Crystalline Dolomite Research Articles

Dolomitization mechanism of Pila Spi formation (middle- late eocene) in the high folded zone, Northern Iraq

Petrographic analysis and geochemistry of the Pila Spi Formation (Middle-late Eocene) were investigated from two outcrop sections (Gali-Zakho and Shaqlawa), within the High Folded Zone, northern Iraq. This formation consists of fine to coarsely crystalline dolomites and limestone sequences. Despite the fact that these strata have been buried and impacted by several episodes of dolomitization and dissolution, some intervals in the Pila Spi Formation have acquired porosity. Dolomicrite, dolomicrosparite, dolosparite, vein, and saddle dolomite are the five primary kinds of dolomite textures recognized based on fabric, grain size, and elemental analysis. Dolomicrite produced the sabkha model in a low-energy hypersaline sedimentary environment, since these rocks had unusually high Sr and Na values, lower Fe and Mn values. The dolomicrosparite and dolosparite developed in a shallow burial environment using the seepage-reflux model, and these rocks had a higher formation temperature, low Sr and Na levels, and a broad range of Fe and Mn values, indicating that they were impacted by meteoric water. The vein and saddle dolomite evolved as a result of the burial model at increased temperature since these rocks had the lowest Sr and Na contents, the greatest MgO and CaO contents, and the highest Fe and Mn values. In selective dissolution processes, the early sabkha, the seepage-reflux model of dolomitization, and the synchronous cycle of meteoric freshwater, many porous intervals were formed.

Sedimentological and Petrographical Characterization of the Cambrian Abbottabad Formation in Kamsar Section, Muzaffarabad Area: Implications for Proto-Tethys Ocean Evolution

The current sedimentological and petrographical research of the Abbottabad Formation has been carried out in order to understand the formation and evolution of the Proto-Tethys Ocean during the Cambrian on the northern margin of the Indian Plate. The Muzaffarabad region is located east of the Upper Indus Basin and the southern part of the Hazara Kashmir Syntaxis. The geological history of the region varies from the Precambrian to the recent period. The Cambrian Abbottabad Formation is well exposed along the Hazara Kashmir Syntaxis at the core of the 500-m-thick Muzaffarabad anticline. The Abbottabad Formation is an unconformity-bounded allo-stratigraphic unit. It has an unconformable lower contact with the Late Precambrian Dogra Formation and an unconformable upper contact with the Paleocene Hangu Formation. The Abbottabad Formation has been divided into four lithofacies, from bottom to top, namely, thinly interbedded dolomite and shale, cherty-stromatolitic dolomite, oxidized limonitic-brecciated zone, and quartzite, with significant lithological changes. Petrographic studies revealed four types of dolomites: fine crystalline dolomite (Dol. I), dolomitic cryptocrystalline chert (Dol. II), algal mat-stromatolitic dolomite (Dol. III), and intraclastic-dolo-grain stone (Dol. IV). The mineral composition of dolostone was analyzed using X-ray diffraction (XRD) and found to be consistent with previous petrographic studies. The dolomite mineral content decreased from base to top, while chert increased towards the top. Elemental weight percentages through energy dispersive X-ray (EDX) analysis show different elements constitute the minerals found in the dolostone, as confirmed by petrographic and XRD analysis. Using outcrop data, facies information, and geochemical data, a modified depositional model of the Abbottabad Formation was developed. During the Early Cambrian period, the formation was deposited in a shallow subtidal to supratidal setting of the Proto-Tethys Ocean. The top of this deposit marks the Cambrian–Paleocene boundary. Because of the progressively coarsening outcrop sequences, this formation seems to be at the very top of the Proto-Tethys Ocean’s shallow marine system.

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.

Multiphase dolomitization mechanisms of the Cambrian upper Changping Formation, North China Platform, China

