Saddle Dolomite Research Articles

Multiphase dolomitization and hydrothermal alteration of the Upper Cambrian-Lower Ordovician carbonates in the Gucheng uplift, Tarim Basin (NW China)

The thick Upper Cambrian-Lower Ordovician carbonates of the Gucheng uplift (southeastern Tarim Basin, northwestern China) have been extensively dolomitized, leading to the formation of massive dolomite reservoirs. The origins of different dolomites and their original fluids were studied based on an integrated analysis of petrology, geochemical data (C, O, and Sr isotopes) and fluid inclusion assemblages (FIAs). The D1 (very finely to finely crystalline planar-e/s dolomite) and the D2 (finely to medium crystalline planar-s/a dolomite) dolomites were formed from the mesosaline to penesaline seawater in the subsurface to shallow burial stage. The increasing temperatures and changes of dolomitizing fluid properties led to the formation of different textures and distributions. Both D1 and D2 dolomites, compared to the matrix limestone, show no obvious improvements in porosity, which indicates that early dolomitizations did not create or preserve the primary pores. CD1 (finely to medium crystalline nonplanar-e/s dolomite) and CD2 (medium to coarsely crystalline nonplanar saddle dolomite) dolomites are dolomite cements of hydrothermal alteration at the deep burial stage. Due to changes in temperature and Mg2+ concentrations of the hydrothermal fluids, the precipitation of CD1 and CD2 dolomites exhibited different textures and filling sequences. In the limited space of matrix dolomite, hydrothermally altered dolomite was dominated by D3 (medium to coarsely crystalline nonplanar-a dolomite). The late calcite fillings were precipitated on account of decreasing Mg2+ concentration at the end of the hydrothermal alteration. Hydrothermal fluids, depleted in Mg2+ and enriched in silica, could be deep-basinal brines driven by the strongly transpressional faults from the Late Caledonian to Early Hercynian. Compared to D1 and D2 dolomites, the higher porosities in hydrothermally altered dolomite indicate that hydrothermal fluids are constructive to the formation of reservoirs. Hydrothermal alteration products showed obvious differences between the dolomite and limestone, and led to different performance in physical characteristics. Hence, the hydrothermally altered reservoirs in the eastern Tarim Basin were mostly distributed in the dolomite but were less developed in the limestone. This study provides new insights into the evolution of dolomitizing fluids that are responsible for multiple dolomitization events and the formation of the hydrtothermal dolomite in the Tarim Basin and elsewhere around the world.

Porosity enhancement potential through dolomitization of carbonate reservoirs, a case of study from the Euphrates Graben fields, East Syria

The dolostones and dolomitic limestones of the lower Cretaceous Judea formation are a key target of hydrocarbons in most of the Euphrates Graben fields. Core materials investigation, thin sections petrographically examinations, and petrophysical evaluations were obtained to determine enhancement of the porosity through dolomitization. Results showed that the lagoon-shallow marine carbonates of the Judea formation are subdivided into two main zones; the upper “limestone zone” is micritic limestones dominated with no reservoir potential, and the lower “dolomite zone” is dolomitic limestones and dolostones dominated with good to very good reservoir potential. Dolomitization of the mudstones and wackestones of the micritic limestones resulted in formation of microcrystalline dolomicrite and early fabric destructive dolomites. Conversely, dolomitization of the packstones and grainstones resulted in formation of the fabric destructive and saddle dolomites. Based on petrography data, dolomitization of the “limestone zone” is interpreted by the seawater dolomitization mechanism at low-temperatures, while dolomitization of the “dolomite zone” is interpreted by the burial dolomitization mechanism under high temperature and pressure conditions. The “limestone zone” is characterized by the poorest reservoir quality, while the “dolomite zone” is characterized by the best reservoir quality. The seawater dolomitization did not significantly enhance the porosity, while the burial dolomitization contributes to enhancing the preserved secondary porosity. Stylolites microfractures and dissolution seams associated with dolomitization played as major factors in porosity enhancement of the dolostones and dolomitic limestones and serving as pathways for hydrocarbon migration. Dissolution processes increased the porosity and more permeability unless they are filled with the precipitated dolomite and/or calcite. Calcification had significant effects on the porosity by blocking the cavities and channels and decreased the effective pore volume.

Diagenetic Modifications and Reservoir Heterogeneity Associated with Magmatic Intrusions in the Devonian Khyber Limestone, Peshawar Basin, NW Pakistan

