Fluid–rock interactions associated with regional tectonics and basin evolution

An integrated approach consisting of fracture analysis, petrography, carbon, oxygen and strontium‐isotope analyses, as well as fluid‐inclusion micro‐thermometry, led to a better understanding of the evolution of fluid–rock interactions and diagenesis of the Upper Permian to Upper Triassic carbonates of the United Arab Emirates. The deposited carbonates were first marked by extensive early dolomitization. During progressive burial, the carbonates were affected by dolomite recrystallization as well as precipitation of vug and fracture‐filling dolomite, quartz and calcite cements. After considerable burial during the Middle Cretaceous, sub‐vertical north–south oriented fractures (F1) were cemented by dolomite derived from mesosaline to hypersaline fluids. Upon the Late Cretaceous maximum burial and ophiolite obduction, sub‐vertical east–west fractures (F2) were cemented by dolomite (Dc2) and saddle dolomite (Ds) derived from hot, highly saline fluids. Then, minor quartz cement has precipitated in fractures from hydrothermal brines. Fluid‐inclusion analyses of the various diagenetic phases imply the involvement of increasingly hot (200°C) saline brines (20 to 23% NaCl eq.). Through one‐dimensional burial history numerical modelling, the maximum temperatures reached by the studied rocks are estimated to be in the range of 160 to 200°C. Tectonically‐driven flux of hot fluids and associated diagenetic products are interpreted to have initiated during the Late Cretaceous maximum burial and lasted until the Oligocene–Miocene compressional tectonics and related uplift. The circulation of such hydrothermal brines led to partial dissolution of dolomites (Dc2 and Ds) and to precipitation of hydrothermal calcite C1 in new (mainly oriented north–south; F3) and pre‐existing, reactivated fractures. The integration of the obtained data confirms that the diagenetic evolution was controlled primarily by the interplay of the burial thermal evolution of the basin and the regional tectonic history. Hence, this contribution highlights the impacts of regional tectonics and basin history on diagenetic processes, which may subsequently affect reservoir properties.     

川西中泥盆统观雾山组白云岩储层储集空间类型、成因及演化

川西地区观雾山组白云岩储层储集空间主要为孔洞和裂缝两类.为弄清观雾山组白云岩储层孔洞成因、孔洞充填期次及演化,首先,通过对孔洞型白云岩储层发育规律与沉积相、层序关系的分析,结合第一期白云石胶结物形成与围岩白云石化的先后顺序,认为川西地区观雾山组白云岩储层孔洞为相控准同生岩溶形成;针对孔洞内不同期次白云石和方解石胶结物的包裹体均一温度、碳氧同位素、激光原位U-Pb同位素定年、锶同位素、稀土元素等分析,认为孔洞内胶结物形成于封闭的成岩环境,成岩流体为受下伏碎屑岩地层水加入改造的中泥盆世海水.观雾山组白云岩储层储集空间经历了三个演化阶段:沉积期-白云石化之前的孔洞及裂缝形成阶段、白云石化期间的围岩白云石化及第一期白云石胶结物形成阶段和中—深埋藏成岩期的孔隙定型阶段,其中中—深埋藏阶段是孔隙减少的主要阶段,造成约250的孔隙损失.     

Multistage dolomitization and distribution of dolomitized bodies in Early Jurassic carbonate platforms (Southern Alps,Italy)

The Early Jurassic dolomitized carbonates are a hydrocarbon exploration target in Northern Italy. Of these carbonates, the Liassic Albenza Formation platform and the overlying Sedrina Formation shelf were studied to define a pervasive dolomitization model and to shed light on dolomite distribution in the sub‐surface. Field work, as well as analyses of well cores, stable isotopes, trace elements and fluid inclusions, was carried out on the outcropping thrust belt and sub‐surface deformed foreland of the Southern Alps. Petrographic analyses showed a first, pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3 and D4). The δ¹⁸O values fall between ?8·2‰ and 0·1‰ Vienna‐Pee Dee Belemnite with the more depleted samples belonging to dolomite cement‐rich dolostones; the δ¹³C ranges from 2·6‰ to 3·7‰ Vienna‐Pee Dee Belemnite. Analysis of trace elements showed different Fe and Mn contents in the sub‐surface and outcropping dolostones, and a higher Fe in the younger dolomite cements. An increase in the precipitation temperature (up to 130 °C from fluid inclusion data) and a decrease in diagenetic fluid salinity (from sea water to brackish) are observed from the first pervasive replacement dolomite to the dolomite cement phases. Field observations indicate that, in the Albenza Formation, dolomitization was limited to palaeohighs or faulted platform margins in the Early Jurassic carbonates. The pervasive replacement phase is interpreted based on a ‘compaction model’; the formation fluids expelled from compacting basinal carbonates could have funnelled along faults into permeable palaeohighs. The high homogenization temperature of the dolomite cements and decreased salinities indicate precipitation at great depth with an influx of meteoric water. These data, along with the thermal history, suggest that the dolomite cements precipitated according to the ‘tectonic squeegee’ dolomitization model. The dolomite precipitation temperature was set against the thermal history of the carbonate platform to interpret the timing of dolomite precipitation. The dolomite precipitation temperatures (90 to 100 °C) were reached in the studied formations first in the thrust fold belt (Early Tertiary, 60 Ma), and then in the foreland succession during the Late Tertiary (10 Ma). This observation suggests that the dolomite precipitation fronts moved southwards over time, recording a ‘diagenetic wave’ linked to the migration of the orogenic system. Observations suggest that the porosity increased during the first phase of replacement dolomitization while the dolomite cementation phases partially occluded the pores. The distribution of porous dolomitized bodies is therefore linked to the ‘compaction dolomitization’ model.     

