Stages Of Dolomitization Research Articles

Genesis of Dolomite from Ma55–Ma510 Sub‐members of the Ordovician Majiagou Formation in the Jingxi Area in the Ordos Basin

AbstractWe clarified three stages of dolomitization and secondary changes by studying the petrology and geochemistry characteristics of dolomite from the Ma55–Ma510 sub‐members of the Ordovician Majiagou Formation in the Jingxi area in the Ordos Basin: (1) Syngenetic microbial dolomitization is characterized by formation of dolomite with a mainly micrite structure and horse tooth‐shape dolomite cements. (2) Seepage reflux dolomitization during the penecontemporaneous period superposed adjustment functions such as recrystallization and stabilization in the middle‐deep burial stage, forming dolomites mainly consisting of micro crystal and powder crystal structure. (3) Powder dolomite, fine dolomite, and medium‐coarse crystalline dolomite formed in pores and fractures in the middle‐deep burial stage. The secondary concussive transgression‐regression under a regressive background is an important condition for the occurrence of many stages of dolomitization in the study area. The basin was an occlusive epicontinental sea environment in the Ma5 member of the Ordovician Majiagou Formation sedimentary period. In the sediments, sulfate content was high, which is conducive to the preservation of microbial activity and microbial dolomitization. Micritic dolomite formed by microbial dolomitization provides good migration pathways for seepage reflux dolomitization. Affected by evaporation seawater with increased Mg/Ca ratio, seepage reflux dolomitization was widely developed and formed large‐scale dolomite, and underwater uplifts and slopes are favorable areas for dolomite. In the middle‐deep burial stage, dolomitizing fluid in the stratum recrystallized or stabilized the previous dolomite and formed a small amount of euhedral dolomite in the pores and fractures.

Multiphase dolomitization of deeply buried Cambrian petroleum reservoirs, Tarim Basin, north‐west China

AbstractCambrian dolostone reservoirs in the Tarim Basin, China, have significant potential for future discoveries of petroleum, although exploration and production planning is hampered by limited understanding of the occurrence and distribution of dolomite in such ancient rocks buried to nearly 8 km. The study herein accessed new drill core samples which provide an opportunity to understand the dolomitization process in deep basins and its impact on Cambrian carbonate reservoirs. This study documents the origin of the dolostone reservoirs using a combination of petrology, fluid‐inclusion microthermometry, and stable and radiogenic‐isotopes of outcrop and core samples. An initial microbial dolomitization event occurred in restricted lagoon environments and is characterized by depleted δ13C values. Dolomicrite from lagoonal and sabkha facies, some fabric‐retentive dolomite and fabric‐obliterative dolomite in the peloidal shoal and reef facies show the highest δ18O values. These dolomites represent relatively early reflux dolomitization. The local occurrence of K‐feldspar in dolomicrite indicates that some radiogenic strontium was contributed via terrigenous input. Most fabric‐retentive dolomite may have precipitated from seawater at slightly elevated temperatures, suggested by petrological and isotopic data. Most fabric‐obliterative dolomite, and medium to coarse dolomite cement, formed between 90°C and 130°C from marine evaporitic brine. Saddle dolomite formed by hydrothermal dolomitization at temperatures up to 170°C, and involved the mixing of connate brines with Sr‐ enriched hydrothermal fluids. Intercrystalline, moldic, and breccia porosities are due to the early stages of dolomitization. Macroscopic, intergranular, vuggy, fracture and dissolution porosity are due to burial‐related dissolution and regional hydrothermal events. This work has shown that old (for example, Cambrian or even Precambrian) sucrosic dolomite with associated anhydrite, buried to as much as 8000 m, can still have a high potential for hosting substantial hydrocarbon resources and should be globally targeted for future exploration.

Dolomitization of the Silurian Niur Formation, Tabas block, east central Iran: Fluid flow and dolomite evolution

