Saddle Dolomite Research Articles

Mineralogical and Maturation Considerations of the Coqueiros Formation (Campos Basin, Brazil): Insights from Multi-Technique Analyses of Source Rocks

The Coqueiros Formation, a strategic stratigraphic unit within the Lagoa Feia Group (LFG) in the Campos Basin offshore Brazil, is known for its lacustrine carbonate deposits, which include both organic-rich shales and economically important “coquina” reservoirs. While coquina facies are widely recognized as reservoirs, the source-rock potential of the intercalated shales remains relatively underexplored. This study aims to characterize the mineralogy and thermal maturity of the Coqueiros Formation to assess its potential as a source rock, using a multi-technique approach integrating X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Raman spectroscopy analyses of shale samples from two wells: 3-BP-11-RJS and 6-DEV-18P-RJS. XRD analyses revealed a heterogeneous mineralogy dominated by carbonates (calcite and dolomite) and quartz, with significant contributions from clay minerals and trace minerals such as pyrite and barite. SEM imaging revealed a heterogeneous fabric with grain size, morphology, and porosity variations, reflecting a dynamic lacustrine depositional setting influenced by storm events and fluctuations in terrigenous input. The presence of authigenic minerals, as reported in other studies, such as saddle dolomite, mega-quartz, and various sulfides, provides evidence for hydrothermal alteration, likely related to Late Cretaceous magmatic activity in the Campos Basin. Raman spectroscopy yielded equivalent vitrinite reflectance (Ro%) values consistently exceeding 1.00, ranging from 1.03 to 1.40, indicating that the organic matter in the Coqueiros Formation shales has attained a high thermal maturity level, surpassing the oil window and reaching the condensate wet gas zone. The mineralogical and equivalent maturation data presented herein provide a valuable foundation for future studies, highlighting the complexity and heterogeneity of the Coqueiros Formation and its potential significance as a source rock within the Campos Basin petroleum system.

Coupled Effects of Paleofluid Evolution on Ultra-deep Microbialite Reservoir Modification: A case study of the upper Ediacaran Deng-2 Member within the Penglai area of central Sichuan Basin, SW China

Paleofluid-rock interaction results in the modification of the ultra-deep reservoir quality, but the coupling effects of the paleofluid evolution on reservoir quality modification are still underestimated. Here, the multi-stage dolomite cements have been studied by means of core observations, thin section identification, fluid inclusions, and in situ elements and isotopic analyses in order to reaveal the paleofluid evolution processes and their effects on the reservoir quality. The fibrous dolomite cement (FDC) and bladed dolomite cement (BDC) both have similar geochemical properties and homogenization temperature (Th) to the matrix dolostone. Subsequent early silicification occurs (QZ1). However, silt- to fine-sized crystalline dolomite cement (CD2) has higher concentration of Fe and medium rare earth element, more negative δ18O than for FDC and BDC, and 103.4 to 150.2 °C in Th. The first petroleum charge episode solid bitumen 1 (SB1) followed the CD2. The medium to coarse sized crystalline dolomite cement (CD3) is characterized by the higher Mn and lower Sr concentration than CD2, positive δEu anomaly, negative δ13C and δ18O, high 87Sr/86Sr ratio and 138.9 °C to 184.3 °C in Th. The solid bitumen 2 (SB2) occurs after CD3. Subsequently, the saddle dolomite (SD) has higher Fe and Mn concentration and lower Na and Sr than others cement, δEu positive anomaly, negative δ13C and δ18O, high 87Sr/86Sr ratio and 177.5 °C to 243.6 °C in Th. Eventually massive silicification (QZ2) took place. The FDC, BDC, QZ1 and SB1 formed in the early diagenetic stage and have little negative effect on the reservoir quality. But the CD2 and CD3 dramatically decreased the reservoir quality related to a quantity of hydrothermal fluids entrance in the mesodiagenetic stage along the strike-slip faults and the unconformity surface at top of the Deng-2 Member during the late stage of the Caledonian orogeny. The reservoir spaces were retained and enlarged during the late diagenetic stage when the peak petroleum charge occurred, massive silicification and thermochemical sulphate reduction took place with SB2, SD and QZ2 being formed. The research outcome may update our understanding on the retention mechanism and the insights for further hydrocarbon exploration of Precambrian ultra-deep microbial reservoirs.

