Natural Hydrocarbon Seeps Research Articles

The spatial scales, distribution, and intensity of natural marine hydrocarbon seeps near Coal Oil Point, California

Natural hydrocarbon seepage from marine environments is an important source of methane and other gases to the atmosphere. Quantifying this flux is necessary for constraining global budgets and understanding local air pollution sources. A field of strong hydrocarbon seepage offshore of Coal Oil Point near Santa Barbara, California produces extensive areas of bursting bubbles at the sea surface. An instrumented buoy was deployed in the Coal Oil Point field to measure directly the atmospheric gas flux from three seeps of varying size and intensity. Spatial scales of continuity of the seeps, quantified by semivariograms, are small, ranging from <1 to about 9 m such that flux values on larger scales are uncorrelated. These de-correlation scales are comparable to the horizontal extents of individual bubble plumes estimated visually at the sea surface. Semivariograms for each seep are used in an ordinary Kriging procedure to interpolate flux measurements onto a regular grid and produce objective maps of the spatial distribution of flux. Spatial integrals yield total flux estimates from the three seeps of ∼7400 m 3 day −1 which amount to 4–13% of the total flux from the Coal Oil Point field based on estimates from previous studies.

REASSESSING POTENTIAL ORIGINS OF SYNTHETIC APERTURE RADAR (SAR) SLICKS FROM THE TIMOR SEA REGION OF THE NORTH WEST SHELF ON THE BASIS OF FIELD AND ANCILLARY DATA

The Timor Sea region of the North West Shelf is one of natural hydrocarbon accumulation and seepage, which has been investigated by integrated remote sensing studies in the past 10 years. One of the primary tools incorporated in these studies has been Synthetic Aperture Radar (SAR). During a recent Geoscience Australia marine survey to the Yampi Shelf area, active hydrocarbon seepage was directly observed in the form of gas plumes rising from the sea-floor. Active seepage was not observed in areas associated with dense clusters of elongated to irregularshaped features in the SAR data, which have previously been interpreted as natural hydrocarbon seepage slicks. These slicks, and another dense cluster of slicks across the Browse–Bonaparte Basin Transition Zone, are reassessed in the context of alternative formational processes.Mapping of bathymetric channels directly beneath the SAR slicks using multi-beam swath bathymetry and measurement of tidal currents using an acoustic doppler current profiler indicates that tidal current flows may have contributed to slick formation over the Yampi Shelf headland. In contrast, coral spawning may have contributed to the formation of annular to crescent-shaped SAR slicks associated with submerged reefs and shoals over the nearby transition zone. Subsequent to identifying potential alternative origins for these two types of SAR features, the remaining slicks across the area were re-categorised on the basis of their size and shape in the context of ancillary hydrographic and environmental data. An alternative nonseepage origin was established for most of the 381 SAR slicks previously identified as being related to natural hydrocarbon seepage. This may necessitate a significant downgrading of the extent and frequency of active hydrocarbon (particularly oil) seepage in the region.

Coastal pollution hazards in southern California observed by SAR imagery: stormwater plumes, wastewater plumes, and natural hydrocarbon seeps

Stormwater runoff plumes, municipal wastewater plumes, and natural hydrocarbon seeps are important pollution hazards for the heavily populated Southern California Bight (SCB). Due to their small size, dynamic and episodic nature, these hazards are difficult to sample adequately using traditional in situ oceanographic methods. Complex coastal circulation and persistent cloud cover can further complicate detection and monitoring of these hazards. We use imagery from space-borne synthetic aperture radar (SAR), complemented by field measurements, to examine these hazards in the SCB. The hazards are detectable in SAR imagery because they deposit surfactants on the sea surface, smoothing capillary and small gravity waves to produce areas of reduced backscatter compared with the surrounding ocean. We suggest that high-resolution SAR, which obtains useful data regardless of darkness or cloud cover, could be an important observational tool for assessment and monitoring of coastal marine pollution hazards in the SCB and other urbanized coastal regions.

