Diagenesis and origin of calcite cement in the Flemish Pass Basin sandstone reservoir (Upper Jurassic): Implications for porosity development

The Flemish Pass Basin is a deep-water basin located offshore on the continental passive margin of the Grand Banks, eastern Newfoundland, which is currently a hydrocarbon exploration target. The current study investigates the petrographic characteristics and origin of carbonate cements in the Ti-3 Member, a primary clastic reservoir interval of the Bodhrán Formation (Upper Jurassic) in the Flemish Pass Basin.The Ti-3 sandstones with average Q_86.0F_3.1R_10.9 contain various diagenetic minerals, including calcite, pyrite, quartz overgrowth, dolomite and siderite. Based on the volume of calcite cement, the investigated sandstones can be classified into (1) calcite-cemented intervals (>20% calcite), and (2) poorly calcite-cemented intervals (porous). Petrographic analysis shows that the dominant cement is intergranular poikilotopic (300–500 μm) calcite, which stared to form extensively at early diagenesis. The precipitation of calcite occured after feldspar leaching and was followed by corrosion of quartz grains. Intergranular calcite cement hosts all-liquid inclusions mainly in the crystal core, but rare primary two-phase (liquid and vapor) fluid inclusions in the rims ((with mean homogenization temperature (T_h) of 70.2 ± 4.9 °C and salinity estimates of 8.8 ± 1.2 eq. wt.% NaCl). The mean δ¹⁸O and δ¹³C isotopic compositions of the intergranular calcite are −8.3 ± 1.2‰, VPDB and −3.0 ± 1.3‰, VPDB, respectively; whereas, fracture-filling calcite has more depleted δ¹⁸O but similar δ¹³C values. The shale normalized rare earth element (REE_SN) patterns of calcite are generally parallel and exhibit slightly negative Ce anomalies and positive Eu anomalies. Fluid-inclusion gas ratios (CO₂/CH₄ and N₂/Ar) of calcite cement further confirms that diagenetic fluids originated from modified seawater. Combined evidence from petrographic, microthermometric and geochemical analyses suggest that (1) the intergranular calcite cement precipitated from diagenetic fluids of mixed marine and meteoric (riverine) waters in suboxic conditions; (2)the cement was sourced from the oxidation of organic matters and the dissolution of biogenic marine carbonates within sandstone beds or adjacent silty mudstones; and (3) the late phases of the intergranular and fracture-filling calcite cements were deposited from hot circulated basinal fluids.Calcite cementation acts as a main controlling factor on the reservoir quality in the Flemish Pass reservoir sandstones. Over 75% of initial porosity was lost due to the early calcite cementation. The development of secondary porosity (mostly enlarged, moldic pores) and throats by later calcite dissolution due to maturation of organic matters (e.g., hydrocarbon and coals), was the key process in improving the reservoir quality.     

Fluid sources and stable isotope signatures in authigenic carbonates from the Northern Apennines,Italy

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

Mineral-forming and diagenetic processes related to Tertiary hydrocarbon seepage at the Bohemian Massif/Outer Western Carpathians interface: Evidence from the Hrabůvka quarry,Moravia, Czech Republic

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

苏北浅滩钙结核的特征及其环境指示意义

对采自苏北浅滩的钙结核样品进行了岩石学、矿物学和碳氧同位素的分析。样品富含石英和长石砂屑,碳酸钙胶结,其碳和氧同位素组成分别为-8.38‰~-8.19‰V-PDB及-5.23‰~-5.03‰V-PDB。根据样品的氧同位素组成,利用碳酸盐-水体系氧同位素方程,结合现今底层水温度,并考虑可能存在的温度变化,计算得到古沉淀流体的δ18 O水范围为-4.72‰~-4.52‰VSMOW,较正常海水偏负,认为钙结核的形成可能受到了淡水影响。根据结核中碎屑矿物的成分及其成熟度,判断其包含的碎屑矿物源自古黄河,认为这些结核样品可能形成于海陆交互环境,形成时间约为7~6.5ka BP左右,当时古海平面高度比现今低10m左右,古黄河河道可能位于苏北浅滩附近。     