The Silurian Niur carbonates at Dahaneh-Kalut and Ozbak-Kuh sections (Tabas block, eastern Central Iran) have been pervasively dolomitized. Four types of dolomite, consisting of replacive dolomite (Rd1–Rd3) and saddle dolomite cements (Cd), were discriminated by texture, cathodolouminscence petrography and geochemical analyses. Rd1 dolomite occurred as a very fine (10–25 μm), planar-e to planar-s dolomite that replaced marine limestones. Rd2 dolomite is a planar-s to planar-e, medium crystalline dolomite (100–400 μm). Rd3 dolomite is a very coarse (400 μm–2 mm), non-planar crystalline dolomite with sweeping extinction, and Cd dolomite occurred as voids- and fractures-filling saddle dolomite (5–10 mm) with strong sweeping extinction. Field and petrographic observations demonstrated that Rd3 and Cd occur only at the Dahaneh-Kalut section and are related to mafic volcanic rocks and fractures at the base of the Niur Formation. The δ18O (−8.45 to −6.06‰ VPDB) and δ13C (−0.01 to 0.99‰ VPDB) values suggest that Rd1 dolomite was precipitated at the early stage of dolomitization from Silurian seawater, whereas Rd2 dolomite formed from similar fluids at higher temperatures upon burial. The δ18O (−13.80 to −9.47‰) and δ13C (−1.66 to 0.76‰) values and fluid inclusion data (Th: 122 to 175 °C, salinity: 18.68 to 24 wt% NaCl eq.) revealed that Rd3 and Cd were precipitated from hot, saline fluids with a magmatic origin. The Mn, Fe, Na and Sr concentrations of Rd3 and Cd, suggest that the responsible dolomitizing fluids were more enriched with respect to these elements and had a different origin relative to the Rd1 and Rd2 dolomitizing fluids. These results revealed that the late Ordovician to early Silurian passive rifting phase generated a local thermal gradient that increased heat flow and thermal convection of diagenetic fluids that led to the precipitation of Rd3 and Cd as pervasive hydrothermal dolomite. The network of fractures and faults that formed during the rifting phase provided open conduits for hydrothermal fluids. These results emphasize the importance of both tectonic setting and thermal conditions in the generation and circulation of dolomitizing fluids as well as in massive dolomitization.

Estimation of Dolomite Formation: Dolomite Precipitation and Dolomitization

Abstract: Reactive-transport models are developed here that produce dolomite via two scenarios: primary dolomite (no CaCO3 dissolution involved) versus secondary dolomite (dolomitization, involving CaCO3 dissolution). Using the available dolomite precipitation rate kinetics, calculations suggest that tens of meters of thick dolomite deposits cannot form at near room temperature (25-35°C) by inorganic precipitation mechanism, though this mechanism will provide dolomite aggregates that can act as the nuclei for dolomite crystallization during later dolomitization stage. Increase in supersaturation, Mg+2/Ca+2 ratio and CO3−2 on the formation of dolomite at near room temperature are subtle except for temperature. This study suggests that microbial mediation is needed for appreciable amount of primary dolomite formation. On the other hand, reactive-transport models depicting dolomitization (temperature range of 40 to 200°C) predicts the formation of two adjacent moving coupled reaction zones (calcite dissolution and dolomite precipitation) with sharp dolomitization front, and generation of >20% of secondary porosity. Due to elevated temperature of formation, dolomitization mechanism is efficient in converting existing calcite into dolomite at a much faster rate compared to primary dolomite formation.

Dolomitization process and its implications for porosity development in dolostones: A case study from the Lower Triassic Feixianguan Formation, Jiannan area, Eastern Sichuan Basin, China

Abstract High-quality reservoirs in the third member of Lower Triassic Feixianguan Formation in the Jiannan area are exclusively restricted to dolostone, which makes it essential to understand origin of porosity in dolostone. However, the studies on the porosity origin of the dolostone in the Jiannan area are rare, and its origin is still controversial. In the paper, the formation mechanisms of porosity in replacement dolostones ever proposed worldwide are summarized, and they can be divided into two main categories based on the formation time of porosity relative to the dolomitization process: syndolomitization and postdolomitization origin. Studies on well cores that have been dolomitized to varying degrees indicate that the porosity in dolostones in the Jiannan area is a syndolomitization origin which is intimately associated with the dolomitization process, depending mainly on the degrees of dolomitization. Four stages of porosity evolution, each with distinctive petrographic and petrophysical properties, were distinguished. In initial stage of dolomitization (stage I), dolomite was formed mainly by volume for volume replacement with porosity constant or slight decrease due to external carbonate source added. In the next stages of progressive dolomitization (stage II), only minor intercrystalline pores were developed and the porosity slightly increased. In more advanced stage (stage III), abundant intercrystalline, as well as some vuggy pores, were formed by selectively dissolution of residual microcrystalline calcites and micritized grains, and the porosity remarkably increased with increasing dolomite content. In the most advanced stage (stage IV), dolomite cements were precipitated in pre-exiting pores in dolostone, resulting in varying decrease in porosity and permeability. Carbonates that must proceed at least in the stage III have the high porosity, while the dolostones in the stage II-2 that display similar rock frameworks only have a minor increase in porosity.

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

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

Dolomitization of Gas Reservoirs: The Upper Permian Changxing and Lower Triassic Feixianguan Formations, Northeast Sichuan Basin, China

