Stable isotope evidence for multiple fluid regimes during carbonate cementation of the Upper Tertiary Hazeva Formation, Dead Sea Graben, southern Israel

Stable carbon- and oxygen-isotope compositions of calcite and dolomite cements have been used to understand porewater evolution in the Upper Tertiary Hazeva Formation within the Dead Sea Graben, southern Israel. Sandstone samples were obtained from four boreholes in three tectonic blocks of the graben over depths of 253–6448 m, a variation that largely reflects differential subsidence of individual fault-bounded blocks. Early carbonate cements dominate diagenesis. Calcite occurs at <1600 m, but was replaced by dolomite at greater depths. Dolomite at 1600–2700 m is Fe-poor (<0.8 mol% FeCO₃), and at 4700–6200 m, Fe-rich (0.5–7.2 mol% FeCO₃). Magnesite, anhydrite and halite are the final diagenetic phases. Calcite has positively correlated δ¹⁸O (+21‰ to +25‰) and δ¹³C (−6‰ to −2‰) values that generally decrease with depth. Dolomite has a wider variation in δ¹⁸O (+18‰ to +30‰) and δ¹³C (−8‰ to −1‰) values, which also generally are lower with increasing depth. However, the δ¹³C and δ¹⁸O values of dolomite from the uppermost 400 m of the Hazeva Formation in the Sedom Deep-1 borehole are anomalous in spanning the entire range of stable carbon and oxygen isotopic compositions over this relatively small interval.The decreasing dolomite δ¹³C values likely indicate an increased contribution of carbon from organic sources with increasing depth. Except for the uppermost 400 m, Hazeva Formation dolomite in the Sedom Deep-1 borehole has stable carbon-isotope compositions that imply initial dolomitization at much shallower levels, prior to the preferential subsidence of this tectonic block. The oxygen isotopic compositions of the calcite cement are best explained by equilibration at present burial temperatures (≤55 °C) with porewater of meteoric origin. Its δ¹⁸O values increased from −5‰ at the shallowest depths to 0‰ at 1600 m. The dolomite oxygen isotopic compositions also reflect equilibration at present burial temperatures with porewaters ranging from 0‰ at 1600 m to +7‰ at 3600 m (100 °C). In the deepest fault block (Sedom Deep-1 borehole), however, increasingly Fe-rich dolomite has (re)equilibrated with porewater whose δ¹⁸O values decreased from +9‰ at 4750 m (120 °C) to +1‰ to +2‰ by 6200 m (150 °C).Much of the dolomite likely formed at relatively shallow depths from saline brines derived from precursors to the Dead Sea. These infiltrated the Hazeva Formation, mixing with and largely displacing meteoric water, and dolomitizing calcite. Rock–water ratios tended to be high during these processes. However, the upper 400 m of the Hazeva Formation in the deepest fault block were likely deposited during its rapid tectonic subsidence, and largely escaped the initial style of dolomitization pervasive elsewhere in the study area. These sediments were also capped by evaporites. This relatively thin interval likely became a preferential conduit for brines that escaped underlying and overlying strata, including the Fe-rich, lower ¹⁸O fluids (evolved seawater?) present in the deepest part of the graben. These rocks present the most promising target for the passage and accumulation of hydrocarbons in the study area.     

Burial-dominated cementation in non-tropical carbonates of the Oligocene Te Kuiti Group, New Zealand

Cementation of bryozoan-echinoid-benthic foraminiferal temperate shelf carbonates of the Oligocene Te Kuiti Group, North Island, New Zealand, occurred mainly during subsurface burial. The calcite cements in the limestones are dominated by equant and syntaxial rim spar which typically becomes ferroan (given an iron supply) and, compared to the skeletal material with normal marine δ¹⁸O values from +2 to −1‰, more depleted in ¹⁸O with depth of burial, the δ¹⁸O composition of bulk cement samples ranging from −1 to −7‰. These trends reflect the establishment in pore waters during sediment burial of reducing conditions and gradually increasing temperatures (20–50°C), respectively. The δ¹³C values (0 to +3‰) of the cements remain the same as the host marine shells, suggesting the source of carbon in the cements was simply redistributed marine carbonate derived from shell dissolution.

