Sedimentary and residual gas geochemical characteristics of the Lower Cambrian organic-rich shales in Southeastern Chongqing,China

To study the sedimentary environment of the Lower Cambrian organic-rich shales and isotopic geochemical characteristics of the residual shale gas, 20 black shale samples from the Niutitang Formation were collected from the Youyang section, located in southeastern Chongqing, China. A combination of geochemical, mineralogical, and trace element studies has been performed on the shale samples from the Lower Cambrian Niutitang Formation, and the results were used to determine the paleoceanic sedimentary environment of this organic-rich shale. The relationships between total organic carbon (TOC) and total sulfur (TS) content, carbon isotope value (δ¹³C_org), trace element enrichment, and mineral composition suggest that the high-TOC Niutitang shale was deposited in an anoxic environment and that the organic matter was well preserved after burial. Stable carbon isotopes and biomarkers both indicate that the organic matter in the Niutitang black shales was mainly derived from both lower aquatic organisms and algaes and belong to type I kerogen. The oil-prone Niutitang black shales have limited residual hydrocarbons, with low values of S₂, I_H, and bitumen A. The carbon isotopic distribution of the residual gas indicate that the shale gas stored in the Niutitang black shale was mostly generated from the cracking of residual bitumen and wet gas during a stage of significantly high maturity. One of the more significant observations in this work involves the carbon isotope compositions of the residual gas (C₁, C₂, and C₃) released by rock crushing. A conventional δ¹³C₁–δ¹³C₂ trend was observed, and most δ¹³C₂ values of the residual gases are heavier than those of the organic matter (OM) in the corresponding samples, indicating the splitting of ethane bonds and the release of smaller molecules, leading to ¹³C enrichment in the residual ethane.     

Geochemical characteristics of gases from the largest tight sand gas field (Sulige) and shale gas field (Fuling) in China

This study performed a detailed geochemical analyses of the components, stable carbon isotopes of alkane gas and CO₂, stable hydrogen isotopes of alkane gas and helium isotopes of reproducing gas from the largest tight gas field (Sulige) and shale gas (Fuling) field in China. The comparative study shows that tight gas from the Sulige gas field in the Ordos Basin is of coal-derived origin, which is characterized by a positive carbon and hydrogen isotopic distribution pattern (δ¹³C₁ > δ¹³C₂ > δ¹³C₃ > δ¹³C₄; δ²H₁ > δ²H₂ > δ²H₃), i.e., the carbon and hydrogen isotopes increase with increasing carbon numbers. Carbon dioxide from this field are of biogenic origin and the helium is crust-derived. Shale gas from the Fuling shale gas field belongs to oil-derived gas which has complete carbon and hydrogen isotopic reversal of secondary alteration origin (δ¹³C₁ < δ¹³C₂ < δ¹³C₃; δ²H₁ < δ²H₂ < δ²H₃), i.e., the carbon and hydrogen isotopes decrease with increasing carbon numbers. Such complete isotopic reversal distribution pattern is due to the secondary alteration like oil or gas cracking, diffusion and so on under high temperature. In that case, positive carbon or hydrogen isotopic distribution pattern will change into complete isotopic reversal as the temperature increases. Carbon dioxide is of abiogenic origin resulting from the thermal metamorphism of carbonates and helium is crust-derived.     

Geochemical characteristics and isotopic reversal of natural gases in eastern Kopeh-Dagh,NE Iran