The oxygen and carbon isotope distribution in the Mediterranean water masses

The oxygen and carbon stable isotope compositions of the present-day Mediterranean waters have been measured in order to evaluate their variability, which is related to the specific climatic and hydrological conditions within the basin. The experimental equation between the δ¹⁸O value and the salinity of water, based on 300 measurements on surface, intermediate, and deep waters sampled during the VICOMED 2 and 3 cruises in the western, central and eastern Mediterranean, has a slope of 0.27, a value which is significantly lower than the slope of 0.45, as defined in the northeast Atlantic Ocean. This difference in the δ¹⁸O–salinity relationship, which occurs immediately in the Alboran basin, is basically a characteristic of the climatic regime of the Mediterranean, i.e., of an excess evaporation over fresh water input. The largest variations of these two parameters, δ¹⁸O of water and δ¹³C of ∑CO₂, are observed in the surface waters, mostly in the western Mediterranean. This evolution mirrors the progressive eastward restriction, which separates the less-evaporated and more-productive western basins from the more-evaporated and less-productive eastern basins. The intermediate waters constitute a homogeneous layer. However, their δ¹⁸O values decrease eastward by 0.35‰ at maximum, due to progressive dilution by mixing with overlying and underlying water masses; their δ¹³C values decrease also eastward by 0.35‰ at maximum, due to an increasing input of nutrients issued from the regeneration of sinking organic particles. The deep waters have similar δ¹⁸O values but slightly higher δ¹³C values (often by less than 0.1‰) than the overlying intermediate waters, indicating generally well ventilated conditions due to active winter convection.     

Authigenic carbonates related to active seepage of methane-rich hot brines at the Cheops mud volcano,Menes caldera (Nile deep-sea fan,eastern Mediterranean Sea)

On the passive margin of the Nile deep-sea fan, the active Cheops mud volcano (MV; ca. 1,500 m diameter, ~20–30 m above seafloor, 3,010–3,020 m water depth) comprises a crater lake with hot (up to ca. 42 °C) methane-rich muddy brines in places overflowing down the MV flanks. During the Medeco2 cruise in fall 2007, ROV dives enabled detailed sampling of the brine fluid, bottom lake sediments at ca. 450 m lake depth, sub-surface sediments from the MV flanks, and carbonate crusts at the MV foot. Based on mineralogical, elemental and stable isotope analyses, this study aims at exploring the origin of the brine fluid and the key biogeochemical processes controlling the formation of these deep-sea authigenic carbonates. In addition to their patchy occurrence in crusts outcropping at the seafloor, authigenic carbonates occur as small concretions disseminated within sub-seafloor sediments, as well as in the bottom sediments and muddy brine of the crater lake. Aragonite and Mg-calcite dominate in the carbonate crusts and in sub-seafloor concretions at the MV foot, whereas Mg-calcite, dolomite and ankerite dominate in the muddy brine lake and in sub-seafloor concretions near the crater rim. The carbonate crusts and sub-seafloor concretions at the MV foot precipitated in isotopic equilibrium with bottom seawater temperature; their low δ¹³C values (–42.6 to –24.5‰) indicate that anaerobic oxidation of methane was the main driver of carbonate precipitation. By contrast, carbonates from the muddy lake brine, bottom lake concretions and crater rim concretions display much higher δ¹³C (up to –5.2‰) and low δ¹⁸O values (down to –2.8‰); this is consistent with their formation in warm fluids of deep origin characterized by ¹³C-rich CO₂ and, as confirmed by independent evidence, slightly higher heavy rare earth element signatures, the main driver of carbonate precipitation being methanogenesis. Moreover, the benthic activity within the seafloor sediment enhances aerobic oxidation of methane and of sulphide that promotes carbonate dissolution and gypsum precipitation. These findings imply that the coupling of carbon and sulphur microbial reactions represents the major link for the transfer of elements and for carbon isotope fractionation between fluids and authigenic minerals. A new challenge awaiting future studies in cold seep environments is to expand this work to oxidized and reduced sulphur authigenic minerals.     

