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.     

Diagenetic alterations and reservoir heterogeneity within the depositional facies: A case study from distributary-channel belt sandstone of Upper Triassic Yanchang Formation reservoirs (Ordos Basin,China)

Diagenesis is of decisive significance for the reservoir heterogeneity of most clastic reservoirs. Linking the distribution of diagenetic processes to the depositional facies and sequence stratigraphy has in recent years been discipline for predicting the distribution of diagenetic alterations and reservoir heterogeneity of clastic reservoirs. This study constructs a model of distribution of diagenetic alterations and reservoir heterogeneity within the depositional facies by linking diagenesis to lithofacies, sandstone architecture and porewater chemistry during burial. This would help to promote better understanding of the distribution of reservoir quality evolution and the intense heterogeneity of reservoirs. Based on an analogue of deltaic distributary channel belt sandstone in Upper Triassic Yanchang Formation, 83 sandstone plug samples were taken from 13 wells located along this channel belt. An integration of scanning electron microscopy, thin sections, electron microprobe analyses, rate-controlled porosimetry (RCP), gas-flow measurements of porosity and permeability, and nuclear magnetic resonance (NMR) experiments, together with published data, were analysed for the distribution, mineralogical and geochemical characteristics of detrital and diagenetic components and the distribution of reservoir quality within the distributary channel belt.Distribution of diagenetic alterations and reservoir heterogeneity within the distributary channel belt sandstones include (i) formation of high quality chlorite rims in the middle part of thick sandstones with coarser grain sizes and a lower content of ductile components resulted from the greater compaction resistance of these sandstones (providing larger pore spaces for chlorite growth), leading to formation of the intergranular pore – wide sheet-like throat and intergranular pore - intragranular pore – wide sheet-like throat (Φ>15%, k>1mD) in the middle part of thick sandstones; (ii) formation of thinner chlorite rims in the middle part of thinner sandstones is associated with the intergranular pore - intragranular pore – narrow sheet-like throat (9%<Φ<14%, 0.2mD<k<0.8mD); (iii) strong cementation by kaolinite in the more proximal sandstones of distributary channel owing to the strong feldspar dissolution by meteoric water, resulting in the intragranular pore - group of interstitial cement pores – narrow sheet-like throat/extremely narrow sheet-like throat (8%<Φ<11%, 0.1mD<k<0.3mD) due to the pore-filling kaolinite occluding porosity; (iv) formation of dense ferrocalcite zones (δ¹⁸O_VPDB = −23.4‰ to −16.6‰; δ¹³ C_VPDB = −4.0‰ to −2.3‰) favoured in the top and bottom of the channel sandstone which near the sandstone-mudstone bouding-surface, destroying pore space (Φ<8%, k<0.1mD); (v) strong compaction in sandstone of distributary channel edge laterally as a result of fine grain size and high content of ductile components in those sandstones, forming the group of interstitial cement pores – extremely narrow sheet-like throat with porosity values less than 8%.     

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.     

The diagenetic history of Oligocene-Miocene sandstones of the Austrian north Alpine foreland basin

Diagenesis is an essential tool to reconstruct the development of reservoir rocks. Diagenetic processes - precipitation and dissolution - have an influence on pore space. The present paper aims to study the diagenetic history of deep-marine sandstones of the Austrian Alpine Foreland Basin. To reach that goal, sediment petrology and diagenetic features of more than 110 sandstone samples from water- and gas-bearing sections from gas fields within the Oligocene-Miocene Puchkirchen Group and Hall Formation has been investigated. Special emphasis was put on samples in the vicinity of the gas-water contact (GWC). The sediment petrography of sandstones of Puchkirchen Group and Hall Formation is similar; hence their diagenesis proceeded the same way. In fact, primary mineralogy was controlled by paleo-geography with increasing transport distance and diverse detrital input.Sediment petrographically, investigated sandstones from the water-bearing horizon seemed quite comparable to the gas-bearing sediments. In general, they can be classified as feldspatic litharenites to litharenites and display porosities of up to 30% and permeabilities of up to 1300 mD. The carbon and oxygen isotopic composition of bulk carbonate cements from these sandstones range from−3.8 to +2.2 and from −7.5 to +0.2‰ [VPDB]. However, near the Gas-Water Contact (GWC) a horizon with low porosities (<3%) and permeabilities (<0.1 mD) is present. This zone is completely cemented with calcite, which has a blocky/homogenous morphology. A slight, but significant negative shift in δ18O isotopy (−2.5‰) is evident.During early diagenesis the first carbonate generations formed. First a fibrous calcite and afterwards a micritic calcite precipitated. Further siliciclastic minerals, such as quartz and feldspar (K-feldspar and minor plagioclase), exhibit corroded grains. Occasionally, clay minerals (illite; smectite, chlorite) formed as rims around detrital grains. Late diagenesis is indicated by the formation of a low permeable zone at the GWC.     

