Replacement Dolomite Research Articles

Multiphase Diagenetic Processes and Their Impact on Reservoir Character of the Late Triassic (Rhaetian) Kingriali Formation, Upper Indus Basin, Pakistan

Multiple episodes of dolomitization of the shallow marine carbonates of the Late Kingriali Formation resulted in regional scale mappable dolostone geobodies in the Kohat and Potwar sub-basins. With the exception of few unaltered patches of the host limestone, more than 90% of the carbonates of the studied formation are diagenetically altered by replacive dolomites with associated dolomite cementation. Petrographical and geochemical data interpretation reveals that during the initial stage of dolomitization, the precursor limestone was significantly modified by the fabric-retentive replacive dolomite (RD-I) and produced bulk dolostones with non-planar-a to planar-s crystals. Neomorphic recrystallization (RD-II) was observed as an overgrowth of the already formed RD-I dolomite crystals during progressive dolomitization. The seawater at shallow depths is enriched with Fe-ions due to its interaction with Fe-rich beds within the studied formation. The modified seawater actively participated in the formation of ferroan replacive dolomites (RD-III). Stable isotopic composition of the unaltered Echinoderm plates, calcite cement (CC-I), and RD-I demonstrates signatures of δ18O and δ13C within the limit of late Triassic marine seawater or modified seawater. Depletion in the stable oxygen isotopic composition (from −0.99‰ to −3.75‰ V-PDB) demonstrates that RD-II and RD-III were formed in a sequence with progressively higher temperature fluids than normal seawater. Precipitation of dolomite cements as cavity filling rhombs (DC-I) and crystal overgrowth (DC-II) with highly depleted δ18O values (−5.44‰ to −7.45‰ V-PDB) illustrates dolomite cementation at higher temperatures and greater depths. The highly depleted values of δ18O (up to −9.16‰ V-PDB) and (up to 0.42‰ V-PDB) for δ13C of saddle dolomite (SD-I) indicate the precipitation of SD-I as a cavity filling dolomite at considerable depth. Calcite cementation and calcitization actively participated in the early, middle, and late diagenetic modifications as interpreted from their petrographic and stable isotopic studies. Porosity enhancement is clearly demonstrated by dissolution, stylolization, fracturing, and replacement dolomitization. Dolomite and calcite cementation had a negative impact on the reservoir character and occluded the dolostone porosity to a greater extent.

Fault-controlled hydrothermal dolomitization of Middle Permian in southeastern Sichuan Basin, SW China, and its temporal relationship with the Emeishan Large Igneous Province: New insights from multi-geochemical proxies and carbonate U–Pb dating

The mechanism of fault-controlled hydrothermal dolomitization has long been controversial due to its complicated tectonic and diagenetic conditions, especially for the source of dolomitizing fluids and its hydrological driving models. In the southeastern Sichuan Basin, fracture-related dolomite bodies are well developed in a Middle Permian carbonate succession, providing an example of dolomitization and its relationship with regional tectonic activities. Previously, many of the dolomites in the Sichuan Basin have been interpreted as of hydrothermal origin but are usually poorly defined since it is hard to determine the nature and timing of dolomitization. In the current study, new petrographic, geochemical, and chronological data of Middle Permian dolomites have been integrated and allowed the classification of those dolomites into two categories (replacive dolomites of D1–D2 and fracture-/vug-filling dolomites of D3–D5). Calcite (C1–C3) and other non‑carbonate cements have also been found. The near-micritic dolomite (D1) was formed at the near-surface to early burial stage, likely from modified seawaters, which was reflected by its similar δ18Ofluid estimates of the parent dolomitizing fluids and rare earth element patterns to matrix lime mudstone, as well as the fabric retentive texture. The fabric destructive (D2) and fracture-/vug-filling dolomites (D3–D5) are characterized by the high estimates of δ18Ofluid of their parent dolomitizing fluids, high 87Sr/86Sr ratios, high Fe and Mn concentrations, positive Eu anomalies, high Th values, and high salinity estimates, indicating a hydrothermal origin. However, the limestone-like δ13CVPDB and strongly negative Ce anomalies of D2–D5 suggest that these dolomites may have originally evolved from seawater-derived fluids which circulated in the basal clastic rocks and flowed upwards by thermal convection through fault systems. New U–Pb ages bracketed a time range of these dolomites from 251 ± 11 Ma to 264 ± 10 Ma, coinciding with the emplacement of the Emeishan Large Igneous Province (~257–260 Ma). This may imply that the contemporary basement faulting and associated abnormal thermal event in southeastern Sichuan Basin still remotely echoed with the activities of the Emeishan Large Igneous Province, although the study area was far away from its eruption center. It facilitated the fluid circulation through fault systems and underlying sandstone aquifers and thus drove fault-controlled hydrothermal dolomitization at shallow burial backgrounds.

