Northern Gulf of Mexico continental slope

The hummocky continental slope in the northwestern Gulf of Mexico is the result of active salt tectonism and accompanying faulting. Fluid and gassy hydrocarbons rise through the sediment column and along faults causing the formation of gas hydrates, gassy sediments, mud volcanoes and mounds, chemosynthetic communities and authigenic carbonates, reefs, and hardgrounds. Salt activity coupled with processes associated with relative sea level fluctuations create a feedback relationship resulting in the above-mentioned phenomena as well as others such as seafloor erosion at great water depths.     

Preliminary assessment of resources and economic potential of individual gas hydrate accumulations in the Gulf of Mexico continental slope

Some global estimates suggest that gas hydrates represent the largest reservoir of fossil fuel. However, only a few studies of the resource and economic potential of individual gas hydrate accumulations exist. Here we estimate the volume of hydrate-bound gas at GC (Green Canyon) blocks 184/185, GC 234/235, GB (Garden Banks) 388, MC (Mississippi Canyon) 798/842, GC 204, MC 852/853, and AT (Atwater Valley) 425/426 sites in the Gulf of Mexico at water depths ∼500–2000 m. The structural accumulations may contain from 4.7×10⁸ to 1.3×10¹¹ m³ of gas at standard temperature and pressure. The resources in individual gas hydrate accumulations are comparable (by volume) with the reserves in very small to major conventional gas fields. Various geologic, technologic, and economic factors affect the economic potential of studied accumulations. The MC 852/853 appears to be characterized by the most favorable combination of these factors, and thus is suggested to have the highest economic potential. The economic potential of gas hydrate accumulations at GC 204, GB 388, and AT 425/426 sites is ranked as ‘average’. Gas hydrate accumulations at GC 234/235, GC 184/185, and MC 798/842 sites contain only small volumes of hydrate-bound gas, and likely have no economic potential. Future gas hydrate research should focus on the detailed study of large structural gas hydrate accumulations from which gas may be profitably recovered (e.g. the MC 852/853 site).     

Physiographic features on the northern Gulf of Mexico continental slope

The continental slope of the northern Gulf of Mexico is diapirically controlled and is comprised of coalescing salt sheets, salt withdrawal basins, salt ridges, salt tongues and sills, and submarine canyons. Bathymetric information from single-beam data has resulted in several published maps. Many of the map areas have been remapped, using multibeam surveys, by the US National Ocean Service, and names have been given to the major physiographic features. The multibeam program was discontinued before complete coverage of the slope was accomplished. We provide charts of the remaining areas with names of features that have been accepted by the US Board of Geographic Names.     

Movement of sediment on the Gulf of Mexico,continental slope and upper continental shelf

Abstract

Grain size, coarse fraction analyses, and depositional environment as interpreted from microfauna are related to the character of sparker reflections at the location of core holes drilled by Exxon, Chevron, Gulf, and Mobil on the continental slope of the northern Gulf of Mexico. Continuous sparker reflections are correlated with slowly deposited, evenly bedded sediments containing bathyal faunas. The coarse fraction is dominated by the tests of foraminifera. Discontinuous, discordant reflections and diffractions are correlated with sediments more rapidly emplaced in the bathyal environment of the continental slope by slumping and sliding from the continental shelf. Their coarse fraction is dominated by terrigenous sand grains. A large portion of the volume of continental slope sediments appears to consist of these “displaced”; sediments, including an area 3–24 km wide and 80 km long, southeast of Corpus Christi, Texas. Comparable processes of movement of sediments are interpreted on the continental shelf south of the Southwest Pass of the Mississippi River. Bathymetry in this area is characterized by a series of subaqueous “gullies”; radiating from the river mouth and leading to terraces at their southern extremities. Side‐scan sonar and PDR surveys show a rough bottom in these “gullies”; and terraces, as contrasted with a relatively smooth bottom elsewhere. The rough bottom is interpreted as indicative of slump and creep of the sediments from shallower water. Some foundation soil borings in this area south of Southwest Pass find a low‐strength material gradually increasing in strength with depth. Other borings find a “crust”; of anomalously strong material 8–15 m below the mudline. The microfauna recovered from the “crust”; has moved to its present position by slump or creep from shallower water along a pattern comparable to the gullies shown in the present‐day bathymetry.     