Original oxygen isotopic composition of planktic foraminifers preserved in diagenetically altered Pleistocene shallow-marine carbonates

The original stable isotopic composition of low-Mg calcitic planktic foraminifer tests is preserved in Pleistocene shallow-marine carbonates (in the Ryukyu Group; Okinawa, Japan) that have been altered by meteoric diagenesis. Whole-rock analyses indicate depleted isotopic values for both δ¹³C (−1.9 to −5.4‰) and δ¹⁸O (−2.9 to −5.2‰), as well as carbonate mineralogy exclusively composed of low-Mg calcite. However, analysis of carefully-extracted planktic foraminifer tests (Globigerinoides sacculifer) that were separated from these whole-rock samples yield heavier δ¹³C values (−0.4 to 1.9‰) and δ¹⁸O values (−3.2 to −1.0‰). The foraminiferal values themselves and comparison of values of various components suggest that the low-Mg calcite tests preserve the original stable isotopic values. Subsequently, the downcore δ¹⁸O change of planktic foraminifers recorded in the Ryukyu Group results from middle Pleistocene glacial–interglacial change. By comparison, isotopic measurements based on whole-rock samples can be obtained diagenetic environmental signals, but misleading with regard to paleoclimatic inferences.     

Mechanism of carbonate cementation and its influence on reservoir in Pinghu Formation of Xihu Sag

Carbonate cements are the most abundant authigenic mineral and impact on physical properties greatly in sandstone reservoir. In this paper, Pinghu Formation of Xihu Sag was taken as a target. Characteristics,distribution and formation of carbonate cements were investigated via optical microscopy, cathodoluminescence(CL), electron probe and in-situ carbon-oxygen isotope. The results showed that carbonate cements varied in types and shapes. Calcite/dolomite mainly present as poikilotopic cements, ...     

Does suspended matter drained from the Isahaya freshwater reservoir cause organic enrichment in the northern Ariake Bay?

The inner part of Isahaya Bay was converted to a freshwater reservoir following the closure of the land claim dike in 1997. Turbid water drains into Isahaya Bay when water levels increase. We investigated whether particulate organic matter (POM) from the reservoir in Isahaya Bay has caused bottom organic enrichment in the northern part of Ariake Bay. Using potential end-members from before to after the rainy seasons, during which a frequent discharge from the reservoir was expected, stable isotope analyses were performed on sediments collected from Isahaya Bay and northern Ariake Bay. Each end-member was isotopically differentiated by δ¹³C and δ¹⁵N (riverine POM: ?28.5 to ?27.2‰ and 3.3–4.6‰; reservoir POM: ?25.7 to ?25.3‰ and 7.4–8.4‰; marine POM: ?21.8 to ?19.7‰ and 6.7–7.6‰; microphytobenthos estimated from consumers: ?16.1 to ?15.9‰ and 5.2–6.1‰, respectively). Sediment isotopic signatures fell within the mixing space defined by the signatures of the end-members. Marine POM contributed greatly to bottom sediments in both seasons in Isahaya Bay and Ariake Bay, ranging from ca. 60–70 and 40–60%, respectively. Reservoir POM contributed around 10% to bottom sediments. This percentage slightly increased in the sediment of Isahaya Bay after the rainy season, but decreased in the sediment of Ariake Bay. Thus, most of the POM discharged from the reservoir would not reach the northern part of Ariake Bay and would not be a major contributor to organic enrichment. This study is the first to quantitatively describe the contribution of drained reservoir POM outside Isahaya Bay.     