Abstract The Lower Triassic Feixianguan Formation, and to a lesser extent the Upper Permian Changxing Formation, contain more than ten giant, recently discovered dolomite gas fields on either side of a paleo-embayment (Kaijiang–Liangping Bay), Sichuan Basin, China. This study documents the origin of the dolomite using a combination of petrology, fluid-inclusion, trace-element geochemistry, and stable-isotope and radiogenic-isotope analysis of samples from outcrop and core samples from producing gas fields. Three stages of dolomitization are revealed. Dolomicrite from the lagoon facies in the Feixianguan Formation and minor fabric-retentive dolomite from reef facies in the Changxing Formation on the northeast side of Kaijiang–Liangping Bay represent the initial dolomite phase, caused by reflux of Feixianguan brine, with salinity close to gypsum saturation (∼ 18 wt % salinity), during the first stage of diagenesis at 30°C. The second and dominant dolomite phase (fabric-retentive dolomite), in both the Feixianguan and Changxing formations on the both sides of Kaijiang–Liangping Bay, is the result of reflux of diagenetic fluids with salinity ranging from normal Feixianguan seawater (3.5% salinity) up to mesohaline water (6.5% salinity) during early burial diagenesis at about 35–40°C, as shown by the dominance of single-phase aqueous inclusions, shallow-formed stylolites that cut the second phase of dolomite, and the Early Triassic marine carbon, oxygen, and strontium-isotope, and rare-earth-element and yttrium (REE + Y) signals. The subordinate third dolomite phase (fabric-obliterative dolomite and dolomite cement) was due to burial diagenetic recrystallization between 80 and 140°C in the Feixianguan and Changxing formations on both sides of Kaijiang–Liangping Bay. The third phase of dolomitization involved water expelled from the slightly younger Lower Triassic Jialingjiang Formation marine evaporites and carbonates on the northeast side of Kaijiang–Liangping Bay, revealed by the high-salinity of the aqueous inclusions that homogenize at 80 to 140°C and elevated strontium-isotope data. The carbon-isotope and REE + Y signals of the third phase of dolomite are similar to earlier dolomite cements confirming the lack of exotic input. Future exploration should be focused on dolomitized, high-energy sediments (e.g., reef and oolitic shoal and bank) in similar Triassic Tethyan-paleogeographic settings that are associated with restricted facies, but underlying dolomitized high-energy Permian strata may also be good exploration targets.

Thermochemical sulfate reduction and fluid evolution of the Lower Triassic Feixianguan Formation sour gas reservoirs, northeast Sichuan Basin, China

The dolomite-hosted, Lower Triassic Feixianguan Formation from the northeast Sichuan Basin, China, is an economically important reservoir that contains sour natural gas. These reservoirs were initially filled with oil, later replaced by gas during burial to 7000 m (22,965 ft) followed by uplift to about 4000 m (13,123 ft). We have studied the souring process (thermochemical sulfate reduction [TSR]) and diagenetic evolution of the Feixianguan Formation using detailed petrology, fluid-inclusion studies, and stable-isotope data from carbonate minerals. Pre-TSR diagenesis included (in time order) the eodiagenetic main stage of dolomitization by a reflux mechanism; fracture-related calcite cementation; barite, quartz, celestite, and fluorite mineralization; and a dolomite recrystallization stage. Thermochemical sulfate reduction resulted in anhydrite replacement by calcite, petroleum destruction, formation of sulfur-rich pyrobitumen and elemental sulfur, and generation of large volumes of and . Diagenesis during TSR can be subdivided into oil-stage TSR and gas-stage TSR, with oil-stage TSR defined by the presence of primary oil and bitumen inclusions in the TSR calcite. Based on aqueous inclusion homogenization temperatures, oil-stage TSR commenced at a temperature of 116°C, with a mode between 130°C and 140°C. Gas-stage TSR started at a temperature of 135°C and continued to maximum burial temperatures of about 220°C. Trace amounts of pyrite, barite, quartz, and celestite grew during TSR. Post-TSR diagenesis was dominated by fracture-related calcite precipitation as well as celestite and anhydrite crystallization. Formation water salinity increased from depositional values (3.5 wt. %) up to 24 wt. % during pre-TSR dolomite recrystallization probably because of an influx of evaporite-associated water from the overlying Jialingjiang Formation, although pre-TSR barite, quartz, celestite, and fluorite mineralization was associated with a transient decrease in water salinity. During TSR, formation water salinity decreased from 26 wt. % to as low as 4 wt. % as a result of water being produced during TSR reactions.

From evaporated seawater to uranium-mineralizing brines: Isotopic and trace element study of quartz–dolomite veins in the Athabasca system

