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1.
The Ringwood evaporite is part of the 900 m.y. old Bitter Springs Formation, a warm-water shallow-marine sequence of stromatolitic dolomite and limestone, microfossiliferous chert, red beds, quartzite, and evaporites. The evaporite at Ringwood comprises two parts: (i) a lower 127 m characterized by brecciated pyritic bituminous dolomite, together with smaller amounts of dolomite-gypsum breccia, friable chloritic dololutite, coarsely crystalline anhydrite, and satin-spar gypsum; and (ii) an upper 133 m which is similar except that bituminous dolomite forms only one bed, and the characteristic rock-type is dolomite-gypsum breccia. The evaporite is overlain by limestone breccia and massive stromatolitic limestone, interpreted as an algal reef. Gypsum is secondary after anhydrite, and the ratio of gypsum to anhydrite increases upwards. The evaporite shows none of the features of a sabkha or desiccated deep ocean basin deposit, and instead is interpreted as the filling of a barred basin which was cut off from the ocean by growth of an algal barrier reef. As circulation became restricted, bituminous dolomite deposited in the lagoon behind the reef, together with pyrite from the destruction by anaerobic bacteria of algal debris derived from the reef. With continued evaporation, brine concentration increased and gypsum precipitated. Occasional dust storms contributed wind-blown clay to the deposit. The barrier reef transgressed diachronously across the evaporite lagoon, and was eventually drowned when normal marine conditions became established. Burial of the evaporite to about 7000 m beneath the succeeding sediments of the Amadeus Basin converted gypsum to anhydrite, and formed chlorite by reaction of clay with dolomite. Late Palaeozoic tectonism folded and brecciated the rocks, and was followed by erosion which eventually exposed the evaporite to ingress of meteoric water. Hydration of anhydrite to gypsum ensued, the reaction becoming less complete with increasing depth from the ground surface.  相似文献   

2.
Depositional theme of a marginal marine evaporite   总被引：1，自引：0，他引：1  
We have reconstructed the depositional environment of the gypsum-carbonate-shale sequence that comprises the Upper Permian Bellerophon Formation of the southeastern Alps in northern Italy. This formation, which reaches a maximum thickness of 600 m, is roughly divided into two facies: (a) a lower dolomite-gypsum facies, and (2) an upper micritic-skeletal limestone facies. It directly overlies, with transitional contact, a thick red-bed sequence (alluvial fanglomerates, fluviatile sandstones and flood-plain siltstones) and is sharply overlain by Lower Triassic calcarenites (oolites, grapestones, pellets, flat-pebble conglomerates). The lower evaporite facies rocks are found in well-defined cycles, each of which, from bottom to top, consists of (A) thin-bedded, worm-burrowed, vuggy ‘earthy’ micritic dolomite, (B) massive to poorly laminated dark grey to black sandy dolomite carrying isolated gypsum nodules, (C) layered (thin-bedded) nodular gypsum (commonly with ‘enterolithic’ folds) with fragmented partings of dolomite, and (D) massive ‘chicken-wire’ nodular gypsum. At Passo di Valles, just east of Predazzo, and 50 km from the basin margin, we measured forty-six consecutive complete cycles, with an average thickness of 3 m per cycle. We interpret the cyclic sequence as having been deposited in a prograding shallow lagoon—sabkha complex. The worm-burrowed ‘earthy’ dolomite mud accumulated in a shallow hypersaline subtidal lagoon. The black sandy dolomite was an ‘intertidal’ sand-flat devoid of algal mats and constantly churned by burrowers (likely crustaceans). As the shoreline prograded lagoonward evaporative concentration of the groundwater induced diagenetic growth of anhydrite nodules (now gypsum) within the porous sandy dolomite. The layered nodular and ‘chicken-wire’ gypsum of the cycle cap is an extreme product of such displacive intra-sediment growth of anhydrite (now gypsum) above the water table of a completely exposed sabkha, such as is found in the Persian Gulf today. We have observed the same cyclically arranged lithologies in two other evaporite sequences in Italy: the Triassic Raibl Formation of the Southern Alps and the Upper Triassic Burano Formation of the central Apennines. We suggest that this mode of deposition is likely a very common one for at least the early stages of marine evaporite accumulation.  相似文献   