Diagenetic and hydrothermal events revealed by an Ediacaran dolomite breccia from the Araras-Alto Paraguai basin, southern Amazon Craton

The establishment of extensive carbonate platforms characterized the Ediacaran period during CaCO3 oversaturation events and the exceptional creation of accommodation space associated with the aftermath of the Marinoan glaciation (∼635 Ma). Carbonate deposits in the Araras Group record the early Ediacaran period, stage 1, in the Amazon Craton. The Serra do Quilombo Formation is an intermediate unit characterized by heavily fractured and faulted dolomites, as well as the presence of dolomite-cemented breccias (CB) overlying thick limestone packages associated with diagenetic and hydrothermal modifications. These features are commonly attributed to structurally controlled hydrothermal dolomitization (HTD) and are of significant economic importance as hydrocarbon reservoirs and Mississippi Valley-type (MVT) lead-zinc mineralization hosts. This study aims to unravel the origin of cemented breccias and the diagenetic/burial processes within the unit, focusing on dolomitization processes. Dolomite samples were analyzed using petrographic, scanning electron microscopy, microprobe, micro-Raman, cathodoluminescence, and isotopic analyses (δ13C, δ18O, 87Sr/86Sr) to unravel their burial history. CBs are sub-vertical to sub-horizontal bodies with complex geometries, generally cutting bedding at high angles, indicating hydrofracturing processes related to vertical flows of hydrothermal fluids (hydraulic breccia) and present the typical cockade texture of expansion breccias in dilatational faults. The substitutive matrix RD1 is the main constituent of the Serra Quilombo Formation, its low correlation between δ13C and δ18O (R2 = 0.009), the well-preserved fabric, and the similarity with the isotopic values (C and Sr) documented for Ediacaran carbonates, suggest that the syndepositional dolomitization. The first generation of dolomite cement (DC1) and the last phase of dolomitic cementation (saddle dolomite - SD) occur, filling pores, CBs, and fractures. The cockade texture of the breccias evidences a low precipitation rate or a pause in precipitation between DC1 and SD. Concurrently, DC1 has isotopic signals of δ18O = −4.34 ± 1.32‰ (n = 18) and 87Sr/86Sr = 0.708831 (n = 2), while SD has values of δ18O = −9.57 ± 2.51‰ (n = 15) and 87Sr/86Sr = 0.711464 (n = 3). The large isotopic fractionation between DC1 and SD suggests different dolomitizing fluids. This relationship shows an increase in 87Sr in the fluid as the temperature increases; moreover, the enrichment in 87Sr of the fluid is explained by the interaction of this fluid with rocks from the crystalline basement. Thus, the main conduit for the ascent of this radiogenic fluid would be faults with deep roots spatially close to tectonically active zones. Lastly, the presence of tectonic stylolites cutting cemented breccias and rotated zebra-like strata-bound structures suggests that brecciation occurred before the installation of fragile post-Ordovician transtensional structures, preceding the establishment of Paleozoic Basins on the South American Platform.

Source and U-Pb Chronology of Diagenetic Fluids in the Permian Maokou Formation Dolomite Reservoir, Eastern Sichuan Basin, China

The hydrothermal dolomitization, facilitated by basement fault activities, had an important impact on the Permian Maokou Formation dolomite in the Sichuan Basin, which experienced complex diagenesis and presented strong reservoir heterogeneity. The source and age of diagenetic fluids in this succession remain controversial. In this study, various analyses were implemented on samples collected from outcrops and wells near the No. 15 fault in the eastern Sichuan Basin to reconstruct the multi-stage fluid activity and analyze the impact on reservoir development, including petrology, micro-domain isotopes, rare earth elements, homogenization temperature of fluid inclusions, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb dating. The homogenization temperature of primary brine inclusions in fine-grained matrix dolomite and saddle dolomite is concentrated between 100 and 150 °C, which indicates that the impacts of abnormally high temperatures of other geological bodies. The δ13C and δ18O value and low 87Sr/86Sr value indicate that the diagenetic fluid of fine-grained matrix dolomite is mainly Permian seawater. The U-Pb ages of fine-grained matrix dolomite are ~260 Ma, which coincides with the age of the main magmatism of Emeishan Large Igneous Province (ELIP), and hydrothermal fluid provided a favorable high-temperature environment in the penecontemporaneous stage. While highly radiogenic 87Sr/86Sr compositions suggests those of saddle dolomite, the high-temperature Sr-rich formation fluid. The U-Pb ages of saddle dolomite are 245–250 Ma, which coincides with the age of the 255~251 Ma magmatism of ELIP. This indicates that those should be the diagenetic products of the ELIP hydrothermal fluid in the shallow burial stage. The U-Pb age of coarse-grained calcite is 190–220 Ma, and it should be the diagenetic product of the deep burial stage. Brine inclusions associated with primary methane inclusions were developed in coarse-grained calcite, with a homogenization temperature range of 140.8–199.8 °C, which indicates that the formation fluid activities were related to hydrocarbon charging. The Permian Maokou Formation dolomite was firstly formed in the penecontemporaneous shallow burial stage, and then it was subjected to further hydrothermal dolomitization due to the basement faulting and the abnormally high heat flow during the active period of ELIP. Hydrothermal dolomitization contributed to the formation and maintenance of intercrystalline and dissolution pores, whereas it also formed saddle dolomite to fill the pores, and reduce the pore space. The influence of deep fluid activities on reservoir evolution is further distinguished.

