Geochemistry and occurrence of inorganic gas accumulations in Chinese sedimentary basins

Inorganic gases are commonly seen in eastern China and occasionally in southern China from the shallow water columns above hot and cold springs. The gases contain 68% to nearly 100% CO₂, with δ¹³C_CO2 and δ¹³C₁ values in the range of −1.18‰ to −6.00‰ and −19.48‰ to −24.94‰, respectively. All of the 34 large inorganic CO₂ and one inorganic methane accumulations discovered in China are distributed in eastern parts of the country, from both onshore and continental shelf basins. No commercial inorganic gas accumulation has been found in central and western China. This is a review of the occurrence and geochemical characteristics of inorganic gas accumulations in Chinese sedimentary basins. A detailed study of gas samples collected from four representative inorganic CO₂ pools and one possible inorganic methane pool indicates that inorganic alkane gases typically show δ¹³C₁ values greater than −10‰ versus PDB (mostly −30‰), with a positive stable carbon isotope sequence of δ¹³C₁ < δ ¹³C₂ < δ¹³C₃ < δ ¹³C₄. In contrast, the δ¹³C₁ values of biogenic alkane gases are lighter than −30‰, with a negative isotope sequence (i.e. δ¹³C₁ > δ¹³C₂ > δ ¹³C₃ > δ¹³C₄). Inorganic gases also tend to show less negative δ¹³C_CO2 values (−10‰) than biogenic gases (<−10‰).     

Stable isotopes of carbon dioxide in soil gas over massive sulfide mineralization at Crandon, Wisconsin

Stable isotope ratios of oxygen and carbon were determined for CO₂ in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO₂ concentrations detected previously by McCarthy et al. (1986). Values of δ¹³C in soil gas CO₂ from depths between 0.5 and 1.0 m were found to range from −12.68‰ to −20.03‰ (PDB). Organic carbon from the uppermost meter of soil has δ¹³C between −24.1 and −25.8‰ (PDB), indicating derivation from plant species with the C₃ (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C₃ and C₄ (Hatch-Slack) plants, together with atmospheric CO₂ are the likely sources of carbon in soil gas CO₂. Values of δ¹⁸O in soil-gas CO₂ range from 32 to 38‰ (SMOW). These δ¹⁸O values are intermediate between that calculated for CO₂ gas in isotopic equilibrium with local groundwaters and that for atmospheric CO₂. The δ¹⁸O data indicate that atmospheric CO₂ has been incorporated by mixing or diffusion. Any CO₂ generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters.The isotopic composition of soil-gas CO₂ taken from directly above the massive sulfide deposit was not distinguishable from that of background samples taken 1 to 2 km away. No enrichment of the δ¹³C value of soil-gas CO₂ was observed, contrary to what would be expected if the anomalous CO₂ were derived from the dissolution of Proterozoic marine limestone country rock or of Paleozoic limestone clasts in glacial till. Therefore, it is inferred that root respiration and decay of C₃ plant material were responsible for most CO₂ generation both in the vicinity of the massive sulfide and in the “background” area, on the occasion of our sampling. Interpretation of our data is complicated by the effects of rainfall, which significantly reduced the magnitude of the CO₂ anomaly. Therefore, we cannot rule out the possible mechanism of carbonate dissolution driven by pyrite oxidation, as proposed by Lovell et al. (1983) and McCarthy et al. (1986). Further work is needed on seasonal and daily variations of CO₂ concentrations and stable isotope ratios in various hydrogeologic and ecologic settings so that more effective sampling strategies can be developed for mineral exploration using soil gases.     

A review of the geochemistry of methane in natural gas hydrate

The largest accumulations on Earth of natural gas are in the form of gas hydrate, found mainly offshore in outer continental margin sediment and, to a lesser extent, in polar regions commonly associated with permafrost. Measurements of hydrocarbon gas compositions and of carbon-isotopic compositions of methane from natural gas hydrate samples, collected in subaquatic settings from around the world, suggest that methane guest molecules in the water clathrate structures are mainly derived by the microbial reduction of CO₂ from sedimentary organic matter. Typically, these hydrocarbon gases are composed of > 99% methane, with carbon-isotopic compositions (δ¹³C_PDB) ranging from − 57 to − 73‰. In only two regions, the Gulf of Mexico and the Caspian Sea, has mainly thermogenic methane been found in gas hydrate. There, hydrocarbon gases have methane contents ranging from 21 to 97%, with δ¹³C values ranging from − 29 to − 57‰. At a few locations, where the gas hydrate contains a mixture of microbial and thermal methane, microbial methane is always dominant. Continental gas hydrate, identified in Alaska and Russia, also has hydrocarbon gases composed of > 99% methane, with carbon-isotopic compositions ranging from − 41 to − 49‰. These gas hydrate deposits also contain a mixture of microbial and thermal methane, with thermal methane likely to be dominant. Published by Elsevier Science Ltd     