Identifying future directions for subsurface hydrocarbon migration research

Subsurface hydrocarbon migration is important for understanding the input and impacts of natural hydrocarbon seepage on the environment. Great uncertainties remain in most aspects of hydrocarbon migration, including some basic mechanisms of this four‐phase flow of tar, oil, water, and gas through the complex fracture‐network geometry particularly since the phases span a wide range of properties. Academic, government, and industry representatives recently attended a workshop to identify the areas of greatest need for future research in shallow hydrocarbon migration.Novel approaches such as studying temporal and spatial seepage variations and analogous geofluid systems (e.g., geysers and trickle beds) allow deductions of subsurface processes and structures that remain largely unclear. Unique complexities exist in hydrocarbon migration due to its multiphase flow and complex geometry, including in‐situ biological weathering. Furthermore, many aspects of the role of hydrocarbons (positive and negative) in the environment are poorly understood, including how they enter the food chain (respiration, consumption, etc.) and “percolate” to higher trophic levels. But understanding these ecological impacts requires knowledge of the emissions' temporal and spatial variability and trajectories.

Kilometer-scale upward migration of hydrocarbons in geopressured sediments by buoyancy-driven propagation of methane-filled fractures

Several lines of evidence support kilometer-scale upward migration of fluids in the Gulf of Mexico Basin: discharge of hypersaline brines at the sea floor; long-term, natural hydrocarbon seeps and microseeps; gas chimneys; lead-zinc mineralization in salt dome cap rocks; and allochthonous brines in Cenozoic sediments. We explore the hypothesis that upward fluid transport in geopressured sediments is caused by buoyancy-driven propagation of isolated methane-filled fractures. In other words, instead of fluid migrating along a fixed network of interconnected pores or fractures, fluid enclosed within an isolated fracture is transported upward by hydrofracturing the mechanically weak geopressured sediments. Thus, the fluid-filled fracture propagates upward through the sediments. Hydrofracture is driven by the pressure difference (buoyancy) between the enclosed methane and the surrounding sediments. Our results show that methane-filled fractures with half-lengths of a few meters should propagate upward through geopressured sediments with velocities of hundreds of meters per year or higher. As methane-filled fractures increase in volume and decrease in density with decreasing confining pressure, they develop the potential to entrain and transport more than 1000 kg/m3 of oil or brine. Methane-filled fractures should propagate to the surface unless they are trapped beneath a layer that has high fracture toughness, such as salt, or are absorbed when they intersect a permeable (>10 md) sand layer.

INFLUENCE OF HYDROCARBON MIGRATION AND SEEPAGE ON BENTHIC COMMUNITIES IN THE TIMOR SEA, AUSTRALIA

The Ashmore Platform–Timor Sea region of Australia’s North West Shelf is an area of significant petroleum exploration potential, with several large commercial oil fields present. Moreover, exploration activity seems likely to continue at current levels for the foreseeable future, and may also extend into deeper water, given high oil prices and improved drilling technologies. The area is also one of high conservation value, with both the Cartier Marine reserve and Ashmore Reef (a Category 1 marine park), as well as numerous other genetically-rich carbonate seed bank systems, closely associated spatially with exploration activities. Balancing the conservation and resource values in this area will present a key challenge into the future.The magnitude of this challenge has been highlighted by recent work undertaken by AGSO, which involved the acquisition and interpretation of assorted remote sensing data, such as high-resolution bathymetry (including sidescan sonar), satellite synthetic aperture radar (SAR), Landsat, water column geochemical sniffer, airborne laser fluorosensor, seismic data and seafloor sediment sampling. These studies have shown that, at both a regional and local scale, the development of these important carbonate systems appears to directly relate to the geological development of the area.At a regional scale, the collision between the Australian and Eurasian crustal plates in the Pliocene (At a local scale, new data also strongly suggest that the locations of the majority of reefs and carbonate banks and build-ups in the area are associated with active and palaeo-hydrocarbon seeps. These seeps are localised over either fault systems which tap the reservoir, along migration fairways, or at the pinch-out of the regional Cretaceous top seal. Our interpretations suggest that the reefs and the build-ups formed by a sequential process. Firstly, hydrocarbon seepage (induced by collisionrelated faulting) localised small seafloor (chemolithotrophic) biological carbonate communities, which ultimately formed topographically positive features. These higher relief features were subsequently preferentially colonised by an assortment of reef-building biota, whose rapid growth progressively kept up with rising sea-level (which was driven principally by collisionrelated subsidence). The most favourable conditions for initial reef colonisation probably occurred during periods of relatively low sea-level, when the areas around the reefs were located at much shallower water depths (Clearly, the fact that the genetically rich carbonate communities in this area are probably causally related to natural hydrocarbon seepage (and the attendant processes which drove that seepage) will present a series of almost unique exploration, development (especially engineering) and conservation challenges.