Origin of facies-controlled dolomite and exploration significance of the Middle Permian Qixia Formation in central Sichuan Basin, Western China

The intraplatform shoal dolomite of the Middle Permian Qixia Formation is currently considered the key target of hydrocarbon exploration in the central Sichuan Basin. To systematically investigate the origin of the stratabound facies-controlled porous dolomites of the Qixia Formation, integrated petrography, logging and seismic analysis were carried out in this work. The results are as following: (1) the dolomite reservoir is universal in the central Sichuan Basin, and its distribution is controlled by intraplatform shoals, with multilayer superposition vertically. Thick massive dolostone may also develop along with the fault. (2) Three replaced dolomites and one dolomite cement were identified: very finely to finely crystalline, anhedral to subhedral dolomite (Rd1); finely to medium crystalline, anhedral to subhedral dolomite (Rd2); coarsely crystalline, subhedral to euhedral dolomite (Rd3) and coarsely crystalline saddle dolomite cement (Sd). Rd2 and Rd3 are partly fabric-retentive, and preserve the original bioclastic ghosts. Sd shows wavy extinction, filled in the breccia veins. (3) The U-Pb dating and homogenization temperatures results indicate that the dolomite and Sd cement are associated with hydrothermal event during the Emeishan large igneous province. The δ13C, 87Sr/86Sr, and seawater-like REEY patterns suggest that the dolomitization and Sd precipitation fluids originate from connate seawater heated by elevated heat-flow. (4) The ELIP triggered large scale thermal anomalies in the basin during the Dongwu movement period. The increased temperature and pressure drove the formation water in the intra-platform shoal facies and overcame the binding effect of Mg2+ hydrate. Moreover, the deep hydrothermal fluid preferentially penetrated into the porous strata of shoal facies along the faults and fractures, mixed with formation water to some extent, and extensive dolomitization occurred. The facies-controlled dolomite reservoir and the underlying Cambrian source rock form a good source-reservoir assemblage, which can be a key replacement option.

U-Pb dating of Barra Velha carbonates reveals the influence of Santos Basin sedimentary and tectonothermal history on pre-salt carbonate ages

The depositional age and diagenetic history of the Upper Barra Velha carbonates are poorly understood. To clarify the geochronology of these events and the behavior of the U-Pb isotopic system in carbonates, we conducted an integrated petrographic, QEMSCAN imaging, and in situ U-Pb dating study of calcites and dolomites from the Barra Velha Formation, in the central Santos Basin (SE Brazil). Our results span three geochronological intervals, which are ∼125 Ma, ∼116-114 Ma, and ∼105-100 Ma. The U-Pb isotopic system of calcite allochems was disturbed, yielding ages of ∼115 Ma and ∼105-100 Ma. These ages coincide with magmatic and hydrothermal events in the pre-salt. Dolomites showed that geochronological disturbance of carbonate components may not be pervasive but rather related to fluid-rock interactions and properties of rocks and their constituents such as mineralogy, porosity and permeability. This is evidenced by the U-Pb system of non-porous, non-permeable, and less reactive dolomitic aggregates remaining undisturbed. Eodiagenetic microcrystalline and fine dolomite rhombs provided evidence of Barremian ages, challenging the belief that the Upper Barra Velha deposits are Aptian. Other eodiagenetic dolomites occur as ∼117 Ma eodiagenetic lamellar dolomites, as pseudomorphic dolomites that underwent U-Pb alteration at ∼100 Ma, and as ∼112 Ma pervasive rhombohedral dolomites associated with Mg-rich fluids percolation in the Upper Barra Velha carbonates. Blocky dolomite and saddle dolomite cements have ages of ∼100 Ma, with saddle dolomite constraining the latest silicification event as > 100 Ma. This research elucidates significant geological events, processes, and factors (including magmatic activity, hydrothermal alteration, fluid-rock interactions, and petrographic and mineralogical controls) that influenced the ages of the Upper Barra Velha carbonates. Moreover, it aids in reconstructing the depositional and diagenetic history of the Barra Velha Formation and enhances the understanding of the Santos Basin tectonothermal evolution.

