Abstract
Gas hydrate is a large potential unconventional energy resource. However, uncertain-ties still remain about gas hydrate system dynamics, including the processes that stimulate growth, development and change within a gas hydrate system. Two contrasting seismic surveys, PEG09 and HKS02, were used to investigate a complex gas hydrate system within the Uruti Basin, SE of the Wairarapa Coast on the upper slope of New Zealand’s Hiku- rangi Margin. The PEG09 survey was conducted in 2009 and 2010 by the New Zealand government and consisted of an acquisition system that included three Bolt APG 8500s airguns and a 10,000-m-long 800-channel streamer. The HKS02 survey was conducted in June 2015 aboard the RV Roger Revelle during voyage RR1508 and incorporated an acqui- sition system that included two 45/90 G/I airguns and a 600-m-long 48-channel streamer. The PEG09 and HKS02 datasets are used to (1) delineate gas hydrate system dynamics in the Uruti Basin, (2) evaluate the economic potential of the Uruti Basin and (3) compare and contrast the seismic response between the two surveys.
A model for gas hydrate system dynamics within the Uruti Basin was built using seismic interpretation methods, high-density velocity analysis, rock physics and seismically interpreted heat flow. The basic structure and stratigraphy within the Uruti Basin is inferred to make up a framework for fluid flow. This framework when combined with active fluid flow, controls the distribution of gas, water and heat throughout the basin which in turn controls the stability and emplacement of gas hydrate. Fluid within the Uruti Basin is inferred to be sourced from the Hikurangi Subduction Zone via the Opouawe-Uruti Fault and associated faults. Upward migration of warm fluids through the deeper fault system causes heat flow anomalies as high as 47.5 mW/m2, locally destabilising gas hydrate at the termination of fault conduits. In the Uruti Basin, fluid is transferred from fault conduits into permeable layers. These permeable layers are inferred to be sand dominated units that are confined between mud dominated layers within a turbidite sequence. Fluids migrate through permeable layers and: (1) accumulate within small structural traps such as anticlines, (2) accumulate beneath BSR upwelling where the base of gas hydrate stability cross cuts dipping stratigraphy, or (3) further migrate upwards within dipping strata. The quantity of free gas hosted within these features was estimated using p-wave velocity for the corresponding numbered feature to be (1) 0.2x109 to 2.9x109 m3, (2) 0.3x109 to 4.4x109 m3 and (3) 3.7x109 to 54.0x109 m3. Dipping converging sediments effectively focus fluid upwards into the shallow SE portion of the basin. Here fluids accumulate against low-permeability ridge sediments until they either (1) migrate to the seafloor through extensional faults or (2) reach supra-lithostatic pressure and form a chimney through hydrostatic fracturing. Either way this results in fluid escape and a “leaky” hydrocarbon system potentially limiting the economic potential of the basin. Three reservoirs within the Uruti Basin are inferred to host concentrated gas hydrate accumulations. The largest of these accumulations was estimated using p-wave velocity to contain around 3.0x109 m3 of free gas at standard conditions. Concentrated gas hydrate is suggested to form from gas entering the regional gas hydrate stability zone through dipping sediment conduits. A short-term temperature anomaly is speculated to be the primary mode allowing temporary free gas stability within the gas hydrate stability zone.
A comparison of the two surveys shows that the PEG09 dataset provided deeper pene- tration and sufficiently detected concentrated gas hydrate accumulations. In contrast, the higher-resolution provided by the HKS02 dataset allowed gas hydrate system dynamics to be analysed near bed scale. In the future; 3D seismic surveys, drilling and higher- frequency surveys over similar gas hydrate systems would enable comprehensive resource assessment and would allow models for gas hydrate system dynamics to be developed further at higher-levels of certainty.