3.
The Middle Muschelkalk (Middle Triassic) of the Catalan Coastal Range (north-east Spain) comprises sandstone, mudstone, anhydrite and minor carbonate layers. Interbedded sandstones and mudstones which are dominant in the north-eastern parts of the basin are terminal alluvial fan deposits. South-westward in the basin, the rocks become dominated by interbedded evaporites and mudstones deposited in sabkha/mudflat environments. The diagenetic and pore water evolution patterns of the Middle Muschelkalk suggest a strong facies control. During eodiagenesis, formation of microdolomite, anhydrite, baryte, magnesite, K-feldspar and mixed-layer chlorite/smectite was favoured within and adjacent to the sabkha/mudflat facies, whereas calcite, haematite, mixed-layer illite/smectite and quartz formed mainly in the alluvial facies. Low δ18OSMOW values for microdolomite (+23.7 to +28.4%) and K-feldspar overgrowths (+17.3 to +17.7%) suggest either low-temperature, isotopic disequilibrium or precipitation from low-18O porewaters. Low-18O waters might have developed, at least in part, during low-temperature alteration of volcanic rock fragments. During mesodiagenesis, precipitation of quartz overgrowths and coarse dolomite occurred in the alluvial sandstones, whereas recrystallization of microdolomite was dominant in the sabkha/mudflat facies. The isotopic compositions of these mesogenetic phases reflect increasing temperature during burial. Upon uplift and erosion, telogenetic calcite and trace haematite precipitated in fractures and replaced dolomite. The isotopic composition of the calcite (δ18OSMOW=+21.5 to +25.6%o; δ13C= 7.7 to - 5.6%o) and presence of haematite indicate infiltration of meteoric waters.  相似文献   

4.
The Early to Middle Cambrian Red Heart Dolomite and lower Arthur Creek Formation of the southern portion of the Georgina Basin, Australia, is an entirely dolomitized succession of shallow-water evaporitic mudflat and deeper-water subtidal lithologies. Three types of dolomite have been identified and are interpreted as: (1) syndepositional dolomite; (2) regional replacement dolomite; and (3) void-filling dolomite (cement). Syndepositional dolomite, derived from saline pore fluids developed in a sabkha environment, is a minor dolomite type with very fine crystal mosaics and has a mottled, non-zoned cathodoluminescence. The widespread regional replacement dolomite ranges from fine- to medium-crystalline forming mainly planar-s and non-planar-a crystal mosaics, and displays blotchy, mottled, non-zoned cathodoluminescence. Void-filling dolomite commonly forms planar-s to planar-e, medium to very coarse crystal mosaics. Rare non-planar-c, very coarsely crystalline saddle dolomite also exists. Void-filling dolomite has a successively zoned cathodoluminescence pattern from non-, to brightly, to dully luminescent. Geochemically, the syndepositional dolomite has δ18O (PDB) values ranging between ? 5.3 and ? 8.6%o. Regional replacement dolomites exhibit a wide range of δ18O values from ? 3.3 to ? 10.9%o whereas void-filling dolomite has δ18O values ranging from ? 10.8 to ? 14.3%o. All three dolomite types have similar δ13C (PDB) values, in the range between +1.7 and ?1.7%o. Three initial dolomitization episodes are interpreted: (1) a sabkha stage, forming the syndepositional dolomite and dolomitizing the evaporitic mudflat lithologies; (2) a brine-reflux stage, replacing the subtidal lithologies; and (3) a burial stage, forming the void-filling dolomite type. Final dolomite stabilization occurred during burial, at elevated temperatures, in the presence of basinal fluids, resulting in progressive recrystallization and stabilization of the earlier-formed syndepositional and replacement dolomites. Both textural and geochemical evolution should be taken into account when studying the origin of dolomites, based on their present geochemical composition. Sulphates are represented by very fine-crystalline syndepositional anhydrite in association with the syndepositional dolomite, and coarse to very coarse anhydrite cement. Evaportic mudflat (sabkha) and burial environments are inferred for the origin of the former and the latter anhydrite types, respectively. Evaporite dissolution breccias, indicative of the former presence of evaporites, are common throughout the succession.  相似文献   

