Late Calcite Cements Research Articles

Chronology and geofluids characteristics of calcite cement in the Upper Triassic Xujiahe Formation tight sandstone reservoir, Western Sichuan Basin, SW China

Geological fluid activities significantly influence diagenesis and hydrocarbon accumulations in reservoir rocks. However, it remains challenging to precisely determine the timing of these activities. This study aims to investigate the chronology and geofluids characteristics of calcite cement that leads to reservoir tightness in the Upper Triassic Xujiahe Formation in the Western Sichuan Foreland Basin, which is a critical exploration target for hydrocarbon resources. We examine calcite cement as a marker of fluid activities, employing petrological analyses, U-Pb dating, and in-situ carbon-oxygen isotope analysis. These methods are integrated with simulations of regional burial and thermal histories to assess pore evolution and hydrocarbon accumulation mechanisms in these tight sandstone reservoirs. Results reveal two main calcite cementation stages within the second member of the Xujiahe Formation (Xu-2 Member): early cements from the Middle Jurassic around 175 ± 11 Ma and late cements from the Late Jurassic between 161 and 156 Ma. Initially, basin-derived alkaline fluids and near-surface atmospheric freshwater supplied the calcium and carbon for early cements during ongoing basin subsidence. Calcium and carbon for the late calcite cements came from the dissolution of carbonate rock fragments by organic acids. The heavier δ13C is primarily associated with in-situ generation and consumption of carbon within the system, while the δ18Ofluid, which is close to modern seawater values, is mainly related to active water-rock reactions induced by processes such as the dissolution of carbonate rock fragments by organic acids. The closed diagenetic system due to calcite cementation in the Late Jurassic led to reservoir tightening. Therefore, basin-scale chronological constraints on calcite cement are of great significance for quantitatively studying sandstone reservoir densification and exploring the mechanism and control factors of hydrocarbon accumulation.

Origin of carbonate cements in Ben Nevis sandstones of Jeanne d’Arc Basin: Insights from geochemistry

The Ben Nevis sandstones in the Jeanne d’Arc Basin in offshore eastern Newfoundland have abundant carbonate cements, and are the primary hydrocarbon reservoirs of the White Rose Oilfield. A multiapproach of petrographic, carbon and oxygen isotopic, and in situ geochemical analyses were applied to investigate the origin of the carbonate cements in the Ben Nevis sandstones (quartzose to litho-quartzose, Well F-04). Petrographic examinations reveal five generations of cements that occur in concretions, which are (from earliest to latest), siderite, early calcite cement (C1), recrystallized fossil (Crf), sparry fossil-filling calcite (Cf), and late calcite cement (C2). The comparable carbon (3.6‰ VPDB) and oxygen isotopic (−1.3‰ VPDB) compositions of siderite to those of the bivalve shells (δ13C = 3.6 ± 0.6‰ VPDB and δ18O = −1.7 ± 0.5‰ VPDB, respectively) indicate an early precipitation from suboxic water near the sediment-seawater interface. The well-preserved intergranular volumes (IGVs up to 41%) and low δ18O (−3.3 ± 1.1‰ VPDB) as well as comparable δ13C values (3.6 ± 0.6‰ VPDB) of the early poikilotopic calcite (C1) argue for a pre-compaction precipitation at a shallow burial setting with carbon derived mainly from the dissolution of bioclasts. The later calcite cement (C2) precipitated during progressive burial with involvement of organic carbon as indicated by its depleted δ18O (−6.7 ± 2.0‰ VPDB) and δ13C (1.8 ± 1.3‰ VPDB) values, and the reduced IGVs (28.2%), as well as the consistent depletion of Sr with enrichment of Mn. The dissolution of aragonitic bioclasts, was likely the main source of Ca for calcite, which is consistent with the high Sr content of C1 (1369 ± 597 ppm), Crf (2005 ± 472 ppm), Cf (1354 ± 48 ppm) and C2 (1831 ± 696 ppm). The parallel shale normalized (REESN) patterns of the cements and the way they mimic, to some extent, that both of seawater and meteoric water, suggest that the diagenetic fluid is bracketed by those of the early Cretaceous marine and meteoric waters, as reflected by the estimated δ18OSW composition of parent fluid for C1. The consistent Eu anomalies of carbonate cements and the increasing δ18OSW composition of pore water during C2 precipitation reflect the evolution of the diagenetic fluids in siliciclastic intervals during progressive burial.The secondary porosity development as the main contributor to the present reservoir porosity seem to be closely related to the dissolution of calcite cements, especially in the lower part of the Ben Nevis interval. The meteoric water influx from the overlying unconformities may have induced the dissolution of calcite concretions.

