Gas Hydrate Stability Zone in Offshore Southern Taiwan

Abstract

Methane hydrates are considered a major potential source of hydrocarbon energy and could be important in meeting natural gas demand in the future. To study the feasibility of recovering methane from the offshore southern Taiwan region and its impact on many geological processes, it is necessary to know the total amount of hydrate in the region; something that is still unclear. Here we take the first step using a bottom-simulating reflector (BSR) to estimate the total volume of the stability field that can hold hydrates in the sediments offshore of southern Taiwan. We used a dense grid of 6-channel and 120-channel reflection profiles to study the distribution and sub-bottom depth of a BSR. BSRs are marked by a reversed polarity reflector that increases in sub-bottom depth with increasing water depth, suggesting that BSRs mark the base of methane hydrate stability zones. For offshore Taiwan a BSR is located in offscraped sediments derived from the Taiwan orogen and the Chinese continental margin. These sediments may have high amounts of organic carbon, thereby providing a source for the methane. We document the areal extent and subbottom depth of BSRs covering a 45000 km2-wide region. The BSRs were classified into three categories based on how well they fit the seismic characteristics of the BSR associated with hydrates. Q1 BSR fits with all the expected seismic attributes to be found at the base of a hydrate boundary, while Q2 and Q3 BSRs are possible and probable reflectors resulting from such a boundary. We then estimate the volume of the hydrate stability zone bounded between the seafloor and the mapped BSRs in the offshore region. At least 1023 km3 of the hydrate stability field is underlain by the highest quality BSRs while as much as 11522 km3 is underlain by all mapped BSRs. We then speculated on the total amount of hydrate stored in the region using published regional porosity-depth relations and assuming a range of saturation values of hydrates in the pore spaces. Hydrate storage can be better estimated once additional porosity and saturation information becomes available.

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