In the present study, an attempt has been made to establish the relationship between diagenetic alterations resulting from magmatic intrusions and their impact on the reservoir properties of the Devonian Khyber Limestone (NW Pakistan). Field observations, petrographic studies, mineralogical analyses, porosity-permeability data, and computed tomography were used to better understand the diagenetic history and petrophysical property evolution. Numerous dolerite intrusions are present in the studied carbonate successions, where the host limestone was altered to dolomite and marble, and fractures and faults developed due to the upwelling of the magmatic/hydrothermal fluids along pathways. Petrographic studies show an early phase of coarse crystalline saddle dolomite (Dol. I), which resulted from Mg-rich hydrothermal fluids originated from the dolerite dykes. Coarse crystalline marble formed due to contact metamorphism at the time of dolerite emplacement. The second phase of dolomitisation (Dol. II) postdates the igneous intrusions and was followed by dedolomitisation, dissolution, and cementation by meteoric calcite. Stable isotope studies likewise confirm two distinct dolomite phases. Dol. I exhibits more depleted δ18O (-15.8 to -9.1‰ V-PDB) and nondepleted δ13C (-2.05 to +1.85‰ V-PDB), whereas Dol. II shows a relatively narrow range of depleted δ18O (-13.9 to -13.8‰) signatures and nondepleted δ13C (+1.58 to +1.89‰ V-PDB). Dolomitic marble shows a marked depletion in δ18O and δ13C (-13.7 to -8.5‰ and -2.3 to 1.95‰, respectively). The initial phase of dolomitisation (Dol. I) did not alter porosity (5.4-6.6%) and permeability (0.0-0.1 mD) with respect to the unaltered limestone (5.6-6.9%; 0.1-0.2 mD). Contact metamorphism resulted in a decrease in porosity and permeability (3.3-4.7%; 0.1 mD). In contrast, an increase in porosity and permeability in Dol. II (7.7-10.5%; 0.8-2.5 mD) and dolomitic marble (6.6-14.7%; 8.2-13.3 mD) is linked to intercrystalline porosity and retainment of fracture porosity in dolomitic marble. Late-stage dissolution and dedolomitization also positively affected the reservoir properties of the studied successions. In conclusion, the aforementioned results reveal the impact of various diagenetic processes resulting from magmatic emplacement and their consequent reservoir heterogeneity.

Geological-petrophysical insights in the deep Cambrian dolostone reservoirs in Tarim Basin, China

Cores, thin sections, and cathodoluminescence (CL) analysis were integrated to document the occurrence and petrology of dolomites, and their pore systems in the Cambrian of Tarim Basin, China. Depositional facies, pore types and dolomitization processes of various dolostone reservoir types are determined. Six types of dolomite are recognized, including microbial dolomite, dolomicrite, fabric retentive dolomite, fabric-obliterative dolomite, fine to medium crystalline dolomite cement, and saddle dolomite cement. Pore systems are dominantly vugs, anhydrite dissolution pores, intercrystalline pores, intercrystalline dissolution pores, fabric dissolution pores and microfractures. Four porous dolostone reservoirs include sabkha dolostone, seepage-reflux dolostone, burial dolostone, hydrothermal dolostone. Fractures play an important role in enhancing reservoir quality in dolostone reservoirs. Conventional wireline logs and image logs are calibrated with cores and related thin sections. Sabkha dolostone reservoirs are characterized by dark and bright spots on the image logs. Seepage-reflux dolostone reservoirs are related to high-energy depositional facies and are characterized by low gamma-ray amplitude, increasing sonic transit time and neutron porosity but reducing bulk density values. Evident dark spots (vugs) are recognized on image logs, and all three porosity logs suggest relatively high reservoir quality in burial dolostone reservoirs. Hydrothermal dolostone reservoirs are recognized by high gamma-ray response caused by hydrothermal minerals (fluorite), and porosity curves indicate good reservoir quality, which is supported by dark spots (vugs) on the image logs. Rapid decrease in resistivity, increasing in sonic transit time values, and the dark sinusoidal waves on the image logs are typical of fractured dolostone reservoirs. The distribution of dolostone reservoirs in each well is predicted using a comprehensive analysis of conventional and image logs, and they are calibrated with oil test data. The research provides insights in the analysis of genetic model of deeply buried dolostone reservoirs and establishes the predictable model for reservoir quality in dolostones via well logs.

Diagenetic conditions and geodynamic setting of the middle Permian hydrothermal dolomites from southwest Sichuan Basin, SW China: Insights from in situ U–Pb carbonate geochronology and isotope geochemistry

The presence of hydrothermal dolomites in the middle Permian carbonates from the southwest Sichuan Basin has been suggested by petrographic features including zebra structure and extensive saddle dolomite cementation. Two major tectono-thermal events, i.e., the intensive volcanism related to the Emeishan large igneous province (ELIP) during the latest middle Permian and the tectonic evolution of the Longmenshan Orogenic Belt since the Late Triassic, might have contributed to the regional fluid flow for hydrothermal dolomitization. Here, we apply LA-ICP-MS U–Pb geochronology, petrography and isotopic geochemistry to constrain the diagenetic and geodynamic conditions of these hydrothermal dolomites. These dolomites resulted from injection of hot fluids up to 95 °C higher than the ambient temperatures. U–Pb ages (from 216.4 ± 7.7 Ma to 206 ± 15 Ma), combined with the highly radiogenic 87Sr/86Sr compositions (up to 0.712151), suggest that the previously proposed ELIP-related dolomitization model may not apply to this site. Instead, our new geochronological data suggest a syn-orogenic flow of basinal brines probably driven by thrust-related compression and/or tectonic valving. Moreover, the indistinguishable U–Pb ages and carbon, oxygen and strontium isotopic compositions among the replacement and cement carbonate phases in these hydrothermal dolomites further suggest that the paragenetic sequence of “replacement dolomite→saddle dolomite→blocky calcite” was originated from a common fluid in evolution. This study emphasizes the merits of LA-ICP-MS U–Pb dating in addressing the discrimination of, as well as diagenetic and geodynamic conditions for, hydrothermal dolomites.

