Seepage System Research Articles

Venting and seepage systems associated with mud volcanoes and mud diapirs in the southern Tyrrhenian Sea

Abstract High resolution swath bathymetry and backscatter data, seismic CHIRP profiles, multibeam water column acoustic measurements and sediment samples were collected on a cold seep province in the southeastern Tyrrhenian Sea, at a water depth of 500–1000 m. The mud volcanoes, characterized by a high backscatter signature, are the site of gas venting at the seafloor that formed a 630-m-high plume in the water column. The mud volcanoes feature a precipitation of iron-oxy-hydroxide crusts and pyritized and Sulfur burrows in the sub-surface and authigenic siderites, also cementing burrows, further down, showing a sharp transition from the oxic zone toward the sulfate-methanogenic zone. The mud flows are characterized by an intermediate backscatter seafloor and by the presence of gas in the sediment only 2 m below the seafloor. The mud flows consist of 1-m-thick drapes of water-rich mud extending downslope from the mud volcanoes. They act as sealing layers that prevent large fluxes of gas venting at the seafloor (low venting) and favor oxic conditions close to the sediment–water interface and the abundant precipitation of post-oxic siderites a few meters below the seafloor. The mud diapirs are characterized by a low backscatter seafloor and large fields of pockmarks. In coincidence with the normal faults, organogenic carbonate crusts form at or very close to the seafloor and are associated with chemosymbiontic bivalves (lucinids). The youngest shells are AMS radiocarbon dated 640–440 BP, suggesting that the seepage activity may have been clogged by the carbonates, only very recently. Similarities between the normal faults in the study area and the tectonic setting of the inland Calabrian Arc show that normal faults can control the location of fluid pathways and, probably, also the rising of the mud diapirs.

Analysis on Seepage Field and Seepage Control Measure around Kunlong Hydropower Project

The Kunlong Hydropower has high water head and complex geologic environment. It is necessary to study the seepage field and the seepage control measure. In order to set up a three dimensional seepage numerical model around the dam site, double fracture seepage systems contained main fractures network and fractured rock masses is adopted. Pipe to represent a drain hole and fissure to represent a line of holes methods are used in simulation of drainage curtain. After setting boundary conditions and calculating parameter appropriately, the whole seepage field around the Kunlong Dam is calculated with the finite element method. The optimal depths of the impervious curtain and drainage curtain are determined by sensitive analysis. The implications of the analysis results on the design of the project are significant.

Using helium and other noble gases in ocean sediments to characterize active methane seepage off the coast of New Zealand

Recently developed analytical techniques to determine the abundances of noble gases in sediment pore water allow noble-gas concentrations and isotope ratios to be measured easily and routinely in lacustrine sediments. We applied these techniques for the first time to ocean sediments to investigate an active cold methane seepage system located in the South Pacific off the coast of the North Island of New Zealand using 3He/4He ratios determined in the sediment pore water. The results show that more 3He-rich fluids are released in the vicinity of the Pacific–Australian subduction zone than at the forearc stations located closer to the New Zealand coast. However, the He isotope signature in the sediment column indicates that only a minor part of the He emanating from deeper strata originates from a depleted mantle source. Hence, most He in the pore water is produced locally by the radioactive decay of U and Th in the sediment minerals or in the underlying crustal rocks. Such an occurrence of isotopically heavy crustal He also suggests that the source of the largest fraction of methane is a near-surface geochemical reservoir. This finding is in line with a previous δ13C study in the water column which concluded that the emanating methane is most likely of biological origin and is formed in the upper few meters of the sediment column. Moreover, the prevalence of isotopically heavy He agrees well with the outcome of other previous studies on island arc systems which indicate that the forearc regions are characterized by crustal He emission, whereas the volcanic arc region is characterized by the presence of mantle He associated with rising magma.