Two gradational burial diagenetic environments influenced by marine-derived porewaters are arbitrarily distinguished: shallow burial phase and moderate burial phase. During the shallow burial phase, down to 500–600 m sub-bottom depth, the carbonates lost at least 25% of their original porosity by mechanical compaction and were selectively cemented by non-ferroan or usually ferroan, variably luminescent, slightly ¹⁸O-depleted sparry calcite cement (δ¹⁸O −2 to −4‰), mainly as syntaxial rims about echinoid grains. These shallow-burial cements form less than about 10% of total cement in the majority of the limestones and their source was probably mainly mild intergranular dissolution of calcitic skeletal fragments accompanying the onset of chemical compaction. During the moderate burial phase, between about 600 and 1100 m sub-bottom depth, porosity loss continued (typically to about 70% of its original value) as a result of pressure-solution of calcitic bioclasts associated with more advanced stages of chemical compaction. This involved development of a wide variety of non-sutured and microstylolitic solution seams, including both single and composite, wispy or continuous, bedding-parallel types and non-parallel reticulate forms. The released carbonate was precipitated as ferroan (or non-ferroan where iron supply was negligible), dull luminescent, strongly ¹⁸O-depleted (δ¹⁸O −4 to −7‰), mainly equant calcite spar cement, occluding available pore space in the limestones.     

Geochemistry and genesis of brine emanations from Cretaceous strata of the Mamfe Basin, Cameroon

The geochemistry of 5 salt springs in the southwestern Mamfe Basin was investigated in order to infer the mineral content of their source and to relate the genesis of the springs to the local geology. Field observations revealed that, they are cold springs (23–28 °C), and are composed of secondary brines that are neutral to alkaline with pH values ranging from 7 to 8.7. Results of chemical analysis show that the springs contain major ions that form evaporite minerals, as well as chalcophile elements. The dominant cation is Na⁺ (>96%), and the dominant anion is Cl⁻ (>99%). Based on correlation coefficients between ions that form evaporites and field occurrence of efflorescences of halite, it is suggested that the ancient evaporites in the Mamfe Basin are composed entirely of carbonate and chloride salts. Meteoric and convective fluid flow processes are responsible for the dissolution of ancient evaporites and subsequent migration of brines to the surface from underground. The brines migrate through permeable strata with migration pathways resulting from a combination of fracture porosity created by post––Cretaceous tectonism and intergranular porosity enhanced by the chemically aggressive migrating brines.     

Origin of coarse-crystalline calcite cement in Early Ordovician carbonate rocks, Ordos basin, northern China: Insights from oxygen and carbon isotopes and fluid inclusion microthermometry

The calcite cement in the Lower Ordovician Majiagou Formation in the Ordos basin in northern China can be subdivided into three groups based on preliminary results of oxygen and carbon isotopes and fluid inclusion microthermometry. Group 1 has low oxygen isotopes (− 14‰ to − 18‰), low T_h values (92–103 °C), and low salinities (1.7–4.9 wt.% NaCl equivalent) and is interpreted to have precipitated during early burial from porewater influenced by meteoric water. Group 2 has much higher oxygen isotope values (− 5‰ to − 8‰), which, coupled with the higher T_h values (136–151 °C), suggest that the calcite was precipitated from fluids that were significantly enriched in ¹⁸O, possibly resulting from fluid–rock reaction during burial. Group 3 occurring along fractures is characterized by high salinities (21–28 wt.% NaCl equivalent) and is interpreted to have been precipitated from locally preserved residual evaporitic brines. The occurrence of primary hydrocarbon inclusions and its low carbon isotopes (− 11‰ to − 15‰) suggest that precipitation of group 3 calcite took place in the presence of hydrocarbons.     

Diagenesis of an Upper Triassic reef complex, Wilde Kirche, Northern Calcareous Alps, Austria

The Wilde Kirche reef complex (Early-Late Rhaetian) grew as an isolated carbonate structure within the shallow Kössen Basin. At the Triassic/Jurassic boundary a single brief (c. 10–50 ka) period of subaerial exposure occurred. The preserved karst profile (70 m thick) displays a vadose zone, enhanced dissolution at a possible palaeo-watertable (5–15 m below the exposure surface), and a freshwater phreatic zone. Karst porosity was predominantly biomouldic. Primary cavities and biomoulds were enlarged and interconnected in the freshwater phreatic zone; cavity networks developed preferentially in patch reef facies. Resubmergence of the reef complex allowed minor modification of the palaeokarst surface by sea floor dissolution and Fe-Mn crust deposition on a sediment-starved passive margin. Fibrous calcite (FC). radiaxial fibrous calcite (RFC) and fascicular optic calcite (FOC) cements preserved as low Mg calcite (LMC) are abundant in primary and karst dissolution cavities. FC cement is restricted to primary porosity, particularly as a synsedimentary cement at the windward reef margin. FC, RFC and FOC contain microdolomite inclusions and show patchy non-/bright cathodoluminescence. δ¹⁸O values of non-luminescent portions (interpreted as near original) are − 1.16 to − 1.82%0 (close to the inferred δ¹⁸O of calcite precipitated from Late Triassic sea water). δ¹³C values are constant (+3 to + 2.2%0). These observations suggest FC, RFC and FOC were originally marine high Mg calcite (HMC) precipitates, and that the bulk of porosity occlusion occurred not in the karst environment but in the marine environment during and after marine transgression. The HMC to LMC transition may have occurred in contact with meteoric water only in the case of FC cement. The most altered (brightly luminescent) portions of RFC/FOC cements yield δ¹⁸O=−2.44 to − 5.8%0, suggesting HMC to LMC alteration at up to 34°C. in the shallow burial environment at depths of 180–250 m. Abundant equant cements with δ¹⁸O =−4·1 to −7.1%0 show crisp, uniform or zoned dull luminescence. They are interpreted as unaltered cements precipitated at 33–36°C at 200–290 m burial depth, from marine-derived fluids under a slightly enhanced geothermal gradient. Fluids carrying the equant cements may have induced the HMC to LMC transition in the fibrous cements.     