Natural gas samples from two gas fields located in Eastern Kopeh-Dagh area were analyzed for molecular and stable isotope compositions. The gaseous hydrocarbons in both Lower Cretaceous clastic reservoir and Upper Jurassic carbonate reservoir are coal-type gases mainly derived from type III kerogen, however enriched δD values of methane implies presence of type II kerogen related material in the source rock. In comparison Upper Jurassic carbonate reservoir gases show higher dryness coefficient resulted through TSR, while presence of C₁C₅ gases in Lower Cretaceous clastic reservoir exhibit no TSR phenomenon. Carbon isotopic values indicate gas to gas cracking and TSR occurrence in the Upper Jurassic carbonate reservoir, as the result of elevated temperature experienced, prior to the following uplifts in last 33–37 million years. The δ¹³C of carbon dioxide and δ³⁴S of hydrogen sulfide in Upper Jurassic carbonate reservoir do not primarily reflect TSR, as uplift related carbonate rock dissolution by acidic gases and reaction/precipitation of light H₂S have changed these values severely. Gaseous hydrocarbons in both reservoirs exhibit enrichment in C₂ gas member, with the carbonate reservoir having higher values resulted through mixing with highly-mature-completely-reversed shale gases. It is likely that the uplifts have lifted off the pressure on shale gases, therefore facilitated the migration of the gases into overlying horizons. However it appears that the released gases during the first major uplift (33–37 million years ago) have migrated to both reservoirs, while the second migrated gases have only mixed with Upper Jurassic carbonate reservoir gases. The studied data suggesting that economic accumulations of natural gas/shale gases deeper than Upper Jurassic carbonate reservoir would be unlikely.     

南海北部莺歌海岭头岬近岸烃类渗漏喷口分布及气体地球化学

Natural hydrocarbon seeps in a marine environment are one of the important contributors to greenhouse gases in the atmosphere,including methane,which is significant to the global carbon cycling and climate change.Four hydrocarbon seep areas,the Lingtou Promontory,the Yinggehai Rivulet mouth,the Yazhou Bay and the Nanshan Promontory,occurring in the Yinggehai Basin delineate a near-shore gas bubble zone.The gas composition and geochemistry of venting bubbles and the spatial distribution of hydrocarbon seeps are surveyed on the near-shore Lingtou Promontory.The gas composition of the venting bubbles is mainly composed of CO_2,CH_4,N_2 and O_2,with minor amounts of non-methane hydrocarbons.The difference in the bubbles' composition is a possible consequence of gas exchange during bubble ascent.The seepage gases from the seafloor are characterized by a high CO_2 content(67.35%) and relatively positive δ~(13)C_(V_PDB) values(-0.49×10~(-3)-0.86×10~(-3)),indicating that the CO_2 is of inorganic origin.The relatively low CH_4 content(23%) and their negative δ~(13)C_(V-PDB) values(-34.43×10~(-3)--37.53×10~(-3)) and high ratios of C_1 content to C_(1-5) one(0.98-0.99)as well point to thermogenic gases.The hydrocarbon seeps on the 3.5 Hz sub-bottom profile display a linear arrangement and are sub-parallel to the No.1 fault,suggesting that the hydrocarbon seeps may be associated with fracture activity or weak zones and that the seepage gases migrate laterally from the central depression of the Yinggehai Basin.     

Molecular and isotopic characteristics of gas hydrate-bound hydrocarbons in southern and central Lake Baikal

We investigated the molecular composition (methane, ethane, and propane) and stable isotope composition (methane and ethane) of hydrate-bound gas in sediments of Lake Baikal. Hydrate-bearing sediment cores were retrieved from eight gas seep sites, located in the southern and central Baikal basins. Empirical classification of the methane stable isotopes (δ¹³C and δD) for all the seep sites indicated the dominant microbial origin of methane via methyl-type fermentation; however, a mixture of thermogenic and microbial gases resulted in relatively high methane δ¹³C signatures at two sites where ethane δ¹³C indicated a typical thermogenic origin. At one of the sites in the southern Baikal basin, we found gas hydrates of enclathrated microbial ethane in which ¹³C and deuterium were both highly depleted (mean δ¹³C and δD of –61.6‰ V-PDB and –285.4‰ V-SMOW, respectively). To the best of our knowledge, this is the first report of C₂ δ¹³C–δD classification for hydrate-bound gas in either freshwater or marine environments.     

