William R. Bryant

Association of gas hydrates and oil seepage in the Gulf of Mexico

Abstract Gas hydrates were recovered from eight sites on the Louisiana slope of the Gulf of Mexico. The gas hydrate discoveries ranged in water depths from 530 to 2400 m occurring as small to medium sized (0.5–50 mm) nodules, interspersed layers (1–10 mm thick) or as solid masses (> 150 mm thick). The hydrates have gas:fluid ratios as high as 170:1 at STP, C1/(C2 + C3) ratios ranging from 1.9 to > 1000 and δ13C ratios from −43 to −71‰. Thermogenic gas hydrates are associated with oil-stained cores containing up to 7% extractable oil exhibiting moderate to severe biodegradation. Biogenic gas hydrates are also associated with elevated bitumen levels (10–700 ppm). All gas hydrate associated cores contain high percentages (up to 65%) of authigenic, isotopically light carbonate. The hydrate-containing cores are associated with seismic “wipeout” zones indicative of gassy sediments. Collapsed structures, diapiric crests, or deep faults on the flanks of diapirs appear to be the sites of the shallow hydrates.

Morphology of a submarine slide, Kitimat Arm, British Columbia

A digitally acquired, scale-corrected side-scan sonar survey yielded high-resolution imagery of a submarine landslide in British Columbia. The landslide, in a fjord-head setting at Kitimat, was last active in 1975 and created a wide area of deformed sea floor. The sediment failure involved shallow rotational movements on the slopes of a fjord-head delta, marginal tearing, translational sliding, compressional folding, and block gliding of fjord-bottom marine clays. The slide is shallow and elongate and appears to have been produced by failure in mobile, low-strength sediments.

Permeability of Unconsolidated and Consolidated Marine Sediments, Gulf of Mexico

ABSTRACT Permeability of a large number of natural marine sediment samples from the Gulf of Mexico was determined through the use of laboratory consolidation tests. The samples were divided into the following groups: Group 1, sediment consisting of more than 80% clay (material 2µ or less in size); Group 2, sediment containing from 60 to 80% clay size material; Group 3, silty clays with less than 60% clay; Group 4, silts and clays that have a significant sand size fraction present (more than 5% sand). The permeabilities of the groups ranged from 10-5 to 10-10 cm/sec with 35 normal seawater being used as the saturating fluid. A statistical analysis of the natural log of permeability versus porosity was used to develop the permeability prediction equation for each of the groups listed. The equation for Group I is k=eP(15.05)-27.37, for Group 2 k=eP(14.18)-26.50, for Group 3 k=eP(15.59)-26.65, for Group 4 k=eP(17.51)-26.93 and for all data k=eP(14.30)-26.30; where P is the porosity (in decimal) and k is the coefficient of permeability. These equations are useful for predicting changes in permeability with depth in fine grained sediments of the Gulf of Mexico. The ability to predict permeability in a continuous sequence, where the deposition history is known, may explain the large variations that we see in the physical properties in sediments similar in grain size and mineralogy. End_of_Record - Last_Page 350-------

Porometry and fabric of marine clay and carbonate sediments: Determinants of permeability

Abstract The porometry of a marine sediment is determined by the fabric, i.e., the shape, orientation, arrangement (spatial distribution) and associations of the solid particles. Fabric ultimately determines the permeability of the sediment by controlling the size, shape and continuity of the pore space. Electron microscopy studies have revealed complex fabrics and porometries of surficial and deeply buried sediments from various geological environments. When viewed in profile using thin sections, the pores of unconsolidated, high-porosity marine clays are often very irregular in shape and span a broad size range. In general, however, the pore profiles of these sediments can be described as having aspect ratios (length-to-width ratios) which approach 1.0. Pore profiles of consolidated, low-porosity clays are characterized by aspect ratios which approach infinity. Coefficients of permeability for marine clays typically range from 10 −5 cm/s (porosities of 70–75%) to 10 −8 cm/s (porosities of about 50%). The permeability coefficient was 10 −13 cm/s and the porosity was 27% for bentonite (a smectite) consolidated in the laboratory to a maximum load of 68.9 × 10 3 kPa ∗ . The pore space characteristics in carbonate sediments vary greatly depending on the constituent particles. Aspect ratios for pore profiles can range from 1.0 in a coccolith ooze to 2.0 in an aragonitic needle matrix consolidated uniaxially at pressures of 4.32 × 10 3 kPa. The permeability of shallow-water (3–4 m) oolitic carbonates is largely controlled by the permeability of the aragonite needle matrix. Despite the large quantity (in one case 94% by weight) of fine-sand-size ooids in these deposits, the comparatively impermeable ooids are matrix supported. This results in a sediment with relatively high porosity and medium permeability, similar to fine sands that are tightly packed and, therefore, grain supported. Permeability coefficients of these oolitic carbonate sediments range from 10 −2 to 10 −4 cm/s. Carbonate sediments recovered from intermediate water depths (970–1980 m) are composed of grains (mostly large shell fragments) embedded in either a coccolith matrix or an aragonite needle matrix. The permeability coefficients for these carbonates range from 10 −5 to 10 −6 cm/s. The sediments studied range in size from sands to clays, and the differences in porosity and permeability observed for these sediments reflect differences both in the fabrics of the fine-grained matrices and in the grain-size distributions. The data presented suggest that the sediment fabric is a function of the characteristics of the constituent particles and the physical and chemical environments of deposition and that the fabric plays a major role in determining the permeability of the deposit. Limited but revealing computerized image analyses have been carried out on scanning and transmission electron micrographs of these sediments. Initial results indicate that this approach may prove useful for quantifying fabric parameters and providing a statistical basis for fabric descriptions.