TAPHONOMIC CONTROLS ON MICROBIALITE TEXTURES FROM THE STEAMBOAT POINT MEMBER, UPPER ORDOVICIAN BIGHORN DOLOMITE, WESTERN TETON MOUNTAINS, USA

ABSTRACT The Bighorn Dolomite is a massive, cliff-forming dolostone unit found across the state of Wyoming and adjacent areas that records the transition from greenhouse to icehouse conditions during the Late Ordovician. The basal Steamboat Point Member of the Bighorn Dolomite contains cm-scale mottled fabrics often attributed to the ichnogenus Thalassinoides, but their origin remains enigmatic in Upper Ordovician strata. The development of mottled fabrics may have significant implications for marine chemistry and paleoecology. We analyzed cm-scale mottled fabrics from the basal Steamboat Point Member of the western Teton Mountains to determine their origin and develop criteria for identification of similar mottled fabrics found in other Upper Ordovician strata. In vertical cross-section, massive, m-scale exposures display vertical columns (approximately 1 cm in diameter) that anastomose and maintain decimeter-scale vertical continuity. Microscopically, the columns are composed of micro-peloidal micritic textures surrounded by spar and microspar, whereas the intercolumnar matrix is composed of dolomicrite grains, skeletal clasts, and intraclasts. We interpret the microscopic peloidal texture as a product of benthic microbial calcification that coalesced to form mesoclots—diagnostic features of thrombolitic microbialites. Paragenetic analyses reveal evidence for early lithified microbial biofilms despite dolomite replacement and aggrading neomorphism. These observations indicate the Steamboat Point Member of the Bighorn Dolomite was deposited in seawater that was highly supersaturated with respect to carbonate, consistent with other studies that suggest deposition during greenhouse conditions prior to the end-Ordovician glaciation. We present a novel ternary diagram that contains criteria to help differentiate between burrows, microbialite textures, or physical processes that may result in mottled fabrics in the rock record.

Temperature regimes during formation of Miocene island dolostones as determined by clumped isotope thermometry: Xisha Islands, South China Sea

Carbonate clumped isotope thermometry is a relatively new type of analysis that has been used to obtain a better understanding of the temperature and salinity of the water that mediated the development of dolostones in the Cenozoic successions of isolated oceanic islands. There is, however, still some uncertainty about the factors that could modify the clumped isotopes even in shallow burial conditions and thereby affect the temperatures and salinities derived from them. Here, focus is placed on a 210.5 m thick succession of Miocene dolostones that is found on the Xisha islands in the South China Sea that have experienced rapid subsidence since the Early Miocene. These dolostones are typically formed of crystals that have a “dirty” core that is encased by a clear outer rim. The formation of the “dirty cores”, the primary replacement dolomite, was probably mediated by normal seawater in near-surface conditions, whereas the later cements formed at higher temperatures from slightly evaporated seawater. Given that it is impossible to separate the “dirty cores” from the clear rimes for analytical purposes, it must be recognized that the clumped isotopes and other geochemical data (e.g., δ18O, 87Sr/86Sr) are averages of these two types of dolomite. This also means that the development of dolomite in these dolostones involved two successive stages of dolomite development that was probably related to sea-level cycles. Despite these issues, clumped isotope thermometry can still provide valuable temperature and salinity information for understanding the dolomitization processes that led to the development of island dolostones.