The Cambrian dolomite reservoirs of the North China Platform offer good exploration prospects, but their occurrence and the impacts of dolomitization on reservoir quality are not yet clearly understood. The study herein assesses outcrop samples in the Cambrian upper Changping Formation, Dingjiatan area, and highlights the important role of multiphase dolomitization in the formation of paleo-reservoirs of acceptable porosity, where oil pools and fields may be discovered. A combination of petrology, fluid inclusion microthermometry, isotopes, and trace element compositions has been used to explain multiphase dolomitization mechanisms and their impacts on dolomite reservoirs. Five types of dolomites are identified through outcrop observation, thin section identification via transmitted light, and cathodoluminescence. The following geochemical analyses reveal various dolomitization mechanisms. In the (pene) contemporaneous stage, microbial dolomite is commonly related to microbial metabolic activities with significant carbon isotopic depletion compared to the Cambrian seawater values. With the influence of microbial dolomitization, dolomicrite corresponds to the sabkha dolomitization mode in a low-salinity seawater environment and early-stage dolomitization. The structureless dolomite (ssD) in the early highstand systems tract (EHST), characterized by elevated 87Sr/88Sr ratios and low oxygen isotopic values, forms from stratum brine water in the shallow-middle burial stage rather than in the (pene) contemporaneous stage. In contrast, ssD in the late highstand systems tract (LHST) undergoes (pene) contemporaneous dolomitization at salinities between 6% and 28% before later pore water transformation, with the participation of atmospheric freshwater through faults and unconformities exhibiting the lowest inclusion temperature and salinity values. The medium to coarse crystalline dolomite (MCD) in the LHST and the saddle dolomite (SD) in the EHST with low REE values are atypical hydrothermal dolomites caused by a combined superposition of middle-deep burial hydrothermal fluids at temperatures >150°C and stratigraphic brines. The MCD is also influenced by terrigenous water characterized by relatively low Eu anomaly values. Finally, the mechanisms of porosity increase are investigated, and it is concluded that the pore increase caused by the (pene) contemporaneous reflux interaction and the later pore retention both lead to better ssD reservoirs in the LHST than in the EHST.

Study on reservoir characteristics, pore-throat structure, and origin of tight oolitic reservoirs: A case study of Triassic Feixianguan Formation, NE Sichuan Basin, SW China

The origin and pore-throat structure of different lithofacies are key issues in exploration and development of tight oolitic reservoirs. Based on core and thin section observation, four types of lithofacies can be recognized in the Feixianguan Formation oolitic reservoir: 1) oolitic limestone with intergranular pores (Lithofacies A), 2) oolitic limestone with mold pores (Lithofacies B), 3) oolitic dolostone (Lithofacies C), and 4) silty crystalline dolomite and fine crystalline dolostone (Lithofacies D). The subsurface core samples from the different lithofacies were studied using mercury injection, 3D CT scanning, and nuclear magnetic resonance, indicating that Lithofacies D possesses the best pore-throat structure and reservoir connectivity. The pore-throat structure of Lithofacies C is very similar to that of D, but the heterogeneity is much stronger. By comparison, the pore-throat structure of Lithofacies A and B is relatively poor. Although the pore heterogeneity of Lithofacies B is weaker than that of Lithofacies A, there is no effective throat connection in Lithofacies B. On the basis of oolitic shoal deposition, the factors controlling the origin of different lithofacies are meteoric freshwater leaching and then dolomitization. Meteoric freshwater leaching dominates the origin of Lithofacies B but does not affect the origin of Lithofacies C. Lithofacies C is jointly controlled by seepage-reflux and hydrothermal dolomitization fluids. The origin of Lithofacies D is only controlled by seepage-reflux dolomitization. Some other diagenesis effects may also have an impact on petrophysical properties of different lithofacies, but they do not play a decisive role in the origin of different lithofacies.

Effects of diagenesis on quality of dengying formation deep dolomite reservoir, Central Sichuan Basin, China: Insights from petrology, geochemistry and in situ U-Pb dating

