Seepage Reflux Research Articles

The Development of Dolomite Within a Sequence Stratigraphic Framework: Cambrian Series 2 Changping Formation, Xiaweidian, China

The Lower Cambrian Changping Formation in the Western Hills of Beijing hosts tidal flat and lagoonal carbonates comprising dolomites, limestones, and dolomitic limestones, reflecting the processes of dolomite cementation and dolomitization within a sedimentary framework. Based on petrographic textures, two types of dolomites were identified: microcrystalline dolomite and fine-mesocrystalline dolomite. Integrating petrological and geochemical data unveils two diagenetic stages. The initial dolomite formation, attributed to hypersaline fluids, occurred in a supratidal-sabkha setting during the early Cambrian. The dolomitization at the top of the Changping Formation, driven by evaporatively concentrated brines from the overlying Mantou Formation, altered peritidal carbonates. This study evaluates the original sedimentary environment and dolomitization within a sequence stratigraphic context, revealing a correlation between dolomitization episodes and the stratigraphic framework in the study area. Factors influencing this framework profoundly impact depositional environments and material composition, leading to micromorphological differences in dolomites. Sabkha dolomite formation, associated with evaporative pumping, predominates near the base of transgressive systems tracts. Seepage reflux dolomite, often linked with evaporative pumping dolomite, constitutes a vertical cycle in the sequence framework. The sequence from bottom to top is sabkha microcrystalline dolomite, limestone and dolomitic limestone, seepage reflux saccharoidal dolostone, and sabkha dolomite.

Linking dolomitization to sequence stratigraphy: Insights from the Upper Jurassic Arab Formation, offshore oilfield, UAE

This petrographic, petrophysical, and geochemical study revealed that dolomitization in the Upper Jurassic Arab Formation reservoir, United Arab Emirates, can be constrained within 2nd, 3rd, and 4th order sequences. The formation represents a 2nd order regressive sequence/cycle (highstand systems tracts, HST) that was deposited across a carbonate ramp with shoal under hot arid climatic conditions during the Kimmeridgian-Tithonian. Deposition occurred subsequent to maximum marine transgression (maximum flooding surface, MFS) and was accompanied by a progressive increase in the degree of restriction of the connectivity between the inner platform (supratidal, upper intertidal and lagoon) and the open sea. Sporadic dolomitization took place in transgressive systems tracts (TST) of the 3rd and 4th order cycles of earliest stages of the 2nd order regression, below the parasequence boundaries and within bioturbation sites. Subsequent rapid and extended 2nd order marine regression cycles were embossed by trends of progressive increase in salinity, which resulted in concomitant systematic decrease in limestone deposition and increase in intensity of seepage reflux/sabkha dolomitization by seepage reflux of hypersaline brines. Dolomitization is most prevalent in the 4th order HST sequences and was accompanied by occlusion of the moldic and intercrystalline pores in the dolostones by gypsum/anhydrite cement. Dolomitization that was accompanied by limited formation of gypsum/anhydrite in the late 2nd order HST is attributed to reflux of penesaline (salinity saturated with Ca-sulfate) /mesohaline (below Ca-sulfate saturation) brines. The reflux of the latter brines is attributed to smaller extent of relative sea-level falls, i.e., partial restriction of the inner platform. The precipitation of dolomite cement in moldic pores and as thin overgrowths around replacive dolomite is attributed to the flow hot basinal brines (i.e., hydrothermal fluids). This interpretation is supported by the presence of saddle dolomite, along with depleted δ18O values, relative enrichment in 87Sr isotope, and high fluid-inclusions homogenization temperatures. The greater amounts of these post-dolomitization cements in the dolopackstones and dolograinstones caused stronger obliteration of their near-surface isotopic signatures compared to the dolomudstones and dolowackestones.This study demonstrates that linking dolomitization of limestones across carbonate ramps under hot-arid climatic conditions to different orders of changes in the relative sea level and lithology of the succession indicates the involvement of various types of dolomitizing fluids and geochemical conditions, including: (i) domination of seepage reflux of hypersaline and mesohaline/penesaline brines, (ii) dolomitization below the seafloor during marine transgression, and (iii) dolomitization within bioturbation sites.

Links between anhydrite precipitation and dolomitization during cycles of sea level change: Insights from the Late Jurassic Arab Formation, Abu Dhabi, United Arab Emirates