Natural seepage of shale gas and the origin of “eternal flames” in the Northern Appalachian Basin, USA

Natural hydrocarbon gas seeps are surface expressions of Petroleum Seepage Systems, whereby gas is ascending through faults from pressurized reservoirs that are typically associated with sandstones or limestones. A spectacular “eternal flame” in western New York State marks a gas macroseep of dominantly thermogenic origin emanating directly from deep shale source rocks, which makes this a rare case in contrast to most Petroleum Seepage Systems where gas derives from conventional reservoirs.The main flaming seep releases about 1 kg of methane per day and may feature the highest ethane and propane (C2 + C3) concentration ever reported for a natural gas seep (∼35 vol. %). The same gas is also released to the atmosphere through nearby invisible and diffuse seepages from the ground. The synopsis of our data with available gas-geochemical data of reservoir gases in the region and the stratigraphy of underlying shales suggests that the thermogenic gas originates from Upper Devonian shales without intermediation of a conventional reservoir. A similar investigation on a second “eternal flame” in Pennsylvania suggests that gas is migrating from a conventional sandstone pool and that the seep is probably not natural but results from an undocumented and abandoned gas or oil well. The large flux of the emitted shale gas in New York State implies the existence of a pressurized gas pool at depth. Tectonically fractured shales seem to express “naturally fracked” characteristics and may provide convenient targets for hydrocarbon exploration. Gas production from “tectonically fracked” systems might not require extensive artificial fracking.

Hydrocarbon seepage during the Messinian salinity crisis in the Tertiary Piedmont Basin (NW Italy)

Seep carbonate deposits of Messinian age have been recently found in the Tertiary Piedmont Basin (NW Italy). These carbonates are preserved as blocks within a chaotic unit emplaced during the Messinian salinity crisis (MSC). They show negative δ13C values (from −27 to −15‰ VPDB) that indicate the involvement of hydrocarbon-rich fluids in their genesis. Three types of carbonates are recognised: (i) vuggy carbonates; (ii) Lucina carbonates; and (iii) tubeworm carbonates. Vuggy carbonates are characterised by carbonate pseudomorphs after gypsum and probably formed during the first stage of the MSC. They are the product of a complex diagenesis, influenced by both hypersalinity and seepage of hydrocarbon rich fluids. These rocks lack chemosymbiotic assemblages, reflecting their formation under extreme environmental conditions, inhospitable for most metazoans. In contrast, Lucina and tubeworm carbonates are characterised by chemosymbiotic macrofauna, represented respectively by Lucina bivalves and putative vestimentiferan tubeworms. The latter have not commonly been documented in ancient seep carbonates and have never been reported from the Messinian sediments of the Piedmont Basin. Both Lucina and tubeworm carbonates are interpreted as the product of hydrocarbon seepage during the second MSC stage. These two types of carbonates formed under less severe conditions than the vuggy carbonates, allowing the survival of seep-dwelling metazoans. During the second MSC stage, the seafloor was probably characterised by an irregular topography and a thin bottom layer of dense anoxic brines, produced by the dissolution of gypsum. It is suggested that vestimentiferan worms were able to thrive on morphologic highs with the posterior part of tubes just below the oxic–anoxic interface, but the anterior part projecting into oxic water. The infaunal Lucina bivalves were only able to live at seeps with an overlying oxic water column. The studied carbonate deposits show features reflecting the uncommon interaction of hydrocarbon-rich seep fluids and sulphate-enriched waters – the latter resulting from both evaporation and dissolution of gypsum – and allow to reconstruct the evolution of a seepage system during the MSC.

Reservoir Fracture Characteristics and its Influence on Oilfield Development in Chang 4+5 Formation in Southern Puziwan Area

Natural fractures controlled by the palaeotectonic stress field and artificial fractures controlled by the current stress field totally determined the distribution of the seepage system in super low permeability reservoir. Synthetically evaluation of the influence of fractures on oilfield development and the research on regularity of natural fractures distribution and the effect of stress field on the fractures are made in this paper, which is of important theoretical and practical significance in improving the development effect in super low permeability reservoir.