The Ediacaran Dengying Formation in Central Sichuan Basin is the deep dolomite gas reservoir with the largest natural gas reserves in China, providing an excellent example for understanding the effect of diagenesis evolution on deep dolomite reservoir quality. By integrating petrology, geochemistry and in situ U-Pb dating, this study aims to reveal the genesis of different rock fabrics and their corresponding diagenetic events, and to discuss the temporal relationship of diagenetic events and their effects on the reservoir quality of Dengying Formation. Two phases of dolomite and three phases of dolomite cement are identified in Ediacaran Dengying Formation deep dolomite reservoirs of Central Sichuan Basin as follows: 1) matrix dolomites (MD), 2) fine-medium crystalline dolomites (FMD), 3) fibrous dolomites (FD), 4) medium-coarse crystalline dolomites (MCD), and 5) saddle dolomite (SD). By analyzing petrographic and temporal relationships between these fabrics, it is suggested that the deep dolomite reservoir of Dengying Formation in Central Sichuan Basin has experienced six diagenetic stages, including 1) syndiagenetic stage, 2) the first-time early diagenetic stage, 3) epidiagenetic stage, 4) the second-time early diagenetic stage, 5) middle diagenetic stage and 6) late diagenetic stage. Microbial dolomitization during syndiagenetic stage produced limited increase in reservoir porosity, but it is of great significance for preservation of reservoir spaces. Silicification is most unfavorable for reservoir formation, but its duration and influence range are very limited. At epidiagenetic stage, the physical properties of Dengying Formation dolomite reservoir have been greatly improved. Two-time early diagenetic stages are both destructive for preservation of reservoir spaces. During middle diagenetic stage, two-stage of hydrothermal alterations occurred, corresponding to Late Silurian to Devonian and Late Permian, respectively. Hydrothermal alteration of the MD is both constructive and destructive, but overall it is constructive, and the improvement of physical properties is limited. For the karstified MD, both two-stage hydrothermal alterations were destructive, resulting in damage of reservoir physical properties caused by cementation by two-stage SD and one-stage MVT minerals.

The effect of meteoric water on the very fine crystalline dolomite reservoir in the shallow burial zone: A case study of the Ma55 submember of Majiagou Formation in Ordos Basin

The meteoric water has obviously changed the physical properties of dolostone reservoirs in the vertical vadose zone and the horizontal phreatic zone, but its influence on the dolostone reservoirs in the shallow burial zone beneath the phreatic surface is still unclear. This study aims to reveal the effect of meteoric water on the dolostone reservoirs in the shallow burial zone through X-ray diffraction, cathodoluminescence, C, O, and Sr isotope using the sample from Majiagou Formation in the Daniudi gas field, Ordos Basin. The diagenesis and paragenesis of the Ma 55 submember were identified and interpreted through petrological study, combined with data from electron probe, X-ray diffraction analysis, and geochemical parameters of diagenetic minerals. The color of the very fine crystalline dolomite under the cathodeluminescence is dark red and red. The order degree of dolomite ranges from 0.54 to 0.91, showing the origin of early seepage-reflux dolomitization. There are a large number of different calcite cements as fills within the pores and fractures. The color of the calcite cement under the cathodoluminescence is orange-yellow, with a zonal structure. Hydrothermal fluid during late diagenesis could be identified by the authigenic fluorite filling in the fractures. According to the assembly of diagenetic minerals, the very fine crystalline dolostones have experienced the seepage-reflux dolomitization, meteoric water dissolution, shallow burial cementation and late cementation. The void spaces of the very fine crystalline dolostones are intercrystalline pores and microfractures. Although a large number of dissolved pores and caves developed in the period of meteoric water dissolution, these caves and dissolved pores has been mostly filled by multi-stages of cementation. Therefore, the effect of meteoric water on dolostone reservoirs in the shallow burial zone beneath the phreatic surface is not obvious. The main controlling factor for the quality of dolostone reservoir was dolomitization. This study provides a new understanding of the influence of meteoric water on reservoir quality in the shallow burial zone during the paleokarst period.

Key factors affecting hydrocarbon accumulation in ancient dolomite gas reservoirs of Xixiangchi formation (Southern Sichuan Basin, China)

The Upper Cambrian Xixiangchi Formation in the Southern Sichuan Basin, China, has favorable hydrocarbon accumulation conditions. The accumulation factors and enrichment conditions of this formation were key considerations in this study. By analyzing core, thin section, seismic, and geochemical data, the research shows that there are many sets of granular and crystalline dolomite reservoirs in the Xixiangchi Formation vertically, with thin thickness of single-reservoir. During the transformation of karst and tectonism, dissolution pores and fractures developed to form an ideal reservoir space. The reservoir of the Xixiangchi Formation is connected to the Lower Cambrian source rock through a fault system. The high-energy shoal facies of the Xixiangchi Formation are located on the oil and gas migration path, providing an appropriate reservoir space for forming the source reservoir configuration relationship between the lower generation and upper reservoir. The key factors affecting hydrocarbon accumulation in the Xixiangchi Formation are sufficient oil and gas supply, development of inherited paleo-uplift, effective transportation system, and favorable reservoir-forming combination. The inherited paleo-uplift controls the distribution of gas reservoirs. Owing to the short migration distance of oil and gas, hydrocarbon is found near source hydrocarbon accumulation, and the paleo-uplift slope area should be targeted for exploration in future studies.