Stable isotope (O, H, C), radiogenic isotope (Sr, Nd) and trace element analyses have been applied to quartz–dolomite veins and their uranium(U)-bearing fluid inclusions associated with Proterozoic unconformity-related UO2 (uraninite) ores in the Athabasca Basin (Canada) in order to trace the evolution of pristine evaporated seawater towards U-mineralizing brines during their migration through sediments and basement rocks.Fluid inclusion data show that quartz and dolomite have precipitated from brines of comparable chemistry (excepted for relatively small amounts of CO2 found in dolomite-hosted fluid inclusions). However, δ18O values of quartz veins (δ18O=11‰ to 18‰) and dolomite veins (δ18O=13‰ to 24‰) clearly indicate isotopic disequilibrium between quartz and dolomite. Hence, it is inferred that this isotopic disequilibrium primarily reflects a decrease in temperature between the quartz stage (∼180°C) and the dolomite stage (∼120°C). The δ13C values of CO2 dissolved in dolomite-hosted fluid inclusions (δ13C=−30‰ to −4‰) and the δ13C values of dolomite (δ13C=−23.5‰ to −3.5‰) indicate that the CO2 dissolved in the mineralizing brines originated from brine–graphite interactions in the basement. The resulting slight increase in the fluid partial pressure of CO2 (pCO2) may have triggered dolomite precipitation instead of quartz.δ18O values of quartz veins and previously published δ18O values of the main alteration minerals around the U-ores (illite, chlorite and tourmaline) show that quartz and alteration minerals were isotopically equilibrated with the same fluid at ∼180°C. The REE concentrations in dolomite produce PAAS-normalized patterns that show some similarities with that of UO2 and are clearly distinct from that of the other main REE-bearing minerals in these environments (monazite, zircon and aluminum phosphate–sulfate (APS) minerals). The radiogenic isotope compositions of dolomite (87Sr/86Sri=0.7053 to 0.7161 and εNd(t)=−8.8 to −20.3) differ from one deposit to another, reflecting both heterogeneity in the basement geology and variable preservation of the original composition of brines. The previously published 87Sr/86Sri and εNd(t) values of UO2 compare with the most evolved dolomites, i.e. dolomites precipitated from brines that exchanged the most with the basement. This reinforces a close genetic link between dolomites and UO2 deposition and implies that UO2 deposition occurred in a cooling system during the transition from quartz to dolomite formation.The δ18O and δD values of the mineralizing brines (δ18O=−1‰ to 8‰ and δD=−150‰ to −50‰) are considerably shifted from that of their theoretical original values acquired during evaporation of seawater (δ18O=∼−3‰ and δD=∼−40‰). The positive δ18O shift is explained by protracted fluid–rock interaction within the basin and basement rocks. The negative δD shift is attributed to incomplete mixing between the U-mineralizing brines and low δD water. This low δD water was likely produced during the abiogenic synthesis of bitumen by Fisher–Tropsch-like reactions involving CO2 derived from brine–graphite interaction in the basement, and radiolytic H2. The resulting low δD brines have been equilibrated with alteration minerals. This may explain why some alteration minerals yield anomalously low δD values whose significance has long been debated.

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

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

Tracing end-member fluid sources in sub-surface iron mineralization and dolomitization along a proximal fault to the dead sea transform

Shear faults in Upper Cretaceous limestones of the central Negev desert adjacent to the Dead Sea Transform (DST) feature extensive ferruginous mineralization and dolomitization. This has been related to topographically driven flow of metalliferous groundwaters through an underlying clastic (Nubian Sandstone) aquifer and rise of the fluids up the fault zones. The present study combines Pb and Sr isotope measurements with detailed sampling and petrography at the eastern end of the Paran fault (Menuha Ridge) in order to identify the types of groundwater and the sources of enriched elements in this regional-scale sedimentary mineralization. Ferroan and non-ferroan dolomitization along the Paran fault caused significant enrichment of several elements (Mg, Cu, Mn, Ni, V, Zn, Pb, and U) and 87Sr/ 86Sr values that are significantly higher than the Upper Cretaceous limestone country rock. The non-ferroan dolomite and the ferroan dolomite sampled at three sites along the Menuha Ridge have similar 87Sr/ 86Sr values 0.7076–0.7089, and 0.7077–0.7086, respectively. Additionally, there is a positive correlation between Mg-content of the dolomites and their 87Sr/ 86Sr values. The isotopic composition of Sr and Pb of dolomite corresponds to the mineralogical type identified in the mineralized rock (non-ferroan dolomite, simple-zoned ferroan dolomite, and complex-zoned ferroan dolomite). The 207Pb/ 204Pb and 206Pb/ 204Pb ratios of Fe oxides and dolomites from the three sites plot on a straight line, where the simple-zoned ferroan dolomite values are at the non-radiogenic end of the line and the complex-zoned ferroan dolomites at the radiogenic end. Both 206Pb/ 204Pb and 207Pb/ 204Pb ratios in dolomites and to a lesser degree in Fe-oxides suggest that a mixing between two end-members controls the behavior of Pb in the mineralization products along the Paran fault. Rather than a single fluid source, the study indicates that two types of metalliferous groundwaters were involved in the dolomitization and mineralization along the Paran fault. The first, and hitherto undocumented, fluid source is the Mg-rich Dead Sea Rift brine, migrating in the sub-surface before dolomitizing the carbonate bedrock. Migration of the brines took a deep path to the site of mineralization, with temperatures reaching 75 °C. Based on the geological history of the region, this probably took place in the Late Miocene–Early Pliocene interval. The second type of groundwater acquired its high solute concentrations from leaching igneous rocks and clastic sediments in the sub-surface, and infiltrated along the Paran fault, precipitating Fe-rich minerals and caused the first stage of dolomitization. This groundwater flowed at shallower depth than the DSR brines, and at lower temperatures ( T ⩽ 50 °C). The study shows that sedimentary mineralization in faults adjacent to active transform fault zones may arise from the combination of several different fluid flow regimes.

