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1.
The saddle dolomites occur more intensely in cores closely to fault than that in cores far away from the fault in Upper Cambrian carbonate of western Tarim basin, suggesting that formation of the saddle dolomites is likely related to fault-controlled fluid flow. They partially fill in fractures and vugs of replacement dolomite. The saddle dolomites exhibit complex internal textures, commonly consisting of core and cortex. In comparison with the matrix dolomites, the saddle dolomites show lower Sr-content and 87Sr/86Sr ratios, higher Fe- and Mn-content, and more negative δ18O values. Combined with high Th (100–130 °C) of primary fluid inclusions, it is suggested that the saddle dolomites precipitated from hydrothermal fluid derived from the deep evaporite-bearing Middle Cambrian strata, and the magnesium source may be due to dissolution of host dolomite during hydrothermal fluid migration. Fault activity resulted in petrographic and geochemical difference of the core and cortex of the saddle dolomites. The cores precipitated from the formation water mixed by deep brines at the early stage of fault activity, and the cortexes precipitated from the deep fluid with higher temperatures through the Middle Cambrian later. In summary, the formation of the saddle dolomites implies a hydrothermal fluid event related to fault activity, which also resulted in high porosity in Upper Cambrian carbonate in western Tarim Basin.  相似文献   

2.
Field observations indicate that tectonic compression, anticline formation and concomitant uplift events of marine Paleogene carbonates in eastern United Arab Emirates, which are related to the Zagros Orogeny, have induced brecciation, karstification, and carbonate cementation in vugs and along faults and fractures. Structural analysis, stable isotopes and fluid inclusion microthermometry are used to constrain the origin and geochemical evolution of the fluids. Fluid flow was related to two tectonic deformation phases. Initially, the flux of moderately 87Sr-rich basinal NaCl–MgCl2–H2O brines along reactivated deep-seated strike-slip faults have resulted in the precipitation of saddle dolomite in fractures and vugs and in dolomitization of host Eocene limestones (δ18OV-PDB −15.8‰ to −6.2‰; homogenization temperatures of 80–115 °C and salinity of 18–25 wt.% eq. NaCl). Subsequently, compression and uplift of the anticline was associated with incursion of meteoric waters and mixing with the basinal brines, which resulted in the precipitation of blocky calcite cement (δ18OV-PDB −22‰ to −12‰; homogenization temperatures of 60–90 °C and salinity of 4.5–9 wt.% eq. NaCl). Saddle dolomite and surrounding blocky calcite have precipitated along the pre- and syn-folding E–W fracture system and its conjugate fracture sets. The stable isotopes coupled with fluid-inclusion micro-thermometry (homogenization temperatures of ≤50 °C and salinity of <1.5 wt.% eq. NaCl) of later prismatic/dogtooth and fibrous calcites, which occurred primarily along the post-folding NNE–SSW fracture system and its conjugate fracture sets, suggest cementation by descending moderately 87Sr-rich, cool meteoric waters. This carbonate cementation history explains the presence of two correlation trends between the δ18OV-PDB and δ13CV-PDB values: (i) a negative temperature-dependent oxygen isotope fractionation trend related to burial diagenesis and to the flux of basinal brines, and (ii) positive brine-meteoric mixing trend. This integrated study approach allows better understanding of changes in fluid composition and circulation pattern during evolution of foreland basins.  相似文献   

3.
The central part of the Zagros Fold-Thrust Belt is characterized by a series of right-lateral and left-lateral transverse tear fault systems, some of them being ornamented by salt diapirs of the Late Precambrian–Early Cambrian Hormuz evaporitic series. Many deep-seated extensional faults, mainly along N–S and few along NW–SE and NE–SW, were formed or reactivated during the Late Precambrian–Early Cambrian and generated horsts and grabens. The extensional faults controlled deposition, distribution and thickness of the Hormuz series. Salt walls and diapirs initiated by the Early Paleozoic especially along the extensional faults. Long-term halokinesis gave rise to thin sedimentary cover above the salt diapirs and aggregated considerable volume of salt into the salt stocks. They created weak zones in the sedimentary cover, located approximately above the former and inactive deep-seated extensional faults. The N–S to NNE–SSW direction of tectonic shortening during the Neogene Zagros folding was sub-parallel with the strikes of the salt walls and rows of diapirs. Variations in thickness of the Hormuz series prepared differences in the basal friction on both sides of the Precambrian–Cambrian extensional faults, which facilitated the Zagros deformation front to advance faster wherever the salt layer was thicker. Consequently, a series of tear fault systems developed along the rows of salt diapirs approximately above the Precambrian–Cambrian extensional faults. Therefore, the present surface expressions of the tear fault systems developed within the sedimentary cover during the Zagros orogeny. Although the direction of the Zagros shortening could also potentially reactivate the basement faults as strike-slip structures, subsurface data and majority of the moderate-large earthquakes do not support basement involvement. This suggests that the tear fault systems are detached on top of the Hormuz series from the deep-seated Precambrian–Cambrian extensional faults in the basement.  相似文献   