Characteristics of fluid inclusions and fluid coupling mineralization of the Pengyang uranium deposit, Ordos Basin

The Pengyang uranium deposit, located in the Tianhuan Depression in the southwestern Ordos Basin, is a large-scale sandstone-type uranium deposit in aeolian sediments. The uranium-bearing rock series in the deposit is mainly sandstone and conglomerate from the Lower Cretaceous Luohe Formation, which is dominated by desert facies. Based on our investigation of the geological characteristics of this deposit, a systematic study of the type, characteristics of the ore-forming fluid and ore genesis was conducted. Through the petrographic study of inclusions, three stages of inclusions have been identified in the quartz overgrowth, carbonate cements, and secondary microfractures in the ore-bearing sandstone of the Luohe Formation, and the third-stage inclusions in the secondary microfractures or late calcite cements represent the ore-forming fluids. There are three types of inclusions captured from ore-forming fluids: liquid-dominated aqueous inclusions, liquid-dominated hydrocarbon inclusions, and natural gas inclusions. Their gaseous components are mainly CH4 and H2O, with a small amount of N2. The homogenization temperatures of liquid-dominated aqueous inclusions range from 58 to 147 ℃, and the salinity ranges from 0.35 to 6.88 wt%. The homogenization temperature of liquid-dominated hydrocarbon inclusions mainly exhibits two peaks (at 60–140 ℃ and 160–190 ℃). Our studies of carbon-hydrogen–oxygen isotopes indicate that the formation of uranium-bearing sandstone is closely related to the participation of organic matter such as oil and gas. Based on the comprehensive researches of fluid inclusions and isotope analysis, it is concluded that the ore-forming fluid of the Pengyang uranium deposit is a mixture of meteoric water and hydrocarbon fluid. The deep hydrocarbon fluid traveled upward along the structural fractures, acting as a reductive agent and participating in the oxidation–reduction reaction of the Pengyang uranium deposit. Although hydrocarbon fluid can bring some uranium from deep area, the uranium from the provenance area of the basin may be the main source of mineralization in the Pengyang uranium deposit. The Pengyang uranium deposit is a sandstone-type uranium deposit related to the reduction of oil and gas. Uranium mineralization is mainly caused by the coupling between the uranium-bearing oxygen-rich fluid infiltrated from the surface and the deep hydrocarbon fluid.

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.

Clumped isotope geothermometry of an Ordovician carbonate mound, Hudson Bay Basin

The Hudson Bay sedimentary basin was overlooked geologically until two decades ago. Recent efforts to understand the palaeogeothermal history of this basin have led to the evaluation of fluid inclusion microthermometry, apatite fission track, organic matter reflectance and Rock–Eval analyses. Although apatite fission track and organic maturity indicators tend to show relatively low maximum burial temperatures (60–80°C), evidence of potential oil slicks on the sea surface and oil and gas shows in offshore wells have been reported across Hudson Bay. Fluid inclusion microthermometry in a carbonate mound sequence suggests homogenization temperatures of 118 ± 25 and 93 ± 10°C for recrystallized synsedimentary marine calcite and late pore-filling burial calcite, respectively. This sequence provides an interesting geological framework to test the application of clumped isotope thermometry against independent geothermometers. Here, we present clumped isotope data acquired on the late calcite cements and diagenetically altered early marine phases. The integration of clumped isotopic data with other thermal indicators allows the reconstruction and refinement of the thermal–diagenetic history of these carbonates by confirming an episode of heating, probably of hydrothermal origin and prior to normal burial diagenesis, that reset both fluid inclusions and the clumped isotope indicators without recrystallization.

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.

Slope destabilization provoked by dissociation of gas hydrates in the Outer Carpathian basin during the Oligocene: Sedimentological, petrographic, isotopic and biostratigraphic record

