INTRODUCTION

The area of investigation for Expedition 311 is the accretionary prism of the Cascadia subduction zone (Fig. F1). The Juan de Fuca plate converges nearly orthogonally to the North American plate at a present rate of ~45 mm/y (e.g., Riddihough, 1984). Seaward of the deformation front, the Cascadia Basin consists of pre-Pleistocene hemipelagic sediments overlain by rapidly deposited Pleistocene turbidites for a total sediment thickness of ~2500 m. Most of the incoming sediment is scraped off the oceanic crust and folded and thrust upward to form elongated anticlinal ridges with elevations as high as 700 m above the adjacent basins. The thrust faults near the deformation front penetrate nearly the entire sediment section (Davis and Hyndman, 1989).

The primary objective of Expedition 311, on the northern Cascadia margin, is to constrain geologic models for the formation of gas hydrate in subduction zone accretionary prisms. This expedition follows the goals for gas hydrate drilling as proposed by the Ocean Drilling Program (ODP) Gas Hydrates Program Planning Group:

The specific objectives of this expedition are to test gas hydrate formation models and constrain model parameters, especially models that account for the formation of concentrated gas hydrate occurrences through upward fluid and methane transport. These objectives require (1) high-quality data on the vertical concentration distribution of gas hydrate and free gas and variation landward in the accretionary prism and (2) estimates of the vertical fluid and methane fluxes through the sediment section as a function of landward distance from the deformation front. The study concentrates on the contrast between dispersed pervasive upward flow and focused flow of fluid and methane in fault zones. The pervasive permeability may be on the sediment grain scale, on a centimeter scale (the scaly fabric observed in previous ODP clastic accretionary prism cores), or in closely spaced faults.

Natural gas hydrate occurs in marine continental slope and onshore Arctic permafrost environments. The Arctic occurrences can exhibit very high gas hydrate concentrations but appear to contain less total gas than marine gas hydrate occurrences. Recent studies have indicated that the largest occurrences of gas hydrate may lie in nearly horizontal layers several hundred meters beneath the seafloor of continental slopes, especially in the large subduction zone accretionary sedimentary prisms. Gas hydrate and underlying free gas produce the ubiquitous bottom-simulating reflectors (BSRs) along numerous continental margins of the world. Gas hydrates do occur on passive margins, but they are less common and appear to be usually at lower concentrations.

The two marine gas hydrate areas that have received the most detailed scientific study, including previous drilling during the ODP, are the Blake Ridge region off the east coast of the United States (a passive margin setting) and the Cascadia margin off the coast of Oregon and Vancouver Island (subduction zone accretionary prism). Important new information on Arctic gas hydrate occurrences has been obtained from the Mallik-1998 and Mallik-2002 drilling projects in northern Canada (Dallimore et al., 1999; Dallimore and Collett, 2005). If our conclusions for the occurrence and distribution of gas hydrate in nature are correct, gas hydrate formed within accretionary prisms is the most important both for understanding the geologic controls on the occurrence of gas hydrate and for the role that natural gas hydrate plays in the global carbon cycle. Within accretionary prisms, the largest amount of gas hydrate is predicted to occur in a widespread layer located just above the BSR (Hyndman and Davis, 1992). However, also important is focusing a portion of the upward methane flux into localized migration conduits or channels to form concentrated near-seafloor gas hydrate accumulations. The amount of gas hydrate in the widespread layer above the BSR, compared to that in local concentrations near the seafloor, is especially important for understanding the response of marine gas hydrate to climate change. Near-seafloor gas hydrate accumulations will respond much more quickly to ocean temperature changes compared to gas hydrate several hundred meters below the seafloor. For the region of ODP Sites 889/890, Taylor et al. (2002) calculated that a 30 m thick hydrate deposit lying at the base of the stability field would dissociate due to seabed warming in ~8000 y. However, near-seafloor gas hydrate could dissociate much more quickly and be much more responsive to human-induced global warming (e.g., Wood et al., 2002). Off Vancouver Island a gas hydrate–related BSR occurs in a 30 km wide band parallel to the coast beneath much of the continental slope (Fig. F1). Gas hydrate is thought to be concentrated in a layer 50–100 m thick, just above the base of the gas hydrate stability field, which is located 200–300 m below seafloor (mbsf) based on earlier interpretations (see, e.g., Hyndman et al., 2001, for a summary of these findings). The gas hydrate concentrations, estimated from downhole logging data collected during ODP Leg 146, may reach ~30% of pore space. The surveys and studies that have been carried out and the evidence for the presence and content of gas hydrate are summarized in two comprehensive review articles (Hyndman et al., 2001; Spence et al., 2000).