Depositional sequences in clastic continental slope deposits,Gulf of Mexico

Tertiary and Quaternary sediments, overlying diapiric older Tertiary shales and Louann Salt on the continental slope in the western Gulf of Mexico, show cyclicity based on seismic-reflection patterns. A set of indistinct parallel reflections or an acoustically semi-transparent zone, normally onlapping onto diapir flanks, alternates with a set of distinct parallel reflections that drape the sea bottom. The indistinct reflections represent deposits employed by bottom transport during a lowering of sea level. Sea level rise and hight stand are characterized by hemipelagic sediments that form blanket-type deposits. Differential sediment loading causes diapiric activity that may reach maximum upward velocities when sea level rises.     

Seismic expression of sedimentary volcanism on the continental slope,northern Gulf of Mexico

High-resolution geophysical data define acoustically amorphous, mounded structures on the upper, middle, and lower continental slope of the northern Gulf of Mexico. Physical samples and observations within this unique seismic facies show gassy sediments, sometimes in hydrated form and, in places, as chemosynthetic communities. The geologic setting of these mounds suggests that the process of formation falls on the continuum of mud volcanoes to mud diapirs.     

Physiographic and bathymetric characteristics of the continental slope,northwest Gulf of Mexico

Bathymetric charts of the continental slope of the northwestern Gulf of Mexico reveal the presence of over 90 intraslope basins with relief in excess of 150 m. The evolution and the general configuration of the basins are a function of halokinesis of allochthonous salt. Intraslope-interlobal and intraslope-superlobal basins occupy the upper and lower continental slope, respectively. Other structures on the slope associated with salt tectonics are the Sigsbee Escarpment, the seaward edge of the Sigsbee salt nappe, and the Alaminos and Keathley canyons. Major erosional features are the Mississippi Canyon and portions of a submarine canyon on the southern extreme of the Sigsbee Escarpment.     

High-frequency near-bottom acoustic reflection signatures of hydrocarbon seeps on the Northern Gulf of Mexico continental slope

Acoustic reflection signatures of four hydro-carbon seeps were classified using near-bottom 25-kHz echosounder profiles. Echo patterns were compared with ground-truth data obtained by submersible observations and shallow coring. Six echo types were distinguished: strong reflections from (1)?exposed or (2)?buried hard substrates, such as authigenic carbonate or gas hydrate; acoustic scattering in (3)?unlayered or (4)?layered sediments owing to gas, shells, or disseminated carbonates; (5)?attenuation caused by gas; and (6)?undisturbed sediments. Echo type distributions suggest that high spatial variability indicates a younger, vigorous seep, whereas extensive hard substrate implies an older, encrusted seep.     

High-frequency near-bottom acoustic reflection signatures of hydrocarbon seeps on the Northern Gulf of Mexico continental slope

 Acoustic reflection signatures of four hydro-carbon seeps were classified using near-bottom 25-kHz echosounder profiles. Echo patterns were compared with ground-truth data obtained by submersible observations and shallow coring. Six echo types were distinguished: strong reflections from (1) exposed or (2) buried hard substrates, such as authigenic carbonate or gas hydrate; acoustic scattering in (3) unlayered or (4) layered sediments owing to gas, shells, or disseminated carbonates; (5) attenuation caused by gas; and (6) undisturbed sediments. Echo type distributions suggest that high spatial variability indicates a younger, vigorous seep, whereas extensive hard substrate implies an older, encrusted seep. Received: 29 May 1998 / Revision received: 7 October 1998     

High-resolution seismic characterization of the gas and gas hydrate system at Green Canyon 955, Gulf of Mexico,USA