Spatial distribution and geochemistry of the nearshore gas seepages and their implications to natural gas migration in the Yinggehai Basin,offshore South China Sea

About 120 gas seepage vents were documented along the west and southwest coast of the Hainan Island, South China Sea, in water depths usually less than 50 m. The principal seepage areas include the Lingtou Promontory, the Yinggehai Rivulet Mouth, Yazhou Bay, the Nanshan Promontory and the Tianya Promontory. They occur along three major zones, reflecting the control by faults and lateral conduits within the basement. It is estimated that the total gas emission from these seepage vents is 294–956 m³/year. The seepage gases are characterized by a high CH₄ content (76%), heavy δ¹³C₁ values (−38 to −33‰) and high C₁/C_1–5 ratios (0.95–1.0), resembling the thermogenic gases from the diapiric gas fields of the Yinggehai Basin. Hydrocarbon–source correlation shows that the hydrocarbons in the sediments from seepage areas can be correlated with the deeply buried Miocene source rocks and sandstone reservoirs in the central depression. The 2D basin modeling results based on a section from the source rock center to the gas seepage sites indicate that the gas-bearing fluids migrated from the source rocks upward through faults or weak zones encompassed by shale diapirism or in up-dip direction along the sandstone-rich strata of Huangliu Formation to arrive to seabed and form the nearshore gas seepages. It is suggested that the seepage gases are sourced from the Miocene source rocks in the central depression of the Yinggehai Basin. This migration model implies that the eastern slope zone between the gas source area of the central depression and the seepage zone is also favorable place for gas accumulation.     

Alternative interpretations as to the origin of the hydrocarbons of the Guajira Basin,Colombia

The origins of the hydrocarbon gases recovered from the Chuchupa, Ballena, and Riohacha fields, located in the Guajira basin, northeast Colombia, are examined. These gases are composed of methane with trace amounts of wet gas components (C₂₊<0.5%). The methane is isotopically light (δ¹³C₁<−50‰), as are the other hydrocarbon components. The Ballena and Riohacha fields are considered to have a biogenic origin and the Chuchupa gas is thought to have a mixed biogenic-thermogenic origin. The presence of a thermogenic contribution at Chuchupa need not, however, indicate that the sedimentary sequence was exposed to elevated temperatures. The generative kitchen located to the northwest of the three gas fields would preferentially feed into the Chuchupa Field.The gases recovered at three onshore seep localities appear to include a thermogenic component. As a result of fractionation it is unclear whether they represent ‘pure’ thermogenic gases or a mixed thermogenic-biogenic origin similar to Chuchupa.An examination of oil microseepage observed in soil samples recovered from the onshore Guajira region point to another hydrocarbon system apparently not related to the three gas fields. Some of these microseeps include ‘fresh’ (nonbiodegraded) oils. These microseeps did not correlate with either of the Tertiary oil families from the Sinú Urabá basin located to the southwest of the study area or with Cretaceous oils from that basin. A better correlation was observed with the La Luna Formation, but significant differences remain implying that the source for the microseeps was less calcareous and deposited in a less restricted environment than associated with ‘classical’ La Luna facies. This could indicate either a facies change or that a different source unit is present within the region.     

Methane-derived authigenic carbonates from active hydrocarbon seeps of the St. Lawrence Estuary,Canada

