Sedimentology of the Cretaceous Maha Sarakham evaporites in the Khorat Plateau of northeastern Thailand

Evaporites of the Cretaceous to early Tertiary Maha Sarakham Formation on the Khorat Plateau of southeast Asia (Thailand and Laos) are composed of three depositional members that each include evaporitic successions, each overlain by non-marine clastic red beds, and are present in both the Khorat and the Sakon Nakhon sub-basins. These two basins are presently separated by the northwest-trending Phu Phan anticline. The thickness of the formation averages 250 m but is up to 1.1 km thick in some areas. In both basins it thickens towards the basin centre suggesting differential basin subsidence preceding or during sedimentation. The stratigraphy, lithological character and mineralogy of the evaporites and clastics are identical in both basins suggesting that they were probably connected during deposition. Evaporites include thick successions of halite, anhydrite and a considerable accumulation of potassic minerals (sylvite and carnallite) but contain some tachyhydrite, and minor amounts of borates. During the deposition of halite the basin was subjected to repeated inflow of fresher marine water that resulted in the formation of anhydrite marker beds. Sedimentary facies and textures of both halite and anhydrite suggest deposition in a shallow saline-pan environment. Many halite beds, however, contain a curious `sieve-like' fabric marked by skeletal anhydrite outlines of gypsum precursor crystals and are the product of early diagenetic replacement by halite of primary shallow-water gypsum. The δ³⁴S isotopic values obtained from different types of anhydrite interbedded with halite range from 14.3‰ to 17.0‰ (CDT), suggesting a marine origin for this sulphate. Bromine concentration in the halite of the Lower Member begins around 70 ppm and systematically increases upward to 400 ppm below the potash-rich zone, also suggesting evaporation of largely marine waters. In the Middle Member the initial concentration of bromine in halite is 200 ppm, rising to 450 ppm in the upper part of this member. The bromine concentration in the Upper Member exhibits uniform upward increase and ranges from 200 to 300 ppm. The presence of tachyhydrite in association with the potassic salts was probably the result of: (1) the large volumes of halite replacement of gypsum, on a bed by bed basis, releasing calcium back into the restricted waters of the basin; and (2) early hydrothermal input of calcium chloride-rich waters. The borates associated with potash-rich beds likely resulted from erosion and influx of water from surrounding granitic terrains; however, hydrothermal influx is also possible. Interbedded with the evaporites are non-marine red beds that are also evaporative, with displacive anhydrite nodules and beds and considerable amounts of displacive halite. The δ³⁴S isotopic values of this anhydrite have non-marine values, ranging from 6.4‰ to 10.9‰ (CDT). These data indicate that the Khorat and Sakhon Nakhon basins underwent periods of marine influx due to relative world sea-level rise but were sporadically isolated from the world ocean.     

Effect of injected CO2 on geochemical alteration of the Altmark gas reservoir in Germany

Capturing CO₂ from point sources and storing it in geologic formations is a potential option for allaying the CO₂ level in the atmosphere. In order to evaluate the effect of geological storage of CO₂ on rock-water interaction, batch experiments were performed on sandstone samples taken from the Altmark reservoir, Germany, under in situ conditions of 125 °C and 50 bar CO₂ partial pressure. Two sets of experiments were performed on pulverized sample material placed inside a closed batch reactor in (a) CO₂ saturated and (b) CO₂ free environment for 5, 9 and 14 days. A 3M NaCl brine was used in both cases to mimic the reservoir formation water. For the “CO₂ free” environment, Ar was used as a pressure medium. The sandstone was mainly composed of quartz, feldspars, anhydrite, calcite, illite and chlorite minerals. Chemical analyses of the liquid phase suggested dissolution of both calcite and anhydrite in both cases. However, dissolution of calcite was more pronounced in the presence of CO₂. In addition, the presence of CO₂ enhanced dissolution of feldspar minerals. Solid phase analysis by X-ray diffraction and Mössbauer spectroscopy did not show any secondary mineral precipitation. Moreover, Mössbauer analysis did not show any evidence of significant changes in redox conditions. Calculations of total dissolved solids’ concentrations indicated that the extent of mineral dissolution was enhanced by a factor of approximately 1.5 during the injection of CO₂, which might improve the injectivity and storage capacity of the targeted reservoir. The experimental data provide a basis for numerical simulations to evaluate the effect of injected CO₂ on long-term geochemical alteration at reservoir scale.     

