Seismic facies analyses as aid in regional gas hydrate assessments. Part-I: Classification analyses

The identification of areas of natural gas hydrate occurrence, defining its concentration and regional distribution, as well as understanding the processes that control gas hydrate formation is a crucial component of regional gas hydrate assessments. In this study, we incorporate an additional element into a regional assessment strategy by including the depositional environment defined through seismic facies classes. The seismic fades classification is attempted using regional 2D seismic data and a 3D seismic volume, as well as core and log-data from two gas hydrate drilling expeditions carried out in the Ulleung Basin, East Sea, to conduct a fully integrated gas hydrate assessment. The facies classification described in this study is part of a procedure to conduct an assessment including: (1) Lithologic description of the recovered core and describe the deposition character of the sediment facies identified; (2) seismic facies classification using 2D and 3D seismic data and their derived various seismic attributes; (3) Linking seismic fades classes to core-derived sedimentological descriptions to define the host-strata most favourable for gas hydrate occurrences; (4) Defining reservoir physical properties and linking these to seismic facies classes for reservoir property description; (5) Calculating regional elements of the gas hydrate petroleum system (top of gas hydrate occurrence and base of gas hydrate stability); (6) Merging seismic fades and reservoir properties, definition of the top and base of the gas hydrate stability zone, to calculate gas hydrate saturations in each seismic fades and the total volume of gas hydrate present in the study area; (7) Defining probabilistic elements of the predicted volume of gas hydrate by combining statistical information of the individual input parameters through exhaustive Monte Carlo simulations. The seismic fades classification algorithm used in this study is based on identifying clusters (classes) that show similar properties in the seismic data and derived attribute characteristics. Our classification scheme uses seven seismic attributes (amplitude, instantaneous frequency and amplitude (also referred to as envelope), sweetness, acoustic impedance, thin-bed- and parallel-bedding indicator) and incorporates the use of principle component analysis for data reduction. The achieved classifications are overall robust and yield identical facies classes for the co-located 2D and 3D data. Linking the seismic fades classes to the recovered core and sedimentological descriptions results in a sedimentological interpretation of the seismic fades. Cross-plots of individual components as well as analysing the Eigen-vectors and -values from the principle component analysis (PCA) helps understand the physical meaning of these seismic facies classes and their seismic character. The five fades classes assigned to the 3D volume and 2D seismic line after PCA represent the mass transport deposit (MTD) units (class #1), hemipelagic mud with some sandy turbidite (class #2), hemipelagic mud with many sandy turbidites (class #3), as well as water-bearing mud with few sand layers and occurring mostly at shallow depth near the seafloor or beneath the base of the gas hydrate stability zone (classes #4 and #5). Within the study area and seismic data sets used, gas hydrate occurrences are only linked to facies classes #2 and #3. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.