开江-梁平地区上二叠统长兴组储层岩石学特征

以露头剖面测量、单井岩芯观察和室内薄片鉴定为依据,结合不同结构和成因类型白云岩(或白云石)的n(Mg)/n(Ca)比值,有序度和C、O、Sr同位素特征,对开江-梁平地区长兴组碳酸盐岩储层成岩作用与孔隙发育和演化规律进行了综合研究,确定研究区对储层破坏最大的成岩作用是胶结、压实-压溶、新生变形和次生矿物的充填等作用,而对储层贡献最大的建设性成岩作用主要为埋藏白云岩化、重结晶、溶蚀和破裂等作用,有效地改善了储层的孔渗性,形成以粒间溶孔、粒内溶孔、铸模孔、晶间溶孔、超大溶孔、溶洞和溶裂缝等多种次生孔隙类型组合的礁、滩相储层基本特征。在再造成岩序列和恢复孔隙演化历史的基础上,对储层发育与孔隙演化的关系可得出如下几点结论:①有利的礁、滩相带决定了储层发育的位置和空间展布规律;②礁、滩复合体的多期次埋藏白云岩化是形成储层的基础;③破裂作用和溶蚀作用是改造储层和提高储层质量的关键。     

Diagenesis of Ordovician carbonates from the north-east Michigan Basin, Manitoulin Island area, Ontario: evidence from petrography, stable isotopes and fluid inclusions

Middle to Late Ordovician subtidal carbonates in the Manitoulin Island area of Ontario are predominantly limestone in composition, but non-ferroan and ferroan dolomite is a common cement as well as a selective or locally pervasive replacement phase. Integration of field, petrographic, geochemical (δ¹³C, δ¹⁸O) and fluid inclusion data indicates that lithification of these carbonates occurred during burial diagenesis, with much of the alteration controlled by regional fracturing and hydrothermal influences. Aqueous (type 1) fluid inclusions in early calcite (pre-dolomite) and dolomite are saline (> 29 wt% NaCl eq.) solutions with Ca and/or Mg in excess of Na and display homogenization temperatures with modes of 95 and 101°C, respectively. These temperatures can be explained by significantly more burial than can be accounted for either by the available stratigraphic information or by an unusually high palaeogeothermal gradient, which also is not well supported. The fluid inclusion temperatures are interpreted to have resulted from hydrothermal fluids which circulated during the burial diagenesis of these strata. Type 1 inclusions in late (post-dolomite) calcite are less saline (<19 wt% NaCl eq.) and have a bimodal distribution of homogenization temperatures with a relatively well defined low temperature peak similar to those in early calcite and dolomite and a broad higher temperature grouping with a mode at 183°C. A small proportion of methane and light hydrocarbon-bearing fluid inclusions (type 2) are present in all stages of carbonate. Dolomitizing fluids were derived from burial compaction of argillaceous sediments in the more central parts of the Michigan Basin and the updip migration of these brines along fractures to the basin margin where the carbonates of the Manitoulin Island area were dolomitized. Alternatively, migration of dolomitizing brines downward from the overlying pervasively dolomitized Silurian sequence into fractures in the Ordovician carbonates may have occurred. Integration of the aqueous fluid inclusion data into the diagenetic history of these carbonates remains equivocal because most of the inclusions are secondary or indeterminate in origin. Nevertheless, high salinities resulting from interaction with evaporitic strata and hydrothermal effects are clearly implicated although the origin of the latter remains unclear. The alteration styles of the Ordovician carbonates in the Manitoulin area are similar to those of Ordovician hydrocarbon reservoirs described from other parts of the Michigan Basin. They indicate that fracture-related diagenesis occurred on a basin-wide scale and that hydrothermal effects were important.     

