New Zealand Miocene Tubular Carbonate Concretions as Evidence of Methane Hydrate Dissociation: A Schematic Model for the Subsurface Plumbing System of Hydrocarbon Seeps

S. L. Nyman¹, C. S. Nelson¹, and K. A. Campbell^2
1University of Waikato
²University of Auckland

Widespread occurrences of tubular carbonate concretions in Cenozoic slope mudstones in New Zealand are interpreted to represent the subsurface plumbing pathways of methane expulsion in hydrocarbon seep systems. The diverse geologic characteristics of the tubular concretions provide a unique opportunity to construct a comprehensive 4-D model of the subsurface development of such a system.

The tubular concretions are carbonate cemented mudstone (50-85% carbonate), and support near-central conduits. They display varying morphologic, mineralogic, petrographic and geochemical characteristics. Additionally, some examples display association with slope instability, faulting, and ancient seafloor seep carbonates.

δ¹³C values of the cement range from –52 to 13‰ PDB and are interpreted to reflect carbonate precipitation from the onset of methane migration to the end of a major fluid migration event. The trend from strongly negative to strongly positive δ¹³C values reflects extensive anaerobic oxidation of methane (AOM), which is confirmed by lipid biomarker evidence. δ¹⁸O values range from -3 to 5‰ PDB suggesting an evolved fluid source influenced by cycles of methane hydrate formation and dissociation.

The resulting schematic model of tubular concretion formation is an analogue for the subsurface fluid migration system of hydrocarbon seeps along the modern Hikurangi Margin off eastern North Island and possibly for other modern and ancient hydrocarbon seep systems. Additionally, they provide some spatial and temporal insight into hydrocarbon migration and gas hydrate formation/dissociation, which could assist in the evaluation of future hydrocarbon resources and any potential hazards associated with their recovery.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas