Martin Hovland

Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned by host sediment properties

The stability of submarine gas hydrates is largely dictated by pressure and temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of deep marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our conceptual model presumes that gas hydrate behaves in a way analogous to ice in a freezing soil. Hydrate growth is inhibited within fine-grained sediments by a combination of reduced pore water activity in the vicinity of hydrophilic mineral surfaces, and the excess internal energy of small crystals confined in pores. The excess energy can be thought of as a “capillary pressure” in the hydrate crystal, related to the pore size distribution and the state of stress in the sediment framework. The base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature (nearer to the seabed) than would be calculated from bulk thermodynamic equilibrium. Capillary effects or a build up of salt in the system can expand the phase boundary between hydrate and free gas into a divariant field extending over a finite depth range dictated by total methane content and pore-size distribution. Hysteresis between the temperatures of crystallization and dissociation of the clathrate is also predicted. Growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, and lenses in muds; cements in sands) can largely be explained by capillary effects, but kinetics of nucleation and growth are also important. The formation of concentrated gas hydrates in a partially closed system with respect to material transport, or where gas can flush through the system, may lead to water depletion in the host sediment. This “freeze-drying” may be detectable through physical changes to the sediment (low water content and overconsolidation) and/or chemical anomalies in the pore waters and metastable presence of free gas within the normal zone of hydrate stability.

Methane-related Carbonate Cements in Pockmarks of the North Sea

ABSTRACT Some pockmarks on the Norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms. Detailed examination of a sandstone sample from a pockmark in Norwegian Block 25/7 in the North Sea has revealed the presence of Mg calcite and aragonite cements, some of the latter forming botryoids. All cement types are characterized by extremely light carbon isotopic compositions, with a mean 13C value of -56.1 PDB, which shows that the cements contain carbonate produced by oxidation of biogenic methane. Oxidation occurred in both the oxic and anoxic diagenetic zones. Trace amounts of interstitial methane (mean 13C = -40.8%) and higher hydrocarbon gases (up to C5) with a C1/Cn ratio of 0.855-0.874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas.

Fault-associated seabed mounds (carbonate knolls?) off western Ireland and north-west Australia

Abstract Groups of seabed mounds, suspected to be modern carbonate knolls or bioherms, have been located on reflection seismic data in the Porcupine Basin off western Ireland and in the Vulcan Sub-basin off north-west Australia. The size of these mounds is impressive, often being over 1 km across and over 100 m high. In the Porcupine Basin they occur in water depths of 650–1000 m, whereas in the Vulcan Sub-basin they occur in water depths of 200–350 m. Shallow seabed coring in the Porcupine Basin has provided data on the age, lithological composition and hydrocarbon concentrations in the upper 3 m layer of sediment both on and off the mounds. In both basins the mounds occur close to faults which may provide conduits for upward migrating hydrocarbons. The seepage of hydrocarbons is suspected to have caused local eutrophication or ‘fertilization’ by providing nutrients to bacteria, which in turn are part of the food chain for higher organisms. The dominant organism in the Porcupine Basin examples appears to be the deep-water coral Lophelia sp., whereas in the Vulcan Sub-basin the mounds appear to be Halimeda-dominated.

The structure and geomorphology of the Dashgil mud volcano, Azerbaijan

The large-scale and small-scale (< 10 m) structure and geomorphology of the active Dashgil mud volcano in central Azerbaijan, have been studied. Through historical records it is known that this mud volcano has major eruptions every 6–32 years. The activity observed during two field trips in 1995, consisted of low-energy venting and debouching of mud, water, oil, and gas from three mud pools (salses) and about 20 individual cones (gryphons) located on the summit plateau of the mud volcano. Besides the bubbling mud pools, ranging from 30 to 75 m across, and the cones, ranging from 0.8 to 3 m height, a 200-m-long string of about 10 sinter mounds was found on the summit plateau. The internal structure of the mud volcano and the processes responsible for the observed Dashgil geomorphology are discussed. Assuming the Dashgil mud volcano to be continuously active, with an average methane production similar to that observed in 1995, between the rare major eruptions, we have conservatively estimated the Dashgil mud volcano to produce an annual amount of at least 800 Sm3 (standard cubic metres) of gas, mainly methane, which is emitted directly into the atmosphere. This estimate indicates that the natural contribution of atmospheric ‘greenhouse’ (radiatively active) gases from the worlds terrestrial and submarine mud volcanoes is highly significant.