Genesis of Upper Permian Changxing Formation dolomites, south of Kaijiang-Liangping Trough, SW China: evidence from petrology, geochemistry, and fluid inclusions

Dolomitization has a profound influence on the quality and distribution of the Changxing Formation reef-shoal gas fields in the south of Upper Permian Kaijiang-Liangping Trough. These gas fields consist of two types of matrix replacement dolomite (D1 and D2) and some saddle dolomite (SD). A comprehensive petrographic and geochemical examination indicates that D1 dolomite is formed by the replacement of matrix limestone by seepage-reflux of penesaline seawater during the syndiagenetic-early diagenetic stage. This interpretation is supported by δ18O values slightly higher than those of dolomites precipitated from coeval seawater and REE patterns similar to those of matrix limestones, reinforced by 87Sr/86Sr ratios that greatly overlap those of matrix limestones and coeval well-preserved fossils. Dolomitization by penesaline seawater rather than by hypersaline evaporated seawater is also supported by the absence of massive evaporites. D2 dolomite, with C-O-Sr isotopes and Fe-Mn concentrations comparable to those of D1, formed through the hydrothermal alteration of D1 at temperatures between 163.2 and 189.3 °C during Early Triassic. SD precipitated directly from hydrothermal fluids at temperatures between 152.4 and 197.8 °C. The hydrothermal origins of D2 and SD are also supported by more negative δ18O values than those of dolomite formed from coeval seawater, higher 87Sr/86Sr ratios, and positive Eu anomalies.

Geological and geochemical constrains on the genesis of the sedimentary-hosted Bou Arfa Mn(-Fe) deposit (Eastern High Atlas, Morocco)

Carbonate-hosted Mn deposits are widespread in North Africa with most of the Mn ores being hosted by Mesozoic dolomitic formations of the Moroccan Atlasic system. The Bou Arfa Mn(-Fe) deposit described herein is located in the eastern High Atlas, close to the North Atlasic Front, and has accounted for the production of ~2 Mt of ore grading from 33 to 82% Mn. The mineralization occurs within Sinemurian flat-lying dolostones as stratabound and karst-filling (veins, lenses, pockets). This mineralization is characterized by the extensive presence of pyrolusite, and minor concentrations of manganite, hausmannite, goethite and hematite. Previous studies suggested that the Bou Arfa Mn-bearing mineralization has a strictly synsedimentary origin, but the intense dolomitization and the occurrence of Mn oxides instead of Mn carbonates question this model. New petrographical, mineralogical and geochemical data show that three dolomitization phases accompanied the ore formation. The early synsedimentary dolomitization was not responsible for strong Mn enrichments, whereas a second dolomitization took place with the formation of hausmannite and manganite during early diagenesis. The precipitation mechanism involves circulation of dissolved Mn2+ in pore waters, and then precipitation under diagenetic suboxic and alkaline conditions. The scarcity of Mn carbonates is likely due to competitiveness of dissolved carbonates species that have favored dolomite precipitation in the early stage. Presence of saddle dolomite in a subsequent alteration stage, crystallography of the Fe oxides, and evolution of δ13C and δ18O reflect burial diagenesis and fluid mixing in the carbonate host rock. These conditions have enabled the transformation of the primary ore into pyrolusite, when diagenetically-derived hydrothermal fluids have generated vein-filling ore. The position of the Bou Arfa site above a paleohigh formed during Atlas rifting, the presence of a faulted zone and the dolomitization of the Sinemurian series have delimited the extension of this diagenetic Mn deposit, which is in some points similar to MVT deposits of North Africa. Weathering is poorly recorded in the Bou Arfa Mn ores due to the stability of the ore-forming minerals under subaerial conditions in relation to the most recent periods of exposure of the High Atlas building.

Middle Permian dolomites of the SW Sichuan Basin and the role of the Emeishan Large Igneous Province in their origin