The Pliocene and Pleistocene sediments at lease block Green Canyon 955 (GC955) in the Gulf of Mexico include sand-rich strata with high saturations of gas hydrate; these gas hydrate accumulations and the associated geology have been characterized over the past decade using conventional industry three-dimensional (3D) seismic data and dedicated logging-while-drilling (LWD) borehole data. To improve structural and stratigraphic characterization and to address questions of gas flow and reservoir properties, in 2013 the U.S. Geological Survey acquired high-resolution two-dimensional (2D) seismic data at GC955. Combined analysis of all available data improves our understanding of the geological evolution of the study area, which includes basin-scale migration of the Mississippi River sediment influx as well as local-scale shifting of sedimentary channels at GC955 in response to salt-driven uplift, structural deformation associated with the salt uplift, and upward gas migration from deeper sediments that charges the main gas hydrate reservoir and shallower strata. The 2D data confirm that the sand-rich reservoir is composed principally of sediments deposited in a proximal levee setting and that episodes of channel scour, interspersed with levee deposition, have resulted in an assemblage of many individual proximal levee deposit “pods” each with horizontal extent up to several hundred meters. Joint analysis of the 2D and 3D data reveals new detail of a complex fault network that controls the fluid-flow system; large east-west trending normal faults allow fluid flow through the reservoir-sealing fine-grained unit, and smaller north-south oriented faults provide focused fluid-flow pathways (chimneys) through the shallower sediments. This system has enabled the flow of gas from the main reservoir to the seafloor throughout the recent history at GC955, and its intricacies help explain the distributed occurrences of gas hydrate in the intervening strata.     

Recognition of areas effected by petroleum seepage: Northern Gulf of Mexico continental slope

The presence of large amounts of gas and/or liquid hydrocarbon seepage in near surface sediments can produce distinct features including an irregular topography (on several scales, ranging from meters to kilometers); seismically transparent/chaotic sediments; oil staining; gas plumes; sediments containing elevated concentrations of extractable organic matter, organic carbon, and calcium carbonate; associated brine seepage and anoxic conditions; extensive bacterial mats; hydrate formation and decomposition; and dense chemoautotrophic communities. Although no single characteristic is always uniquely associated with seepage, the co-occurrence of several of these features is strongly suggestive of an area being exposed to non-indigenous upward migrating hydrocarbons.     

Numerical simulations of depressurization-induced gas production from gas hydrate reservoirs at the Walker Ridge 313 site,northern Gulf of Mexico

In 2009, the Gulf of Mexico (GOM) Gas Hydrates Joint-Industry-Project (JIP) Leg II drilling program confirmed that gas hydrate occurs at high saturations within reservoir-quality sands in the GOM. A comprehensive logging-while-drilling dataset was collected from seven wells at three sites, including two wells at the Walker Ridge 313 site. By constraining the saturations and thicknesses of hydrate-bearing sands using logging-while-drilling data, two-dimensional (2D), cylindrical, r-z and three-dimensional (3D) reservoir models were simulated. The gas hydrate occurrences inferred from seismic analysis are used to delineate the areal extent of the 3D reservoir models. Numerical simulations of gas production from the Walker Ridge reservoirs were conducted using the depressurization method at a constant bottomhole pressure. Results of these simulations indicate that these hydrate deposits are readily produced, owing to high intrinsic reservoir-quality and their proximity to the base of hydrate stability. The elevated in situ reservoir temperatures contribute to high (5–40 MMscf/day) predicted production rates. The production rates obtained from the 2D and 3D models are in close agreement. To evaluate the effect of spatial dimensions, the 2D reservoir domains were simulated at two outer radii. The results showed increased potential for formation of secondary hydrate and appearance of lag time for production rates as reservoir size increases. Similar phenomena were observed in the 3D reservoir models. The results also suggest that interbedded gas hydrate accumulations might be preferable targets for gas production in comparison with massive deposits. Hydrate in such accumulations can be readily dissociated due to heat supply from surrounding hydrate-free zones. Special cases were considered to evaluate the effect of overburden and underburden permeability on production. The obtained data show that production can be significantly degraded in comparison with a case using impermeable boundaries. The main reason for the reduced productivity is water influx from the surrounding strata; a secondary cause is gas escape into the overburden. The results dictate that in order to reliably estimate production potential, permeability of the surroundings has to be included in a model.     