Geochemical variations within a laminated evaporite deposit: evidence for brine composition during formation of the Permian Castile Formation, Texas and New Mexico, USA

The Upper Permian Castile Formation of the Delaware Basin in northwest Texas and New Mexico consists of up to 600 m of evaporites and is subdivided into units of anhydrite overlain by halite. The Castile Formation has commonly been interpreted as a deep-water, deep-basin deposit in which sediments were laid down in several hundred metres of water or brine. Recent textural observations within anhydrite units, in which the thick-bedded anhydrite horizons have been interpreted as being of shallow-water origin, have challenged this assumption. This geochemical study of the oldest anhydrite unit in the Castile Formation (the Anhydrite 1 Member) attempts to resolve some of the problems regarding brine depth and evolution in the basin. The Anhydrite 1 Member has been subdivided into five major cycles on the basis of the distribution of stratigraphic units of thick-bedded anhydrite.

Stable isotopic analyses of sulphur from anhydrite, and oxygen and carbon from calcite show that the basin waters were chemically homogeneous during precipitation of anhydrite, and do not indicate any significant input of meteoric, continental-derived waters. Throughout the section studied progressive enrichment of ¹⁸O upwards within cored intervals indicates continuous evaporation of the water body. Carbon isotopes appear to indicate fluctuations in organic activity within the cycles. Trace elemental analyses of Fe, Mg, Sr, Mn, Al, Ba, Zn, Pb and Cu from the sulphate fraction of the samples show a very high variability. There is a distinct increase in trace elemental abundances at the tops of cycles which may indicate variations in precipitation kinetics. Analyses of texturally defined cycles show that up-core trends for many of the trace elements correlate with changes in δ¹⁸O, indicating a progressive increase in the influence of evaporation. In addition, cyclical variations in trace elemental composition indicate changes in basin conditions with around a 350-year cyclicity. These changes are independent of δ¹⁸O values. The geochemical data do not provide conclusive proof of water depth during deposition of the Castile Formation. The data are interpreted as reflecting small-scale changes in conditions of deposition, despite the fact that water input remained essentially constant in terms of chemical composition.     

Origin and evolution of formation water at the Jujo–Tecominoacán oil reservoir,Gulf of Mexico. Part 1: Chemical evolution and water–rock interaction

The origin and evolution of formation water from Upper Jurassic to Upper Cretaceous mudstone–packstone–dolomite host rocks at the Jujo–Tecominoacán oil reservoir, located onshore in SE-Mexico at a depth from 5200 to 6200 m.b.s.l., have been investigated, using detailed water geochemistry from 12 producer wells and six closed wells, and related host rock mineralogy. Saline waters of Cl–Na type with total dissolved solids from 10 to 23 g/L are chemically distinct from hypersaline Cl–Ca–Na and Cl–Na–Ca type waters with TDS between 181 and 385 g/L. Bromine/Cl and Br/Na ratios suggest the subaerial evaporation of seawater beyond halite precipitation to explain the extreme hypersaline components, while less saline samples were formed by mixing of high salinity end members with surface-derived, low salinity water components. The dissolution of evaporites from adjacent salt domes has little impact on present formation water composition. Geochemical simulations with Harvie-Mφller-Weare and PHRQPITZ thermodynamic data sets suggest secondary fluid enrichment in Ca, HCO₃ and Sr by water–rock interaction. The volumetric mass balance between Ca enrichment and Mg depletion confirms dolomitization as the major alteration process. Potassium/Cl ratios below evaporation trajectory are attributed to minor precipitation of K feldspar and illitization without evidence for albitization at the Jujo–Tecominoacán reservoir. The abundance of secondary dolomite, illite and pyrite in drilling cores from reservoir host rock reconfirms the observed water–rock exchange processes. Sulfate concentrations are controlled by anhydrite solubility as indicated by positive SI-values, although anhydrite deposition is limited throughout the lithological reservoir column. The chemical variety of produced water at the Jujo–Tecominoacán oil field is related to a sequence of primary and secondary processes, including infiltration of evaporated seawater and original meteoric fluids, the subsequent mixing of different water types and the formation of secondary minerals by water–rock interaction. A best fit between measured and calculated reservoir temperatures was obtained with the Mg–Li geothermometer for high salinity formation water (TDS > 180 g/L), whereas Na–K, Na–Ka–Ca and quartz geothermometers are partially applicable for less salinite water (TDS < 23 g/L).     

