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1.
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   

2.
The Precambrian Dengying Formation is maximum buried carbonate reservoir in the Sichuan Basin. Reservoir types are thought to be dominated by sedimentary facies or karst controlled. Precursory sedimentary fabrics have been intensively superimposed by medium-to coarse-grained dolomite in most areas. Dolomitized intervals contain planar and saddle dolomite, quartz, and few hydrothermal replacive minerals. Fluid inclusion analyses of dolomite suggests that rocks are formed at temperatures ranging from 132.6°C to 218.7°C in the presence of dense brines, while the dolomite phases are demonstrated by negative δ18O and δ13C VPDB values. Strontium isotopes enriched in 87Sr, and the fluid source could support the conclusion. The dolomites of the Dengying Formation in central Sichuan Basin that formed around basement-rooted wrench faults, in turn mainly oriented towards the North-South and East-West strike-slip faults, are detectable. Lastly, the grabens take the form of negative flower structures-the result of an intra-cratonic rift that took place during the Sinian and early Cambrian period through tensional faulting.Our primary contention is that basement fault, which resulted in the magmatic or deep clastic fluids migration, was key for the formation of the obvious high-temperature coarse dolomite and saddle dolomite replacement. Subsequently, hot fluids that circulated within the matrix dolomite were aided by fractures or vugs and (1) leached into the dolomite, producing vugs and pores; (2) precipitated saddle dolomite, and (3) led to hydrofracturing, fractures enlargement, and further brecciation. The dolomite eventually formed porous hydrocarbon reservoirs through diagenesis. This model better illustrates how fluids that originated from deep basin migration along strike-slip transfer faults and fractures flowed out to structures in Precambrian basement-rooted faults, inheriting the rift in the Cambrian. The data involved offers a fresh perspective pertinent to deep hydrocarbon exploration of dolomite reservoirs in Southwestern China.  相似文献   

3.
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   

4.
Shallow marine carbonate sedimentation dominated during the Albian in the western part of the Basque Cantabrian Basin in Northern Spain, forming the large Ramales Platform. This platform originated on a less subsiding tectonic block facing deeper and more subsiding areas to the south and east, which were created by tectonic activity in the Basin. Fracture-related hydrothermal dolomites hosted in these Albian carbonates are well exposed in the Asón valley area. Mapping in the studied area revealed several dolomite bodies related to main faults that cut the stratification almost at right angles. The bodies show a vertical development along fault-strike up to 900 m thick from which kilometre-scale branches expand following the stratification. Dolomitization is pervasive and independent of the limestone facies. Main dolomite facies are fine replacive, sucrosic and saddle. Petrography, C, O and Sr isotopes and fluid-inclusion analysis support a polyphase hydrothermal dolomitization at fluid temperatures between 75 °C and 240 °C and highly variable salinity of up to 22 wt.% NaCl. Fine dolomite replaced limestone first and then, sucrosic and saddle dolomites replaced part of the first dolomite and cemented newly created fracture porosity together with different calcite cements. Zebra dolomites and hydroclastic breccias are products of this later stage. Burial analysis of the host rock supports maximum burial temperatures of 80 °C and intense tectonic activity from the Albian to Turonian with a latest Albian peak subsidence. Albian stretching of the crust and subsequent ascent of the isotherms in the area is suggested to have produced sufficient heat to the dolomitizing fluids. The structural analysis indicates a strong transtensional tectonic activity in the studied area during Albian to Turonian time with the creation of an overstep between W–E trending and N–S trending faults. Fluids moved from subsiding areas to fractured uplifted parts of the Ramales Platform, enhanced by diapiric activity.  相似文献   

