Coarse Crystalline Dolomite Research Articles

Diagenetic archives of the deeply-buried Cambrian Xiaoerbulake Formation microbialite reservoirs, Bachu-Tazhong area, Tarim Basin, China

The deep-burial Cambrian Xiaoerbulake Formation microbialites reservoir hosts significant hydrocarbon in the Tarim Basin. However, limited understandings of fluid-rock interactions from deposition to deep-burial regimes hamper petroleum exploration. Here, petrological observations, in-situ elemental contents and porosity-permeability analyses were performed to understand the complex interactions and the microbialites performance. Dolo-mudstone, crystalline dolostone, and four kinds of microbialites are distinguished as dolostones types. In the depositional stage, microbialites are characterized by (evaporated) seawater dolomitization with the mediation of microbial activity. The dolostone matrix shows seawater-like δ13C, δ18O and REY patterns. Thrombolitic and foam spongy dolostones from high-energy subtidal to intertidal zones show large framework pores compared to stratiform stromatolitic dolostones from low-energy intertidal zones. In near-surface settings, meteoric diagenesis generates secondary pores, while fine-crystalline dolomite cements partially fill pores. During the shallow to intermediate burial regimes, medium-to coarse-crystalline dolomite cements yield negative δ18O values and slightly flat REY patterns. The precipitation of this dolomite phase further decreases reservoir porosity. In deep burial settings, one type of saddle dolomite is likely to be formed by replacement from carbonate precursors, while the second type might be directly precipitated from hydrothermal fluids. These two types of saddle dolomites show different cathodoluminescent characteristics and geochemical data. Subsequently, two types of calcite cements are formed by the calcification of dolomite and yield significant negative δ13C values due to thermochemical sulfate reduction. Finally, the third calcite phase is likely to be a direct precipitate from burial brines. Hydrothermal activity and thermochemical sulfate reduction generate and/or re-distribute secondary pores. Based on the plethora of depositional environments and fluid-rock interactions, thrombolitic dolostone generally yields higher porosity-permeability than other types of microbialites. Thrombolite reservoir has high potential for petroleum exploration and is significant for those concerned with microbialite hydrocarbon fields.

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.

Influencing factors of rock thermal conductivity and applicability evaluation of its mixing law predictive models

The thermal conductivity (TC) of rocks is a very important parameter in geothermal theory and applied research, its value is related to rock composition, mineral grains characteristics, rock porosity, and pore fluid etc., and simultaneously is affected by formation thermodynamic environment (temperature, pressure,etc.). Statistical analysis results of the thermophysical property parameters of borehole samples from Jizhong Depression indicated that the TC is mainly controlled macroscopically by lithology, compared with sandstone, mudstone, conglomerate, and gneiss, dolomite has a greater TC. In the dry state, the TC of dolomite samples showed a decrease with increasing temperature, the values of TC decreased by 12.3–27.5% in the temperature interval of 30–130 °C. Moreover, under the same temperature conditions, excluding the effects of water content and mineral fraction, the TC of medium–coarse crystalline dolomite, medium–fine crystalline dolomite, microcrystalline dolomite, and mud-powder crystalline dolomite decreased successively, reflecting the influence of the mineral grain size on TC. The validity and applicability of six commonly used mixing law predictive models for TC were evaluated based on X-ray diffraction (XRD) data from 70 borehole samples. The results showed that Geometric mean model displayed the best fit (goodness of fit, R2 = 0.878) for rocks, and Voigt-Ruess-Hill average also showed a good fit, with an R2 of 0.868, and had the best prediction for mudstone (AME = 10.16, RMSE = 0.25). In cases where the calculated values deviate significantly from the measured values, it may reflect the limitation that the mixing models consider only the composition of minerals but ignores the structural characteristics of rocks.

Early dolomitization and subsequent hydrothermal modification of the middle Permian Qixia Formation carbonate in the northwest Sichuan Basin