An explanation for the varves of the Castile evaporites (Upper Permian), Texas and New Mexico, USA

Abstract Extraordinary sequences of conspicuous, pervasive and laterally persistent varves characterize the Castile evaporites. They occur as singlets (calcite laminae), couplets (calcite laminae interstratified with anhydrite laminae), thick couplets (calcite laminae interstratified with thin anhydrite beds) and triplets (calcite and anhydrite laminae interstratified with thin halite beds). The varves accumulated in a deep (initially ≈ 550 m), persistently stratified, saline lake surrounded by an extinct reef. The lake had formed when the reef grew across a channel between an embayment and the ocean. Although located virtually on the palaeo-equator, the lake experienced negligible meteoric influx and extreme seasonality. During the season of high relative humidity, more marine groundwater entered the lake through the permeable reef barrier than exited as reflux and, secondarily, as evaporation. Consequently, the lake level rose by up to several metres to sea level. The ‘refreshening’ decreased salinity and replenished dissolved CO₂– the critical nutrient limiting growth of indigenous phytoplankton. Algae proliferated, pH increased and CaCO₃ precipitated. It mixed with organic matter to form a thin, dark lamina. During the season of low relative humidity, tens of cubic kilometres of water evaporated from and, secondarily, leaked out through the surrounding reef. More water exited than entered, brine level fell below sea level, and salinity of the upper brine layer increased. Gypsum usually precipitated and rained onto the basin floor forming a couplet; infrequently, halite also precipitated forming a triplet. Every few thousand years, for <50 to several hundred years, the lake became unstratified during the dry season, and wind-induced overturn allowed a layer of gypsum crystals up to ≈ 2 cm high to precipitate on the basin floor. Each layer, now thin beds of anhydrite nodules and anhydrite pseudomorphs after gypsum, and an underlying lamina of CaCO₃ and admixed organic matter formed a thick couplet. The different varve types recur with a period of 1800–3000 years reflecting climatic changes on a millennial time scale.     

New constraints on Precambrian ocean composition

The Precambrian record of carbonate and evaporite sedimentation is equivocal. In contrast to most previous interpretations, it is possible that Archean, Paleoproterozoic, and to a lesser extent, Meso to Neoproterozoic seawater favored surplus abiotic carbonate precipitation, as aragonite and (hi-Mg?) calcite, in comparison to younger times. Furthermore, gypsum/anhydrite may have been only rarely precipitated prior to halite precipitation during evaporation prior to about 1.8 Ga. Two effects may have contributed to these relationships. First, sulfate concentration of seawater may have been critically low prior to about 1.9 Ga so the product mCa++ x mSO4-- would not have produced gypsum before halite, as in the Mesoproterozoic to modern ocean. Second, the bicarbonate to calcium ratio was sufficiently high so that during progressive evaporation of seawater, calcium would have been exhausted before the gypsum field was reached. The pH of the Archean and Paleoproterozoic ocean need not have been significantly different from the modern value of 8.1, even at CO2 partial pressures of a tenth of an atmosphere. Higher CO2 partial pressures require somewhat lower pH values.     

美国新墨西哥州钾盐矿床及其开发

新墨西哥州卡尔斯巴德 (Carlsbad)钾盐矿床是美国最早发现的古钾盐矿床 ,该矿床一直是美国钾盐的主要供给地。含钾蒸发岩系产出于得克萨斯州西部、新墨西哥州东南部Delaware盆地上二叠统海相地层中。含钾蒸发岩系可分为 4个建造。由下向上分别为 :①Castile建造 ,由石盐岩和其夹层硬石膏岩或石灰岩构成 ;②Salado建造 ,由钾盐、石盐岩、含泥石盐岩、硬石膏岩、杂卤石岩、白云岩、泥岩构成 ;③Rustler建造。由石盐岩、石膏岩、硬石膏岩、硅质岩、白云岩及石灰岩构成 ;④DeweyLake红层建造 ,由红色泥岩和砂岩构成。其中 ,Salado建造 ,厚 6 70m ,含 12个矿带 ,面积为 492 0km2 ,钾盐矿体主要由钾石盐和无水钾镁矾以及石盐等矿物所组成。次生的钾盐矿物有 :钾盐镁矾和钾镁矾等     