Dolomitization of the Middle Permian carbonate platform in South China occurred in successive phases, partially influenced by the emplacement of the Emeishan Large Igneous Province. The main replacement dolomites are: (1) finely crystalline (˂50 μm) Md I, (2) mesocrystalline (50–250 μm), non-planar-a to non-planar-s Md II; and (3) coarse crystalline (250–500 μm), non-planar-s to non-planar-a Md III. Dolomite cement (Cd) is characterized by pores and/or fractures filled with saddle crystals within the Md III dolomites. The Md I dolomites share similar rare earth element patterns and carbon and strontium isotopes to host limestones and δ18O values that are in equilibrium with the limestone. These characteristics indicate that Md I dolomite was formed via the seepage/reflux dolomitization of evaporated seawater during the penecontemporaneous phase. The Md II dolomites share δ13C values and 87Sr/86Sr ratios similar to those of typical Permian carbonates, characterized by a range of low δ18O values. Analyses of fluid inclusions in these Md II dolomites showed homogenization temperatures of 90–130 °C and salinities of 4–9 wt% NaCl equivalent. These results suggest that Md II represents typical replacive dolomitization occurring within convective half-cells under geothermal conditions. They are particularly pervasive when the platform is heated by magmatic underplating and/or igneous emplacement to form thermal convection. The coarser Md III dolomites are characterized by high porosity and subsequently partially filled with saddle dolomite cement (Cd). These Md III and Cd dolomites are rich in radiogenic strontium, have low δ13C values, very low δ18O values, positive Eu anomalies, and contain fluid inclusions indicating high homogenization temperatures and salinities. These characteristics suggest that the Md III and Cd dolomites precipitated from high-temperature hydrothermal fluids, potentially associated with the emplacement of the Emeishan Large Igneous Province (LIP). Regional heating from the Emeishan LIP induced basin uplift and faulting. These were important in channeling the upwelling deep brine fluids into the overlying platform carbonates, leading to the localized intense dolomitization. This suite of Middle Permian dolomites is an excellent example of the successive phases of different dolomitization processes, including that associated with hydrothermal and thermal convection during the emplacement of a LIP.

Integration of stable isotopes, radiometric dating and microthermometry of saddle dolomite and host dolostones (Cretaceous carbonates, Kurdistan, Iraq): New insights into hydrothermal dolomitization

This paper integrates Mg-, C-, O-, Sr-isotopes, fluid-inclusions microthermometry, and radiometric dating of dolostones and vug-filling saddle dolomite and calcite of the Upper Cretaceous Bekhme Formation, the Kurdistan Region of Iraq, provide new insights into the origin, timing and geochemical evolution of hydrothermal dolomitizing fluids. Similar isotopic compositions and fluid-inclusion microthermometry of the dolostones and saddle dolomite suggest formation by similar fluids. The U–Pb ages of dolostones and saddle dolomite suggest that dolomitization (74.8 Ma) was accomplished in 300 Ka at near Earth's surface by flow of hydrothermal fluids followed by dolostone dissolution and precipitation of vug-filling saddle dolomite (73.8 Ma). Mg isotopic compositions of the dolomites (26MgDSM3=−0.81‰ to −0.76‰) are interpreted to indicate derivation of Mg from marine connate waters entrapped in overpressured basinal mudstones. Relatively high Sr-isotope ratios of the dolomites precludes the role of ophiolites as an important source of Mg in the basin.

Genesis of Dolomite in Middle Permian Maokou Formation in Eastern Sichuan: Constraints from In Situ Geochemistry, Sr-Mg Isotopes, and Fluid Inclusions

The dolostone reservoir of the Middle Permian Maokou Formation in Eastern Sichuan has good prospects for oil and gas exploration. Study of dolomitizing genesis of the Maokou Formation is essential for predicting the distribution of the dolostone reservoir. Petrography, in situ geochemistry, Sr-Mg isotopes, and fluid inclusions were carried out on samples from the Maokou Formation in Eastern Sichuan in order to discuss the dolomitizing process. Based on mineral and textural characteristics, dolomites were divided into four components: partially clouded dolomite (PCD), mosaic-like dolomite (MLD), cloudy-centered and clear-rimmed dolomite (CACD), and saddle dolomite (SDD). Results indicate that the Maokou Formation in Eastern Sichuan mainly experienced two stages of dolomitization. PCD, MLD, and cloudy-centered dolomite (CCD) were formed during the early dolomitization. They all show turbid crystal planes and bright orange-red CL and have similar trace element contents, 87Sr/86Sr ratios, and rare-earth patterns, indicating that they might be formed in the same fluid. This is a period when dolomitizing fluids mainly migrated along pores or microcracks and replaced protogenetic calcites, which occurred in the shallow burial stage of the Maokou Formation before the Late Permian. Clear-rimmed dolomite (CRD) and SDD were formed in the late stage of dolomitization. They all have clean crystal planes and darkly red CL. CRD of the ERY profile has trace element contents, 87Sr/86Sr ratios, and rare-earth patterns similar to SDD of the HLCH profile and Well TL6, inferring that both may be formed in the same fluid. Combined with high SrO contents and homogenous temperatures of fluid inclusions of CRD and SDD and Mg-isotopic compositions, they were generated by hydrothermal dolomitization. The hydrothermal fluid stage is related to the movement of the Emeishan Large Igneous Province, which was made up of basaltic magmatic fluids mixing with the surface water. The hydrothermal fluid mainly migrated upwards along structural fractures or faults and filled in structural fractures, occurring in the Late Permian to Middle-Late Triassic.