4.
Thick Upper Cambrian-Lower Ordovician carbonates were deposited on a shallow marine platform in the northern Tarim Basin, which were extensively dolomitized, particularly for the Upper Cambrian carbonates. The resulting dolomite rocks are predominantly composed of matrix dolomites with minor cement dolomites. Based on petrographic textures, matrix dolomites consist of very finely to finely crystalline, nonplanar-a to planar-s dolomite (Md1), finely to medium crystalline, planar-e(s) dolomite (Md2), and finely to coarsely crystalline, nonplanar-a dolomite (Md3). Minor cement dolomites include finely to medium crystalline, planar-s(e) dolomite (Cd1) and coarsely crystalline, nonplanar saddle dolomite (Cd2), which partially or completely fill dissolution vugs and fractures; these cements postdate matrix dolomites but predate later quartz and calcite infills. Origins of matrix and cement dolomites and other diagenetic minerals are interpreted on the basis of petrography, isotopic geochemistry (O, C and Sr), and fluid inclusion microthermometry. Md1 dolomite was initially mediated by microbes and subsequently precipitated from slightly modified brines (e.g., evaporated seawater) in near-surface to very shallow burial settings, whereas Md2 dolomite was formed from connate seawater in association with burial dissolution and localized Mg concentration (or cannibalization) in shallow burial conditions. Md3 dolomite, however, was likely the result of intense recrystallization (or neomorphism) upon previously-formed dolomites (e.g., Md1 or Md2 dolomite) as the host carbonates were deeply buried, and influenced by later hydrothermal fluids. Subsequent cement dolomite and quartz crystals precipitated from higher-temperature, hydrothermal fluids, which were contributed more or less by the extensive Permian large igneous province (LIP) activity in Tarim Basin as evidenced by less radiogenic Sr in the cement and parts of matrix dolomites. This extensive abnormal hydrothermal activity could also have resulted in recrystallization (or neomorphism) on the previous matrix dolomites. Faults/fractures likely acted as important conduit networks which could have channeled the hydrothermal fluids from depths. However, the basin uplift triggered by the Late Hercynian Orogeny from the Late Permian would have facilitated downward infiltration of meteoric water and dilution of hydrothermal fluids, resulting in precipitation of later calcites in which lighter C and more radiogenic Sr components demonstrate such a switch of fluid properties. This study provides a useful analogue to understand the complicated dolomitizing processes and later hydrothermal alteration intimately related to the Permian LIP activity within Tarim Basin and elsewhere.  相似文献   

5.
At Matienzo (Basque–Cantabrian Basin, northern Spain), a large stratabound HTD body (4 by 2 km2 and 80–400 m thick) delimited by two parallel sinistral strike-slip faults is exposed in Aptian carbonates. The margins of the HTD body are characterised by dolomite “tongues” indicating that some limestone beds were more prone to dolomitisation. However, no clear relationship between HTD occurrence and precursor limestone facies can be established. Massive limestone beds, as found at the top of the HTD body, act as barriers to hydrothermal processes, since no dolomite is present in or above these beds. Three types of dolomites have been differentiated, i.e. 1) matrix, 2) coarse crystalline and 3) zebra dolomite. The distribution of the dolomite types is attributed to ascending fluid flow and changing degree of dolomite oversaturation.The dolomite body was formed by two dolomitisation phases under burial conditions. No indications for a synsedimentary/early diagenetic dolomitisation have been observed. The first dolomitisation phase is characterized by ferroan dolomite and the second by non-ferroan dolomite. The two HTD phases are characterised by depleted δ18O-values (ranging between −10‰ and −16‰ V-PDB), δ13C-values similar to the Aptian–Albian marine signature and homogenisation temperatures of primary fluid inclusions between 120 °C and 150 °C. The dolomitising fluid was enriched in 87Sr compared to Aptian seawater, excluding the latter as an unmodified fluid source for dolomitisation. Microthermometry of primary fluid inclusions indicates that the dolomitising fluid evolved from a moderate saline (9.7 – 14.0 wt% NaCl) to a more saline (10.9 – 21.0 wt% NaCl) H2O–NaCl brine. The dolomitising fluid likely originated from evaporated seawater. Fluid circulation through the dolomitised strata is inferred to have taken place during the tectonically active period of the late Albian throughout which important sinistral-strike slip movements along basement faults occurred.  相似文献   