OBSERVATIONS OF SALINITY PATTERNS IN SHALLOW GROUNDWATER AND DRAINAGE WATER FROM AGRICULTURAL LAND IN THE NORTHERN PART OF THE NETHERLANDS

ABSTRACTIn studies concerning rainwater lenses on a parcel scale in brackish polder areas it is assumed that infiltrated rainwater flows through the rainwater lens and seeps to the watercourses. This freshwater system is supposed to be superimposed on the brackish diffuse upward seepage system. This study investigates the influence of field drainage on the dynamic behaviour of fresh rainwater lenses and the risk of salinization of the root zone. Observations presented in this paper show that shallow water and salt movements in clayey polders are more complex and more dynamic than previously thought.On 18 parcels 2D geo‐electrical resistivity imaging profiles were measured, which give a good spatial and temporal representation of the salinity distribution of the groundwater. It appears that during wet periods upconing of brackish groundwater appears under pipe drains and ditches, which drain a mixture of fresh meteoric water and deep brackish groundwater. Between drainpipes small rainwater lenses develop. During the dry season, when the groundwater table drops below the drainage level, brackish soil moisture stays behind. Transpiration of the crops rooted in the clayey polder soils result in strong capillary rise of salts, which may even reach the root zone in some places. Copyright © 2011 John Wiley & Sons, Ltd.

Study on the chaotic behavior of mining rock seepage system

One dimensional non-steady, non-Darcy flow of water in a rock stratum was reduced into a system described by six ordinary differential equations involving five controlling parameters. Through response computations and time series analysis, chaotic behavior in the reduced system was discussed in details. Firstly, the dynamical response of the reduced system under a set of parameters was calculated, and the power spectrum of the attractor was obtained through fast Lagrangian transformation; then the phase space was reconstructed by fixing embedding dimension to be 6 and delay time to range from 1 to 20, and the correlation dimension of the attractor was calculated based on the curves under the coordinates of logarithm of correlation integral vs. logarithm of covering radius; and lastly, the Lyapunov indices of the attractor were calculated by using Gram-Schmit’s orthogonalization method. The results show that the power spectrum of the attractor is continuous; the correlation dimension of the attractor is equal to 2.36; among the Lyapunov indices, LE1, LE2, LE3 are positive, LE5, LE6 are negative, and LE4 fluctuates near zero. All the analysis indicates that there may exist chaos in the system of non-steady, non-Darcy flow.

Characterization of seafloor pockmark seepage of hydrocarbons employing fractal: A case study from the western continental margin of India

Here we apply quantitative technique to describe the seafloor seepages based on the multi-beam backscatter and bathymetric investigations to characterize the pockmark morphology. The variable seafloor backscatter strength for coarser seafloor sediments are related to the diagenesis derived from biodegraded seepages. In this regard, box counting method is used to estimate ‘fractal dimension’ for backscatter imagery data of 398 blocks. These blocks are further sub-grouped into six classes depending on the spread of pockmark related seepages. The study area lies 102 km west off Marmagao along the central west coast of India which contains pre-dominantly (70%) gas-charged sediments. Comparison between the estimated self-similar fractals reveals that there is approximately 97% correlation between the box ( D box) and information ( D info) dimensions. Box dimension–derived fractal dimension values, suggest that the seepages are more along the fault trace in deeper waters, in comparison to sparsely distributed shallow water seepages. Besides, this poor seepage is confined within the smooth to moderately rough seafloor. It is established that the high backscatter strength along the upper slope of the pockmark region having higher fractal dimensions reflects multifractal behavior of seepage distribution. Entire area indicates patchy seepage patterns as supported by estimated fractal values showing intermittent fluctuations, which emphasizes non-linear behavior. Estimated self organizing criticality (SOC) parameters for six representative blocks reveal that the nature of pockmark, fault trace, sediment nature coupled with slumping of pockmark’s wall, sediment movement due to bottom currents are controlling the dynamic balance in the area seepage system. Further, our study emphasizing the multifractal behavior of seepage blocks, clearly depicts the drift in the seepage pattern.