Gas accumulation from oil cracking in the eastern Tarim Basin: A case study of the YN2 gas field

Oil and gas exploration in eastern Tarim Basin, NW China has been successful in recent years, with several commercial gas accumulations being discovered in a thermally mature to over-mature region. The Yingnan2 (YN2) gas field, situated in the Yingnan structure of the Yingjisu Depression, produces gases that are relatively enriched in nitrogen and C₂₊ alkanes. The δ¹³C₁ (−38.6‰ to −36.2‰) and δ¹³C₂ values (−30.9‰ to −34.7‰) of these gases are characteristic of marine sourced gases with relatively high maturity levels. The distributions of biomarkers in the associated condensates suggest close affinities with the Cambrian–Lower Ordovician source rocks which, in the Yingjisu Sag, are currently over-mature (with 3–4%Ro). Burial and thermal maturity modeling results indicate that paleo-temperatures of the Cambrian–Lower Ordovician source rocks had increased from 90 to 210 °C during the late Caledonian orogeny (458–438 Ma), due to rapid subsidence and sediment loading. By the end of Ordovician, hydrocarbon potential in these source rocks had been largely exhausted. The homogenization temperatures of hydrocarbon fluid inclusions identified from the Jurassic reservoirs of the YN2 gas field suggest a hydrocarbon emplacement time as recent as about 10 Ma, when the maturity levels of Middle–Lower Jurassic source rocks in the study area were too low (<0.7%Ro) to form a large quantity of oil and gas. The presence of abundant diamondoid hydrocarbons in the associated condensates and the relatively heavy isotopic values of the oils indicate that the gases were derived from thermal cracking of early-formed oils. Estimation from the stable carbon isotope ratios of gaseous alkanes suggests that the gases may have been formed at temperatures well above 190 °C. Thus, the oil and gas accumulation history in the study area can be reconstructed as follows: (1) during the late Caledonian orogeny, the Cambrian–Lower Ordovician marine source rocks had gone through the peak oil, wet gas and dry gas generation stages, with the generated oil and gas migrating upwards along faults and fractures to form early oil and gas accumulations in the Middle–Upper Ordovician and Silurian sandstone reservoirs; (2) since the late Yanshanian orogeny, the early oil accumulations have been buried deeper and oil has undergone thermal cracking to form gas; (3) during the late Himalayan orogeny, the seals for the deep reservoirs were breached; and the gas and condensates migrated upward and eventually accumulating in the relatively shallow Jurassic reservoirs.     

Environmental control of carbon isotope variations in Pennsylvania black-shale sequences, Midcontinent, U.S.A.

A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and ratios) and display the lightest δ¹³C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ¹³C_kerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ¹³C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO₂-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO₂” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments.     

Cryogenic carbonates in cave environments: A review

Cryogenic cave carbonate (CCC) represents a specific type of speleothem. Its precipitation proceeds at the freezing point and is triggered by freezing-induced concentration of solutes. Compared to classical speleothems (stalagmites, flowstones), CCC occurs as accumulations of loose uncemented aggregates. The grain sizes range from less than 1 μm to over 1 cm in diameter. Karst groundwater chemistry and its freezing rate upon entering the cave are responsible for highly variable grain morphology. Rapid freezing of water results in the formation of CCC powders with grain size typically below 50 μm. Slow freezing of water in caves (usually in systems where the CO₂ escape is partly restricted; e.g., ice covered water pools) results in the formation of large mineral grains, with sizes from less than 1 mm to about 20 mm. The range of carbon and oxygen stable isotope compositions of CCC is larger than for a typical carbonate speleothem. Rapid freezing of water accompanied by a quick kinetic CO₂ degassing results in large ranges of δ¹³C of the CCC powders (between –10‰ and +18‰ PDB). Slow freezing of water, with a restricted CO₂ escape results in gradual increase of δ¹³C values (from −9‰ to +6‰ PDB; data ranges in individual caves are usually much more restricted), accompanied by a δ¹⁸O decrease of the precipitated carbonate (overall range from −10‰ to −24‰ PDB). These unusual trends of the carbonate δ¹⁸O evolution reflect incorporation of the heavier ¹⁸O isotope into the formed ice. New isotope data on CCC from three Romanian ice caves allow better understanding of the carbon and oxygen isotope fingerprint in carbonates precipitated from freezing of bulk water. CCCs are proposed as a new genetic group of speleothems.     