Burial dolomitization and porosity development in a mixed carbonate-clastic sequence: an example from the Bowland Basin, northern England

Widespread dolomitization and leaching occur in the Asbian to Brigantian (Dinantian) sequence of the Bowland Basin. Within this mudrock-dominated succession, dolomite is developed in calcarenites and limestone breccia/conglomerates deposited in a carbonate slope environment (Pendleside Limestone) and also within graded quartz wackes deposited by density currents in a generally ‘starved’ basin environment (Pendleside Sandstone). The dolomitized intervals range in thickness from less than one metre to several tens of metres and have a stratabound nature. All stages of calcite cement pre-date dolomitization and calcite veins are dolomitized. Dolomite crystals replace neomorphic spar and may also contain insoluble residues that were concentrated along stylolites. Thus dolomitization was a late stage process within the carbonate diagenetic sequence. A late-stage diagenetic origin is also indicated within the sandstones, with dolomite post-dating the development of quartz overgrowths. Six main textural styles of dolomite are observed: (1) scattered; (2) mosaic; (3) subhedral to euhedral rhombic; (4) microcrystalline; (5) single crystal and (6) saddle. The style of dolomite developed is dependent on the host rock mineralogy, on whether it is space-filling or replacive and also on temperature. Chemically the dolomite varies from near stoichiometric compositions to ankeritic varieties containing up to 20 mole % FeCO₃. Generally the dolomites have isotopic compositions depleted in δ¹⁸O compared to the host limestone, with similar or lighter δ¹³C values. Initial dolomite was of the scattered type, but with progressive replacement of the host a mosaic dolostone with a sucrosic texture was produced. There was a general increase in the Fe and Mn content and reduction in δ¹⁸O ratio of the crystals during dolomitization. Leaching is restricted to partly dolomitized horizons, where calcite, feldspars, micas, clays and, to some extent, dolomite have been leached. This has produced biomouldic and vuggy secondary porosity within the carbonates, whereas in the sandstones honeycombed, corroded and floating grains associated with oversized pores occur. Porosity within both carbonates and sandstones is reduced by ferroan dolomite/ankerite cements. Field, petrographic and chemical characteristics indicate that dolomitizing solutions were predominantly derived from the enclosing mudrocks (Bowland Shales) during intermediate/deep burial. Fluid migration out of the mudrocks would have been sided by dehydration reactions and overpressure, the fluids migrating along the most permeable horizons—the coarse grained carbonates and sandstones that are now dolomitized and contain secondary porosity.     

Dolomitization of the Capitan Formation forereef facies (Permian, west Texas and New Mexico): seepage reflux revisited

Abstract Interpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines. This study looked at the partially dolomitized forereef facies of the Capitan Formation to test the extent of reflux dolomitization and evaluate the possible role of the near‐backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids. The Capitan Formation forereef facies ranges from 10% to 90% dolomite. Most of the dolomite is fabric preserving and formed during early burial after marine cementation, before and/or during evaporite cementation and before stylolitization. Within the forereef facies, dolomite follows depositional units, with debris‐flow and grain‐flow deposits the most dolomitized and turbidity‐current deposits the least. The amount of dolomite increases with stratigraphic age and decreases downslope. Within the reef facies, dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and, in contrast, is completely dolomitized. This dolomite distribution supports dolomitization by sinking fluids. Oxygen isotopic values for fabric‐preserving dolomite (δ¹⁸O = 0·9 ± 1·0‰, N = 101) support dolomitization by sea water to isotopically enriched sea water. These values are closer to the near‐backreef dolomite (δ¹⁸O = 2·1 ± 0·7‰, N = 48) than the hypersaline backreef dolomite (δ¹⁸O = 3·6 ± 0·9‰, N = 11). Therefore, the fabric‐preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near‐backreef carbonate lagoon. The occurrence of mesosaline brine reflux in the Capitan Formation has important implications for dolomitization in forereef facies and elsewhere. First, any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present. Secondly, such brines may travel significant distances vertically provided permeable pathways (such as fractures) are present. Therefore, the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration.     

Multiple Dolomitization and Fluid Flow Events in the Precambrian Dengying Formation of Sichuan Basin,Southwestern China

