Magnetotelluric study of Howley Basin, Newfoundland

Abstract

The aim of this study is to develop a better understanding of the Howley Basin and its hydrocarbon potential using a magnetotellurics (MT) survey. This sedimentary basin is located in western Newfoundland, and is covered by the Howley formation which is only found in this basin. Very little is know about the basin and its structure. The Rocky Brook formation, which consists of organic rich mudstone that is known for generating hydrocarbons, is found in the adjacent Cormack Basin and is believed to be present in the Howley Basin as well. The MT method is a passive electromagnetic method that is sensitive to the electrical conductivity structure of the subsurface. The source for MT is completely natural, resulting from the flow of charged particles in the ionosphere (e.g. from solar storms) and from lightning strikes. These cause natural variations in the Earth’s magnetic field which in turn result in induced currents in the subsurface. MT sounding of an 18 km long profile was conducted in August and September 2013, crossing from the Topsail Igneous Complex formation into the Howley formation at about 3 km from the southeast end of the survey and extending to the anticipated location of the boundary between the Howley and Cormack Basins in the northwest. The Howley Basin is characterized by higher conductivities than the surrounding igneous basement. Strike analysis reveals the basin has a preferred geo-electric orientation of 9⁰ with the deeper structures having a preferred strike of 33⁰. A range of 2D inversions were performed for both strikes. TM mode only inversion yielded the best results with misfit (least square fit to the observed impedances) of 3.9 for the strike of 9⁰ and 2.4 for the strike orientation of 33⁰. The MT data suggest a maximum thickness of the basin along the profile of approximately 2 km and that the basin is decreasing in thickness towards its eastern boundary. There are also indications of a conductive feature or features extending beneath the basin, possibly related to faulting through the basin and into the basement or of a step in the base of the basin.