High-quality dolomite reservoir are widely developed in Permian Qixia Formation in northwest Sichuan Basin. Understanding the genesis of dolomite is significant to predicting high-quality reservoir distribution. Based on the petrographical and geochemical characteristics, three types of dolomites and one type of dolomite cement were identified in the Qixia Formation in northwest Sichuan Basin: micritic to fine-crystalline dolomites (D1), medium-crystalline dolomite (D2), medium-to coarse-crystalline dolomite (D3) and saddle dolomite cement.Type D1 dolomites are characterized by micritic to fine-sized crystals, lamellar structure, generally dull to no luminescence, and relatively high strontium (Sr) concentrations. These characteristics suggest that type D1 dolomites were initially formed during the penecontemporaneous phase. The wide range of depleted δ18O values, slightly higher 87Sr/86Sr ratios and slightly positive europium (Eu) anomalies indicate that type-D1 dolomites were subsequently subjected to hydrothermal modification. Type D2 dolomite is characterized by unimodal middle-sized crystals and crystal texture dominated by nonplanar crystals. It usually occurs as thin or thick layers in the study area. The δ18O and δ13C values of type D2 dolomite generally fall within the estimated Middle Permian marine dolomite and show a significantly positive offset compared to those of coeval seawater. The 87Sr/86Sr ratios of type D2 dolomite are slightly higher than the coeval seawater. These features combined with the flat rare earth element (REE) profile and relatively high Sr concentrations (both are similar to the host limestone) indicate that type D2 dolomites first precipitated in the slightly evaporated Middle Permian seawater during the near-surface to shallow burial phase. Type D3 dolomites are mainly characterized by interlocking mosaic crystals with crystal sizes varying from 300 μm to 900 μm. It mainly occurs as patches in type D2 dolomites or as a thick layer sandwiched by type D2 dolomites. These features combined with its high 87Sr/86Sr ratios, δ18O depletion and positive Eu anomalies (which are very similar to those of the saddle dolomites) demonstrate that it is formed by hydrothermal recrystallization upon type D2 dolomites. Saddle dolomite generally occurs as the filling cement of pores, vugs, and fractures and is usually combined with matrix dolomite to form zebra dolomite and hydrothermal dolomite breccia. It has a curve crystal face, demonstrates wavy extinction under cross-polarization and red luminescence under a cathodoluminescence microscope. These features, together with the highest Th, most negative δ18O value and highest 87Sr/86Sr ratio compared to those of all types of dolomites in the study area as well as the positive Eu anomalies suggest that it should have been formed by hydrothermal fluid precipitation.Type D1 and type D3 dolomites are volumetrically minor and the pores are poorly developed. Saddle dolomite cemented and filled the pores and vugs of matrix dolomite, resulting in reservoir damage. Type D2 dolomites constitute the main body of high-quality reservoir in Qixia Formation. It precipitated in the slightly evaporated penesaline Middle Permian seawater during the near-surface to shallow burial phase. Potential high-quality reservoirs may occur in the shoal around the limited inter-shoal sea.

Evolution of diagenetic fluids in the deeply buried, upper Ediacaran Dengying Formation, Central Sichuan Basin, China

Deeply buried Precambrian carbonate reservoirs host economic petroleum reserves. The upper Ediacaran Dengying Formation (maximum burial depth >8 km) is predominantly composed of dolostone with porosity ranging from 2 to 4%, and minor limestone with a porosity of <2%. Detailed petrological observations and high-resolution geochemical analyses show the evolution from depositional fluids to late diagenetic fluids. Dolostone is subdivided into dolo-mudstone, dolo-laminite, stromatolitic dolostone, thrombolitic dolostone and oncoidal dolostone. These dolostones were precipitated from the Ediacaran seawater and yield rare earth elements and yttrium (REY) patterns similar to those of modern seawater. In the early diagenetic regimes, fibrous dolomite cement, characterised by superchondritic Y/Ho ratios, was first precipitated from marine porewaters, and then fine-crystalline dolomite cement, characterised by low Y/Ho ratios, was formed during meteoric diagenesis. Bladed dolomite cement was subsequently precipitated from (modified) seawater-dominated porewaters. In the intermediate burial regimes, medium- to coarse-crystalline dolomite cement was formed during reduction of Mn oxides. In the deep-burial regimes, saddle dolomite was precipitated from hydrothermal fluids and yields REY patterns characterised by MREE enrichment. Its superchondritic Y/Ho ratios were likely caused by the fractionation between Y and Ho during the co-precipitation of fluorite. Calcite cement has similar parent fluids to those of saddle dolomite. These dolomite phases have similar δ13C values, while their δ18O values gradually decrease during the paragenetic sequence. Multiphase dolomitisation led to the increase in Ca/Mg ratios of formation water and reservoir performance. This study is significant for those concerned with the deeply buried dolostone reservoirs. KEY POINTS Microbialites and five types of dolomite cements have different REY patterns. Co-precipitation of fluorite led to the superchondritic Y/Ho ratios of dolomite. Dolomite and calcite grains show micro-scale geochemical variations. This dolostone reservoir underwent multiphase fluids, including meteoric water, seawater and hydrothermal fluid.

Nature and evolution of diagenetic fluids in the deeply buried Cambrian Xiaoerbulake Formation, Tarim Basin, China

Significant amounts of petroleum are found in the deeply buried Cambrian Xiaoerbulake Formation in the Tarim Basin. However, limited understanding of origin of diagenetic fluids hampers petroleum exploration. In this study, petrological and high-resolution geochemical methods have been performed to understand the origin and evolution of diagenetic fluids, and their effects on the formation of dolostone reservoir. The Xiaoerbulake Formation is characterised by microbialites. The matrix is characterised by modern seawater-like rare earth element and yttrium (REY) patterns and δ18O values of −6.02 ± 2.11‰ Vienna Pee Dee Belemnite (VPDB). Fine-crystalline dolomite cements have similar δ13C values and heavy rare earth element enrichment patterns to that of matrix dolomite, yet with lower δ18O values of −7.47 ± 1.2‰ VPDB. In the deep-burial regimes, medium- to coarse-crystalline dolomite cements and saddle dolomites were precipitated at temperatures of 90–120 and 130–190 °C, respectively, lower δ18O values, distinct middle rare earth element enrichment, and high Fe and Mn contents compared with matrix and fine-crystalline dolomite cements. These imply thermochemical reduction of Fe- or Mn-rich oxides and/or hydrothermal fluids. Compared with other types of dolomite cements, calcite cements are characterised by low δ13C values (as low as −6‰ VPDB), light rare earth elements enrichment and significantly elevated Y/Ho ratios (86 ± 59.3), likely related to the oxidation of organic matter during thermochemical sulfate reduction (TSR). Consequently, diagenetic fluid may have evolved from: Cambrian seawater, meteoric water, shallow-buried seawater-dominated pore water, medium-deep-buried ferrous-rich pore water, lately high-salinity hydrothermal fluid-dominated pore water and finally to acidic fluids related to TSR dissolution. Reservoir quality is enhanced by hydrothermal pulses and TSR in deep-burial settings. This study is significant for those concerned with the formation of deep-buried carbonate reservoirs.