Prediction of diagenetic facies using well logs – A case study from the upper Triassic Yanchang Formation,Ordos Basin,China

Understanding diagenetic heterogeneity in tight sandstone reservoirs is vital for hydrocarbon exploration. As a typical tight sandstone reservoir, the seventh unit of the Upper Triassic Yanchang Formation in the Ordos Basin (Chang 7 unit), central China, is an important oil-producing interval. Results of helium porosity and permeability and petrographic assessment from thin sections, X-ray diffraction, scanning electron microscopy and cathodoluminescence analysis demonstrate that the sandstones have encountered various diagenetic processes encompassing mechanical and chemical compaction, cementation by carbonate, quartz, clay minerals, and dissolution of feldspar and lithic fragments. The sandstones comprise silt-to medium-grained lithic arkoses to feldspathic litharenites and litharenites, which have low porosity (0.5%–13.6%, with an average of 6.8%) and low permeability (0.009 × 10⁻³ μm² to 1.818 × 10⁻³ μm², with an average of 0.106 × 10⁻³ μm²).This study suggests that diagenetic facies identified from petrographic observations can be up-scaled by correlation with wire-line log responses, which can facilitate prediction of reservoir quality at a field-scale. Four diagenetic facies are determined based on petrographic features including intensity of compaction, cement types and amounts, and degree of dissolution. Unstable and labile components of sandstones can be identified by low bulk density and low gamma ray log values, and those sandstones show the highest reservoir quality. Tightly compacted sandstones/siltstones, which tend to have high gamma ray readings and relatively high bulk density values, show the poorest reservoir quality. A model based on principal component analysis (PCA) is built and show better prediction of diagenetic facies than biplots of well logs. The model is validated by blind testing log-predicted diagenetic facies against petrographic features from core samples of the Upper Triassic Yanchang Formation in the Ordos Basin, which indicates it is a helpful predictive model.     

Saddle dolomite and calcite cements as records of fluid flow during basin evolution: Paleogene carbonates,United Arab Emirates

Field observations indicate that tectonic compression, anticline formation and concomitant uplift events of marine Paleogene carbonates in eastern United Arab Emirates, which are related to the Zagros Orogeny, have induced brecciation, karstification, and carbonate cementation in vugs and along faults and fractures. Structural analysis, stable isotopes and fluid inclusion microthermometry are used to constrain the origin and geochemical evolution of the fluids. Fluid flow was related to two tectonic deformation phases. Initially, the flux of moderately ⁸⁷Sr-rich basinal NaCl–MgCl₂–H₂O brines along reactivated deep-seated strike-slip faults have resulted in the precipitation of saddle dolomite in fractures and vugs and in dolomitization of host Eocene limestones (δ¹⁸O_V-PDB −15.8‰ to −6.2‰; homogenization temperatures of 80–115 °C and salinity of 18–25 wt.% eq. NaCl). Subsequently, compression and uplift of the anticline was associated with incursion of meteoric waters and mixing with the basinal brines, which resulted in the precipitation of blocky calcite cement (δ¹⁸O_V-PDB −22‰ to −12‰; homogenization temperatures of 60–90 °C and salinity of 4.5–9 wt.% eq. NaCl). Saddle dolomite and surrounding blocky calcite have precipitated along the pre- and syn-folding E–W fracture system and its conjugate fracture sets. The stable isotopes coupled with fluid-inclusion micro-thermometry (homogenization temperatures of ≤50 °C and salinity of <1.5 wt.% eq. NaCl) of later prismatic/dogtooth and fibrous calcites, which occurred primarily along the post-folding NNE–SSW fracture system and its conjugate fracture sets, suggest cementation by descending moderately ⁸⁷Sr-rich, cool meteoric waters. This carbonate cementation history explains the presence of two correlation trends between the δ¹⁸O_V-PDB and δ¹³C_V-PDB values: (i) a negative temperature-dependent oxygen isotope fractionation trend related to burial diagenesis and to the flux of basinal brines, and (ii) positive brine-meteoric mixing trend. This integrated study approach allows better understanding of changes in fluid composition and circulation pattern during evolution of foreland basins.     