The Precambrian Dengying Formation is a set of large-scale, extensively dolomitized, carbonate reservoirs occurring within the Sichuan Basin. Petrographic and geochemical studies reveal dolomitization was a direct result of precipitation by chemically distinct fluids occurring at different times and at different intensities. Based on this evidence, dolomitization and multiple fluid flow events are analyzed, and three types of fluid evolution models are proposed. Results of analysis show that Precambrian Dengying Formation carbonates were deposited in a restricted peritidal environment(630–542 Ma). A high temperature and high Mg~(2+) concentration seawater was a direct result of dolomitization for the micrite matrix, and for fibrous aragonite in primary pores. Geochemical evidence shows low δ~(18)O values of micritic dolomite varying from-1.29‰ to-4.52‰ PDB, abundant light rare earth elements(REEs), and low dolomite order degrees. Microbes and meteoric water significantly altered dolomite original chemical signatures, resulting in algal micritic dolomite and the fine-grained, granular, dolosparite dolomite having very negative δ~(18)O values. Finely crystalline cement dolomite(536.3–280 Ma) and coarsely crystalline cement dolomite have a higher crystallization degree and higher order degree. The diagenetic sequence and fluid inclusion evidence imply a linear correlation between their burial depth and homogenization temperatures, which closely resemble the temperature of generated hydrocarbon. Compared with finely crystalline dolomite, precipitation of coarsely crystalline dolomite was more affected by restricted basinal fluids. In addition, there is a trend toward a more negative δ~(18)O value, higher salinity, higher Fe and Mn concentrations, REE-rich. Two periods of hydrothermal fluids are identified, as the exceptionally high temperatures as opposed to the temperatures of burial history, in addition to the presence of high salinity fluid inclusions. The early hydrothermal fluid flow event was characterized by hot magnesium-and silicon-rich fluids, as demonstrated by the recrystallized matrix dolomite that is intimately associated with flint, opal, and microcrystalline quartz in intergranular or intercrystalline pores. This event was likely the result of a seafloor hydrothermal chimney eruption during Episode I of the Tongwan Movement(536.3±5.5 Ma). In contrast, later hydrothermal fluids, which caused precipitation of saddle dolomite, were characterized by high salinity(15–16.05 wt% NaCl equivalent) and homogenization temperatures(250 to 265°C), δ~(18)O values that were more enriched, and REE signatures. Geochemical data and the paragenetic sequence indicate that this hydrothermal fluid was related to extensive Permian large igneous province activity(360–280 Ma). This study demonstrates the presence of complicated dolomitization processes occurring during various paleoclimates, tectonic cycles, and basinal fluids flow; results are a useful reference for these dolomitized Precambrian carbonates reservoirs.     

Field geometry, petrography and geochemistry of a dolomitization front (Late Jurassic, central Lebanon)

This contribution describes the field geometry, petrography and geochemistry of a well-exposed dolomitization front in Upper Jurassic carbonates, and attempts to highlight the sedimentological, structural and relative sea-level controls on multiphase dolomitization and related diagenetic events. The data presented reflect the superposition of various diagenetic phases which have resulted in a single dolostone body, whose dimensions are well defined in the field. Local microbial intraclastic dolomites of Late Tithonian age accumulated in a hypersaline lagoon during relative sea-level fall. These pre-date beige hydrothermal dolostones (51 to 55 mol% CaCO₃; δ ¹⁸O: −9·3 to −4·0‰ V-PDB; δ ¹³C: −1·5 to +2·1‰ V-PDB; ⁸⁷Sr/⁸⁶Sr: 0·70742; matrix porosity: ≈6%; Klinkenberg permeability: ≈0·5 mD), whose dolomitizing fluid circulated along faults and invaded the nearby facies. First, the burrows were dolomitized, then the bulk rocks, resulting in the investigated 'tongue'-shaped dolomite body. Upon Late Jurassic–Early Cretaceous uplift, near-surface water percolated through – and altered – the underlying beige dolostones. This event was followed by a ferroan dolomite cement phase, which occurred during further burial. This contribution, featuring a well-defined geometric pattern of a dolomitization front with a large petrographic and geochemical data set, may also serve as a case study illustrating the complexity of superimposed diagenetic processes which have to be taken into account in modelling exercises of multiphase hydrothermal dolomitization.     

Application of numerical modelling to a case of compaction-driven dolomitization: a Jurassic palaeohigh in the Po Plain, Italy

