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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 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   

3.
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   

4.
The Zagros-Taurus fold and thrust belt hosts a prolific hydrocarbon system. Most hydrocarbon reserves are stored in naturally fractured reservoirs and such fracture systems can therefore have a significant impact on reservoir performance. Fractures are one of the most important paths for fluid flow in carbonate reservoirs, and industrial geoscientists and engineers therefore need to understand and study fracture patterns in order to optimise hydrocarbon production. The observed fracture patterns in outcrops may have implications on fluid flow and reservoir modelling in subsurface reservoirs, and we have therefore undertaken a case study of fracturing associated with regional folding in Iraqi Kurdistan. In this area, some exploration wells currently target Upper Triassic dolostones (Kurra Chine Formation) and/or Lower Jurassic limestones and dolomitised limestones (Sehkaniyan Formation). In both units hydrocarbon production comes mainly from secondary porosity created by dolomitisation, dissolution and fracturing. Both formations have undergone multiple phases of deformation associated with burial, uplift, folding and thrusting. We investigate some fracture pattern characteristics and some petrophysical properties of these units using selected outcrops around the Gara, Ora and Ranya anticlines that form folds directly traceable for 25–70 km. Our outcrop data is compared with subsurface fracture and petrophysical datasets reported from wells in the nearby Shaikhan and Swara Tika Fields. The 1-2-3D fracture attributes collected from outcrops are fracture orientation, type, spacing, intensity, length and cross-cutting and abutting relationships. Fracture orientations show a clear relationship to the local fold axis in both the outcrop and subsurface, although in some cases they appear to relate more to the present day in-situ maximum horizontal stress direction or local strike-slip faulting. Three stages of fracturing are proposed: pre-folding, early-folding and post-folding fractures. In addition, we report petrophysical properties - porosity, permeability and acoustic velocity of both the Kurra Chine and Sehkaniyan formations in relation to their structural position within folds and faults and stratigraphic level. The highest porosities and permeabilities are recorded in the hinges and backlimbs of the Gara Anticline. The best reservoir quality (highest porosity and permeability) is often found in areas associated with replacement dolomite i.e. solution vugs and intercrystalline porosity. The Kurra Chine Formation displays similar trends in velocity-porosity data at both outcrop and the subsurface. However, the Sehkaniyan Formation displays lower acoustic velocity for a given porosity at outcrop compared to the subsurface.  相似文献   

5.
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   

6.
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   

7.
Facies and diagenetic heterogeneities in carbonate reservoir rocks affect both, fracture distribution and fracture permeability. Many studies focussed on fracture patterns in limestone–marl alternations, as e.g. fluid flow models, are based on laterally continuous beds. Here we examine 4010 fractures in multiple layers of limestone–marl alternations using a modified scan-line method. The studied successions belong to the Blue Lias Formation (Hettangian–Sinemurian), exposed on the coast of the Bristol Channel, United Kingdom. We combine methods of sedimentology and structural geology with rock physics to gain a better understanding of the role of facies, diagenesis and petrophysical properties (tensile and compressive strength, hardness, porosity) on the distribution of fractures (fracture orientation, density, spacing and height). Fracture distribution varies significantly despite similar bed thicknesses, indicating that planar bedding planes (i.e. well-bedded limestones, WBL) and beds with bedding plane irregularities (i.e. semi-nodular limestones, SNL) must be distinguished. SNL show higher percentages of non-stratabound fractures (67%) while they are more stratabound in WBL (57%). Additionally, beds with variable bed thicknesses (in scale of 15 m long beds) exhibit a wide range of fracture spacing, whereas fractures in beds with more continuous bed thicknesses are more regularly spaced. Considering all lithologies, the percentage of non-stratabound fractures increases proportionally with CaCO3 content. Three subsections studied in detail reveal different main sedimentological and diagenetic features (from early lithified over differentially compacted to physically compacted). All of them are characterised by dissimilar percentages of stratabound and non-stratabound fractures in limestone beds and marl interbeds. Our findings demonstrate that the distribution of fractures in individual well-bedded limestones is not necessarily representative for successions of limestone–marl alternations; multiple layers should therefore be studied in outcrop analogues as basis for fluid flow models of reservoirs composed of such lithologies.  相似文献   