Reservoir heterogeneities due to diagenesis in Jurassic Samana Suk Formation Kahi section Nizampur Basin North West Himalayas Pakistan

The porosity and permeability determine the reservoir quality of sedimentary rocks, which is fundamentally influenced by both depositional and diagenetic processes. The Jurassic carbonates are targeted in many regions for oil and gas exploration. The current study is carried out to elaborate on the diagenetic alterations and their effect on reservoir properties. A thick outcrop of the Jurassic Samana Suk Formation is studied in the Kahi section of Nizampur Basin Northwest Himalayas, Pakistan, to study and relate the trend of lithological variations and depositional settings. Field investigation revealed that the Samana Suk Formation is extensively distributed in the area and primarily made up of interbedded limestone and dolomite units. The original unaltered limestone has a thick-bedded and oolitic to bioclastic nature. Different types of dolomites have been recognized based on colour contrast and sedimentary features. Moreover, saddle dolomite cement, calcite cementation, and mechanical and chemical compaction have also been observed. The petrographic studies show different types of diagenetic alterations that affected the Samana Suk Formation, including micritization, bioturbation, mechanical and chemical compaction in the form of fractures and stylolites, various calcite cementation, which includes the various cement types that range from isopachous, blocky, granular equant, fibrous, and dog tooth cementation, along with dissolution that occurred in different diagenetic realms. Pyritization was rarely observed. Moreover, different phases of dolomites were identified, ranging in size and shape, i.e., finely crystalline to coarse crystalline and planar euhedral to non-planar anhedral. The stable oxygen isotope values of these dolomites show depletion from original marine signatures and suggest burial-related fault-controlled dolomitization events. Overall, diagenetic processes like dissolution, fracturing and dolomitization increased the reservoir potential. On the contrary, the overburden and cement precipitation result in a decrease in the reservoir properties.

Effects of basin tectonic evolution on multi-phase dolomitization: Insights from the Middle Permian Qixia Formation of the NW Sichuan Basin, SW China

The platform-marginal shoal carbonates of the Middle Permian Qixia Formation in the NW Sichuan Basin, as important ultra-deep (>5000 m) hydrocarbon reservoirs, experienced multi-phase dolomitization and recrystallization during the long-term and complex basin tectonic evolution; however, there is no consensus on the dolomitization mechanism. Four types of dolomites have been identified: medium- and coarsely-crystalline dolomites within patchy dolomitized limestone (LD) at the top of the carbonate strata, massive medium to coarsely-crystalline dolomites categorized as porous dolomites (MD1) and tight dolomites (MD2) at the bottom of the carbonate strata, and cement coarsely saddle dolomites (SD) in the vugs and fractures. All dolomites exhibit similar REESN patterns to the host limestone with an obviously positively Ce anomaly, and show Sr isotopes falling in the range of Permian seawater. However, the LD dolomites display a higher Sr but lower Mn concentration, and more positive δ13C and δ18O than other types of dolomites. The petrographic and geochemical results suggest the dolomites of the Qixia Formation replaced precursor grainstone by the seawater-derived fluids. A comprehensive dolomitization model is proposed, incorporating seawater-reflux, intermediate-burial and tectonic-squeegee, and hydrothermal dolomitization for forming different types of dolomites. LD dolomites are early diagenetic products after selective dissolution under a near-surface or shallow burial environment, in which the weak-evaporated Permian seawater mixed with minor meteoric water accounting for dolomitizing fluids. MD1 and MD2 dolomites are likely to form due to the residually buried seawater for widely dolomitization driven by the Late Triassic tectonic compression. Subsequently, the dolomitic strata underwent hydrothermal modification due to rejuvenation of the thrust belt to enable saddle dolomites filling pre-forming porosity. This study integrates outcrop, petrographic, geochemical and dating analysis with paleogeography and tectonic events, providing a new perspective to establish a conceptual model for multi-phase dolomitization as an analog in comparative tectonic settings worldwide.

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.

The interplay between dolomitizing fluids, tectonically-controlled saddle dolomite and calcite cements in lower Cambrian to Furongian strata in the Tazhong Uplift

The interplay between dolomitizing fluids, tectonically-controlled saddle dolomite and calcite cements in Lower-Cambrian to Furongian strata in the Tazhong Uplift, Tarim Basin were interpreted from petrography, geochemistry and fluid inclusions microthermometry. Stochiometric replacement dolomites (RD1, RD2 and RD3) and saddle dolomite (DC) cement were identified. Equant-drusy calcite (ECAL) and blocky-to columnar calcite (LCAL) cements were identified. Weak relationships exist between δ13C, δ18O, 87Sr/86Sr, Mg/Ca versus Mn/Sr ratios, which are tools used to evaluate the impact of diagenetic environment on C–O–Sr isotope composition of the carbonates. The replacement dolomites and DC with high Mg/Ca ratios (0.97–1.21) have low depleted δ18O, δ13C values, compared to the ECAL and LCAL with more depleted δ18O, δ13C values, indicating that the C–O–Sr isotope composition suffered diagenetic alteration by higher-temperature and equilibrium isotopic fractionation. The carbonates fluid system is characterized by negative shift in C–O-isotopic composition with hypersaline signatures compared to Early Paleozoic evaporative seawater. Moreover, the increasing trend in (Cl, Br, I)/(CaO + MgO) ratios with Mg/Ca ratios, couple with positive relationship between Cl and Br, suggests fluids contamination by halogen-rich inclusions. The RD1, RD2 and RD3 were precipitated by relatively higher-temperature saline fluids at depth during shallow-deep burial dolomitization. The DC have average Th-values (141.5–199.2 °C) mostly higher than the estimated ambient temperatures (165 °C) of the studied strata, meaning their growth in fractures resulted from higher-temperature Mg-enriched saline basinal fluids conveyed by hydrothermal convection and short-lived geothermal squeegee-type flow. The LCAL were precipitated by Ca/Mg-enriched basinal brine fluids diluted by influx of meteoric water, cementing fractures in host rocks, as dolomite was replaced by calcite (i.e., calcitization). The conceptual model reveals that dolomitizing fluids and calcitization fluids were conveyed through permeable horizons resulting from strong extensional tectonics and weak compressional tectonics, forming irregular networks of saddle dolomite and calcite cement in host rocks.