Geochemical characterization of secondary microbial gas occurrence in the Songliao Basin, NE China

A suite of natural gases from the northern Songliao Basin in NE China were characterized for their molecular and carbon isotopic composition. Gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation, with very high dryness (C₁/C₂₊ > 100), high C₂/C₃ and i-C₄/n-C₄ ratios, high nitrogen content and variable carbon dioxide content. Isotopic values show wide range variations (δ¹³C_CH4 from −79.5‰ to −45.0‰, δ¹³C_C2H6 from −53.7‰ to −32.2‰, δ¹³C_C3H8 from −36.5‰ to −20.1‰, δ¹³C_nC4H10 from −32.7‰ to −24.5‰, and δ¹³C_CO2 from −21.6‰ to +10.5‰). A variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence. Secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures. System openness also was considered for shallow biodegraded gas accumulations. Gases from the Daqing Anticline are relatively wet with ¹³C enriched methane and ¹³C depleted CO₂, representing typically thermogenic origin. Gases within the Longhupao-Da’an Terrace have variable dryness, ¹³C enriched methane and variable δ¹³C of CO₂, suggesting dominant thermogenic origin and minor secondary microbial methane augment. The Puqian-Ao’nan Uplift contains relatively dry gas with ¹³C depleted methane and ¹³C enriched CO₂, typical for secondary microbial gas with a minor part of thermogenic methane. Gas accumulations in the Western Slope are very dry with low carbon dioxide concentrations. Some gases contain ¹³C depleted methane, ethane and propane, indicating low maturity/primary microbial origin. Recognition of varying genetic gas types in the Songliao Basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems.     

A geochemical study on mud volcanoes in the Junggar Basin, China

A comprehensive study was performed to characterize, for the first time, the mud, water, and gases released from onshore mud volcanoes located in the southern margin of the Junggar Basin, northwestern China. Chemical compositions of mud, along with the geology of the basin, suggest that a source of the mud is Mesozoic or Cenozoic shale. Oxygen and H isotope compositions of the released water suggest a local meteoric origin. Combined with the positive Eu anomalies of the water, a large ¹⁸O shift of the water suggests extensive interaction with rocks. Gases discharged from the mud volcanoes are predominantly thermogenic hydrocarbons, and the high δ¹³C values (>+20‰ VPDB) for CO₂ gases and dissolved carbonate in muddy water suggest secondary methanogenesis with CO₂ reduction after oil biodegradation.The enrichments of Eu and ¹⁸O in water and the low thermal gradient of the area suggest that the water-rock interactions possibly occur deeper than 3670 ± 200 m. On the other hand, considering the relationship to the petroleum reservoir around the mud volcanoes, the depth of the gases can be derived from about 3600 m, a depth that is greater than that generally estimated for reservoirs whose gas is characterized by ¹³C-enriched CO₂. Oil biodegradation with CO₂ reduction likely occurs at a shallower depth along the seepage system of the mud volcano. The results contribute to the worldwide data set of gas genesis in mud volcanoes. Moreover, they further support the concept that most terrestrial mud volcanoes release thermogenic gas produced in very deep sediments and may be early indicators of oil biodegradation, an important problem in the petroleum industry.     

Isotopic and molecular composition of coal-bed gas in the Amasra region (Zonguldak basin—western Black Sea)

Previous studies on the coal-bed methane potential of the Zonguldak basin have indicated that the gases are thermogenic and sourced by the coal-bearing Carboniferous units. In this earlier work, the origin of coal-bed gas was only defined according to the molecular composition of gases and to organic geochemical properties of the respective source rocks, since data on isotopic composition of gases were not available. Furthermore, in the western Black Sea region there also exist other source rocks, which may have contributed to the coal-bed gas accumulations. The aim of this study is to determine the origin of coal-bed gas and to try a gas-source rock correlation. For this purpose, the molecular and isotopic compositions of 13 headspace gases from coals and adjacent sediments of two wells in the Amasra region have been analyzed. Total organic carbon (TOC) measurements and Rock-Eval pyrolysis were performed in order to characterize the respective source rocks. Coals and sediments are bearing humic type organic matter, which have hydrogen indices (HI) of up to 300 mgHC/gTOC, indicating a certain content of liptinitic material. The stable carbon isotope ratios (δ¹³C) of the kerogen vary from −23.1 to −27.7‰. Air-free calculated gases contain hydrocarbons up to C₅, carbon dioxide (<1%) and a considerable amount of nitrogen (up to 38%). The gaseous hydrocarbons are dominated by methane (>98%). The stable carbon isotope ratios of methane, ethane and propane are defined as δ¹³C₁: −51.1 to −48.3‰, δ¹³C₂: −37.9 to −25.3‰, δ¹³C₃: −26.0 to −19.2 ‰, respectively. The δD₁ values of methane range from −190 to −178‰. According to its isotopic composition, methane is a mixture, partly generated bacterially, partly thermogenic. Molecular and isotopic composition of the gases and organic geochemical properties of possible source rocks indicate that the thermogenic gas generation took place in coals and organic rich shales of the Westphalian-A Kozlu formation. The bacterial input can be related to a primary bacterial methane generation during Carboniferous and/or to a recent secondary bacterial methane generation. However, some peculiarities of respective isotope values of headspace gases can also be related to the desorption process, which took place by sampling.     