Ahermatypic coral banks off mid-Norway; evidence for a link with seepage of light hydrocarbons

Large (up to 31-meter high) coral banks (or bioherms) occur on the continental shelf off mid-Norway at water depths between 220 and 310 meters. They are built up by the cold-water, ahermatypic, scleractinian coral Lophelia pertusa (L.). A 3-km-wide and 200-km-long traverse was mapped geophysically across a large part of the mid-Norway shelf. A total of 57 suspected individual banks were found. Although they occur in local clusters of up to 9 banks per km 2 , the mean density along the whole transect is only 0.09 suspected banks per km 2 , with the highest regional density (1.2 banks per km 2 ) occurring above subcropping presumed Paleocene bedrock. A detailed investigation employing an ROV (remotely operated vehicle) was conducted of a cluster consisting of 9 individual banks. Based on geophysical, visual, geochemical, radiocarbon, and other analyses, we conclude that at least some of the coral banks have been forming at the same locality for over 8,000 years, and that there is a strong correlation between coral-bank occurrence and relatively high values of light hydrocarbons (methane, ethane, propane, and n-butane) in near-surface sediments. To explain the structure and distribution of these coral banks, we propose a model where they form as a consequence of local fertilization that results from focused hydrocarbon micro-seepage of deep thermogenic hydrocarbons migrating to the surface along inclined, permeable sedimentary strata. A direct corollary of this model is that if and when the source of local fertilization is shut off, the bioherms die out. This possibly could be the reason why extinct bioherms are more common than live ones in some areas of the ocean.

Characteristics of two natural gas seepages in the North Sea

Two occurrences of active gas seepages are described from the North Sea. The southernmost one, situated above a salt diapir in Norwegian block 19, has been studied and sampled by use of a remotely operated vehicle (ROV). This seepage consists of about 120 single seeps located within a diameter of ∼ 100 m. It is estimated to produce ∼ 24 m3 of methane gas per day (at ambient pressure, 75 m water depth). Isotope values of the methane gas and higher hydrocarbon gases in the surrounding seafloor sediments, show that their origin is from a deep seated, thermogenic source. No typical gas-induced erosion features are found on the seafloor at this location, probably due to the lack of very fine grained material. The second occurrence is located in U.K. block 1525 (Geoteam, 1984), where the seepage is associated with a very large pockmark depression, measuring 17 m in depth and 700×450 m in width. This depression represents an eroded fine grained sediment volume of ∼ 7.105 cubic metres. No detailed inspection or sampling of the gas has been performed here. However seismic reflection anomalies are seen on airgun seismic records at various levels down to a depth of at least 1100 m below seafloor. The seeping gas, possibly mixed with liquids, at this location is therefore also expected to be of a thermogenic origin.

Gas hydrate and free gas volumes in marine sediments: Example from the Niger Delta front