Heritage Stone 8. Formation of Pinolitic Magnesite at Quartz Creek, British Columbia, Canada: Inferences from Preliminary Petrographic, Geochemical and Geochronological Studies

Rocks in the Late Proterozoic Horsethief Creek Group at Quartz Creek in British Columbia display rare ‘pinolitic’ textures resembling those described in some sparry magnesite deposits elsewhere in the world. Elongated white magnesite crystals up to 30 cm long occur in a contrasting, dark, fine-grained matrix of dolomite, chlorite, organic material, clay minerals and pyrite. The rocks are aesthetically appealing for use in sculpture and as dimension stone. The term ‘pinolite’ is derived from the superficial similarities between these unusual textures and pinecones. Petrographic examination indicates that these textures formed when metasomatic fluids replaced primary sedimentary dolomite with magnesite. Fluids moved along fractures and bedding planes with repeated fracturing yielding magnesite crystals oriented in opposite directions on either side of annealed fractures, and broken magnesite crystals adjacent to later fractures. Magnesite contains dolomite microinclusions and has elevated Ca contents that are consistent with its formation by replacement of dolomite. Low concentrations of Cr, Ni, Co, Ti, Sr, and Ba in magnesite also imply formation in a metasomatic rather than a sedimentary environment. The rare earth element (REE) concentrations in the Quartz Creek magnesite are higher than those in most evaporitic magnesite and REE patterns lack the Ce and Eu anomalies that characterize carbonate rocks from sedimentary environments. Enrichment in light REE relative to heavy REE, and the similarities between dolomite, chlorite, and magnesite REE profiles, imply that metasomatic fluids modified the original sedimentary geochemical signature of the dolostones during formation of the pinolite rocks. A Late Ordovician to Early Silurian U–Pb age (433 ± 12 Ma), for titanite in the black matrix surrounding the sparry magnesite is younger than the local host rocks, and also younger than the Mesoproterozoic to Middle Cambrian stratigraphic ages of the host rocks for nearby magnesite deposits. The ca. 433 Ma titanite overlaps the ages for numerous fault-associated diatremes and volcaniclastic deposits in the area. Possibly the igneous activity furnished heat for, and/or was the source for, metasomatic fluids that produced the pinolite deposits.

Petrography and Geochemistry of Dolomites of Samanasuk Formation, Dara Adam Khel Section, Kohat Ranges, Pakistan

Replacement dolomite occurs in Jurassic Samanasuk Formation in Dara Adam khel area of Kohat ranges, North-Western Himalayas, Pakistan. This study, for the first time, document the process of dolomitization and evolution of strata bound dolomitic bodies. Field investigation, petrography and geochemistry helped in unraveling the formation of several dolomitic bodies. Petrographically dolomites comprises of: (1) medium grain crystalline planer subhedral dolomite (Dol-I); (2) fine grained crystalline anhedral non-planer dolomite rhombs (Dol-II); (3) medium to coarse grained crystalline subhedral-anhedral non-planer dolomite (Dol-III) and coarse to very coarse grained crystalline saddle dolomite cements (SD). The saddle dolomites (SD) postdate the replacement dolomites and precede telogenetic calcite (TC) cements. Stable O and C isotope analysis shows that these dolomites have δ18Ovpdb ranging from -4.09% to -10.4 whereas the δ13Cvpdb ranges from +0.8 to +2.51. Major and trace elements data show that Sr concentrations of 145.5 to 173 ppm; Fe contents of 2198 to 8215 ppm; and Mn contents of 93.5 to 411 ppm. Petrographically replacive dolomites, saddle dolomite, and δ18Ovpdb values depicts neomorphism of replacement dolomites that were formed earlier were exposed to late dolomitizing fluids. As a result of basin uplift during the Himalayan orogeny in Eocene time, dolomitization event was stopped through occurrence of meteoric water. The Main Boundary Thrust (MBT) and its splays were most likely essential conduits that channelized dolomitizing fluids from siliciclastic rocks that were buried deeply into the Jurassic carbonates rocks, leading to more extreme dolomitization.