Despite extensive research in the field of evaporative dolomitization, the close link between anhydrite precipitation and dolomitization is still poorly explored within the context of sequence stratigraphy. This comprehensive diagenetic and sequence stratigraphic study of the Late Jurassic Arab Formation, Abu Dhabi, United Arab Emirates, provides important insights into the relationship between anhydrite precipitation and dolomitization within the 2nd, 3rd, and 4th orders of relative sea-level cycles. Initial good connectivity between the inner ramp and open sea during 2nd order late transgressive-early regression cycles accounts for the limestone-dominated lithology with limited anhydrite formation. Conversely, the following progressive 2nd order fall in the relative periodic restriction of the inner platform during 2nd order early regressive cycles (early highstand systems tracts, HST) caused seepage reflux dolomitization and the precipitation of scattered to pervasive anhydrite cement in the dolostones. Variations in the extent of dolomitization were controlled by the permeability and reactivity of the precursor limestones, resulting in the formation of what is known as dolomite fingers. The most laterally extensive dolomitization during late 2nd order regression resulted in the formation of microcrystalline dolostones with nodular and chickenwire anhydrite by a combination of evaporative sabkha pumping and the seepage reflux of lagoon brines. The δ1³C and δ1⁸O of calcite and dolomite reveal the influence of degree of restriction of the inner ramp, and related extent of seawater evaporation. The lower limestone-dominated interval (3rd order HST) is characterized by lower δ1³C values (+2.0‰ to +2.5‰) owing to periodic restriction of seawater circulation, which resulted in oxidation of organic matter during aging of the seawater. This study approach provides important insights into the genetic links between interbedded platform limestones, dolostones, and anhydrites, and hence a better understanding and prediction of reservoir quality distribution and compartmentalization.

Dolomitization history and fluid evolution of end-ediacaran multi-phase dolomites from the near-surface to deep burial depths in the tarim craton, northwestern China

The deeply buried (>8 km) end-Ediacaran Qigebrak Formation dolostones in the Tarim Basin, China, have significant petroleum potential, but have experienced multi-stage dolomitization that has resulted in the occurrence of diverse dolomite phases. The features and origins of the different dolomite types were merely investigated, which hinders an in-depth understanding of the complex dolomitization processes. This paper presents an integrated petrological, geochemical (C–O–Sr isotope and in situ trace element), fluid inclusion microthermometric, and dolomite U–Pb dating study of outcrop and core samples, to investigate the dolomitization mechanisms, and the nature and evolution of the dolomitizing fluids. The results indicate the occurrence of early dolomitization triggered by seepage–reflux of mesosaline seawater, which retained the primary marine geochemical properties of most of the matrix dolomite. Subsequently, during early diagenesis, fibrous dolomite cement was precipitated from marine porewaters, which have similar geochemical characteristics to the matrix dolomites. Following this, during intermediate-depth burial diagenesis in the Ordovician, fine-to medium-crystalline dolomite cements formed at temperatures of 80–117 °C in association with dissolution–reprecipitation of the matrix and early fibrous cements or diagenetic interactions with clay minerals of the lower Cambrian Yurtus Formation shales. During deep burial diagenesis and dolomitization in the Carboniferous, coarse-crystalline dolomite cements formed at temperatures of 104–138 °C from saline brines enriched in middle rare earth elements. Saddle dolomites, formed by hydrothermal dolomitization (162–184 °C) that was closely related to Permian–Triassic volcanic activity, have positive Eu anomalies and depleted δ18O values, that resulted from the mixing of basinal brines with 87Sr-enriched hydrothermal fluids. This study provides a detailed description of multiple dolomitization events in deep time that affected deeply buried dolostones.

Resistance of eogenetic dolomites to geochemical resetting during diagenetic alteration: A case study of the lower Qiulitage Formation of the Late Cambrian, Tarim Basin

Rare earth elements (REEs), carbon, oxygen and Mg isotopes have been widely used to explain the origin of dolomite, but dolomite may undergo complex diagenetic alteration during long geological processes, and whether the geochemical signals of precursor dolomite can be preserved is still unknown. In order to further explore the formation mechanism of dolomite and the influence of diagenetic alteration processes on geochemical parameters, this paper takes the thick-layer dolomite of the Lower Qiulitage Formation of the Late Cambrian in the Tazhong uplift area as an example and divides the dolomite into two categories through petrologic observation. One category is fabric-retentive dolomite (FRD), including algal clotted dolomite, algal psammitic or psammitic grain dolomite, and aphanocrystalline dolomite; the other category is fabric-destructive dolomite (FDD), including crystalline dolomite (CD) and siliceous dolomite (SD). Cation ordering, unit cell parameters, major/trace elements, REEs as well as C–O–Mg isotopes of the pure matrix powders of different types of dolomites are further analyzed. The results show that the dolomites are near-source/far-source seepage reflux type and sabkha type eogenetic origin, and the magnesium ions mainly come from Late Cambrian seawater; dolomite crystals of different sizes are the result of the precursor dolomite suffering varying degrees of recrystallization during the early diagenesis period, inheriting the original geochemical characteristics; hydrothermal alteration occurs in the late stage of diagenesis, with limited magnesium ions in the hydrothermal fluid. Its impact on the dolomite is mainly reflected in local dolomite distortion and a large amount of aphanocrystalline silica and coarse-grained quartz cement filling, which fails to obviously reset the geochemical information of the matrix dolomite. The above results demonstrate that, under favorable conditions, early genetic marine dolomites may resist geochemical resetting over time intervals of more than 400 million years.