Adsorptive removal of aqueous fluoride by liner minerals from SPL-landfill leachate during the seepage process

The breakthrough curve (BTC) of F− through a lining layer composed of kaolinite and montmorillonite was investigated in a one-dimensional seepage system as functions of the initial F− concentration, liner thickness, seepage velocity and presence of a co-solute. The removal of F− from the seepage system was much greater than that estimated from the batch adsorption result, probably due to the high liner mass-leachate volume ratio. By lowering the F− feed concentration from 30 to 15mg/L, the peak retardation (μ′1) was enhanced by 30–60%, which was consistent with the nonlinear F− adsorption (N<1) for the given materials. From a column with 1.5-times thicker liner (6cm), more than 80% of the peak concentration was reduced with both kaolinite and montmorillonite compared to the thinner layer (4cm). However, when the seepage velocity was doubled from 3.4×10−3 to 6.8×10−4cm/s, the peak concentration was increased by 20–30% and an early peak arrival was detected. The presence of 0.035mM [Fe(CN)6]4− showed no significant impact, as shown in the batch adsorption system. In conclusion, the thickness of the liner and seepage rate through the liner layer should be primary factors in determining the efficacy of F− removal from SPL-induced leachate.

Diversity and distribution of cold-seep fauna associated with different geological and environmental settings at mud volcanoes and pockmarks of the Nile Deep-Sea Fan

The Nile Deep-Sea Fan (NDSF) is located on the passive continental margin off Egypt and is characterized by the occurrence of active fluid seepage such as brine lakes, pockmarks and mud volcanoes. This study characterizes the structure of faunal assemblages of such active seepage systems of the NDSF. Benthic communities associated with reduced, sulphidic microhabitats such as sediments and carbonate crusts were sampled by remotely operated vehicles during two cruises in 2006 (BIONIL) and 2007 (MEDECO). Environmental conditions and biological factors including family-level faunal composition, density and diversity were measured at local and regional scales. Significant differences were detected at different spatial scales: (1) the fauna of reduced habitats differed substantially in activity, diversity and biomass from the non-seep environment at similar water depth, (2) cold seep microhabitats showed differences in community structure and composition related to substratum type as well as to the intensity and location of fluid emissions.

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

Abstract 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 18 O shift of the water suggests extensive interaction with rocks. Gases discharged from the mud volcanoes are predominantly thermogenic hydrocarbons, and the high δ 13 C values (>+20‰ VPDB) for CO 2 gases and dissolved carbonate in muddy water suggest secondary methanogenesis with CO 2 reduction after oil biodegradation. The enrichments of Eu and 18 O 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 13 C-enriched CO 2 . Oil biodegradation with CO 2 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.

Methane in shallow cold seeps at Mocha Island off central Chile

We studied for the first time the intertidal and subtidal gas seepage system in Mocha Island off Central Chile. Four main seepage sites were investigated (of which one site included about 150 bubbling points) that release from 150 to 240 tonnes CH 4 into the atmosphere per year. The total amount of methane emitted into the atmosphere is estimated in the order of 800 tonnes per year. The gases emanated from the seeps contain 70% methane, and the stable carbon isotopic composition of methane, δ 13C–CH 4 averaged −44.4±1.4‰ which indicates a major contribution of thermogenic gas. Adjacent to one of the subtidal seeps, rocky substrates support a diverse community of microbial filaments, macroalgae, and benthic organisms. While stable carbon isotopic compositions of marine benthic organisms indicate a dominant photosynthesis-based food web, those of some hard-substrate invertebrates were in the range −48.8‰ to −36.8‰, suggesting assimilation of methane-derived carbon by some selected taxa. This work highlights the potential subsidy of the trophic web by CH 4–C, and that its emission to the atmosphere justifies the need of evaluating the use of methane to support the energy requirements of the local community.