Abstract Bottom simulating reflectors (BSRs) detected on reflection seismic records from various deep-water locations world-wide, are known to occur as a response to the formation of gas hydrates above the reflector, and accumulation of small amounts of free gas below the reflector. Although estimates of enormous potential energy resources, and dramatic climate change scenarios have been discussed on account of the BSRs, our conclusions with respect to the commercially recoverable amounts of energy from gas hydrates and free gas associated with BSRs on the Niger Delta front, are disappointingly negative. We base our estimates on recent results from BSR-penetrating scientific drilling, a review of natural gas hydrate observations, and on theoretical considerations of gas hydrate formation and host sediment conditions. The main conclusions are as follows: 1. 1. Natural gas hydrates and BSRs most probably occur as a direct consequence of focused and diffusive vertical fluid escape. 2. 2. The mean maximum amount of gas hydrates residing in sediments above the BSR is most probably only 3% by volume. 3. 3. The mean maximum amount of free gas charge in the sediments below even well developed (strong) BSRs is only 5% by volume. 4. 4. Currently there is no commercial potential for recovering and exploiting ‘trapped’ energy sources in the form of BSR-associated gas hydrates and free gas on the Niger Delta front, mainly because they occur as dispersed, rather than concentrated deposits.

Cold-water corals—are they hydrocarbon seep related?

Numerous cold-water ahermatypic coral reefs (so-called Lophelia reefs), up to 30m high, 150m wide and 400m long, occur at waterdepths from 100 to 350 m off Mid Norway. After the discovery of relatively high interstitial methane concentrations (204 ppb) in sediments collected at the base of one of these reef structures a causative link between the presence of the reefs and local micro-seepage through seabed of light hydrocarbons is here suggested. Shallow seismic records gathered across this and other reef colonies off Mid Norway also indicate the existence of gas-charged sediments below the reefs.

A large methane plume east of Bear Island (Barents Sea): Implications for the marine methane cycle

A pockmark field extending over 35 km2 at 74°54′N, 27°3′E, described by Solheim and Elverhøi (1993), was re-surveyed and found to be covered with more than 30 steep-sided craters between 300 and 700 m in diameter and up to 28 m deep. The craters are thought to have been formed by an explosive gas eruption. Anomalously high concentrations of methane in the shelf waters around the craters suggest that a strong methane source near this area is still active today. Methane enrichment more than 10 km away from the crater field indicates the large dimensions of a plume and the amount of gas released from sources below the seafloor of the Barents Sea shelf. From the characteristic vertical decrease of methane towards the sea surface, it is concluded that biota are extensively using this energy pool and reducing the methane concentration within the water column by about 98% between 300 m depth and the sea surface. Degassing to the atmosphere is minimal based on the shape of the methane concentration gradient. Nevertheless, the net flux of methane from this area of the Barents Sea is about 2.9 × 104 g CH4 km−2 yr−1 and thus in the upper range of the presently estimated global marine methane release. This flux is a minimum estimate and is likely to increase seasonally when rough weather leads to more effective vertical mixing during autumn and winter. The amount of methane consumed in the water column, however, is about 50 times greater and hence should significantly contribute to the marine carbon inventory.

Characteristics of pockmarks in the Norwegian Trench

A 3 km wide corridor across the Norwegian Trench was surveyed in detail with high-resolution, side-scan sonar and subbottom profiler (boomer) in order to study pockmarks. The pockmarks in the Norwegian Trench are dish-shaped depressions found in the soft, silty clays. They range from 2 m to 300 m in diameter and appear with a maximum density of 40–50 per km2. Lineated depressions and strings of small pockmarks were discovered. The strings are generally composed of individual pockmarks 10–15 m in diameter. Each string can be up to 400 m long. Some medium-sized pockmarks (40–50 m in diameter) were found in groups, often associated with a large parental pockmark. The deep-towed, subbottom profiler records indicated columnar disturbances (“fountains”) in the sediments below some of the pockmarks. These fountains are interpreted as vents or fissures through which gas or liquids migrate. Buried pockmarks and anomalies interpreted as boulders, possibly gas bubbles trapped in the substrata, were also found. The recordings bear evidence of gas or liquid migration originating from layers lying beneath the soft-clay sequence. A large hydrocarbon gas reservoir has recently been found during exploration drilling in the immediate vicinity of the studied pockmark area. Pockmarks may be indicators of deep-lying gas or oil reservoirs and therefore prove interesting features in relation to hydrocarbon prospecting.