Sedimentological, biostratigraphic, petrographic, and stable C and O isotope study was carried out on chaotic complexes hosting carbonate concretions in the uppermost Oligocene successions of the Outer Carpathians. These chaotic complexes reveal a range of sedimentary features indicative of intrabasinal, submarine mass-wasting deposition, including soft-sediment deformation, brecciation, homogenization and fluidization of the parent material. Apart from carbonate concretions, they enclose blocks of rocks having lithologies typical for the underlying Oligocene strata. The concretions exhibit extremely depleted δ13C (by down to −51.3‰) indicating anaerobic oxidation of methane, whereas their δ18O values are strongly enriched relative to coeval brackish basin water indicating dissociation of gas hydrates as the main fluid source. These data and the geotectonic context imply that the chaotic complexes formed as submarine slumps on a N-inclined paleoslope of the basin in consequence to tectonically-induced dissociation of gas hydrates. Hydrates were destabilized within the sediments due to synorogenic uplift, related to northward progradation of the accretionary prism. These slumps and hosted concretions were dated by the combination of calcareous nannoplankton and organic-walled dinoflagellate cysts. The age of concretions indicated by dinoflagellate cyst assemblages always overlaps with that of the surrounding slump matrix determined by both micropaleontological groups. Together with sedimentological and petrographic evidence, this confirms that the concretions formed in situ within the sediment shortly after deposition and prior to slumping. Hydrate dissociation, seepage of methane-charged fluids and the resultant slope destabilization was not an isolated event, but a continuous process occurring diachronously in front of N-prograding accretionary prism during Oligocene, at least from early NP23 to late NP25 biozones.The stable C and O isotope analysis of carbonate concretions was crucial for this research, because the low δ13C and high δ18O values were the only preserved indicators of anaerobic oxidation of methane and dissociation of gas hydrates in the basin, respectively. Because, the concretions are composed of a mixture of dolomite and calcite, a semiautomatic method for measuring C and O isotope composition selectively for calcite and dolomite (Baudrand et al., 2012) was applied, which provided conclusive results. These measurements were combined with detailed petrographic investigations, which enabled assessment of isotopic composition for various carbonate constituents separately, e.g. sedimentary calcite (biogenic and detrital), early and late calcite cements, dolomite cement. They showed that the concretionary microcrystalline calcite was the main cement that precipitated due to hydrate dissociation and subsequent methane oxidation. This approach can provide valuable information when applied to other authigenic carbonates composed of a mixture of dolomite and calcite.

Fluid compartmentalization of Devonian and Mississippian dolostones, Western Canada Sedimentary Basin: petrologic and geochemical evidence from fracture mineralization

Integrated petrographic, geochemical, and fluid inclusion study of fracture mineralization and associated host rock in selected Mississippian and Devonian carbonates extending from southeastern Alberta to northwestern British Columbia, Canada, aims to quantify the type and nature of fluid precipitated saddle dolomite and late calcite cement and their origin. Petrographic and isotopic evidence from both the Devonian and Mississippian fracture-filling carbonates indicate the presence of a hydrothermal fluid source. The δ18O isotopic values for the Devonian saddle dolomite (−14.62‰ to −3.75‰ VPDB, average −11.12‰) combined with enriched 87Sr/86Sr isotopic ratios (0.70827–0.71599, average 0.71006) and higher homogenization temperatures (Th = 74–194.6 °C, average 126.8 °C) and salinity values (7.7–26.6 wt.% NaCl, average 16.2 wt.% NaCl) show significant differences from the Mississippian saddle dolomite, which is characterized by less negative δ18O isotopic values (−12.53‰ to −7.82‰ VPDB, average −9.14‰), less radiogenic 87Sr/86Sr isotopic ratios (0.70859–0.70943, average 0.70887), and lower homogenization temperatures (Th) and salinity values of fluid inclusions (87.6–214.2 °C, average 136.3 °C; 2.0–13.2 wt.% NaCl, average 9.6 wt.% NaCl). Later fracture- and vug-rimming blocky calcite cement records comparable or slightly lower values of δ18O (−16.31‰ to −4.08‰ VPDB, average −9.76‰) and 87Sr/86Sr (0.70784–0.709743, average 0.70868) and much lower salinity values (0–22.5 wt.% NaCl, average 2.86 wt.% NaCl) for samples mostly from the Mississippian age group. These results possibly suggest two different hydrothermal episodes related to early (Antler) and late (Laramide) tectonic events that affected the Western Canada Sedimentary Basin with possible compartmentalization of hydrothermal systems and their associated brines in the basin.

Activation of stylolites as conduits for overpressured fluid flow in dolomitized platform carbonates

Abstract This study investigates the Late Aptian–earliest Albian platform carbonates of the Benicàssim area (Maestrat Basin, Spain) in order to assess the relationship between bed-parallel stylolites and the flow of diagenetic fluids during dolomitization and subsequent hydrothermal alteration. Dolostones and burial dolomite and calcite cements were studied by a combination of field geology and standard petrographic and isotope analysis. Field data indicate that dolostones are closely associated with seismic-scale synsedimentary faults, preferentially replace grain-dominated facies and typically show wavy dolomitizing fronts that mostly correspond to bed-parallel stylolites. The dolostones are corroded and contain bed-parallel pores that are filled with hydrothermal saddle dolomite and blocky calcite cements. This late calcite cement frequently engulfs clasts of the host dolostones, suggesting that hydraulic brecciation likely associated with overpressured fluid occurred. Results indicate that stylolites play a key role in the distribution of dolostones and subsequent hydrothermal mineralization. During the replacement stage, stylolites acted as baffles for the dolomitzing fluids controlling lateral fluid flow and resulting in the stratabound dolostone distribution. During the post-dolomitization stage, stylolites became preferred pathways for overpressured hydrothermal corrosive and mineralizing fluids that likely came from the underlying basement, and increased bed-parallel stylolitic porosity and probably also permeability.