Origin and flux of a gas seep in the Northern Alps (Giswil, Switzerland)

AbstractNatural gas seeps in the Alpine region are poorly investigated. However, they can provide useful information regarding the hydrocarbon potential of sedimentary Alpine units and related geofluid migration, typically controlled by pressurized gas accumulations and tectonics. A gas seep located near Giswil, in the Swiss Northern Alps, was investigated, for the first time, for molecular and isotopic gas composition, methane flux to the atmosphere, and gas flux variations over time. The analyses indicated that the gas was thermogenic (CH4 &gt; 96%; δ13C1: −35.5‰ to −40.2‰) and showed evidence of subsurface petroleum biodegradation (13C‐enriched CO2, and very low C3+ concentrations). The source rock in the region is marine Type II kerogen, which is likely the same as that providing thermogenic gas in the nearby Wilen shallow well, close to Lake Sarnen. However, the lack of δ13CCO2 and δ13C3 data for that well prevented us from determining whether the Wilen and Giswil seeps are fed by the same reservoir and seepage system. Gas fluxes from the Giswil seep, measured using a closed‐chamber system, were significant and mainly from two major vents. However, a substantial gas exhalation from the soil occurs diffusely in an area of at least 115 m2, leading to a total CH4 output conservatively estimated to be at least 16 tonnes per year. Gas flux variations, monitored over a 1‐month period by a special tent and flowmeter, showed not only daily meteorological oscillations, but also an intrinsic ‘pulsation’ with periods of enhanced flux that lasted 2–6 h each, occurring every few days. The pulses are likely related to episodes of gas pressure build‐up and discharge along the seepage system. However, to date, no relationship to seismicity in the active Sarnen strike‐slip fault system has been established.

Diagenetic and epigenetic alteration of Cretaceous to Paleogene organic-rich sedimentary successions in northwestern Jordan, typical of the western margin of the Arabian Plate

ABSTRACTFour reference sections through the calcareous-siliciclastic rocks of the Lower Cretaceous Kurnub Group and the Amman and Muwaqqar formations of the Upper Cretaceous to Paleogene Belqa Group in Jordan document the various processes of accumulation and alteration of organic matter (OM). Sections at Jerash, Sultani, Wadi Isal, and in the Kharazeh area were investigated by means of sedimentary petrography/mineralogy, organic petrography, and organic chemistry, and correlated with equivalent deposits in Syria and Egypt. The impacts of oxidation potential (Eh), acidity/basicity (pH) and temperature variations during the post-depositional alteration of these organic concentrations were assessed using x-y plots. Syngenetic Aptian-Albian coaly and organic-rich amber beds near Jerash developed in a tide-dominated delta under marginally alkaline conditions and were altered under slightly acidic conditions at temperatures of less than 100°C. Environmental analysis focused on Maastrichtian-Paleocene (?), oil shales in the Sultani area that were deposited in a small restricted basin on the continental shelf of the Neo-Tethys Ocean. Abnormally high contents of vanadium (V), phosphorus (P), zinc (Zn), and uranium (U) gave rise to yellow uranium ore minerals, phosphates and zinc sulfides and so bridge the gap between OM concentrations and those of uranium. The western part of the Arabian Peninsula is known for its uraniferous phosphorites and U-bearing calcretes. Reducing conditions during deposition of syngenetic OM in the oil shales may have shifted, in places, toward more oxidizing conditions in the course of post-depositional alteration at temperatures well below 200°C. Epigenetic fault-related concentration of OM was responsible for the Wadi Isal Aptian-Albian tar sand deposit and the oil seepage system in calcareous wall rocks of the Kharazeh Fault. The alteration of the tar sands (fluvial sandstones) is designated a high-sulfidation type (introduced aluminum sulfate minerals), whereas the oil seepage in Campanian shallow-marine carbonates is a low-sulfidation type (removal of aluminum sulfate minerals). Both alteration patterns may have implications for structure-bound base- and precious-metal deposits whose emplacement involves hydrocarbons as a carrier of metals in the mineralizing fluids. The Jordanian oil shales, tar sands, and uranium mineralization are possible sources of energy. Recent economic deals on the recovery and use of oil shales and uranium in Jordan are reviewed.