Origin of methane in high-arsenic groundwater of Taiwan – Evidence from stable isotope analyses and radiocarbon dating

Groundwaters in the confined aquifers of the Chianan and Ilan coastal plains of Taiwan are rich in dissolved methane (CH₄). Serious endemic “blackfoot disease”, which occurred in the Chianan plain, especially during AD1950-1970, has been demonstrated to have arisen from drinking highly reducing groundwater with abnormal arsenic and humic substance levels. In order to explore the origin of CH₄ and its hydrological implications, stable carbon isotope ratios (δ¹³C) and radiocarbon (¹⁴C) ages of exsolved CH₄, dissolved inorganic carbon (DIC), and sedimentary biogenic sediments from a total of 34 newly completed water wells at 16 sites were determined. The main results obtained are as follows: (1) The δ¹³C_CH4 (−65‰ to −75‰) values indicate that, except for one thermogenic sample (δ¹³C_CH4=38.2‰) from the Ilan plain, all CH₄ samples analyzed were produced via microbially mediated CO₂ reduction. Many δ¹³C_DIC values are considerably greater than −10‰ and even up to 10‰ due to Rayleigh enrichment during CO₂ reduction. (2) Almost all the ¹⁴C ages of CH₄ samples from the shallow aquifer (I) (<60 m depth) are greater than the ¹⁴C ages of coexisting DIC and sediments, suggesting the presence of CH₄ from underlying aquifers. (3) The ¹⁴C ages of coexisting CH₄, DIC and sediments from aquifer (II) of the Chianan plain are essentially equal, reflecting in-situ generation of CH₄ and DIC from decomposition of sedimentary organic matter and sluggishness of the groundwater flow. On the other hand, both CH₄ and DIC from each individual well of the relatively deep aquifers (III) and (IV) in the Chianan plain are remarkably younger than the deposition of their coexisting sediments, indicating that current groundwaters entered these two aquifers much later than the deposition of aquifer sediments. (4) Each CH₄ sample collected from the Ilan plain is older than coexisting DIC, which in turn is distinctly older than the deposition of respective aquifer sediments, demonstrating the presence of much older CO₂ and CH₄ from underlying strata.     

Geochemistry and episodic accumulation of natural gases from the Ledong gas field in the Yinggehai Basin, offshore South China Sea

The Ledong gas field, consisting of three gas pools in a shale diapir structure zone, is the largest gas discovery in the Yinggehai Basin. The gases produced from the Pliocene and Quaternary marine sandstone reservoirs show a considerable variation in chemical composition, with 5.4–88% CH₄, 0–93% CO₂, and 1–23.7% N₂. The CO₂-enriched gases often display heavier methane δ¹³C values than those with low CO₂ contents. The δ¹⁵N values of the gases range from −8 to −2‰, and the N₂ content correlates negatively with the CO₂ content. The high geothermal gradient associated with a relatively great burial depth in this area has led to the generation of hydrocarbon and nitrogen gases from the Lower–Middle Miocene source rocks and the formation of abundant CO₂ from the Tertiary calcareous-shales and pre-Tertiary carbonates. The compositional heterogeneities and stable carbon isotope data of the produced gases indicate that the formation of the LD221 gas field is attributed to three phases of gas migration: initially biogenic gas, followed by thermogenic hydrocarbon gas, and then CO₂-rich gas. The filling processes occurred within a short period approximately from 1.2 to 0.1 Ma based on the results of the kinetics modeling. Geophysical and geochemical data show that the diapiric faults that cut through Miocene sediments act as the main pathways for upward gas migration from the deep overpressured system into the shallow normal pressure reservoirs, and that the deep overpressure is the main driving force for vertical and lateral migration of the gases. This gas migration pattern implies that the transitional pressure zone around the shale diapir structures was on the pathway of upward migrating gases, and is also a favorable place for gas accumulation. The proposed multiple sources and multiple phases of gas migration and accumulation model for the Ledong gas field potentially provide useful information for the future exploration efforts in this area.     

Geochemistry and origin of the giant Quaternary shallow gas accumulations in the eastern Qaidam Basin, NW China

