Deep Free‐Gas Accumulation Beneath the Chatham Rise, New Zealand—An AVO Study

Abstract

Abstract Subduction zones serve as carbon recycling centers, where vast amassments of geologic carbon accrete or subduct through thermogenic gas windows over millions of years. We focus on New Zealand's Chatham Rise, a fossilized accretionary wedge remnant of the ∼400 Myr‐active East Gondwanan margin. We undertake an amplitude‐variation‐with‐offset (AVO)‐based seismic analysis of the abiogenic Mesozoic sedimentary sequence (MES) and overlying Sequence Y chalk interval, which span the Chatham Rise's northwestern slope. Two‐term AVO attribute analysis resulted in the interpretation of distinct AVO Class III–IV reflection anomalies, which demarcate the siliciclastic MES from overlying Sequence Y chalks. Unified through their strong negative intercept amplitudes, Class III anomalies increase in absolute amplitude with angle, while Class IV anomalies decrease in absolute amplitude with angle of incidence. Simultaneous AVO inversion of seismic data highlighted the presence of P‐impedance anomalies, which directly underlie the regionally occurring Sequence Y chalk interval. Class III anomalies are modeled and interpreted as the result of a previously undefined coarse‐grained lithofacies, bearing low saturations (2%–10%) of free gas. Co‐occurring ClassIV AVO anomalies are modeled to provide evidence for a fine‐grained upper MES, bearing similarly low saturations of free‐gas in pore space. We speculate on the gas' origin, which could be from the Hikurangi subduction margin, in situ ancient microbial activity, or a new undetermined source related to the ancient East Gondwanan subduction margin and accretionary wedge.

Keywords

• subduction margins
• biogenic gas
• thermogenic gas
• Chatham Rise
• Gondwana
• amplitude‐variations‐with‐offset