墨西哥湾天然气水合物矿产资源预测模型及其应用

墨西哥湾是世界上研究天然气水合物较深入的海区,调查资料丰富,已在50多处采集到天然气水合物样品,具备建立天然气水合物矿产资源预测模型的条件。选择34处已知天然气水合物矿点和34处已知无矿点作为训练区,建立矿点存在与否的预测模型。模型的相关系数值表明墨西哥湾天然气水合物与盐底辟关系密切。将该模型应用于整个墨西哥湾北部,初步获得了天然气水合物存在可能性概率分布图;概率大于0.7的预测单元包含已知矿点中的30个,利用该阈值圈定了墨西哥湾天然气水合物潜在资源分布区。     

Distribution and expression of gas seeps in a gas hydrate province of the northeastern Sakhalin continental slope, Sea of Okhotsk

Multidisciplinary surveys were conducted to investigate gas seepage and gas hydrate accumulation on the northeastern Sakhalin continental slope (NESS), Sea of Okhotsk, during joint Korean–Russian–Japanese expeditions conducted from 2003 to 2007 (CHAOS and SSGH projects). One hundred sixty-one gas seeps were detected in a 2000 km² area of the NESS (between 53°45′N and 54°45′N). Active gas seeps in a gas hydrate province on the NESS were evident from features in the water column, on the seafloor, and in the subsurface: well-defined hydroacoustic anomalies (gas flares), side-scan sonar structures with high backscatter intensity (seepage structures), bathymetric structures (pockmarks and mounds), gas- and gas-hydrate-related seismic features (bottom-simulating reflectors, gas chimneys, high-amplitude reflectors, and acoustic blanking), high methane concentrations in seawater, and gas hydrates in sediment near the seafloor. These expressions were generally spatially related; a gas flare would be associated with a seepage structure (mound), below which a gas chimney was present. The spatial distribution of gas seeps on the NESS is controlled by four types of geological structures: faults, the shelf break, seafloor canyons, and submarine slides. Gas chimneys that produced enhanced reflection on high-resolution seismic profiles are interpreted as active pathways for upward gas migration to the seafloor. The chimneys and gas flares are good indicators of active seepage.     

Hydrocarbon-derived carbonate buildups of the northern Gulf of Mexico continental slope: A review of submersible investigations

Hydrocarbon-derived and microbially mediated authigenic carbonates occur over the entire depth range of the northern Gulf of Mexico slope. These carbonates consist of nodules and incipient nodules in surface sediments, hardgrounds and isolated slabs, and moundlike buildups of up to 10–20 m relief above the surrounding seafloor. The authigenic carbonates are characterized by ¹³C negative values in the range –18 to –55 (PDB) suggesting mixing of seawater carbon with¹³C-depleted carbon sources ranging from crude oil to biogenic methane. Near the shelf edge, carbonates are diluted with biogenic material produced by reefs—bioherms developed at low sea level stands. Fossil-poor carbonates over salt diapirs of the upper and middle slope formed in the shallow subsurface and have been exhumed by the combined processes of uplift and physical erosion. Middle and lower slope carbonates are generally rich in fossil shells of chemosynthetic organisms. Mg calcite pelloidal matrix and acicular to botryoidal aragonitic void-filling cements are common petrographic features of these hydrocarbonderived carbonates. At two sites carbonates are mixed with barite.     

Acoustical and geological characteristics of near-surface sediments,upper continental slope of northern Gulf of Mexico

The continental slope off Texas and Louisiana is the most structurally and sedimentologically complex area in the Gulf of Mexico. This 120,000 km² area is dominated by diapiric tectonics, resulting in numerous faults, oil and gas seeps, and other geological phenomena that affect near-surface sediments. Bottom gradients range from 0 to 20°. High-resolution seismic surveys, foundation borings, and drop cores reveal several mass-movement features, acoustic wipeout zones, sea floor erosion, faults, hardgrounds, bioherms, reefs, and outcrops. Recent data indicate that most geological activity takes place during relative sea level lowering and low stands.     