鄂尔多斯盆地南部富县地区三叠系延长组长8油层组油气成藏期次与时限

多旋回叠合盆地的油气成藏期次与时限研究是成藏地质学研究热点及前缘领域。鄂尔多斯盆地延长组长8油层组作为深层油气勘探的重点层位，其成藏期次与时限鲜有报道。根据含油气砂岩样品的包裹体薄片岩相学与荧光分析分析，研究区存在两期不同产状的烃类流体包裹体(FI)，第一期烃类FI呈带状分布在石英加大边、石英矿物早期裂缝中，显示褐色或浅绿色荧光；第二期烃类FI分布在切穿石英及其加大边裂缝，或晚期方解石胶结物，显示浅黄色或黄绿色荧光。根据与烃类FI同期的盐水FI均一温度的测试分析，早期FI均一温度分布在85℃~135℃，峰值接近105℃；晚期分布在135℃~155℃，峰值接近145℃。结合构造热演化史，该区存在两期幕式充注成藏过程，主要成藏时限为164 Ma和 127 Ma。为盆地延长组油气成藏机理及过程研究提供了新认识。 Timing and stages of oil-gas accumulation (OGA) are hot spots and key frontier for petroleum accumulation geology in multi-cycle superimposed basin. As the important deep oil-gas reservoir of exploration, timing and stages of OGA of Chang 8 member of Yanchang formation in Ordos Basin are rarely reported. According to the lithofacies and fluorescence analysis of hydrocarbon fluid inclusions (FI) from sandstone samples, there are two different occurrence fluid inclusions for Chang 8 member of Yanchang formation in the study area. The first occurrence of hydrocarbon FI is banded in the quartz plus side, quartz mineral early cracks, showing brown or light green fluorescence. The second occurrence of the hydrocarbon FI is distributed in the cut-off quartz and its increased fissures, or late calcite cements, showing light yellow or yellowish green fluorescence. According to the test analysis of the homogeneous temperature of the salt water FI with the hydrocarbon FI, the early FI homogeneous temperature distribution is in a range of (85˚C - 135˚C) with the peak of 105˚C; the late distribution is in a range of (135˚C - 155˚C) with the peak of 145˚C. Combined with the history of tectonic thermal evolution, it reveals that the Chang 8 reservoirs went through 2-stage OGA of 164Ma and 127 Ma, which provides a new understanding for the mechanism and process of hydrocarbon accumulation in the Yanchang Formation.

A comprehensive review of hydrocarbons and genetic model of the sandstone‐hosted Dongsheng uranium deposit, Ordos Basin, China

AbstractThe Dongsheng uranium deposit, the largest in situ leach uranium mine in the Ordos Basin, geometrically forms a roll‐front type deposit that is hosted in the Middle Jurassic Zhiluo Formation. The genesis of the mineralization, however, has long been a topic of great debate. Regional faults, epigenetic alterations in surface outcrops, natural oil seeps, and experimental findings support a reducing microenvironment during ore genesis. The bulk of the mineralization is coffinite. Based on thin‐section petrography, some of the coffinite is intimately intergrown with authigenic pyrite (ore‐stage pyrite) and is commonly juxtaposed with some late diagenetic sparry calcite (ore‐stage calcite) in primary pores, suggesting simultaneous precipitation. Measured homogenization temperatures of greater than 100°C from fluid inclusions indicate circulation of low‐temperature hydrothermal fluids in the ore zone. The carbon isotopic compositions of late calcite cement (δ13CVPDB = −31.0 to −1.4‰) suggest that they were partly derived from sedimentary organic carbon, possibly from deep‐seated petroleum fluids emanating from nearby faults. Hydrogen and oxygen isotope data from kaolinite cement (δD = −133 to −116‰ and δ18OSMOW = 12.6–13.8‰) indicate that the mineralizing fluids differed from magmatic and metamorphic fluids and were more depleted in D (2H) than modern regional meteoric waters. Such a strongly negative hydrogen isotopic signature suggests that there has been selective modification of δD by CH4±H2S±H2 fluids. Ore‐stage pyrite lies within a very wide range of δ34S (−39.2 to 26.9‰), suggesting that the pyrite has a complex origin and that bacterially mediated sulfate reduction cannot be precluded. Hydrocarbon migration and its role in uranium reduction and precipitation have here been unequivocally defined. Thus, a unifying model for uranium mineralization can be established: Early coupled bacterial uranium mineralization and hydrocarbon oxidation were followed by later recrystallization of ore phases in association with low‐temperature hydrothermal solutions under hydrocarbon‐induced reducing conditions.