Dolomitization Mechanism in the Chorgali Formation (Eocene), Pakistan: Evidences from Field observations, Petrography, Mineralogical and SEM analysis

The subject of dolomite formation has always put researchers in the challenge. In this study, the origin of the dolomites of the Chorgali Formation near Chorgali Pass was explored through field observation, petrographic characteristics, mineralogical and geochemical analysis. Analytical analyses used in the present studies include XRD, SEM-EDS, and stable isotopes studies. The dolomites were categorized into three major genetic types based on their textural and structural features showing their distinct origins. These dolomites are named fine crystalline dolomite (Df), medium crystalline dolomite (Dm), and coarse crystalline dolomite (Dc). The analytical approaches linked with field observations and petrographic examinations identified that; the first type of dolomites (i.e., Df) is related to the early stage of diagenesis and contain the presence of mineral dolomite (40%), gypsum (27%), quartz (17%) and albite (16%) and are low stochiometric low ordered in character as recognized by XRD results. The SEM-EDS analysis identified low Mg, Ca, O, Na, Si, and Fe concentrations. Further, the stable isotopes (δ18O) values for Df (i.e., -5.95 to -396‰V-PDB) are less depleted. Moreover, Dm and Dc revealed 100% mineral dolomite and are stochiometric ordered dolomites. High concentrations of Mg, Ca and O are observed in Dm and Dc respectively. Stable isotopes (δ18O) result in exhibiting highly depleted values for Dm (-7.947‰V-PDB) and Dc (-9.227 to -8.302‰V-PDB) showing its formation with the elevated temperature at depth. In addition, δ13C values of Df, Dm, and Dc lie in the range of the original marine signature of Eocene times.

Control of climate, sea-level fluctuations and tectonics on the pervasive dolomitization and porosity evolution of the Oligo-Miocene Asmari Formation (Dezful Embayment, SW Iran)

The Asmari reservoir properties in various hydrocarbon fields across the Dezful Embayment (SW Iran) are modified by dolomitization. Petrographic and geochemical analyses of the dolomitized Asmari Formation units highlight their origin in the Bibi Hakimeh and Gachsaran oilfields. The petrographic studies revealed that dolomite in the Asmari Formation can be classified texturally (based on crystal sizes) into three main groups: (i) very fine crystalline (D1, <20 μm), (ii) fine to medium crystalline (D2, 20–150 μm) and (iii) coarse crystalline dolomites (D3, >150 μm). Petrography, fluid inclusions, stable isotopes (δ13C, δ18O), radiogenic isotopes (86Sr/88Sr) and major and trace elemental analyses were used to interpret the origin and paragenetic sequence of each dolomite group. Pervasive dolomitization prevails in the upper part of the Asmari Formation where very fine and medium crystalline dolomites with partially and fully fabric-retentive and fabric destructive textures, replaced micrite and allochems of the platform-top facies, respectively. The parent fluids of the very fine dolomites originated from contemporaneous Oligo-Miocene seawaters produced in a sabkha environment. The fine to medium dolomites (D2) most likely shared the same seawater origin and relate to refluxing, sabkha-derived fluids. The medium to coarse crystalline dolomite has more depleted oxygen isotopes than the former, very fine to fine crystalline, dolomites suggesting a burial dolomitization origin. The estimated high salinities (≈19 to 20 wt% NaCl) together with compatibility of oxygen and strontium isotope data with evaporative seawater values, suggest Oligo-Miocene seawater was the parent fluid for precipitation of coarse and cement phase of the medium crystalline dolomites. This study reveals that reactivated basement faults with associated fractures have probably played an important role in convecting and heating evaporative seawater along their planes. This heated seawater provided the required fluid for recrystallization of coarser matrix and cement phases of medium (D2) and coarse (D3) dolomites at depth. Therefore, the interaction of climate, sea-level changes, and tectonics played a critical role in formation and development of pervasive dolomitization in the Asmari Formation. Additionally, dissolution (partly as a consequence of dolomitization) was one of the key processes during early diagenesis in the formation of different types of porosity such as biomolds, vugs and intraparticles porosity. This study shows that the reservoir quality improves from finely crystalline dolomite towards samples dominated by coarsely crystalline dolomites.