A strontium, oxygen and hydrogen isotopic composition of brines, Michigan and appalachian basins, Ontario and Michigan

⁸⁷Sr/⁸⁶Sr ratios of brine from samples from the Michigan and Appalachian Basins, in Ontario and Michigan, covering the stratigraphic interval from the Cambrian to Mississippian, vary from 0.708 to 0.711. With the exception of the salt units of the Salina Formation (Silurian), most values are greater than seawater for the time in question, indicating water-rock interaction. The sources of the radiogenic Sr has not been identified. All samples plot below the GMWL in δ¹⁸O−δ²H space, with the Cambrian and Ordovician samples closest to the line. Mixing of brines meteoric and glacial (Pleistocene) water is indicated in some cases. The more concentrated brines from each stratigraphic unit show a very narrow spread in values. All the Ordovician brines show a narrow range over a 200 km area for Sr, O and H isotopes, indicating extensive lateral migration of the fluids.Strontium in the brine has not equilibrated isotopically with its host rock. In some cases the late-stage minerals saddle dolomite, calcite and anhydrite have the same ⁸⁷Sr/⁸⁶Sr ratios as the brine, indicating that they precipitated from the brine in isotopic equilibrium.     

Evidence for episodic cementation and diagenetic recording of seismic pumping events,North Coles Levee,California, U.S.A.

Bands of calcite and dolomite cements alternating with zones of nearly carbonate-free sand occur in the Stevens sandston aat North Coles Levee, San Joaquin, Valley, California. Temperatures calculated from O isotopes suggest that the calcite cement bands were emplaced episodically as a result of repeated injections of hot water from deeper in the section. Burial analysis suggests that these cements precipitated from 7 Ma to the present over the temperature range of 45 to ∼95°C.Carbon isotope data suggest that the C in the cements is a mixture derived from two sources, detrital shell material (δ¹³C(PDB)≈) and CO₂ liberated from maturing kerogen (δ¹³C ≈ −24). Plots of δ¹³C vs time and depth of crystallization show that the cementation sequence was: (1) dolomite cements, possibly concretionary, precipitated at depths <1–2 km and at temperatures <45°C; (2) calcite cements with δ¹³C(PDB) values as low as −13, crystallized from depths between 1220 and 1820 m (4000 and 6000 ft) and at temperatures between 45 and 80°C; (3) calcite cements with δ¹³C(PDB) values approaching zero and calculated temperatures of crystallization up to the present reservoir temperature of 95±3°C.A log of δ¹³C vs calculated depth of crystallization correlates with the stratigraphic column at North Coles Levee. If the correlation is valid the light δ¹³ in each cement sample can be tied to its source. A model based on this interpretation suggests that the early, light C was derived from maturing kerogen in the Kreyenhagen Formation (Eocene) as it passed through the oil window between 4 and 5 Ma. The subsequent passage of younger sediments with less organic material produced correspondingly smaller amounts of light CO₂ which was reflected in the relatively heavier C isotopes in the later cements.It is suggested that the epidsodic injections of hot water carried dissolved gases and minerals, principally calcite, upward from rocks as deep as 2–3 km below the Stevens sandstone and reprecipitated the calcite in more permeable zones in the rock. Degassing of CO₂ from rising pore waters likely triggered the precipitation and accounts for the relatively large volumes of cement. The Sibson model for seismic pumping of pore fluids is considered a likely explanation for the observed cementation.     