Nearly 2000 pockmarks with diameters ranging from a few tens of meters up to 700 m are present on the seafloor of the St. Lawrence Estuary in eastern Canada. Coring of some pockmarks resulted in the recovery of various-sized and shaped carbonate concretions in a predominantly silty mud matrix. Petrographic and geochemical data on four authigenic carbonate concretions are reported as well as data from shell material in the unconsolidated sediment. Video observations and echo-sounder images indicate that the sampled pockmarks are actively gas venting. The video images show significant look-alike microbial mats in areas where gas is venting. The carbonate concretions are primarily made up of carbonate cements with varying percentage of shell fragments, micrite particles and fine-grained clastics. Orthorhombic crystal morphology and diagenetic fabrics including isopachous layers and botryoids characterize the aragonite cement. Oxygen isotopes ratios for the cement crusts do not record any thermal anomaly at the site of precipitation with δ¹⁸O_VPDB ratios (+3‰) in equilibrium with cold (5 °C) deep marine waters, whereas significant negative δ¹³C_VPDB ratios (−9.9 to −33.5‰) for cement and shell material within concretions indicate that the carbonates largely derive from the microbial oxidation of methane. The δ¹³C_VPDB ratios of aragonite shells (−2.7 to −5.6‰) taken from unconsolidated sediments at some distance from the concretions/vents show variable dilution of HCO₃⁻ with negative δ¹³C_VPDB ratios derived from microbial oxidation of methane with isotopically normal (0‰) marine bicarbonate. These results are in agreement with other lines of evidence suggesting that pockmarks formed through the recent and still active release of gas from a reservoir within the Paleozoic sedimentary succession.     

The characteristics of gas hydrates recovered from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

Systematic analyses have been carried out on two gas hydrate-bearing sediment core samples, HYPV4, which was preserved by CH₄ gas pressurization, and HYLN7, which was preserved in liquid-nitrogen, recovered from the BPXA-DOE-USGS Mount Elbert Stratigraphic Test Well. Gas hydrate in the studied core samples was found by observation to have developed in sediment pores, and the distribution of hydrate saturation in the cores imply that gas hydrate had experienced stepwise dissociation before it was stabilized by either liquid nitrogen or pressurizing gas. The gas hydrates were determined to be structure Type I hydrate with hydration numbers of approximately 6.1 by instrumentation methods such as powder X-ray diffraction, Raman spectroscopy and solid state ¹³C NMR. The hydrate gas composition was predominantly methane, and isotopic analysis showed that the methane was of thermogenic origin (mean δ¹³C = −48.6‰ and δ_D = −248‰ for sample HYLN7). Isotopic analysis of methane from sample HYPV4 revealed secondary hydrate formation from the pressurizing methane gas during storage.     

Hydrocarbon gases in the crystalline rocks of the Gravberg-1 well,Swedish deep gas project

Desorbed gas analyses of cuttings from the Gravberg—1 well, the culmination of the Swedish deep gas project, were undertaken every 100 m from 219 to 5907 m. A sample at 6517 m, from sidetrack 2, was also included. The desorbed gas method performed by hot acid treatment in an evacuated system was superior to both ball mill crushing and thermodesorption methods. Two types of hydrocarbon gases were found in trace quantities. One type associated with the dolerite sills was an isotopically heavy, δ¹³C₁: −11 to −15‰, dry gas, with a methane content up to 98%. The other type, occurring throughout the granitic rocks, was an isotopically lighter gas, δ¹³C₁: −21 to −39 ‰, containing 30–45% of C₂C₄ olefins and paraffins were present in almost equal amounts in the second type of gas. The dry gas observed in the dolerites is assumed to be abiogenic gas existing in inclusions of basic minerals which react with acid during the analytical procedure. The other type of hydrocarbon gas is thought to be formed from H₂ and CO₂ by a catalytic reaction since it is mainly associated with the magnetic fraction of the rock. A Fischer-Tropsch reaction over a magnetite catalyst is the most likely reaction since it produces both paraffins and olefins. Studies on thin sections of cores and coarse cuttings suggest that the wet gas is not isolated in inclusions and the changes with time observed for a few thin sections indicate that it diffuses quite freely. The potential risks of contamination from drilling fluids and bit metamorphism were examined by comparing the hydrocarbon results with changes in the mud system, rate of penetration and bit life. Hydrocarbon analyses of a few mud additives were included as well. The result of these examinations plus the results of hydrocarbon analyses of cores from pilot core holes in the Siljan Crater suggest that the hydrocarbons observed in the cuttings are indigeneous.Comparing the results of the present study with other hydrocarbon occurrences outside of the Siljan Crater indicates that the hydrocarbons found in the Gravberg—1 well occur widespread in crustal rocks.     