Gassy sediment occurrence and properties: northern gulf of Mexico

Free gas is ubiquitous at shallow sediment depths of the northern margin of the Gulf of Mexico. Gassy sediment patches are between 250 and 500 m in horizontal size. Often the gassy layers are within 100 m from the sea floor and are only a few meters thick. Both biogenic and thermogenic gas hydrates have been recovered. Stability values of temperature and pressure indicate that hydrates can exist in water depths less than 500 m. Gassy sediment geoacoustic parameter values are not well constrained because of a lack of concurrent measurements of acoustic properties and sediment gas content. For Gulf of Mexico gassy sediment, some reportedin situ values of sound speed are reduced by an order of magnitude below values for water saturated sediments. More commonly, sound speed is reduced from water saturated sediment values by only 15 to 50 percent.

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.

Slope-instability processes caused by salt movements in a complex deep-water environment, Bryant Canyon area, northwest Gulf of Mexico

Halokinetic and slope-instability processes have sculpted numerous morphological features on the flanks of the intraslope basins in the Bryant Canyon area. High-resolution geophysical data and long sediment cores (as much as 20 m [66 ft] long) were used to define the time and spatial evolution of sediment failures and their relationship to halokinetic processes. Two episodes of increased salt-tectonic activity are defined: (1) The first acted at the beginning of interglacial oxygen isotope stage 5 as salt adjusted to the abandoned environments of the Bryant and Eastern Canyon systems, and (2) the second occurred during the last glacial period and is characterized by the seaward propagation of salt masses. Three types of slopes are recognized in the intraslope basins: (1) highly inclined slopes with low-relief morphologic features resulting from shallow, translational slump complexes, (2) highly inclined slopes with high-relief morphologic features resulting from deep, rotational slump complexes, and (3) highly inclined slopes dissected by high-relief canyonlike landslide troughs resulting from channelized rotational slumps. The first two slope types occur mainly on the northern flanks of the basins, whereas the third type occurs on the southern flanks. We propose that the slump complexes on types 1 and 2 slopes were triggered by the oversteepening of the flanks by the seaward mobilization of underlying salt masses. The channelized rotational slumps on type 3 slopes are interpreted to result from the development of salt diapir bulges that lead to locally increased gradients on the basin flanks. Most of the sediment failures have been transformed into debris flows and led to the most recent phase of infilling of the basin floors.

Consolidation of Marine Clays and Carbonates

Consolidation tests performed on a large number of marine sediments from the Gulf of Mexico indicate that high void ratio marine clay sediments exhibit a linear void ratio-pressure relation in contrast to the non-linear relation as ordinarily observed in clay soils. The use of this linear relation will provide a more accurate evaluation of preconsolidation pressures of marine sediments.

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.

Origin, physical, and mineralogical nature of red clays: the Pacific ocean basin as a model

Extensive examination of North Pacific Basin red clays by scanning and transmission electron microscopy reveals that the mean constituent of the red clays are illite-rich argillaceous or shale clasts, quartz and authigenic smectite. The main source of the shale clasts and quartz are aeolian in nature and are derived mainly from African and Asian shales. Illite-rich argillaceous or shale clasts are identifiable by their morphology (high degree of roundness), selected area diffraction, and their unique fracture characteristics created by an ultra thin-sectioning process. This allows for the identification and differentiation of illite-rich shale clasts from other clays, including detrital illite, kaolinite, and smectite. Geotechnical examination of the red clays indicate that they are overconsolidated: the preconsolidation stress is in all cases larger than the vertical effective stress. The overconsolidation is attributed to the strong bonding of argillaceous or shale clasts, quartz and other particulate matter by x-ray amorphous and well developed crystalline sheets of authigenic smectite characterized by high surface activity.