Mineral equilibria in a six-component seawater system,Na-K-Mg-Ca-SO4-Cl-H2O,at 25°C

Phase relations in the 6-component system Na-K-Mg-Ca-SO₄-Cl-H₂O have been calculated for halite saturation, 25°C and 1 atm pressure. Using a Jänecke projection with the apices Ca-Mg-K₂-SO₄, 27 stable invariant points have been located which are connected by 69 univariant curves. Polyhalite is the only quaternary solid, but anhydrite occupies the bulk of the interior tetrahedral space. Consequently, 24 of the invariant points lie very close to the Ca-free base, Mg-K₂-SO₄. The remaining three points involve tachyhydrite and/or antarcticite. All points but two (20,27) represent peritectic conditions. Metastable equilibria have been calculated for the Ca-free system and yield relations corresponding to the solar diagram.Seawater lies in the subspace anhydrite-halite-carnallite-kieserite-bischofite (point 20) and its evaporation has been discussed for conditions of equilibrium and fractional crystallization. After gypsum is converted to anhydrite, halite precipitates. The next phase, under equilibrium conditions, is glauberite, crystallizing at the expense of anhydrite. Continued evaporation leads to glauberite resorption and eventual replacement by polyhalite. Then follow the magnesium sulfates epsomite, hexahydrite and kieserite, which are joined by carnallite. Polyhalite is replaced by anhydrite and bischoflte is added at the final invariant condition. Kainite does not appear as a primary phase under equilibrium conditions, but it is an important phase during fractional crystallization, where Ca-phases are not allowed to back-react with the brine.Up to the appearance of glauberite, thickness ratios of halite: anhydrite couplets (equilibrium or fractionation) can vary from 0 to 7, the relative amount of halite increasing with more intense evaporation. During evaporation, the activity of H₂O decreases from 0.98 (seawater) to 0.34 (final invariant brine). The data provided can be used to evaluate the effects of mineral precipitation, evaporation and brine mixing for a wide variety of natural brines.     

川东北黄金口背斜三叠系深部杂卤石特征及成因探讨

川东北早中三叠纪具备成盐成钾条件,黄金口背斜地区发现深部富钾卤水已经有近40年的历史,但是尚无对深层杂卤石的系统研究。近年,四川省启动了新一轮的找钾工作,并在黄金口背斜地区施工了钾盐探井,文章依托钻孔编录及测井资料,对该孔含盐系地层沉积特征进行了研究,并系统采集了3500 m以深的杂卤石、石盐、硬石膏等蒸发岩样品进行了分析测试。该孔杂卤石主要赋存于嘉陵江组五段二亚段与雷口坡组一段一亚段含盐地层中,可细分为4个成盐旋回,共发现杂卤石矿层8层,累计厚度33.5 m。该孔杂卤石主要呈薄层状产于硬石膏中,部分呈斑点状、团块状、薄层状产于石盐中,镜下呈半自形-他形粒状、板状、柱状结构,结合粉晶X衍射及扫描电镜观察,初步认为该孔杂卤石为原生沉积成因。     

The anatomy of a sulphate platform and adjacent basin system in the Leba sub-basin of the Lower Werra Anhydrite (Zechstein, Upper Permian), northern Poland

The Lower Werra Anhydrite (Zechstein, Upper Permian) deposits of the teba area originated in a deep basin setting, in shallow to deep water conditions. Facies changes occur within small distances and suggest fluctuating boundaries between well defined basins and platforms. This pattern of local platforms and adjacent basins developed during deposition. In basinal areas, the sequence is clearly transgressive, whereas on platforms accumulation kept pace with subsidence after an initial transgression. Nodular anhydrite represents a polygenetic deposit which formed at different times with respect to deposition. Massive anhydrite with pseudomorphs after upright-growth gypsum crystals suggest rapid precipitation in a subaqueous environment and/or fluctuating, but generally high, salinity conditions. Massive clastic sulphate originated due to periodic high energy events and resedimentation, or due to brecciation possibly connected with salinity fluctuations and the dissolution of halite. Massive, textureless anhydrite is locally porous and passes upward into breccia, indicating a strongly saline environment. Bedded anhydrite is considered to form in shallow water environments and laminated anhydrite in deep water. Bedded anhydrites contain portions which are graded. Intercalations of sulphate turbidites and upright-growth gypsum suggest fluctuating water depths, with comparatively deep water during turbidite deposition, but shallower conditions during upright-growth gypsum deposition. The sequence observed in slope zones at platform-basin margins, detrital (parautochthonous) sulphate sand to graded beds to basinal laminites, indicates that redistribution processes were important. At the onset of the Lower Werra Anhydrite deposition bathymetric relief existed between the central part of the basin and its margins, where carbonate platforms remained subaerially exposed. Formation of local platforms and adjacent basins required a relatively high subsidence rate, as pre-existing relief cannot account for the total accumulated thickness of the Lower Werra Anhydrite deposits. One implication of this is that the main argument against ‘the shallow water - shallow basin’ evaporite basin model, i.e.,a very fast rate of subsidence, may not be valid for the Łeba Lower Werra Anhydrite basin.     