Gas hydrate concentrations were previously estimated from downhole acoustic and resistivity logs, multichannel seismic (MCS) data, vertical seismic profile (VSP) velocities, and pore water freshening (e.g., Yuan et al., 1996, 1999; Hyndman et al., 1999). Gas hydrate concentrations determined from the different methods vary slightly but were estimated to be on average 20%–35% of the pore space over a 100 m thick interval above the BSR. These are high concentrations for a marine gas hydrate occurrence and are not observed on other margins. ODP drilling and logging at the Blake Ridge offshore South Carolina (passive margin environment) showed gas hydrate concentrations that were on average <10% (Paull, Matsumoto, Wallace, et al., 1996). Results from Leg 204 at southern Hydrate Ridge located on the southern Cascadia margin offshore Oregon, with a very similar tectonic environment to offshore Vancouver Island, showed very low gas hydrate concentrations of <5% except for the unusual summit of Hydrate Ridge (Trehu et al., 2004). An earlier approach by Ussler and Paull (2001) for interpreting chlorinity data from Leg 146 by using a smooth chlorinity baseline suggested lower gas hydrate concentrations at Sites 889/890 but apparently contradicted other geophysical results using electrical resistivity and seismic velocity. In preparation for Expedition 311, the gas hydrate concentrations along the northern Cascadia margin were recalculated using Leg 146 acoustic/electrical resistivity logs and pore water chlorinity/salinity data (Riedel et al., 2005). New estimates show that the concentrations could alternatively be between 5% and 10% on average of the pore volume from ~130 mbsf to the BSR (~230 mbsf).

No conclusive decision can be made using interstitial water chlorinity data from Leg 146 to assess the occurrence of gas hydrate without a more complete analysis of the baseline chlorinity data and without additional pore water chemistry data (lithium and strontium data) to confirm the model by Torres et al. (2004) derived from Leg 204 results. Therefore, gas hydrate concentrations along the northern Cascadia margin as estimated from chlorinity data may either be as low as a few percent, as suggested earlier by Ussler and Paull (2001), or as high as 40%, previously suggested by Hyndman et al. (1999). Gas hydrate concentrations also were calculated from the resistivity data using Archie's equation. Archie's equation consists of several empirical parameters (referred to as a, m, and n), which were determined to be a = 1.4, m = 1.76, and n = 1.0 by Hyndman et al. (1999) using the Leg 146 core data at Sites 888 and 889/890. However, Collett (2000) redefined these parameters to a = 1.0, m = 2.8, and n = 1.9 by using the Site 889 resistivity and neutron porosity logs. The differences in these empirical parameters result in highly different estimated gas hydrate concentrations. The Leg 146 logging data are of relatively poor quality and neither the new parameters nor the previous analyses fit all data well, and a large uncertainty remains in the results. However, results from Leg 204, obtained from a tectonically similar environment, suggest that the Archie parameters by Collett (2000) may be preferable.

Finally, new reference velocity profiles were calculated from the Site 889 porosities using various published empirical relations between porosity and velocity (Jarrard et al., 1995; Hyndman et al., 1993) and the Lee et al. (1993) weighted equation. All the newly proposed baselines are significantly shifted toward higher seismic velocities relative to the former baseline defined by Yuan et al. (1996) and Hyndman et al. (2001). Thus, they result in gas hydrate concentrations that are only between 5% and 10% of the pore volume. However, significant uncertainty remains in the applicability of the empirical parameters for each of the individual equations. If the model by Torres et al. (2004) proposed for Hydrate Ridge (Leg 204) is applicable to Site 889 and if the Archie parameters by Collett (2000) are correct, these two methods (although individually very uncertain) confirm results from the acoustic velocity analyses of Lee et al. (1993). Thus, the total gas hydrate concentration at Sites 889/890 may be much lower than previously assumed.

A general model for deep-sea gas hydrate formation by removal of methane from upwardly expelled fluids was developed earlier for the Expedition 311 area (Fig. F2) (Hyndman and Davis, 1992). Mainly microbial methane, inferred to be produced over a thick sediment section, migrates vertically to form gas hydrate when it enters the stability field. The gas hydrate concentration is predicted to be greatest just above the BSR. A model has also been proposed for how free gas and the resulting BSR will be formed as the base of the gas hydrate stability moves upward due to post-Pleistocene seafloor warming, uplift, and sediment deposition (e.g., Paull and Ussler, 1997; von Huene and Pecher, 1998). In addition, physical and mathematical models have been developed for the formation of gas hydrate from upward methane advection and diffusion (e.g., Xu and Ruppel, 1999).