Fluid circulation and formation of minerals and bitumens in the sedimentary rocks of the Outer Carpathians – Based on studies on the quartz–calcite–organic matter association

Different methods have been used to examine minerals and/or solid bitumens in three adjacent Carpathian regions of Poland, Ukraine and Slovakia. The minerals fill smaller and larger veins and cavities, where they occur either together or separately. They usually co-occur with the solid bitumens. All δ¹³C_PDB values measured for calcite lie in a relatively wide interval between −6.25‰ and +1.54‰, while most values fall into the narrower interval from below 0 to about −3‰. The general range of calcite δ¹⁸O results for the whole studied region is between +17.13‰ and +25.23‰ VSMOW or from about −11 to −5‰ VPDB, while the majority of these values are between +20.0 and 23.5‰ VSMOW (−10.53 and −8.00‰ PDB, respectively). δ¹⁸O_VSMOW results for quartz vary between +23.2 and 27.6. The carbonate percentage determined in some samples falls between from <2% CaCO₃ to >90% CaCO₃, while the TOC values changes from 0.09% to over 70%.The aliphatic fraction predominates in all studied samples, mainly in bitumens and oils. The composition of the aliphatic fraction is relatively homogeneous and points to a strong aliphatic, oil-like paraffin character of the bitumens. Such a composition is characteristic of the Carpathian oils and different from the rocks studied that contain the higher percentage of a polar fraction. The content of the aliphatic fraction in bitumens is only slightly higher than that in two oils used for comparison. The distribution of n-alkanes is variable in rocks, solid bitumens as well as inclusions in quartz and calcite. Two groups of bitumens may be distinguished. Those with a predominance of long-chain n-alkanes in the C₂₅–C₂₇ interval (in some cases from C₂₃–C₂₅ and without or with a very low concentration of short-chain n-alkanes in the interval of C₁₄–C₂₁) show also a high content of isoprenoids i.e. of pristane (Pr) and phytane (Ph). In all but one bitumen samples, Pr predominates over Ph. The second group comprises oils and rock samples with a characteristic predominance of short-chain n-alkanes in the interval from C₁₃–C₁₉ and a low percentage of the long-chain n-alkanes from the n-C₂₇–n-C₃₃ interval. Pristane and phytane exhibit a concentration comparable to that of C₁₇ and C₁₈ n-alkanes with a Pr predominance over Ph. Due to high maturity, only small amounts of the most stable compounds from the hopane group have been observed in the samples, also oleanane in one case. Among the aromatic hydrocarbons, phenanthrene and its methyl- and dimethyl-derivatives are dominant in bitumens, source rocks and inclusions in calcite and quartz. Occurrence of cyclohexylbenzene and its alkyl-derivatives as well as cyclohexylfluorenes in solid bitumens suggest that they formed from oil accumulations under the influence of relatively high temperatures in oxidizing conditions.Homogenization temperatures for aqueous/brine inclusions in quartz within the Dukla and Silesian units (Polish and Ukrainian segments) are between 125 and 183.9 °C, while salinities are low in the interval of 0.2–5.5 wt% NaCl eq. The inclusions in calcite homogenize at higher temperatures of almost 200 °C and the brine displays higher salinity than the fluid in the quartz. Two quartz generations may be distinguished by inclusion and isotope characteristics and the macroscopic diversity. Oil inclusions homogenize at 95 °C. One phase inclusions in quartz contain methane, CO₂ and nitrogen in variable proportions.     