5.
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   

6.
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   

7.
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   

8.
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   

9.
Deeply buried (4500–7000 m) Ordovician carbonate reservoirs in the Tazhong area, Tarim Basin, NW China show obvious heterogeneity with porosity from null in limestones and sweet dolostones to 27.8% in sour dolostones, from which economically important oils, sour gas and condensates are currently being produced. Petrographic features, C, O, Sr isotopes were determined, and fluid inclusions were analyzed on diagenetic calcite, dolomite and barite from Ordovician reservoirs to understand controls on the porosity distribution. Ordovician carbonate reservoirs in the Tazhong area are controlled mainly by initial sedimentary environments and eo-genetic and near-surface diagenetic processes. However, vugs and pores generated from eogenetic and telogenetic meteoric dissolution were observed to have partially been destroyed due to subsequent compaction, filling and cementation. In some locations or wells (especially ZG5-ZG7 Oilfield nearby ZG5 Fault), burial diagenesis (e.g. thermochemical sulfate reduction, TSR) probably played an important role in quality improvement towards high-quality reservoirs. C2 calcite and dolomite cements and barite have fluid inclusions homogenization temperatures (Ths) from 86 to 113 °C, from 96 to 128 °C and from 128 to 151 °C, respectively. We observed petrographically corroded edges of these high-temperature minerals with oil inclusions, indicating the dissolution must have occurred under deep-burial conditions. The occurrence of TSR within Ordovician carbonate reservoirs is supported by C3 calcite replacement of barite, and the association of sulfur species including pyrite, anhydrite or barite and elemental sulfur with hydrocarbon and 12C-rich (as low as −7.2‰ V-PDB) C3 calcite with elevated Ths (135–153 °C). The TSR may have induced burial dissolution of dolomite and thus probably improved porosity of the sour dolostones reservoirs at least in some locations. In contrast, no significant burial dissolution occurred in limestone reservoirs and non-TSR dolostone reservoirs. The deeply buried sour dolostone reservoirs may therefore be potential exploration targets in Tarim Basin or elsewhere in the world.  相似文献   

10.
One hundred and ten carbon and nineteen strontium isotopic values of outcropping Cambrian Series 2 and Series 3 carbonate rocks in the Penglaiba section of the Keping area were analyzed. Effective isotopic data with little influence of diagenesis were used to address the global correlation. The δ13C values exhibit two major positive excursions (peaking at 3.1‰ and 3.3‰) and three major negative excursions (peaking at −3.0‰, −4.2‰ and −3.2‰). The carbon isotope excursions (peaking at −3.0‰ and −4.2‰) across the Cambrian Series 2-Series 3 boundary show good correlations with similar variations reported in Siberia, Laurentia, North China and South China. In contrast, the other three carbon isotope excursions (peaking at 3.1‰, 3.3‰ and −3.2‰) do not have a fairly good global correlation because of the lack of biostratigraphic data. With respect to the 87Sr/86Sr ratios, they show good correlation with those reported for Laurentia, and further support the global δ13C comparison. On the basis of these new data, it is showed that the combination of δ13C curves and 87Sr/86Sr variations serves as a powerful tool for correlation and subdivision of Cambrian strata in the Tarim Basin of northwest China, and provides new data for global correlation.  相似文献   

