Groundwater mineralisation processes in Mediterranean semi‐arid systems (Cap‐Bon,North east of Tunisia): hydrogeological and geochemical approaches

The groundwater of the Korba plain represents major water resources in Tunisia. The Plio‐Quaternary unconfined aquifer of the Cap‐Bon (north‐east Tunisia) is subject to the intensive agricultural activities and high groundwater pumping rates due to the increasing of the groundwater extraction. The degradation of the groundwater quality is characterized by the salinization phenomena. Groundwater were sampled and analysed for physic‐chemical parameters: Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^‐, SO₄^2‐, HCO₃^‐, NO₃^‐, pH, electrical conductivity (EC), and the temperature (T°). The hydrochemical analysis is coupled with the calculation of the saturation indexes (SI gypsum, SI halite, SI calcite and SI dolomite), ionic derivation and with the ion correlations compared to chloride concentrations: Na⁺/ Cl^‐, Ca²⁺/ Cl^‐ and Mg²⁺/ Cl^‐ ratios. Seawater fractions in the groundwater were calculated using the chloride concentration. Those processes can be used as indicators of seawater intrusion progression. EC methods were also conducted to obtain new informations on the spatial scales and dynamics of the fresh water–seawater interface of coastal groundwater exchange. The mixing zone between freshwater and saltwater was clearly observed from the EC profile in the investigated area where a strong increase in EC with depth was observed, corresponding to the freshwater and saltwater interface. Results of hydrochemical study revealed the presence of direct cation exchange linked to seawater intrusion and dissolution processes associated with cations exchange. These results, together with EC investigation, indicated that the groundwater is affected by seawater intrusion and is still major actor as a source of salinization of the groundwater in Korba coastal plain. Further isotopic and hydrological investigations will be necessary to identify and more understood the underlying mechanisms. Copyright © 2012 John Wiley & Sons, Ltd.     

How have catch shares been allocated?

A unique database was created that describes the methods used to allocate shares in nearly every major catch share fishery in the world. Approximately 54% of the major catch share fisheries in the world allocated the Total Allowable Catch (TAC) solely on the basis of historical catch records, 3% used auctions, and 6% used equal sharing rules. The remaining 37% used a combination of methods, including vessel-based rules. These results confirm the widely-held belief that nearly all catch share programs have “grandfathered” private access to fishery resources: 91% of the fisheries in the database allocated some fraction of the TAC on the basis of historical catch. This publicly available database should be a useful reference tool for policymakers, academics, and others interested in catch shares management in Hawai'i and across the globe.     

Ground penetrating radar imaging of cap rock, caliche and carbonate strata

Field experiments show ground penetrating radar (GPR) can be used to image shallow carbonate stratigraphy effectively in a variety of settings. In south Florida, the position and structure of cap rock cover on limestone can be an important control on surface water flow and vegetation, but larger scale outcrops (tens of meters) of cap rock are sparse. GPR mapping through south Florida prairie, cypress swamp and hardwood hammock resolves variations in thickness and structure of cap rock to 3 m and holds the potential to test theories for cap rock–vegetation relationships. In other settings, carbonate strata are mapped to test models for the formation of local structural anomalies. A test of GPR imaging capabilities on an arid caliche (calcrete) horizon in southeastern Nevada shows depth penetration to 2 m with resolution of the base of caliche. GPR profiling also succeeds in resolving more deeply buried (5 m) limestone discontinuity surfaces that record subaerial exposure in south Florida.     

构造变形过程中盖层封闭性研究

研究了埋藏-成岩-抬升变形过程中盖层封闭性的演化特点,开展了构造变形过程中盖层破裂实验.认为泥岩在高演化阶段封闭性变差并不具有普遍性,在深埋条件下和构造稳定区,高演化泥岩仍可具有良好的封闭性;构造抬升是盖层封闭性发生破坏的主要因素;力学实验表明,膏盐岩盖层在极小的埋深(1 000 m左右)下就有很强的塑性,其极限强度很...     