Pleistocene paleoclimate of the arid region of Israel as recorded in calcite deposits along regional transverse faults and in veins

The δ¹⁸O and δ¹³C values of the calcites associated with E-W and NE-SW transverse faults in the Negev, Israel, indicate that calcite was deposited from meteoric water. A regional change in the δ¹⁸O and δ¹³C values was observed. The ¹⁸O content in the calcite increases, from the southwestern (δ¹⁸O = −17.8‰) to the northeastern (δ¹⁸O = −2.9‰) part of the region. The δ¹³C values show the opposite trend of the ¹³C content decrease: from +2‰ in the south to −10‰ in the northeast. These trends had to reflect changes in regional paleoclimate, suggesting a change in the isotopic composition of the solution from which the calcite was deposited in different periods. The variations in the δ¹⁸O values reflect shifts in the δ¹⁸O values of precipitation and are associated with a change in the source of moist air masses which came from the equatorial Atlantic in the early Pleistocene and from the Mediterranean during a later period. Variations in δ¹³C values reflect changes from humid to arid conditions. Two modes of calcite deposition are suggested: (1) precipitation of calcite minerals in the unsaturated zone following the dissolution in the soil or (2) calcite deposition that occurred as CO₂ was lost during emergence of paleogroundwater from Lower Cretaceous and Jurassic aquifers.     

Isotopic geochemistry of burial-metamorphosed volcanogenic sediments,Great Valley sequence,northern California

Isotopic and mineralogic data from an 8500-m thick section of the Great Valley sequence, northern California, indicate that changes in the δ¹⁸O values of authigenic minerals resulted from the conversion of smectite to a 10 Å clay-mineral as temperature increased with burial in the Jurassic- Cretaceous outer-arc basin. The clay-mineral assemblage in mudstone is characterized by a proportional increase of the 10 Å clay-mineral with increasing stratigraphic depth, and by a depletion in the δ¹⁸O value of the mixed-layer smectite/10 Å clay-mineral with descending stratigraphic position from +21.9 to + 15.5%. SMOW. Modeling of the oxygen isotopic data from authigenic phases, based on equilibrium fractionation during clay-mineral diagenesis, indicates that δ¹⁸O values of calcite in mudstones and of calcite cements in sandstone precipitated along a temperature gradient of about 25°C/km during maximum burial to about 6–7 km. δD values of the mixed-layer smectite/10 Å clay-mineral range between ?69 to ?44%. SMOW. Using temperatures calculated from the oxygen isotopic data, the deuterium and oxygen isotopic data indicate that the smectite underwent late-stage dehydration and probably buffered the composition of formation waters from sea water values to isotopic compositions of δ¹⁸O ≈ +8%. SMOW and δD ≈ ?25%. SMOW. The δ¹³C values of calcite from mudstone and sandstone imply that crystallization of authigenic calcite was linked to organic diagenesis during which dissolved HCO^t-₃ was continuously enriched in ¹³C as temperature increased with burial. At the base of the sequence and immediately overlying the ophiolitic basement rocks, several hundred meters of strata were altered by more oxygen-depleted (δ¹⁸O ? +4 to +5%.) hydrothermal fluids emanating from the ophiolitic rocks, probably at maximum burial depth.     

Diagenesis and reservoir quality analysis in the Lower Cretaceous Qishn sandstones from Masila oilfields in the Sayun–Masila Basin,eastern Yemen

Lower Cretaceous sandstones of the Qishn Formation have been studied by integrating sedimentological, petrological and petrophysical analyses from wells in the Masila oilfields of eastern Yemen. These analyses were used to define the origin, type of diagenesis and its relation to reservoir quality. The sandstones of the Qishn Formation are predominately quartz arenite to subarkose arenite with sublitharenite and quartz wackes displaying a range of porosities, averaging 22.33%. Permeability is likewise variable with an average of 2844.2 mD. Cementation coupled with compaction had an important effect on porosity destruction after sedimentation and burial. The widespread occurrence of early calcite cement suggests that the sandstones of the Qishn Formation lost significant primary porosity at an early stage of its diagenetic history. In addition to poikilotopic calcite, several different cements including kaolinite, illite, chlorite and minor illite–smectite occur as pore‐filling and pore‐lining cements, which were either accompanied by or followed the development of the early calcite cement. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar grains. The new data presented in this paper suggest the reservoir quality of Qishn sandstones is strongly linked to their diagenetic history; hence, the reservoir quality is reduced by clay minerals, calcite and silica cements but is enhanced by the dissolution of the unstable grains, in addition to partial or complete dissolution of calcite cements and unstable grains. Copyright © 2015 John Wiley & Sons, Ltd.     