Sedimentary control on diagenetic paths of dolomite reservoirs in a volcanic setting: A case study of the Permian Chihsia Formation in the Sichuan Basin, China

The formation mechanisms of multi-stage dolomites and hydrocarbon reservoirs associated with volcanic activity are complex. This study investigated the Permian Chihsia Formation in the Sichuan Basin, China, which is the location of recent discoveries of giant natural gas reserves. We undertook a petrological and geochemical study, including trace element and C–O–Sr isotopic analysis. The quality of the dolomite reservoirs associated with volcanic activity was controlled by different diagenetic pathways associated with the sediment types and fault systems. Outcrop, core, and thin-section observations identified three types of replacement dolomite: very finely to finely crystalline dolomite (Rd1), medium crystalline patchy dolomite (Rd2), and coarsely to medium crystalline dolomite (Rd3). The tidal flat sediments only experienced limited seepage reflux dolomitization, which resulted in the formation of the dense very finely to finely crystalline dolomite (Rd1). The bioclastic grain shoal sediments deposited in a high-energy shallow-water environment contain abundant intergranular and dissolution pores, and vugs, which allowed convection of seawater and dolomitization during the initial phases of volcanic activity of the late early Permian. Subsequently, the bioclastic grain shoal sediments were mostly replaced by coarsely to medium crystalline dolomite (Rd3), which formed the highest-quality reservoirs in the study area. In addition, the incompletely dolomitized medium crystalline patchy dolomite (Rd2) was formed due to the low original porosity of micritic, peloidal, grain shoal sediments during a transgression in the middle and late depositional stages. During the peak of volcanism at the end-Guadalupian, deep faults allowed seawater to mix with ascending hydrothermal fluids, which resulted in the precipitation of dolomite cement in pores near the fault fracture zones, which blocked the pores. Therefore, these high-energy shoal sediments deposited in the slope break zone far from strike-slip faults have a relatively high porosity. Our results provide new insights into the origin of dolomite reservoirs formed in association with a volcanic setting, and are applicable to deep dolomite reservoirs worldwide.

Variations in extent, distribution and impact of dolomitization on reservoir quality of Upper Cretaceous foreland-basin carbonates, Abu Dhabi, United Arab Emirates

Petrography, petrophysics, and geochemistry of an Upper Cretaceous, foreland-basin carbonate reservoir, Abu Dhabi, United Arab Emirates, are used to constrain the spatio-temporal variations in the extent of dolomitization and its impact on reservoir quality. Dolomitization in highstand systems tracts (HST) is attributed to repeated tidal and evaporative pumping as well as seepage reflux of penesaline brines during restriction of the platform due to 4th and 5th order cycles of relative sea-level fall and particularly below parasequence boundaries. This interpretation is supported by the presence of rare poikilotopic gypsum cement and scattered laths and micro-nodules of calcitized gypsum in the dolomitized peritidal dolostones. Dolomitization along bioturbation sites, which is most common in the transgressive systems tracts (TST), is attributed to the development of suitable localized geochemical conditions (e.g., microbial sulfate reduction and related increase in carbonate alkalinity). Porosity and permeability of the dolostones are strongly controlled by depositional textures of precursor limestones and by subsequent diagenetic evolution. Dolomitization of mudstones, wackestones and matrix-rich packstones has resulted in the formation of micropore-dominated microcrystalline dolostones whereas dolomitization of the shoal grainstones resulted in the formation of coarse-crystalline dolostones with abundant well-connected intercrystalline and moldic macropores. Mesogenetic alterations of the dolostones, which are attributed to the flow of hot basinal brines along steep faults, include dolomite cementation and, subsequently, dissolution and calcitization of dolomite (dedolomitization) and calcite cementation. The lack of systematic differences in porosity and permeability of dolostones between the oil and water-saturated dolostones suggest that most diagenesis occurred prior to completion of oil emplacement and/or reflect the shallow maximum burial depths of the formation (around 1.3 km). This study demonstrates that variations in the distribution and extent of dolomitization within a sequence stratigraphic context across an oilfield should be considered as primary control on the spatio-temporal reservoir lithology and heterogeneity in carbonate successions.

Genesis of Cambrian Dolomites in the Bachu Area, Tarim Basin, NW China: Constraints from Petrology, Geochemistry, and Fluid Inclusions