Diagenesis and Reservoir Evolution Model of the Ediacaran Dengying Formation in the Sichuan Basin: Evidence from Laser Ablation U-Pb Dating and In Situ Isotope Analysis

The microbial dolomite of the Ediacaran Dengying Fm., Sichuan Basin, SW China is endowed with abundant oil and gas resources. A complex diagenetic history greatly complicates the development of such microbial dolomite reservoirs, and has severely restrained hydrocarbon exploration. This study focused on key field profiles in Eastern Sichuan and aimed to decipher the origin and evolution of dolomites, using novel techniques, including elemental mapping, laser ablation U-Pb dating as well as carbon, oxygen, and strontium isotope analyses. Additionally, we compared the diagenesis and reservoir development model for the Dengying microbial dolomites of the Central Sichuan to other areas, with the aim to provide practical guidance for oil and gas exploration across the entire Sichuan Basin. Our results have shown that the Ediacaran microbial dolomite in Eastern Sichuan experienced four stages of diagenetic modification–specifically: (1) syn-sedimentary dolomitization; (2) penecontemporaneous cementation of fibrous dolomite cement; (3) bladed dolomite and brown-dark very fine crystalline dolomite cementation during the Ordovician; and (4) fine crystalline dolomite and medium-coarse crystalline dolomite cementation during the Silurian-Devonian. Petrology and geochemistry data imply that each diagenetic phase of dolomitization was mainly sourced from marine-dominated fluids. The influence of tectonically related fluids was rarely noted. Significantly, U-Pb ages of these dolomites reveal that the history of dolomite infill to pores within the Eastern Sichuan Basin clearly post-dated that in the Central Sichuan region, resulting in high porosities (6%–11%) that favored hydrocarbon accumulation. This study suggests that the platform margin at the eastern edge of the Xuanhan-Kaijiang paleo-uplift is favorable as the next key exploration target for microbial dolomites in the Sichuan Basin.

Petrographic, Rare Earth Elemental and Isotopic Constraints on the Dolomite Origin: A Case Study from the Middle-Upper Cambrian Xixiangchi Formation in Eastern Sichuan Basin, Southwest China

The Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin is regarded as an important reservoir with great potential for hydrocarbon exploration. It is previously indicated that the Xixiangchi carbonates have experienced extensive dolomitization, however, the origin of dolomitizing fluids and the dolomitization mechanism still remain uncertain. In this study, a set of petrographic and geochemical examinations, including rare earth elements (REE) and isotopic (C, O, and Sr) compositions were used to trace the origins of dolomitizing fluids and associated diagenetic processes. The petrographic examination revealed three types of matrix dolomites (D1, D2, D3) and one cement saddle dolomite (SD). These phases have crystal size ranges of less than 30 μm (very fine to fine crystals, D1), 30–100 μm (fine to medium crystals, D2), 100–300 μm (medium to coarsely crystalline dolomite, D3), and 0.3–4 mm (fracture filling cements, SD), respectively. D1 is characterized by non to very weak luminescence, weakly negative Ce anomalies (Ce/Ce* = 0.84 ± 0.02), strongly negative Eu anomalies (Eu/Eu* = 0.65 ± 0.03), and high 87Sr/86Sr ratios (0.71062 ± 0.00122). In combination with δ13C (−1.5‰ ± 0.2‰) and δ18O (−9.7‰ ± 0.5‰) compositions, D1 is interpreted to be formed by penecontemporaneous dolomitization in the near-surface environment with seawater as the dolomitizing fluid. In contrast, D2 is characterized by intercrystalline pores, dirty crystal surfaces, similar δ13C (−1.4‰ ± 0.4‰) compositions but higher δ18O (−8.9‰ ± 0.7‰) compositions, and lower 87Sr/86Sr ratios (0.70992 ± 0.00035), similar Ce anomalies (Ce/Ce* = 0.87 ± 0.04) and higher Eu anomalies (Eu/Eu* = 0.85 ± 0.04). The coarser D2 is regarded to be formed by the post-penecontemporaneous seepage-reflux dolomitization or by recrystallization of D1 dolomite in a near-surface or shallow burial environment. D3 is distinguished by a cloudy core with clear rims, showing slightly higher Eu anomalies (Eu/Eu* = 0.88 ± 0.02) and similar Ce anomalies (Ce/Ce* = 0.88 ± 0.02) than those of D1 and D2. Combined with the δ18O compositions (−10.4‰ ± 0.4‰) and 87Sr/86Sr ratios (0.70989 ± 0.00048), D3 is thought to be formed by the overgrowth or recrystallization of D1 and D2 dolomites in a shallow to moderate burial environment. The fractures filling SD dolomite consists of nonplanar and much coarser crystals with undulatory extinctions and brighter red luminescence. The lower δ18O (−11.1‰ ± 0.3‰) compositions, lower negative Eu anomalies (Eu/Eu* = 0.70 ± 0.01) of SD than the matrix dolomites, and similar Ce anomalies (Ce/Ce* = 0.83 ± 0.01) are indicative of hydrothermal dolomitization, with possible fluids associated with the magma during the period of Emei taphrogenic movement. In addition, the 87Sr/86Sr ratios (0.70941 ± 0.00003) of SD suggest probable origin from the coeval seawater partially. Therefore, SD dolomite is interpreted to be formed by hydrothermal dolomitization with mixed dolomitizing fluid of seawater and hydrothermal fluids. In summary, all the matrix dolomites have almost the same ΣREE concentrations and exhibit similar near-flat REE partition patterns with weak LREE enrichments, weakly negative Ce anomalies, and negative Eu anomalies. Such characteristics of REE compositions are indicative of similar evolved dolomitizing fluid, such as seawater or seawater- derived fluids. By contrast, SD dolomites have a different REE partition pattern with left-leaning characteristics, LREE depletions, and negative Eu anomalies, thus suggesting a different dolomitizing fluid source from the matrix dolomites. In addition, the development of intercrystalline pores associated with D2 dolomite makes it more likely to be a potential reservoir, indicating that the dolomitizing history of carbonate has a strong influence on the quality of potential dolomite reservoirs.