High-resolution characterization and integrated study of a reservoir formation: the danian carbonate platform in the Aquitaine Basin (France)

This paper provides an example of an integrated multi-scale study of a carbonate reservoir. The Danian Lower R2 carbonate reservoir is located in the South of the Aquitaine Basin (France) and represents a potential underground gas storage site for Gaz de France. The Danian Lower R2 reservoir was deposited as a prograding carbonate platform bordered by a reef barrier. The effects of sedimentary and diagenetic events on the reservoir properties, particularly dolomitization, were evaluated. In this study, the reservoir quality has been assessed by seismic analyses at the basin scale, by log-analysis at the reservoir scale, by petrographic methods and by petrophysical tools at the pore-core scale. Two dolomitization stages, separated by a compaction event with associated fracturing and stylolites, have been identified. These diagenetic events have significantly improved the Lower R2 carbonate reservoir properties. It is demonstrated that the reservoir quality is mainly controlled by the pore-geometry, which is determined by various diagenetic processes. The permeability values of the reservoir range over 4 orders of magnitude, from 0.1 to 5600 mD and the porosity values range between 2 and 42%. Reservoir unit 4 (a karstic dolomite) shows the best reservoir properties with average porosity values ranging between 11.1% and 19.3% and an average permeability ranging between 379 and 766 mD. Reservoir unit 2 (a fine-grained limestone) shows the worst reservoir properties. The cementation factors range from 1.68 to 2.48. The dolomitic crystal carbonate texture (mainly units 3 and 4) shows the highest value of the cementation factor (1.98–2.48) and formation factor (9.54–36.97), which is due to its high degree of cementation. The saturation exponents vary between 1.2 and 3.4. Using these experimental electrical parameters and the resistivity laterolog tool we predicted the water saturation in the various reservoir units. The permeability was predicted by combining the formation factor with the micro-geometric characteristic length. The best fit is obtained with the Katz and Thompson's model and for a constant of 1/171.

Geology and Geochemistry of the Reocin Zinc-Lead Deposit, Basque-Cantabrian Basin, Northern Spain

The Reocin Zn-Pb deposit, 30 km southwest of Santander, Spain, occurs within Lower Cretaceous dolomitized Urgonian limestones on the southern flank of the Santillana syncline. The Reocin deposit is one of the largest known strata-bound, carbonate-hosted, zinc-lead deposits in Europe. The total metal endowment of the deposit, including past production and remaining reserves, is 62 Mt of ore grading 8.7 percent Zn and 1.0 percent Pb. The epigenetic mineralization consists of sphalerite and galena, with lesser marcasite and trace pyrite with dolomite as gangue. Microprobe analyses of different generations of dolomite revealed nonstoichiometric compositions with various amounts of iron (up to 14 mol % of FeCO3). Replacement of host dolomite, open-space filling of fractures, and cementation of breccias derived from dissolution collapse are the principal types of ore occurrence. Detailed cross-section mapping indicates a stratigraphic and structural control on the deposit. A stratiform morphology is present in the western part of the orebody (Capa Sur), whereas mineralization in the eastern part is highly discordant but strata bound (Barrendera). Stratigraphic studies demonstrate that synsedimentary tectonic activity, related to the rifting of the North Atlantic (Bay of Biscay), was responsible for variation in sedimentation, presence of unconformities (including paleokarsts), local platform emergence and dolomitization along the N60 fault trend. In the Reocin area, two stages of dolomitization are recognized. The first stage is a pervasive dolomitization of the limestone country rocks that was controlled by faulting and locally affected the upper part of the Aptian and the complete Albian sequence. The second dolomitization event occurred after erosion and was controlled by karstic cavities. This later dolomitization was accompanied by ore deposition and, locally, filling of dolomite sands and clastic sediments in karstic cavities. The circulation of hydrothermal fluids responsible for sulfide deposition and the infilling of karst cavities were broadly contemporaneous, indicating a post-Albian age. Vitrinite reflectance data are consistent with previously measured fluid inclusion temperatures and indicate temperatures of ore deposition that were less than 100°C. Carbon and oxygen isotopic data from samples of regional limestone, host-rock dolostone and ore-stage dolomite suggest an early hydrothermal alteration of limestone to dolostone. This initial dolomitization was followed by a second period of dolomite formation produced by the mixing of basinal metal-rich fluids with local modified seawater. Both dolomitization events occurred under similar conditions from fluids exhibiting characteristics of basinal brines. The δ 34S values of sulfides are between –1.8 and +8.5 per mil, which is consistent with thermochemical sulfate reduction involving organic matter as the main source of reduced sulfur. Galena lead isotope compositions are among the most radiogenic values reported for Zn-Pb occurrences in Europe, and they are distinct from values reported for galena from other Basque-Cantabrian deposits. This suggests that a significant part of the lead was scavenged from the local underlying Asturian sediments. The stratigraphic and structural setting, timing of epigenetic mineralization, mineralogy, and isotopic geochemistry of sulfide and gangue minerals of the Reocin deposit are consistent with the features of most of Mississippi Valley-type ore deposits.