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.     

Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands

Hydrocarbons in the form of gases, oils and waxes have been observed in the basalts of the Faeroe Islands in the North Atlantic. Gases and traces of oil were observed in the outflowing water of the deep Lopra-1 well drilled in 1981. The hydrocarbon gas was fairly dry, with stable isotopic ratios of δ¹³C₁₋₃=−41.4; −32.4; −26.5% respectively, typical of a thermogenic gas. High temperature gas chromatography of the oil showed that it consisted mostly of C₁₃₋₆₀ n-alkanes. Biomarker distribution observed by GC-MS indicates that the oil was derived from a mature source rock deposited in an anoxic environment; this suggests that the source rock must lie beneath the known basalts.Waxes exhibiting bright yellow fluorescence under UV light were observed as coatings on zeolite minerals widely distributed on the Faeroe Islands. The waxes consist predominantly of higher n-alkanes shown by HTGC. The fluorescence indicates the presence of aromatic compounds. Biomarker distribution indicated that the waxes were derived at least in part from a source rock containing some terrestrial organic matter astestified by the low amounts of oleanane present. The waxes were probably deposited from traces of oil present in deeply circulating waters in fractures within the basalts.Coals which had been suspected to generate some of the hydrocarbons observed in the Faeroese basalts were also examined. Vitrinite values of R_o=0.5% as well as GC-MS analyses of the Suduroy coal extracts showed that these coals are immature and have not generated significant hydrocarbons.     

Molecular and isotopic signatures in sediments and gas hydrate of the central/southwestern Ulleung Basin: high alkalinity escape fuelled by biogenically sourced methane

Natural marine gas hydrate was discovered in Korean territorial waters during a 2007 KIGAM cruise to the central/southwestern Ulleung Basin, East Sea. The first data on the geochemical characterization of hydrate-bound water and gas are presented here for cold seep site 07GHP-10 in the central basin sector, together with analogous data for four sites (07GHP-01, 07GHP-02, 07GHP-03, and 07GHP-14) where no hydrates were detected in other cores from the central/southwestern sectors. Hydrate-bound water displayed very low concentrations of major ions (Cl⁻, SO₄²⁻, Na⁺, Mg²⁺, K⁺, and Ca²⁺), and more positive δD (15.5‰) and δ¹⁸O (2.3‰) signatures compared to seawater. Cl^– freshening and more positive isotopic values were also observed in the pore water at gas hydrate site 07GHP-10. The inferred sulfate–methane interface (SMI) was very shallow (<5 mbsf) at least at four sites, suggesting the widespread occurrence of anaerobic oxidation of methane (AOM) at shallow sediment depths, and possibly high methane flux. Around the SMI, pore water alkalinity was very high (>40 mM), but the carbon isotopic ratios of dissolved inorganic carbon (δ¹³C_DIC) did not show minimum values typical of AOM. Moreover, macroscopic authigenic carbonates were not observed at any of the core sites. This can plausibly be explained by carbon with high δ¹³C values diffusing upward from below the SMI, increasing alkalinity via deep methanogenesis and eventually escaping as alkalinity into the water column, with minor precipitation as solid phase. This contrasts, but is not inconsistent with recent reports of methane-fuelled carbonate formation at other sites in the southwestern basin sector. Methane was the main hydrocarbon component (>99.85%) of headspace, void, and hydrate-bound gases, C₁/C₂₊ ratios were at least 1,000, and δ¹³C_CH4 and δD_CH4 values were in the typical range of methane generated by microbial reduction of CO₂. This is supported by the δ¹³C_C2H6 signatures of void and hydrate-bound gases, and helps clarify some contradictory interpretations existing for the Ulleung Basin as a whole. In combination, these findings suggest that deep biogenic gas and pore waters migrate upward through pathways such as hydrofractures, and measurably influence the shallow carbon cycle. As a result, cation-adjusted alkalinity/removed sulfate diagrams cannot always serve to estimate the degree of alkalinity produced by sulfate reduction and AOM in high methane flux areas.     