This study provided an overview of the geological setting and geochemical characteristics of the Pleistocene shallow gas accumulations in the eastern Qaidam Basin, NW China. The five largest gas accumulations discovered in this region have a combined enclosure area of about 87 km² and 7.9 trillion cubic feet (tcf) of proven gas reserves. The dominance of methane (mostly more than 99.9%) and the δ¹³C and δD values of methane (−68.51 to −65.00‰ and −227.55 to −221.94‰, respectively) suggest that these gases are biogenic, derived from the degradation of sedimentary organic matter by methanogens under relatively low temperatures (<75 °C). A sufficient supply and adequate preservation of organic matter in the Pleistocene sediments was made possible by the lake basin’s high altitude (2600–3000 m), high water salinity (>15% TDS) and strong stratification. The deposition and extensive lateral occurrence of lacustrine – shoreline sands/silts in beach sand sheets and sand bars provided excellent reservoirs for the biogenic gas generated from adjacent rocks. Effective but dynamic gas seals are provided by a combination of factors, such as the intermittent vertical variation in the sediment lithologies, hydraulic trapping due to the mudstone water saturation, the hydrocarbon gradient created as the result of gas generation from potential caprocks, and the presence of a regional caprock consisting of 400–800 m of muds and evaporites. It appears that the most favorable traps for large gas accumulations occur on structural slopes near the major gas kitchen, and the prolific gas pools are often those large gentle anticlines with little faulting complication.     

Worldwide distribution and significance of secondary microbial methane formed during petroleum biodegradation in conventional reservoirs

Around half of world’s endowment of in-place oil and bitumen experienced biodegradation, which is now believed to be largely an anaerobic methanogenic process. However, the distribution and scale of methanogenic biodegradation in the world’s petroleum accumulations and the significance of its terminal product, secondary microbial methane, in the global gas endowment and carbon cycle are largely unknown. Here, I present geological and geochemical criteria to recognize secondary microbial methane in conventional petroleum reservoirs. These include the presence of biodegraded oil (as pools, legs or shows) in the reservoir or down-dip, the relatively dry (methane dominated) gas containing methane with δ¹³C values between −55‰ and −35‰ and, most importantly, CO₂ with δ¹³C > +2‰. Based on these criteria, the presence of secondary microbial methane is apparent in 22 basins, probable in 12 basins and possible in six basins worldwide. Reservoirs apparently containing secondary microbial methane are mostly Cenozoic and clastic and occur at depths of 37-1834 m below surface/mudline and temperatures of 12-71 °C. Using the current global endowment of in-place oil and bitumen and reasonable assumptions about conversion of oil into methane during biodegradation, I estimated that ∼65,500 tcf of secondary microbial methane could have been generated in existing worldwide accumulations of oil and bitumen through their geological history. From 1461-2760 tcf in-place (845-1644 tcf recoverable) of secondary microbial methane may be accumulated as free and oil-dissolved gas in petroleum reservoirs. I also updated the inventory of primary microbial methane and estimated that the global primary microbial gas endowment (free and oil-dissolved) is from 676-797 tcf in-place (407-589 tcf recoverable). Secondary microbial methane may account for ∼5-11% of the global conventional recoverable gas endowment and appears more abundant than primary microbial gas (∼3-4% of the global gas endowment). Most of the generated secondary microbial methane probably is aerobically and anaerobically oxidized to CO₂ in the overburden above petroleum reservoirs. However, some secondary microbial methane may escape from shallow reservoirs into the atmosphere and affect present and past global climate.     

Geochemical features and origin of natural gas in heavy oil area of the Western Slope,Songliao Basin,China

The Western Slope of the Songliao Basin is rich in heavy oil resources (>70 × 10⁸ bbl), around which there are shallow gas reservoirs (∼1.0 × 10¹² m³). The gas is dominated by methane with a dryness over 0.99, and the non-hydrocarbon component being overwelmingly nitrogen. Carbon isotope composition of methane and its homologs is depleted in ¹³C, with δ¹³C₁ values being in the range of −55‰ to −75‰, δ¹³C₂ being in the range of −40‰ to −53‰ and δ¹³C₃ being in the range of −30‰ to −42‰, respectively. These values differ significantly from those solution gases source in the Daqing oilfield. This study concludes that heavy oils along the Western Slope were derived from mature source rocks in the Qijia-Gulong Depression, that were biodegraded. The low reservoir temperature (30–50 °C) and low salinity of formation water with neutral to alkaline pH (NaHCO₃) appeared ideal for microbial activity and thus biodegradation. Natural gas along the Western Slope appears mainly to have originated from biodegradation and the formation of heavy oil. This origin is suggested by the heavy δ¹³C of CO₂ (−18.78‰ to 0.95‰) which suggests that the methane was produced via fermentation as the terminal decomposition stage of the oil.     