Characteristics and main controlling factors of dolomite reservoirs in the Upper Cambrian Sanshanzi Formation, eastern Ordos Basin, China

The Cambrian system in the Ordos Basin has a similar development degree of carbonate strata to that in the Sichuan and the Tarim Basins; however, no significant oil and gas breakthroughs similar to those in the latter two basins have been made in the Ordos Basin yet, and the characteristics, genesis, and differences in the reservoir need to be explored. Field outcrop observations reveal that the oil and gas shows are common in the Upper Cambrian Sanshanzi Formation in the eastern Ordos Basin, indicating important oil and gas exploration potential. Analysis of the characteristics and genesis of dolomite reservoirs can greatly support the exploration and development of the Cambrian carbonate oil reservoirs in the Ordos Basin. Based on the field outcrop observations and comprehensive laboratory tests and analyses, the reservoir characteristics and main control factors are systematically discussed. This study reveals that the reservoir lithology mainly comprises crystalline dolomite, from fine crystalline dolomite to coarse crystalline dolomite; this results in reservoirs with low porosity, extreme-low porosity, and ultralow permeability dominated by intergranular denudation, caves, and microfractures. They can be further divided into two types of reservoirs: type I reservoirs that have low porosity, ultralow permeability, and medium pore-throat radii, and type II reservoirs that have extreme-low porosity, ultralow permeability, and small pore-throat radii. The physical properties of fine-medium crystalline and medium crystalline dolomite reservoirs are slightly better, and the throats tend to be larger with increasing grain size. The reservoirs have not experienced deep burial processes; dolomitization mainly occurs in the marine-sourced fluid environment with weak oxidation and weak reduction, influenced by the atmospheric fresh water and thermal fluid transformation during the late period to some extent, whereas the hydrocarbons mainly charged during the late period. The crystalline dolomite in residual grains superimposes the denudation and cataclasis in the epigenetic karst stage that are the main controlling factors of the reservoirs. The crystalline cataclasis of dolomite can aid in the formation and reconstruction of the reservoirs. The spatial and temporal distribution laws of the reservoirs and key formation periods need to be studied further.

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.