Microbial biomineralization of iron seepage water: Implication for the iron ores formation in intertidal zone of Zhoushan Archipelago, East China Sea

The biogeochemcical reactions responsible for fossilized minerals preservation in ancient geological conditions are very often debatable, because little is known about the in situ processes at geo-historical period. In the present study, we describe the formation of iron ores collected in the intertidal zone of the Zhujiajian Island, Zhoushan Archipelago in the East China Sea. Morphological, mineralogical and geochemical analyses were performed on the iron ores and the surrounding geological materials. The results show that the iron ores, composed of spherical ferrihydrite and fibrous aggregates of goethite, presented morphological characteristics reminiscent of bacterial activity. The biominerization process in the seepage system is believed to represent an analogue mechanism for the biogenic formation of iron ores. The degradation of the ancient wood layer provided humic substances which accelerated the leaching process of iron from the surrounding bedrock and soils. The abundant leaching iron not only provided the sufficient iron material source, but also created the ideal conditions for the survival of the iron-oxidizing bacteria. The presence of Leptothrix-like sheaths and Gallionella-like stalks in the present-day seepage environment promoted the oxidization of Fe 2+ to Fe 3+ and the rapid precipitation of bacteriogenic iron oxides (BIOS) on bacterial sheaths and stalks, allowing the preservation of the morphological characteristics of the bacteria. As time went by, the amorphous biomineralization product (ferrihydrite) can further transfer to more crystalline goethite and therefore be preserved in the ores permanently, representing as the imprints of bacterial activity during the formation of iron ores. The present findings should help elucidate the role of bacteria in the formation of biogenic iron ores in different environments during geo-historical context.

オホーツク海のメタンハイドレートとプルーム

Methane hydrates exist beneath the sea bottom near cold seeps NE off the Sakhalin in the Sea of Okhotsk. Multidisciplinary field operations were performed at a study area (approximately 16 × 20 km2) to investigate seepage characteristics and understand gas hydrate formation mechanisms. A continuous profiling survey was conducted to obtain a distribution map of seepage structures on the floor by using a deep-tow, side-scan-sonar equipment. The distribution map reveals that the dense area of seepage structures coincides with a sea-floor area of deformed sediments caused possibly by repeated sediment slumping and debris flows in the past. We speculate that this deformation may have created shallow faults that are suitable to conduits for the migration and discharge of gas and fluid. Three seepage structures were selected to study about their fluid-seep conditions around the sea floor level. Hieroglyph seepage structure is located at the northern end of the dense area of the structures. Kitami and Chaos structures are located about 2 and 7 km respectively apart from the Hieroglyph structure within the dense area. Large plumes on echograms and higher methane contents in the water column confirm gas seepage activities at the three structures. There observed at least two and four plumes at the Hieroglyph and Chaos structures, respectively. Each gas chimney image in seismic reflection profiles was traced to connect each BSR and seepage structure. Both pull-up and disturbed structures of BSR around the gas chimney images were interpreted as to be indications of significant heat flows caused by ascending fluid at both Kitami and Chaos structures. On the other hand, almost no pull-up/disturbance of BSR was observed at the Hieroglyph structure, suggesting little water seepage. The seep activity may vary with time off the Sakhalin. The Hieroglyph structure is located at the edge of a dense area of the seepage structures. It might serve as an indicator for the long-term activity of the fluid seepage system off the Sakhalin.

Evidence of subsurface anaerobic biodegradation of hydrocarbons and potential secondary methanogenesis in terrestrial mud volcanoes