The dolomitization of carbonate rocks has always been a hot topic in the study of the dolomite reservoir. In this study, the genesis of Cambrian dolomite in the Bachu area, Tarim Basin, was assessed through petrographic examinations, isotope compositions (C, O, and Sr), trace and rare earth elements, and fluid inclusion microthermometry. Microscopic analysis revealed three types of dolomites: very fine-crystalline, nonplanar dolomite (D1); fine-crystalline, nonplanar to planar-s dolomite (D2); and medium- to coarse-crystalline, planar-e to planar-s dolomite (D3). D1 dolomite exhibits well-preserved original sedimentary features, such as algal laminae, stromatolite, and evaporite streak, and is characterized by the 87Sr/86Sr value and δ18O value in equilibrium with the coeval seawater, its high Sr and Na content, and its low Mn content. This indicates that D1 dolomite is primarily a penecontemporaneous dolomite in tidal flat or lagoon environments, and its dolomitizing fluid is mainly evaporated mesosaline to penesaline seawater. D2 dolomite shows ghosts of precursor particles; features δ13C values in equilibrium with the coeval seawater, high 87Sr/86Sr values, low Sr content, and positive Eu anomaly; and is widely distributed close to stylolite. This illustrates that D2 dolomite was principally formed by seepage–reflux dolomitization, and is closely related to hydrothermal activity and pressure dissolution. D3 dolomite displays a crystal texture with a cloudy core and compositional zoning, and the original sedimentary fabrics cannot be identified. It has similar δ13C values and REE patterns to the calcite precipitated from coeval seawater, high 87Sr/86Sr values, low Sr contents and high Mn/Sr ratios, which suggests that D3 dolomite is chiefly related to the recrystallization of the precursor dolomite during the deep burial stage, and the deep circular brine provides Mg ions through the fluid–rock reaction. This study shows that the Cambrian dolomite in the Bachu area is mainly formed in the coeval seawater environment during the penecontemporaneous and shallow burial stages, and has extensively suffered from recrystallization and burial diagenesis due to long-term deep burial, which was further strengthened in the fracture-enriched area.

Effects of diagenesis on quality of deep dolomite reservoirs: A case study of the Upper Cambrian Xixiangchi Formation in the eastern Sichuan Basin, China

With the Upper Cambrian Xixiangchi Formation in the eastern Sichuan Basin as the target, this study investigates various diagenetic events during different diagenetic stages in deep dolomite reservoirs, accompanied by evaluations of their effects on the formation and evolution of the reservoir rock. A series of experiments are implemented on core and outcrop samples, including petrologic analysis, fluid inclusion analysis, rare earth and minor element investigation, and carbon and oxygen isotope test. During the syngenetic (syndepositional and penecontemporaneous) diagenesis stage, dolomitization is closely related to evaporation concentration and seepage reflux of high-salinity seawater, which facilitates the reservoir rock development by greatly enhancing the permeability of the reservoir. Meanwhile, a small number of secondary pores are generated in the sediments subjected to episodic atmospheric exposure and thus affected by meteoric water. During the early diagenesis stage, recrystallization transforms part of the granular dolomite into the crystalline dolomite with or without the phantom of the grain texture. It also alters the original rock’s pore structure and improves the effective primary porosity. Thus, recrystallization is key in forming the crystalline dolomite reservoir rock. However, compaction, cementation, and filling lead to the loss of massive early-formed primary pores and some secondary pores. During the mesodiagenesis-late diagenesis stage, the burial karstification, related to organic matter maturation, is the most direct control factor of the effective reservoirs space formation, and its alteration effect on the reservoir rock is related to the early process. This research helps to better identify the impact of various diagenetic processes during different diagenetic stages upon the formation and evolution of the deep dolomite reservoir rock, and it also helps analyze the relationships among these diagenetic processes. The findings of this research provide valuable references for investigating the formation mechanism of the deep dolomite reservoir rock in the Sichuan Basin.

Early dolomitization and subsequent recrystallization of Middle Ordovician carbonates in the western flank of the Ordos Basin, Northern China: Implications for hydrocarbon exploration

The pervasive dolostone from the Middle Ordovician Zhuozishan Formation in the Ordos Basin (Northern China) are considered as potential hydrocarbon reservoirs. Three types of matrix dolomites (Md) are recognized: 1) very fine to fine crystalline, nonplanar-a to planar-e dolomite (Md1); 2) fine to medium crystalline, nonplanar-a to planar-e dolomite (Md2); and 3) medium crystalline, planar-s to nonplanar-a dolomite (Md3). The initial dolomitization of Md1 and Md2 dolomites is inferred to have taken place in the presence of seepage reflux of slightly evaporated seawater in the near-surface to shallow burial settings, and insignificant recrystallization occurred in the process of being buried again, according to petrography, trace elements, and oxygen-carbon-strontium isotopic geochemistry. Coarser crystals with cloudy cores and clear rims consist of the Md3 dolomites, and the texture shows irregular contacts between planar-s and nonplanar-a crystal. Geochemical analysis reveals that the Md3 dolomites have a negative shift in δ18O values with increasing Mn contents, and a negative δ18O trend with depth in the same well, while the δ13C does not tend to correlate with the oxygen isotopes. Additionally, the 87Sr/86Sr ratios of Md3 dolomites are relatively higher due to contribution of radiogenic 87Sr. Petrological attribute and geochemical evidence suggest that the massive Md3 dolomites can be due to significant recrystallization of earlier formed dolomites during burial period. The unevenly distributed pores in Md3 dolomites suggest the heterogeneous structure of the precursor dolomites, which may be the Md2 dolomites. The residual particles of Md2 dolomites indicate the precursor limestones were deposited in a relatively high-energy part of the shoal. Hence, the wide distributed Md3 dolomites show salient facies-controlled characteristics, which suggests strategies for hydrocarbon exploration of potential heterogeneous dolomite reservoir.