6.
The lower Ordovician St. George Group in Western Newfoundland consists of a sequence of subtidal and peritidal carbonates, which are extensively dolomitized. The current study investigates the diagenetic evolution of the Catoche Formation from the Port aux Choix and Port au Port peninsulas in order to study the controls on reservoir quality in western Newfoundland. The Catoche Formation dolomites are classified into three main generations. Early and pervasive replacement dolomite (D1) indicates that dolomitization began during early stages of diagenesis. Stable isotope and trace element data indicate significant variations between D1 dolomite on the Port aux Choix and Port au Port peninsulas. The depleted δ18O signature of D1 dolomite fluids (−8.7 ± 1.3‰ VPBD) on the Port aux Choix Peninsula is consistent with partial dolomitization associated with mixing of seawater and meteoric waters on the flanks of structural highs. In contrast δ18O values (−6.1 ± 0.7‰ VPBD) and trace element data from the Port au Port Peninsula indicate that pervasive D1 was associated with mixing of possibly post evaporitic brines with meteoric waters.Later-stage replacement dolomites (D2) are associated with enhancement in porosity through the development of intercrystalline pores, while latest stage saddle dolomite (D3), significantly occluded the pores in some horizons. D2 dolomite formed due to the influx of warm (>100 °C), saline (>15 eq. wt% NaCl) fluids. Intercrystalline porosity in D2 formed due to the dolomitization of precursor calcite, due to the lower molar volume of dolomite compared to calcite. Therefore porosity development is lower on the Port au Port Peninsula, with no significant volume change during the recrystallization of the pervasive early (D1) dolomicrite. Similarly, extensive porous horizons on the Port aux Choix Peninsula are related to the limited extent of D1 dolomitization. This suggests that the quality of a potential dolomite reservoir is strongly controlled by tectonic and diagenetic history of host carbonates.  相似文献   

7.
The rift zone??s relief, the spreading kinematics, and the experimental modeling of the Knipovich Ridge??s formation were analyzed. Its rift zone is formed in a transtension environment. Faulting is predominant in its northern part, while strike-slip is characteristic for the south. A system of short extension basins connected by deep strike-slip U-shaped troughs is observed in the south. A system of volcanic rises connected by short shallow basins is observed in the north. The rift valley is V-shaped. According to the experimental modeling data, these extension kinematics provide the formation of short extension basins connected by strike-slips and transtension faults. Their length and orientation depend on the spreading obliquity of each segment.  相似文献   

8.
Fluid flow in fractures and host rocks has been investigated in shallow buried Miocene alluvial fan deposits. A structural, petrographical (optical, CL, SE microscopes and XRD) and geochemical (microprobe and δ18O-δ13C stable isotopes) study has been performed in normal faults affecting Serravalian-Tortonian siliciclastic rocks of the Vallès-Penedès basin. These faults formed during the development of the Vallès-Penedès fault-related syncline, which caused the rotation of the earliest fractures. Faulting occurred continuously before, during and after host rock cementation. Rocks affected by faulting are represented by clay-rich gouges, which formed thanks to the high phyllite clast content within the otherwise clean and mature sandstones and conglomerates. Despite the low permeability of these rocks, cross-fault and fault-parallel fluid flows occurred in most of the faults.Host rocks and veins were cemented by two generations of calcite, i.e. Cc1 and Cc2. Cc1 precipitated from meteoric waters at shallow burial conditions whereas Cc2 precipitated from meteoric waters in a confined aquifer.Palygorskite has been identified in shear zones within the gouges indicating their later formation by interaction of Mg-rich fluids with previous smectites. These fluids probably derived from Miocene seawater expelled from the underlying Transitional–Marine Complex “TMC” by compaction.Sedimentation, fracturing and cementation occurred in a very short lapse time of about 6–7 Ma, between the Serravalian-Tortonian age of the sediments and the end of the extensional tectonics in the Vallès-Penedès fault (Pliocene).  相似文献   