Geochemistry,origin and accumulation of natural gases in the deepwater area of the Qiongdongnan Basin,South China Sea

The Qiongdongnan Basin, South China Sea has received huge thickness (>12 km) of Tertiary-Quaternary sediments in the deepwater area to which great attention has been paid due to the recent discoveries of the SS22-1 and the SS17-2 commercial gas fields in the Pliocene-Upper Miocene submarine canyon system with water depth over 1300 m. In this study, the geochemistry, origin and accumulation models of these gases were investigated. The results reveal that the gases are predominated by hydrocarbon gases (98%–99% by volume), with the ratio of C₁/C_1-5 ranging from 0.92 to 0.94, and they are characterized by relatively heavy δ¹³C₁ (−36.8‰ to −39.4‰) and δD_CH4 values (−144‰ to −147‰), similar to the thermogenic gases discovered in the shallow water area of the basin. The C_5-7 light hydrocarbons associated with these gases are dominated by isoparaffins (35%–65%), implying an origin from higher plants. For the associated condensates, carbon isotopic compositions and high abundance of oleanane and presence of bicadinanes show close affinity with those from the YC13-1 gas field in the shallow water area. All these geochemical characteristics correlate well with those found in the shales of the Oligocene Yacheng Formation in the Qiongdongnan Basin. The Yacheng Formation in the deepwater area has TOC values in the range of 0.4–21% and contains type II_b–III gas-prone kerogens, indicating an excellent gas source rock. The kinetic modeling results show that the δ¹³C₁ values of the gas generated from the Yacheng source rock since 3 or 4 Ma are well matched with those of the reservoir gases, indicating that the gas pool is young and likely formed after 4 Ma. The geologic and geochemical data show that the mud diapirs and faults provide the main pathways for the upward migration of gases from the deep gas kitchen into the shallow, normally pressured reservoirs, and that the deep overpressure is the key driving force for the vertical and lateral migration of gas. This gas migration pattern implies that the South Low Uplift and the No.2 Fault zone near the deepwater area are also favorable for gas accumulation because they are located in the pathway of gas migration, and therefore more attention should be paid to them in the future.     

The impact of organic fluids on the carbon isotopic compositions of carbonate-rich reservoirs: Case study of the Lucaogou Formation in the Jimusaer Sag,Junggar Basin,NW China

The processes involved in the interaction between organic fluids and carbonates, and the resulting effect on reservoir quality during the evolution and maturation of organic matter remain unclear despite the fact that these processes influence the carbon and oxygen isotopic compositions of carbonates. Here, we provide new insights into these processes using data obtained from a detailed analysis of a mixed dolomitic–clastic and organic-rich sedimentary sequence within the middle Permian Lucaogou Formation in the Junggar Basin of NW China. The techniques used during this study include drillcore observations, thin section petrography, scanning electron microscopy (SEM) and electron probe microanalysis, and carbon and oxygen isotope analyses. Oil grades and total organic carbon (TOC) contents represent the amount of oil charging and the abundance of organic fluids within a reservoir, respectively, and both negatively correlate with the whole-rock δ¹³C and δ¹⁸O of the carbonates in the study area, indicating that organic fluids have affected the reservoir rocks. Secondary carbonates, including sparry calcite and dolomite overgrowths and cements, are common within the Lucaogou Formation. Well-developed sparry calcite is present within dark mudstone whereas the other two forms of secondary carbonates are present within the dolomite-rich reservoir rocks in this formation. Comparing thin section petrology with δ¹³C compositions suggests that the carbon isotopic composition of matrix carbonates varies little over small distances within a given horizon but varies significantly with stratigraphic height as a result of the development of secondary carbonates. The net change in whole-rock δ¹³C as a result of these secondary carbonates ranges from 1.8‰ to 4.6‰, with the secondary carbonates having calculated δ¹³C compositions from −18.6‰ to −8.5‰ that are indicative of an organic origin. The positive correlation between the concentration of Fe within matrix and secondary carbonates within one of the samples suggests that the diagenetic system within the Lucaogou Formation was relatively closed. The correlation between δ¹³C and δ¹⁸O in carbonates is commonly thought to be strengthened by the influence of meteoric water as well as organic fluids. However, good initial correlation between δ¹³C and δ¹⁸O of whole rock carbonates within the Lucaogou Formation (resulted from the evaporitic sedimentary environment) was reduced by organic fluids to some extent. Consequently, the δ¹³C–δ¹⁸O covariations within these sediments are not always reliable indicators of diagenetic alteration by organic fluids or meteoric water.The characteristics and δ¹³C compositions of the sparry calcite within the formation is indicative of a genetic relationship with organic acids as a result of the addition of organic CO₂ to the reservoir. Further analysis suggests that both carbonate and feldspar were dissolved by interaction with organic CO₂. However, dissolved carbonate reprecipitated as secondary carbonates, meaning that the interaction between organic fluids and dolomites did not directly improve reservoir quality, although this process did enhance the dissolution of feldspar and increase porosity. This indicates that the δ¹³C and δ¹⁸O of secondary carbonates and their influence on whole-rock carbonate isotopic values can be used to geochemically identify the effect of organic fluids on closed carbonate-rich reservoir systems.     