Dolomitization of the Waulsortian Limestone (Lower Carboniferous) in the Irish Midlands

The Waulsortian Limestone (Lower Carboniferous) of the southern Irish Midlands is dolomitized pervasively over a much larger region than previous studies have documented. This study indicates a complex, multistage, multiple fluid history for regional dolomitization. Partially and completely dolomitized sections of Waulsortian Limestones are characterized by finely crystalline (0·01–0·3 mm) planar dolomite. Planar replacive dolomite is commonly followed by coarse (≥0·5 mm) nonplanar replacive dolomite, and pervasive void‐filling saddle dolomite cement is frequently associated with Zn–Pb mineralization. Planar dolomite has average δ18O and δ13C values (‰ PDB) of –4·8 and 3·9 respectively. These are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the Waulsortian (δ18O=–2·2, δ13C=3·7). Mean C and O isotope values of planar replacive dolomite are also distinct from those of nonplanar and saddle dolomite cement (–7·0 and 3·3; –7·4 and 2·4 respectively). Fluid inclusions indicate a complex history involving at least three chemically and thermally distinct fluids during dolomite cementation. The petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin, possibly in equilibrium with modified Carboniferous sea water. Where the Waulsortian was exposed to hydrothermal fluids (70–280 °C), planar dolomite underwent a neomorphic recrystallization to a coarser crystalline, planar and nonplanar dolomite characterized by lower δ18O values. Void‐filling dolomite cement is isotopically similar to nonplanar, replacive dolomite and reflects a similar origin from hydrothermal fluids. This history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the Irish Midlands and provides a framework upon which to test competing models of regional vs. localized fluid flow.

Dolomitization and Dolomite Neomorphism: Trenton and Black River Limestones (Middle Ordovician) Northern Indiana, U.S.A.

ABSTRACT The Trenton and Black River Limestones are dolomitized extensively along the axis of the Kankakee Arch in Indiana, with the proportion of dolomite decreasing to the south and southeast of the arch. Planar and nonplanar dolomite replacement textures and rhombic (type 1) and saddle (type 2) void-filling dolomite cements are present. Three stages of dolomitization, involving different fluids, are inferred on the basis of petrographic and geochemical characteristics of the dolomites. Nonferroan planar dolomite has relatively high 18O values (-1.8 to -6.1o/oo PDB) and has 87Sr/86Sr ratios (0.70833 to 0.70856) that overlap those of Middle Ordovician seawater. Petrography, geochemistry, and the geometry of the dolomitized body suggest that the planar dolomite was formed in Middle and Late Ordovician seawater during the deposition of the overlying Maquoketa Shale. Ferroan planar and nonplanar dolomite occurs in the upper few meters of the Trenton Limestone, confined to areas underlain by planar dolomite. This dolomite contains patches of nonferroan dolomite with cathodoluminescence (CL) characteristics similar to underlying planar dolomite. Ferroan dolomite has relatively low 18O values (-5.1 to -7.3o/oo PDB) and has slightly radiogenic 87Sr/86Sr ratios (0.70915 to 0.70969) similar to those obtained for the overlying Maquoketa Shale. These data indicate that ferroan dolomite formed by neomorphism of nonferroan planar dolomite as fluids were expelled from the overlying Maquoketa Shale during burial. The absence of ferroan dolomite at the Trenton-Maquoketa contact, in areas where the earlier-formed nonferroan planar dolomite also is absent, indicates that the fluid expelled from the overlying shale did not contain enough Mg2+ to dolomitize limestone. Type 1 dolomite cement has isotopic compositions similar to those of the ferroan dolomite, suggesting that it also formed from shale-derived burial fluids. CL growth zoning patterns in these cements suggest that diagenetic fluids moved stratigraphically downward and toward the southeast along the axis of the Kankakee Arch. Type 2 saddle dolomite cements precipitated late; their low 18O values (-6.0 to -7.0o/oo PDB) are similar to those of the type 1 dolomite cement. However, fluid-inclusion data indicate that the saddle dolomite was precipitated from more saline, basinal fluids and at higher temperatures (94° to 143°C) than the type 1 cements (80° to 104°C). A trend of decreasing fluid-inclusion homogenization temperatures and salinities from the Michigan Basin to the axis of Kankakee Arch suggests that these fluids emerged from the Michigan Basin after precipitation of type 1 cement.