Microfacies analysis and depositional environment of the Upper Devonian Dankovo-Lebedyansky sediments, Tatarstan, Volga-Ural Basin, Russia

The Upper Devonian, Middle Famennian Dankovo-Lebedyansky sediments in Southeast Tatarstan territory are a carbonate sequence composed of limestone and dolomite, which is important hydrocarbon reservoir units, therefore, it is necessary to conduct in-depth study on its microfacies and depositional environment. In this study, a multidisciplinary approach that combines core observation with thin section examination is used. The limestone contains abundant skeletal grains (echinoderms, foraminifera, algae, gastropods, and calcispheres), as well as non-skeletal grains (intraclasts and peloids). On the basis of detailed petrographic investigations, six sedimentary microfacies can be identified, including (i) peloidal grainstone (MF 1), (ii) cemented bioclastic peloidal grainstone (MF 2), (iii) echinoderm-concentrated packstone (MF 3), (iv) algae packstone (MF 4), (v) bioclastic wackestone (MF 5), (vi) whole-fossil wackestone (MF 6), as well as dolomite or dolostone as diagenetic facies (MF 7). Based on microfacies analysis, the Dankovo-Lebedyansky sediments were deposited in three distinct sedimentary facies belts (shoal, lagoon and open marine environment). In order to reflect dispositional energy condition, the microfacies were grouped into facies associations: (i) low-energy microfacies associations including MF 5 and MF 6, (ii) moderate energy microfacies associations including MF 1, MF 2, MF 3, and MF 4. The dolomite, or diagenetic facies (MF 7), is the result of slightly to extensively dolomitization of limestone in the Dankovo-Lebedyansky sediments. Most of frequent dolostone types are euhedral planar-e and subhedral planar-s. According to petrographic characteristic of the dolostone, a seepage reflux model can be used to explain the dolomitization process of Dankovo-Lebedyansky sediments.

Effects of Deep Fluids on Middle Permian Dolomite of the Western Sichuan Basin

Deep fluid activity is closely related to carbonate reservoir transformation as well as oil and gas accumulation. The large-scale deep fluid activity caused by Emeishan large igneous province (ELIP) brought a lot of deep material and energy to Sichuan Basin and changed the paleogeothermal and geochemical field of the region they flowed through, which had an important impact on the formation of the Middle Permian dolomite reservoir in Western Sichuan. Through comprehensive analyses of petrographic, geochemical, and fluid inclusion data from two drill cores and three outcrop sections of the Middle Permian Formation in the Western Sichuan Basin, the dolomitizing fluid, dolomite genesis, and effect of deep fluid were discussed. The dolomite was divided into four types which are matrix dolomites (Md1, Md2, and Md3) and cement dolomites (Cd). Among them, the Md1 was formed in penecontemporaneous seawater by seepage reflux dolomitization, while Md2 was formed in heated Permian seawater through thermal convection. In the northwest of the study area, the Md3 and Cd were formed by higher-temperature thermal convection without hydrothermal. In the southwest of the study area, the lower δ13C value and Sr contents, higher Mn contents and 87Sr/86Sr ratios, and positive Eu anomalies as well as much higher Th and salinity values of inclusions show that hydrothermal participated in the dolomitization, forming the Md3 and Cd through structure-hydrothermal dolomitization and the thermal convection of seawater mixed with hydrothermal. The deep fluid activities of ELIP provided both the material source (Mg2+) for the dolomitization process and the migration motive power for the dolomitization fluid. From the southwest to northwest, with the deep fluid activity decreasing, the abundance of Md3 and Cd, which are closely related to the hydrothermal solution, reduced.

Massive dolomitization driven by MgSO4-rich seawater and its effects on thermochemical sulfate reduction, Upper Permian Changxing Formation, northeastern Sichuan, China