9.
The Tournasian age Pekisko carbonates in the Normandville Field (northwestern Alberta) form waulsortian-like, bryozoan/crinoid mounds that developed in fairly deep, low energy, cool water systems, close to the ramp margin. Three main depositional environments occur: (1) crinoidal apron with wackestone, grainstone and floatstone facies; (2) mound flank with grainstone, wackestone, packstone and floatstone facies dipping 35°; and (3) bryozoan mound core, composed of rudstone and floatstone facies with fenestrate bryozoa, minor crinoids and carbonate mud. Local highs due to fault-bounded blocks, created from the collapse of the Devonian Peace River High, may have controlled the location of mound nucleation.Diagenesis of the bryozoan/crinoid mounds included calcite cementation, compaction, dolomitization, silicification, and hydrocarbon emplacement events. The mound core facies contains submarine fascicular optic calcite and bladed/prismatic calcite cements, and later ferroan, brightly luminescent, pore-filling blocky spar cement. The crinoid apron facies contains syntaxial cement associated with crinoids, and the ferroan blocky spar cement. The mounds are dominantly limestone; however, in one well, dolomite dominates the lower section. Four types of dolomite have been identified: partial replacive; chemical-compaction-related, pervasive dolomite and saddle dolomite cement. All dolomites are non-stoichiometric (CaCO3 mole% 56.6–62.6). The partial, zoned replacive dolomite replaces micrite and syntaxial rim calcite in mound flank and crinoid apron facies. The chemical compaction-related dolomite is found along dissolution seams and stylolites and has similar CL characteristics to the replacive dolomite. The pervasive dolomite is fabric destructive and has dull cores and bright rims in CL. Saddle dolomite (0.15 mm) has brightly-luminescent, concentric zoning and occurs in vugs and fossil pore spaces.Chemical and isotopic analysis of the bryozoan/crinoid mounds indicate that the original marine signatures in micrite, early cements, some crinoids and brachiopods have been preserved. However, carbon isotopic values for some crinoids, matrix and dolomite show more positive values compared to known Mississippian carbonate values. Recrystallization during shallow burial has reset the oxygen isotopic composition of some crinoids and micrite. Oxygen and carbon isotopic compositions of most dolomites overlap with altered crinoids and early calcite cements. However, saddle dolomites have lighter δ18O values, similar to saddle dolomites from the Devonian Wabamun Group in this area. The isotopic variations in later ferroan calcite cements show an inverted-J trend, possibly due to variable amounts of water-rock interaction. While the Sr-isotopic ratio of submarine calcite cement coincides with that of Mississippian seawater, the later ferroan calcite cement is more radiogenic, indicating a different source of fluids.  相似文献   