Origin and distribution of hydrogen sulfide in the Yuanba gas field, Sichuan Basin, Southwest China

The Yuanba gas field in the Permian Changxing Formation (P2c), which exhibits wide variations in its hydrogen sulfide (H2S) concentration (1.20–12.16%), is a typical sour gas field in the northern Sichuan Basin. The sulfur-rich reservoir's solid bitumen (atomic S/C ratios are 0.032–0.142), and late calcite cement δ13C values, which are smaller than the δ13C values of the host dolostone, indicate that the H2S originated from thermal sulfate reduction (TSR) and oil was involved in TSR. The gas souring index (GSI) of P2c's gases is generally lower than 0.1. The ethane δ13C values increase as the GSI increases, although no obvious increase was observed in the methane δ13C values. The calcite cements' δ13C values (−15.36 to +4.56‰) in dolostone are heavier than the typical reported values, which implies that only limited heavy hydrocarbon gases were involved in TSR. No anhydrites developed in P2c's reservoirs, and dissolved sulfate anions (SO42−) were mainly enriched during dolomitization. Insufficient dissolved SO42− most likely caused the lower H2S concentrations in the Permian to Triassic reservoirs in the northeastern Sichuan Basin compared to the Permian Khuff Formation in Saudi Arabia and the Jurassic Smackover Formation in Mississippi. Except for the SO42− in residual water in paleo-oil zones, SO42− from bottom water may also be involved in TSR; therefore, oil reservoirs with bottom water have more SO42− and can produce more H2S than pure oil reservoirs. This phenomenon may be the main cause of the great difference in the H2S concentrations between reservoirs, while gravitational differentiation during late uplift most likely creates differences in H2S concentrations in a single reservoir. Carbon dioxide (CO2), which has a relatively heavy δ13C value (−3.9 to −0.3‰), may be the combined result of TSR, the balance between CO2 and inorganic fluid systems, and carbonate decomposition.

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

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

Geochemistry of Phanerozoic diagenesis on Victoria Island, NWT, Canada

Victoria Island, in Arctic Canada, is one of the largest islands in the world, but its geology has remained largely unmapped and unstudied owing to its remoteness. Base-metal and hydrocarbon showings have been reported from the island, but the origin and prospectivity of these showings remain enigmatic. Regionally extensive, void-filling Phanerozoic diagenetic phases (dolomite, calcite, quartz) are conspicuous in two of the most widespread carbonate units, the Neoproterozoic Wynniatt Formation and the Cambro–Ordovician Victoria Island formation, recording repeated post-depositional movement of fluid through the rocks. These phases were studied to (a) determine whether they could have been associated with movement of metalliferous or petroliferous fluids, (b) establish a diagenetic base-line for a very large and economically unexplored area, and (c) determine whether the fluid-flow events could have been related to known episodes of deformation and mineralisation elsewhere in the Canadian Arctic archipelago. Using an innovative protocol combining in situ SIMS analysis of O and S isotopes with LA-ICP-MS trace-element analysis, the geochemical conditions attending the diagenetic evolution of host dolostone and cement phases were determined. The Wynniatt Formation dolostone contains four Phanerozoic cements: saddle dolomite, brown dolomite, replacive calcite and late calcite. Average δ18O (SMOW) values of Wynniatt Formation saddle and brown dolomite, replacive calcite, and late calcite cements are 24.7‰, 7.7‰, and 6.9‰, respectively. The PAAS-normalised rare earth element patterns of the dolostone, dolomite, and replacive calcite are smooth and flat with slightly positive Ce and Y anomalies and MREE-enrichment; late calcite cement, in contrast, has negative Ce and positive Y anomalies. Recrystallisation of the host dolostone by a reduced, saddle-dolomite-precipitating fluid in a fluid-dominated system altered the isotopic and REE pattern of the dolostone to saddle-dolomite values. This fluid had interacted with shale at depth and then mixed with a relatively high-salinity fluid. Brown dolomite precipitated from this fluid after a decrease in salinity, and oxygenated meteoric water later infiltrated the system and precipitated the calcite cements.The Victoria Island formation dolostone contains two cements: quartz and dolomite. Host dolostone, quartz, and dolomite have average δ18O (SMOW) values of 31.7‰, 18.7‰, and 18.6‰, respectively, whereas δ34S (V-CDT) for framboidal pyrite in the quartz cement averages −7.5‰. The host dolostone has flat PAAS-normalised REE patterns with a positive Eu anomaly, no Ce anomaly, and a zig-zag HREE pattern, whereas the dolomite cements have two REE patterns: one similar to the dolostone (flat) and the other with LREE-enrichment, and negative La and Eu anomalies. These data suggest that a reduced, hydrothermal fluid altered and partially silicified the host dolostone. Quartz precipitated from a seawater-sourced fluid that had incorporated metals at depth and mixed with bacterially reduced sulphur at the site of precipitation. The dolomite cement records the change from a rock-dominant system to a fluid-dominant system. This fluid was sourced from seawater and interacted with REE-phosphate minerals.The results suggest that precipitation of the diagenetic cements took place before peak burial associated with the Ellesmerian Orogeny (late Devonian–early Carboniferous). The most probable cause of fluid migration, therefore, is the Ellesmerian Orogeny for the majority of the diagenetic phases. High-latitude meteoric fluid then infiltrated the system at some time after the Ellesmerian Orogeny. Similar ages, temperatures, compositions, and precipitation mechanisms as those associated with the Cornwallis zinc district (Polaris mine), highlight a possible relation to known base-metal mineralising fluids.The results of this study provide much needed baseline knowledge of the post-depositional history of Victoria Island. It also illustrates that a “less-is-more” approach, using detailed micro-analytical methods on carefully selected and well-constrained samples, allows one to decipher a complex diagenetic history involving the mixing of fluids from different reservoirs and frequent changes in redox state, both of which may be overlooked using conventional macro-scale approaches.