Origin of light volatile hydrocarbon gases in mud volcano fluids, Gulf of Cadiz — Evidence for multiple sources and transport mechanisms in active sedimentary wedges

Widespread mud volcanism across the thick (≤ 14 km) seismically active sedimentary prism of the Gulf of Cadiz is driven by tectonic activity along extensive strike–slip faults and thrusts associated with the accommodation of the Africa–Eurasia convergence and building of the Arc of Gibraltar, respectively. An investigation of eleven active sites located on the Moroccan Margin and in deeper waters across the wedge showed that light volatile hydrocarbon gases vented at the mud volcanoes (MVs) have distinct, mainly thermogenic, origins. Gases of higher and lower thermal maturities are mixed at Ginsburg and Mercator MVs on the Moroccan Margin, probably because high maturity gases that are trapped beneath evaporite deposits are transported upwards at the MVs and mixed with shallower, less mature, thermogenic gases during migration. At all other sites except for the westernmost Porto MV, δ¹³C–CH₄ and δ²H–CH₄ values of ~ − 50‰ and − 200‰, respectively, suggest a common origin for methane; however, the ratio of CH₄/(C₂H₆ + C₃H₈) varies from ~ 10 to > 7000 between sites. Mixing of shallow biogenic and deep thermogenic gases cannot account for the observed compositions which instead result mainly from extensive migration of thermogenic gases in the deeply-buried sediments, possibly associated with biodegradation of C₂₊ homologues and secondary methane production at Captain Arutyunov and Carlos Ribeiro MVs. At the deep-water Bonjardim, Olenin and Carlos Ribeiro MVs, generation of C₂₊-enriched gases is probably promoted by high heat flux anomalies which have been measured in the western area of the wedge. At Porto MV, gases are highly enriched in CH₄ having δ¹³C–CH₄ ~ − 50‰, as at most sites, but markedly lower δ²H–CH₄ values < − 250‰, suggesting that it is not generated by thermal cracking of n-alkanes but rather that it has a deep Archaeal origin. The presence of petroleum-type hydrocarbons is consistent with a thermogenic origin, and at sites where CH₄ is predominant support the suggestion that gases have experienced extensive transport during which they mobilized oil from sediments ~ 2–4 km deep. These fluids then migrate into shallower, thermally immature muds, driving their mobilization and extrusion at the seafloor. At Porto MV, the limited presence of petroleum in mud breccia sediments further supports the hypothesis of a predominantly deep microbial origin of CH₄.     

Coal seam gas distribution and hydrodynamics of the Sydney Basin,NSW, Australia

This paper reviews various coal seam gas (CSG) models that have been developed for the Sydney Basin, and provides an alternative interpretation for gas composition layering and deep-seated CO₂ origins. Open file CSG wells, supplemented by mine-scale information, were used to examine trends in gas content and composition at locations from the margin to the centre of the basin. Regionally available hydrochemistry data and interpretations of hydrodynamics were incorporated with conventional petroleum well data on porosity and permeability. The synthesised gas and groundwater model presented in this paper suggests that meteoric water flow under hydrostatic pressure transports methanogenic consortia into the subsurface and that water chemistry evolves during migration from calcium-rich freshwaters in inland recharge areas towards sodium-rich brackish water down-gradient and with depth. Groundwater chemistry changes result in the dissolution and precipitation of minerals as well as affecting the behaviour of dissolved gases such as CO₂. Mixing of carbonate-rich waters with waters of significantly different chemistries at depth causes the liberation of CO₂ gas from the solution that is adsorbed into the coal matrix in hydrodynamically closed terrains. In more open systems, excess CO₂ in the groundwater (carried as bicarbonate) may lead to precipitation of calcite in the host strata. As a result, areas in the central and eastern parts of the basin do not host spatially extensive CO₂ gas accumulations but experience more widespread calcite mineralisation, with gas compositions dominated by hydrocarbons, including wet gases. Basin boundary areas (commonly topographic and/or structural highs) in the northern, western and southern parts of the basin commonly contain CO₂-rich gases at depth. This deep-seated CO₂-rich gas is generally thought to derive from local to continental scale magmatic intrusions, but could also be the product of carbonate dissolution or acetate fermentation.     