Hydrothermal dolomitization in the Middle Permian in the Central Sichuan Basin, SW China: evidence from petrology, geochemistry, and fluid inclusions

Limestone reservoirs formed by epigenetic karst and dolostone reservoirs formed by dolomitization are both developed in the Middle Permian of Central Sichuan Basin. In recent years, high-yield industrial gas flow has been obtained from these dolostone reservoirs, which makes the Middle Permian become the most realistic succeeding carbonate strata in the basin. Based on petrographic features, geochemical analyses (including C-O-Sr isotopes, trace elements, rare earth elements, and fluid-inclusions), seismic data, and previous studies, origin of dolomite in Middle Permian dolostone reservoirs in Central Sichuan Basin is studied. Three conditions of structurally controlled hydrothermal dolomitization (including two-stage strike-slip basement-rooted faults, underlying Cryogenian aquifers (or Pre-Sinian basement) and overlying Longtan Formation aquitard) are met. The Middle Permian dolostone reservoirs consist of saddle dolomites, fine-medium crystalline dolomites, and hydrothermal minerals, which are formed by hydrothermal dolomitization. The hydrothermal fluid replaces matrix limestone to form fine-medium crystalline dolomite, while saddle dolomite and other hydrothermal minerals are directly precipitated from hydrothermal fluid. Hydrothermal dolomitization fluid reforms matrix limestone in three ways, including dissolution and filling, replacement, and hydraulic fracturing, resulting to various types of reservoir spaces including dissolution vugs (or pores). Hydrothermal dolomitization of the Middle Permian in Central Sichuan Basin mainly occurs at the intersection of high-angle basement-rooted strike-slip faults, and the seismic section shows negative flower structure style.

Indication of hydrocarbon migration in the Western Mecsek Mountains evidenced by fluid inclusion chemostratigraphy

Primary and secondary hydrocarbon-bearing fluid inclusion (HCFI) assemblages occur in the Middle Triassic Lapis Limestone in the Szuadó Valley of the Western Mecsek Mts. The primary HCFIs were trapped in saddle dolomite crystals, and the secondary HCFIs were enclosed in calcite neospar and fracture-filling calcite. Solid bitumen is also present along fractures. The volatile compounds liberated from fluid inclusions are characterized by non-hydrocarbon and hydrocarbon species. The fluorescent properties of HCFIs, the occurrence of the solid bitumen, as well as the composition of inclusion oils indicate the migration of light oils through the Lapis Limestone. Petrographic observations suggest a prolonged oil charge event, which resulted in HCFIs beeing trapped under evolving diagenetic conditions.

Origin of Middle-Upper Cambrian dolomites in eastern Laurentia: A case study from Belle Isle strait, western Newfoundland

The Middle and Upper Cambrian Port au Port Group of western Newfoundland consists mainly of mixed carbonates and siliciclastics and is composed of the March Point, Petit Jardin, and Berry Head formations. Samples were collected from the core NF-02 that spans the uppermost Hawke Bay (40 m), the March Point (~83 m thick) and the lowermost Petit Jardin (~47 m). Petrographic examination of carbonates of the March Point and Petit Jardin carbonates reveals three generations of dolomite: (i) micritic to near-micritic (D1, 4 μm −35 μm) fabric-retentive dolomite, (ii) coarse crystalline sucrosic (D2, 60 μm−150 μm) fabric-destructive dolomite (sub-to euhedral zoned rhombs with clear rim and cloudy core), and (iii) coarse anhedral (D3, >0.5 mm) fracture-filling saddle dolomite. The microthermometric measurements of primary two-phased fluid inclusions retained in D2, and D3 samples yielded homogenization temperatures (Th) showing a general increase with burial. The mean values of Th and salinity are 89.1 ± 6.4 °C vs 21.2 ± 2.2 eq. wt% NaCl, and 110 ± 8.2 °C vs 21.1 ± 4.7 eq. wt% NaCl, respectively. The investigated dolomites show a decrease of Sr from D1 (102 ± 49 ppm) to D3 (47 ± 15 ppm) which reflects the influence of progressive burial. The mean δ18O values decrease from D1 (−6.3 ± −1.1‰ VPDB), to D3 (−9.7 ± 1.7‰ VPDB) where D2 (−7.6 ± 1.2‰ VPDB) is in the middle. The estimated δ18O values of the D1 parent fluid (−5 to −10‰ VSMOW) are bracketed by those suggested for the Middle Cambrian meteoric and seawaters. The fluid-inclusion gas analyses provided CO2/CH4 and N2/Ar ratios coherent with the conclusions from the petrographic examination and geochemical compositions.

Preservation of organic carbon in dolomitized Cambrian stromatolites and implications for microbial biosignatures in diagenetically replaced carbonate rock