Determination of Hydrocarbon Charging History by Diagenetic Sequence and Fluid Inclusions: A Case Study of the Kongquehe Area in the Tarim Basin

Petrographic analysis of hydrocarbon inclusions in reservoirs is the basis and prerequisite for study of hydrocarbon charge history using fluid inclusion analysis. Samples from Silurian reservoirs in the Kongquehe area were studied with microscopy, cathode luminescence and scanning electron microscopy, and the paragenetic sequence of diagenetic events was established. Aqueous and oil inclusions were found in four different occurrences, i.e., ① in healed cracks in detrital quartz grains, ② in quartz overgrowths that were formed relatively early in diagenesis, ③ in healed cracks crosscutting quartz overgrowths and detrital quartz, and ④ in paragenetically late calcite cements. Solid bitumens were found in intergranular pores and in late fractures, whereas gas inclusions occur in healed cracks crosscutting quartz overgrowths and detrital quartz. The homogenization temperatures of aqueous (Thaq) and oil incluisons (Tho) within individual fluid inclusion assemblages are very consistent, suggesting that the microthermometric data are reliable. The Thaq values are generally larger than Tho, indicating the oil charging events took place at significant depths. The results suggest that there were at least two episodes of hydrocarbon charging in the Kongquehe area: the early hydrocarbon charging occurred in late Caledonian, dominated by oil, and the late hydrocarbon charging occurred in the Yanshan‐Himalayan, first by oil and then gases. In addition, two episodes of hydrocarbon reservoir adjustment and destruction occurred in the Hercynian and Himalayan, respectively, forming solid bitumen.

Fault-controlled and stratabound dolostones in the Late Aptian–earliest Albian Benassal Formation (Maestrat Basin, E Spain): Petrology and geochemistry constrains

Fault-controlled hydrothermal dolomitization of the Late Aptian to earliest Albian Benassal Fm shallow water carbonates resulted in the seismic-scale stratabound dolostone geobodies that characterize the Benicàssim case study (Maestrat Basin, E Spain). Petrological and geochemical data indicate that dolomite cement (DC1) filling intergranular porosity in grain-dominated facies constituted the initial stage of dolomitization. The bulk of the dolostone is formed by a replacive nonplanar-a to planar-s dolomite (RD1) crystal mosaic with very low porosity and characteristic retentive fabric. Neomorphic recrystallization of RD1 to form replacive dolomite RD2 occurred by successive dolomitizing fluid flow. The replacement sequence DC1-RD1-RD2 is characterized by a depletion in the oxygen isotopic composition (mean δ18O(V-PDB) values from −6.92, to −8.55, to −9.86‰), which is interpreted to result from progressively higher temperature fluids. Clear dolomite overgrowths (overdolomitization) precipitated during the last stage of replacement. Strontium isotopic composition suggests that the most likely origin of magnesium was Cretaceous seawater-derived brines that were heated and enriched in radiogenic strontium and iron while circulating through the Paleozoic basement and/or Permo-Triassic red beds. Burial curves and analytical data indicate that the replacement took place at burial depths between 500 and 750 m, and by hydrothermal fluids exceeding temperatures of 80 °C. Following the partial dolomitization of the host rock, porosity considerably increased in dolostones by burial corrosion related to the circulation of acidic fluids derived from the emplacement of the Mississippi Valley-Type deposits. Overpressured acidic fluids circulated along faults, fractures and open stylolites. Saddle dolomite and ore-stage calcite cement filled most of the newly created vuggy porosity. Subsequent to MVT mineralization, precipitation of calcite cements resulted from the migration of meteoric-derived fluids during uplift and subaerial exposure. This late calcite cement destroyed most of the dolostone porosity and constitutes the main cause for its present day poor reservoir quality.