Geochemical hydrocarbon exploration — a new/old exploration tool

The search for petroleum has evolved into a highly sophisticated technology where today almost every scientific discipline known is being brought to bear upon the endeavour. Yet, the use of geochemical hydrocarbon exploration remains a peripheral exploration tool. The trend toward scientific integration has led the petroleum explorationist to the point of being a specialist. It would seem that our petroleum scientists have focussed their interests mainly on the investigation of principles and less on their ultimate purpose of discovering new and larger oil and gas reserves. So, it is not by chance, that leading geochemists have been speaking more and more freely of the necessity to integrate our tools of exploration and thereby do a better job. The theoretical basis for hydrocarbon geochemistry is complex, and, as with all exploration tools, the problems and difficulties of interpreting the data will never be completely eliminated.This article considers the importance of using the ΔC method in geochemical hydrocarbon exploration which has been employed successfully for over 40 years. The addition of carbon-isotope ratios and trace-element analysis to this method has added a new dimension to geochemical hydrocarbon exploration. The theoretical basis of the ΔC method has been presented earlier by the author and will only be touched upon briefly here.Very simply, the basis of all geochemical hydrocarbon exploration is based on the much debated premise that the lighter hydrocarbon gases and their components migrate vertically from a trap through the overlying sedimentary pile to the surface. Upon reaching the surface, through oxidation, they leave their signatures in one form or another that can be detected by physicochemical methods. These physicochemical signatures are discernable as “geochemical haloes”.From soil samples, collected from 2–3 m deep, what is measured is the result of absorption and adsorption by soil particles that are altered to CO₂ by oxidation and form a unique, stable, carbonate system with the surface and near-surface material. This is unlike other carbonate systems and when subjected to a differential thermal technique, dissociates into CO₂ surface material is cumulative and indicates where maximum hydrocarbon leakage has taken place over the life span of the material sampled. It is durable and unaffected by pressure and temperature variation or recent hydrocarbon contamination.Values are expressed in terms of millivolts which are proportional to the CO₂ given off by the dissociation of the carbonate system under standard conditions. Frequency curves are constructed for all values for the determination of significant contour levels above the normal geochemical background for mapping.After significant ΔC anomalies are located, they can be further verified by use of carbon-isotope ratios. As methane migrates to the surface from underlying hydrocarbon accumulations, there is a progressive selection or fractionation that causes enrichment of the carbon-13 isotope. The methane, thus reaching the near-surface, is isotopically lighter. When oxidized in accordance with the equation CH₄ + 2O₂ → 2H₂O + CO₂, the carbon having been converted to carbon dioxide, is taken up in the pore-filling carbonate cements that are found in the near-surface soils and sediments.High carbon dioxide values (ΔC) in the geochemical halo are related the δ ¹³C carbon-isotope ratios from underlying hydrocarbon accumulations. This is observed over fields containing hydrocarbon accumulations where δ ¹³C values in the pore-filling carbonate cements become increasingly negative (lighter) toward the crests of traps (i.e. exhibiting lower ΔC values). This indicates enrichment of ¹²C relative to the PDB standard. Whereas, positive values of δ ¹³C indicate depletion in ¹²C or enrichment in ¹³C (i.e. exhibiting higher ΔC values away from the crests of the traps).The observed ΔC anomalies and δ ¹³C anomalies leave an indelible pattern in the near-surface sediments and soils which are herein referred to as geochemical hydrocarbon haloes.Trace-element associations, that form organometallic compounds, are found “haloed” or concentrated over or around underlying hydrocarbon reservoirs. These associations seem to have occurred from vertically migrating methane that has acted as a “carrier” sweeping up the trace elements on the pathways to the surface. Vanadium, nickel, chromium, iron, cobalt, copper, manganese, strontium, barium are various trace element ratios seen to also halo and indicate subsurface hydrocarbon accumulations.An example presented from the Ocho-Juan Field, a producing reef field, located in Scurry and Fisher Counties, Texas shows that the combination of ΔC, δ ¹³C and trace-element analysis from near-surface soil sampling is a significant step forward in improving geochemical hydrocarbon exploration methods.     

The effect of minor to moderate biodegradation on C5 to C9 hydrocarbons in crude oils

A suite of 18 oils from the Barrow Island oilfield, Australia, and a non-biodegraded reference oil have been analysed compositionally in order to detail the effect of minor to moderate biodegradation on C₅ to C₉ hydrocarbons. Carbon isotopic data for individual low molecular weight hydrocarbons were also obtained for six of the oils. The Barrow Island oils came from different production wells, reservoir horizons, and compartments, but have a common source (the Upper Jurassic Dingo Claystone Formation), with some organo-facies differences. Hydrocarbon ratios based on hopanes, steranes, alkylnaphthalenes and alkylphenanthrenes indicate thermal maturities of about 0.8% R_c for most of the oils. The co-occurrence in all the oils of relatively high amounts of 25-norhopanes with C₅ to C₉ hydrocarbons, aromatic hydrocarbons and cyclic alkanes implies that the oils are the result of multiple charging, with a heavily biodegraded charge being overprinted by fresher and more pristine oil. The later oil charge was itself variably biodegraded, leading to significant compositional variations across the oilfield, which help delineate compartmentalisation. Biodegradation resulted in strong depletion of n-alkanes (>95%) from most of the oils. Benzene and toluene were partially or completely removed from the Barrow Island oils by water washing. However, hydrocarbons with lower water solubility were either not affected by water washing, or water washing had only a minor effect. There are three main controls on the susceptibility to biodegradation of cyclic, branched and aromatic low molecular weight hydrocarbons: carbon skeleton, degree of alkylation, and position of alkylation. Firstly, ring preference ratios at C₆ and C₇ show that isoalkanes are retained preferentially relative to alkylcyclohexanes, and to some extent alkylcyclopentanes. Dimethylpentanes are substantially more resistant to biodegradation than most dimethylcyclopentanes, but methylhexanes are depleted faster than methylpentanes and dimethylcyclopentanes. For C₈ and C₉ hydrocarbons, alkylcyclohexanes are more resistant to biodegradation than linear alkanes. Secondly, there is a trend of lower susceptibility to biodegradation with greater alkyl substitution for isoalkanes, alkylcyclohexanes, alkylcyclopentanes and alkylbenzenes. Thirdly, the position of alkylation has a strong control, with adjacent methyl groups reducing the susceptibility of an isomer to biodegradation. 1,2,3-Trimethylbenzene is the most resistant of the C₃ alkylbenzene isomers during moderate biodegradation. 2-Methylalkanes are the most susceptible branched alkanes to biodegradation, 3-methylalkanes are the most resistant and 4-methylalkanes have intermediate resistance. Therefore, terminal methyl groups are more prone to bacterial attack compared to mid-chain isomers, and C₃ carbon chains are more readily utilised than C₂ carbon chains. 1,1-Dimethylcyclopentane and 1,1-dimethylcyclohexane are the most resistant of the alkylcyclohexanes and alkylcyclopentanes to biodegradation. The straight-chained and branched C₅–C₉ alkanes are isotopically light (depleted in ¹³C) relative to cycloalkanes and aromatic hydrocarbons. The effects of biodegradation consistently lead to enrichment in ¹³C for each remaining hydrocarbon, due to preferential removal of ¹²C. Differences in the rates of biodegradation of low molecular weight hydrocarbons shown by compositional data are also reflected in the level of enrichment in ¹³C. The carbon isotopic effects of biodegradation show a decreasing level of isotopic enrichments in ¹³C with increasing molecular weight. This suggests that the kinetic isotope effect associated with biodegradation is site-specific and often related to a terminal carbon, where its impact on the isotopic composition becomes progressively ‘diluted’ with increasing carbon number.     