5.
In the Tarim Basin, dolomite, which formed during the middle Cambrian associated with evaporites, has been attributed to the sabkha-style dolomite formed during the syndepositional period. The sedimentary microfacies suggests dolomite formation in the middle Cambrian is an ancient analogue of the sabkha of Abu Dhabi. Poorly crystallised dolomite spheroids or ovoids within or on the surface of dolomite crystals are a common phenomenon that can be widely observed in different stromatolites in the upper part of the intertidal zone and strongly resemble the morphology in modern sabkha dolomite-producing microbial mats and in microbial culture experiments. These lines of evidence suggest organic substrates for dolomite nucleation. Dolomite formation in the middle Cambrian in the Tarim Basin has been considered a classic analogue for carbonate and evaporate assemblages. The extent of microbial dolomite in ancient sabkha environments is proposed as an alternative model for dolomite formation, in which the mineral properties of organic substrates play a crucial role.  相似文献   

6.
鄂尔多斯盆地奥陶系白云岩是重要的天然气储集层,对其白云岩成因和储层发育规律的研究对于碳酸盐岩深层勘探理论的发展具有重要意义。文章基于大量岩心、薄片的观察,并结合稳定碳氧同位素、稀土元素、白云岩有序度、地层水、包裹体特征等地球化学分析,对盆地奥陶系存在的两种典型白云岩储层的发育特征和形成机理进行了详细研究。分析表明,含硬石膏结核或柱状晶的泥粉晶白云岩主要为潮坪蒸发环境的萨布哈成因,其规模储层的发育主要受到长时不整合的控制,不整合面以下发育大量硬石膏结核和柱状晶溶模孔;颗粒滩白云岩主要为回流渗透成因,其储层孔隙的发育主要受到海平面高频升降的影响,发育多旋回的粒（晶）间孔隙型储层。虽然两种白云岩的白云石化流体都为浓缩海水,但由于白云石化的机制和速率不同,二者的原始孔隙类型和特征迥异。两种白云岩孔隙在后期的成岩改造和演变过程中,既有相同之处,又有明显的区别。这一认识也将对深层的白云岩储层勘探提供指导作用。  相似文献   

7.
Sabkhas are ubiquitous geomorphic features in eastern Saudi Arabia. Seven brine samples were taken from Sabkha Jayb Uwayyid in eastern Saudi Arabia. Brine chemistry, saturation state with respect to carbonate and evaporate minerals, and evaporation-driven geochemical reaction paths were investigated to delineate the origin of brines and the evolution of both brine chemistry and sabkha mineralogy. The average total dissolved solids in the sabkha brines is 243 g/l. The order of cation dominance is Na+   >>  Mg2+ >>  Ca2+>K+, while anion dominance is Cl− >> SO4 2− >> HCO3 −. Based on the chemical divide principle and observed ion ratios, it was concluded that sabkha brines have evolved from deep groundwater rather than from direct rainfall, runoff from the surroundings, or inflow of shallow groundwater. Aqueous speciation simulations show that: (1) all seven brines are supersaturated with respect to calcite, dolomite, and magnesite and undersaturated with respect to halite; (2) three brines are undersaturated with respect to both gypsum and anhydrite, while three brines are supersaturated with respect to both minerals; (3) anhydrite is a more stable solid phase than gypsum in four brines. Evaporation factors required to bring the brines to the halite phase boundary ranged from 1.016 to 4.53. All reaction paths to the halite phase boundary follow the neutral path as CO2 is degassed and dolomite precipitates from the brines. On average, a sabkha brine containing 1 kg of H2O precipitates 7.6 g of minerals along the reaction path to the halite phase boundary, of which 52% is anhydrite, 35.3% is gypsum, and 12.7% is dolomite. Bicarbonate is the limiting factor of dolomite precipitation, and sulfate is the limiting factor of gypsum and anhydrite precipitation from sabkha brines.  相似文献   

8.
石膏对白云岩溶解影响的实验模拟研究   总被引：25，自引：2，他引：25  
黄思静  杨俊杰 《沉积学报》1996,14(1):103-109
表生到埋藏成岩作用的温度与压力（４０-１３０℃、常压－３０ＭＰａ）条件下，含膏与不含膏白云岩的溶解实验证明:在表生与相对浅埋藏的温压条件（低于７５℃，２０ＭＰａ）下，石膏（或硬石膏）的存在可不同程度地加速白云岩的溶解，随着实验温度和压力的升高，石膏（或硬石膏）对白云岩溶解的这种积极作用逐渐降低。在相对深埋藏的温压条件（高于７５℃、２０ＭＰａ）下，石膏（或硬石膏）的存在显著阻止白云岩的溶解，随着实验温度和压力的继续升高，石膏（或硬石膏）对白云岩溶解的这种消极作用也逐渐增加。从实验的这种结果可以预测，在近地表条件下和埋藏成岩作用的早期阶段，由溶解作用造成的含膏白云岩地层的次生孔隙将比不含膏的白云岩地层更为发育，因而在经历了古风化作用的地层中，含膏白云岩层更易形成良好的储层；与之相反，在相对高温高压的深埋藏成岩阶段，不含膏的白云岩地层中将更容易因酸性水的溶解作用而形成次生孔隙。因而在非蒸发沉积环境中形成的白云岩体（如正常海沉积环境的灰岩中的白云岩透镜体）更易因深埋藏溶蚀作用而形成良好的储层。  相似文献   