Stromatolites have been a major focus in the search for ancient microbial life, however, the organic carbon biosignatures of dolomitized stromatolites have not yet been fully characterized or correlated with their dolomitizing conditions. Although dolomitization rarely preserves microbial morphology, the presence of organic carbon can provide valuable information for characterization of fossils' biogenicity, syngenicity, and indigeneity to their host rock. The Cambrian Allentown Formation in New Jersey, USA, is an excellent example of dolomitized stromatolites and thrombolites containing diagenetically modified microbial biosignatures. Based on XRD and EPMA data, the dolomite composition is typically stoichiometric, with varying degrees of cationic ordering. The outcrop underwent early dolomitization in a marginal-marine setting and later burial diagenesis resulting in multi-generational dolomite formation: (1) microspar dolomite formed by early diagenetic replacement at or near the surface, (2) zoned dolomite formed penecontemporaneously with the microspar phase as rhombohedral crystals by infilling primary pore spaces within the microspar matrix. The rhombic crystals continued to grow outward in alternating stages of Fe-enriched and -depleted fluids, which were preserved in zoned rims and revealed by cathodoluminescence, and (3) saddle dolomite formed during late stage deep burial with Fe- and Mn-rich fluids, and occurs as a void-filling, high-temperature phase. Organic carbon, characterized using confocal Raman microscopy, has an exclusive distribution within the microspar dolomite, and the D and G bands' characteristics reveal similar thermal alteration to the host rock, indicating that the mapped organic carbon is indigenous and syngenetic with the Cambrian carbonates. The findings presented in this study reveal organic matter found within microspar of various dolomitized facies deriving from different source pools of organic carbon. This study sheds light on biosignatures in secondary dolostones and may aid biosignature detection in older carbonate rocks on Earth and Mars.

Diagenetic evolution and associated dolomitization events in the middle Jurassic Samana Suk Formation, Lesser Himalayan Hill Ranges, NW Pakistan

The Jurassic carbonates of the Samana Suk Formation are extensively exposed in the foreland areas of Himalayas and form major reservoir of the upper Indus basin. These carbonates are composed of oolitic, pelitic fossiliferous and micritic limestone units which have been extensively modified by diagenetic alterations, particularly dolomitization. Field observations show two distinct types of dolostone geobodies (i) bedding parallel stratiform, and (ii) patchy dolostone units respectively. Bedding parallel stratiform dolostones are present in the basal part of the formation, while patchy dolostones are present at the middle and upper parts of the Samana Suk Formation. The dolomitization intensity of both geobodies increases from NW to SE in the study area. Petrographic studies reveal six phases of dolomites and three phases of calcites based on texture, crystal size and morphology. These phases are: matrix replacive dolomites (MD-I to MD-III); cementing dolomites include, replacive cementing dolomite (RD), saddle cementing dolomite (SD) and late stage cementing dolomites (CD); and calcite phases include CC-I and CC-II. XRD analyses reveal that stratiform matrix dolomites (MD-I, MD-II) are stoichiometric (51.08–51.86 mol percent of CaCO3) and contain up to 95% of the mineral dolomite. The patchy dolomite cement is non-stoichiometric (33.39–55.08 mol% of CaCO3) and contains around 65% of the mineral dolomite, whereas saddle dolomites is also non-stoichiometric (51.57 to 53.50 mol% of CaCO3) in origin. Stable isotope studies reveal non-depleted δ18O and δ13C values of matrix dolomites (MD-I, MD-II) represents coeval sea-water signatures of Jurassic carbonates, hence may have been formed by evaporative process. Dolomite cements (RD, SD) shows depleted δ18O values which represent elevated temperature, related to hydrothermal fluid source for their formation. The fracture filling calcite (CC-II) exhibits less depleted values indicative of meteoric fluids affected by shallow to moderate burial. The dedolomites shows depleted δ13C values suggests their formation from the meteoric water. Field, petrographic and geochemical studies suggest that diagenetic evolution of the Samana Suk Formation is the multistage process. In the first phase, marine diagenetic processes including marine cementation, stratiform dolomitization may have formed due to surface processes of marine water in peritidal to intertidal settings, while the second phase of diagenesis is due to burial associated processes which includes hydrothermal dolomitization occurred due to movement of magnesium rich fluids along weak planes such as fractures, faults, bedding planes and stylolites. Last stage includes formation of hydrothermal fracture filling calcites, replacive pyrites and dedolomites due to the uplift related processes.

Geochemical and Dynamic Model of Repeated Hydrothermal Injections in Two Mesozoic Successions, Provençal Domain, Maritime Alps, SE-France

A field, petrographic and geochemical study of two Triassic–Jurassic carbonate successions from the Maritime Alps, SE France, indicates that dolomitization is related to episodic fracturing and the flow of hydrothermal fluids. The mechanism governing hydrothermal fluids has been documented with the best possible spatio-temporal resolutions specifying the migration and trapping of hydrothermal fluids as a function of depth. This is rarely reported in the literature, as it requires a very wide range of disciplines from facies analysis (petrography) to very diverse and advanced chemical methods (elemental analysis, isotope geochemistry, microthermometry). In most cases, our different recognized diagenetic phases were mechanically separated on a centimetric scale and analyzed separately. The wide range of the δ18OVPDB and 87Sr/86Sr values of diagenetic carbonates reflect three main diagenetic realms, including: (1) the formation of replacive dolomites (Type I) in the eogenetic realm, (2) formation of coarse to very coarse crystalline saddle dolomites (Types II and Type III) in the shallow to deep burial mesogenetic realm, respectively, and (3) telogenetic formation of a late calcite cement (C1) in the telogenetic realm due to the uplift incursion of meteoric waters. The Triassic dolomites show a lower 87Sr/86Sr ratio (mean = 0.709125) compared to the Jurassic dolomites (mean = 0.710065). The Jurassic calcite (C1J) shows lower Sr isotopic ratios than the Triassic C1T calcite. These are probably linked to the pulses of the seafloor’s hydrothermal activity and to an increase in the continental riverine input during Late Cretaceous and Early Cenozoic times. This study adds a new insight into the burial diagenetic conditions during multiple hydrothermal flow events.