The assessment of gas origin in mud volcanoes and related petroleum systems must consider post-genetic processes which may alter the original molecular and isotopic composition of reservoir gas. Beyond eventual molecular and isotopic fractionation due to gas migration and microbial oxidation, investigated in previous studies, we now demonstrate that mud volcanoes can show signals of anaerobic biodegradation of natural gas and oil in the subsurface. A large set of gas geochemical data from more than 150 terrestrial mud volcanoes worldwide has been examined. Due to the very low amount of C 2+ in mud volcanoes, isotopic ratios of ethane, propane and butane (generally the best tracers of anaerobic biodegradation) are only available in a few cases. However, it is observed that 13C-enriched propane is always associated with positive δ 13C CO2 values, which are known indicators of secondary methanogenesis following anaerobic biodegradation of petroleum. Data from carbon isotopic ratio of CO 2 are available for 134 onshore mud volcanoes from 9 countries (Azerbaijan, Georgia, Ukraine, Russia, Turkmenistan, Trinidad, Italy, Japan and Taiwan). Exactly 50% of mud volcanoes, all releasing thermogenic or mixed methane, show at least one sample with δ 13C CO2 > +5‰ (PDB). Thermogenic CH 4 associated with positive carbon isotopic ratio of CO 2 generally maintains its δ 13C-enriched signature, which is therefore not perturbed by the lighter secondary microbial gas. There is, however, high variability in the δ 13C CO2 values within the same mud volcanoes, so that positive δ 13C CO2 values can be found in some vents and not in others, or not continuously in the same vent. This can be due to high sensitivity of δ 13C CO2 to gas–water–rock interactions or to the presence of differently biodegraded seepage systems in the same mud volcano. However, finding a positive δ 13C CO2 value should be considered highly indicative of anaerobic biodegradation and further analyses should be made, especially if mud volcanoes are to be used as pathfinders of the conditions indicative of subsurface hydrocarbon accumulations in unexplored areas.

Seep-carbonates in a thrust-related anticline at the leading edge of an orogenic wedge: The case of the middle–late Miocene Salsomaggiore Ridge (Northern Apennines, Italy)

Seep-carbonates are important indicators of submarine fluid expulsion and dewatering of sediments along convergent margins and accretionary prisms. In the Northern Apennine compressional wedge, seep-carbonates usually occur in pelitic successions from different settings of the middle late Miocene foreland basin system. In Serravallian foredeep deposits of the Salsomaggiore area, seep-carbonates cemented and/or encrusted coarse-grained sandstones and conglomerates. Fluid expulsion, strictly related to variable intensities in tectonic stresses, generated seep-carbonate facies and sediment instabilities, depending on the position of the fluid-pathway feeders within the Salsomaggiore Ridge, a middle–late Miocene intra-basinal high, at the leading edge of the Apenninic orogenic wedge. This site provides a unique opportunity for analyzing depositional characters, and the three-dimensional distribution and evolution of a seepage system in a thrust-related anticline. The resulting evolutionary model shares common features with recent ridges in other accretionary wedges. In this model a strict relationship between tectonics, seepage and sediment instability during a major tectonic phase is outlined.

Significance of hydrocarbon seepage relative to petroleum generation and entrapment

Near-surface indications of migrating hydrocarbons provide the petroleum systems analyst critical information about source (organic matter type), maturation (organic maturity), migration (migration pathway delineation), and in selected geologic settings, specific prospect hydrocarbon charge. All petroliferous basins exhibit some type of near-surface signal, but the hydrocarbon leakage to surface is not always detectable with conventional seep detection methods. Understanding the Petroleum Seepage System, hence petroleum dynamics of a basin, is key to understanding and using near-surface geochemical methods for basin assessment and prospect evaluation. The relationships between near-surface hydrocarbon seepage and subsurface petroleum generation and entrapment are often complex. The petroleum seepage system contain four key elements: seepage activity (qualitative expressions of relative leakage rates, active vs passive, and episodic vs continuous), seepage type (concentration of migrated thermogenic hydrocarbon relative to in situ material, macro vs micro), migration focus (major direction of bulk flow leakage relative to the subsurface hydrocarbon generation and/or entrapment), and near-surface seep disturbances (near-surface processes which can greatly alter or block seepage signals). The rate and volume of hydrocarbon seepage to the surface greatly control near-surface geological and biological responses, and thus are the best method of sampling and analysis to detect hydrocarbon leakage effectively. To properly predict subsurface petroleum properties, interpretation of near-surface geochemical data must recognize many potential problems including recent organic matter input, transported hydrocarbons, bacterial alteration, mixing, contamination, and fractionation effects. Surface geochemical data should always be integrated with other geological data. Calibration datasets to determine the utility of near-surface geochemical techniques within particular basinal settings are essential when evaluating prospects for hydrocarbon charge.