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.

Petrography and origin of dedolomites of the Ordovician Majiagou formation in the southeastern Ordos Basin, China: implications for reservoir quality

Dedolomites, the replacement of dolomite by calcite, are widely distributed in the Ordovician Majiagou Formation of the southeastern Ordos Basin, China, which critically affects reservoir quality throughout the region. Two types of dedolomites were recognized in the upper 100 m of the Majiagou Formation, namely brecciated dedolomite and crystalline dedolomite. The petrographic and geochemical data reveal that the investigated dedolomites are often associated with clay minerals and dissolution-collapse breccia, indicating their close relationship with erosional unconformities. The relative negative δ18O and δ13C values and relative high Fe, Mn contents of dedolomite with respect to corresponding dolomite are interpreted as being result from meteoric phreatic water involvement. The dedolomitization process related to subaerial exposure is conventionally interpreted to be formed by dolomite dissolution and calcite precipitation. The rhombic shape crystals in the brecciated dedolomite was formed by an Mg2+-loss process under evaporite solution conditions, which retain the rhombic shape of the dolomite. The crystalline dedolomites, however, were formed by dissolution/precipitation and recrystallization process to form medium to coarse calcite mosaic with very low intercrystalline porosity. The resulting fabrics are mainly controlled by the paleo-topography. The precursor dolomite of the crystalline dedolomite is primarily recrystallized mudstone, which is more soluble and favorable for calcite precipitation. The crystalline dedolomite mostly occurs in the low paleotopographic locations, where the karst-saturated water with respect to CaCO3 is more concentrated, occluding the remaining porosity when excess calcium is supplied. It’s, therefore, recommended to implement drilling in tectonic highland and avoid low paleotopographic locations.

The Teena Zn-Pb Deposit (McArthur Basin, Australia). Part II: Carbonate Replacement Sulfide Mineralization During Burial Diagenesis—Implications for Mineral Exploration

AbstractThe Teena Zn-Pb deposit is located in the Carpentaria Zn Province (Australia), which contains some of the largest clastic dominant (CD-type) massive sulfide Zn-Pb deposits in the world. The timing of the main stage of hydrothermal sulfide mineralization in the Teena subbasin is constrained to the midstage of burial diagenesis, during a period of short-lived regional extension. To distinguish hydrothermal alteration from spatially and temporally overlapping burial diagenetic alteration, and to establish the primary controls on hydrothermal mass transfer, it is necessary to evaluate the various foot- and hanging-wall alteration assemblages that formed between early- (eogenesis) and late- (mesogenesis) stage diagenesis. To achieve this, we have statistically evaluated a large lithogeochemistry dataset (n >2,500) and selected a subset (n = 65) of representative samples for detailed mineralogical (X-ray diffraction, illite crystallinity) and petrographic (scanning electron microscopy) analyses; hyperspectral core imaging data were then used to upscale key paragenetic observations.We show that sulfide mineralization was predated by multiple diagenetic alteration assemblages, including stratiform pyrite, dolomite nodules and cement, disseminated hematite and authigenic K-feldspar. These assemblages formed during eogenesis in multiple subbasins across the broader McArthur Basin and are not part of the synmineralization alteration footprint. Whereas pyrite and dolomite formed primarily from the in situ degradation of organic matter, feldspar authigenesis was the product of K metasomatism that was focused along permeable coarse-grained volcaniclastic sandstone beds within the host-rock sequence. The immature volcaniclastic input is broadly representative of the siliciclastic compositional end member in the subbasin, which formed the protolith for phyllosilicate (illite, phengite, chlorite) formation during burial diagenesis. There is no evidence of extensive phyllosilicate alteration in any of the geochemical, mineralogical (illite crystallinity), or petrographic datasets, despite some evidence of K-feldspar replacement by sphalerite in the Lower and Main mineralized lenses. Rather, the high Zn grades formed via dolomite replacement, which is resolvable from a chemical mass balance analysis and consistent with petrographic observations.There are significant exploration implications associated with carbonate-replacement sulfide mineralization during mesogenesis: (1) the capacity for secondary porosity generation in the host rock is as important as its sulfate-reducing capacity; (2) hydrothermal mineralization has a short-range cryptic lateral and vertical synmineralization alteration footprint due to acid neutralization by a carbonate-rich protolith; and (3) the distribution and chemistry of premineralization phases (e.g., pyrite, dolomite nodules) cannot be directly related to the mineralization footprint, which is localized to the 4th-order subbasin scale. Future exploration for this deposit style should therefore be focused on identifying units that contain a mixture of organic carbon and carbonate in the protolith, at favorable stratigraphic redox boundaries, and proximal to feeder growth faults.