9.
This study represents a preliminary investigation of the late Messinian subsurface Marsa Zouaghah Formation in the Western Libyan Offshore, Central Mediterranean Sea. The formation was deposited in three major environmental settings: (a) Marginal Sabkha; (b) Open Lagoon; and (c) Hypersaline Lagoon. The marginal sabkha and open lagoonal settings are locally interrupted by intertidal oolitic shoal deposits. The marginal sabkha facies is replaced in central parts of the Libyan offshore by a narrow zone of aeolian-fluvial facies, the distribution and thickness of which is entirely controlled by a local uplift, the Tripoli Nose'. The marginal sabkha facies broadly defines the late Messinian palaeoshoreline lying parallel to, and north of, the E-W Jifarah fault system which dominated the southern part of the Libyan offshore. This fades is, thus, interpreted as being structurally controlled by fault systems. The hypersaline lagoonal facies is developed in areas of relatively higher rates of subsidence than that of adjacent facies belts. It is therefore, related to restriction formed by continuous subsidence and evaporation. The Marsa Zouaghah Formation constitutes saltern evaporates deposited over a wide platform in sabkha and lagoonal settings, forming part of the 'basin-wide-evaporites' of the Mediterranean Basin deposited during the Messinian salinity crisis. The evaporites formed during a major relative sea-level fall within a subsiding basin situated on the northern continental margin of the African plate. Local, vertical and lateral variations in lithofacies and thickness within the Messinian deposits of the north-west Libyan offshore were controlled by contemporaneous strike-slip movements in addition to sea-level change.  相似文献   

10.
Cambrian dolostone reservoirs in the Tarim Basin, China, have significant potential for future discoveries of petroleum, although exploration and production planning is hampered by limited understanding of the occurrence and distribution of dolomite in such ancient rocks buried to nearly 8 km. The study herein accessed new drill core samples which provide an opportunity to understand the dolomitization process in deep basins and its impact on Cambrian carbonate reservoirs. This study documents the origin of the dolostone reservoirs using a combination of petrology, fluid‐inclusion microthermometry, and stable and radiogenic‐isotopes of outcrop and core samples. An initial microbial dolomitization event occurred in restricted lagoon environments and is characterized by depleted δ13C values. Dolomicrite from lagoonal and sabkha facies, some fabric‐retentive dolomite and fabric‐obliterative dolomite in the peloidal shoal and reef facies show the highest δ18O values. These dolomites represent relatively early reflux dolomitization. The local occurrence of K‐feldspar in dolomicrite indicates that some radiogenic strontium was contributed via terrigenous input. Most fabric‐retentive dolomite may have precipitated from seawater at slightly elevated temperatures, suggested by petrological and isotopic data. Most fabric‐obliterative dolomite, and medium to coarse dolomite cement, formed between 90°C and 130°C from marine evaporitic brine. Saddle dolomite formed by hydrothermal dolomitization at temperatures up to 170°C, and involved the mixing of connate brines with Sr‐ enriched hydrothermal fluids. Intercrystalline, moldic, and breccia porosities are due to the early stages of dolomitization. Macroscopic, intergranular, vuggy, fracture and dissolution porosity are due to burial‐related dissolution and regional hydrothermal events. This work has shown that old (for example, Cambrian or even Precambrian) sucrosic dolomite with associated anhydrite, buried to as much as 8000 m, can still have a high potential for hosting substantial hydrocarbon resources and should be globally targeted for future exploration.  相似文献   