Vodyanitskii mud volcano,Sorokin trough,Black Sea: Geological characterization and quantification of gas bubble streams

Vodyanitskii mud volcano is located at a depth of about 2070 m in the Sorokin Trough, Black sea. It is a 500-m wide and 20-m high cone surrounded by a depression, which is typical of many mud volcanoes in the Black Sea. 75 kHz sidescan sonar show different generations of mud flows that include mud breccia, authigenic carbonates, and gas hydrates that were sampled by gravity coring. The fluids that flow through or erupt with the mud are enriched in chloride (up to ∼650 mmol L⁻¹ at ∼150-cm sediment depth) suggesting a deep source, which is similar to the fluids of the close-by Dvurechenskii mud volcano. Direct observation with the remotely operated vehicle Quest revealed gas bubbles emanating at two distinct sites at the crest of the mud volcano, which confirms earlier observations of bubble-induced hydroacoustic anomalies in echosounder records. The sediments at the main bubble emission site show a thermal anomaly with temperatures at ∼60 cm sediment depth that were 0.9 °C warmer than the bottom water. Chemical and isotopic analyses of the emanated gas revealed that it consisted primarily of methane (99.8%) and was of microbial origin (δD-CH₄ = −170.8‰ (SMOW), δ¹³C-CH₄ = −61.0‰ (V-PDB), δ¹³C-C₂H₆ = −44.0‰ (V-PDB)). The gas flux was estimated using the video observations of the ROV. Assuming that the flux is constant with time, about 0.9 ± 0.5 × 10⁶ mol of methane is released every year. This value is of the same order-of-magnitude as reported fluxes of dissolved methane released with pore water at other mud volcanoes. This suggests that bubble emanation is a significant pathway transporting methane from the sediments into the water column.     

Cause of shale gas geochemical anomalies and mechanisms for gas enrichment and depletion in high-maturity shales

This article reviews the abnormal characteristics of shale gases (natural gases produced from organic-rich shales) and discusses the cause of the anomalies and mechanisms for gas enrichment and depletion in high-maturity organic-rich shales. The reported shale gas geochemical anomalies include rollover of iso-alkane/normal alkane ratios, rollover of ethane and propane isotopic compositions, abnormally light ethane and propane δ¹³C values as well as isotope reversals among methane, ethane and propane. These anomalies reflect the complex histories of gas generation and associated isotopic fractionation as well as in-situ “mixing and accumulation” of gases generated from different precursors at different thermal maturities. A model was proposed to explain the observed geochemical anomalies. Gas generation from kerogen cracking at relatively low thermal maturity accounted for the increase of iso-alkane/normal alkane ratios and ethane and propane δ¹³C values (normal trend). Simultaneous cracking of kerogen, retained oil and wet gas and associated isotopic fractionation at higher maturity caused decreasing iso-alkane/normal alkane ratios, lighter ethane and propane δ¹³C and corresponding conversion of carbon isotopic distribution patterns from normal through partial reversal to complete reversal. Relatively low oil expulsion efficiency at peak oil generation, low expulsion efficiency at peak gas generation and little gas loss during post-generation evolution are necessary for organic-rich shales to display the observed geochemical anomalies. High organic matter richness, high thermal maturity (high degrees of kerogen-gas and oil-gas conversions) and late-stage (the stage of peak gas generation and post-generation evolution) closed system accounted for gas enrichment in shales. Loss of free gases during post-generation evolution may result in gas depletion or even undersaturation (total gas content lower than the gas sorption capacity) in high-maturity organic-rich shales.     

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.     