Massive dolostones replacing build-ups and containing H2S of thermochemical sulfate reduction (TSR) origin occur in the Upper Permian reef and bank deposits, northeastern Sichuan, China, despite the absence of gypsum or anhydrite deposits, which are usually present in TSR cases. Fluid chemistry from fluid incluions and δ13C, δ18O and 87Sr/86Sr values from different diagenetic phases were measured to determine the dolomitization regime, and to assess the relationships between the occurrence of dolomitization and H2S accumulation. Dolomitization was initiated by seawater with slightly increased salinities (penesaline) at shallow depths prior to chemical compaction. The micro- to fine-crystalline cloudy dolomite formed with relatively high Na and Sr contents, and with δ13C, δ18O and 87Sr/86Sr values (2.2‰∼4.8‰, −3.9‰∼-5.0‰ and 0.70724∼0.70746, respectively) inherited from seawater. The evaporation of Permian seawater in back-reef and inter-reef lagoons during sea-level fall and the subsequent seepage reflux into reef-beach bodies led to greater Mg2+ and SO42− concentrations in higher-salinity pore waters. Further massive dolomitization was promoted by compactional flow of hotter residual seawater at shallow to intermediate depths and resulted in the formation of fine- to medium-crystalline clean dolomite with lower Na and Sr contents, more depleted δ18O values (-5.1‰∼-6.0 ‰), andδ13C (2.5‰∼4.8 ‰) and 87Sr/86Sr values (0.70726∼0.70741) similar to those of the coeval seawater. The relatively closed hydrodynamic system during burial facilitated SO42− preservation. The whole dolomitization process enriched porewater SO42−, which have been almost exhausted by subsequent TSR, accounting for the high present-day concentrations of H2S. The output of this study shows that similar scenarios involving dolomitization driven by condensed MgSO4 seawater and H2S accumulations can occur in evaporite-free settings across a broad range and deserves special attention during deep oil/gas exploitation.

Characteristics of multiple dolomitizing fluids and the genetic mechanism of dolomite formation in the Permian Qixia Formation, NW Sichuan Basin

The ultra-deep dolomite reservoir of the Permian Qixia Formation in northwest Sichuan is important for hydrocarbon reservoirs, but the mechanism of dolomite genesis is complex and has long been controversial, thus requiring further elucidation. By studying the dolomite with different types in the Permian Qixia Formation, northwest Sichuan Basin, this paper, based on outcrop sections observation and core description, seeks to find out the dolomitizing fluids' source and the dolomite's genetic mechanism by analyzing the dolomite's rock-mineral and geochemical characteristics. Various technologies are applied, including rock thin section observation, cathode luminescence, carbon and oxygen isotope analyses, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and inclusion homogenization temperature and salinity. Results show that the dolomite is mainly classified into silty-fine dolomite (Md1), fine-medium dolomite (Md2), medium-coarse dolomite (Md3), and saddle dolomite (SD) filled in the pores. Md1 is formed in the early shallow-burial stage; the dolomitizing fluids are mainly derived from the early seawater sealed in carbonate sediments; and the penesaline seawater is formed by the seepage reflux under a moderate evaporitic environment. Md2 and Md3 are formed in the middle and late shallow-burial stages. Of these two types, Md2 is formed earlier, and the dolomitizing fluids are primarily derived from the seawater sealed in the early stage and partially derived from the residual evaporated seawater and hydrothermal fluids. In contrast, Md3 is formed later, and the dolomitizing fluids are the mixture of the seawater sealed in the early stage and the hydrothermal fluids formed in the late stage. In addition, Md3 was affected by the hydrothermal process more significantly during the formation. SD is deposited and formed when the saturated hydrotherm upwells along with the fracture system in the late stage. This study provides a significant reference for understanding the source and action mechanism of multi-stage dolomitizing fluids and expands the exploration field of the deep dolomite reservoir.

Dolomitization Controlled by Paleogeomorphology in the Epicontinental Sea Environment: A Case Study of the 5th Sub-Member in 5 Member of the Ordovician Majiagou Formation in Daniudi Gas Field, Ordos Basin

The 5th sub-member in 5 Member Ordovician Majiagou Formation in Daniudi Gas Field, Ordos Basin, is deposited in an environment consisting of an ancient epicontinental sea, where very fine crystalline dolostone reservoir has developed. In this study, based on the petrological and geochemical characteristics, the genesis of the dolomite developed in M55 were studied by analyzing the properties and sources of the dolomitization fluids, and the influence of the paleogeomorphology differences on the distribution of dolostone was also discussed in order to clarify the distribution of the dolostone developed in the lime flat of the epicontinental sea. The dolostone of the M55 had a crystal structure, mainly including microcrystalline and very fine crystalline. The content of MgO and CaO in dolomite was negatively correlated, indicating that it was the result of replacement. The dolomite was dark red under cathode luminescence, and the distribution mode of rare earth elements showed the negative anomaly of Ce and Eu, indicating that the dolomitization fluid was sea-sourced fluid. The δ13C, δ18O, and 87Sr/86Sr isotope range of limestone was similar to that of Ordovician seawater in the study area, whereas the δ13C, δ18O, and 87Sr/86Sr of dolostone were obviously more positive than that of limestone. The substitute index of the salinity (Z) of the dolomitization fluid was higher than 122, which is higher than limestone (Z = 120.5), indicating that the dolomitization fluid was slightly evaporated seawater. The wormholes observed on the core and the gypsum in the penecontemporaneous period observed in the thin sections indicated that the dolostone was formed in a period when the sea level was relatively low, and it was the result of seepage–reflux dolomitization. By analyzing the correlation between the thickness of dolostone and the paleogeomorphology of the M55 of the sedimentary period, it was found that the thickness of dolostone at relatively high altitude was significantly larger than that of other areas. The development of dolostone was controlled by sea level, and the local paleogeomorphology controls the distribution of dolostone during the period of low sea level. There were many more limestone–dolostone cycles and larger cumulative thicknesses of dolostone at relatively higher topography. This study provides a theoretical basis for the prediction of the distribution of dolostone reservoirs in the carbonate tidal flat environment dominated by lime flats under the background of the ancient epicontinental sea.