11.
Microbial mediation is the only demonstrated mechanism to precipitate dolomite under Earth surface conditions. A link between microbial activity and dolomite formation in the sabkha of Abu Dhabi has, until now, not been evaluated, even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences. Such an evaluation is the purpose of this study, which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of Abu Dhabi, along a transect from the intertidal to the supratidal zone. This investigation revealed a close association between microbial mats and dolomite, suggesting that microbes are involved in the mineralization process. Observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats. In current models, microbial dolomite precipitation is linked to an active microbial activity that sustains high pH and alkalinity and decreased sulphate concentrations in pore waters. Such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone. By contrast, these models cannot be applied to the supratidal zone, where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity. As no abiotic mechanism is known to form dolomite at Earth surface conditions, two different hypotheses can reconcile this result. In a first scenario, all of the dolomite present in the supratidal zone formed in the past, when the mats were active at the surface. In a second scenario, dolomite formation continues within the buried and inactive mats. In order to explain dolomite formation in the absence of active microbial metabolisms, a revised microbial model is proposed in which the mineral‐template properties of exopolymeric substances play a crucial role.  相似文献   

12.
The Lower Triassic Kangan Formation in the Persian Gulf (South Pars Gas Field) and its adjacent areas are composed of carbonate–evaporite sequences. These sediments were deposited in a shallow marine homoclinal ramp. Study of the anhydrite-bearing intervals shows various structures and textures. The anhydrite structures are mainly bedded, massive, chicken-wire and nodular type and the main textures are felted, sparse crystal, needle shape, lath shape, equant and fibrous. Pervasive and poikilotopic cement together with replacement and porphyroblastic gypsum are accounted as the most common diagenetic features in anhydrite. Evaluation of anhydrite occurrences and features support both primary and secondary formations. The nodular to chicken-wire anhydrite formed under synsedimentary sabkha conditions, whereas anhydrite cements occurred during the late stages of diagenesis (shallow burial stage). Massive to bedded anhydrite could have been formed under subaqueous conditions or originated by coalescing and continued growth of anhydrite nodules in the sabkha zone. Anhydrite fabrics impose a significant control on the reservoir quality of the Kangan carbonates at the South Pars Gas Field. Thick massive and bedded anhydrite could have been formed as an intraformational seals and anhydrite cements occluded pore spaces and reduced the poroperm values. The sequence stratigraphic analysis revealed two depositional sequences in the studied intervals, which are composed of TST and HST. Investigation of anhydrite throughout depositional sequences indicates a change in the content and style of anhydrite texture. Anhydrite content (volume) decreases upward through transgressive system tract (sea-level rise) whereas, it enhances during highstand system tract (sea-level fall). Pervasive and poikilotopic anhydrite cements together with replacement by anhydrite are prevalent features during transgressive and early highstand system tract. At the late HST, with a progradational stacking pattern, anhydrite value increases and felted, radial, equant, crystalline and mosaic texture are the most common anhydrite fabrics. Sequence boundaries that indicate maximum sea level fall and exposure of successions are marked by the broad anhydrite deposits with massive to bedded and chicken-wire structures and various textures that located in late HST package. There is an unambiguous relationship between the microfacies associations, the evaporite textures, and the sea-level fluctuations. This relationship could lead to a predictable pattern that can be of use as a general guide for the sequence stratigraphic interpretations in the area.  相似文献   

13.
《Applied Geochemistry》2000,15(3):265-279
The conditions for mineral alteration and formation damage during CO2 treatment of Tensleep sandstone reservoirs in northern Wyoming, USA, were examined through core-flooding laboratory experiments carried out under simulated reservoir conditions (80°C and 166 bars). Subsurface cores from the Tensleep sandstone, which were cemented by dolomite and anhydrite, and synthetic brines were used. The brines used were (Ca, Mg, Na)SO4–NaCl solution (9.69 g/l total dissolved solids) for Run 1 and a 0.25 mol/l NaCl solution for Run 2. The solution used in Run 1 was saturated with respect to anhydrite at run conditions, which is characteristic of Tensleep Formation waters.Three major reactions took place during flooding, including (1) dissolution of dolomite, (2) alteration of K-feldspar to form kaolinite, and (3) precipitation (in Run 1) or dissolution (in Run 2) of anhydrite. All sample solutions remained undersaturated with respect to carbonates. The permeability of all the cores (except one used in Run 2) decreased during the experiments despite the dissolution of authigenic cement. Kaolinite crystal growth occurring in pore throats likely reduced the permeability.Application of the experimental results to reservoirs in the Tensleep Formation indicates that an injection solution will obtain saturation with respect to dolomite (and anhydrite) in the immediate vicinity of the injection well. The injection of NaCl-type water, which can be obtained from other formations, causes a greater increase in porosity than the injection of Tensleep Formation waters because of the dissolution of both dolomite and anhydrite cements.  相似文献   