Early diagenetic evolution based on petrography and stable isotope analysis in the Barra Velha Formation of the Brazilian Pre‐salt

AbstractMineralogy and texture of diagenetic phases in the Aptian Pre‐Salt Barra Velha Formation are described, quantified and compared by facies and structural setting to understand their spatial and temporal distribution, and to develop predictive concepts for their genesis. This study examined data from eight wells from one oil‐field in the Santos Basin. Calcite is the most abundant mineral and occurs with fibro‐radial texture as spherulites and shrubs and sparse microcrystalline mudstone. The δ18O values from calcite spherulites and shrubs suggest water of similar composition and temperature, but they have different δ13C values. Mudstones show lower δ18O, suggesting warmer lake water and/or lower evaporation, whereas δ13C values indicate a variable, but commonly strong influence of biogenic CO2. Dolomite with rhombohedral habit was the first to precipitate, followed by lamellar, saddle and anhedral varieties. Rhombohedral dolomites show a positive δ13C‐δ18O correlation and a similar range in values to spherulites and shrubs, suggesting similar lake water. The lamellar dolomite is related to biofilms, whereas anhedral dolomite is attributed to mixing of pore and meteoric waters. Lamellar and anhedral dolomites have similar isotopic values, precipitating after rhombohedral dolomite in slightly warmer and/or less evaporatively concentrated pore water. Saddle dolomite is related to hydrothermal fluids that percolated the formation during early diagenesis. Silica occurs as replacement of primary calcite and Mg‐clay, but also as a cement and more rarely as a depositional chert. Both dolomite and silica display a complex range of petrographic textures, many of which are facies dependent. This study focusses on the most important phases of the paragenetic sequence that took place pre‐burial and are (1) formation of Mg‐clay, calcite spherulites and shrubs, (2) partial dolomitisation of shrubs and spherulites and precipitation of rhombohedral and lamellar dolomites, (3) precipitation of saddle dolomite, matrix and carbonate dissolution and (4) anhedral dolomite and all textures of precipitated or substituted silica.

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.

Disconformity-controlled hydrothermal dolomitization and cementation during basin evolution: Upper Triassic carbonates, UAE

Petrography, fluid-inclusion microthermometry, stable isotope analyses, and radiometric (206Pb/238U) dating of Upper Triassic dolostones, saddle dolomite, and quartz and calcite cements were used to constrain the timing and conditions of dolomitization and cementation in the context of the tectonic evolution of a basin in the northern United Arab Emirates. Dolomitization (ca. 152.4 Ma) and precipitation of saddle dolomite (ca. 146.8 Ma), calcite (ca. 144.6 Ma), and quartz cements are attributed to focused synrifting flow of hot basinal brines into grain-supported limestones in which permeability was enhanced by incursion of meteoric waters beneath a disconformity surface. Another calcite cement generation (ca. 99.7 Ma) was formed by flow of hot brines during tectonic compression related to the obduction of Oman ophiolites in the Late Cretaceous. Thus, this paper provides new insights into (1) stratigraphic controls on and timing of hydrothermal (hot basinal brines) dolomitization, (2) the origin of closely associated intraformational limestones and dolostones, and (3) linkages between diagenesis and thermochemical modifications of basinal brines during tectonic evolution of sedimentary basins.