Late-stage calcites in the Permian Capitan Formation and its equivalents, Delaware Basin margin, west Texas and New Mexico: evidence for replacement of precursor evaporites

Comparison of Upper Guadalupian fore-reef, reef and back-reef strata from outcrops in the Guadalupe Mountains with equivalent subsurface cores from the northern and eastern margins of the Delaware Basin indicates that extensive evaporite diagenesis has occurred in both areas. In both surface and subsurface sections, the original sediments were extensively dolomitized and most primary and secondary porosity was filled with anhydrite. These evaporites were emplaced by reflux of evaporitic fluids from shelf settings through solution-enlarged fractures and karstic sink holes into the underlying strata. Outcrop areas today, however, contain no preserved evaporites in reef and fore-reef sections and only partial remnants of evaporites are retained in back-reef settings. In their place, these rocks contain minor silica, very large volumes of coarse sparry calcite and some secondary porosity. The replacement minerals locally form pseudomorphs of their evaporite precursors and, less commonly, contain solid anhydrite inclusions. Some silicification, dissolution of anhydrite and conversion of anhydrite to gypsum have occurred in these strata where they are still buried at depths in excess of 1 km; however, no calcite replacements were noted from any subsurface core samples. Subsurface alteration has also led to the widespread, late-stage development of large- and small-scale dissolution breccias. The restriction of calcite cements to very near-surface sections, petrographic evidence that the calcites post-date hydrocarbon emplacement, and the highly variable but generally ‘light’carbon and oxygen isotopic signatures of the spars all indicate that calcite precipitation is a very late diagenetic (telogenetic) phenomenon. Evaporite dissolution and calcitization reactions have only taken place where Permian strata were flushed with meteoric fluids as a consequence of Tertiary uplift, tilting and breaching of regional hydrological seals. A typical sequence of alteration involves initial corrosion of anhydrite, one or more stages of hydration/dehydration during conversion to gypsum, dissolution of gypsum and precipitation of sparry calcite. Such evaporite dissolution and replacement processes are probably continuing today in near-outcrop as well as deeper settings. This study emphasizes the potential importance of telogenetic processes in evaporite diagenesis and in the precipitation of carbonate cements. The extensive mineralogical and petrophysical transformations which these strata have undergone during their uplift indicates that considerable caution must be exercised in using surface exposures to interpret subsurface reservoir parameters in evaporitic carbonate rocks.     

Formation mechanisms of calcite cements in tight sandstones of the Jurassic Lianggaoshan Formation,northeastern Central Sichuan Basin

The characteristics and formation mechanism of calcite cements in the tight sandstone of the Jurassic Lianggaoshan Formation in the northeastern Central Sichuan Basin were analysed using petrographic and isotopic techniques. In the tight sandstone of the Lianggaoshan Formation, cements are mostly calcite and occur as poikilitic, pore-filling, fracture-filling and replacement of clastic particles. Contents of Al, Si, Fe and Mn in the poikilitic calcites are significantly less than that in the dissolution pore-filling and metasomatic calcites. Three stages (early, middle and late) of authigenic calcites correspond to temperature ranges of <60, 60–100 and ≥100?°C, respectively, with most calcite cements formed under lower temperature (<100?°C) conditions. The δ¹⁸O values of the early–middle authigenic calcites are in equilibrium with connate water, and the δ¹⁸O values of late calcites are depleted in ¹⁸O indicating equilibrium at higher temperatures. The early authigenic calcites precipitated in a relatively open system associated with CO₂ from bacterial fermentation at an immature to low-mature stage, and a Ca²⁺- and alkaline-rich environment owing to hydration–hydrolysis and dissolution of silicate minerals during phase A of eodiagenesis. The middle–late authigenic calcites precipitated in a relatively closed system with CO₂ from decarboxylation of organic acids and Ca²⁺ from dissolution of silicate minerals and transformation of clay minerals during phase B of eodiagenesis to mesodiagenesis. Calcite cements mainly occur in the medium and fine sandstones of sand flats and beach bars. Authigenic calcite dissolution is extremely weak, and calcite cementation is pore-space destructive.     