Temporal variation in natural methane seep rate due to tides, Coal Oil Point area, California

Two large steel tents (each 30 m by 30 m), open at the bottom to the seafloor, capture ∼16,800 m3 d−1 (594 MCF) of primarily methane from a large natural hydrocarbon seep, occurring a kilometer offshore in 67 m of water. The gas is piped to shore where it is metered and processed. The seep flow rate was monitored hourly for 9 months. Our results show that the tidal forcing causes the flow rate to vary by 4–7% around the mean. These results are the first quantitative documentation of the effect of tides on natural gas seepage in relatively deep water. Time series analyses of the 9 month record clearly show four principal tidal components with periods of 12.0, 12.4, 23.9, and 25.8 hours. High tide correlates with reduced flow, and low tide correlates with increased flow. The correlation indicates that each meter increase of sea height results in a decrease of 10–15 m3 hr−1 or 1.5–2.2% of the hourly flow rate. The observed changes are best accounted for by a pore activation model, whereby gas is released from small pores at low pressures but is inhibited at higher pressure. Pressure‐dependent gas solubility changes are a less likely cause of flow variation. Our study implies that sea level differences, on a tidal timescale, can significantly change the gas seepage rate from sediments. Lower sea level in the last hundred thousand years would presumably allow higher gas loss from the sediment, assuming sufficient gas present, because of reduced hydrostatic pressure at the sediment‐sea interface. The magnitude of this long‐term change cannot be extrapolated from our tidal data.

A Gas-Capture Buoy for Measuring Bubbling Gas Flux in Oceans and Lakes

The design, calibration, and deployment of a buoy and gas-capture assembly for measuring bubbling gas flux in oceans and lakes are described. The assembly collects gas in a chamber while continuously measuring the position of the gas–water interface that forms as gas accumulates. Interface position is determined from the differential pressure between the chamber and ambient seawater. A spar buoy provides flotation and stability to reduce vertical motion from surface waves. The gas-collection assembly and spar, referred to as a flux buoy, is suitable for deployment from small boats under conditions of light wind and small waves. The flux buoy is being used to determine the spatial distribution of natural hydrocarbon seepage off the south-central California coast. Hydrocarbon seepage from continental shelves may be an important source of atmospheric methane.

Modifications of the local environment by natural marine hydrocarbon seeps

The bubble gas partial pressures, dissolved gas and oil, and fluid motions within the rising bubble streams of three shallow (&lt;70 m) natural hydrocarbon seeps in the Santa Barbara Channel were measured. Aqueous methane concentrations near the surface in the bubble plumes were substantially greater (&gt;108times) than atmospheric equilibrium values; at two of the seeps it was slightly supersaturated with respect to the bubble's partial pressure. This indicates that the rate limiting step for methane, CH4, transfer into the water column was not bubble gas transfer, but rather turbulence transfer of water out of the saturated bubble stream to the bulk ocean. Strong upwelling flows were observed, as well as elevated fluorescence indicative of dissolved oils. At the bottom, bubbles are circa 90% CH4, but at the surface they were circa 60% CH4, 30% air, and 10% higher hydrocarbons.