The distribution of phenylalkanes in the modern sediment associated with gas hydrate from the Gulf of Mexico

Phenylalkanes with carbon numbers between 16 and 19, characterized by the main carbon-18, have been identified in the modern sediments collected from gas hydrate area from the Gulf of Mexico. The structure of phenylalkanes with four isomers for every carbon number was determined by means of their mass spectra and previous studies. The distribution of the series characterized by a low molecular mass was similar to the distribution of n-alkane, alkylcyclohexanes and alkylbenzenes in each sample. There were differences in the distribution of the phenylalkane series between the S - 1, S - 4, S - 7, S - 9 samples and the S - 8, S - 10 and S - 11 samples. The phenylalkanes might be derived from Archaea associated with anaerobic oxidation of methane （AOM） processes in S - 1, S -4, S -7 and S -9 samples according to their distribution resembled with the distribution of the extract from a type of Archaea. The distribution of alkylcyclohexanes and alkylbenzenes in S - 1, S - 4, S - 7 and S - 9 sample was found to be similar to each other. The odd-over-even predominance of alkylcyclohexanes was seen as the input of some bac- terial.     

Acoustic evidences of gas in the continental slope sediments of the Gulf of Cadiz (E Atlantic)

Detailed reviews of high-resolution acoustic studies in the continental slope of the Gulf of Cadiz has revealed the following gas-related features: acoustic turbidity and blanking, bright spots, ancient and modern pockmarks, high-amplitude diffractions, acoustic plumes and turbidity in the water column, and BSRs. The origin of the gas is believed to be biogenic and thermogenic. The BSR-like acoustic anomalies occur intermittently in some areas of the upper slope and tend to occur in the volcanoes/diapirs. The pressure–temperature conditions deduced for the location of those acoustic anomalies do not correspond to the conditions of stability of gas hydrates. It is suggested that these volcanoes/diapirs intrusions may locally induce anomalously higher pore pressure conditions on the immediately surrounding sediments, affecting the stability field of the gas hydrates.     

Active diapirism and slope steepening,northern gulf of Mexico continental slope

Abstract

Large diapiric and nondiapiric masses of Jurassic salt and Tertiary shale underlie the northern Gulf of Mexico continental slope and adjacent outer continental shelf. These masses show evidence of being structurally active at present and in the very recent geologic past. Local steepening of the sea floor in response to the vertical growth of these structures is a serious concern to those involved in the site selection and the construction of future oil and gas production and transportation facilities in this frontier petroleum province.

The seabed of the northern Gulf slope is hummocky and consists of many hillocks, knolls, and ridges interspersed by topographic depressions and canyon systems. Topographic highs and lows relate respectively to vertical diapiric growth and to withdrawal of large volumes of salt and shale. Topographic highs vary considerably in shape and size, but all have very limited areas of nearly flat sea floor. Intraslope topographic lows consist of three principal types: (1) remnants of submarine canyons blocked by diapiric uplift that terminated active downslope sediment transport common during stages of low sea level; (2) closed depressions formed by subsidence in response to salt and shale withdrawal and flow into surrounding diapiric uplifts; and (3) small collapse basins formed by faulting in strata arched over structural crests of diapirs.

Distribution patterns of both diapiric features and sediment accumulations on the slope are the result of the complex relationship that exists between sediment loading and diapirism. Diapiric activity is proportional to the thickness of salt or underconsolidated shale available for mobilization, and to the sedimentary load distribution on these highly plastic deposits. Variations in overburden load, in turn, are dependent on rates, volumes, and bulk densities of depo‐sitional influx; proximity to sources of supply, erosion, and distribution of sediments; and topographic control of sediment accumulation. Sediment capture in diapirically controlled interdomal basins and canyon systems localizes overburden load, thus inducing further diapiric growth, and complex structural and stratigraphic patterns are induced throughout the continental slope region.

Drill cores in the slope province indicate that most of the slope sediments are fine‐grained muds; appreciable quantities of sand‐size sediment are present principally in canyon axes. Turbidite sand layers drilled on a topographic high adjacent to the Gyre Basin reflect uplift far above their original deposition level, and calculations yield rates of uplift that average 2 to 4 m per 100 years. Seismic reflection profiles provide considerable evidence of “fresh”; slumps and ero‐sional surfaces on the flanks of many topographic highs not yet blanketed by a veneer of young sediments. This evidence thus supports our conclusion that the present continental slope region of the northern Gulf of Mexico is undergoing active diapirism and consequent slope steepening. Because most of the sediment on the flanks of diapiric structures consists of underconsolidated muds, slumping will take place regularly in response to further diapiric movement.