Mg and Sr isotopic evidence for basin wide alteration of early diagenetic dolomite in the Williston Basin by ascending crustal fluids

Dolomite is a predominantly diagenetic mineral that forms by replacing marine calcite and aragonite during burial. Dolomitization requires subsurface fluid flow to deliver Mg and remove product Ca, and only forms abiotically in laboratory experiments at temperatures above ~100°C, equivalent to burial depths of 3 km or greater. Limited sources of Mg at these depths would appear to restrict most dolomite formation to shallow burial depths using seawater as a source of reactive Mg, despite the kinetic limitation imposed by lower temperatures. The Williston Basin is replete with evaporites and dolomites that appear to have formed using Mg from evaporatively concentrated seawater at near surface temperatures. However, two dolomite bodies examined in this study record basin wide gradients in 87Sr/86Sr ratios and δ26Mg values that are inconsistent with dolomitization by seawater. We evaluate the processes that may have created these gradients. We propose a model whereby seepage reflux created early diagenetic protodolomite that was later altered during deep burial by 87Sr and Mg-enriched fluids ascending from the deep structural center of the basin with the degree of alteration decreasing towards the edges. The ascending fluid flow event is tentatively linked to anomalous heating in the Late Devonian / Early Carboniferous. Vertical faults are hypothesized to have behaved as conduits for channeling 87Sr and Mg-enriched crustal fluids upwards into the bottom of the Williston Basin, pressurizing the basal aquifer system and commencing up-dip directed fluid flow through confined aquifers. Clumped isotope and fluid inclusion thermometry indicate that the ascending fluids were hydrothermal with an estimated δ26Mg value of –0.65 ±0.20‰, which is much lower than estimates of early Paleozoic seawater δ26Mg values of +0.09 to +0.23‰. Two sources of crustal Mg are consistent with the low inferred δ26Mg values of the ascending fluid: (1) chlorite, which preferentially releases isotopically light Mg, with literature values in the range of –0.78‰ to –1.8‰, and (2) magnesite, which forms during carbonation of serpentinite with literature values in the range –0.7‰ to –1.1‰. Ultramafic rocks belonging to the Thompson Nickel Belt pass beneath the center of the Williston Basin in North Dakota, indicating that dissolution of both chlorite and magnesite may have contributed reactive Mg to the Williston Basin during the ascending fluid flow event. As dolomite is a mineral that is easily altered during burial, our findings warrant caution in choosing dolomites for reconstructing the 26Mg history of seawater.

Dolomitization of the Middle Ordovician Ma55 Sub-member of the Majiagou Formation and implications for hydrocarbon exploration in the northern Ordos Basin, NW China

The Middle Ordovician Majiagou Formation in the northern Ordos Basin consists of a sequence of subtidal and peritidal carbonates that are extensively dolomitized. The diagenetic evolution of the Ma55 Sub-member was investigated to determine the factors controlling reservoir quality in the northern Ordos Basin. Dolomite in the Ma55 Sub-member is classified into three main types on the basis of crystal size: dolomicrite (D1), micritic–silty and fine silty dolomite (D2), and silty (and coarser) dolomite (D3). The Dolomite D1 was produced by dolomitization in a low-salinity diagenetic environment and transformed by near-surface freshwater leaching during shallow burial. The Dolomite D2 developed as a result of post-penecontemporaneous seepage–reflux dolomitization and was affected by later burial dolomitization. The Dolomite D3 was produced during burial dolomitization. Although the original porosity of the Dolomite D1 was low, freshwater diagenesis increased the porosity significantly. As a result of the transformation of montmorillonite to illite, there was a deficiency in the Mg2+ necessary for Dolomite D2 formation, resulting in the nonuniform dolomitization of this type of dolomite. The intercrystalline pores of the Dolomite D2 are mostly filled with montmorillonite and illite, reducing its porosity and permeability and consequently making the Dolomite D2 a poor reservoir. The Dolomite D3 has a higher porosity and contains various types of dissolution and intercrystalline pores, generating a higher permeability. This dolomite is a potential reservoir in the study area and a possible target for oil and gas exploration. Overall, the results of the study suggest that the quality of a potential dolomite reservoir is strongly controlled by the diagenetic history of the host carbonates.