Methane and methane carbon isotope ratios in the Northeast Atlantic including the Mid-Atlantic Ridge (50°N)

Previous work has shown that methane anomalies frequently occur within the rift valley of the Mid-Atlantic Ridge (MAR). The plumes appear confined within the high, steep walls of the valley, and it is not known whether methane may escape to the open ocean outside. In order to investigate this question, the concentration and ¹³C/¹²C ratio of methane together with CCl₃F concentration were measured in the northeastern Atlantic including the rift valley near 50°N. This segment contained methane plumes centered several 100 m above the valley floor with δ¹³C values mostly between –15‰ and –10‰. A limited number of helium isotope measurements showed that δ³He increased to 17% at the bottom of the valley, which suggests the helium and methane sources may be spatially separated. In the eastern Atlantic away from the ridge (48°N, 20°W), the methane concentration decreased monotonically from the surface to the bottom, but the methane δ¹³C exhibited a mid-water maximum of about –25‰. The bottom water methane contained a significantly lower δ¹³C of about –36‰. Thus, it appears that isotopically heavy methane escapes from the MAR into North Atlantic Deep Water (NADW) that contacts the ridge crest while circulating to the east. The formation of NADW supplies isotopically light methane that dilutes the input of heavy carbon from the ridge. We employed a time-dependent box model to calculate the extent of isotope dilution and thereby the flux of MAR methane into the NADW circulation. The degree of methane oxidation, which affects the ¹³C/¹²C of methane through kinetic isotope fractionation, was estimated by comparing methane and CFC-11 model results with observations. The model calculations indicate a MAR methane source of about 0.06×10⁻⁹ mol L⁻¹ yr⁻¹ to waters at the depth of the ridge crest. Assuming this extends to a 500 m thick layer over half of the entire Atlantic, the amount of methane escaping from the MAR to the open ocean is estimated to be about 1×10⁹ mol yr⁻¹. The total production of methane within the rift valley is likely much greater than the flux from the valley to the outside because of local oxidation. This implies that serpentinization of ultramafic rocks supports much of methane production in the rift valley because the amount expected from basalt degassing in association with mantle helium (<0.6×10⁹ mol CH₄ yr⁻¹) is less than even the net amount escaping from the valley. The model results also indicate the methane specific oxidation rate is about 0.05 yr⁻¹ in open waters of the northern Atlantic.     

Enrichment and distribution of shale oil in the Cretaceous Qingshankou Formation,Songliao Basin,Northeast China

The Songliao Basin is a large-scale petroliferous basin in China. With a gradual decline in conventional oil production, the exploration and development of replacement resources in the basin is becoming increasingly important. Previous studies have shown that the Cretaceous Qingshankou Formation (K₂qn) has favorable geological conditions for the formation of shale oil. Thus, shale oil in the Qingshankou Formation represents a promising and practical replacement resource for conventional oil. In this study, geological field surveys, core observation, sample tests, and the analysis of well logs were applied to study the geochemical and reservoir characteristics of shales, identify shale oil beds, build shale oil enrichment models, and classify favorable exploration areas of shale oil from the Cretaceous Qingshankou Formation. The organic matter content is high in shales from the first member of the Cretaceous Qingshankou Formation (K₂qn¹), with average total organic carbon (TOC) content exceeding 2%. The organic matter is mainly derived from lower aquatic organisms in a reducing brackish to fresh water environment, resulting in mostly type I kerogen. The vitrinite reflectance (Ro) and the temperature at which the maximum is release of hydrocarbons from cracking of kerogen occurred during pyrolysis (T_max) respectively range from 0.5% to 1.1% and from 430 °C to 450 °C, indicating that the K₂qn¹ shales are in the low-mature to mature stage (Ro ranges from 0.5% to 1.2%) and currently generating a large amount of oil. The favorable depth for oil generation and expulsion is 1800–2200 m and 1900–2500 m, respectively as determined by basin modeling. The reserving space of the K₂qn¹ shale oil includes micropores and mircofractures. The micropore reservoirs are developed in shales interbedded with siltstones exhibiting high gamma ray (GR), high resistivity (Rt), low density (DEN), and slightly abnormal spontaneous potential (SP) in the well-logging curves. The microfracture reservoirs are mainly thick shales with high Rt, high AC (acoustic transit time), high GR, low DEN, and abnormal SP. Based on the shale distribution, geochemical characteristics, reservoir types, fracture development, and the process of shale oil generation and enrichment, the southern Taikang and northern Da'an are classified as two favorable shale oil exploration areas in the Songliao Basin.     