Types, Petrophysical Properties and Pore Evolution of Late Ediacaran Microbial Carbonates, Tarim Basin, NW China

AbstractThe Upper Ediacaran microbial carbonates of the Tarim Basin are potential reservoir geobodies for future hydrocarbon exploration with rising interest in exploration for deeply‐buried reserves. However, little knowledge has been acquired on the types of microbial carbonates that are present, the properties of the reservoir and the pore evolution, hampering predictions of high‐quality reservoirs in these carbonates. Integrated with petrography and in‐situ U‐Pb dating geochronology analyses, this study aims to clarify the types of microbial carbonates present and to reconstruct the pore evolution processes of the potential reservoir rocks. The Upper Ediacaran microbial carbonates of the Tarim Basin can be divided into four types, based on their features in terms of different scales (macro‐ to micro‐): microbial laminite, stromatolite, spongiomicrobialite and microbial‐peloidal wackestone/mudstone. Petrophysical properties show that all these microbial carbonates have low porosity and very low permeability with poor connectivity. These carbonates were subject to long‐term and complex diagenetic processes, mainly consisting of dissolution, compaction, pervasive dolomitization, cementation and fracturing. The most important reservoir spaces are contributed by vugs and dissolution‐enlarged pores, which are likely to have been associated with the widespread uplift of the Aksu area in the terminal Ediacaran. In contrast, the cementation of the fine‐to‐medium crystalline dolomite greatly reduced the pre‐existing pores. Pore types are closely related to different microbial fabrics, which played an important role in the pore evolution of the microbial carbonates.

Analysis of the Diagenetic Fluid of Deep Dolomitic “Leopard-Spot” Limestones in the Middle Permian Qixia Formation in Shuangyushi Region in Northwest Sichuan Basin, China

A thorough investigation of the characteristics and formation mechanism of dolomite is greatly significant in assessing the validity of dolomitization theory and dolomite reservoirs. Extensive development of dolomites has been found in the organic bank of the Middle Permian Qixia Formation in the northwest of the Sichuan Basin. For that reason, field profile and drilling core samples were collected from the common dolomitic “leopard-spot” limestones in the Middle Permian Qixia Formation in the study area and observed in this work. The diagenetic fluid and formation factors of dolomites were analyzed through major elements, microelements, carbon, oxygen and strontium (Sr) isotope, and fluid inclusion, as well as order degree of dolomite. From the extracted outcomes, valuable insights can be derived. More specifically, in the study area, dolomitic “leopard-spot” limestones were mainly developed in the upper part of Section II in the Qixia Formation. Interestingly, the vertical distribution plays a dominant role since it was transited gradually and gently to the thick-layered lumpy crystalline dolomites downward. The dolomitic “leopard-spots” are mainly composed of dolosiltite-fine crystalline dolomites with residual fabrics, and the base limestones were mainly formed by biomicrites (debris). In addition, the average δ13C of dolomitic “leopard-spot” and base limestones was 3.06‰ and 3.31‰, respectively, whereas their average δ18O was −4.29‰ and −3.16‰ and the 87Sr/86Sr (0.70734 in average) was higher than those of the contemporaneous seawater and mantle provenance range. Moreover, the contents of the major elements in dolomitic “leopard-spot” and base limestones were basically consistent. The uniform temperature of inclusion ranges between 70°C and 115°C. According to the geochemical data and the previously reported studies on the regional paleogeotemperature environment, the diagenetic fluid of the dolomitic “leopard-spot” limestones in the Qixia Formation in the northwest Sichuan Basin was mainly attributed to contemporaneous sea flows. However, influenced by the local high-temperature environment, the terrestrial atmosphere, and the fresh water, it is speculated that the dolomitic “leopard-spot” limestones in the study area might belong to dolomitization involving fresh water from the penecontemporaneous stage to the early shallow burial stage.

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.

Restoration of formation processes of dolomite reservoirs based on laser U-Pb dating: A case study of Ordovician Majiagou Formation, Ordos Basin, NW China