Early-stage marine dolomite altered by hydrothermal fluids in the Middle Permian Maokou Formation in the eastern Sichuan Basin, Southern China

Deciphering the dolomitization process is the key to understanding the formation of massive dolostones on shallow-marine platforms, it also improves our understanding of dolostone reservoirs. Hydrothermal fluids are suggested to be a potential mechanism for dolomitization; however, determining whether hydrothermal fluids are responsible for dolomitization or the recrystallization of an early dolostones is poorly understood. The dolomite of Maokou Formation (Guadalupain) in the eastern part of the Sichuan Basin is a good case study on the hydrothermal dolomitization and recrystallization. Based on petrographic observation, the massive dolostone (MD) consists of fine-crystalline dolomite (D1), fine to medium crystalline dolomite (D2) and coarse-crystalline dolomite (D3), and the patchy dolostone (PD) which is composed of medium to coarse dolomite with an euhedral to subhedral texture. Since the fine-crystalline dolomite is cross-cut by low amplitude stylolites, and is composed of euhedral dolomite with a dull red luminescence, dolomitization is interpreted to have occurred in near surface condition. While the Fe, Mn, and ∑REE concentrations, δ13CVPDB values, 87Sr/86Sr ratios, and REE Pattern of D1 are comparable to those of the coexisting limestone, and the δ18OVPDB values are slightly higher than that of the coeval limestone, indicating that the dolomitization fluid of D1 was derived from Middle Permian seawater. In contrast, zebra fabrics and saddle dolomites are common in D2 and D3, and their geochemical characteristics and cathluminescence are quite different from those of D1 but they are similar to those of the saddle dolomites. Besides, the fluid inclusions in D2 and D3 have high homogenization temperatures and salinities, which are also similar to those of the saddle dolomite. Moreover, the D2 shows an inequigranular texture, and there are no obvious boundaries between D1, D2 and D3. Therefore, the D2 and D3 were recrystallized upon D1 by hydrothermal fluids related to the Emeishan magmatism. The patchy dolostone exhibits a non-stoichiometric composition and low degree of cation ordering, suggests rapid and incomplete dolomitization. The dolomitization of PD is interpreted to be the replacement caused by hydrothermal fluids as the PD exhibits negative δ18OVPDB values, high homogenization temperatures and salinity, positive Eu anomalies and REE Patterns similar to those of the saddle dolomites. The porosity and stoichiometry of the D1 are higher and lower, respectively, than those of D2 and D3, and the vugs are almost entirely filled. Thus, the hydrothermal fluids were destructive for the formation of the dolostone reservoir of Maokou Formation in the study area.

On the dolomite reservoirs formed by dissolution: Differential eogenetic versus hydrothermal in the lower Permian Sichuan Basin, southwestern China

Dolomite reservoirs formed by dissolution are significant for hydrocarbon exploration, but their origins are poorly understood because of their complexity. This study investigated the lower Permian Qixia Formation of the Sichuan Basin (northern Longmenshan Mountains), southwestern China, which has recently been the location of giant natural gas discoveries. Petrological and geochemical analyses were undertaken, including trace elements and C, O, and Sr isotopes. The results indicate that the reservoirs were formed by differential eogenetic karstification and hydrothermal dissolution of grain dolomites. Core and thin-section observations identified three types of dolomite based on grain size: fine- to medium-crystalline dolomite (Dol-1), medium- to coarse-crystalline dolomite (Dol-2), and coarse- to megacrystalline saddle dolomite (Dol-3). The Dol-2 types can be further divided into three subtypes according to the grain structure and degree of pore development: euhedral medium- to coarse-crystalline dolomite with pores (Dol-2a), porous subhedral–anhedral medium- to coarse-crystalline dolomite (Dol-2b), and tight subhedral–anhedral medium- to coarse-crystalline dolomite (Dol-2c). The Dol-1 types appear to have formed because of the release of and subsequent metasomatism by Mg2+ in trapped seawater along stylolites at shallow burial conditions (500–1000 m). The Dol-2a types resulted mainly from eogenetic karstification of original limestone and subsequent dolomitization by marine-related fluids. These dolomites are geochemically characterized by relatively flat rare-earth element patterns and relatively low Y/Ho values (mean e 37.1). The Dol-2b and Dol-3 types were formed mainly by hydrothermal fluids and have similar rare-earth element patterns with large positive Eu anomalies (mean e 5.7). The Dol-2c types experienced metasomatism by marine-related fluids mixed with minor hydrothermal fluids. All these formation mechanisms reflect the influence of multistage diagenetic and hydrothermal fluids. The Dol-2a types form the most favorable reservoirs in the study area, followed by the Dol-2b types. The distribution of the Dol-2a types was controlled by eogenetic karstification of grain shoals, whereas that of the Dol-2b types reflects the influence of tectonically derived hydrothermal fluid dissolution of grain shoals. As such, both primary sedimentary features and later differential dissolution (eogenetic and hydrothermal) contributed to reservoir development, which involved factors such as the sedimentary facies (grain shoal), sea-level fluctuations (eogenetic karst development), and faulting (hydrothermal dissolution). Therefore, the formation of dolomite and associated reservoirs is complex. The results of this study provide an improved understanding of the origins and complexity of dolomite reservoirs formed by dissolution, which is relevant to exploration for such reservoirs.