Fluid flow record from fracture-fill calcite in the Eocene limestones from the South-Pyrenean Basin (NE Spain) and its relationship to oil shows

The presence of oil shows associated with fractures provides a significant opportunity to a) unravel the type, origin and evolution of fluids involved in fracture-fills, and b) examine how they relate to oil migration. Two stages of calcite cement (C1 and C2) were distinguished in the fractures of the Eocene Armàncies platform carbonates; C1 is characterised by fence-like crystals, exhibits dull red luminescence and contains abundant twin planes, inclusions and δ¹⁸O values that range from − 6.2‰ to − 4.8‰ VPDB. C2 consists of blocky clean crystals, is characterized by dark brown-red luminescence that alternates with yellow bands, and contains hydrocarbon fluid inclusions with homogenisation temperatures of approximately 120 °C. δ¹⁸O values range from − 9.6‰ to − 8.9‰ VPDB. The remaining porosity after C2 precipitation is filled with liquid oil that reached 115 °C. This would seem to indicate that free oil and fluid inclusions oil probably come from the same migration pulse. Oil migration timing was coeval with C2 and continued after calcite cementation was completed.     

准噶尔盆地腹部超压顶面附近深层砂岩碳酸盐胶结作用和次生溶蚀孔隙形成机理

准噶尔盆地腹部深层超压顶面附近(现今埋深4400~6200m, 温度105~145℃) 砂岩中广泛出现的碳酸盐胶结作用和次生溶蚀孔隙与超压流体活动关系密切.由于超压封闭和释放引起其顶面附近砂岩中的孔隙压力和水化学环境的周期性变化可导致碳酸盐沉淀和易溶矿物溶解过程交替出现.根据腹部地区超压顶面附近深埋砂岩成岩作用、碳酸盐胶结物含量、储集物性、碳酸盐胶结物碳、氧同位素和烃源岩热演化模拟等资料的综合研究表明: 含铁碳酸盐胶结物是主要的胶结成分, 长石类成分的次生溶蚀孔隙是主要的储集空间类型, 纵向上深层砂岩碳酸盐胶结物出现15%~30%含量的地层厚度范围约在邻近超压顶面之下100m至之上大于450m, 碳酸盐胶结物出现大于25%含量高值带分布在靠近超压顶面向上的250~300m的地层厚度范围, 超压顶面附近砂岩中碳酸盐胶结物含量高值的深度范围也是次生溶蚀孔隙(孔隙度10%~20%) 发育带的范围; 因晚白垩世以来深部侏罗纪煤系有机质生烃增压作用, 导致在超压顶面附近砂岩晚成岩作用阶段深部富含碳酸盐的超压流体频繁活动, 所形成的碳酸盐胶结物受到了明显的与深部生烃增压有关的超压热流体和有机脱羧作用的影响; 超压流体多次通过超压顶面排放, 使得超压顶面附近砂岩遭受了多期酸性流体溶蚀作用过程, 形成了次生溶蚀孔隙发育带.      

Diagenesis in the Middle Jurassic Khatatba Formation sandstones in the Shoushan Basin,northern Western Desert,Egypt

The Middle Jurassic Khatatba Formation acts as a hydrocarbon reservoir in the subsurface in the Western Desert, Egypt. This study, which is based on core samples from two exploration boreholes, describes the lithological and diagenetic characteristics of the Khatatba Formation sandstones. The sandstones are fine‐ to coarse‐grained, moderately to well‐sorted quartz arenites, deposited in fluvial channels and in a shallow‐marine setting. Diagenetic components include mechanical and chemical compaction, cementation (calcite, clay minerals, quartz overgrowths, and a minor amount of pyrite), and dissolution of calcite cements and feldspar grains. The widespread occurrence of an early calcite cement suggests that the Khatatba sandstones lost a significant amount of primary porosity at an early stage of its diagenetic history. In addition to calcite, several different cements including kaolinite and syntaxial quartz overgrowth occur as pore‐filling and pore‐lining cements. Kaolinite (largely vermicular) fills pore spaces and causes reduction in the permeability of the reservoir. Based on framework grain–cement relationships, precipitation of the early calcite cement was either accompanied by or followed the development of part of the pore‐lining and pore‐filling cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar. Late kaolinite clay cement occurs due to dissolved feldspar and has an impact on the reservoir quality of the Khatatba sandstones. Open hydraulic fractures also generated significant secondary porosity in sandstone reservoirs, where both fractures and dissolution took place in multiple phases during late diagenetic stages. The diagenesis and sedimentary facies help control the reservoir quality of the Khatatba sandstones. Fluvial channel sandstones have the highest porosities and permeabilities, in part because of calcite cementation, which inhibited authigenic clays or was later dissolved, creating intergranular secondary porosity. Fluvial crevasse‐splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolinite matrix and pervasive, shallow‐burial calcite and quartz overgrowth cements, respectively. Copyright © 2013 John Wiley & Sons, Ltd.     