Dolomitization of a carbonate platform can occur at different times and in different diagenetic environments, from synsedimentary to deep burial settings. Numerical simulations are valuable tools to test and select the model that, among different hypotheses compatible with field and geochemical data, best honour mass balance, kinetic and thermodynamic constraints. Moreover, the simulation can predict the distribution of the dolomitized bodies in the subsurface and evaluate porosity changes; valuable information for the oil industry. This study is the first attempt to reproduce and investigate the compaction dolomitization model. The diagenetic study of the Jurassic carbonate basin and palaeohigh system of the Po Plain indicates that the carbonates of the palaeohighs were dolomitized by basin compaction fluids. The main goal of the simulations is to evaluate the origin and evolution of the dolomitizing fluids and to provide insights regarding the distribution of the potential reservoir‐dolomitized bodies in the Po Plain. The modelling process is subdivided into two steps: basin modelling and reactive transport modelling. The SEBE3 basin simulator (Eni proprietary) was used to create a three‐dimensional model of the compacting system. The results include compaction fluid flow rate from the basin to the palaeohigh, compaction duration and a determination of the total amount of fluid introduced into the palaeohigh. These data are then used to perform reactive transport modelling with the TOUGHREACT code. Sensitivities on dolomite kinetic parameters suggest that dolomitization was an efficient process even at low temperatures, with differences mainly related to the dynamics of the process. Fluid composition is one of the main constraints, the sea water derived compaction fluid is proven to be efficient for dolomitization due to its relatively high Mg content. Simulations also confirmed that permeability is the most important factor influencing fluid flow and, consequently, the dolomite distribution in the formation. Permeable fractured zones have a strong influence, diverting the dolomitizing fluids from their normal path towards overlying or lateral zones. Moreover, the simulations showed that, after dolomite replacement is complete, the dolomitizing fluids can precipitate dolomite cement, causing over‐dolomitization, with related localized plugging effects in the zone of influx. Mass balance calculations indicate that in the dolomitization compaction model, the amount of compaction water fluxed from the basin to the carbonate is the main constraint on dolomitization efficiency. This observation implies that the ratio between the volume of the basin undergoing compaction and the volume of the palaeohigh is a limiting factor on the final size of the dolomitized bodies. An isolated palaeohigh could be an ideal site for pervasive replacement dolomitization due to the large volume of compaction fluids available compared with the carbonate rock volume. In the case of large platforms, the more permeable margin lithofacies are the most likely sites for compaction model dolomitization. The combined use of a basin simulator and reactive transport modelling has proved to be a successful method to verify model reliability and it provides insights into the volumetric distribution of diagenetic products.     

Genesis of Dolomite from Ma55–Ma510 Sub-members of the Ordovician Majiagou Formation in the Jingxi Area in the Ordos Basin

We clarified three stages of dolomitization and secondary changes by studying the petrology and geochemistry characteristics of dolomite from the Ma55–Ma510 sub-members of the Ordovician Majiagou Formation in the Jingxi area in the Ordos Basin: (1) Syngenetic microbial dolomitization is characterized by formation of dolomite with a mainly micrite structure and horse tooth-shape dolomite cements. (2) Seepage reflux dolomitization during the penecontemporaneous period superposed adjustment functions such as recrystallization and stabilization in the middle-deep burial stage, forming dolomites mainly consisting of micro crystal and powder crystal structure. (3) Powder dolomite, fine dolomite, and medium-coarse crystalline dolomite formed in pores and fractures in the middle-deep burial stage. The secondary concussive transgression-regression under a regressive background is an important condition for the occurrence of many stages of dolomitization in the study area. The basin was an occlusive epicontinental sea environment in the Ma5 member of the Ordovician Majiagou Formation sedimentary period. In the sediments, sulfate content was high, which is conducive to the preservation of microbial activity and microbial dolomitization. Micritic dolomite formed by microbial dolomitization provides good migration pathways for seepage reflux dolomitization. Affected by evaporation seawater with increased Mg/Ca ratio, seepage reflux dolomitization was widely developed and formed large-scale dolomite, and underwater uplifts and slopes are favorable areas for dolomite. In the middle-deep burial stage, dolomitizing fluid in the stratum recrystallized or stabilized the previous dolomite and formed a small amount of euhedral dolomite in the pores and fractures.     

基于激光U-Pb定年的埋藏白云岩形成过程——以塔里木盆地永安坝剖面下奥陶统蓬莱坝组为例

埋藏白云石化作用是形成厚层块状白云岩的主要机制之一,但其形成过程一直存在争议。本文以塔里木盆地永安坝剖面蓬莱坝组为例进行解剖,在露头和薄片岩石学研究的基础上,利用激光U-Pb定年和同位素分析,剖析了蓬莱坝组白云岩形成时期及演化过程,取得三个方面的认识：（1）蓬莱坝组发育四种类型白云岩：藻纹层白云岩、自形-半自形细中晶白云岩、雾心亮边自形中晶白云岩和他形粗晶白云岩,不同类型白云岩垂向互层发育;（2）U-Pb定年结果显示蓬莱坝组受三期云化作用改造,分别为准同生期云化作用、晚奥陶世到志留纪浅埋藏云化作用（464±12Ma到433±22Ma）及泥盆纪埋藏云化作用（382±29Ma）,浅埋藏云化作用会对准同生白云石造成重结晶,而埋藏云化作用表现为白云石次生加大,存在寒武系云化流体卷入,影响U-Pb定年;（3）规模白云岩的发育为沉积环境和构造埋藏演化史共同作用的结果,提出塔中北斜坡和塔北南缘为规模白云岩发育区,这对本区油气勘探具有重要的指导意义。     