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.

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.

Regional fault-controlled shallow dolomitization of the Middle Cambrian Cathedral Formation by hydrothermal fluids fluxed through a basal clastic aquifer

AbstractThis study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin. Kilometer-scale dolomite bodies within the Cathedral Formation carbonate platform are composed of replacement dolomite (RD), with saddle dolomite-cemented (SDC) breccias occurring along faults. These are overlain by the Stephen Formation (Burgess Shale equivalent) shale. RD is crosscut by low-amplitude stylolites cemented by SDC, indicating that dolomitization occurred at very shallow depths (<1 km) during the Middle Cambrian. Clumped isotope data from RD and SDC indicate that dolomitizing fluid temperatures were >230 °C, which demonstrates that dolomitization occurred from hydrothermal fluids. Assuming a geothermal gradient of 40 °C/km, due to rift-related basin extension, fluids likely convected along faults that extended to ∼6 km depth. The negative cerium anomalies of RD indicate that seawater was involved in the earliest phases of replacement dolomitization. 84Kr/36Ar and 132Xe/36Ar data are consistent with serpentinite-derived fluids, which became more dominant during later phases of replacement dolomitization/SDC precipitation. The elevated 87Sr/86Sr of dolomite phases, and its co-occurrence with authigenic quartz and albite, likely reflects fluid interaction with K-feldspar in the underlying Gog Group before ascending faults to regionally dolomitize the Cathedral Formation. In summary, these results demonstrate the important role of a basal clastic aquifer in regional-scale fluid circulation during hydrothermal dolomitization. Furthermore, the presence of the Stephen Formation shale above the platform facilitated the build-up of fluid pressure during the final phase of dolomitization, leading to the formation of saddle dolomite-cemented breccias at much shallower depths than previously realized.

Application of fluorescent light microscopy to enhanced lithofacies analysis of Jurassic carbonate rocks, SE Bohemian Massif

Upper Jurassic limestones and dolomites of the Vranovice Formation occur on the southeastern slopes of the Bohemian Massif below the Western Carpathian thrust belt. They represent a special type of reservoir rocks in the potential geothermal systems and exhibit highly variable porosity, permeability, and flow mechanisms due to different type of diagenesis and pervasive dolomitization. Fluorescence and translucent light microscopy was used for more detailed petrographic analysis of Vranovice limestones and dolomites. Isopachous, syntaxial and dolomitic cements were distinguished along with fossils recrystallization. Two generations of dolomite were identified – replacement dolomite and zonal cementation dolomite, what suggests a number of changes in the formation water chemistry during the dolomitization process. Extent of the observed dolomitization is inversely related to the porosity of the Jurassic rocks. Postdiagenetic generation of fissures cutting the strata due to tectonic deformation is documented by bright fluorescent color. In the studied samples of Vranovice limestones and dolomites the fissures represent the most important migration pathways, more than pore throats themselves. Further genetic information on petoleum migration is interpreted from intercrystalline porosity filled with precipitated solid bitumen. Present results provide basis for further investigations in pore-water-rock interactions and changes in chemical conditions during diagenesis.

