4 300 m,侏罗系烃源岩现今成熟度（Ro%）范围为0.6%～0.9%,温度范围分布在90 ℃～125 ℃;5）超压层段的侏罗系烃源岩现今生烃转化率最高可达70%,现今平均生油率可达90 mg/g·TOC,仍具有较强的生烃能力;6）泥质烃源岩本身对超压具有很好的封闭作用;7）侏罗系烃源岩内发育大量低角度或高角度微裂缝;8）超压储层主要以油层为主,无水层,准噶尔盆地东道海子北凹陷砂岩中超压主要是通过烃源岩中排出的高压流体运移至储层发生超压传递的结果。     

江西雅山和下桐岭钨矿床辉钼矿Re-Os年龄及其地质意义

雅山钨矿(W-Mo)和下桐岭钨钼铋铍矿(W-Mo-Bi-Be)是江西武功山地区典型的石英脉型钨矿床,二者的成矿元素组合略有差异,但二者的成矿时代是否存在差异尚不十分清楚.为了确定二者成矿年代,通过选取雅山钨矿和下桐岭钨矿石英脉中与黑钨矿伴生的辉钼矿进行了Re-Os同位素定年,获得了雅山钨矿3个辉钼矿Re-Os同位素等时线年龄和模式加权平均年龄分别为156.0± 2.8 Ma和156.0± 2.4 Ma;下桐岭钨矿4个辉钼矿Re-Os同位素等时线年龄和模式加权平均年龄分别为156.4± 8.7 Ma和156.1± 2.1 Ma;在误差范围内,说明二者的成矿时代基本一致.研究表明,雅山钨矿和下桐岭钨矿床与武功山地区浒坑钨矿成矿年代一致,与华南地区大规模钨锡成矿作用的时代基本一致(170 ~ 150 Ma),均形成于晚侏罗世.