Hydrothermal dolomitization in platform and basin carbonate successions during thrusting: A hydrocarbon reservoir analogue (Mesozoic of Venetian Southern Alps,Italy)

The Early Jurassic dolomitized carbonates in the Venetian Alp, represent a surface analogue of the hydrocarbon exploration targets in Adriatic offshore and Po Plain, Italy. Dolomitization affected the carbonate platform of Monte Zugna Formation (Lower Jurassic) and the Neptunian dikes breccia in the pelagic Maiolica Formation (Uppermost Jurassic–Lower Cretaceous) improving the poro-perm characteristics. Petrography, stable isotope, strontium isotope ratio, trace element and fluid inclusion analyses were carried out on samples from the Monte Grappa Anticline, which is the direct analogue for subsurface. The petrographic analyses showed a first pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3, DS). The same, quite wide range of oxygen isotope (?9 to ?2‰ V-PDB) is observed in all dolomite types. The δ¹³C range is in the positive field of marine derived carbonate (from +0.5 to +3.2‰ PDB). The trace element analysis showed a slight enrichment in Fe and Mn contents in the Monte Zugna dolostones with respect the original limestone. The same dolomite precipitation temperature (up to 105 °C Th) was observed in the replacement and cement dolomites, suggesting a unique dolomitization event. This temperature, largely higher than the maximum burial temperature (about 50 °C), supports a hydrothermal origin of the dolomitizing fluids, which had a seawater to brackish composition. The data collected suggest a hydrothermal dolomitization occurring during to the South Alpine thrusting according to the “squeegee model”. The interpretation is consistent with the dolomitization model proposed for similar Jurassic successions in the Central Southern Alps. This study indicates that the deformed foreland and thrust fold belts carbonates in Po Plain and Adriatic offshore are suitable to be dolomitized, and therefore reflect an efficient hydrocarbon exploration play.     

Variations of methane induced pyrite formation in the accretionary wedge sediments offshore southwestern Taiwan

The accretionary wedge of offshore southwestern Taiwan contains abundant deposits of gas hydrate beneath the sea floor. High concentrations of methane in pore waters are observed at several locations with little data concerning historical methane venting available. To understand temporal variation of methane venting in sediments over geologic time, a 23-m-long Calypso piston core (MD05-2911) was collected on the flank of the Yung-An Ridge. Pore water sulfate, dissolved sulfide, dissolved iron, methane, sedimentary pyrite, acid volatile sulfide, reactive iron, organic carbon and nitrogen as well as carbonate δ¹³C were analyzed.Three zones with markedly different pyrite concentration were found at the study site. Unit I sediments (>20 mbsf) were characterized with a high amount of pyrite (251–380 μmol/g) and a δ¹³C-depleted carbonate, Unit II sediments (15–20 mbsf) with a low pyrite (15–43 μmol/g) and a high content of iron oxide mineral and Unit III sediments (<10 mbsf) by a present-day sulfate–methane interface (SMI) at 5 m with a high amount of pyrite (84–221 μmol/g) and a high concentration of dissolved sulfide.The oscillation records of pyrite concentrations are controlled by temporal variations of methane flux. With an abundant supply of methane to Unit I and III, anaerobic methane oxidation and associated sulfate reduction favor diagenetic conditions conducive for significant pyrite formation. No AOM signal was found in Unit II, characterized by typical organically-limited normal marine sediments with little pyrite formation. The AOM induced pyrite formation near the SMI generates a marked pyrite signature, rendering such formation of pyrite as a useful proxy in identifying methane flux oscillation in a methane flux fluctuate environment.     