USING INTEGRATED REMOTE SENSING TECHNOLOGIES TO EVALUATE AND CHARACTERISE HYDROCARBON MIGRATION AND CHARGE CHARACTERISTICS ON THE YAMPI SHELF, NORTHWESTERN AUSTRALIA: A METHODOLOGICAL STUDY

As part of a major interpretative study of the Yampi Shelf, northeastern Browse Basin, the Australian Geological Survey Organisation (AGSO) acquired regional seismic, water column geochemical sniffer (WaSi), RadarSat Synthetic Aperture Radar (SAR), and high resolution bathymetric data over the area between 1995 and 1998. These data have been integrated in order to better understand both the hydrocarbon charge and top seal capacity characteristics of the area, and the relative responses of assorted remote sensing technologies to hydrocarbon migration, leakage and seepage. The area investigated comprises part of a Palaeozoic to Mesozoic flexural ramp margin, which dips northwest away from the flanking cratonic Kimberley Block. The basement has a rugose topography, with some horst blocks elevated 500 m above the surrounding basement. Progressive onlap of Cretaceous post-rift seals onto the ramp causes the regional seal to become thinner and sandier margin- ward. In some of these more margin-ward locations, some prominent basement highs are 'bald' of seal, whereas in others, the seal thins dramatically onto topographically prominent, landward-dipping tilt blocks. Natural hydrocarbon seepage was confirmed independently by SAR and WaSi, though the relative response and sensitivity of each technology to seepage of varying rates and compositions are quite different. Overall, the principal seeping hydrocarbon is dry (

Importance of Remote-Sensing Data in Structural Geologic Analysis of Oil- and Gas-Bearing Regions of Azerbaijan

In addition to surface geologic and geophysical investigations, Panchromatic SPOT and Landsat Thematic Mapper (TM) data were merged using an IHS approach to produce a satellite composite image to use as a basis for structural geologic analysis of the oil- and gas-bearing regions of Azerbaijan, including the South Caspian and Kura Basins. With the development of new exploration technology, various nonseismic reconnaissance methods, such as remote sensing, gravity, and magnetics for oil exploration have been substantiated and improved. The Azerbaijan and surrounding mountain ranges are favorable for studying natural hydrocarbon seepages. Most of the seepage occurs in faulted and mud volcano areas onshore of the Caspian Sea in Azerbaijan. Determining the geometry of the fault system from analysis of remote-sensing data allows prediction of (1) vertical oil migration from source rocks into tectonic traps in overlying formations and (2) quantitative volume of oil reservoirs.

Abstract:&nbsp; Comparison of Methods to Determine Gaseous Emissions to the Atmosphere Above Natural Marine Hydrocarbon Seeps&nbsp;

Abstract:&nbsp; Comparison of Methods to Determine Gaseous Emissions to the Atmosphere Above Natural Marine Hydrocarbon Seeps&nbsp;

Abstract: Decrease in Natural Hydrocarbon Seepage Offshore Coal Oil Point, California, Associated with Oil Production&amp;nbsp;

Abstract: Decrease in Natural Hydrocarbon Seepage Offshore Coal Oil Point, California, Associated with Oil Production&amp;nbsp;

Decrease in natural marine hydrocarbon seepage near Coal Oil Point, California, associated with offshore oil production

Prolific natural hydrocarbon seepage occurs offshore of Coal Oil Point in the Santa Barbara Channel, California. Within the water column above submarine vents, plumes of hydrocarbon gas bubbles act as acoustic scattering targets. Using 3.5 kHz sonar data, seep distribution offshore of Coal Oil Point was mapped for August 1996, July 1995, and July 1973. Comparison of the seep distributions over time reveals more than 50% decrease in the areal extent of seepage, accompanied by declines in seep emission volume, in a 13 km 2 area above a producing oil reservoir. Declines in reservoir pressure and depletion of seep hydrocarbon sources associated with oil production are the mechanisms inferred to explain the declines in seep area and emission volume.

Reduction in Natural Hydrocarbon Seepage from the Offshore South Ellwood Field near Coal Oil Point, California, Due to Oil Production: ABSTRACT