Exhumed hydrocarbon-seep authigenic carbonates from Zakynthos Island (Greece): Concretions not archaeological remains

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

Facies evolution and stratigraphic correlation in the early Oligocene Tard Clay of Hungary as revealed by maceral,biomarker and stable isotope composition

Hydrocarbon source rocks (i.e. Tard Clay Formation), containing type-II organic matter, were deposited in the Hungarian Paleogene Basin during Lower Oligocene. A major contribution of aquatic organisms (green algae, dinoflagellates, Chrysophyte algae) and minor inputs from macrophytes and land plants to organic matter accumulation is indicated by n-alkane distribution patterns, composition of steroids, and δ¹³C of hydrocarbon biomarkers. Microbial communities included heterotrophic bacteria, cyanobacteria, chemoautotrophic bacteria, as well as green sulphur bacteria. The presence of methanotrophic bacteria is indicated by ¹³C-depleted hop-17(21)-ene. Higher inputs of terrestrial organic matter occurred during deposition of the lower and uppermost units of the Tard Clay Formation. The terpenoid hydrocarbon composition argues for angiosperm-dominated vegetation in the area of the Hungarian Paleogene Basin. Diterpenoid hydrocarbons, derived from the resins of conifers, are about 2–3‰ enriched in ¹³C compared to the angiosperm biomarkers.Environmental conditions changed from marine to brackish, accompanied by oxygen-depletion in the lower parts of the water column. Organic carbon accumulation during this period was a consequence of stagnant bottom water conditions in the Hungarian Paleogene Basin due to salinity stratification. This is indicated by low pristane/phytane ratios (varying from 0.27 to 1.44), enhanced ratios of dimethylated versus trimethylated MTTCs (0.14–0.59), and the presence of aryl isoprenoids (from 0.4 to 14.1 μg/g TOC). A major marine incursion is evidenced by stable isotope ratios of organic matter and carbonates. In the uppermost member of the Tard Clay, a transition from brackish towards normal marine conditions is proposed.Up to 3 anoxic cycles are recognized in the drill core sections, separated by minima in pristane/phytane ratios and maxima in the depth trends of di-/tri-methylated MTTCs and aryl isoprenoid concentrations. In combination with the position of maxima of δ¹³C of carbonate and organic matter and an abrupt decrease in perylene concentrations, the cycles can be used for intra-formational correlation of the Tard Clay.     

Origin and distribution of hydrogen sulfide in the Yuanba gas field,Sichuan Basin,Southwest China

The Yuanba gas field in the Permian Changxing Formation (P₂c), which exhibits wide variations in its hydrogen sulfide (H₂S) concentration (1.20–12.16%), is a typical sour gas field in the northern Sichuan Basin. The sulfur-rich reservoir's solid bitumen (atomic S/C ratios are 0.032–0.142), and late calcite cement δ¹³C values, which are smaller than the δ¹³C values of the host dolostone, indicate that the H₂S originated from thermal sulfate reduction (TSR) and oil was involved in TSR. The gas souring index (GSI) of P₂c's gases is generally lower than 0.1. The ethane δ¹³C values increase as the GSI increases, although no obvious increase was observed in the methane δ¹³C values. The calcite cements' δ¹³C values (−15.36 to +4.56‰) in dolostone are heavier than the typical reported values, which implies that only limited heavy hydrocarbon gases were involved in TSR. No anhydrites developed in P₂c's reservoirs, and dissolved sulfate anions (SO₄²⁻) were mainly enriched during dolomitization. Insufficient dissolved SO₄²⁻ most likely caused the lower H₂S concentrations in the Permian to Triassic reservoirs in the northeastern Sichuan Basin compared to the Permian Khuff Formation in Saudi Arabia and the Jurassic Smackover Formation in Mississippi. Except for the SO₄²⁻ in residual water in paleo-oil zones, SO₄²⁻ from bottom water may also be involved in TSR; therefore, oil reservoirs with bottom water have more SO₄²⁻ and can produce more H₂S than pure oil reservoirs. This phenomenon may be the main cause of the great difference in the H₂S concentrations between reservoirs, while gravitational differentiation during late uplift most likely creates differences in H₂S concentrations in a single reservoir. Carbon dioxide (CO₂), which has a relatively heavy δ¹³C value (−3.9 to −0.3‰), may be the combined result of TSR, the balance between CO₂ and inorganic fluid systems, and carbonate decomposition.     