11.
Eocene–Oligocene dolomite concretions and beds from the Grybów and Dukla units of the Polish Outer Carpathians were studied. These rocks occur in the organic carbon-rich, marine and fine-grained deposits of hemipelagic or turbiditic origin. Mineralogic, elemental and stable C and O isotopic composition of the dolomites was determined. Results indicate that the rocks were formed by precipitation of predominantly Fe-rich dolomite cement close to the sediment-water interface prior to significant compaction. The main source of bicarbonate for dolomite formation was bacterial methanogenesis as evidenced by the high δ13C values up to 16.6‰. The main source of alkalinity was probably weathering of silicate minerals which might have also liberated Ca and Mg ions for the dolomites to form. The distribution of these dolomites indicates that microbial methane production was widespread in the Silesian basin. Moreover, formation of some dolomites in the Eastern part of the Dukla unit was probably associated with gas hydrates as suggested by the elemental and oxygen isotopic composition of dolomitic matrix. Therefore, the dolomites may serve as a proxy of areas where biogenic methane was produced, where the rocks had high hydrocarbon potential, and where hydrates could have existed.Detailed mineralogic and petrographic analyses allowed for the reconstruction of the diagenetic sequence and the evolution of pore fluids. Textural relationships between successive cement generations indicate that the central parts of the composite dolomite crystals experienced corrosion and that the latest ankerite cement filled the secondary intragranular cavities within those crystals. This observation shows that reconstructions of pore fluid evolution based on core-to-rim analyses of such composite crystals may lead to wrong interpretations. Septarian cracks developed in the dolomites are often filled with multistage cements. The earliest generations are ferroan dolomite and ankerite cements which precipitated within the cracks simultaneously to the ferroan dolomite and ankerite cements from the matrix of the dolomitic rocks which shows that septarian cracking occurred very early, during the final stages of concretionary formation. These cements were followed by the late-diagenetic precipitates, mainly quartz, kaolinite and blocky calcite. This calcite is commonly associated with bitumen which shows that it precipitated during or after oil migration in the decarboxylation zone.  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.
The Early Jurassic dolomitized carbonates in the Venetian Alp, represent a surface analogue of the hydrocarbon exploration targets in Adriatic offshore and Po Plain, Italy. Dolomitization affected the carbonate platform of Monte Zugna Formation (Lower Jurassic) and the Neptunian dikes breccia in the pelagic Maiolica Formation (Uppermost Jurassic–Lower Cretaceous) improving the poro-perm characteristics. Petrography, stable isotope, strontium isotope ratio, trace element and fluid inclusion analyses were carried out on samples from the Monte Grappa Anticline, which is the direct analogue for subsurface. The petrographic analyses showed a first pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3, DS). The same, quite wide range of oxygen isotope (?9 to ?2‰ V-PDB) is observed in all dolomite types. The δ13C range is in the positive field of marine derived carbonate (from +0.5 to +3.2‰ PDB). The trace element analysis showed a slight enrichment in Fe and Mn contents in the Monte Zugna dolostones with respect the original limestone. The same dolomite precipitation temperature (up to 105 °C Th) was observed in the replacement and cement dolomites, suggesting a unique dolomitization event. This temperature, largely higher than the maximum burial temperature (about 50 °C), supports a hydrothermal origin of the dolomitizing fluids, which had a seawater to brackish composition. The data collected suggest a hydrothermal dolomitization occurring during to the South Alpine thrusting according to the “squeegee model”. The interpretation is consistent with the dolomitization model proposed for similar Jurassic successions in the Central Southern Alps. This study indicates that the deformed foreland and thrust fold belts carbonates in Po Plain and Adriatic offshore are suitable to be dolomitized, and therefore reflect an efficient hydrocarbon exploration play.  相似文献   