14.
ALI M'RABET 《Sedimentology》1981,28(3):331-352
Combined field, sedimentological, mineralogical, isotopic and geochemical study of the Lower Cretaceous dolomites of Central Tunisia has demonstrated considerable diversity in origin. Environments of dolomite formation include deep phreatic, karst, lacustrine and evaporitic sabkha. All four groups of dolomite are composed of non-stoichiometric and/or disordered crystals which are more or less rich in calcium and in iron. Petrographic fabrics are of three types: replacement, recrystallization and cementation. These three fabrics are proposed among the various criteria for the different environments of dolomitization. Average isotope (δ18 O and δ13 C) contents for these four dolomite groups range from ?10·5 to +0·4%o (PDB) and ?3·9 to + 3·7%o respectively. The distribution of strontium is related both to the degree of recrystallization and to the palaeosalinity. Ferrous iron, also very common, is regarded as an indicator of relatively deep reducing conditions, mainly in meteoric groundwaters. Sodium distribution is related to inclusions within the dolomite, its distribution being relatively constant in all four groups; it cannot be regarded as a reliable criterion for palaeosalinity of dolomitizing fluids. This study confirms that dolomitization may occur under widely different palaeoenvironments, either at the surface or during burial. With the exception of the sabkha environment, dolomitizing fluids seem to have been essentially meteoric.  相似文献   

15.
Diagenetically altered Pleistocene dolomite occurs in the shallow subsurface of the Arabian Gulf, offshore of Al Jubayl, Saudi Arabia. This dolomite accumulated in relatively shallow marine to sabkha depositional environments. In contrast with the thin extent of most other Quaternary sabkha and sabkha-related dolomite deposits, these deposits comprise a thick (>56 m) accumulation. Additionally, this Pleistocene dolomite displays a high degree of ordering and has a more nearly ideal stoichiometric composition than the dolomite from the depositionally and diagenetically analogous Abu Dhabi sabkha complex. The Pleistocene dolomite also has lower δ13 and δ18O values than the modern Abu Dhabi sabkha dolomite, and higher values than those commonly reported for analogous dolomite from the ancient rock record. The low δ18O values, in conjunction with the geological setting, indicate that the diagenetic waters were meteoric or mixed meteoric and marine in composition. Thus, the degree of ordering, stoichiometric and stable isotopic values indicate that this dolomite has undergone diagenetic alteration relative to its presumed Holocene precursor.  相似文献   

16.
The isotopic composition of diagenetic carbonates, sulfates, illites and associated diagenetic constituents of the Carboniferous Juruá sandstones, Solimões Basin, largest hydrocarbon reservoirs of northern Brazil, was analyzed in conjunction with petrographic characterization and chemical composition. Eolian Juruá sandstones, deposited in a coastal sabkha of a wide cratonic sag, are the best reservoirs, but diagenetic processes introduced strong and complex heterogeneities, which affect hydrocarbon production. Dolomite and anhydrite cementation, together with compaction, exerts the main control on the quality of the reservoirs. Early dolomite cements precipitated under strong evaporation, while late dolomites are related to thermal decarboxylation. Late anhydrite cements are related to the interbedded evaporites and possibly to the oxidation of dissolved sulfide. Authigenic illite K–Ar ages are related to the voluminous Triassic basic magmatism (around 200 Ma), and to the Jurassic–Cretaceous tectonism (around 150 Ma), which may have also affected the late anhydrite and dolomite–ankerite cementation. This isotopic study revealed the major conditions of the diagenetic processes that strongly affected the Juruá reservoirs, which are essential for the development of models that will optimize the production, as well as decrease the risks in the exploration for new Juruá reservoirs.  相似文献   