四川盆地龙岗气田三叠系飞仙关组储集层特征及成因机理*

根据岩心、岩石铸体薄片、岩石物性分析化验资料并结合测井资料,对四川盆地龙岗气田三叠系飞仙关组碳酸盐岩储集层特征进行了研究。结果表明,研究区飞仙关组孔隙较发育、物性较好、能作为优质储集层的基本上都是白云岩类,尤其是鲕粒白云岩。初步认为这类鲕粒白云岩的形成主要与早期白云化作用（可能为混合水白云化）有关。结合区域地质背景,认为飞仙关期相对海平面呈振荡下降,其对早期白云化有重要控制作用,并在垂向上形成飞三1、飞二1两个白云化旋回,而优质储集层主要发育于这两个旋回中。     

Reactivity of dolomitizing fluids and Mg source evaluation of fault-controlled dolomitization at the Benicàssim outcrop analogue (Maestrat basin,E Spain)

The mechanisms responsible for the formation of huge volumes of dolomitized rocks associated with faults are not well understood. We present a case study for high-temperature dolomitization of an Early Cretaceous (Aptian–Albian) ramp in Benicàssim (Maestrat basin, E Spain). In this area, seismic-scale fault-controlled stratabound dolostone bodies extend over several kilometres away from large-scale faults. This work aims at evaluating different Mg sources for dolomitization, estimating the reactivity of dolomitizing fluids at variable temperature and quantifying the required versus available fluid volumes to account for the Benicàssim dolostones. Field relationships, stable ¹³C and ¹⁸O isotopes, as well as radiogenic ⁸⁷Sr/⁸⁶Sr isotopes, indicate that dolomitization at Benicàssim was produced by a high-temperature fluid (>80 °C). ¹³C and ¹⁸O isotopic compositions for dolomite vary from +0.5 to +2.9‰ V-PDB and from +21.1 to +24.3 V-SMOW, respectively. A Mg source analysis reveals that the most likely dolomitizing fluid was seawater-derived brine that interacted with underlying Triassic red beds and the Paleozoic basement. Geochemical models suggest that evolved seawater can be considerably more reactive than high-salinity brines, and the maximum reactivity occurs at about 100 °C. Mass-balance calculations indicate that interstitial fluids with high pressure and/or high temperature relative to the normal geothermal gradient cannot account for the volume of dolomite at Benicàssim. Instead a pervasive fluid circulation mechanism, like thermal convection, is required to provide a sufficient volume of dolomitizing fluid, which most likely occurred during the Late Cretaceous post-rift stage of the Maestrat basin. This study illustrates the importance of fluid budget quantification to critically evaluate genetic models for dolomitization and other diagenetic processes.     

准噶尔盆地乌夏地区二叠系风城组云质岩类成因研究

准噶尔盆地乌夏地区风城组发育的云质岩类是一套特殊成因的、重要的油气储层。岩石学、扫描电镜能谱和稳定同 位素分析表明,发生白云岩化作用的母岩是火山凝灰物质,白云岩化的流体来自风城组干旱气候条件下形成的咸化滨浅湖 卤水以及下伏佳木河组和石炭系残留的富镁海水。凝灰岩中富含大量的斜长石,其中的富钙长石在二氧化碳参与下水解蚀变, 沉淀方解石（即方解石化）。后期富镁卤水渗滤早期形成的方解石,成岩交代形成白云岩。富镁流体运移受火山喷发产生的 热流驱动,产生热对流环。同时,乌夏断裂同生逆掩活动加快深层富镁流体上侵,并使流体在层间快速渗透、流动。构造 作用产生的裂缝或断裂对白云岩的形成及优质储层分布起到决定性控制作用。     

