NS51B-01
The Presence of Gas Hydrate Layers Inferred From BSR at the West Margin of the Baja Californian Peninsula
Studies of hydrate occurrence have been very limited at the Mexican Pacific margins. Five seismic reflection profiles across the west margin of the Baja California Peninsula were examined to determine the temperature and pressure at sediment layers containing gas hydrate, inferred from the presence of BSR events. During March and April, 2002, a French-Mexican marine geophysical expedition (FAMEX-2002) was carried out in the Eastern Pacific margin to survey the sea floor structures in the Mexican region near the peninsula [22N-29N]. More than 4,000 nautical miles of seismic reflection sections were gathered during FAMEX-2002. BSR were well identified at several seismic reflection profiles across the peninsula western slope [26N and 23.5N], at depths greater than 3,000 m bsl. The maximum length of BSR seen in the margin is greater than 30 km long. Using phase diagrams of the hydrate stability zones and range of temperatures measured at the sea floor, we were able to estimate interactively the upper and bottom limits of the BSR temperatures to preserve the hydrate state at depths greater than 3,200 m bsl and 200 m bsf.
NS51B-02
Relative Permeability of Gas Hydrate Bearing Sediments
Permeability controls the rate of fluid flow through porous media and relative permeability describes the phase interference when more than one phase is present in the pore space. The presence of gas hydrate in sediments reduces the effective permeability (product of intrinsic and relative permeability) of water and gas released from hydrate dissociation. Knowledge of the relative permeabilities of gas and water or brine as a function of gas hydrate, gas, and water saturation is essential in the prediction production of natural gas from hydrate-bearing reservoirs. We have measured the effective permeability of confined dry sand samples (with mineral grain-to-grain contact) and of the same samples containing water, ice, and hydrate under a variety of conditions to determine the impact of hydrate in the pore space and estimate relative permeabilities. In our tests, the presence of hydrate strongly affects the gas relative permeability, which decreases as the hydrate saturation increases. Hydrate saturations near 50 percent resulted in a very low effective permeability. We conducted experiments using two different sands, with differences in the hydrate formation procedure resulting in changes in hydrate saturation morphology. In addition, we performed water floods of our hydrate-bearing samples, and used x-ray CT scanning to monitor water saturation changes at several locations. These data were used to estimate properties of the hydrate- bearing porous medium by means of inverse modeling.
NS51B-03
Remagnetization and Cementation of Unconsolidated Sediments in the Mallik 5L-38 Well (Canadian Arctic) by Solute Exclusion During Gas Hydrate Formation
Bulk magnetic properties provide a sensitive measure of sedimentary diagenesis related to the stability and growth of gas hydrates. The deposit at Mallik (Mackenzie Delta, Canadian Arctic) occurs in unconsolidated Tertiary sands, but is absent in interstratified silt layers. A detailed sampling of the JAPEX/JNOC/GSC Mallik 5L-38 core tested the use of magnetic properties for detecting diagenetic changes related to the hydrate. Petrographic studies reveal that the sands are well sorted and clean, with quartz > chert >> muscovite and little fines content. Excepting a few rare bands of indurated dolomite in the midst of the gas hydrate zone, there is little or no cementation in the sands. Detrital magnetite is the dominant magnetic mineral, comprising up to a few percent of the sand grain population. In contrast, the muddier layers have a somewhat different detrital grain composition, richer in lithic (sedimentary and metamorphic) grains, feldspar, and clays. They are extensively diagenetically altered (to as much as 30- 40%) and cemented with carbonates, clays, chlorite and the iron sulphide greigite (the dominant magnetic mineral). The greigite is recognized by its isotropic creamy-white reflectance, cubic to prismatic habit, and characteristic tarnish to faintly bluish bireflectant mackinawite. Habits range from disseminated cubes and colliform masses to inflationary massive sulphide veins and clots. Rare detrital grains of magnetite were observed among the silt grains, but never in a reaction relationship or overgrown. Instead the greigite has nucleated separately, in tensional fractures and granular masses up to 4 mm across. In this particular sediment sequence, being so quartz and chert rich, there is insufficient local source for the introduced cements (calcite, dolomite, greigite, clays, jarosite), so ions must have been introduced by fluid flow. Magnetic studies reveal a bi-modal character related to the lithology (sands versus silts) and their magnetic mineralogy. Silt samples are significantly stronger than sand samples in saturation magnetization and magnetic susceptibility. The silt samples have single-domain to pseudo-single domain coercivity ratios whereas the gas hydrate bearing sands have a more multi-domain nature. Sands with current gas hydrate concentrations > 80% have less magnetic material and single domain characteristics. The source of the greigite, carbonates, and other diagenetic minerals was apparently concentrated solutes excluded from formation waters by the freezing and formation of the water dominated gas hydrate. The hydrates served as a cementing agent for the unconsolidated sediments, allowing them to fracture. Some layers have been so inflated by the introduction carbonate and sulfide cements that they resemble hydrothermal tufa and skarns with floating sand grains. In the silts, the magnetic properties reflect the mixture of primary detrital magnetite and diagenetic greigite in various grain sizes and concentrations. At Mallik, the magnetic properties are sensitive to the diagenetic mineralogy and redox state associated with the transport of methane and pore fluids and the creation of gas hydrates. Hypersaline brines, produced by solute exclusion from pore waters, fractured and inflated less permeable sediments and forced rapid disequilibrium growth of greigite without dissolving primary detrital magnetite grains.
NS51B-04
Effects of Attenuation of Gas Hydrate-bearing Sediments on Seismic Data: Example from Mallik, Northwest Territories, Canada
Wave attenuation is an important physical property of hydrate-bearing sediments that is rarely taken into account in site characterization with seismic data. We present a field example showing improved images of hydrate- bearing sediments on seismic data after compensation of attenuation effects. Compressional quality factors (Q) are estimated from zero-offset Vertical Seismic Profiling data acquired at Mallik, Northwest Territories, Canada. During the last 10 years, two internationally-partnered research drilling programs have intersected three major intervals of sub-permafrost gas hydrates at Mallik, and have successfully extracted core samples containing significant amount of gas hydrates. Individual gas hydrate intervals are up to 40m in thickness and are characterized by high in situ gas hydrate saturation, sometimes exceeding 80% of pore volume of unconsolidated clastic sediments having average porosities ranging from 25% to 40%. The Q-factors obtained from the VSP data demonstrate significant wave attenuation for permafrost and hydrate- bearing sediments. These results are in agreement with previous attenuation estimates from sonic logs and crosshole data at different frequency intervals. The Q-factors obtained from VSP data were used to compensate attenuation effects on surface 3D seismic data acquired over the Mallik gas hydrate research wells. Intervals of gas hydrate on surface seismic data are characterized by strong reflectivity and effects from attenuation are not perceptible from a simple visual inspection of the data. However, the application of an inverse Q-filter increases the resolution of the data and improves correlation with log data, particularly for the shallowest gas hydrate interval. Compensation of the attenuation effects of the permafrost likely explains most of the improvements for the shallow gas hydrate zone. Our results show that characterization of the Mallik gas hydrates with seismic data not corrected for attenuation would tend to overestimate thicknesses and lateral extent of hydrate-bearing strata and hence, the volume of hydrates in place.