To address the issue of non-unique interpretation of dolomite reservoir diagenetic and porosity evolution in the previous qualitative or semi-quantitative studies, we investigate two dolomite reservoir types, i.e. weathering-crust karstic reservoirs and mound-beach reservoirs, in the Ordovician Majiagou Formation, Ordos Basin by using in-situ laser ablation U-Pb dating as well as carbon and oxygen isotopic composition analysis. The results show that: (1) According to the dating of 8 reservoir samples, the Majiagou Formation experienced 5 diagenetic stages (Stage 1: deposition of matrix dolomite or penecontemporaneous dolomitization, in 444.0–494.0 Ma; Stage 2: dogtooth- or blade-shaped dolomite cementation, in 440.0–467.0 Ma; Stage 3: dolomitic silt filling, in 316.5–381.0 Ma; Stage 4: crystalline dolomite filling, in 354.0 Ma; Stage 5: crystalline calcite filling, in 292.7–319.0 Ma). (2) Supra-salt weathering-crust karstic dolomite reservoirs went through several diagenetic processes including penecontemporaneous dolomitization, compaction, weathering-crust karstification, packing, and rupturing in succession. Gypsum mold pores formed in the phase of hypergenic karstification and were filled with such minerals as dolomitic silts and calcites, and thus the porosity decreased from 10%–40% to 3%–8%. (3) Sub-salt mound-beach dolomite reservoirs went through the diagenetic processes including penecontemporaneous dolomitization, compaction, subsea cementation, penecontemporaneous corrosion, infiltration backflow dolomitization, packing, and rupturing. The porosity of reservoirs was originally 10%–30%, decreased to 0–6% due to seawater cementation, rose back to 5%–15% owing to penecontemporaneous corrosion, and finally declined to 2%–6% as a result of crystalline dolomites and calcites packing. The above methodology for the restoration of dolomitization and porosity evolution may be helpful for the restoration of porosity evolution in other basins or series of strata.

Fluid Properties and Genesis of Dolomites in the Devonian Guanwushan Formation of Upper Yangtze Platform, SW China

The Guanwushan Formation (GWSF) of Devonian dolomite are extensively developed in the northwest of Sichuan Basin in the Upper Yangtze region, but the properties of dolomitization fluid and the geneses are still unclear. Three types of dolomites can be divided by petrological characteristics: the fine-microcrystalline dolomites (FMD), the fine crystalline dolomites (FCD) and the medium crystalline dolomites (MCD). The order degree of these three types of dolomites increased in turn, and they all showed dark cathodoluminescence (CL) luminescence. The total amount of Rare Earth Elements (∑REE) of the dolomite was low, while the dolomite enriched with light REE and lacking heavy REE presented a distribution pattern consistent with that of limestone. The weak negative anomalies of the Ce and Eu indicated that the dolomites were formed in a weak redox environment with relatively low temperature. The dolomitization fluids were inherited from the original seawater. The respective δ13CPDB values of the three types of dolomites varied a little, indicating that they were not affected by the biological effects. Specifically, the δ18OPDB values of the FMD and FCD dolomites were higher than that of the limestone, indicating that the dolomitization fluid was influenced by evaporation at the penecontemporaneous stage. The interpretations were also supported by the 87Sr/86Sr ratios, as the 87Sr/86Sr ratios of FMD comparable to the Middle Devonian seawater. The δ18OPDB value of the MCD dolomite was lower than that of the limestone. It also showed poor automorphic extent, which clarified that the dolomite experienced more intense dolomitization in greater burial depth and at higher temperatures.

INSIGHTS INTO THE ORIGIN OF SHALLOW MARINE DOLOSTONES, LONGWANGMIAO FORMATION (LOWER CAMBRIAN), NORTHEAST SICHUAN BASIN, CHINA: PETROGRAPHIC AND GEOCHEMICAL CONSTRAINTS

For a better understanding of the origin of Early Cambrian dolomites in the Sichuan Basin, which serve as valuable oil and gas reservoirs, we present and interpret complex multiparameter petrographic and geochemical analyses of dolomites in the Cambrian Longwangmiao Formation at the Yangsiqiao section. Our data show that these dolomites mainly occur in the middle and upper parts of the Longwangmiao Formation, and we identify two types of dolomites: micrite and fine-medium crystalline dolomites. The micritic dolomites, occasionally coupled with quartz and mica minerals, show dirty crystal surfaces, nonautogenous crystal shapes, and dark cathodoluminescence and mainly occur in pure micrite and granular dolostones, which retain the original sedimentary texture. Contrarily the fine-medium crystalline dolomites, mainly developed in crystalline and grain dolostones and generally coupled with residual calcites and altered sedimentary textures, have a dark heart-bright boundary texture under a polarized microscope and occasional zonal texture under cathode luminescence. Furthermore, the micritic dolomites have CaO and MgO contents approximately similar to those of the ideal stoichiometric dolomite, relatively high Sr concentration, Σ rare-earth element (REE) +Y concentrations, and δ13C and δ18O values located in the corresponding range of dolomite deposited from Early Cambrian marine water. However, light-REE enrichment (LREE), heavy-REE (HREE) depletion patterns, and high Al2O3 and SiO2 contents distinguish these micritic dolomites from typical marine carbonate deposits. Notably, the fine-medium crystalline dolomites have higher CaO content and lower MgO, Al2O3 and SiO2 contents, lower Sr and higher Fe and Mn concentrations than those of micritic dolomites. The ΣREE+Y concentrations and REE patterns of these fine-medium crystalline dolomites belong to the typical marine carbonate deposits. These features indicate considerable differences in origins between micrite and fine-medium crystalline dolomites. The micritic dolomites show a significant genetic response to the precipitation of seawater in shallow marine with the injection of terrestrial water during the syngenetic period, while the fine-medium crystalline dolomites have a typical response to the secondary metasomatism products of original calcites at the shallow burial stage.

