Distribution and fractionation of light hydrocarbons related to gas hydrate occurrence and biogenic production at Hikurangi Margin (IODP Site U1517), New Zealand

Abstract

<p>The investigation of the gas hydrate system and hydrocarbon distribution were targets of IODP expeditions 372 and 375 on the Hikurangi Margin offshore New Zealand. Isotopic and molecular signatures clearly indicate a biogenic signature of methane at all sites drilled along a section crossing the accretionary wedge and basin sediments. The gas void and headspace samples from depth of a few meters up to 600 m below the seafloor have varying amounts of light hydrocarbons with high amounts of methane and changing ratios of C<sub>2</sub>:C<sub>3</sub>. The best example is the high-resolution profile gained from gas voids collected at Site U1517. Drilling at U1517 reached through the creeping part of the Tuaheni Landslide Complex (TLC), the base of the slide mass, and the Bottom Simulation Reflector (BSR) just above the base of the hole. Whereas gas hydrates could not be observed macroscopically, the distribution of gas hydrates was determined by logging while drilling (LWD) and pore water data revealing the occurrence of gas hydrates at roughly 105 – 160 mbsf with elevated saturations in thin coarse-grained sediments. The application of cryo-Scanning Electric Microscopy (cryo-SEM) on samples preserved in liquid nitrogen enabled the visualization of gas hydrates.</p><p> </p><p>At Site U1517 the high-resolution void sampling reveals molecular and isotopic fractionation of hydrocarbons in close relation to the gas hydrate occurrences and allows for drawing conclusions on the recent history of the gas hydrate system and absence of free gas transport from below at the site. The molecular and isotopic composition further indicates ongoing propanogenesis.</p>