8.
The middle Permian Lucaogou Formation in the Jimusaer Sag of the southeastern Junggar Basin, NW China, was the site of a recent discovery of a giant tight oil reservoir. This reservoir is unusual as it is hosted by lacustrine mixed dolomitic-clastic rocks, significantly differing from other tight reservoirs that are generally hosted by marine/lacustrine siliciclastic–calcitic sequences. Here, we improve our understanding of this relatively new type of tight oil reservoir by presenting the results of a preliminarily investigation into the basic characteristics and origin of this reservoir using field, petrological, geophysical (including seismic and logging), and geochemical data. Field and well core observations indicate that the Lucaogou Formation is a sequence of mixed carbonate (mainly dolomites) and terrigenous clastic (mainly feldspars) sediments that were deposited in a highly saline environment. The formation is divided into upper and lower cycles based on lithological variations between coarse- and fine-grained rocks; in particular, dolomites and siltstones are interbedded with organic-rich mudstones in the lower part of each cycle, whereas the upper part of each cycle contains few dolomites and siltstones. Tight oil accumulations are generally present in the lower part of each cycle, and dolomites and dolomite-bearing rocks are the main reservoir rocks in these cycles, including sandy dolomite, dolarenite, dolomicrite, and a few dolomitic siltstones. Optical microscope, back scattered electron, and scanning electron microscope imaging indicate that the main oil reservoir spaces are secondary pores that were generated by the dissolution of clastics and dolomite by highly acidic and corrosive hydrocarbon-related fluids.  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.
Understanding the distribution of natural fractures in sedimentary systems is of high relevance for the exploration and production of fluids in the subsurface. This study focuses on a Kimmeridgian mixed siliciclastic-carbonate ramp system, which is part of the Jurassic limestones outcropping in the northeastern Iberian Chain. The study area is located north of the Ricla village, fifty kilometers southwest of Zaragoza. The outcrop stretches over six kilometers in length, it allows for recording detailed fracture patterns and facies variations. A GIS-based software-package 'DigiFract' is used to measure and digitize fractures in vertical outcrops. Fracture orientation measurements obtained from vertical and horizontal outcrops are used to create a conceptual three-dimensional image of the fracture distribution of the mixed ramp system. The fracture data are processed by integrating outcrop logs and sample-data obtained from thin-sections and rock property analysis. The continuous facies belts of the shallow low-angle ramp system show limited lateral variations. The studied sediments are subdivided in four main facies tracts (FT's): (FT-1) Bioclastic siltstones, (FT-2) alternating sandy limestones and marls, (FT-3) cross-bedded and channelled oolitic-bioclastic sand- and grainstones and (FT-4) coralgal float-to mudstones with eventites. Measured fracture orientations of the vertical and horizontal outcrops are identical throughout the entire exposure. Two main fracture sets are identified, the first set (Set 1) has a N–S direction and the second set (Set 2) has a NE–SW direction. The lateral homogeneity of the sedimentary system, thus facies, layer thickness and slope angle, can be translated to the observed fracture patterns. Within one single facies belt, fractures tend to behave the same in the proximal, middle and distal part of the ramp system. However, vertical facies variations are an important factor for the measured vertical fracture-heterogeneity. Fine-grained mud-supported facies correspond to periods of a sea-level highstand; coarse-grained cemented facies on the other hand are related to a sea-level lowstand. The physical contrast of the sediments caused by sea-level fluctuations forces fractures to solely concentrate in the brittle layers. Fracture density and termination patterns observed on this ramp stand in contrast to fracture geometries observed in flat-topped carbonate platforms. Lateral facies heterogeneity and platform anatomy of flat-topped carbonate platforms are key parameters for the eventual fracture distribution. For the studied mixed ramp-type system the vertical facies variations are key parameter.  相似文献   

11.
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   

12.
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   

13.
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   

14.
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   

15.
The Mississippian Barnett Shale (Texas, USA), consisting of organic-rich shales and limestones, hosts the largest gas fields of North America. This study examines sealed fractures from core and outcrop samples of the Barnett Shale of the Fort Worth Basin and aims to: 1) characterize the phases occurring in the fractures from samples having experienced different burial histories; 2) establish a paragenetic sequence to relate the timing of fracture origin and sealing with the burial history of the basin; and 3) contribute to the understanding of the mechanisms of fracture formation in shales, including overpressure origin.Four fracture generations were distinguished in the most deeply buried core samples by characterizing the sealing minerals petrographically and geochemically. The generations were inserted into the framework of a reconstructed burial history for the Fort Worth Basin, which allowed a time sequence for fracture development to be established. This in turn allowed inference of conditions of fracture development, and consideration of fracture mechanisms as well as the origin of the parent fluids of sealing minerals.Type 1 fractures formed during early mechanical compaction (at a few 10 s to 100 m of depth) of still not fully cemented sediments. Type 2 fractures formed during moderate burial (∼2 km), from slightly modified seawater. Their timing is consistent with overpressure generated during rapid deposition and differential compaction of Pennsylvanian lithologies during the onset of the Ouachita compressional event. Type 3 fractures formed during deep burial (>3 km) from silica-rich basinal brines possibly derived from clay diagenesis. Type 4 fractures formed at very deep burial (>4 km), from hot and 18O-rich fluids, carrying light oil (20-30 API) and record the opening of the fluid system after hydrocarbon migration.Differences are highlighted between the timing and thermal regimes under which fractures formed in Barnett lithologies from different areas of the basin, this suggesting that extrapolation of outcrop observations to subsurface must be used with due care.  相似文献   