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 and elemental indicators of nutrient sources and status of coastal habitats in the Caribbean Sea,Yucatan Peninsula,Mexico

Nutrient inputs associated with coastal population growth threaten the integrity of coastal ecosystems around the globe. In order to assess the threat posed by rapid growth in tourism, we analyzed the nutrient concentrations as well as the δ¹⁵N of NO₃⁻ and macrophytes to detect wastewater nitrogen (N) at 6 locations along a groundwater-dominated coastal seagrass bed on the Caribbean coast of Mexico. We predicted that locations with greater coastal development would have higher concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (P), as well as δ¹⁵N of NO₃⁻, reflecting wastewater sources of N. However, concentrations of NO₃⁻ were not significantly different between developed (3.3 ± 5.3 μM NO₃⁻) and undeveloped (1.1 ± 0.7 μM) marine embayments. The most important control on DIN concentration appeared to be mixing of fresh and salt water, with DIN concentrations negatively correlated with salinity. The δ¹⁵N of NO₃⁻ was elevated at an inland pond (7.0 ± 0.42‰) and a hydrologically-connected tide pool (7.6 ± 0.57‰) approximately 1 km downstream of the pond. The elevated δ¹⁵N of NO₃⁻ at the pond was paralleled by high δ¹⁵N values of Cladophora sp., a ubiquitous green alga (10 ± 1‰). We hypothesize that inputs of nitrogen rich (NO₃⁻ > 30 μM) groundwater, characterized by ¹⁵N enriched signatures, flow through localized submarine groundwater discharges (SGD) and contribute to the elevated δ¹⁵N signatures observed in many benthic macrophytes. However, changes in nitrogen concentrations and isotope values over the salinity gradient suggest that other processes (e.g. denitrification) could also be contributing to the ¹⁵N enrichments observed in primary producers. More measurements are needed to determine the relative importance of nitrogen transformation processes as a source of ¹⁵N to groundwaters; however, it is clear that continued inputs of anthropogenic N via SGD have the potential to severely impact ecologically and economically valuable seagrass meadows and coral reefs along the Caribbean coast of Mexico.     

Molecular signals for anaerobic methane oxidation in Black Sea seep carbonates and a microbial mat

Linked to gas seeps on the Ukrainian shelf (northwestern Black Sea), massive authigenic carbonates form as a result of anaerobic methane oxidation. Lipid distributions in these ‘cold seep’ carbonates and an associated microbial mat were investigated for process markers reflecting the presence and metabolic activity of distinctive methane-related biota. The samples contain free, irregular isoprenoid hydrocarbons, namely the tail-to-tail linked acyclic C₂₀-isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), its C₂₅-homologue 2,6,10,15,19-pentamethylicosane (PMI), and several unsaturated derivatives thereof. Furthermore, specific acyclic and cyclic C₄₀-isoprenoids were released upon ether cleavage of the polar fraction from the carbonate. The abundance of these compounds indicates a pronounced role of particular Archaea in the biogeochemical cycling of carbon at methane seeps. Stable carbon isotopic analyses of these lipids reveal extraordinary depletions in ¹³C corresponding to δ-values in the range of −100±30‰ PDB, whereas other compounds show isotopic compositions normally observed for marine lipids (around −30‰ PDB). The isotope data imply that the biosynthesis of the archaeal isoprenoids occurred in situ and involved the utilization of isotopically depleted, i.e. methane-derived, carbon. Apart from archaeal markers, the carbonate and the mat contain authigenic, framboidal pyrite and isotopically depleted fatty acids, namely iso-, and anteiso-branched compounds most likely derived from sulphate-reducing bacteria (SRB). The indications for a tight association of these normally competitive organisms support a model invoking a syntrophic relationship of SRB with Archaea responsible for the anaerobic oxidation of methane. The biomarker patterns obtained from the Black Sea samples were further compared to those from a Oligocene seep carbonate (Lincoln Creek Formation, WA, USA) in order to evaluate their biomarker potential for ancient settings. The prominent occurrence of isotopically light crocetane (−112‰) and PMI (−120‰) meets the findings for the contemporary materials. Thus, isotopically depleted isoprenoids provide diagenetically stable fingerprints for the reconstruction of carbon cycling in both, modern and ancient methane seep systems.     