湖北秭归地区震旦系灯影组白云岩地球化学特征及其成因意义

白云岩化流体性质与成岩作用是近年来碳酸盐岩成岩作用研究的热点,研究白云岩的成因有利于进一步认识白云岩储层的形成机制并为优质储层的预测提供依据。通过岩石学、矿物学(X射线衍射)、地球化学(微量元素、稀土元素、碳氧同位素)方法,系统研究湖北秭归地区灯影组不同类型白云岩的成因,并分析了可能的白云岩化模式。样品的微量元素特征显示,秭归地区白云岩未受到陆源碎屑物质的影响,形成于气候干旱、海水咸度较大且氧化的沉积环境中;Sr含量特征显示白云岩发生了较为彻底的白云岩化,其成岩环境为温度较高的埋藏环境,成岩过程中未受到淋滤作用的影响;秭归地区白云岩化流体主要来源于海水。结合蒸发白云岩(萨布哈)及埋藏白云岩化模式解释了秭归地区泥微晶白云岩及晶粒白云岩的形成过程。     

Dolomite characteristics and diagenetic model of the Calcari Grigi Group (Asiago Plateau,Southern Alps – Italy): an example of multiphase dolomitization

A multidisciplinary study, conducted over the carbonate platform deposits of the Liassic Calcari Grigi Group (Southern Alps), highlighted how the use of outcrop analogues can contribute to better define the distribution of dolomitic bodies related to fault networks, to characterize the petrophysical properties of the dolomitic sequence and unravel a complex diagenetic history. This study was carried out in the Asiago Plateau (southernmost part of the eastern Southern Alps, northern Italy) which provides excellent outcrops of the Jurassic Calcari Grigi Group. The dolomitization of the Jurassic sequence is variable in terms of stratigraphic extension and geographic distribution. In the studied localities the dolomitization is generally limited to the Mount Zugna Formation and is characterized by an undulatory front, with ‘sub‐vertical dolomitic chimneys’ along the major faults. Within this unit, and often associated with faults, stacked high‐porosity and permeability bed‐parallel dolomitic bodies are developed that show excellent petrophysical properties. The dolomitic intervals are characterized by pervasive unimodal and patchy polymodal dolomite crystals. Thin section, cathodoluminescence, isotopic and fluid inclusion analyses were used to constrain the paragenetic evolution of the sequence which is similar in all the studied localities. The first dolomitization stage is marked by zoned dolomite crystals with a dull luminescent core. The porosity is thought to have increased after this stage, with dark blue luminescent dolomite accompanied by the corrosion of older crystals. The appearance of saddle dolomite marks the onset of the porosity reduction stage, ending with the infilling of vugs and the remaining open pores with calcite cement. The diagenetic evolution locally stopped at the saddle dolomite stage with the complete occlusion of the remaining pores. Paragenetic and fluid‐inclusion data suggest an evolutionary trend of increasing temperatures and decreasing salinity toward brackish fluids responsible for dolomite and calcite precipitation. The integration of the available data seem to indicate that the diagenetic evolution of the study area is related to: (i) the interplay between evolving fluids (from marine to brackish); (ii) the burial of the sequence (increasing temperature); and (iii) the evolution of the hydrogeological system (fault and fracture network, fluid mixing). This complex paragenetic evolution is strongly linked to the evolution of the porosity framework that evolved from a good, widespread network in the early stages of the burial history to a confined system in the later stages due to reduction of porosity by the deposition of late calcite and dolomite cements.     

Dolomite-rock textures and secondary porosity development in Ellenburger Group carbonates (Lower Ordovician), west Texas and southeastern New Mexico

Pervasive early- to late-stage dolomitization of Lower Ordovician Ellenburger Group carbonates in the deep Permian Basin of west Texas and southeastern New Mexico is recorded in core samples having present-day burial depths of 1.5–7.0 km. Seven dolomite-rock textures are recognized and classified according to crystal-size distribution and crystal-boundary shape. Unimodal and polymodal planar-s (subhedral) mosaic dolomite is the most widespread type, and it replaced allochems and matrix or occurs as void-filling cement. Planar-e (euhedral) dolomite crystals line pore spaces and/or fractures, or form mosaics of medium to coarse euhedral crystals. This kind of occurrence relates to significant intercrystalline porosity. Non-planar-a (anhedral) dolomite replaced a precursor limestone/dolostone only in zones that are characterized by original high porosity and permeability. Non-planar dolomite cement (saddle dolomite) is the latest generation and is responsible for occlusion of fractures and pore space. Dolomitization is closely associated with the development of secondary porosity; dolomitization pre-and post-dates dissolution and corrosion and no secondary porosity generation is present in the associated limestones. The most common porosity types are non-fabric selective moldic and vuggy porosity and intercrystalline porosity. Up to 12% effective porosity is recorded in the deep (6477 m) Delaware basin. These porous zones are characterized by late-diagenetic coarse-crystalline dolomite, whereas the non-porous intervals are composed of dense mosaics of early-diagenetic dolomites. The distribution of dolomite rock textures indicates that porous zones were preserved as limestone until late in the diagenetic history, and were then subjected to late-stage dolomitization in a deep burial environment, resulting in coarse-crystalline porous dolomites. In addition to karst horizons at the top of the Ellenburger Group, exploration for Ellenburger Group reservoirs should consider the presence of such porous zones within other Ellenburger Group dolomites.     