Mottled dolomite in the lower Cambrian Longwangmiao formation in the Northern Sichuan Basin, South China

At the end of Early Cambrian time, the Sichuan basin (South China) was located in a wide carbonate platform, with hundreds of meters of carbonate deposited. The Longwangmiao Formation carbonate in Sichuan basin is partially to completely dolomitized, displaying a mottled texture in the northern area of the exposure. The mottled dolomitic limestone developed parallel to bedding, with shape irregular boundaries with limestone that has not been dolomitized. The mottled dolomite is composed of powder crystalline and finely crystalline dolomite, while the matrix limestone is composed of micritic calcite. the isotopic composition of mottled dolomite (δ13C = +0.29‰PDB, δ18O = −1.15‰PDB) is similar to that of micrite calcite (δ13C = −0.49‰PDB, δ18O = −1.45‰PDB). Both isotopic values and trace element data indicate that the dolomitized fluid is originated from sea water. Some beds contain gypsum pseudomorphs and mud cracks, indicating a shallow and evaporative environment with local high salinity during deposition. Dolomitization likely took place early, in part as a result of sea water salinity concentration. Trace fossils thalassinoides horizontalis, thalassinoides callianassa and planolites developed in the Longwangmiao Formation, and the sharp edges of mottled dolomite are similar to these trace fossils. The beds are intensely bioturbated. In the burrow network, the sediments and burrow fill were coarse and loose with little clay, and it is interpreted here as being easier to be dolomitized than the surrounding sediments. Partial dolomitization is thus interpreted to have occurred in the burrow system, and the degree of dolomitization was related to the degree of bioturbation, which is controlled by the trace-making creatures.

Reactive Transport Modeling of Reflux Dolomitization of Carbonate Platforms: Enlightenment from Yingshan Formation in Shunnan Area, Tarim Basin

Dolomite plays an important role in carbonate reservoirs. The topography in the study area creates conditions for reflux dolomitization. The northeastward paleogeomorphy during the deposition of the Yingshan Formation was favorable for reflux dolomitization. Furthermore, the petrological and geochemical evidence indicated that the formation of finely crystalline dolomites was penecontemporaneous to sedimentation. The content of powder crystal dolomites increases from grainstone, to packstone, to mudstone. Previous studies only analyzed the origin of dolomites based on traditional geological methods, but did not analyze the spatial influence of reflux dolomitization on the reservoir quality. In this study, the reflux dolomitization of platform carbonate sediments was evaluated using three-dimensional reactive transport models. The sensitivity of dolomitization to a range of intrinsic and extrinsic controls was also explored. The reflux dolomitization involves replacement dolomitization and over-dolomitization. The porosity change is the result of the abundance change of dolomite and anhydrite. The fluid flow pattern in the model is related to the injection rate and geothermal gradient. According to the spatial and temporal change of mineral, ionic concentration, and physical property, the reflux dolomitization could be divided into five stages. From the sensitivity analysis, high permeability promotes dolomitization only in the initial stage, while low permeability and high porosity means stronger dolomitization. Besides, the injection rate, reactive surface area (RSA), geothermal gradient, and brine salinity are all proportional to the dolomitization. Differently from porosity change, the permeability change is concentrated in the upper part of the numerical model. The location of “sweet spot” varies with the locations of change centers of porosity and permeability. In the stage-1 and 4 of dolomitzation, it overlaps with porosity and permeability growth centers. While in the stage-2, 3 and 5, it lies between the porosity and permeability growth/reduction centers.