Prolific natural gas seepage, a significant air pollution source in Santa Barbara County, occurs offshore from Coal Oil Point, near Santa Barbara. Seepage rates are quantified by measuring the acoustic return of sonar sources from the gas bubbles rising through the water column, and by measuring the dissolved concentrations of hydrocarbons downcurrent from the gas seep vents. In 1995 we digitally recorded 3.5 kHz data in the same location as a 1972-1973 survey. Comparison of the data suggests a substantial reduction in natural seepage within one mile of Platform Holly since oil production began in 1972 from the Monterey Formation reservoir, which is the source of the gas. Dissolved concentrations of hydrocarbons were measured in the ocean 2-6 miles downcurrent from Platform Holly in an area where concentrations were previously measured in a 1981 SNIFFER survey. The dissolved propane concentrations in 1995 had a peak value of 7 ppm, whereas in 1981 the peak value was 65 ppm. A reduction in natural gas seepage rate is also indicated by the 60% decline in the amount of seep gas captured by a seep containment device located one mile east of Platform Holly. The reduction in gas seepage rate may be attributedmore » to a 40% reduction in subsurface reservoir pressure under Platform Holly that resulted from withdrawal of 43 MMBO and a similar volume of water from the Monterey Formation since 1972.« less

Distribution and abundance of benthic and demersal macrofauna within a natural hydrocarbon seep

Natural oil seeps provide a heightened supply of organic carbon that can potentially result in higher numbers of infaunal organisms, yet toxic effects of oil or its by-products could have negative impacts. These contrasting influences make oil seeps an ideal setting to investigate the effects of a tradeoff between enrichment and toxicity. We explored small-scale (1 to 10 m) spatial patterns of distribution of infauna and physical and chemical characteristics of sediments within an oil seep area located near Santa Barbara, California, USA. Correlation and principal components analyses revealed that physicochemical parameters of the sedirnents (dissolved sulfide, volatile solids, organic carbon and nitrogen, sediment grain size) generally covaried with one another; oil-rich samples had large grain size and high concentrations of organic material. Densities of nematodes were positively correlated with amount of oil present, whereas all other major taxa (polychaetes, gammarids, oligochaetes, bivalves, copepods, ostracods, cumaceans) were negatively correlated. These results contrasted with patterns that we observed at a much larger spatial scale (up to 1 km) where the number of organisms coloniz~ng trays of clean sediments was greater at 2 seepage sltes compared to 2 sites that lacked seeps. A field experunent tested the effects of hydrocarbon seepage on the distnbution and abundance of macrofauna. Colon~zat~on by infauna was assessed in trays of sediments that varied in organic enrichment (clean sediment, sediment with fresh oil, weathered oil, or sulfide). The results were consistent with small-scale (within-seep) patterns observed in the field survey; after 24 h, nematode densities were 3 times higher in the enriched trays, whereas all other taxa were most abundant in the clean trays. The results suggest that the biological effects of oil seepage are scale-dependent and that behavioral mechan~sms involving microhabitat selection may contribute to the observed spatial patterns of d~stributlon.

Remote Sensing Study of Historical Changes in Natural Oil Slick Volumes in the Santa Barbara Channel: ABSTRACT

Large natural oil slicks are continually forming in the Santa Barbara Channel as a result of natural hydrocarbon seeps that occur above subsurface oil accumulations. Evaporation of volatile constituents in these oil slicks is a significant source of air pollution. In this study we employed a suite of remote sensing data along with a variety of image processing and geographic information system techniques to analyze the location, extent and changes in sea surface oil slicks during the period of 1975 - present. An extensive collection of airborne and spaceborne remote sensor data was accessed and examined for this study, including aerial photography, airborne spectral and microwave data, Landsat MSS and TM data and Shuttle Imaging Radar. Areas of oil slick accumulations were identified, delineated, georeferenced, measured and mapped to a common scale using both manual and digital techniques. A Gaussian mathematical model was employed to provide quantitative estimates of surface oil accumulations identified within each image over the entire twenty year period. This study indicates that variances in mapped oil slick location and extent as well as the estimated amounts of oil present within each image may be attributed to image type and quality as well as environmental conditions.