Dolomitization history and porosity evolution of a giant, deeply buried Ediacaran gas field (Sichuan Basin, China)

The deeply buried (>7 km) upper Ediacaran (Sinian) Dengying Formation (ca. 551.1–541 Ma) in the Sichuan Basin, China, is the largest Precambrian dolostone gas reservoir worldwide. Gas exploration from the Dengying Formation, however, is hampered by a limited understanding of its complex dolomitization history and porosity evolution. New petrological, geochemical and petrophysical analyses were performed and are discussed here to develop a better understanding of the formation’s complexities. Microbialite, dolo-grainstone, and crystalline dolostone lithologies from the platform margin have high primary porosities relative to dolo-mudstone and less common microbialite lithologies from the low-energy platform interior. Spatially-variable primary porosity was subsequently overprinted by meteoric dissolution and dolomitization, often inducing secondary porosity. The various forms of dolomitization (sabkha, reflux, and burial dolomitization, and cementation) increased the resistivity of these rocks to chemical and physical compaction and porosity destruction. In the deep burial domain, medium- to coarse-crystalline dolomite cements and saddle dolomite precipitated at fluid temperatures of 120–160 °C and 160–220 °C, respectively. The significance of deep burial dissolution is critically discussed and constrained by: (i) corrosion of late diagenetic minerals, (ii) pores cross-cutting (or forming around) stylolites, and (iii) the occurrence of solid bitumen in the center of secondary pores. Vuggy, inter- and intracrystalline pores developed during a late and deep burial stage, and comprise ca. 20% of the overall Dengying reservoir porosity. Mechanisms which induced corrosion at this burial stage include hydrothermal pulses and thermochemical sulfate reduction. Similar to many carbonate reservoirs in the Phanerozoic, this study documents that the platform margin has better porosities and higher gas production compared to the platform interior due to its specific carbonate rock properties and diagenesis. Data shown here document the complex multiphase dolomitization history and high potential for gas production from deeply buried Precambrian dolostone reservoirs.

Petrological and geochemical constraints on the origin of dolomites: a case study from the early Cambrian Qingxudong Formation, Sichuan Basin, South China

Several types of dolomite had occurred pervasively in the lower Cambrian Qingxudong Formation in the Sichuan Basin. However, the origin of these dolomites remains particularly controversial until now. Here, we describe the petrographic and geochemical characteristics of dolomites from the lower Cambrian Qingxudong Formation in the southeastern Sichuan Basin, which can potentially provide useful information to constrain the origin and evolution of dolomitization fluid. Three fabric types and five dolomite generations were identified in this study: fabric-retentive dolomites (dolomicrite, Dol-1, and dolomitic microbialite, Dol-2), fabric-destructive dolomites (sucrosic dolomite, Dol-3) and pore-filling dolomites (medium- to coarse-crystalline dolomite, Dol-4, and saddle dolomite, Dol-5). The δ13C and δ18O values, 87Sr/86Sr ratios and rare earth element (REE) pattern of Dol-1, Dol-2 (except the δ13C values) and Dol-3 are approximately consistent with original seawater values, suggesting that the dolomitization fluids were mediated by seawater. The calculated dolomitization temperature (25.66 °C) and Z values (125.2) of Dol-1 with evaporite minerals indicate that the dolomitization fluid was high-salinity concentrated seawater derived from evaporation. The calculated dolomitization temperature (24.20 °C) and Z value (121.8) of Dol-2, and no traces of evaporite minerals in Dol-2 imply that the dolomitization fluid was slightly hypersaline seawater. The more negative δ13C and abundant microbial features of Dol-2 indicate that microbial mats were involved in dolomite formation. The dolomitization temperature (30.55 °C) and Z value (125) of Dol-3 also indicate that the dolomitization fluid was high-salinity concentrated seawater. However, Dol-3 is commonly distributed in reef and shoal facies (high porosity and permeability) throughout the Qingxudong Formation. Moreover, the abundant organic matters in the dolomitization fluid likely overcame the kinetic and thermodynamic barriers in the process of dolomitization. The geochemical data (except the δ18O values) of Dol-4 are approximately consistent with fabric-retentive dolomites, suggesting that the Dol-4 had been inherited from precursor carbonates. The negative δ18O values of Dol-4 revealed a further burial environment. High LREE/HREE ratios, positive europium anomalies and negative δ18O values of Dol-5 suggest that these dolomites were affected by hydrothermal fluids under deep burial conditions. Thus, Dol-1, Dol-2 and Dol-3 are likely attributed to sabkha, organogenic (microbial) and seepage-reflux dolomitization models during early-stage diagenesis, respectively. Dol-4 and Dol-5 are commonly attributed to burial and hydrothermal dolomitization models during deep burial diagenesis. This study provides new insights into the origin of dolomites and is especially useful for the further understanding of dolomite reservoir formation in the Qingxudong Formation.

Dolomitization and hydrotectonic model of burial dolomitization of the Furongian-Lower Ordovician carbonates in the Tazhong Uplift, central Tarim Basin, NW China: Implications from petrography and geochemistry

