The goal of this project is to determine structural and stratigraphic controls on hydrate occurrence and distribution in Green Canyon (GC) 955 and Walker Ridge (WR) 313 blocks with special emphasis on hydrate-bearing sand reservoirs. Structural and stratigraphic controls on hydrate distribution will be examined by jointly analyzing surface-towed, multichannel seismic data and well logs through a combination of pre-stack depth migration (PSDM), full-waveform inversion (FWI), and rock physics modeling methods.
Oklahoma State University, Stillwater, OK 74078-1026
Enormous volumes of hydrate and associated free gas may exist beneath the seafloor along continental margins making hydrates a potential energy resource, a deep-water geohazard, and a potential component of the carbon cycle. Thus, hydrate research has a multi-dimensional significance.
Geophysical and subsurface investigations have indicated that hydrates are heterogeneously distributed and demonstrate multiple arrangement styles such as freely floating within pore spaces, embedded in rock matrix, cementing outside mineral grains, creating and filling fractures, and forming massive seafloor outcrops. Hydrates affect elastic velocities of their host sediments. The magnitude of velocity change, however, depends both on the in situ volume and arrangement styles of hydrate.
The most ambitious and successful hydrate research in the Gulf of Mexico (GoM) was carried out in 2009 in the second leg of the Joint Industrial Project (JIP) where several boreholes in WR313 and GC955 were completed using a comprehensive set of logging-while-drilling (LWD) tools. JIP Leg 2 inferred hydrate in at-least two arrangement styles: pore-filling in sand-dominated reservoirs and fracture-filling in clay-dominated reservoirs. Presence of hydrate in highly concentrated form within sand dominated reservoirs was confirmed indicating a high possibility of hydrate recovery with near-future technology.
Researchers currently do not have detailed knowledge of structural and stratigraphic controls on hydrate distribution in the study area, and it is critical to address this knowledge gap to fully appreciate both the feasibility of hydrate extraction from a resource perspective and the environmental impact of perturbing these natural systems. This effort will address this knowledge gap by resolving questions related to heterogeneity of hydrate distribution within sand reservoirs and association of fracture-dominated and sand-dominated reservoirs and hydrate and free-gas interaction, which in turn will enable elucidation of hydrates as an energy source and their significance in natural environments.
Project personnel will use 2-D seismic profiles that are co-located with GC955 and WR313 wells to construct high-fidelity depth images using PSDM and high-resolution velocity and attenuation models using FWI. The depth image, the velocity, and the attenuation models will be used for detailed interpretation of the study areas to identify the structural and stratigraphic features and facies relevant to hydrate occurrence and distribution. A rock physics template will be used to relate hydrate saturation and growth style to the LWD sonic velocities. The sonic log and the rock physics template will be up-scaled to seismic wavelengths. Hydrate saturation will be extended outwards from the well using the FWI velocity model to create a high-resolution map of hydrate saturation in the study area. The saturation maps will be analyzed in conjunction with previously created structural, stratigraphic, and facies models to understand how, where, and why hydrates form and concentrate in different locations within the study area.
The project will greatly advance the tools and techniques used for delineating specific hydrate prospects. Results from the proposed effort can be incorporated into other DOE-supported projects on modeling the potential productivity and commercial viability of hydrate from sand-dominated reservoirs.
Application of Full Wave Inversion (FWI), pre-stack depth migration (PSDM), and rock physics-based seismic quantification will greatly increase knowledge of hydrate and free gas systems in the northern GoM in general, and at sites WR313 and GC955 in particular. A combination of reflectivity attributable from the PSDM image and the FWI velocity model will unravel the heterogeneity of hydrate distribution within the sand reservoir. The structural (fault, fractures, etc.) and stratigraphic (pinch-outs, channel-levee architecture, etc.) interpretation of the PSDM image combined with the attenuation from FWI will help establish the relationship between fracture-dominated and sand-dominated environments, including the extent of their inter-fingering. Rock physics-based seismic quantification will help determine the stratigraphic controls on hydrate and free gas distribution.
This effort will provide insight into the geological controls on hydrates and will shed light on fundamental in situ properties (porosity, pore-interconnectivity, and mineralogy) of hydrate-bearing sediments. These results will be of great interest to industries that view hydrate as either an energy source or geo-hazard.
The seismic data procured from M/S CGG-Veritas Inc. were loaded into the processing system. The navigation quality control and data binning was completed in February 2013. The preliminary data processing (filtering and muting) has been completed. The normal move-out correction and brute stack have been created and used to create the first depth migrated image. The velocity model has been updated, the data re-migrated, and the initial interpretation of BSR completed in late May 2013.
Researchers will complete the data processing to provide the initial velocity model and initial depth image needed for the reflection traveltime tomography and the full waveform tomography. The depth images from migration, and the velocity and the attenuation models from tomography, will be used for detailed interpretation of the study areas to identify the structural and stratigraphic features and facies relevant to hydrate occurrence and distribution.