Dating and characterizing primary gas accumulation in Precambrian dolomite reservoirs, Central Sichuan Basin, China: Insights from pyrobitumen Re-Os and dolomite U-Pb geochronology

Primary gas accumulation with trillions of cubic meters of gas volumes was discovered recently in the Precambrian Dengying Formation of the Gaoshiti-Moxi block, Central Sichuan Basin, China. In this study we investigated the pervasive saddle dolomite cements and solid bitumens within karst vugs in the Dengying dolomite reservoirs. Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in situ U-Pb dating, we obtained a high-precision absolute age of 259.4 ± 3.0 Ma for the saddle dolomite cementation, consistent in timing with the eruption of Emeishan flood basalts (262–251 Ma). Combined with high homogenization temperatures (Th) and salinities of primary fluid inclusion, as well as rare earth element (REE) compositions, we consider that the saddle dolomite precipitation was likely related to hydrothermal activity driven by the Emeishan mantle plume.The identification of rare surviving oil inclusions and extensive solid bitumens within the dolomite vugs is indicative of the presence of paleo-oil reservoirs. These bitumens are characterized by high equivalent vitrinite reflectance (Requ), no fluorescence, and deformed fibrous textures, typical of highly mature pyrobitumens. Gas geochemistry data indicates that only minor thermochemical sulfate reduction (TSR) has occurred. Thus the pyrobitumen was interpreted to result mainly from the thermal cracking of paleo-oils, and its associated Re-Os isochron age of 154 ± 21 Ma, representing the timing of dry gas formation. Petrographical evidence suggests that the paleo-oil charge must have taken place after sparry dolomite cementation but prior to pyrobitumen generation. This, combined with coeval aqueous inclusion Th data and published thermal-burial models, constrains the time of paleo-oil emplacement to ca. 222–205 Ma, suggesting an oil-filling event related to the Indosinian Orogeny. This study documented the first chronological data constraining the complex evolution history of a petroleum system in one of the oldest primary gas reservoirs in the world.

LA-ICP-MS U-Pb geochronology and clumped isotope constraints on the formation and evolution of an ancient dolomite reservoir: The Middle Permian of northwest Sichuan Basin (SW China)

Recent advances in laser-ablation inductively coupled-plasma mass spectrometry (LA-ICP-MS) in-situ U-Pb radiometric dating and clumped isotope thermometry (Δ47) of carbonate minerals provide potential for refining the fluid flow and diagenetic history of carbonate successions. In this study, the Middle Permian dolomites proximal to the Longmenshan fold and thrust belt in northwest Sichuan Basin, southwest China, were investigated using combined U-Pb geochronology, clumped isotope thermometry, and routine isotopic (δ13C, δ18O and 87Sr/86Sr) and elemental geochemistry, in an attempt to reveal the possible relevance of carbonate diagenesis and porosity evolution to the basin-scale tectonic/fluid flow events in the framework of absolute time. Overall, formation and evolution of these dolomites temporally correlated well with major tectonic episodes of the Longmenshan fold and thrust belt. Contrary to the previously-assumed volcanic-related model, U-Pb dating and Δ47 analyses suggest a mid- to late-Triassic replacive dolomitization event (U-Pb ages of 240 ± 12 Ma to 233.8 ± 6.4 Ma) by a hot (Δ47 temperatures 88– 104 °C) basinal brine, which was likely driven by thrust-related compression of the Longmenshan fold and thrust belt during the Triassic. Replacive dolomitization was immediately followed by cementation of euhedral dolomites (U-Pb age of 228 ± 10 Ma) and blocky calcites (U-Pb ages of 224.8 ± 1.8 Ma to 213.4 ± 3.3 Ma) precipitated from the basinal brine in the same tectonic regime. Afterwards, there was long-term cessation of diagenesis that was coincident with, and thus may well be attributed to, tectono-thermal quiescence during most of the Mesozoic Era and Paleogene. Finally, saddle dolomite cements yield Miocene ages (U-Pb ages of 16.40 ± 0.74 Ma to 12.3 ± 1.2 Ma) and precipitated in pre-existing vugs, representing a hydrothermal fluid flow event (Δ47 temperatures up to 170 °C) during the reactivation of thrusting. In addition, similarities in U-Pb ages and geochemical signatures between host rocks (replacive dolomites) and the vug-lining dolomite cements suggest that most porosity in these dolomite reservoirs was inherited from the precursor limestones, which probably experienced meteoric leaching during late Permian uplift. This study highlights that there are geodynamic controls on fluid flow and consequent diagenetic evolution of carbonates in tectonically active sedimentary basins. It illustrates that burial alone in this setting is not a sufficient driver for diagenetic alteration and porosity reduction. The combination of U-Pb radiometric dating, clumped isotope thermometry and routine geochemical analyses is a useful approach in refining the diagenetic and porosity evolution history of ancient carbonate successions.