Early Diagenetic Dolomite as a Potential Archive of Paleo-redox Fluctuations in an Early Tonian Marine Basin?

The Tonian Period witnessed important environmental changes and critical evolutionary innovations. Published iron speciation data suggest a global redox transition of mid-depth seawaters from euxinic to ferruginous in early Tonian, but details of this transition remain unknown. This study explores Tonian stromatolitic carbonates as a possible archive of paleoenvironmental changes, through the investigation of dolomitic limestones and dolostones associated with stromatolites of the Weiji Formation in the Huaibei region of North China. Three types of dolomitization are recognized on the basis of petrographic and geochemical data. Type I and II dolomitization resulted in dolomitic limestones characterized by LREE depletions, MREE enrichments, positive yttrium anomalies, and a lack of europium anomalies, indicating early diagenetic dolomitization, possibly in the iron reduction zone and under the influence of bottom seawater. The lack of cerium anomalies in these carbonates suggests anoxia in shallow marine environments. The coexistence of ferroan/non-ferroan dolomite crystals and overgrowth bands is interpreted as possible evidence for rapid fluctuations between iron-rich and iron-depleted conditions in pore-waters or seawaters. In contrast, type III dolomitization resulted in pervasively dolomitized stromatolitic carbonates and likely represents late diagenetic processes. This study highlights the potential of early diagenetic dolomite as an archive for paleoenvironmental investigations.     

Burial dolomitization of the Woo Dale Limestones Formation (Lower Carboniferous), Derbyshire, England

Dolomite occurs in open-shelf packstones and grainstones of the Vincent House Member of the Woo Dale Limestones Formation (Lower Carboniferous) in the Wye Valley area, east of Buxton, Derbyshire, Central England. Two stages of dolomitization can be recognized. Stage 1 consists of limpid, non-luminescent dolomite lacking detectable iron and manganese. Stage 2 comprises cloudy, inclusion-rich, orange-red luminescing dolomite with significant iron and manganese. The contact between the two stages is irregular and suggests a hiatus between dolomite growth stages. Dolostones lacking any unreplaced limestone may contain up to 35% calcite cement rilling intercrystal porosity. This cement shows a characteristic zonal sequence and is normally in optical continuity with the surrounding dolomite crystals. In some cases, precementation compaction of the dolomite mosaic caused fracturing of the crystals. Sherds of dolomite on the floors of pore-spaces then provided the nucleus for growth of calcite cement. The petrographic character and stratigraphic distribution of the dolomite suggest that dolomitization occurred at depth, during burial of the Woo Dale Limestones in the Late Carboniferous. Dolomitizing solutions, expelled from basinal shales, moved up-dip along permeable limestones or along the sub-Carboniferous unconformity. Fluids responsible for the generation of stage 2 dolomite may have been contaminated by ions such as iron and manganese released from the alteration of volcanic rocks which occur beneath the Carboniferous Limestone in the Wye Valley area.     

Tectonically driven dolomitization of Cambrian to Lower Ordovician carbonates of the Quruqtagh area,north‐eastern flank of Tarim Basin,north‐west China