Isotopic composition of gas hydrates in subsurface sediments from offshore Sakhalin Island, Sea of Okhotsk

Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C₁/_C2+ from all the seepage sites were in the range of 1,500–50,000, while δ¹³C and δD values of methane ranged from ?66.0 to ?63.2‰ VPDB and ?204.6 to ?196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO₂. δ¹³C values of ethane and propane (i.e., ?40.8 to ?27.4‰ VPDB and ?41.3 to ?30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3–16.6‰, while there were no differences in δ¹³C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.     

A contribution to the reconstruction of Miocene seepage from authigenic carbonates of the northern Apennines (Italy)

Authigenic carbonates from outcrops of the northern Apennines consist of small and irregular lenses and exhibit numerous features indicative of cold-seep settings. Detailed petrographic, mineralogical and geochemical studies from two Miocene deposits are presented. The first carbonate outcrop, named Fosso Riconi, is located in the foredeep basin of the Apenninic chain, whereas the second deposit represents a satellite basin called Sarsetta. The stable isotope data from specific carbonate minerals show a wide range of values well known from other palaeoseeps of the Apennine Mountains. The majority of seep carbonates are formed by low-Mg calcite and ankerite. Those minerals have δ¹³C values between ?7 and ?23‰ V-PDB, suggesting variable amounts of carbonate derived from oxidized methane, seawater (dissolved inorganic carbon) and sedimentary organic matter. Dolomite samples have the lowest δ¹³C values (?30.8 to ?39.0‰ V-PDB), indicating methane as the main carbon source. The findings suggest an evolutionary formation of the seeps and development of authigenic carbonates influenced by the activity of chemosynthetic organisms, of which large lucinid clams are preserved. Bioirrigation by the clams controlled the sediment–water exchange, and is here considered as an explanation for the anomalous Mg content of the calcite. We hypothesize that the seep carbonates were formed during periods of active methane-rich seepage, whereas during periods of slow seepage carbonate formation was reduced. Despite different geological settings, the two examined deposits of Sarsetta and Fosso Riconi show similar features, suggesting that a common pattern of fluid circulation played a major role in carbonate formation at both seep sites.     

Gas flux and carbonate occurrence at a shallow seep of thermogenic natural gas

The Coal Oil Point seep field located offshore Santa Barbara, CA, consists of dozens of named seeps, including a peripheral ～200 m² area known as Brian Seep, located in 10 m water depth. A single comprehensive survey of gas flux at Brian Seep yielded a methane release rate of ～450 moles of CH₄ per day, originating from 68 persistent gas vents and 23 intermittent vents, with gas flux among persistent vents displaying a log normal frequency distribution. A subsequent series of 33 repeat surveys conducted over a period of 6 months tracked eight persistent vents, and revealed substantial temporal variability in gas venting, with flux from each individual vent varying by more than a factor of 4. During wintertime surveys sediment was largely absent from the site, and carbonate concretions were exposed at the seafloor. The presence of the carbonates was unexpected, as the thermogenic seep gas contains 6.7% CO₂, which should act to dissolve carbonates. The average δ¹³C of the carbonates was ?29.2?±?2.8‰ VPDB, compared to a range of ?1.0 to +7.8‰ for CO₂ in the seep gas, indicating that CO₂ from the seep gas is quantitatively not as important as ¹³C-depleted bicarbonate derived from methane oxidation. Methane, with a δ¹³C of approximately ?43‰, is oxidized and the resulting inorganic carbon precipitates as high-magnesium calcite and other carbonate minerals. This finding is supported by ¹³C-depleted biomarkers typically associated with anaerobic methanotrophic archaea and their bacterial syntrophic partners in the carbonates (lipid biomarker δ¹³C ranged from ?84 to ?25‰). The inconsistency in δ¹³C between the carbonates and the seeping CO₂ was resolved by discovering pockets of gas trapped near the base of the sediment column with δ¹³C-CO₂ values ranging from ?26.9 to ?11.6‰. A mechanism of carbonate formation is proposed in which carbonates form near the sediment–bedrock interface during times of sufficient sediment coverage, in which anaerobic oxidation of methane is favored. Precipitation occurs at a sufficient distance from active venting for the molecular and isotopic composition of seep gas to be masked by the generation of carbonate alkalinity from anaerobic methane oxidation.