The origin and formation model of Permian dolostones on the northwestern margin of Junggar Basin, China

This study investigates the mechanism of dolostone formation and establishes a dolomitization model in the Permian strata on the northwestern margin of Junggar Basin, China. Dolomitic rock samples are collected from the Permian Fengcheng Formation in Urho area and then characterized by petrological, mineralogical, carbon and oxygen isotope, and trace element geochemical analyses. Results show that the major types of dolomitic rocks include dolomitic mudstone, dolomitic siltstone, dolomitized tuffaceous siltstone, and dolomitized tuffaceous mudstone. The dolomitized rocks are dominated by euhedral or subhedral powder- and fine-crystal dolomites formed by replacement lacustrine argilla-calcareous and siliceous (tuffaceous) components and commonly filled with residual and late calcite cements. The parameters of dolomitic rocks show great variations, including the V/Ni ratio (1.02–4.88), Sr content (95.9–783.6μg/g), Mg/Ca ratio (0.68–5.13), degree of ordering (0.39–1.00), δ18OPDB (−14.8‰ to 3.2‰), and δ13CPDB (−1‰ to 5.2‰). The dolomitic rocks have multi-stage origins and were formed in a semi-closed continental brackish-saltwater bay with weak hydrodynamic processes, deep water bodies, and relatively quiet conditions. In the Permian depositional stage, a combination of complex tectonic activities, fault development, hot subtropical climate, and frequent volcanic activities provided not only Mg2+ source for dolomitization but also channels for rapid flow and seepage of Mg-rich fluids. The origins of dolostones in the study area include penecontemporaneous dolomitization, burial dolomitization, and hydrothermal dolomitization. This study lays a foundation for further studies on dolomite formation and dolomite reservoir, and provides effective methods for researching complex dolostone (tuffaceous, shale and silty dolomite) formation.

Diagenetic evolution and associated mineralization in Middle Devonian carbonates, southwestern Ontario, Canada

Abstract The Middle Devonian Lucas Formation of southwestern Ontario, Canada, forms the main hydrocarbon reservoir rocks in the region. Integrated field, petrographic, fluid inclusion and isotope geochemistry reveal a variety of diagenetic processes that include the formation of dolomite, calcite, celestine and fluorite under diverse geochemical conditions. The host carbonate strata display two types of replacive dolomite fabrics: 1) fine-crystalline matrix dolomite (5–20 μm); and 2) medium-crystalline dolomite (50–150 μm) with an average size of 100 μm. Stable oxygen and carbon isotope results for both dolomite fabrics show a wide range of values (δ18O = −9.4‰ to −3.9‰ VPDB, δ13C = 0.4‰ to 3.4‰ VPBD). For late fracture-filling calcite, δ 18O and δ 13C values range from −9.4 to −5.9‰ VPDB and −7.7‰ to −0.04‰ VPDB, respectively. The 87Sr/86Sr ratios of selected dolomite and calcite samples range from 0.70798–0.70838, preserving Middle Devonian seawater values while some samples have slightly more radiogenic ratios than contemporaneous seawater. Field and petrographic evidence and Sr isotope ratios of dolomite suggest early dolomitization from evaporated seawater in a sabkha setting (shallow seepage reflux or evaporitive pumping). However, the depletion in δ18O (varying from −3.9 to −9.4‰ VPDB), and radiogenic Sr isotope values of the Lucas Formation dolomite are considered to be the result of recrystallization, which is interpreted to occur at elevated temperatures in a burial environment. The 18O-depleted values and slightly radiogenic 87Sr/86Sr ratios of calcite cements suggest precipitation from hot, saline basinal/hydrothermal brines. In addition, the 13C-depleted calcites indicate that carbon was possibly formed by oxidation of organic matter. Fluid inclusions in some of the late calcite cements fluoresce under UV light, indicating the presence of hydrocarbons. Homogenization temperatures of these inclusions vary from 87.1°C to 171.9°C with an average value of 126°C. Average salinity of fluid inclusions is 19.7 wt. % NaCl eq. Celestine samples have lower salinities relative to calcite, averaging 14.5 wt. % NaCl eq. with an average homogenization temperature of 219.5°C. Based on the geochemical and fluid inclusion results, hot, saline basinal/hydrocarbon bearing brines originated from central part of the basin and migrated to Devonian carbonates via fractures and faults. Celestine is a product of thermochemical sulfate reduction and formed due to mixing of saline, Sr and sulfate bearing brines and meteoric waters.

