Assessment of spectral attributes in identifying gas hydrates in seismic data from the Pegasus Basin, offshore New Zealand

• Main Author: Jackson, Emily
• Other Authors: Bedle, Heather, Pranter, Matthew, Elwood Madden, Megan
• Language: English
• Published: 2023
• Subjects: Geophysics; Gas Hydrates; Seismic attributes; Machine learning; New Zealand; permafrost
• Online Access: https://shareok.org/handle/11244/337512

Gas hydrates are formed in the subsurface along shallow ocean basins or in permafrost settings, and are commonly identified in the seismic data by the bottom-simulating reflector (BSR). Various methods have been employed in the past to measure gas hydrates from lab analyses, well log, or velocity data, but few studies have demonstrated methods to identify gas hydrates in seismic data when the BSR is sparse or lacking. One approach is to measure the expected attenuation, or the reduction in the seismic waveform, caused by hydrates in the gas hydrate stability zone (GHSZ). This study proposes the application of two statistical attributes—skewness and kurtosis—that measure the asymmetry of the seismic amplitude spectrum in order to quantify the attenuation responses throughout the GHSZ. Although the study area does not contain well log data, there are numerous studies that confirm hydrates exist throughout the Pegasus Basin. These attributes, in addition to other instantaneous and amplitude-related attributes, demonstrate that frequency-related variations are the major contributors to attenuation response, rather than seismic amplitude or geology effects. The spectral attribute results show that strong positive skewness and kurtosis variations above the high amplitude BSR is likely due to attenuation through an interval of hydrates. Negative skewness and kurtosis may correspond to an interval that does not contain hydrates, therefore suggesting that the GHSZ in the Pegasus Basin consists of discontinuous intervals of hydrates, rather than one continuous layer from ocean bottom to BSR.