South Atlantic and benthic foraminifer δ13C deviations: implications for reconstructing the Late Quaternary deep-water circulation

Benthic foraminifer species Cibicidoides wuellerstorfi and related genera are assumed to secrete calcite very close to the carbon isotope values of the ambient bottom-water ΣCO₂. Recently, attention has been focused on substantial productivity-linked δ¹³C depletions. To examine further the productivity effect on benthic δ¹³C deviations, we present data from the South Atlantic between 15 and 35°S, including water samples from 10 hydrographic stations and related surface-sediment measurements on C. wuellerstorfi. We compare open-ocean data with observations in the Namibia Upwelling area. As a result, δ¹³C_ΣCO2 values as well as phosphate concentrations in water samples of the upwelling realm differ significantly from those of the open-ocean realm at least in the upper and mid-depth water masses (SACW, AAIW, UCDW). However, deviations from the Redfield fractionation, caused by air–sea fractionation, remain constant within each water mass, which means that the carbon isotope changes toward upwelling areas are exclusively determined by biological cycling. In addition to lower δ¹³C_ΣCO2 values in upwelling areas, a depletion in the δ¹³C of epibenthic foraminifer calcite is observed, which is most likely explained by the decay of organic matter, reducing the ¹³C/¹²C ratio in the pore water and influencing the carbon isotopic composition of the C. wuellerstorfi shells of highly productive areas. The paleoceanographic implication of this effect for reconstructing the Late Quaternary deep-water circulation is discussed using carbon isotope records of several sediment cores within and outside the Namibia upwelling area.     

Nitrogen and carbon isotopic composition of marine and terrestrial organic matter in Arctic Ocean sediments:: implications for nutrient utilization and organic matter composition

Relationships between organic carbon, total nitrogen and organic nitrogen concentrations and variations in δ¹³C_org and δ¹⁵N_org are examined in surface sediments from the eastern central Arctic Ocean and the Yermak Plateau. Removing the organic matter from samples with KOBr/KOH and determining residual as well as total N shows that there is a significant amount of bound inorganic N in the samples, which causes TOC/N_total ratios to be low (4–10 depending on the organic content). TOC/N_org ratios are significantly higher (8–16). This correction of organic TOC/N ratios for the presence of soil-derived bound ammonium is especially important in samples with high illite concentrations, the clay mineral mainly responsible for ammonium adsorption. The isotopic composition of the organic N fraction was estimated by determining the isotopic composition of the total and inorganic nitrogen fractions and assuming mass-balance. A strong correlation between δ¹⁵N_org values of the sediments and the nitrate concentration of surface waters indicates different relative nitrate utilization rates of the phytoplankton in various regions of the Arctic Ocean. On the Yermak Plateau, low δ¹⁵N_org values correspond to high nitrate concentrations, whereas in the central Arctic Ocean high δ¹⁵N_org values are found beneath low nitrate waters. Sediment δ¹³C_org values are close to −23.0‰ in the Yermak Plateau region and approximately −21.4‰ in the central Arctic Ocean. Particulate organic matter collected from meltwater ponds and ice-cores are relatively enriched in ¹³C (δ¹³C_org=−15.3 to −20.6‰) most likely due to low CO₂(aq) concentrations in these environments. A maximum terrestrial contribution of 30% of the organic matter to sediments in the central Arctic Ocean is derived, based on the carbon isotope data and various assumptions about the isotopic composition of the potential endmembers.