A Marine CSEM Study of Methane Hydrates at Pockmark Field CN03 Nyegga, Mid-Offshore Norwegian Sea

Eric Attias

The Nyegga Region is situated at the mid-Norwegian continental margin at a water depth between 700 and 800 m, on the northern flank of the Storegga slide, at the southeast part of the Voring plateau. Several recurring rifting episodes and continental break up until the Late Palaeocene/Early Eocene have formed this region. The Nyegga Region contains numerous complex chimney pockmarks that formed due to fluid migration from overpressure and is assumed to accommodate large amounts of gas hydrate, with a low saturation state and a large lateral spread. This field has a prominent bottom-simulating reflector (BSR). The pipe structure pockmark CNO3 in the Nyegga region features a central crater. It is characterized by high hydrate concentration and there is evidence that it may still be discharging methane into the water column. It is, not clear how free gas and gas hydrate can coexist within the gas hydrate stability zone, however, several hypotheses have been proposed including temperature anomalies, salinity anomalies, and a water starved model. The concurrent seismic and electromagnetic data distinguish between high temperature anomalies, (low resistivity anomaly), presence of highly saline water (low resistivity anomaly) and hydrate (high resistivity and high p-wave anomaly) and gas distribution (high resistivity and low p wave anomaly). In May 2012, an extensive controlled-source electromagnetic (CSEM) experiment was carried out at CNO3 aiming to investigate the hydrate and free gas distribution in the top 300 m of sediments in this region. This data set will complement existing seismic data sets and will be used to quantify the amount of gas and methane hydrate in the chimney and its immediate surroundings, as well as studying the methane hydrate layers at depth over a larger regional scale. A component of the CSEM survey was conducted using the University of Southampton’s CSEM system, which included three types of equipment. A deep-towed Active Source Instrument (DASI) transmitter provided frequency domain signals that were recorded by fixed sea-bottom receivers (LEMURs) and also by the deep-towed VULCAN instrument. Eight LEMUR receivers were deployed in a pattern extending up to 1.5 km from the central fluid escape feature. Four were deployed within the central zone. The other four were placed at 1.5 km distance. The DASI system with Vulcan was then towed along each 4 survey lines. Here we will present preliminary pseudosections of the CSEM data and begin to interpret the results in light of the extensive geophysical and geochemical data sets that exist in this region.

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