Inverse geochemical modeling of groundwater salinization in Azraq Basin,Jordan

The aim is to define the mechanism of chemical reactions that are responsible for the salinization of the Azraq basin along groundwater flow path, using inverse modeling technique by PHREEQC Interactive 2.8 for Windows. The chemical analysis of representative groundwater samples was used to predict the causes of salinization of groundwater. In addition, the saturation indices analysis was used to characterize the geochemical processes that led to the dissolution of mineral constituents within the groundwater aquifer system. According to the modeling results, it was noted that the groundwater at the recharge area was undersaturated with respect to calcite, dolomite, gypsum, anhydrite, and halite. Thus, the water dissolved these minerals during water rock interaction, and therefore, the concentration of Ca, Mg, Na, and SO₄ increased along the groundwater flow path. Furthermore, the groundwater at the discharge area was oversaturated with respect to calcite and dolomite. This meant that the water would precipitate these minerals along the flow path, while the water was undersaturated with respect to gypsum and halite throughout the simulated path; this showed the dissolution processes that take place during water-rock interaction. Therefore, the salinity of the groundwater increased significantly along the groundwater flow paths.     

Diagenetic origin of Basal Anhydrite in the Cretaceous Maha Sarakham salt: Khorat Plateau, NE Thailand

Development of a diagenetic anhydrite bed at the base of the Cretaceous Maha Sarakham Saline Formation (the `Basal Anhydrite' member) of the Khorat Plateau in north-eastern Thailand took place due to leaching and/or pressure dissolution of salt at the contact between an underlying active sandstone aquifer system and an overlying massive halite-dominated evaporite sequence. Basal evaporites composed of halite with intercalated anhydrite of the latter sequence are undergoing dissolution as a result of subsurface flushing, with anhydrite produced as the insoluble residue. The result is a 1·1 m thick interval of nodular anhydrite displaying unique, basin-wide continuity. Observed textures, petrographic features and chemical data from the anhydrite and associated authigenic minerals support the origin of the Basal Anhydrite Member as an accumulation residue from the dissolution of the Maha Sarakham salts. Petrographically, the anhydrite in this unit is made up of crystals that are blocky and recrystallized, sheared, generally elongated and broken, and is bounded at the bottom by organic-rich stylolite surfaces. Authigenic and euhedral dolomite and calcite crystals are associated with the anhydrite. Traces of pyrite, galena and chalcopyrite are present along the stylolite surfaces suggesting supply of fresh water from the underlying sandstone at highly reducing conditions of burial. The δ<sup>34</sup>S of sulphate in the Basal Anhydrite averages 15 ‰ (CDT) and falls within the isotopic composition of the anhydrite in the Cretaceous Maha Sarakham Formation proper and the Cretaceous values of marine evaporites. Measured δ<sup>18</sup>O in dolomite range from ?4·37 to ?14·26‰ (PDB) suggesting a re-equilibration of dolomite with basinal water depleted in <sup>18</sup>O and possible recrystallization of dolomite under relatively elevated temperatures. The δ<sup>13</sup>C, however, varies from +1·57 to ?2·53‰ (PDB) suggesting a contribution of carbon from oxidation of organic matter. This basal anhydrite bed, similar to basinwide beds found at the bottom of many giant evaporite sequences, has always been considered to be depositional. Here, at the base of the Maha Sarakham Formation, we demonstrate that the anhydrite is diagenetic in origin and was formed by accumulation of original anhydrite by dissolution of interbedded halite from waters circulating though the underlying aquifer: it represents an `upside-down' caprock.     

