Seismic Detection of Permafrost Ice, Gas Hydrates and Conventional Hydrocarbons across Arctic CA

Christopher Harrison

Deep terrestrial PFI extent is a proxy to estimate geotherms and constrain deeper seismic indicators to infer GH and deeper gas. Most Sverdrup Basin islands, have distinct seismic reflectors at the base PFI within thick porous sandstones (Devonian, Triassic and Jurassic), where maximum inferred PFI thickness is 974 m on Isle Vanier. In shale dominated bedrock, the base of PFI may be shown by amplitude decay on dipping reflectors of encased thin porous beds. Wells provide sonic, resistivity and temperature data confirming the PFI inferences. Lack of PFI offshore produces a seismic time shift at shorelines up to 190ms. GH seismic indicators occur deeper and an example on Ellef Ringnes Island infers gas hydrate and deeper free gas from observing 1.) base PFI reflector within 350m thick porous Hassel Fm (Cretaceous). 2) Amplitude decay on dipping intra-Hassel Fm. reflector indicating a GH/water level and 3) a basal Hassel dipping reflector showing amplitude/polarity change, beneath the inferred GH stability zone, interpreted to be a conventional gas/water DHI in porous media. Regional well data of frozen and unfrozen velocities allow depth conversion of levels and a GH stability zone to be inferred. Other GH inferences are on Southwest Ellesmere Island in Devonian clastic strata seen as base PFI and a deeper base GH indicator. Seismic on the Drake point gas field of Melville Island shows amplitude decay of the event at reservoir level as it dips to the north. This DHI location is interpreted to indicate the limit of conventional gas. An example of strong anomalous seismic amplitude is observed in the Awingak Fm. (Late Jurassic) on seismic shot on the sea ice offshore western Mackenzie King Island. A Beaufort Sea seismic example traverses a linear series of moat-surrounded mound features at sea bed (Gary Knolls) thought to be fluid/gas escape features. The geothermal setting along the profile provides a thickness of relict submarine PFI ranging from over 300 m to 0. The profile exhibits deeper fault-bound amplitude anomalies interpreted as conventional gas reservoirs. Below sea bed a variety of features including seismic amplitude variation, reflector time sag and vertical continuity disruptions are interpreted to be shallow free gas or relict PFI, velocity effect of thinned or absent PFI, and vertical gas conduits (chimneys), respectively. An example of seismic data in Baffin Bay near the Scott Inlet oil seep indicates deeper anomalous amplitude, shallow cross-cutting seismic events, consistent with conventional DHI and GH indicators.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013