Possible errors in helium analyses of soil gases by mass spectrometers having constant-pressure inlet systems

Published data suggest that soil gas helium concentrations of 5.28–5.34 ppm v/v over uranium and hydrocarbon deposits are significantly anomalous compared to the ambient atmospheric background of 5.24 ppm. However, analyses for helium by mass spectrometers having constant-pressure inlet systems, from which most of these data are derived, are subject to errors of equivalent magnitude. These errors arise when the major component composition of unknown and standard gases differ, for the different gases have different flow rates through the inlet system — relative rates being O₂ < dry air < water-saturated air < N₂ < CO₂ CH₄. Soil gas compositions can vary greatly and, compared to a dry air standard, the flow-rate of a water-saturated gas containing 10% biogenic CO₂ will increase, enhancing the apparent He content to 5.33 ppm. Accurate helium analyses can be achieved by using a constant-volume inlet and integrating the detector response over the period of the samples' passage through the detector.     

Detailed compositional analysis of gas seepage at the National Carbon Storage Test Site,Teapot Dome,Wyoming, USA

A baseline determination of CO₂ and CH₄ fluxes and soil gas concentrations of CO₂ and CH₄ was made over the Teapot Dome oil field in the Naval Petroleum Reserve No. 3 (NPR-3) in Wyoming, USA. This was done in anticipation of experimentation with CO₂ sequestration in the Pennsylvanian Tensleep Sandstone underlying the field at a depth of 1680 m.The baseline data were collected during the winter, 2004 in order to minimize near-surface biological activity in the soil profile. The baseline data were used to select anomalous locations that may be the result of seeping thermogenic gas, along with background locations. Five 10-m holes were drilled, 3 of which had anomalous gas microseepage, and 2 were characterized as “background.” These were equipped for nested gas sampling at depths of 10-, 5-, 3-, 2-, and 1-m depths. Methane concentrations as high as 170,000 ppmv (17%) were found, along with high concentrations of C₂H₆, C₃H₈, n-C₄H₁₀, and i-C₄H₁₀. Much smaller concentrations of C₂H₄ and C₃H₆ were observed indicating the beginning of hydrocarbon oxidation in the anomalous holes. The anomalous 10-m holes also had high concentrations of isotopically enriched CO₂, indicating the oxidation of hydrocarbons. Concentrations of the gases decreased upward, as expected, indicating oxidation and transport into the atmosphere. The ancient source of the gases was confirmed by ¹⁴C determinations on CO₂, with radiocarbon ages approaching 38 ka within 5 m of the surface.Modeling was used to analyze the distribution of hydrocarbons in the anomalous and background 10-m holes. Diffusion alone was not sufficient to account for the hydrocarbon concentration distributions, however the data could be fit with the addition of a consumptive reaction. First-order rate constants for methanotrophic oxidation were obtained by inverse modeling. High rates of oxidation were found, particularly near the surface in the anomalous 10-m holes, demonstrating the effectiveness of the process in the attenuation of CH₄ microseepage. The results also demonstrate the importance of CH₄ measurements in the planning of a monitoring and verification program for geological CO₂ sequestration in sites with significant remaining hydrocarbons (i.e. spent oil reservoirs).