Rock-water interaction during the exploitation of oil pools in subsalt and intersalt rocks: The pripyat depression as an example

Subsalt and intersalt oil pools, which are exploited with the application of water injection technique, are characterized by the dissolution of catagenetic halite and sulfates in pores, cracks, and caverns inside productive rocks, the precipitation of neogenic minerals, and the decomposition of dolomites. These processes lead to a substantial change in the initial filtration and reservoir properties of the rocks. Methods, algorithms, and computer program, which are proposed to estimate scales of the lithohydrogeochemical processes, make it possible to control changes in productive rocks, along with their filtration and reservoir properties, during the flow of waters from the injection zone to the drainage zone. It has been demonstrated that the dissolution of halite fillings is the most essential process leading to a significant increase in the filtration and reservoir properties of rocks during the exploitation of flooded oil pools. Thus, during the whole period of exploitation of some pools in Belarussian oil fields, up to 1.2 million m³ of sodium chloride, which was previously present as halite cement, were extracted together with the associated water. It is concluded that the currently popular lithohydrogeochemical approach to the study of processes in water-saturated rocks systems can be successfully used for solving many problems related to not only natural geological processes but comparable (in scale) technogenic processes induced by water-rock interactions during the exploitation of oil pools.     

Variations in abundances and distributions of isoprenoid chromans and long-chain alkylbenzenes in sediments of the Mulhouse Basin: a molecular sedimentary record of palaeosalinity

The abundances and distribution patterns of mono-, di- and trimethylated 2-methyl-2-(4,8,12-trimethyltridecyl) chromans (MTTCs) and long-chain alkylbenzenes in extracts of marl (66 samples), anhydrite (15) and halite (1) strata of the Salt IV Formation of the Oligocene Mulhouse Basin are reported. The distributions of the methylated MTTCs indicate salinity changes of the upper part of a density stratified water column of the basin. These variations are explained by a tectonically or climatically induced change in the supply of water of relative lower salinity to form a layer overlying the deeper water brine. Hence, it is suggested that mesohaline (3.5–15%) conditions in the surface waters were established as a result of periodic incursions of marine water and subsequent evaporation. Conversely, during periods when the surface water was derived mainly from fresh water from the hinterland, lower average salinity in the surface layer resulted. The distributions of long-chain alkylbenzenes also appear to record these changes.     

Halite saltern in the Canning Basin, Western Australia: a sedimentological analysis of drill core from the Ordovician-Silurian Mallowa Salt

The Late Ordovician-Early Silurian Mallowa Salt of the Carribuddy Group, Canning Basin, north-west Australia, is the largest halite deposit known in Australia, attaining thicknesses of 800 m or more within an area of approximately 200 000 km². Study of 675 m of drill core from BHP-Utah Minerals’ Brooke No. 1 well in the Willara Sub-basin indicates that the Mallowa Salt accumulated within a saltern (dominantly subaqueous evaporite water body) that was subject to recurrent freshening, desiccation and exposure. Textures and bromine signatures imply a shallow water to ephemeral hypersaline environment typified by increasing salinity and shallowing into evaporitic mudflat conditions toward the top of halite-mudstone cycles (Type 2) and the less common dolomite/anhydrite-halite-mudstone cycles (Type 1). The borate mineral priceite occurs in the capping mudstones of some cycles, reinforcing the idea of an increasing continental influence toward the top of mudstone-capped halite cycles. The rock salt in both Type 1 and Type 2 cycles typically comprises a mosaic of large, randomly orientated, interlocking halite crystals that formed during early diagenesis. It only partially preserves a primary sedimentary fabric of vertically elongate crystals, some with remnant aligned chevrons. Intraformational hiati, halite karst tubes and solution pits attest to episodic dissolution. Stacked Type 2 cycles dominate; occasional major recharges of less saline, perhaps marine, waters in the same area produced Type I cycles. The envisaged saltern conditions were comparable in many ways to those prevailing during the deposition of halite cycles of the Permian Salado Formation in New Mexico and the Permian San Andres Formation of the Palo Duro Basin area in Texas. However, in the Canning Basin the cycles are characterized by a much lower proportion of anhydrite, implying perhaps a greater degree of continental restriction to the basin. The moderately high level of bromine in the Mallowa Salt (156·5 ± 43·5 ppm Br for primary halite, 146·1 ± 54·7 ppm Br for secondary halite) accords with evolved continental brines, although highly evaporative minerals such as polyhalite and magnesite are absent. The bromine levels suggest little or no dissolution/reprecipitation of primary halite and yet, paradoxically, there is little preservation of the primary depositional fabric. The preservation of early halite cements and replacement textures supports the idea of an early shutdown of brine flow paths, probably at burial depths of no more than a few metres, and the resultant preservation of primary bromine values in the secondary halite.     