Multistage dolomitization influenced by the Emeishan large igneous province: Petrographical and geochemical evidence

The formation of large igneous provinces (LIPs) seems to require a series of conditions conducive to massive dolomitization; however, the specific mechanisms remain unclear. Based on petrographic, geochemical and fluid-inclusion microthermometry analyses, this study identifies the types of dolomites in the Maokou Formation (Middle Permian) in the eastern Sichuan Basin. It analyses the related dolomitization fluids and discusses the influencing mechanisms of the Emeishan large igneous provinces (ELIP) on dolomitization at different stages. Three types of dolomite were identified: (1) fabric-retentive, very finely crystalline, planar-s to nonplanar-a dolomite (MD1); (2) fabric-retentive, fine to medium crystalline, planar-s to nonplanar-a dolomite (MD2); and (3) medium to coarsely crystalline, nonplanar (saddle) dolomite (SD). MD1 exhibited dark-red luminescence under cathodoluminescence and rare earth element patterns that are similar to those of lime mudstone, indicating that its dolomitization fluid was medium-salinity Permian seawater. MD2 displayed orange-red luminescence under cathodoluminescence, with a high Ba content (mean: 397.5 ppm) and positive Eu anomalies (mean: 2.34), indicating that its dolomitization fluid was a mixture of shallowly buried seawater and a small amount of external hydrothermal fluid. SD showed red luminescence under cathodoluminescence, with the highest Ba content (mean: 9820.4 ppm) and significant positive Eu anomalies (mean: 3.165), indicating that its dolomitization fluid was mainly ELIP-related hydrothermal fluid with a small amount of residual seawater. Before the ELIP activity, no dolomite occurred in the study area. In the initial stage of ELIP activity, upwelling of the Emeishan mantle plume led to a shift in sedimentary patterns, promoting the occurrence of reflux dolomitization and the formation of MD1. In the intermediate stage of ELIP activity, the activation of basement faults promoted the occurrence of thermal convection dolomitization, resulting in MD2 formation. In the peak stage of ELIP activity, large-scale eruption of Emeishan basalt led to the development of hydrothermal dolomitization, forming SD. This study provides an approach to the investigation of the influences of LIPs on dolomitization processes.

Multiple Paleokarst Events in the Cambrian Potosi Dolomite, Illinois Basin

The Cambrian (Furongian) Potosi Dolomite (100-183 m) in Illinois is part of the Cambro-Ordovician Knox Group. It is a uniformly dolomitized unit with very low intercrystalline porosity but contains very permeable vug, fracture/cavern porosity intervals. Here, we interpret the characteristics of the widespread porous zones in the Potosi as paleokarst features formed by rising hypogenic basinal/hydrothermal fluids. The conformity bounded Potosi Dolomite is characterized by massive dolomitization, overdolomitization and occlusion of previously generated intercrystalline porosity, void filling mineralization, and extensive dissolution and formation of cavity-conduit systems. The pore spaces are typically lined with drusy quartz or are characterized by partial to complete infilling with chalcedonic silica and/or dolomite cements. Clay minerals may partially fill pore spaces; physical properties and thorium-potassium crossplot suggest chlorite as the main clay mineral present. Dolomite crystals typically are planar-s or nonplanar with open-space filling, inclusion rich saddle dolomite displaying curved and zigzag crystal faces. Void filling cement does not exhibit sign of pressure solution and in places vug porosity is developed along bedding parallel stylolite indicating post burial origin of these features. Cavern reservoirs in the Potosi are laterally extensive and often stacked with intervening very low porosity dolomite; very low bulk density, excursion of caliper log signature from the baseline, and loss of fluid circulation during drilling in these intervals signify anomalously high porosity and permeability interpreted as being the result of cavern forming multiple paleokarst events. Post burial origin of cavities and void filling cements, association of saddle dolomite and chlorite, and occurrence of Mississippi Valley-type (MVT) ore deposits in Missouri suggest karstification by hypogenic warm basinal/hydrothermal fluids. Dissolution and mineralization likely occurred by flow of deep basinal formation waters and hydrothermal fluids (sourced from the crystalline basement underlying the Reelfoot Rift and the Illinois Basin) along numerous basement-rooted normal, reverse, and strike-slip faults, and the associated fold and fractures. Expansion and contraction because of fault-related seismicity likely developed fracture porosity in brittle host dolomite and possibly ruptured any underling impermeable units to enable large-scale upward and outward fluid movement. The Potosi fracture/cavern porosity intervals are confined by thick very low porosity dolomite intervals that could serve as effective seal. There is no report of any show of oil in the Potosi Dolomite, but the unit has an excellent potential to serve as a combined reservoir and seal for storing anthropogenic CO2 and waste material.

