Magnetotellurics for onshore petroleum exploration: a case study from the Officer Basin

Abstract

The Officer Basin, South Australia, is a sedimentary basin that contains several possible salt diapiric structures that have been partially defined from two-dimensional (2D) seismic transects. This paper reports a case study to assess the potential of using magnetotellurics (MT) and gravity data to delineate salt diapiric structures. The study aimed to develop an economical and low impact technique for greenfields exploration of basins in which hydrocarbon resources may be structurally controlled by salt tectonics. Twenty-six MT sites were deployed across two orthogonal transects (seismic lines 85-0009 and 86-0194), one that crosses a known salt structure and the other with no salt wall structure. For the line 85-0009, the depth to the top of the salt dome was 700 m with a width 2.5 km, and appears to be quite disseminated resulting in low acoustic impedance contrasts. Each 18 km line was two-dimensionally modeled to a depth of 3.5 km and evaluated against the seismically imaged basin model. We show that the salt diapiric structure is only marginally defined by the MT and gravity data, but with more sites and better quality data the resolution will undoubtedly improve. The salt structure was imaged in the same location as the seismic anomaly, and appears as a slightly more resistive body (>200 .m) compared to the porous sedimentary host (~2-200 .m). The resistivity model imaged the depth of the dipping basement, and is consistent with the gravity data. We conclude that in areas of salt structure poorly imaged by seismic methods, MT may be a significant new exploration tool to delineate potential targets.