Dolomitization of the Ordovician subsalt Majiagou Formation in the central Ordos Basin, China: fluid origins and dolomites evolution

The Middle Ordovician subsalt Majiagou Formation in the Ordos Basin comprises pervasively dolomitized shallow marine limestone and is a major reservoir rich in natural gas resources. Four types of dolomite matrix and cement were identified based on petrographic textures: (very) finely crystalline, non-planar to planar-s matrix dolomite (Md1); finely to medium crystalline, planar-s to planar-e matrix dolomite (Md2); microbialites comprising dolomite microcrystals (Md3); and finely to coarsely crystalline dolomite cement (Cd). The Md1 and Md2 dolomites were controlled by alternating lagoon-shoal facies and have δ13C values (− 1.89 to + 1.45‰ VPDB for Md1, − 1.35 to + 0.42‰ VPDB for Md2) that fall within or are slightly higher than the coeval seawater, suggesting the dolomitizing fluid of evaporated seawater. Md2 dolomite was then subjected to penecontemporaneous karstification by meteoric water and burial recrystallization by sealed brines during diagenesis, as indicated by its relatively lower δ18O values (− 8.89 to − 5.73‰ VPDB) and higher 87Sr/86Sr ratios (0.708920–0.710199). Md3 dolomite comprises thrombolite and stromatolite and is interpreted to form by a combination of initial microbial mediation and later replacive dolomitization related to evaporated seawater. Cd dolomite was associated with early-formed karst system in the Md2 host dolomite. The lowest δ18O values (− 11.78 to − 10.18‰ VPDB) and 87Sr/86Sr ratios (0.708688–0.708725) and fluid inclusion data (Th: 123–175 °C) indicate involvement of hydrothermal fluid from which the Cd dolomite precipitated during deep burial. These results reveal the multi-stage dolomitization history of the Majiagou Formation and provide new constraints on fluid origins and dolomites evolution during deep burial in old superimposed basins, such as the Ordos Basin and elsewhere.

Episodic dolomitization of Paleozoic limestones in the Kinta Valley, Malaysia: Implications on porosity evolution and reservoir properties

Carbonates undergo a variety of physical, mineralogical, chemical changes during depositional and post-depositional stages that are of interest to exploration geologists and sedimentologists. World over, about 60% of hydrocarbon reserves occur in carbonates with the dominant proportion associated with dolomitic reservoirs, in particular fault-associated dolomites which appear to be proven hydrocarbon reservoirs. Dolomitization as the critical diagenetic process thus plays an important role in determining reservoir characteristics and in designing exploration strategies. The buried Paleozoic dolomites exhibit relatively higher porosity than their counterparts, Paleozoic limestones. Fault-associated dolomites are usually targets of hydrocarbon exploration due to their good reservoir quality. High heterogeneity in carbonate reservoirs due to varied porosity types and distribution makes reservoir characterization a daunting task. However, the key elements that control porosity development and evolution in these dolomites remain debatable. Studying the influence of multiphase dolomitization, associated diagenetic events, hydrothermal alteration and their causative thermo-tectonic events on porosity evolution and reservoir quality is essential to understanding the critical processes and controlling factors that result in diverse impacts on reservoir quality. In this paper, an attempt is made to characterize dolomitization in homogeneous carbonate mud and its relationship with porosity evolution and reservoir property. In the field, six facies types excluding host limestone are recognized, which are in turn characterized into seven petrographically recognizable phases, namely the host limestone, early replacement dolomite, sucrosic dolomite, metamorphosed dolomite, late replacement dolomite, brecciated limestone and dolomite and late stage calcite. Mineralogical analyses reveal the dominance of stoichiometric dolomites, followed by calcareous dolomites and magnesium calcites. Among all the field-based facies and petrographic phases, the sucrosic dolomites show appreciable porosity within a range from 4% to 8%, whereas the other phases show either nil or insignificant porosity under megascopic and petrographic observations. Our study confirms that in the process of dolomitization that substantially modifies porosity, dolomitic crystal morphologies play a significant role in enhancing or reducing the porosity.