Vertical distribution of microbial and meiofaunal populations in sediments of a natural coastal hydrocarbon seep

We studied the vertical distribution of microbes and meiofauna in natural hydrocarbon seep sediments to determine if there was a relationship between profiles of benthic trophic structure and the unique biogeochemical conditions present at the seep. Three stations in the Santa Barbara Channel represented a gradient of natural petroleum seepage, from very active, to moderate, to none. Seasonal differences were examined by sampling in the three major oceanographic seasons, upwelling (April), mixed (July), and Davidson (December). Densities of microbes and meiofauna were highest in July, and decreased in winter. All population sizes decreased with increasing depth in the sediment. Harpacticoids and Chi a were practically restricted to the surface sediments. Harpacticoids and ChI a were more dense (number per unit volume or strata of sediment) and abundant (number per unit area of sediment or sum of the strata) at the comparison site than at the seep sites. Density and abundance of nematodes, bacteria cell counts, and bacterial biomass were greater at the station with the most active seepage rates. Bacterial biovolumes appeared constant among sediment depths and stations, but cell biovolumes were larger in July. The data are consistent with the hypothesis that organic enrichment via petroleum utilization is responsible for increased abundances of bacteria and nematodes at the seep. There were strong correlations between densities of harpacticoids and microalgae, and densities of nematodes and bacteria. These links indicate that seeping petroleum might have an enhanced effect on the detrital (bacterial based) food web, but a toxic effect on the grazing (microaIgaI based) food web.

Potential Contribution of Natural Hydrocarbon Seepage to Benthic Productivity and the Fisheries of Atlantic Canada

Bacterial processes involving methane and other low molecular weight hydrocarbons may be a substantial contribution to the overall benthic productivity in areas where natural seepage from the seabed occurs. This process may be sufficiently extensive on the continental shelf off the east coast of Canada to be of importance to the fishery.

Detection of Shallow Hydrocarbons with High-Frequency Seismic Reflection Data: ABSTRACT

High-frequency (200 to 15,000 Hz), high-resolution seismic reflection profiles can be a valuable tool in the exploration for offshore hydrocarbon accumulations. The occurrence of hydrocarbons (particularly natural gas) either in the water column or shallow subbottom can result in strong acoustic impedance (density × velocity) contrasts which may produce a number of anomalous seismic responses including: (1) water-column anomalies, which are strong reflections that rise from the ocean bottom into the overlying water column as a result of natural gas seepage; (2) subbottom amplitude anomalies (bright spots) that, when found along geologic structures, may indicate shallow hydrocarbon accumulations; (3) seismic smears, which are turbid, chaotic, high-amplitude events that may indicate shallow, gas-charged sediments; and (4) seismic wipeouts, transparent zones commonly found below seismic smears or water-column anomalies indicating total reflection and/or absorption of seismic energy in an overlying zone of gas. A high-frequency, high-resolution seismic reflection profile survey of the continental margin off northern Santa Cruz County, California, using both a 300-joule uniboom and a 1-kilojoule sparker has resulted in the detection of all four of these seismic anomalies. The shelf here is cut diagonally by the San Gregorio fault zone which marks the southeast boundary of the Outer Santa Cruz basin. Northeast of the San Gregorio fault, the shelf is dissected by the Monterey Bay fault zone. More than 100 water-column anomalies (gas seeps) have been detected, some of which rise over 50 m (160 ft) into the overlying water column. Most water-column anomalies correlate with subbottom geologic structures such as anticlines, faults, and truncated, tilted strata. Several subbottom amplitude anomali s, seismic smears, and seismic wipeouts have also been detected and correlated with structures. All these anomalies have been found in association with the middle Miocene Monterey Formation, the late Miocene Santa Cruz Mudstone, and the Pliocene Purisima Formation. Samples of natural gas have also been collected from a shallow End_Page 521------------------------------ coastal water well. Geochemical analyses indicate the presence of 74 to 91% methane, 7 to 23% nitrogen, ~ 2% carbon to dioxide, and < 1% ethane. The gas appears to be thermogenic in origin as it has ^dgr13C values of -29.51 to -32.5 ^pmil PDB. Oil-saturated sandstones have also been dredged from a depth of 700 m (2,300 ft) in Ascension submarine canyon, further indicating the natural seepage of hydrocarbons in this area. We interpret these natural hydrocarbon seeps to be the result of migration from depth along active faults within the San Gregorio and Monterey Bay fault zones. High-frequency, high-resolution seismic reflection surveys in unexplored offshore areas may yield evidence for hydrocarbons that are not detectable with conventional, deep-penetrating, low-frequency (< 100 Hz) seismic reflection systems, and thus may be very valuable in the search for oil and gas accumulations. End_of_Article - Last_Page 522------------