Burial dolomitization driven by modified seawater and basal aquifer‐sourced brines: Insights from the Middle and Upper Devonian of the Western Canadian Sedimentary Basin

AbstractDolomitization in the Western Canadian Sedimentary Basin has been extensively researched, producing vast geochemical datasets. This provides a unique opportunity to assess the regional sources and flux of dolomitizing fluids on a larger scale than previous studies. A meta‐analysis was conducted on stable isotope, strontium isotope (87Sr/86Sr), fluid inclusion and lithium‐rich formation water data published over 30 years, with new petrographic, X‐ray diffraction, stable isotope and rare‐earth element (REE+Y) data. The Middle to Upper Devonian Swan Hills Formation, Leduc Formation and Wabamun Group contain replacement dolomite (RD) cross‐cut by stylolites, suggesting replacement dolomitization occurred during shallow burial. Stable isotope, REE+Y and 87Sr/86Sr data indicate RD formed from Devonian seawater, then recrystallized during burial. Apart from the Wabamun Group of the Peace River Arch (PRA), saddle dolomite cement (SDC) is more δ18O(PDB) depleted than RD, and cross‐cuts stylolites, suggesting precipitation during deep burial. SDC 87Sr/86Sr data indicate contributions from 87Sr‐rich basinal brines in the West Shale Basin (WSB) and PRA, and authigenic quartz/albite suggests basinal brines interacted with underlying clastic aquifers before ascending faults into carbonate strata. The absence of quartz/albite within dolomites of the East Shale Basin (ESB) suggests dolomitizing fluids only interacted with carbonate strata. We conclude that replacement dolomitization resulted from connate Devonian seawater circulating through aquifers and faults during shallow burial. SDC precipitated during deep burial from basinal brines sourced from basal carbonates (ESB) and clastic aquifers (WSB, PRA). Lithium‐rich formation waters suggest basinal brines originated as residual evapo‐concentrated Middle Devonian seawater that interacted with basal aquifers and ascended faults during the Antler and Laramide Orogenies. These results corroborate those of previous studies but are verified by new integrated analysis of multiple datasets. New insights emphasize the importance of basal aquifers and residual evapo‐concentrated seawater in dolomitization, which is potentially applicable to other regionally dolomitized basins.

Characteristics and formation mechanisms of the unconformity-related paleokarst reservoirs in the Upper Sinian, Northwestern Tarim Basin, China

In the Late Sinian, the Tarim Basin experienced regional tectonic uplift and subaerial exposure. A regional unconformity was extensively distributed during the succession from the Sinian to Cambrian. As a result, a large number of secondary pores were developed in the Upper Sinian Qegebulake Formation. An integrated study based on petrographical and petrophysical analysis, geochemistry, karstology and fluid inclusions was performed to provide insight into the characteristics and formation mechanism of the Upper Sinian carbonate reservoirs. The reservoir rocks mainly include microbial dolomite, crystalline dolomite and breccias. Reservoir matrix porosity and permeability range from 0.30% to 11.06% and from 0.00156 mD to 1.49 mD respectively.Geological and geochemical characteristics suggest that three phases of dissolution were determined to be responsible for the development of the reservoirs within about 70 m beneath the unconformity. They are meteoric dissolution during subaerial exposure, and deep-seated hydrothermal dissolution and organic acid dissolution during the burial stage. The Upper Sinian dolomite underwent strong meteoric dissolution, which is supported by the appearance of the unconformity and the associated caves. The geochemical characteristics of the cave-filling calcite further indicate the diagenetic overprint from meteoric water, including the relatively negative values of δ18O (−18.46 to −8.5‰, VPDB) and wide range of δ13C (−8.11 to −0.24‰, VPDB), low ƩREE (1.77–4.23 ppm) and Yb–La and Yb–Ca variation. The occurrence of saddle dolomite as well as analytical results such as δ13C (0.67–2.52‰, VPDB) and δ18O (−13.21 to −9.81‰, VPDB) isotope signature and high homogenization temperature of fluid inclusions in quartz (87.2 to > 220 °C) and saddle dolomite (82.3–135.4 °C) suggest that the reservoirs rocks experienced interaction with hydrothermal fluids. Only bitumen occurred in the edge of secondary pores formed by dissolution of quartz and saddle dolomite, which suggests that organic acid dissolution likely occurred during the evolution of reservoirs.A geological model for the evolution of the paleokarst reservoirs is proposed through the analysis of major reservoir controlling factors. The primary pore types and reservoir petrophysical property of the Qigebulake Formation mainly depend on the sedimentation. Supergene karstification is the most important process for reservoir formation. Deep-seated hydrothermal activity has both positive and negative attribution on the formation of reservoirs. Organic acid dissolution plays an active role in the formation and preservation of reservoirs. This study provides a useful reference for hydrocarbon exploration in the deep buried Sinian carbonate reservoirs.