Abstract The nature of the dolomitizing fluids and genesis of the Furongian-Lower Ordovician dolomites and dolomite cements are interpreted on the basis of petrography, geochemistry (trace elements, REEs, O-C-Sr isotopes) and fluid inclusion microthermometry. These carbonate strata were deposited on a shallow marine platform in the Tazhong Uplift in central Tarim Basin and were extensively dolomitized. Based on field investigations and petrographic examination, two types of fabric-retentive dolomitic textures (dolomicrites (DM) and microbial dolomudstone (DMS)) and three types of fabric-obliterated dolomitic textures (very fine- to fine-crystalline planar-e(s) dolomite (RD1), fine- to medium-crystalline planar-e(s) dolomite (RD2) and medium- to coarse-crystalline nonplanar-a, planar-e(s) dolomite (RD3)) are identified. Two types of dolomite cements (medium- to coarse-crystalline dolomite cement (DC1) and coarse- to very coarse-crystalline saddle dolomite cement (DC2) are identified. The REE patterns of the analyzed dolomites and calcite cements are characterized using seawater normalization standard. The DC2 dolomite cement and later-stage calcite cement (LSCAL) display positive Europium anomalies (δEu) and positive Cerium anomalies (δCe). Whereas the DM, DMS, RD1, RD2, RD3 and early-stage calcite cement (ESCAL) samples display positive δCe, light REE (LREE) enrichment and negative δEu. The DM and DMS were formed through seawater flux and mesosaline brine derived from evaporative seawater during early-burial seepage-reflux dolomitization and microbial dolomitization, respectively. The preferential occurrence of RD1 along low-amplitude stylolites suggest that their formation may have linked to pressure dissolution by which Mg-ions were probably released for replacement dolomitization during late early-burial to early shallow-burial dolomitization. Moreover, the tightly packed and curved crystal faces of RD2 and RD3 with irregular overgrowth rims and increasing crystal sizes presumably resulted from the recrystallization and replacement upon the RD1 and/or from the cortical overgrowth and dissolution of remaining precursor limestones by the influx of dolomitizing fluids during late shallow-burial dolomitization. The average range of the homogenization temperatures (Th) of CD2 is 132.4 °C–198.7 °C; and mostly higher than the estimated ambient temperature (95 °C–150 °C) for the Furongian-Lower Ordovician strata, implying that the growth of DC2 in the study area was the result of hydrothermal dolomitization. The comparable isotopic (δ18O, δ13C and 87Sr/86Sr) values between RD2, RD3, DC1, DC2 dolomites and coeval Neoproterozoic to Early Phanerozoic seawater values, in addition to the relatively high Th and salinity levels in these dolomites, indicates that their dolomitizing fluid were derived from radiogenic 87Sr-enrich coeval seawater during late shallow-burial dolomitization and hydrothermal dolomitization. The hydrotectonic driving model of burial dolomitization of the Furongian-Lower Ordovician carbonates in the Tazhong Uplift resulted from compressional tectonics that affected the Tarim Basin during Cambrian to Middle Ordovician and Late Ordovician period. This model is characterized by fluid flow in response to significant thermal heating, pressure-loading and enhanced fluid expulsion by squeezing, and the resultant higher-temperature Mg-rich connate waters and hydrothermal basinal brines are conveyed by convective-advective flows along permeable horizons, fractures and overthrust fault planes, resulting in extensive replacement dolomitization of carbonate materials and recrystallization of early formed dolomites; forming irregular networks of dolomites and calcite cements in the host carbonate rocks.

Fluid system and pressure evolution study based on isotope and fluid inclusion geochemistry: a case study on the Sinian Dengying and the Cambrian Longwangmiao Formation in the Gaoshiti-Moxi structure, Central Sichuan Basin

Systematic microscopic observation and analysis reveal multi-stage mineral fillings and fluid charging in the Sinian (Ediacaran) Dengying and the Cambrian Longwangmiao Formation in the Gaoshiti-Moxi structure, Central Sichuan Basin. Reservoir pore space is filled with five different stages of minerals from the edge to the center: fine crystalline dolomite, rimmed bitumen, coarse crystalline dolomite, stripped or oil droplet-like bitumen, and calcite or quartz, respectively. Sr, C, and O isotopic analysis for fine crystalline dolomite and the late stage calcite and quartz filled in reservoir pore space indicate that, prior to oil-generating window of the Cambrian Qiongzhusi Formation, the Sinian Dengying and the Cambrian Longwangmiao Formation were within the same connected and open fluid system. But after the gas-generating window, the Cambrian Longwangmiao Formation evolves gradually into a relatively independent and closed fluid system. By restoring the paleo fluid pressure, it is found that the paleo fluid pressure coefficient of the Sinian Dengying Formation has gone from 1.0, 1.1 to 1.2, 1.2, 1.2 to 1.3, and 1.0 to 1.1 at different evolution stages of source rock (at corresponding Ro value of 0.5, 1.3, 1.7, 2.0, and over 2.8, respectively), and the paleo fluid pressure coefficient of the Cambrian Longwangmiao Formation has varied from 1.0, 2.1 to 2.2, 1.8 to 1.9, 1.6 to 1.8, and 1.55 to 1.75. The pressure restoration result reveals that overpressure has never occurred in the Sinian Dengying Formation during the geological history, but overpressure commonly occurred in the Cambrian Longwangmiao Formation, and the values varied at different evolution stages of source rock. This pressure evolution process also demonstrates that the Sinian Dengying and the Cambrian Longwangmiao Formation, prior to the oil-generating window, were within an identical normal pressure fluid system. Meanwhile, it also matches well with the present exploration discoveries: the overpressured gas pools of the Cambrian Longwangmiao Formation are preserved up to now and hold hundreds of billions cubic meters of gas reserves, while the Sinian Dengying Formation contains residual normal pressure gas pools that formed after the adjustment of natural gas over a large area.