Dolomite characteristics and diagenetic model of the Calcari Grigi Group (Asiago Plateau, Southern Alps – Italy): an example of multiphase dolomitization

AbstractA multidisciplinary study, conducted over the carbonate platform deposits of the Liassic Calcari Grigi Group (Southern Alps), highlighted how the use of outcrop analogues can contribute to better define the distribution of dolomitic bodies related to fault networks, to characterize the petrophysical properties of the dolomitic sequence and unravel a complex diagenetic history. This study was carried out in the Asiago Plateau (southernmost part of the eastern Southern Alps, northern Italy) which provides excellent outcrops of the Jurassic Calcari Grigi Group. The dolomitization of the Jurassic sequence is variable in terms of stratigraphic extension and geographic distribution. In the studied localities the dolomitization is generally limited to the Mount Zugna Formation and is characterized by an undulatory front, with ‘sub‐vertical dolomitic chimneys’ along the major faults. Within this unit, and often associated with faults, stacked high‐porosity and permeability bed‐parallel dolomitic bodies are developed that show excellent petrophysical properties. The dolomitic intervals are characterized by pervasive unimodal and patchy polymodal dolomite crystals. Thin section, cathodoluminescence, isotopic and fluid inclusion analyses were used to constrain the paragenetic evolution of the sequence which is similar in all the studied localities. The first dolomitization stage is marked by zoned dolomite crystals with a dull luminescent core. The porosity is thought to have increased after this stage, with dark blue luminescent dolomite accompanied by the corrosion of older crystals. The appearance of saddle dolomite marks the onset of the porosity reduction stage, ending with the infilling of vugs and the remaining open pores with calcite cement. The diagenetic evolution locally stopped at the saddle dolomite stage with the complete occlusion of the remaining pores. Paragenetic and fluid‐inclusion data suggest an evolutionary trend of increasing temperatures and decreasing salinity toward brackish fluids responsible for dolomite and calcite precipitation. The integration of the available data seem to indicate that the diagenetic evolution of the study area is related to: (i) the interplay between evolving fluids (from marine to brackish); (ii) the burial of the sequence (increasing temperature); and (iii) the evolution of the hydrogeological system (fault and fracture network, fluid mixing). This complex paragenetic evolution is strongly linked to the evolution of the porosity framework that evolved from a good, widespread network in the early stages of the burial history to a confined system in the later stages due to reduction of porosity by the deposition of late calcite and dolomite cements.

Stable carbon and oxygen isotope signatures of pedogenic needle fibre calcite

Carbon and oxygen isotope compositions of the needle fibre calcite (NFC) from two soils observed in quarries (Villiers and Savagnier) in the Swiss Jura Mountains are used to investigate the processes involved in the formation of these calcite crystals in pedogenic environments. NFC occurs in soils with different macroscopic and microscopic morphologies. Three groups of microscopic morphologies are distinguished: simple needles (SN), simple needles with overgrowths (SNO), and simple needles with nanofibres (SNN), nanofibres being either organic or mineral in nature. These groups correspond to different stages in the formation and evolution of NFC. Evaluation of the δ 13C and δ 18O values of NFC and late calcite cements (LCC) in relationship to the composition of the carbonate host rock (CHR), and δ 13C DIC from the soil waters, indicate that both NFC and LCC are precipitated in isotopic equilibrium with the soil waters. Similar δ 13C and δ 18O values of the elongated NFC and the rhombohedral calcite crystals of the LCC suggest that the precipitation of these different calcite habits is not due to changes in physicochemical conditions. The growth of NFC crystals inside an organic mould formed by the fungal hyphae could explain the formation of calcite crystals in the shape of a needle in isotopic equilibrium with the local environment. The comparison of the δ 13C and δ 18O signatures of the various NFC morphological groups suggests that they likely evolve in the soil solution from calcite crystals liberated during the degradation of fungal hyphae walls. The NFC may be a proxy of the soil conditions (temperature and vegetation) during its precipitation, and also an indicator of biogenic (fungal) activity.