10.
The processes involved in the interaction between organic fluids and carbonates, and the resulting effect on reservoir quality during the evolution and maturation of organic matter remain unclear despite the fact that these processes influence the carbon and oxygen isotopic compositions of carbonates. Here, we provide new insights into these processes using data obtained from a detailed analysis of a mixed dolomitic–clastic and organic-rich sedimentary sequence within the middle Permian Lucaogou Formation in the Junggar Basin of NW China. The techniques used during this study include drillcore observations, thin section petrography, scanning electron microscopy (SEM) and electron probe microanalysis, and carbon and oxygen isotope analyses. Oil grades and total organic carbon (TOC) contents represent the amount of oil charging and the abundance of organic fluids within a reservoir, respectively, and both negatively correlate with the whole-rock δ13C and δ18O of the carbonates in the study area, indicating that organic fluids have affected the reservoir rocks. Secondary carbonates, including sparry calcite and dolomite overgrowths and cements, are common within the Lucaogou Formation. Well-developed sparry calcite is present within dark mudstone whereas the other two forms of secondary carbonates are present within the dolomite-rich reservoir rocks in this formation. Comparing thin section petrology with δ13C compositions suggests that the carbon isotopic composition of matrix carbonates varies little over small distances within a given horizon but varies significantly with stratigraphic height as a result of the development of secondary carbonates. The net change in whole-rock δ13C as a result of these secondary carbonates ranges from 1.8‰ to 4.6‰, with the secondary carbonates having calculated δ13C compositions from −18.6‰ to −8.5‰ that are indicative of an organic origin. The positive correlation between the concentration of Fe within matrix and secondary carbonates within one of the samples suggests that the diagenetic system within the Lucaogou Formation was relatively closed. The correlation between δ13C and δ18O in carbonates is commonly thought to be strengthened by the influence of meteoric water as well as organic fluids. However, good initial correlation between δ13C and δ18O of whole rock carbonates within the Lucaogou Formation (resulted from the evaporitic sedimentary environment) was reduced by organic fluids to some extent. Consequently, the δ13C–δ18O covariations within these sediments are not always reliable indicators of diagenetic alteration by organic fluids or meteoric water.The characteristics and δ13C compositions of the sparry calcite within the formation is indicative of a genetic relationship with organic acids as a result of the addition of organic CO2 to the reservoir. Further analysis suggests that both carbonate and feldspar were dissolved by interaction with organic CO2. However, dissolved carbonate reprecipitated as secondary carbonates, meaning that the interaction between organic fluids and dolomites did not directly improve reservoir quality, although this process did enhance the dissolution of feldspar and increase porosity. This indicates that the δ13C and δ18O of secondary carbonates and their influence on whole-rock carbonate isotopic values can be used to geochemically identify the effect of organic fluids on closed carbonate-rich reservoir systems.  相似文献   

11.
Marine dolostones of Carboniferous Huanglong Formation constitute major gas reservoir rocks in eastern Sichuan Basin. However, the investigation with respect to sources of dolomitizing and diagenetic fluids is relatively underexplored. The current study attempts to investigate the REE characteristics of dolomites using seawater normalization standard, and therefore discusses the origins of dolomitizing and diagenetic fluids, on the basis of continuous 47.33-m-long core samples from the second member of Huanglong Formation (C2h2) in eastern Sichuan Basin. Low Th, Sc, and Hf concentrations (0.791 × 10−6, 4.751 × 10−6, and 0.214 × 10−6, respectively), random correlation between total REE concentration (ΣREE) and Fe or Mn abundance, and seawater-like Y/Ho ratios (mean value of 45.612) indicate that the carbonate samples are valid for REE analysis. Based on petrographic characteristics, four dolomite types are identified, including micritic-sized dolomite (type Dol-1), fine-to medium-sized dolomite (type Dol-2), medium-to coarse-sized dolomite (type Dol-3), and coarse-to giant-sized saddle dolomite (type Dol-4). Dol-1 dolomites, characterized by positive Ce anomaly (mean value of 6.398), light REE (LREE) enrichment, and heavy REE (HREE) depletion with mean LREE/HREE ratio of 12.657, show micritic calcite-like REE patterns, indicating seawater origin of their dolomitizing fluids. Dol-1 dolomites were formed in sabkha environment whereas the dolomitizing fluids originated from evaporative brine water due to their micritic crystal sizes and tight lithology. Dol-2 dolomites, particularly subtype Dol-2a barely developing vuggy porosity, also show micritic calcite-like REE patterns, suggesting their dolomitizing fluids were seawater or seawater-derived fluids. This inference is confirmed by low Fe and Mn concentrations, which range from 651 μg/g to 1018 μg/g (mean value of 863 μg/g) and 65 μg/g to 167 μg/g (mean value of 105 μg/g), respectively, whereas homogenization temperatures (Th, mean value of 103 °C) indicate that Dol-2 dolomites were formed under burial environment. Dol-3 dolomites, in form of cements of Dol-2 dolomites, show similar REE patterns to their host minerals (i.e., Dol-2 dolomites), indicating their parent source was possibly derived from Dol-2 dolomites. Dol-3 dolomites have high Fe and Mn concentrations with mean values of 3346 μg/g (ranging from 2897 μg/g to 3856 μg/g) and 236 μg/g (ranging from 178 μg/g to 287 μg/g), respectively, indicating the involvement of meteoric water. Meanwhile, it confirms that the dissolution in Dol-2 dolomites was caused by meteoric water leaching. Positive Eu anomalies (mean value of 1.406) in Dol-4 dolomites, coupled with high homogenization temperatures (mean value of 314 °C), suggest that Dol-4 dolomites precipitated from hydrothermal fluids. High Fe and Mn concentrations (mean values of 2521 μg/g and 193 μg/g, respectively) in Dol-4 dolomites likely results from interactions of hydrothermal fluids with deep burial clastic rocks.  相似文献   