Chronic hypoxia and low salinity impair anti-predatory responses of the green-lipped mussel Perna viridis

The effects of chronic hypoxia and low salinity on anti-predatory responses of the green-lipped mussel Perna viridis were investigated. Dissolved oxygen concentrations ranged from hypoxic to normoxic (1.5 ± 0.3 mg l⁻¹, 3.0 ± 0.3 mg l⁻¹ and 6.0 ± 0.3 mg l⁻¹), and salinities were selected within the variation during the wet season in Hong Kong coastal waters (15‰, 20‰, 25‰ and 30‰). The dissolved oxygen and salinity significantly affected some anti-predatory responses of mussel, including byssus production, shell thickness and shell weight, and the adductor diameter was only significantly affected by salinity. Besides, interactive effects of dissolved oxygen and salinity on the byssus production and shell thickness were also observed. In hypoxic and low salinity conditions, P. viridis produced fewer byssal threads, thinner shell and adductor muscle, indicating that hypoxia and low salinity are severe environmental stressors for self-defence of mussel, and their interactive effects further increase the predation risk.     

The metabolic balance between autotrophy and heterotrophy in the western Arctic Ocean

The goal of this study was to explore how net community production (NCP) is influenced by the relationship between primary production and community respiration in the western Arctic Ocean. Plankton NCP and respiration were determined by measuring changes in oxygen in light and dark bottle incubations, respectively. Rates of NCP averaged over shelf, slope and basin waters were positive in summer 2002 (57±191 mmol O₂ m⁻² d⁻¹) and spring 2004 (85±86 mmol O₂ m⁻² d⁻¹) and negative in summer 2004 (−25±176 mmol O₂ m⁻² d⁻¹). Determinations of NCP obtained from bottle incubations were similar to rates inferred from in situ changes in dissolved inorganic carbon. An examination of the spatial variability of primary production and community respiration indicated that respiration is distributed more uniformly than primary production. A spatial offset between photosynthesis and respiration from the shelf to the Arctic basin was present in spring 2004, but was not seen at other times. NCP and the potential for export appear to be dependent on an uncoupling of primary production and community respiration. NCP continued into the summer after the stock of NO₃⁻ had been depleted. Our data suggest that the uniform distribution of respiration relative to primary production is an important factor influencing NCP and the potential for export in the western Arctic.     

The supply and preservation of ancient and modern components of organic carbon in the Canadian Beaufort Shelf of the Arctic Ocean

The provenance of organic matter in sediments from the Mackenzie River and Beaufort Shelf was investigated using the stable carbon and radiocarbon isotopic compositions of bulk organic matter and the stable carbon isotopic compositions of individual organic compounds, including lignin-derived phenols and lipid-derived fatty acids. Most river suspended sediments and shelf surface sediments contained organic carbon characterized by highly depleted Δ¹⁴C values that were consistent with average radiocarbon ages exceeding 7000 years. The stable carbon isotopic signatures of lignin phenols were uniformly depleted (−25≥δ¹³C≥−32‰), indicating the predominant contributions of C₃ vascular plant sources. The isotopic compositions of C₁₄ and C₁₆ fatty acids exhibited important contrasts between the river (−36‰ to −40‰) and shelf (−25‰ to −29‰) sediments that were consistent with contributions from freshwater algae and/or vascular plants in the former and marine phytoplankton in the latter. Using ¹⁴C isotopic mass balance, the abundances of modern and ancient organic matter were quantitatively constrained. The fate of organic matter in the Beaufort Shelf was explored by normalizing these abundances to the specific surface area of sediments. Ancient organic carbon, which may include old pre-aged soil material as well as fossil bitumen or kerogen, accounted for the majority (∼70%) of the particulate organic matter exported by the Mackenzie River and deposited in surface sediments of the Beaufort Shelf. Modern organic carbon accounted for ∼30% in both river and shelf sediments, with significant contributions from vascular plant-derived materials in both river and shelf samples and from marine algae in the shelf sediments. Respiration (and/or leaching) of particle-bound marine organic matter dominates the carbon metabolism in the Mackenzie Delta/Beaufort Shelf region. However, land-derived pools, including modern carbon derived from vascular plants as well as ancient carbon also appeared to undergo a degree of post-depositional degradation prior to burial in the shelf. These novel source apportionments are reflected in an updated carbon budget for the study area.     