The sources and evolution of mineralising fluids in iron oxide–copper–gold systems, Norrbotten, Sweden: Constraints from Br/Cl ratios and stable Cl isotopes of fluid inclusion leachates

We have analysed the halogen concentrations and chlorine stable isotope composition of fluid inclusion leachates from three spatially associated Fe-oxide ± Cu ± Au mineralising systems in Norrbotten, Sweden. Fluid inclusions in late-stage veins in Fe-oxide–apatite deposits contain saline brines and have a wide range of Br/Cl molar ratios, from 0.2 to 1.1 × 10⁻³ and δ³⁷Cl values from −3.1‰ to −1.0‰. Leachates from saline fluid inclusions from the Greenstone and Porphyry hosted Cu–Au prospects have Br/Cl ratios that range from 0.2 to 0.5 × 10⁻³ and δ³⁷Cl values from −5.6‰ to −1.3‰. Finally, the Cu–Au deposits hosted by the Nautanen Deformation Zone (NDZ) have Br/Cl molar ratios from 0.4 to 1.1 × 10⁻³ and δ³⁷Cl values that range from −2.4‰ to +0.5‰, although the bulk of the data fall within 0‰ ± 0.5‰.The Br/Cl ratios of leachates are consistent with the derivation of salinity from magmatic sources or from the dissolution of halite. Most of the isotopic data from the Fe-oxide–apatite and Greenstone deposits are consistent with a mantle derived source of the chlorine, with the exception of the four samples with the most negative values. The origin of the low δ³⁷Cl values in these samples is unknown but we suggest that there may have been some modification of the Cl-isotope signature due to fractionation between the mineralising fluids and Cl-rich silicate assemblages found in the alteration haloes around the deposits. If such a process has occurred then a modified crustal source of the chlorine for all the samples cannot be ruled out although the amount of fractionation necessary to generate the low δ³⁷Cl values would be significantly larger.The source of Cl in the NDZ deposits has a crustal signature, which suggests the Cl in this system may be derived from (meta-) evaporites or from input from crustal melts such as granitic pegmatites of the Lina Suite.     

Depositional environment, sand provenance, and diagenesis of the Basal Salina Formation (lower Eocene), northwestern Peru

The Basal Salina Formation is a lower Eocene transgressive sequence consisting of interbedded shales, siltstones, and conglomeratic sandstones. This formation occurs in the Talara basin of northwestern Peru and is one of a series of complexly faulted hydrocarbon-producing formations within this extensional forearc basin. These sediments were probably deposited in a fan-delta complex that developed along the ancestral Amotape Mountains during the early Eocene. Most of the sediment was derived from the low-grade metamorphic and plutonic rocks that comprise the Amotape Mountains, and their sedimentary cover. Detrital modes of these sandstones reflect the complex tectonic history of the area, rather than the overall forearc setting. Unlike most forearc sediments, these are highly quartzose, with only minor percentages of volcanic detritus. This sand is variably indurated and cemented by chlorite, quartz, calcite, and kaolinite. Clay-mineral matrix assemblages show gradational changes with depth, from primarily detrital kaolinite to diagenetic chlorite and mixed-layered illite/smectite. Basal Salina sandstones exhibit a paragenetic sequence that may be tied to early meteoric influx or late-stage influx of thermally driven brines associated with hydrocarbon migration. Much of the porosity is secondary, resulting from a first-stage dissolution of silicic constituents (volcanic lithic fragments, feldspar, and fibrous quartz) and a later dissolution of surrounding carbonate cement. Types of pores include skeletal grains, grain molds, elongate pores, and fracture porosity. Measured porosity values range up to 24% and coarser samples tend to be more porous. Permeability is enhanced by fractures and deterred by clay-mineral cements and alteration residues.     

Diagenesis of Pennsylvanian phylloid algal mounds from the southern Cantabrian Zone (Spain)

The Pennsylvanian phylloid algal mounds exposed in the Cervatina Limestone of the Cantabrian Zone (NW Spain) developed during the highstands of high-frequency shallowing-upward cycles and lack evidence of subaerial exposure at their tops. Mound core facies are composed of massive bafflestones with variable amounts of calcite cements and anchicodiacean phylloid algae with cyathiform thalli preserved in growth position. Through standard petrographic, isotopic (δ¹⁸O and δ¹³C), major and trace element (Ca, Mg, Fe, Mn, Sr) and cathodoluminescence analyses, five calcite cement phases (cement 1 (C1)–cement 5 (C5)) have been identified filling primary and secondary pores. Early marine diagenesis is represented by micritization and non-luminescent to mottled-dull luminescent high-Mg calcite fibrous marine cement (C1). A dissolution phase then occurred and created vuggy and moldic pores. Based on the absence of field or petrographical or geochemical evidence of exposure, it is inferred that dissolution occurred in near-surface undersaturated marine waters with respect to aragonite related to progressive organic matter oxidation. Secondary porosity was subsequently filled by dull-bright-dull bladed high-Mg calcite (C2), which precipitated in the early shallow burial from marine-derived pore waters. Remaining porosity was occluded by shallow-burial precipitates consisting of non-luminescent scalenohedral low-Mg calcite (C3) followed by non-ferroan dull luminescent calcite spar (C4). Latter phases of calcite spar exhibiting non- and dull luminescence (C5) are associated with burial calcite veins. Low δ¹⁸O values (around ?8‰), moderately depleted δ¹³C values (around 0.5‰) and the homogeneity of trace element contents of carbonate matrix, cements and vein-filling calcites suggest burial isotopic re-equilibration and recrystallization, probably in Early Permian times during post-thrusting orocline formation.     