Dolomites occur extensively in Cambrian to Lower Ordovician carbonates in the Tienshan orogen of the Quruqtagh area, north‐east Tarim Basin, where thick (up to 1 km), dark grey lenticular limestones of semi‐pelagic to pelagic origin are prominent. The dolomites generally occur as beige, anastomosed geobodies that cross‐cut well‐stratified limestones. Based on detailed field investigations and petrographic examination, three types of matrix dolomite are identified: fine crystalline planar‐e (Md1), fine to medium crystalline planar‐s (e) (Md2) and fine to coarse crystalline non‐planar‐a (Md3) dolomites. One type of cement dolomite, the non‐planar saddle dolomite (Cd), is also common. The preferential occurrence of Md1 along low‐amplitude stylolites points to a causal link to pressure dissolution by which minor Mg ions were probably released for replacive dolomitization during shallow burial compaction. Type Md2, Md3 and Cd dolomites, commonly co‐occurring within the fractured zones, have large overlaps in isotopic composition with that of host limestone, implying that dolomitizing fluids inherited their composition from remnant pore fluids or were buffered by the formation water of host limestones through water–rock interaction. However, the lower δ¹⁸O and higher ⁸⁷Sr/⁸⁶Sr ratios of these dolomites also suggest more intense fluid–rock interaction at elevated temperature and inputs of Mg and radiogenic Sr from the host limestones with more argillaceous matter and possibly underlying Neoproterozoic siliciclastic strata. Secondary tensional faults and fractures within a compressional tectonic regime were probably important conduits through which higher‐temperature Mg‐rich fluids that had been expelled from depth were driven by enhanced tectonic compression and heating during block overthrusting, forming irregular networks of dolomitized bodies enclosed within the host limestones. This scenario probably took place during the Late Hercynian orogeny, as the Tarim block collided with Tienshan island arc system to the north and north‐east. Subsequent downward recharges of meteoric fluids into the dolomitizing aquifer probably terminated dolomitization as a result of final closure of the South Tienshan Ocean (or Palaeo‐Asian Ocean) and significant tectonic uplift of the Tienshan orogen. This study demonstrates the constructive role of notably tensional (or transtensional) faulting/fracturing in channelling fluids upward as a result of intense tectonic compression and heating along overthrust planes on the convergent plate margin; however, a relatively short‐lived, low fluid flux may have limited the dolomitization exclusively within the fractured/faulted limestones in the overthrust sheets.     

Multiple dolomitization events along the Pozalagua Fault (Pozalagua Quarry,Basque–Cantabrian Basin,Northern Spain)

The Pozalagua Quarry in the Basque–Cantabrian Basin of northern Spain exposes a unique set of fault‐associated dolomites that can be studied on a decametre scale. The dolomites developed along the Pozalagua Fault system in slope‐deposited limestones of Albian age. Following marine phreatic diagenesis, the limestones were subject to meteoric karst formation. The resulting cavities were filled either by angular limestone fragments in a black clay‐rich matrix, or by cave floor/pond (now dolomitized) sediments. The subsequent diagenetic history reflects repeated periods of fracturing, fluid expulsion, dissolution and cementation. Contrasting fluid pulses resulted in the formation of a network of hydrothermal karst and the subsequent development of coarse‐crystalline calcite cement, zebra dolomite, recrystallized coarse‐crystalline dolomite, elongated blue–grey coarse‐crystalline dolomite cement in the open fault and, finally, coarse‐crystalline saddle dolomite. Decimetre‐size reworked host‐rock fragments present in the latter two dolomite phases probably reflect roof collapse fragments of a cave system that developed along the Pozalagua Fault system. However, there are also metre‐scale host‐rock fragments that apparently ‘float’ in the coarse‐crystalline saddle dolomites, implying that either fragment assimilation was a widespread process or violent expulsion of fluids occurred along the Pozalagua Fault system. The presence of pre‐dolomite and post‐dolomite stylolites, parallel to bedding, supports a linkage between the diagenetic events and the Late Albian tectonism that affected the region.     

Hydrothermal dolomitization in a complex geodynamic setting (Lower Palaeozoic,northern Spain)

Burial hydrothermal dolomitization is a common diagenetic modification in sedimentary basins with implications for oil and gas reservoir performance. Outcrop analogues represent an easily accessible source of data to refine the genetic models and assess risk in hydrocarbon exploration and production. The Palaeozoic succession of northern Spain contains numerous excellent exposures of epigenetically dolomitized limestones, particularly in the Carboniferous and Cambrian. The epigenetic dolomites in the Cambrian carbonates of the Láncara Formation are volumetrically small, but have a large aerial distribution across different tectonic units of the Variscan fold and thrust belt. Coarse crystals, abundant saddle dolomite cement, negative δ¹⁸O and fluid inclusion homogenization temperatures between 80°C and 120°C characterize these dolomites, which are petrographically and geochemically similar to the tens of kilometre‐sized hydrothermal dolomites replacing the Upper Carboniferous succession in the same area. In both cases, the dolomitizing fluids are derived from highly evaporated sea water, modified to a limited degree through fluid‐rock interaction. The dolomitization events affecting both Cambrian and Carboniferous strata are probably related to the same post‐orogenic hydrothermal fluid flow. The formation of the post‐collisional (latest Carboniferous) Cantabrian arc fostered dolomitization: the extension related to bending of the arc generated deep‐reaching faults and strike‐slip movements, which favoured the circulation of hot dolomitizing fluids in the outer parts of this orocline. A similar dolomitization process affected other areas of Europe after the main stages of the Variscan orogeny. Dolomitization was a continuous, uninterrupted, isochemical process. Limestone replacement resulted in a major porosity redistribution and focused the fluid flow into the newly created porous zones. Replacement was followed immediately by partial to complete cementation of the pores (including zebra fabrics and vugs) with saddle dolomite. The amount of porosity left depends on the volume of cement and therefore on the volume of fluids available.