Interactions between Clays and Carbonates in the Aptian Pre-Salt Reservoirs of Santos Basin, Eastern Brazilian Margin

The giant Pre-salt reservoirs represent most of the oil production in Brazil. The main Aptian sag reservoirs were deposited in a unique and highly complex hyper-alkaline lacustrine setting. These deposits are essentially constituted by fascicular and spherulitic calcite precipitated in a magnesian clay matrix (stevensite, kerolite, and saponite/hectorite). Although vital for understanding the origin and main reservoir quality control, the genesis and interactions of clays and carbonates are still poorly constrained. The detailed petrographic description was focused on 812 thin sections from five wells drilled in the Santos Basin Aptian Barra Velha Formation, combined with cathodoluminescence, UV epifluorescence, and X-ray diffraction analyses. The main syngenetic processes were the deposition of finely laminated peloidal and ooidal Mg-clays, the formation of fascicular calcite crusts on the sediment–water interface, and the redeposition of these materials as intraclasts. Abundant clay peloids engulfed in syngenetic shrubs indicate that calcite and clay precipitation was concomitant, though with highly variable rates. Eodiagenetic phases include matrix-replacive and -displacive spherulites and fascicular shrubs; matrix-replacive blocky calcite and dolomite; lamellar carbonates filling matrix shrinkage pores; and microcrystalline calcite, dolomite, and silica replacing the Mg-clay matrix. The preferential dolomitization and calcitization of peloidal layers were most likely due to their higher permeability and larger specific surface. Matrix-replacive saddle dolomite, macrocrystalline calcite, and dawsonite are interpreted as mesodiagenetic or hydrothermal phases after significant matrix dissolution. Unraveling the processes of the formation and alteration of the carbonates and clays and their interactions in the Pre-salt deposits is essential for constraining the depositional and diagenetic conditions in their unique environments and their diagenetic overprinting and for decreasing the exploration risks and increasing the production of those extraordinary reservoirs.

Characteristics and main controlling factors of intra-platform shoal thin-layer dolomite reservoirs: A case study of Middle Permian Qixia Formation in Gaoshiti–Moxi area of Sichuan Basin, SW China

Based on the study of the distribution of intra-platform shoals and the characteristics of dolomite reservoirs in the Middle Permian Qixia Formation in the Gaoshiti–Moxi area of the Sichuan Basin, SW China, the controlling factors of reservoir development were analyzed, and the formation model of “intra-platform shoal thin-layer dolomite reservoir” was established. The Qixia Formation is a regressive cycle from bottom to top, in which the first member (Qi1 Member) develops low-energy open sea microfacies, and the second member (Qi2 Member) evolves into intra-platform shoal and inter-shoal sea with decreases in sea level. The intra-platform shoal is mainly distributed near the top of two secondary shallowing cycles of the Qi2 Member. The most important reservoir rock of the Qixia Formation is thin-layer fractured-vuggy dolomite, followed by vuggy dolomite. The semi-filled saddle dolomite is common in fracture-vug, and intercrystalline pores and residual dissolution pores combined with fractures to form the effective pore-fracture network. Based on the coupling analysis of sedimentary and diagenesis characteristics, the reservoir formation model of “pre-depositional micro-paleogeomorphology controlling shoal, sedimentary shoal controlling dolomite, penecontemporaneous dolomite benefiting preservation of pores, and late hydrothermal action effectively improving reservoir quality” was systematically established. The “first-order high zone” micro-paleogeomorphology before the deposition of the Qixia Formation controlled the development of large area of intra-platform shoals in Gaoshiti area during the deposition of the Qi2 Member. Shoal facies is the basic condition of early dolomitization, and the distribution range of intra-platform shoal and dolomite reservoir is highly consistent. The grain limestone of shoal facies is transformed by two stages of dolomitization. The penecontemporaneous dolomitization is conducive to the preservation of primary pores and secondary dissolved pores. The burial hydrothermal fluid enters the early dolomite body along the fractures associated with the Emeishan basalt event, makes it recrystallized into medium–coarse crystal dolomite. With the intercrystalline pores and the residual vugs after the hydrothermal dissolution along the fractures, the high-quality intra-platform shoal-type thin-layer dolomite reservoirs are formed. The establishment of this reservoir formation model can provide important theoretical support for the sustainable development of Permian gas reservoirs in the Sichuan Basin.

Episodic hydrothermal alteration on Middle Permian carbonate reservoirs and its geological significance in southwestern Sichuan Basin, SW China

To analyze the episodic alteration of Middle Permian carbonate reservoirs by complex hydrothermal fluid in southwestern Sichuan Basin, petrology, geochemistry, fluid inclusion and U-Pb dating researches are conducted. The fractures and vugs of Middle Permian Qixia–Maokou formations are filled with multi-stage medium−coarse saddle dolomites and associated hydrothermal minerals, which indicates that the early limestone/dolomite episodic alteration was caused by the large-scale, high-temperature, deep magnesium-rich brine along flowing channels such as basement faults or associated fractures under the tectonic compression and napping during the Indosinian. The time of magnesium-rich hydrothermal activity was from the Middle Triassic to the Late Triassic. The siliceous and calcite fillings were triggered by hydrothermal alteration in the Middle and Late Yanshanian Movement and Himalayan Movement. Hydrothermal dolomitization is controlled by fault, hydrothermal property, flowing channel and surrounding rock lithology, which occur as equilibrium effect of porosity and permeability. The thick massive grainstone/dolomites were mainly altered by modification such as hydrothermal dolomitization/recrystallization, brecciation and fracture-vugs filling. Early thin–medium packstones were mainly altered by dissolution and infilling of fracturing, bedding dolomitization, dissolution and associated mineral fillings. The dissolved vugs and fractures are the main reservoir space under hydrothermal conditions, and the connection of dissolved vugs and network fractures is favorable for forming high-quality dolomite reservoir. Hydrothermal dolomite reservoirs are developed within a range of 1 km near faults, with a thickness of 30–60 m. Hydrothermal dolomite reservoirs with local connected pore/vugs and fractures have exploration potential.