Replacement of evaporites within the Permian Park City Formation, Bighorn Basin, Wyoming, USA

The Permian Park City Formation consists of cyclically bedded subtidal to supratidal carbonates, cherts and siltstones. Early diagenesis of Park City Formation carbonates occurred under the influence of waters ranging from evaporative brines to dilute meteoric solutions and resulted in evaporite emplacement (syndepositional nodules and cements), as well as dolomitization, silicification and leaching of carbonate grains. Major differences are seen, however, in the diagenetic patterns of subsurface and surface sections of Park City Formation rocks. Subsurface samples are characterized by extensively preserved evaporite crystals and nodules, and preserve evidence of significant silicification (chert, chalcedony and megaquartz) and minor calcitization of evaporites. In outcrop sections, the evaporites are more poorly preserved, and have been replaced by silica and calcite and also leached. The resultant mouldic porosity is filled with widespread, very coarse, blocky calcite spar. These replacements appear to be multistage phenomena. Field and petrographic evidence indicates that silicification involved direct replacement of evaporites and occurred during the early stages of burial prior to hydrocarbon migration. Siliceous sponge spicules provided a major source of silica, and the fluids involved in replacement were probably a mixture of marine and meteoric waters. A second period of replacement and minor calcitization is inferred to have occurred during deep burial (under the influence of thermochemical sulphate reduction), although the presence of hydrocarbons probably retarded most other diagenetic reactions during this time interval. The major period of evaporite diagenesis, however, occurred during late stage uplift. The late stage replacement and pore-filling calcites have δ¹³C values ranging from 0·5 to -25·3%, and δ¹⁸O values of -16·1 to -24·3₀ (PDB), reflecting extensive modification by meteoric water. Vigorous groundwater flow, associated with mid-Tertiary block faulting, led to migration of meteoric fluids through the porous carbonates to depths of several kilometres. These waters reacted with the in situ hydrocarbon-rich pore fluids and evaporite minerals, and precipitated calcite cements. The Tosi Chert appears to have been an even more open system to fluid migration during its burial and has undergone a much more complex diagenetic history, as evidenced by multiple episodes of silicification, calcitization (ferroan and non-ferroan), and hydrocarbon emplacement. The multistage replacement processes described here do not appear to be restricted to the Permian of Wyoming. Similarly complex patterns of alteration have been noted in the Permian of west Texas, New Mexico, Greenland and other areas, as well as in strata of other ages. Thus, multistage evaporite dissolution and replacement may well be the norm rather than the exception in the geological record.     

济阳坳陷古近系成岩作用及其对储层质量的影响?

大量的储层分析化验资料研究表明，济阳坳陷古近系砂岩储层经历了压实、胶结等多种成岩作用。压实作用以机械压实为主，压溶作用较少。胶结作用主要有十分普遍的石英次生加大胶结、碳酸盐胶结、粘土矿物胶结。碳酸盐胶结物有早、晚两期，早期泥晶方解石主要分布在浅层，连晶方解石充填的深度分布范围大，中深部最为发育。溶蚀作用主要有碳酸盐胶结物、长石颗粒和少量岩屑溶蚀。储层孔隙经历了由原生到次生的演化过程，在成岩演化过程中，长石的溶蚀作用、碳酸盐矿物的沉淀与溶解作用是影响孔隙大小的关键因素，早期方解石的胶结有利于后期次生孔隙的发育。不同凹陷次生孔隙发育的深度不同。在济阳坳陷西部的惠民凹陷次生孔隙主要发育于1500-2400m深度，向东至东营凹陷则主要发育于1650-2450m，再向东到沾化凹陷为2300-3500m，车镇凹陷为2200-2700m。从西向东、由南向北次生孔隙的发育深度逐步增大是受盆地沉积沉降中心的迁移规律控制的。