Stratigraphic Patterns of Deep-water Dolomite, Northeast Australia

ABSTRACT In ODP Hole 815A, located along the edge of the Townsville Trough and marginal to the Marion Plateau of northeast Australia, dolomite forms about 5-20 wt % of carbonate within a distal part of a muddy contourite succession of Pliocene (2.6-4.0 Ma) age. Integration of seismic, lithic, and biostratigraphic data identifies a history of current-controlled episodic deposition subsequently shut down during rapid subsidence by ~2.6 Ma. Overlying pelagic-periplatform sediment contains no or trace amounts of dolomite. High-order, rhythmic (1-5 m thick) variation in dolomite, quartz, and carbonate content within the contourite succession defines a past, high-order stratigraphic variation in diagenetic potential for dolomite. d13C values of dolomite reflect incorporation of at least two sources of bicarbonate mixed with seawater: (1) in more carbonate-rich sediment, d13C values (1 to -2.0%) indicate likely contribution from dissolution of metastable carbonate; (2) in more siliciclastic-rich sediment, with locally elevated total organic content, d13C values (-4 to -6%) reflect contribution from organic-matter diagenesis. The majority of d18O values of dolomite over the entire contourite succession vary between 3.2 and 2%, and are interpreted to reflect near-surface to shallow (tens of meters) subsurface, marine-derived diagenesis. A few dolomites with lower (2 to 0%) values are restricted to two stratigraphic intervals, and reflect one or some combination of the following processes: a second stage of dolomitization at greater depths; near-surface diagenesis with much warmer bottom waters; or burial diagenesis with mixture of meteoric- and marine-derived pore fluids. Magnesian calcite is absent today in the contourite succession. However, the depositional and diagenetic frameworks of the contourite succession suggest that metastability of aragonite and magnesian calcite, variable current activity, and seawater diffusion across the sediment-water interface all promoted dolomitization. Compilation of dolomite stratigraphy along slopes off northeast Australia illustrates that dolomite distribution in Hole 815A is one of four intervals (D1-D4) in strata younger than 4 Ma: D1, 0.3-0.8 Ma; D2, 1-1.3 Ma; D3 (D3a, ~ 1.8 and D3b, 2.0-2.4 Ma); and D4, 2.6-4.0 Ma. The stratigraphy is linked to temporal variation in supply of shelf- and pelagic-derived metastable carbonate regulated by changes in paleoceanography, eustasy, and tectonics. D1, D3b, and D4 are in sediments with ages similar to previously documented dolomitization stages in shallow-water platforms elsewhere in the world, and suggest a coincidence between elevated diagenetic potential for deep-water dolomitization in platform margins and substantive changes in ocean c volume transport linked to eustatic change in sea level.

Burial dolomitization and dedolomitization of the late Cambrian Wagok Formation, Yeongweol, Korea

The Cambrian Wagok Formation in Yongweol, Korea, involved two stages of dolomitization, separated by an intermediate phase of dissolution, dolomite cementation, and dedolomitization. Textural, isotopic, and chemical investigations of the dolomites and calcite indicate that they all formed or recrystallized in a burial diagenetic environment. Cathodoluminescence petrography shows two stages of dolomitization (Types I & II). Type I dolomite is non- to dully luminescent with a heavily corroded outre rim, comprising more than 90% of the whole sequence, whereas Type II dolomite is brightly luminescent and overgrew mostly on the corroded Type I dolomite. Replacement of Type I dolomite by calcite before the Type II dolomitization is supported by the following textural evidences: 1) the tip of Type I dolomite is heavily corroded; 2) relic crystal boundaries as well as ghosts of Type I dolomite can be observed within calcite; 3) calcite contains relic crystals of Type I dolomite; and 4) Type II dolomites always show sharp crystal surfaces, and cross-cut into calcite. Carbon isotopic compositions of Type I dolomite are in the range of −0.6 to +0.8‰ (PDB), which suggests that carbon isotopes were buffered by preexisting marine carbonates. Lighter oxygen isotopic values (−9.3 to −8.2‰, PDB) as well as xenotopic texture indicate that Type I dolomites formed in a burial diagenetic environment. Type I & II dolomites and dedolomite are Fe-, Mn-, Na-, and Sr-poor, all less than 300 ppm. Thus, similar trace and minor elemental contents of the three phases suggest that the diagenetic fluids responsible for all the phases were chemically similar in terms of trace and minor elements. The higher initial87Sr/86Sr ratio of Type I dolomite (0.7102–0.7105) than that of Cambrian seawater (0.7088–0.7092) suggests that the dolomitizing fluids were basin-derived.