Pore-fillings of dolomite reservoirs in Sinian Dengying Formation in Sichuan basin

Abstract The Sinian Dengying Formation in the Sichuan basin is an important potential reservoir for oil and gas exploration in the periphery of the Sichuan basin. Its reservoirs were well developed. And many kinds of fillings which lead to the decrease of pores can be seen in reservoir pores and caves. In this paper, the pore fillings of the Dengying Formation reservoir in Sichuan basin are the object of study. The types and characteristics of the fillings are studied in detail by means of rock slices analysis, cathode luminescence, trace elements and inclusion temperature measurement. The study shows that the fillings include dolomite fillings and siliceous quartz fillings and bitumen fillings. Among them, dolomite fillings can be divided into botryoidal dolomite fillings, foliated dolomite fillings, finely-medium crystalline dolomite fillings and coarse crystalline dolomite fillings. 50%–80% of the pores are filled by the dolomite fillings with botryoidal structure, and the pores can be partially full filled; 0%–20% of pores are filled by foliated dolomite fillings; 20–80% of the pores are filled by finely-medium dolomite fillings; 20–80% of the pores are filled by coarse crystalline dolomite fillings. 1%–5% of secondary pores are often filled by siliceous quartzs, and the pores can be partially full filled. Bitumen fillings often occur after the above-mentioned fillings, and the residual pores are often semi-filled by bitumens, which makes the original pores and throat smaller and reduces the effective porosity and permeability of the reservoir. So, it is a destructive diagenesis. The pore fillings make the secondary porosity in the reservoir significantly lower, which is not conducive to the formation and preservation of the Dengying Formation reservoir.

Multiphase dolomitization in the Jutana Formation (Cambrian), Salt Range (Pakistan): evidences from field observations, microscopic studies and isotopic analysis

Excellent dolomite exposures are observed in the eastern Salt Range (Pakistan), where the Cambrian Jutana Formation consists of two distinct units (i.e. oolitic – pisolitic unit and massive dolomite unit). Field observations revealed that the lower, oolitic-pisolitic unit mostly comprises medium to thick bedded, interlayered brown yellowish dolostone containing ooids/pisoids and faunal assemblages, and grey whitish sandstone with distinct depositional sedimentary features (i.e. trough-, herringbone- and hhummocky crossbedding). The upper massive dolostone unit consists of thick bedded to massive dolostone. These two units are separated by shale. Petrographic studies identified three dolomite types, which include: fine crystalline dolomite (Dol. I), medium-coarse crystalline dolomite (Dol. II) and fracture associated, coarse crystalline dolomite (Dol. III). Stable isotope studies indicate less depleted δ 18 O values for Dol. I (-6.44 to -3.76‰V-PDB), slightly depleted δ18O values for Dol. II (-7.73 to -5.24‰V-PDB) and more depleted δ 18 O values for Dol. III (-7.29 to -7.20‰V-PDB). The δ 13 C values of the three dolomite phases are well within the range of Cambrian sea-water signatures. Furthermore, δ 26 Mg-δ 25 Mg signatures (Dol. I; δ 26 Mg=-1.19 to -1.67, δ 25 Mg=-0.61 to -0.86 and Dol. II; δ 26 Mg=-1.34 to -1.59, δ 25 Mg=-0.70 to -0.83) indicate three phases of dolomitization in different diagenetic settings. First, an initial stage of dolomitization during the early Cambrian resulted from altered marine, Mg-rich fluids associated with the mixing zone mechanism. Second, a late stage of dolomitization was associated with burial during late Permian. A third dolomitization phase was related to post-Eocene times.

Early dolomitization in the Lower Cretaceous shallow-water carbonates of Southern Apennines (Italy): Clues about palaeoclimatic fluctuations in western Tethys

A multidisciplinary study of the dolomitized bodies present in the Lower Cretaceous platform carbonates of Mt. Faito (Southern Apennines – Italy) was carried out in order to explore the connection between early dolomite formation and fluctuating climate conditions.The Berriasian-Aptian investigated succession is 466m thick and mainly consists of shallow-water lagoonal limestones with frequent dolomite caps. The dolomitization intensity varies along the succession and reaches its peak in the upper Hauterivian-lower Barremian interval, where it is present a completely dolomitized interval about 100-m-thick. Field relations, petrography, mineralogy, and geochemistry of the analyzed dolomite bodies allowed identifying two populations of early dolomites, a fine-medium crystalline (FMdol) and a coarse crystalline dolomite (Cdol), both interpreted as the product of mesohaline water reflux. According to our interpretation, FMdol precipitated from concentrated brines in the very early stage of the reflux process, producing typical sedimentary features as dolomite caps. In the successive step of the process, the basin-ward ‘latent’ reflux precipitated Cdol from less concentrated brines.A peculiar feature of the studied succession is the great consistency between stratigraphic distribution of dolomite bodies and their geochemical signature. The completely dolomitized Hauterivian-Barremian interval, in fact, is characterized by geochemical values suggesting an origin from distinctly saltier brines. Considering that the observed near-surface dolomitization process is controlled by physical and chemical parameters reflecting the paleoenvironmental and paleoclimatic conditions during dolomite formation, we propose that the stratigraphically controlled dolomitization intensity reflects periodic fluctuations in the salinity of dolomitizing fluid, in turn controlled by long-term climate oscillations.The present work highlights that the stratigraphic distribution of early diagenetic dolomite may be used as proxy to define the climatic fluctuations that have influenced the sedimentary dynamics in the Early Cretaceous. Moreover, considering that a comparable early dolomite distribution is present also in the Dinaric Platform, we suggest that a regional scale climate control acted on early dolomite formation and distribution. Refining the knowledge of such a key control may have a significative impact on hydrocarbon reservoir characterization and exploration in the Periadriatic area.