12.
The Palaeozoic sedimentary sequence of the Prague synform (Ordovician–Devonian) in the centre of the Bohemian massif underwent Variscan deformation and thermal overprint events. Variscan veins widespread throughout the sedimentary strata have precipitated from syntectonic aqueous and hydrocarbon-rich fluids. Homogenization temperatures of aqueous inclusions increased from 70 up to 226 °C in the Cambrian rocks underlying the Prague synform. Seawater, modified due to intensive water–rock interaction, was the main fluid component. Fluid flow was limited and restricted to the lithostratigraphic compartments forming a rock-buffered system. Stable isotopic modelling (C, O) and final interpretations of the confined hydrostratigraphic fluid migration was supported by the 87Sr/86Sr ratio in veins and wall rocks. Siliciclastic Cambrian and Ordovician rocks and the associated intersecting veins yielded similar isotopic signatures, and consequently the fluid migration is restricted to layer- and formation-scales. Gradually downwards increasing fluid temperature and compositional changes reflect burial at oil-window conditions. An open fluid system could be expected in proximity of major faults of the Prague syncline and at the top of the sedimentary sequence.  相似文献   

13.
In Zakynthos Island (Greece), authigenic cementation of marine sediment has formed pipe-like, disc and doughnut-shaped concretions. The concretions are mostly composed of authigenic ferroan dolomite accompanied by pyrite. Samples with >80% dolomite, have stable isotope compositions in two groups. The more indurated concretions have δ18O around +4‰ and δ13C values between −8 and −29‰ indicating dolomite forming from anaerobic oxidation of thermogenic methane (hydrocarbon seep), in the sulphate-methane transition zone. The outer surfaces of some concretions, and the less-cemented concretions, typically have slightly heavier isotopic compositions and may indicate that concretion growth progressed from the outer margin in the ambient microbially-modified marine pore fluids, inward toward the central conduit where the isotopic compositions were more heavily influenced by the seep fluid. Sr isotope data suggest the concretions are fossil features, possibly of Pliocene age and represent an exhumed hydrocarbon seep plumbing system. Exposure on the modern seabed in the shallow subtidal zone has caused confusion, as concretion morphology resembles archaeological stonework of the Hellenic period.  相似文献   

14.
In Western Canada Sedimentary Basin (WCSB), large scale and focused fluid flow that caused hydrothermal dolomitization have been suggested with different timing and intensity. In this study, we conducted a petrographic and geochemical comparison between the Middle Devonian Sulphur Point and Slave Point carbonates from northwestern Alberta. The results demonstrate the presence of both an early fluid flow event associated with the Late Devonian to Mississippian Antler Orogeny as well as a later event coincident with the Late Cretaceous to Early Tertiary Laramide Orogeny. Early fluid flow event is characterized by high salinity fluids, and high temperatures and oxygen isotopic values of marine or slightly enriched values, as demonstrated in saddle dolomite from the Slave Point Formation. In contrast, later fluids that caused the precipitation of saddle dolomite in the Sulphur Point formation are characterized by having slightly saline values, comparable homogenization temperatures but more enriched δ18O values and slightly depleted δ13C signatures. Geochemical data indicate that this later hydrothermal fluid was mixture of Middle Devonian brines and radiogenic basement fluids.  相似文献   