Facies, stratigraphic architecture, and faults - The controls on the cement distribution in the Devonian Sappington Formation in southwestern Montana

Diagenesis has a profound effect on the porosity and permeability of sediments, and it is necessary to holistically evaluate the diagenetic history in the context of depositional, stratigraphic, and structural frameworks. This outcrop study from the late Devonian to early Mississippian Sappington Formation of southwestern Montana, USA, investigates the complex interplay of these different geologic factors and their controls on the distribution of diagenetic alterations.Our results suggest that the principal control on the distribution of diagenetic minerals is the thickness of the Lower, Middle, and Upper Sappington Shale Members, respectively. The sedimentary architecture plays a secondary role as clinoform boundaries and stratigraphic surfaces compartmentalized fluid flow. Structural features are of local importance and depositional facies, other than the previously mentioned shales, display little effect on the distribution of diagenetic minerals.Six dominant cement types were recognized in the Sappington Formation. Quartz and clay cements are the earliest cements and formed on the surfaces of detrital grains during early diagenesis and are relatively minor phases (<5% vol). These are postdated by a series of carbonate cements that dominate the diagenetic mineral assemblage (>60% vol) and include two types of dolomite, and two types of calcite. Dolomite is the most abundant diagenetic mineral and is observed as zoned, euhedral to subhedral rhombs with a nearly stoichiometric core and a series of ferroan rims and as subhedral to anhedral ferroan rhombs without zonation. Calcite cements are most common as a rim on the outer extents of dolomite rhombs but also occur as a replacement of dolomite or detrital feldspar.The outcrop-based model presented here – the first comprehensive diagenetic model of the Sappington Formation in SW-Montana, USA – can serve as a guide to hydrocarbon development in tight hybrid plays, such as the Bakken Formation in the Williston Basin, USA. It provides a more holistic understanding of the relationship between the depositional architecture and the diagenetic alterations and helps to decrease uncertainty in the second and third dimensions.

Multistage dolomitization and formation of ultra-deep Lower Cambrian Longwangmiao Formation reservoir in central Sichuan Basin, China

The deeply buried dolostones in the Lower Cambrian Longwangmiao Formation at Central Sichuan Basin of China are a crucial hydrocarbon exploration target. However, the origin of various types of dolomite for these carbonate reservoirs has not been well understood. In this study, petrological studies, geochemical analyses including C, O and Sr isotopes, in-situ trace elements and rare earth elements, and fluid inclusions analyses are performed in order to uncover the dolomitization mechanisms. The results show that an initial pervasive replacive dolomitization phase (Md, D1 and D2) was followed by volumetrically less important cement phases (D3, D4, D5, D6 and calcite). The δ18O values and element concentrations (e.g., Na and Sr) appear to suggest that this early dolomitization was caused by reflux of slightly modified seawater. In the shallow burial realm, influx of meteoric water led to a decrease in Fe content (from 1000 ppm down to 70 ppm) and ∑REE content (from 34.22 ppm down to 9.56 ppm) in early diagenetic D3 dolomite with more negative δ18OSMOW values. This early meteoric water flux may have created considerable dissolution porosities. The D3 and D6 dolomite cements were interpreted as the result of dolomite dissolution and reprecipitation due to burial compaction. The D4, D5 dolomites and late calcite cements display markedly positive Eu anomalies suggesting the presence of hydrothermal fluid. Some late diagenetic calcites with negative δ13C values as low as −5‰ V-PDB and relatively higher Y/Ho ratio of ca. 36.9 were interpreted to be derived from thermochemical sulfate reduction (TSR). The effects of hydrothermal fluid and TSR on reservoir qualities seem to be minor in this case. Hence, this study emphasizes the role of late diagenetic preservation of primary and early diagenetic porosity in the formation of economically important, ultra-deep dolostone reservoirs in sedimentary basins.

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.