Origin of Dolomitization in Demirkaz&amp;#305k Formation (Adana-Turkey)

Demirkaz&#305k formation consisting of the limestones and dolomites is common in the Adana, Turkey. In this study, it is aimed to find the origin of dolomitization by preparing a detailed geological map. For this purpose, firstly, thin section/polished section and, chemical analysis of the twenty-one samples characterizing dolomitization of the field were prepared and examined. In the chemical analyzes, 21 samples were determined MgO-CaO ratio. Geological map was prepared by field study and MgO contents of the samples that are known coordinates in the chemical data, and the regions where dolomitization was effective were determined. Dolomitization in these carbonates occurs at early stages and late stages. Two dolomite types are petrographically determined; (1) fine crystalline dolomite (TD-1) with 10 - 25 μm crystal size and (2) coarse crystalline dolomite (TD-2) with 50 - 120 μm crystal size. TD-1 is formed by synsedimentary replacement and TD-2 derived from TD-1. The evidence of dolomitization due to pressure changes in the TD-2 dolomites is evidence of late diagenetic. There are three types of carbonate by chemical data; limestone, dolomitic limestone, calcitic dolomite and MgCO3 ratio of these is increased respectively. The rate of dolomitization is very variable and at the fracture regions, the Mg ratio increases, indicating that tectonism is the cause of dolomitization.

Impact of early dolomitization on multi-scale petrophysical heterogeneities and fracture intensity of low-porosity platform carbonates (Albian-Cenomanian, southern Apennines, Italy)

Petrographic, petrophysical and fracture analyses were carried out on middle Cretaceous platform carbonates of the southern Apennines (Italy) that represent an outcrop analogue of the Val d’Agri and Tempa Rossa reservoirs of the Basilicata region. The studied outcrops, which are made of interlayered limestones and dolomites of inner platform environment, were selected to study the impact of dolomitization on reservoir properties and the control of dolomite texture on fracture development. Two types of dolomites – both formed during very early diagenesis – were found interlayered, at a metre scale, with micrite-rich limestones (mainly mudstones and wackestones). Dolomite A is fine-to medium crystalline and makes non-planar mosaics. Dolomite B is coarse-crystalline and makes planar-s and planar-e mosaics. The intercrystalline space of the planar-e subtype of dolomite B is either open or filled by un-replaced micrite or by late calcite or saddle dolomite cement. Dolomite A and dolomite B have similar average porosities of 3.7 and 3.1% respectively, which are significantly higher than the average porosity of limestones (1.4%). Their poro-perm relationships are similar, with the notable exception of planar-e type B dolomites, which generally display higher permeability values.The intensity of top bounded fractures is distinctly lower in coarse-crystalline dolomites than in fine-crystalline dolomites and limestones, both at the macro- and the micro-scale. On the other hand neither lithology (i.e. limestone vs. dolomite) nor dolomite crystal size control the intensity of perfect bed-bounded fractures, which is strictly controlled by the fracture layer thickness.Our results provide information that could be used as guidance for the characterization and modelling of fractured carbonate reservoirs made of interlayered limestones and dolomites.

Exploring the geological features and processes that control the shape and internal fabrics of late diagenetic dolomite bodies (Lower Khuff equivalent – Central Oman Mountains)

The Lower Permian carbonate rock succession of the Oman Mountains represents a stratigraphical surface analogue for Middle Eastern Lower Khuff hydrocarbon reservoirs. The characteristics of the carbonate rocks are highly variable both laterally and vertically due to the superposition of several episodes of dolomitization. A stratabound early dolomitization event affected the well-bedded limestones during Triassic times. Subsequently, a later burial dolomitization led to the emplacement of several hundreds of meter wide and tens of meter thick dolomite bodies. The goal of our study is to understand the distribution and genetic controls on the formation of late diagenetic dolomite bodies.Field mapping in Wordian to Capitanian beds was conducted in the Jebal Akhdar tectonic window in the Central Oman Mountains to determine the shape, internal fabrics and associated structures of late diagenetic dolomite bodies. The longest extension of late diagenetic dolomite bodies was found to follow predominantly the strike direction of fracture planes, bed-to-bed contacts, the Precambrian–Permian unconformity and the bottom rim of early diagenetic dolomite. Two groups of diagenetic fabrics were observed in association to fractures and bedding planes. First, vug related fabrics comprise vuggy pores and zebra dolomite. Second, breccia related fabrics consist of either small-scale crackle/mosaic breccia fabrics and centimeter sized vugs followed by bedding-parallel vugs lined up. Petrographic analysis revealed three types of diagenetic phases referred to as D1, D2 and D3. The matrix replacive phase D1 represents the main diagenetic phase formed during burial. Coarse crystalline dolomite rhombs (phase D2) and saddle dolomite (phase D3) were found along stylolites, in vugs and in fractures. Based on the presence of saddle dolomite and zebra dolomite textures, late diagenetic dolomite bodies are interpreted to have formed by the circulation of warm dolomitizing fluids of at least 60 °C.