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.     

Bathymetric influence on dissolved methane in hydrothermal plumes revealed by concentration and stable carbon isotope measurements at newly discovered venting sites on the Central Indian Ridge (11–13°S)

Methane is a useful tracer for studying hydrothermal discharge, especially where the source fluids are of low temperature and lack metal precipitates. However, the dual origins of deep-sea methane, both chemical and biological, complicate the interpretation of methane observations. Here, we use both the concentration and stable carbon isotopic composition (δ¹³C) of dissolved methane to trace hydrothermal plumes and identify the source and behavior of methane at two sites of newly discovered hydrothermal activity on the Central Indian Ridge (11–13°S). At both sites, methane and optical anomalies between 2500 and 3500 m at all stations indicate active hydrothermal discharge. We compared methane concentrations and δ¹³C at three stations, two (CTIR110136 and CTIR110208) with the most prominent anomalies at each site, and a third (CTIR110140) with near-background methane values. At stations CTIR110136 and CTIR110208, the concentration and δ¹³C of methane in distinct plumes ranged from 3.3 to 42.3 nmol kg⁻¹ and −30.0 to −15.4‰, respectively, compared to deep-water values of 0.5 to 1.2 nmol kg⁻¹ and −35.1 to −28.9‰ at the station with a near-background distal plume (CTIR110140). δ¹³C was highest in the center of the plumes at CTIR110136 (−15.4‰) and CTIR110208 (−17.8‰). From the plume values we estimate that the δ¹³C of methane in the hydrothermal fluids at these stations was approximately −19‰ and thus the methane was most likely derived from magmatic outgassing or the chemical synthesis of inorganic matter. We used the relationship between δ¹³C and methane concentration to examine the behavior of methane at the plume stations. In the CTIR110208 plume, simple physical mixing was likely the major process controlling the methane profile. In the CTIR110136 plume we interpret a more complicated relationship as resulting from microbial oxidation as well as physical mixing. We argue that this difference in methane behavior between the two areas stems from a distinct bathymetric dissimilarity between the two stations. The location of CTIR110208 on the open slope of a ridge allowed rapid plume dispersion and physical mixing, whereas the location of CTIR110136 in a small basin surrounded by wall structures inhibited physical mixing and enhanced microbial oxidation.     

Thermal maturity,source rock potential and kinetics of hydrocarbon generation in Permian shales from the Damodar Valley basin,Eastern India

This study investigates the source rock characteristics of Permian shales from the Jharia sub-basin of Damodar Valley in Eastern India. Borehole shales from the Raniganj, Barren Measure and Barakar Formations were subjected to bulk and quantitative pyrolysis, carbon isotope measurements, mineral identification and organic petrography. The results obtained were used to predict the abundance, source and maturity of kerogen, along with kinetic parameters for its thermal breakdown into simpler hydrocarbons.The shales are characterized by a high TOC (>3.4%), mature to post-mature, heterogeneous Type II–III kerogen. Raniganj and Barren Measure shales are in mature, late oil generation stage (Rr%_Raniganj = 0.99–1.22; Rr%_{Barren Measure} = 1.1–1.41). Vitrinite is the dominant maceral in these shales. Barakar shows a post-mature kerogen in gas generation stage (Rr%_Barakar = 1.11–2.0) and consist mainly of inertinite and vitrinite. The δ¹³C_org value of kerogen concentrate from Barren Measure shale indicates a lacustrine/marine origin (−24.6–−30.84‰ vs. VPDB) and that of Raniganj and Barakar (−22.72–−25.03‰ vs. VPDB) show the organic provenance to be continental. The δ¹³C ratio of thermo-labile hydrocarbons (C₁–C₃) in Barren Measure suggests a thermogenic source.Discrete bulk kinetic parameters indicate that Raniganj has lower activation energies (ΔE = 42–62 kcal/mol) compared to Barren Measure and Barakar (ΔE = 44–68 kcal/mol). Temperature for onset (10%), middle (50%) and end (90%) of kerogen transformation is least for Raniganj, followed by Barren Measure and Barakar. Mineral content is dominated by quartz (42–63%), siderite (9–15%) and clay (14–29%). Permian shales, in particular the Barren Measure, as inferred from the results of our study, demonstrate excellent properties of a potential shale gas system.