15.
Authigenic carbonates are frequently associated with methane cold-seep systems, which extensively occur in various geologic settings worldwide. Of interest is the relation between the fluids involved in their formation and the isotopic signals recorded in the carbonate cements. Along the Northern Apennines foothills (Italy), hydrocarbons and connate waters still seeping nowadays are believed to be the primary sources for the formation of fossil authigenic carbonate found in Plio-Pleistocene marine sediments. Four selected outcrops of dolomitic authigenic carbonates were analysed to compare signature of seeping fluids with fractionation of stable carbon and oxygen isotopes recorded in the carbonate.Along the foothills, deep methane-rich fluids spontaneously rise to the surface through mud volcanoes or are exploited in wells drilled nearby to the fossil Plio-Pleistocene authigenic carbonates. The plumbing system providing fluids to present-day cold seeps was structurally achieved in Late Miocene and Plio-Pleistocene. δ13C values of methane, which vary from −51.9 to −43.0‰ VPDB, indicate that gas composition from the deep hydrocarbon reservoirs is relatively uniform along the foothills. On the contrary, δ13C in fossil authigenic carbonates strongly varies among different areas and also within the same outcrop.The different carbon sources that fed the investigated carbonates were identified and include: thermogenic methane from the deep Miocene reservoirs, 13C-enriched CO2 derived from secondary methanogenesis and microbial methane from Pliocene successions buried in the Po Plain. The δ13C variability documented among samples from a single outcrop testifies that the authigenic carbonates might represent a record of varying biogeochemical processes in the hydrocarbon reservoirs. The sources of stable oxygen isotopes in authigenic carbonates are often ascribed to marine water. Oxygen isotopic fractionation in the dolomite cements indicates that marine pore water couldn't be the sole source of oxygen. δ18O values provide a preliminary evidence that connate waters had a role in the carbonates precipitation. The concomitant occurrence of active cold seepages and fossil record of former plumbing systems suggests that generation and migration of hydrocarbons are long-lasting and very effective processes along the Northern Apennines foothills.  相似文献   

16.
The Anisian–Ladinian Latemar platform, northern Italy, presents a spectacularly exposed outcrop analogue for dolomitized carbonate reservoirs in relation to fracture-controlled igneous intrusions. Although the Latemar is one of the best studied carbonate platforms worldwide, timing and evolution of dolomitization and the link to fractures and dikes have not been explored in detail. Previous dolomite observations are based on a stratigraphically limited portion of the platform. This study extends observations to the complete exposed interval in which dolomite bodies occur, including those within the less accessible Valsorda valley.Numerous parallel mafic dikes crosscut the Latemar platform and border several of its large dolomite bodies (50 m wide, 100 m high). Within dikes and along dike-carbonate contacts, there are abundant dolomite veins that are geochemically related to surrounding dolomite bodies. Dolomitization is the result of limestone interaction with hydrothermal fluids delivered along these dikes. At dike boundaries, impermeable marble aureoles exist derived from contact metamorphism. The marble aureoles have locally shielded surrounding limestone from dolomitizing fluid. Dolomite occurs only where the ‘protective’ marble is missing or crosscut by fractures. Based on geometric relationships, we conclude that dikes and their damage zones formed the pathways for the dolomitizing fluids and functioned as boundaries for dolomite bodies.From field observations and petrography, we established a detailed paragenesis. Dolomitization started shortly after dike emplacement. There is an evolution in the Fe content of matrix dolomite and dolomite veins, from highly ferroan dolomite to non-ferroan (saddle) dolomite, alternating with episodes of silica cementation. Non-ferroan calcite precipitation followed dolomitization, possibly indicating concurrent depletion in Mg. This stage likely resulted in further limestone recrystallization rather than dolomitization. Stable and radiogenic isotopes suggest that the dolomitizing fluid comprised Carnian seawater with elevated Fe and Mg from interaction with other lithologies (possibly the nearby Predazzo intrusion).  相似文献   

17.
Methane-derived rocks in Monferrato and the Tertiary Piedmont Basin (NW Italy) consist of seep carbonates, formed by gas seepage at the seafloor, and macroconcretions resulting from the cementation of buried sediments crossed by gas-rich fluids. These rocks are characterized by both negative δ13C values and a marked enrichment in δ18O. Petrographic features not commonly described and that point to enigmatic depositional and diagenetic conditions have been observed in both types of rocks: inhomogeneous distribution of cements within cavities; dolomite crystals floating within cavity-filling calcite spar; non-gravitational fabrics of internal sediments plastering cavity walls; open framework within microbial crusts. These features suggest the former presence of gas hydrates in sediments. During their dissociation, new space was formed and filled with authigenic carbonates or injected sediments. Analogous mechanisms of clathrate freeze-and-thaw processes have been inferred for the genesis of zebra and stromatactis structures and particular kinds of carbonate breccias. The term melt-seal structure is proposed for this kind of diagenetic structure. The fabrics of gas hydrates and the geochemical conditions of sediments, in turn depending on the relative rates of supply of methane-rich fluids and normal seawater, conditioned the final aspect of the rocks.  相似文献   