The Effect of the Emeishan Mantle Plume on the Genetic Mechanism of the Maokou Formation Reservoir in the Central Sichuan Region

Thin layers and high-yield dolomite reservoirs were recently discovered in the Permian Guadeloupian Maokou Formation. The genetic mechanism of this reservoir is controversial because of its complex sedimentation and diagenesis in the Maokou Formation. Traditionally, the genesis has focused on sedimentation, karst, and fracture, whereas the influence of the Emeishan mantle plume activity (EMP) has been ignored. In this study, we enumerated petrographic (grouped into micritic bioclastic limestone, limy dolomite, grain dolomite, dolomite cement, calcite cement, and saddle dolomite) and geochemical data (δ13C, δ18O, REE, and 87Sr/86Sr) from a microscopic perspective to support the impact of EMP on reservoirs. We conclude that EMP activity altered the sedimentary environment and induced a complex diagenesis. The paleogeomorphic reconstruction data indicate that the EMP caused an uplift zone in the NE–SW direction, depositing advantageous high-energy beach facies. In terms of diagenesis, the abnormally high 87Sr/86Sr ratios and REE with positive Eu anomalies suggest that dolomitization was influenced by both seawater and hydrothermal fluids. Based on the above evidence, we established a reservoir genetic model for the Maokou Formation related to the intensity of the EMP. This study provides a new perspective on the mantle plume activity for the study of carbonate reservoir genesis.

Temporal constraints on hydrothermal circulation associated with strike-slip faulting in the Permian Maokou carbonates, central Sichuan Basin (SW China)

Despite the ubiquity of hydrothermal fluid circulation in the crust, its timing is difficult to precisely determine using traditional thermometric dating due to strongly disturbed geothermal fields. We present a case study from the mid-Permian hydrocarbon-bearing dolostone reservoirs, situated proximally to strike-slip faults in the central Sichuan Basin (China), to highlight the utility of carbonate in-situ U-Pb geochronology in delineating the timing of hydrothermal fluid flow. Matrix and void-filling cement dolomite phases were identified in the porous Maokou Formation carbonates. Predominantly manifesting as saddle dolomites along fractures, these cements yielded statistically homogeneous U-Pb ages (242.5 ± 3.1 Ma, 238.9 ± 8.2 Ma, 244 ± 15 Ma, and 239.2 ± 6.5 Ma), slightly postdating the established stratigraphic timeline (ca. 273 to 259 Ma). The narrow temporal span suggests early emplacement following a short burial, compatible with the petrographic observation of burial stylolites crosscutting saddle dolomites. Integrating shallow emplacement depths inferred from a comparison between the dolomite U-Pb ages and a published burial history, with fluid-inclusion microthermometry, elucidates the hydrothermal system that was once present in this field. The pervasive occurrence of hydrofracturing, along with dolomite geochemical indicators represented by bell-shaped (REE + Y) PAAS patterns and elevated Y/Ho ratios, provide additional evidence for the hydrothermal fluid injection and resulting dolomitization. The dolomites' positive Eu anomalies and the parent fluids’ 18O enrichment indicate a source from deeper basinal pore waters that have interacted significantly with ambient rocks. The overlying Triassic evaporites may have provided Mg-rich brines, mixing into the circulating hot fluids responsible for hydrothermal dolomitization. Our dolomite U-Pb ages coincide with the seismically estimated timing of transtensional strike-slip fault activation. This temporal correspondence, combined with spatial associations and lateral slickensides along fracture planes, suggests that the hydrothermal fluid movement is highly relevant to the active strike-slip faults. This observation reveals a previously unrecognized early-middle Triassic, structurally controlled hydrothermal activity in the region. We propose that this episode of hydrothermal circulation was driven by extensional tectonism in response to the spreading of the Proto-Tethys Ocean. This finding is not in line with the earlier research that considered late Permian Emeishan volcanism to have been the main trigger for hydrothermal fluid influx. The long-standing view regarding hydrothermal dolomitization in this region may need to be re-evaluated.