Cenozoic coastal carbonate deposits of Qatar: Evidence for dolomite preservation bias in highly‐arid systems

AbstractIn the ancient rock record, early replacement of metastable marine calcium carbonate deposits by dolomite has long been associated with evidence of arid depositional environments. Such associations led to the development of the seepage reflux dolomitization model, whereby magnesium‐rich marine waters concentrated by evaporation descend into underlying sediments, replacing primary aragonite and calcite deposits with dolomite through rock–water interaction. In the modern arid coastal systems of Qatar, where marine waters concentrate to halite saturation, both aqueous geochemical measurements and mineralogical investigations show that replacement of calcium carbonate deposits by dolomite is not occurring to any consequential degree. At best dolomite formation is minor and localized. Instead, modern and mid‐Holocene coastal deposits retain their original mineralogy, showing little evidence of carbonate precipitation reactions with the notable exception of beachrock formation. Pleistocene coastal deposits are mostly absent in comparison with both Qatar Holocene coastal deposits, and Pleistocene coastal deposits from more humid settings. The lack of onshore Pleistocene‐aged carbonate outcrops in Qatar, as well as regionally, is interpreted to reflect coastal sediment denudation during emergence due to: (i) the absence of coastal marine lithification; and (ii) the absence of meteoric cementation and root stabilization in the highly‐arid realm. In contrast, marine Palaeogene and Miocene carbonate deposits are preserved in outcrop in Qatar, having suffered marine lithification by dolomite, thus promoting retention of carbonate strata through subsequent lowstands. These older deposits formed during times of ocean acidification, which, based on natural system and laboratory investigations, is interpreted to promote metastable carbonate dissolution and dolomite formation. The highly‐arid Holocene and Pleistocene coastal systems of Qatar represent limestone factories, but these deposits are not being retained to the rock record. Based on these observations, the association between the occurrence of dolomite and arid depositional settings may be to some degree a preservation bias.

Mg, C and O isotopic compositions of Late Cretaceous lacustrine dolomite and travertine in the northern Tianshan Mountains, Northwest China

The Junggar Basin was characterized by high temperature and strong tectonic activities during the Late Cretaceous, in which a dolomite-forming lacustrine depositional system associated with travertine was deposited in its southern sector adjoining the northern Tianshan Mountains. Two broad types of dolomite are identified in terms of mineralogy, including the homogeneous and anhedral dolomite-A and high order degree and euhedral dolomite-B, which are also largely varied in mineral size (10–20 μm and 30–50 μm in size, respectively). The dolomites display a low degrees of dolomite order (mean = 0.48) and Ca-rich (mean = 55.8). Compared with δ18O values (PDB) of limestones in the shore to shallow facies (−7.0 to −11.1‰), the apparent positive δ18O values of dolomites (−4.2 to −5.9‰) indicate a stronger evaporation environment, while the values of −6.8 to −8.1‰ in the salt marsh facies dolomitic limestone may reflect transitional conditions. Moreover, the more positive δ26Mg values of dolomites (ranging from −1.52 to −2.91‰) than those of limestone and travertine (−3.58‰ and −4.31‰, respectively) also indicate an origin related fractionation effect. In addition, based the occurrence of low-temperature hydrothermal minerals, including chlorite, barite, siderite and silver in the dolostone and travertine, we interpret a new travertine-related origin for the lacustrine dolomite. Thermal water entering saline and alkaline lake environments subject to strong evaporation promoted dolomitization. From the simulation of the Rayleigh and seepage reflux models, we speculate that the lacustrine dolomites formed in a near-surface environment (25–50 °C). Moreover, the results contain abundant information on palaeoenvironmental and palaeoclimatic changes, which are very important for reconstructing and deciphering palaeoenvironments and expanding the dolomitization model under thermal water influenced evaporative lacustrine conditions.

The effects of diagenesis on the petrophysical and geochemical attributes of the Asmari Formation, Marun oil field, southwest Iran

The distribution of good reservoir quality and its causes is the main challenges in carbonate reservoir characterization. This study investigates the effects of diagenetic processes on the reservoir quality of the carbonate successions of the Asmari Formation, in the Marun oil field, southwest Iran. The study applies an integrated approach including core petrography, petrophysical rock typing, stable carbon and oxygen isotopes as well as major and trace elements analyses. Petrographic studies and geochemical analyses express that the Asmari limestones have been affected mainly by compaction, dissolution, recrystallization, calcite and anhydrite cementation and dolomitization. Among those diagenetic overprints, dolomitization and dissolution played an important role to enhance the reservoir quality of the formation. Moreover, four types of dolomites were recognized and the rate of dolomitization increases toward the top of the Asmari carbonate successions. Possible models for dolomitization include mixing zone, brine reflux, seepage reflux and tidal pumping of seawater. Employing Flow Zone Index and Discrete Rock Type concepts led to classification of the Asmari reservoir into seven reservoir rock types. Integrating reservoir rock typing with petrographic studies and geochemical analyses also confirms that reservoir quality of the Asmari Fm. would have been mainly controlled by diagenetic processes. Moreover, stable isotopes, trace elements and facies analyses support the idea that carbonate intervals of the Asmari Formation were deposited in a warm, shallow-water environment under a saline condition.