老挝他曲地区石盐流体包裹体特征、氢氧同位素组成及成盐物质补给方式

对老挝他曲地区钻孔中石盐流体包裹体特征、类型以及氢氧同位素组成进行了研究,用以重塑盆地成盐过程中的古环境,并在此基础上对盆地成盐物质的补给方式进行了探讨。结果显示,钻孔中塔贡组下盐段原生石盐岩主要包括人字形和漏斗形晶体两种类型。其中人字形石盐晶体在下盐段中广泛发育,漏斗形晶体数量较少,但通常与人字形石盐晶体产于同一层位,指示了下盐段沉积时盆地处于一种极浅水环境。石盐原生流体包裹体氢氧同位素组成明显偏离全球大气降水线,集中分布于其右下方,反映盆地成盐过程中处于强烈蒸发条件之下。氢氧同位素组成在纵向上的变化,可能揭示了在盐湖演化至钾盐沉积阶段大气温度具有升高趋势。白垩纪中期全球海平面达到极大值,而在呵叻盆地内部,持续的坳陷和强烈的蒸发作用将导致湖平面快速下降。这样,外海与盐盆之间则会产生水位差,由此产生的水力梯度将为外海海水通过障壁向盆地方向进行渗透提供潜在的动力。此外,早白垩世呵叻高原广泛发育的沙漠沉积,则为外海海水大规模的侧向渗透进入盐盆提供了可能的通道。因此,结合现有的资料,本文提出,除海侵补给外,外海通过障壁侧向渗透补给对老挝钾盐盆地成盐物质的供给也具有重要作用。     

热化学硫酸盐还原作用对碳酸盐岩气藏的化学改造——以川东北地区长兴组-飞仙关组气藏为例

目前在川东北地区长兴组—飞仙关组已发现普光、渡口河、铁山坡、罗家寨等多个高含H2S的大、中型气田。通过天然气地球化学特征、流体包裹体盐度和岩心及薄片的镜下详细观察后认为,川东北地区长兴组—飞仙关组的大多数气藏遭受了热化学硫酸盐还原作用(TSR)的化学改造,TSR的改造主要表现在3个方面:1使C2 重烃相对于CH4、12C相对于13C优先被消耗,造成天然气干燥系数变大和碳同位素变重;2由于TSR产生的大量淡水的加入,使气藏的原生地层水被稀释,造成地层水盐度降低;3TSR相关流体(烃类和H2S等)与储层岩石之间的相互作用使储层被溶蚀和硬石膏发生蚀变,造成储层孔隙度增大,从而对改善其物性具有重要意义。     

Geochemical Features of Vein Anhydrite from the Kakkonda Geothermal System, Northeast Japan

Abstract. Cathodoluminescence (CL) color, rare earth element (REE) content, sulfur and oxygen isotopes and fluid inclusions of anhydrite, which frequently filled in hydrothermal veins in the Kakkonda geothermal system, were investigated to elucidate the spatial, temporal and genetical evolution of fluids in the deep reservoir. The anhydrite samples studied are classified into four types based on CL colors and REE contents: type-N (no color), type-G (green color), type-T (tan color) and type-S (tan color with a high REE content). In the shallow reservoir, only type-N anhydrite is observed. In the deep reservoir, type-G anhydrite occurs in vertical veins whereas type-T and -N in lateral veins. Type-S anhydrite occurs in the heat-source Kakkonda Granite. The CL textures revealed that type-G anhydrite deposited earlier than type-T in the deep reservoir, implying that fracture system was changed from predominantly vertical to lateral.
Studies of fluid inclusions and δ³⁴S and δ¹⁸O values of the samples indicate that type-N anhydrite deposited from diluted fluids derived from meteoric water, whereas type-G, -T and -S anhydrites deposited from magmatic brines derived from the Kakkonda Granite with the exception of some of type-G with recrystallization texture and no primary fluid inclusion, which deposited from fossil seawater preserved in the sedimentary rocks. Type-G, -T and -S anhydrites exhibit remarkably different chondrite-normalized REE patterns with a positive Eu anomaly, with a convex shape (peak at Sm or Eu) and with a negative Eu anomaly, respectively. The difference in the patterns might result from the different extent of hydrothermal alteration of the reservoir rocks and contribution of the magmatic fluids.     