18.
In the paper “Formation mechanism of deep Cambrian dolomite reservoirs in the Tarim basin, northwestern China” (Zhu et al., 2015), we concluded that hydrothermal alteration further enhanced porosity in the deep Cambrian dolomite reservoirs in the Tarim Basin, NW China. Professor Ehrenberg and Bjørlykke made comments that the hydrothermal dolomitization and increase in porosity were not well supported and casted doubt on the exploration potential. We insist the influence of hydrothermal alteration on and large exploration potential in the deep Cambrian dolomite reservoirs due to the unique geological conditions and the recent exploration results.  相似文献   

19.
Extensive, large-scale pervasive cementation in the form of cement bodies within fluvial strata has rarely been documented although fluvial strata commonly act as important hydrocarbon reservoirs, as well as groundwater aquifers. Here, we present outcrop, petrographic and geochemical data for pervasive ferroan dolomite cement bodies up to 250 m in size from Upper Cretaceous Desert Member and Castlegate Sandstone fluvial strata exposed in the Book Cliffs in Utah. These cement bodies are present with coastal plain fluvial strata within both the Desert and Castlegate lowstand sandstones and are most abundant in the thin, distal fluvial strata. Cement bodies are almost entirely absent in updip, thicker, fluvial strata. Petrographic observations suggest a predominantly early diagenetic timing to the mildly ferroan dolomite, with a component of later burial origin. δ13C values for the cement (+4.8 to −5.7‰ V-PDB) suggest a marine-derived source for the earliest phase with a burial organic matter source for later cement. δ18O data (−6.3 to −11.8‰ V-PDB) suggest precipitation from freshwater dominated fluids. It is proposed here that dolomite was derived from leaching of detrital dolomite under lowstand coals and cementation took place in coastal aquifers experiencing mixed meteoric-marine fluids as a result of base-level fluctuations. This data presented here shows that large cement bodies can be an important component within fluvial sandstones with a potentially significant impact upon both reservoir quality and fluid flow within reservoirs, especially at the marine-non-marine interface.  相似文献   

20.
Ancient hydrocarbon seepage occurred in the Hrabůvka quarry at the boundary between the basement of the Bohemian Massif (represented by folded Lower Carboniferous siliciclastics of the Culm facies) and Tertiary sedimentary cover of the Carpathian Foredeep (formed by Lower Badenian siliciclastics and calcareous clays). The unconsolidated Lower Badenian sediments contain lithified domains composed of limestone and breccias with limestone cement, whereas the basement rocks are cut by subvertical neptunic dykes filled up by limestone and calcite-marcasite-pyrite veinlets representing sealed fluid conduits. The deeply negative δ13C values of both vein calcite and limestone (down to −38.1‰ V-PDB) indicate that oxidation of hydrocarbons was the major source of carbon for authigenic mineralization. A fluid inclusion study suggests low fluid temperatures (<50 °C) and low and variable salinities of aqueous fluids associated with hydrocarbons (0.7–6.7 wt. % NaCl eq.). The variability of δ18O values of authigenic carbonates (−1.7 to −8.2‰ V-PDB) could reflect either slight changes in temperature of escaping fluids (mostly within 15 °C), and/or some mixing with meteoric waters. The low δ34S values of vein marcasite (∼–20‰ V-CDT) are consistent with bacterial reduction of sulfate in the hydrothermal system. Low C1/(C2+C3) ratios in hydrocarbon gas extracted from authigenic carbonates (9.9 and 5.8) as well as the high δ13C values of methane (−31.8 and −32.4‰ V-PDB) are compatible with a thermogenic source of hydrocarbons. REE data indicate sequestration of REE from finely dispersed detrital material in the apical part of the hydrothermal system. The available data are compatible with two possible scenarios of fluid origin. The hydrocarbons could have been leached from underlying Paleozoic sedimentary sequence by aqueous fluids that infiltrated into the basement after Tertiary tectonic reactivation. Alternatively, an external source of hydrocarbon-bearing fluids can be found in the adjacent Outer Western Carpathians flysch nappes containing petroleum-producing lithologies. Nevertheless, a regional flow of hydrocarbon-bearing fluids is evidenced by the occurrence of very similar hydrocarbon-bearing vein mineralizations in a wider area.  相似文献   

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