17.
The Kimmeridgian‐Tithonian aged Arab Formation, as the main reservoir of the Jurassic succession in the Balal oilfield, located in the offshore region of the Iranian sector of the Persian Gulf, is investigated in this study. The formation is composed of dolomites and limestones with anhydrite interbeds. Based on detailed petrographic studies, six microfacies are recognized, which are classified in four sub‐environments including supratidal, intertidal, lagoonal and the high energy shoal of a homoclinal carbonate ramp. The main diagenetic features of the studied succession include dolomitization, anhydritization, cementation, micritization, fracturing and compaction. Based on stable isotope data, dolomitization of the upper Arab carbonates is related to sabkha settings (i.e. evaporative type). In terms of sequence stratigraphy, three shallowing‐upward sequences are recognized, based on core and wireline log data from four wells of the studied field. Considering depositional and diagenetic effects on the reservoir quality, the studied facies are classified into eight reservoir rock types (RRT) with distinct reservoir qualities. Dolomitization has played a major role in reservoir quality enhancement, whereas anhydritization, carbonate cementation, and compaction have damaged the pore throat network. Distribution of the recognized RRTs in time and space are discussed within the context of a sequence stratigraphic framework.  相似文献   

18.
鄂尔多斯盆地中奥陶统马五4亚段是靖边气田重要的天然气勘探层段,目前相关的岩相古地理图多以段和亚段为单位、以盆地为尺度,作图精度不能满足油气勘探开发的需要。以靖边潜台西侧为研究区,根据该区的古地貌特征、标志性矿物（硬）石膏的类型及其环境意义,分马五43、马五42和马五41三个小层开展岩相古地理工作。根据沉积背景和地层等厚图将该区中部厚度较大、地形略陡的区域解释为洼地,将洼地周边厚度较小、地形平缓的区域解释为坪。目的层段（硬）石膏有块状、球状结核和晶体等三种类型,块状硬石膏与暗色泥质藻纹层白云岩互层,代表了一种浅水水下蒸发、间或遭海水漫侵的潮上环境;球状硬石膏结核分散于浅黄色泥粉晶白云岩中,为准同生成因,代表了蒸发、偏氧化、变盐度的潮上环境;石膏晶体相对较少,多为柱状,或与球状硬石膏结核混生,代表的沉积环境与球状硬石膏结核基本相同。块状硬石膏主要分布于洼地中,为潮上带硬石膏洼地,根据硬石膏的含量进一步细分为含块状硬石膏白云岩洼地、块状硬石膏质白云岩洼地和白云质块状硬石膏洼地;球状硬石膏结核及石膏晶体主要分布于古地貌的坪中,结合其局限蒸发潮上带的背景,将其命名为球状硬石膏结核白云岩潮坪。马五43、马五42和马五41的岩相古地理格局基本一致,但从下向上,硬石膏洼地范围逐渐收缩,块状硬石膏的含量也逐渐下降,反映了沉积过程中水体逐渐蒸发变浅的过程。  相似文献   

19.
Recent supratidal anhydrite (CaS04) is reportedly confined to a latitudinal range of ˜ 24–30°N. Because of this apparent constraint, it is problematical to relate known distributions of ancient deposits to palaeomagnetically determined latitudes. Recent work on the Red Sea coast of the Sudan shows that anhydrite is present south of 18°N and possibly down to 15°N latitude. In Arabia and North America ancient sabkha anhydrites occupy near-equatorial positions, which imply climatic belts within subtropical high-pressure cells. There is no reason why anhydrite should not occur at lower latitudes, provided the depositional area is within an effective rain-shadow where evaporation far exceeds rainfall (as in the Red Sea region).  相似文献   

20.
塔里木盆地寒武系广泛发育白云岩,其中中寒武统地层沉积了巨厚的蒸发岩,蒸发岩和白云岩可以构成很好的储盖组合,该领域也是塔里木盆地未来勘探的新领域。萨布哈白云岩和渗透回流白云岩是两种与蒸发岩相关的白云岩,其储层发育的主控因素也都与蒸发岩存在着直接或间接的联系。通过对大量岩芯、薄片、主量元素、稀土元素、碳、氧稳定同位素及锶稳定同位素等地球化学测试和测井资料的细致研究,系统总结了它们的岩石学特征、地球化学特征。萨布哈白云岩储层的岩性以含石膏的泥 粉晶白云岩为特征,孔隙类型主要为膏模孔和溶塌角砾砾间孔,萨布哈白云石化作用及伴生石膏的沉淀作用和准同生期大气淡水溶蚀作用是该类储层的主控因素;渗透回流白云岩储层的岩性以保留原岩颗粒或藻（丘）格架结构的粉晶白云岩为特征,孔隙类型主要为粒间孔、铸模孔、格架孔,渗透回流白云石化作用和大气淡水溶蚀作用是该类储层的主控因素。两者主要发育于塔里木盆地干旱气候背景下的中-下寒武统地层,其分布规律主要受沉积相控制。  相似文献   

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