Petrographic evidence of the past occurrence of gas hydrates in the Tertiary Piedmont Basin (NW Italy)

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

Hydrothermal dolomite reservoir in the Precambrian Dengying Formation of central Sichuan Basin,Southwestern China

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.     

A new TOC,Rock-Eval,and carbon isotope record of Lower Jurassic source rocks from the Slyne Basin,offshore Ireland

This paper presents an integrated assessment of total organic carbon, Rock-Eval pyrolysis, carbonate δ¹³C and δ¹⁸O, and total organic carbon δ¹³C record of Upper Sinemurian–Aalenian age from the Slyne Basin (well 18/25-1), offshore Ireland. Total organic carbon (up to 5.2 wt%) and Rock-Eval pyrolysis data highlights the Portree Shale Formation Equivalent (Lower–?Middle Toarcian) as a mature (T_max ∼440 °C) source rock dominated by type II kerogens. This interval is more than 90 m thick.In a previously unexplored paleogeographical domain regarding stable carbon isotopes, the δ¹³C record shows positive values during the Upper Sinemurian, a negative trend followed by a minor positive trend during the uppermost Sinemurian–Lower Pliensbachian, a negative trend during the lowermost Lower Toarcian, and then the return to more positive δ¹³C values. Despite the biostratigraphic uncertainty, the coarse resolution of the dataset, and the complex assessment of diagenetic effects, we suggest that the δ¹³C record of the studied well records the Raricostatum Zone positive CIE, the Sinemurian-Pliensbachian Boundary Event, and the negative CIE associated with the T-OAE. The T-OAE positive CIE is not clearly defined in the obtained dataset. The slight post T-OAE negative CIE recovery to more positive values and the continuation of a relatively high organic content is coeval with a regional organic matter preservation interval observed in several Northern-European locations.These new datasets and their interpretation add to the understanding of the paleoenvironmental dynamics during the Early–?Middle Jurassic in the Northern European domain and opens new possibilities for hydrocarbon exploration in Ireland's offshore.     

Authigenic carbonates from active methane seeps offshore southwest Africa

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5?m length indicates a maximum age of about 60,000–80,000?years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0?<?δ¹³C ‰ V-PDB?<?–40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4?<?δ¹⁸O ‰ V-PDB?<?+6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ¹⁸O values might reflect the contribution of ¹⁸O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca²⁺ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ¹⁸O–δ¹³C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.     

Authigenic Mg-calcite at a cold methane seep site in the Laptev Sea

Authigenic minerals were studied in Holocene shelf sediments of the Laptev Sea (cold methane seep site, water depth 71 m). The study presents the first finds of large hard carbonate concretions with Mg-calcite cement in recent sediments of the Arctic shelf seas. These concretions differ from previously reported glendonites and concretions from bottom sediments of the White Sea, Kara Sea, Sea of Okhotsk, etc. A study of the morphology, microstructure, and composition of these newly reported concretions revealed the multistage formation of carbonates (structural varieties of Mg-calcite and aragonite). It was shown that organic matter played an important role in the formation of authigenic carbonates, i.e., in the formation of sedimentary–diagenetic Mg-calcite. The role of methane as a possible source for authigenic carbonate formation was estimated. It was found that methane-derived Mg-calcite accounts for 17–35% of concretion materials. Mg-calcite had δ¹³С-С_carb values between–24 and–23‰ and δ¹³С-С_org values between–44.5 and–88.5‰.