Timing- and origin of dedolomite in upper Wappinger Group (Lower Ordovician) strata, southeastern New York

Detailed petrographic-, geochemical-, and geologic (including surface- and subsurface) study of the massive regional-scale dolostones of the upper Wappinger Group (Lower Ordovician part of Sauk Sequence, southeastern New York) enables us to recognize the products of three stages of dolomitization and of one episode of dedolomitization. The three dolomite stages are: (1) earliest syndepositional dolomite, very fine crystalline dolomite (10 to 30 μm) and some tiny cores of coarser rhombs and displaying δ18O mean value of −7.2‰ and δ13C mean value of −1.2‰; (2) second-stage, shallow-burial dolomites including both replacement-type phases (fine- to coarsely crystalline dolomite), and some dolomite cement that was precipitated between crystals of replacement origin and displays δ18O mean value of −7.6‰ and δ13C mean value of −1.7‰; and (3) late-stage dolomite that fills fractures and crosscuts earlier-stage dolomites and that probably formed at intermediate depth of burial.

Jabali, a Zn-Pb-(Ag) carbonate-hosted deposit associated with Late Jurassic rifting in Yemen

The Jabali deposit (3.8 Mt at 16% Zn, 2% Pb and 132 g/t Ag) is hosted by dolomitized platform carbonates of Kimmeridgian age at the southwestern edge of the oil-producing Wadi al Jawf rift basin in northern Yemen. Paleogeographical reconstructions demonstrate that tensional synsedimentary tectonic activity from the Late Jurassic to Early Cretaceous was responsible for the thick accumulation of argillaceous and evaporitic sediments in the subsident rift basin, the unstable margin of which was the site of rapid facies changes, local disconformities and periods of emergence, as well as of dolomitization along the WNW- and NNW-striking boundary fault system. In the Jabali area, the upper part of the Jurassic sequence underwent two stages of dolomitization before emergence and deep karstic erosion. Solution cavities and depressions in the eroded surface were filled by dolomite sand and black pyritic mudstone prior to a last marine transgression of limited extent. Subsequent ore deposition and associated late dolomitization sealed the network of solution cavities, impregnating the dolomite sands and the host dolomites. Sphalerite I and wurtzite, followed by silver-bearing zoned sphalerite II associated with galena, crystallized from a cyclic influx of low-temperature (75-100°C) saline solutions. Lead isotope geochemistry indicates that the lead, zinc and silver probably originated from an Early Proterozoic basement. The dissolved metals were likely derived from the basal aquifer (detrital material of basement origin) of the evaporite-bearing sequence filling the Wadi al Jawf trough. Migrating metalliferous brines from the basin to the uplifted Jabali area, where ore deposition was favoured by a reducing environment, were probably channelled by the boundary fault system during the last stages of synsedimentary tectonic activity.

Dolomitization and Porosity Development in the Middle and Upper Wabamun Group, Southeast Peace River Arch, Alberta, Canada

Dolomitization and fracturing are critical to porosity development in most Wabamun Group reservoirs in the southern part of the Peace River arch. Cores from Peoria, Normandville, and Eaglesham fields and adjacent areas were examined to determine the relationship between depositional facies, dolomitization, fracturing, and porosity. Three-dimensional (3-D) seismic data indicate that most upper Wabamun dolomites are coincident with mound-like structural highs at the top of the Wabamun, with structural relief due largely to differential compaction between dolomite and adjacent limestone. The dolomite bodies generally are small, only 100-300 m across, which makes 3-D seismic data very helpful in Wabamun exploration. When this depositional/diagenetic model for dolomitization w s integrated into 3-D seismic interpretations, Canadian OccidentaPs discovery rate in the Wabamun increased from approximately 25 to 80%. Dolomitization and porosity are laterally quite variable in the Wabamun with the 200-250-m Wabamun section changing laterally from porous dolomite to nonporous limestone in less than 1 km (sometimes as little as 150 m). Two generations of replacive dolomite were identified, early near-surface and late hydrothermal. The early dolomite is facies, fabric, and mineralogy selective with some burrow fills and aragonitic mollusks being preferentially dolomitized in dolomitic limestones. Complete dolomitization is common in unfossiliferous carbonates (mudstones and peloid wackestones and packstones), which are interpreted as being deposited on paleotopographic highs and dolomitized by seawater or slightly evaporated seawater during shallow burial. Most fracturing occurred after early, repla ive dolomitization. Late hydrothermal dolomitization was not strictly facies or fabric selective. Features associated with hydrothermal dolomites include vugs, fractures, collapse breccias, white dolomite cement, anhydrite, and coarse calcite cement, although anhydrite and coarse calcite cement precipitated distinctly after hydrothermal dolomitization. Late dolomitization, karst-like dissolution and collapse apparently were localized near the margins of the early dolomites where adjacent limestones were dolomitized or dissolved. Dissolution of calcite in partially dolomitized limestones resulted in geopetal accumulations of dolomite rhombs (porous sucrosic dolomite) in some fractures and burrow fills. Hydrothermal fluids apparently moved updip through the underlying Winterburn and/or Leduc formations, an then upward through porous, early dolomites in the Wabamun, causing dolomitization of adjacent limestones and karst-like dissolution. Stable oxygen isotope ratios of the dolomites are quite variable (^dgr18O ranges from -2.0 to -11.4^pmil, PDB), supporting multiple stages of dolomitization during progressive burial.