16.
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   

17.
In conventional studies of tracing dolomitization and diagenetic fluids, REEs of dolomites were widely used as been normalized by PAAS, NASC or chondrite. However, most dolomites are formed in seawater or seawater-derived fluids. Thus, we conduct a new attempt to normalize the REEs of dolomite using seawater standard, based on case studies on 36 Triassic limestone–dolomite samples from the Geshan section of southeast China and 26 Permian–Triassic dolomite samples from the Panlongdong section of northeastern Sichuan Basin, southwest China.The Geshan seawater-normalized (SN) REE patterns are characterized by notable positive CeSN (average CeSN/Ce* = 6.823, SD = 0.192) and negative PrSN anomalies (average PrSN/Pr* = 0.310, SD = 0.010), and slightly negative GdSN anomalis (average GdSN/Gd* = 0.864, SD = 0.053), with no obvious EuSN anomaly (average EuSN/Eu* = 1.036, SD = 0.094). The signatures of REE patterns barely changed during the dolomitization process. For the REE compositions of the Panlongdong dolomite, it can be found that (1) the recrystallization process can result in varied total REE concentrations (between 7.16 ppm and 37.87 ppm), but do not alter the REE patterns, including consistent positive CeSN anomalies (average = 4.074, SD = 0.27) and LREE enrichment (average NdSN/YbSN = 3.164, SD = 0.787); (2) meteoric incursion can reverse Ce anomaly, from the strong positive Ce anomalies (CeSN/Ce* = 5.059) to slightly positive (CeSN/Ce* = 2.459) or even negative Ce anomalies; and (3) hydrothermal fluid altered REE pattern is complicated by fluctuated distribution curve, negative Ce anomaly and positive Eu anomaly (EuSN/Eu* = 1.862). These results suggest that the seawater normalized REE patterns of dolomite can serve as an index to study the source of the dolomitization fluids and distinguish complex diagenetic processes, providing a complement to previous works.  相似文献   

18.
The Jiaolai Basin (Fig. 1) is an under-explored rift basin that has produced minor oil from Lower Cretaceous lacustrine deltaic sandstones. The reservoir quality is highly heterogeneous and is an important exploratory unknown in the basin. This study investigates how reservoir porosity and permeability vary with diagenetic minerals and burial history, particularly the effects of fracturing on the diagenesis and reservoir deliverability. The Laiyang sandstones are tight reservoirs with low porosity and permeability (Φ < 10% and K < 1 mD). Spatial variations in detrital supply and burial history significantly affected the diagenetic alterations during burial. In the western Laiyang Sag, the rocks are primarily feldspathic litharenites that underwent progressive burial, and thus, the primary porosity was partially to completely eliminated as a result of significant mechanical compaction of ductile grains. In contrast, in the eastern Laiyang Sag, the rocks are lithic arkoses that were uplifted to the surface and extensively eroded, which resulted in less porosity reduction by compaction. The tectonic uplift could promote leaching by meteoric water and the dissolution of remaining feldspars and calcite cement. Relatively high-quality reservoirs are preferentially developed in distributary channel and mouth-bar sandstones with chlorite rims on detrital quartz grains, which are also the locations of aqueous fluid flow that produced secondary porosity. The fold-related fractures are primarily developed in the silt–sandstones of Longwangzhuang and Shuinan members in the eastern Laiyang Sag. Quartz is the most prevalent fracture filling mineral in the Laiyang sandstones, and most of the small-aperture fractures are completely sealed, whereas the large-aperture fractures in a given set may be only partially sealed. The greatest fracture density is in the silt–sandstones containing more brittle minerals such as calcite and quartz cement. The wide apertures are crucial to preservation of the fracture porosity, and the great variation in the distribution of fracture-filling cements presents an opportunity for targeting fractures that contribute to fluid flow.  相似文献   

19.
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   

20.
西非下刚果盆地为一典型被动大陆边缘含盐盆地,下刚果盆地北部海域在白垩系海相碳酸盐岩层系获得丰富油气发现。研究区海相碳酸盐岩领域油气勘探面临的核心瓶颈问题,即白垩系碳酸盐岩的沉积模式、演化规律、储层特征以及沉积储层发育控制因素。综合钻井、地震、区域地质等资料,分析认为自下向上相对海平面的上升控制了沉积演化,沉积体系演化模式为浅海碳酸盐岩台地→浅海混积陆棚→半深海-海底扇。下刚果盆地碳酸盐岩储层展布在纵向及平面上均可以划分为内中外3个储层发育带,碳酸盐岩储层最主要发育于下白垩统Albian阶下Sendji组。该时期研究区整体发育浅海碳酸盐岩混积缓坡台地沉积体系,沉积亚相可进一步划分为混积滨岸、后缓坡、浅水缓坡以及深水缓坡4种类型,其中浅水缓坡亚相颗粒滩微相与后缓坡亚相台内浅滩、砂质浅滩微相储层最为发育。碳酸盐岩储层岩性组合主要包括颗粒灰岩、砂岩、砂质灰岩、白云岩4种类型;储层发育主要受沉积相带的控制,并受成岩作用的影响。  相似文献   

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