回流渗透模式白云岩研究历史与进展*

回流渗透白云岩化是一类重要的白云岩化模式,其应用广泛,成岩储集性良好,具有重要的研究意义。近年来,国内外对回流渗透模式白云岩的研究不断深入,取得了许多新的进展。作者从回流渗透模式的研究历史出发,详细讨论了其发展和应用现状,总结了该模式下白云岩的Mg来源、水文模式和沉积岩石学特征,并着重讨论了中等盐度下的回流渗透白云岩化等新的研究和应用,指出大规模的回流渗透白云岩化作用可在中等盐度的条件下形成,其强度往往受控于海平面变化。     

渤海湾盆地塘沽地区古近系沙河街组湖相白云岩特征及成因*

渤海湾盆地塘沽地区古近系沙河街组三段沉积早期为半封闭的湖湾沉积环境,底部发育一套白云岩储集层,其产出受区域内北东向古潜山基底断裂控制,沿古潜山两侧发育,分布面积较大,累计厚度可达100多米。系统的岩石学、矿物学和地球化学特征研究表明,该套白云岩常与泥岩互层产出,白云石主要为铁白云石和含铁白云石,普通显微镜下呈微晶状,扫描电镜下可见菱面体状晶型;X射线衍射分析表明其形成于准同生期,是快速结晶的产物;碳氧同位素分析及稀土元素分析显示,这类微晶白云岩形成于较为封闭的高盐度蒸发环境中,具统一的流体来源和相同成因机理。基于上述研究成果,认为塘沽地区沙河街组三段微晶白云岩的形成主要经历了准同生期回流渗透作用和成岩期埋藏环境下类质同象交代作用两个阶段。     

The porosity origin of dolostone reservoirs in the Tarim,Sichuan and Ordos basins and its implication to reservoir prediction

Origin of dolostone remained a controversial subject, although numerous dolomitization models had been proposed to date. Because of the dolomitization’s potential to be hydrocarbon reservoirs, one debatable issue was the role of dolomitization in porosity construction or destruction. Based upon case studies of dolostone reservoirs in various geological settings such as evaporative tidal flat (Ordos Basin, NW China), evaporative platform (Sichuan Basin, SW China), and burial and hydrothermal diagenesis (Tarim Basin, NW China), here we systematically discuss the origin of porosity in dolostone reservoirs. Contrary to traditional concepts, which regarded dolomitization as a significant mechanism for porosity creation, we found two dominant factors controlling reservoir development in dolostones, i.e., porosity inherited from precursor carbonates and porosity resulted from post-dolomitization dissolution. Actually, dolomitization rarely had a notable effect on porosity creation but rather in many cases destroyed pre-existing porosity such as saddle dolostone precipitation in vugs and fractures. Porosity in dolostones associated with evaporative tidal flat or evaporative platform was generally created by subaerial dissolution of evaporites and/or undolomitized components. Porosity in burial dolostones was inherited mostly from precursor carbonates, which could be enlarged due to subsequent dissolution. Intercrystalline porosity in hydrothermal dolostones was either formed during dolo- mitization or inherited from precursor carbonates, whereas dissolution-enlarged intercrystalline pores and/or vugs were usually interpreted to be the result of hydrothermal alteration. These understandings on dolostone porosity shed light on reservoir prediction. Dolostone reservoirs associated with evaporative tidal flat were laterally distributed as banded or quasi-stratified shapes in evaporite-bearing dolostones, and vertically presented as multi-interval patterns on tops of shallowing-upward cycles. Dolostone reservoirs associated with evaporative platform commonly occurred along epiplatforms or beneath evaporite beds, and vertically presented as multi-interval patterns in dolostones and/or evaporite-bearing dolostones of reef/shoal facies. Constrained by primary sedimentary facies, burial dolostone reservoirs were distributed in dolomitized, porous sediments of reef/shoal facies, and occurred vertically as multi-interval patterns in crystalline dolostones on tops of shallowing-upward cycles. Hydrothermal dolomitization was obviously controlled by conduits (e.g., faults, unconformities), along which lenticular reservoirs could develop.