Possible causes of weak BSRs on the Hikurangi Margin. Could they be caused by the presence of gas or hydrates in fractures?

Abstract

Gas hydrates, methane ices, may constitute a source of energy in the future and increased knowledge of where and how gas hydrates occur is important. The presence of bottom simulating reflections (BSRs, seismic reflections with reversed phase from the interface between hydrate- and gas-filled rocks) is commonly used to indicate occurrence of gas hydrates in subseafloor sediments. Furthermore, the strength of BSRs, and the seismic amplitude versus offset (AVO) anomalies may indicate the quality of a gas hydrate reservoir. The Hikurangi Margin, a promising gas hydrate province offshore of the eastern North Island, New Zealand, often has weak BSRs compared to other areas globally. Previous studies have attributed this pattern to low concentration of gas in a patchy distribution (probably, gas in fractures). In order to clarify whether weak BSRs could be caused by the presence of gas and/or hydrates in fractures, and further research on the possible causes of weak BSRs, rock models with various porefilled inclusions into a sediment matrix are applied to evaluate the strength of BSRs. This is performed using the Kuster-Toksöz effective moduli and a newly developed approximation for the reflection coefficient. Results show that there are several other lithological factors that could lead to weak BSRs. They are: a low ratio of bulk to shear modulus for matrix, a high density for sediments, small modulus contrasts between inclusion and matrix for rocks, small modulus contrasts for inclusions across the BSR, a small density contrast for inclusions across the BSR, and a high aspect ratio for gas or hydrates. Generally, the properties of the rock matrix, gas presence, and shape of pore spaces are extremely important for the strength of BSRs besides distribution style and fraction of gas or hydrates. Furthermore, fractures saturated with gas have similarly effects as gas in a patchy distribution and could lead to weak BSRs. Weak BSRs do not always mean low concentration of gas hydrates. Based on the geological background in the study area, there is a convincing argument that a combination of a relatively low ratio of bulk to shear modulus for sediments, a relative low concentration of gas mainly present in fractures, and a relatively high density for sediments, cause weak BSRs. Both of the low ratio and high density maybe indicate older, well compacted rock.