Recently, evidence for focused fluid/gas flow and gas hydrate formation has been identified on the Vancouver margin. The most studied site is an active cold vent field associated with near-surface gas hydrate occurrences close to Sites 889/890 (e.g., Riedel et al., 2002). Studies include high-resolution subbottom profiling, three-dimensional (3-D) seismic surveys, piston coring, and ocean-bottom video surveying and sampling with the remotely operated vehicle ROPOS. These vents represent fault-related conduits for focused fluid and/or gas migration associated with massive gas hydrate formation within the fault zone and represent, therefore, the opposite mechanism to the widespread fluid flow. It is thus far unknown how important these cold vents are in the total budget of fluid flow in an accretionary prism. Drilling at the vent field will help constrain the significance of fault-related fluid and/or gas flow.

During Expedition 311, a series of four holes was drilled along a margin-perpendicular transect (Figs. F3, F4) representing different stages in the evolution of the gas hydrate stability zone; an additional fifth site was established to examine an apparent active cold vent.

At the southwestern end of the transect, Site U1326 (prospectus Site CAS-03C) is located on top of the first uplifted ridge, followed by Site U1325 (prospectus Site CAS-02C) located in the first slope-sediment basin. The third Site U1327 (prospectus Site CAS0-01B) is located near Leg 146 Site 889. The fourth site of the transect, Site U1329 (prospectus Site CAS-05D), represents the eastward limit of the gas hydrate occurrence on the margin. The additional fifth site is located at an active cold vent (Site U1328; prospectus Site CAS-06A).

Recently acquired bathymetry data reveal a collapse structure near the originally proposed primary Site CAS-03B (Fig. F5) where we now expect a more complicated geologic history. We decided to switch the former alternate location Site CAS-03C to primary status to avoid coring directly into the slump feature.

The original proposal 553-Full2 "Cascadia Gas Hydrate" included long-term monitoring experiments using Circulation Obviation Retrofit Kit (CORK)-II, Advanced CORK (ACORK), modular formation dynamic testing (MDT), and distributed temperature sensing (DTS). An extra site for a dual-hole hydrogeologic ACORK experiment was proposed near Site 889 (prospectus Site CAS-01C). With this shortened expedition approach under the Integrated Ocean Drilling Program (IODP), the long-term monitoring experiments were postponed until a second (yet unscheduled) expedition. Other components of the original proposal that had to be deferred included coring, wireline logging, and logging-while-drilling operations at prospectus Sites CAS-04B and CAS-05B and a walk-away VSP at prospectus Site CAS-01B.

At each site we tried to accomplish a three-hole conventional coring, specialized coring, and logging approach to maximize the scientific objectives. The first hole (Hole A) was always dedicated to logging-while-drilling/measurement-while-drilling (LWD/MWD) operations. All LWD/MWD holes were drilled within the first week of the expedition, followed by coring and conventional wireline operations. The second hole (Hole B) in most cases was for continuous coring, temperature measurements, and pressure core sampler (PCS) pressure coring to establish complete downhole profiles of gas hydrate proxies (such as chlorinity, infrared images, etc.). The third hole (Hole C), when included, was dedicated to special tool deployments, especially the two HYACINTH pressure coring systems, the Fugro Pressure Corer (FPC) and HYACE Rotary Corer (HRC), and additional PCS pressure coring.

During this expedition, we conducted LWD/MWD operations prior to coring each site. It was essential to know before deploying the available pressure core systems what the gas hydrate concentrations and vertical distribution were to choose the optimum depths for pressure coring operations. Because these holes were drilled prior to coring, the LWD/MWD data had to be monitored to detect gas entering the hole, since the standard method of hydrocarbon safety management during IODP is core gas ratio measurements. The primary measurement used during Expedition 311 for gas monitoring during LWD/MWD operations was the annular pressure while drilling (APWD) measured by the EcoScope tool in the borehole annulus (the space between the drill string and the borehole wall). On the basis of a calculation of the effect of free gas on the borehole fluid density, it was determined that a pressure decrease of >100 psi from the general trend of fluid pressure indicated that a significant amount of gas had been released into the drilling fluid, requiring some preventative action. For example, a pressure decrease of 100 psi corresponds to 25% gas saturation in a borehole drilled to 300 mbsf. It was also decided to monitor sudden pressure increases of >100 psi, which had been reported as precursors to gas flow into the annulus (Aldred et al., 1998). Figure F6 shows the results for LWD/MWD APWD measurements acquired during the expedition. For the entire LWD/MWD operations, no APWD response was >30 psi above or below the baseline, and hence, no corrective actions were required under the adopted LWD/MWD safety protocol. The observed positive APWD excursions were all related to drilling-related difficulties, such as an increase in pumping rates during drilling hard intervals or cuttings restricting flow in the annulus. Interestingly, small negative excursions were noted at some sites at depths below the BSR (e.g., Site U1328) that were also associated with loss coherence of the sonic waveforms acquired by the SonicVision tool, which may be the result of the presence of free gas.