Neogene evaporites in desert volcanic environments: Atacama Desert, northern Chile

The Upper Miocene and Pliocene evaporite deposits of the Atacama Desert of northern Chile (Hilaricos and Soledad Formations) are among the few non‐marine evaporites in which aridity not only formed the deposits, but has also preserved them almost unaltered under near‐surface conditions. These deposits are largely composed of displacive Ca sulphate and halite together with minor amounts of glauberite, thenardite and polyhalite. However, at the base and top of these deposits, there are also beds of gypsum crystal pseudomorphs that originally formed as free‐growth forms within shallow brine bodies, rather than as displacive sediments. The halite is present as interstitial cement, displacive cubes and shallow‐water, bottom‐growth chevron crusts. Most of the calcium sulphate is presently anhydrite, pseudomorphous after gypsum, that was the primary depositional sulphate mineral. The secondary anhydrite formed under early diagenetic conditions after slight burial (some metres) resulting from the effect of strongly evolved pore brines. The anhydrite has been preserved without rehydration during late diagenetic and exhumation stages on account of the arid environment of the Atacama Desert. Both the Hilaricos and the Soledad Formations contain geochemical markers indicating that these Neogene evaporites had a largely non‐marine origin. Bromine content in the halite is very low (few p.p.m.), indicating neither a sedimentological relation with sea water nor the likelihood of direct recycling of prior marine halites. Moreover, the δ³⁴S of sulphates (+4·5‰ to +9‰) also reflects a non‐marine origin, with a strong volcanic influence, although some recycling of Mesozoic marine sulphates cannot be ruled out. δ³⁴S of dissolved sulphate from hot springs and streams in the area commonly displays positive values (+2‰ to +10‰). Leaching of oxidized sulphur and chlorine compounds from volcanoes and epithermal ore bodies, very common in the associated drainage areas, have been the main contribution to the accumulation of evaporites. The sedimentary and diagenetic evolution of the Hilaricos and Soledad evaporites (based on lithofacies analysis) provides information about the palaeohydrological conditions in the Central Depression of northern Chile during the Neogene. In addition, the diagenesis and exhumation history of these evaporites confirms the persistence of strongly arid conditions from Late Miocene until the present. A final phase of tectonism took place permitting the internal drainage to change and open to the sea, resulting in dissolution and removal of a significant portion of these deposits. Despite the extensive dissolution, the remaining evaporites have undergone little late exhumational hydration.     

Salt Crystallization Sequences of Nonmarine Brine and Their Application for the Formation of Potassium Deposits

The salt assemblages precipitated during evaporation of concentrated brine collected from Gasikule Salt Lake (GSL) were studied to better understand the formation of potassium deposits in the Qaidam Basin. The study included isothermal evaporation at 25 °C in the laboratory and solar evaporation in the ponds at GSL field. Brines increased in density and became moderately acidic (pH?≈?5.30) while major ion geochemistry and precipitate mineralogy all showed broad agreement between both systems. Four salt assemblages were identified in the isothermal evaporation experiment: halite?→?halite?+?hexahydrite?→?halite?+?bischofite?+?carnallite?→?bischofite. Alternately, three salt assemblages were recognized in the solar evaporation: halite?→?halite?+?epsomite?+?carnallite?→?halite?+?carnallite?+?bischofite. The key difference in salt assemblages between the two systems is attributed to differences in relative humidity and temperature conditions. Although the GSL has deep spring inflow recharge, the high abundance of MgSO₄ salts demonstrates that the salt assemblages are similar to normal seawater evaporation. Thus, different proportions of deep spring inflow and river water could form both MgSO₄-deficient potassium evaporite and normal seawater potassium evaporites. Therefore, nonmarine water may form diverse potassium evaporite deposits in continental basins when the geological structure as well as hydrogeological and climatic conditions is appropriate.