14.
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   

15.
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.  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.
The Esino Limestone of the western Southern Alps represents a differentiated Ladinian-Lower Carnian (?) carbonate platform comprised of margin, slope and peritidal inner platform facies up to 1000 m thick. A major regional subaerial exposure event lead to coverage by another peritidal Lower Carnian carbonate platform (Breno Formation). Multiphase dolomitization affected the carbonate sediments. Petrographic examinations identified at least three main generations of dolomites (D1, D2, and D3) that occur as both replacement and fracture-filling cements. These phases have crystal-size ranges of 3–35 μm (dolomicrite D1), 40–600 μm (eu-to subhedral crystals D2), and 200 μm to 5 mm (cavity- and fracture-filling anhedral to subhedral saddle dolomite D3), respectively.The fabric retentive near-micritic grain size coupled with low mean Sr concentration (76 ± 37 ppm) and estimated δ18O of the parent dolomitizing fluids of D1 suggest formation in shallow burial setting at temperature ∼ 45–50 °C with possible contributions from volcanic-related fluids (basinal fluids circulated in volcaniclastics or related to volcanic activity), which is consistent with its abnormally high Fe (4438 ± 4393 ppm) and Mn (1219 ± 1418 ppm) contents. The larger crystal sizes, homogenization temperatures (D2, 108 ± 9 °C; D3, 111 ± 14 °C) of primary two-phase fluid inclusions, and calculated salinity estimates (D2, 23 ± 2 eq wt% NaCl; D3, 20 ± 4 eq wt% NaCl) of D2 and D3 suggest that they formed at later stages under mid-to deeper burial settings at higher temperatures from dolomitizing fluids of higher salinity, which is supported by higher estimated δ18O values of their parent dolomitizing fluids. This is also consistent with their high Fe (4462 ± 4888 ppm; and 1091 ± 1183 ppm, respectively) and Mn (556 ± 289 ppm and 1091 ± 1183 ppm) contents, and low Sr concentrations (53 ± 31 ppm and 57 ± 24 ppm, respectively).The similarity in shale-normalized (SN) REE patterns and Ce (Ce/Ce*)SN and La (Pr/Pr*)SN anomalies of the investigated carbonates support the genetic relationship between the dolomite generations and their calcite precursor. Positive Eu anomalies, coupled with fluid-inclusion gas ratios (N2/Ar, CO2/CH4, Ar/He), high F− concentration, high F/Cl and high Cl/Br molar ratios suggest an origin from diagenetic fluids circulated through volcanic rocks, which is consistent with the co-occurrence of volcaniclastic lenses in the investigated sequence.  相似文献   

18.
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   

19.
Significant volumes of the known hydrocarbon reserves are found in carbonate rocks, many of these dolomitized. The spatial distribution of diagenesis on these rocks is one of the main challenges in oil reservoir modeling. Reactive transport models can be a powerful tool to understand the active diagenetic processes and their effects on the quality of these reservoirs. In this study it was used, for the first time, the CMG-GEM simulator to model diagenetic evolution of a carbonate sequence, subjected to compaction-driven and geothermal flow in a simulated period of 200 thousand years. It was simulated carbonate cementation, dolomitization and dissolution, with and without presence of faults. Among the analyzed variables, the volume of circulating fluid was the most important factor. For both mechanisms, flow simulated velocities obtained had magnitudes smaller than 10−6 m/day. Diagenesis was insignificant for these low speeds at simulated time interval. Only dolomitized facies presented relevant diagenesis in form of calcite dissolution and dolomite precipitation. Simulations with flow rates of 1 m/day revealed a considerable increase in observed diagenesis, especially in carbonate cementation and in porosity enhancement. Diagenesis was more pronounced in more permeable sediments, highlighting the role of fluid flow in diagenetic reactions. Relative dissolution was greatly reduced during simulations performed with absence of dolomite and dolomitization reactions. The presence of faults strongly influences spatial distribution of diagenesis, especially relatively to dissolution. More permeable facies were more dissolved near fault, decreasing with increasing distance. Low permeability facies, as mudstones, are not dissolved, even near fault. Spatial distribution of diagenesis would then be dependent mainly on the quality of original pore structure, of fault presence and mineral composition of rock.  相似文献   

20.
Several hundred hydrothermal vent complexes were formed in the Vøring Basin as a consequence of magmatic sill emplacement in the late Palaeocene. The 6607/12-1 exploration well was drilled through a 220-m-thick sequence of Eocene–Miocene diatomites with carbonate nodules above the apex of one of these vent complexes. Analysed calcites and dolomites from this interval have isotopic signatures typical for methane seep carbonates, with low 13C signatures of –28 to –54 PDB. The data suggest that the vent complex acted as a fluid migration pathway for about 50×106 years after its formation, leading to near-surface microbial activity and seep carbonate formation.  相似文献   

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