The application of high resolution aeromagnetic surveys to petroleum exploration in the Western Otway Basin

Abstract

h 1993 the South Australian Department of Mines and Energy (MESA) commissioned a high resolution aeromagnetic survey over part of the Western Otway Basin (SAEI region, P1). The survey area consisted of a section along the S.A./Vic. border, approximately 25 krrr wide, from north of Penola to offshore south of Mt. Garnbier (-3'7" 15' S to -38' 10' S and 140" 40' E to i41' E). The survey was intended as a test case to determine whether a high resolution survey (400 m line spacing and 60 m flight height) would permit the detection of weakly magnetic, intersedimentary horizons and allow the mapping of magnetic lineations that might correspond to faults and shear zones, thus assisting in the detection of structural traps that may contain significant reservoirs of hydrocarbons. The economic potential of the study area is demonstrated by recent gas discoveries. The Otway Basin formed as a result of the separation of Australia and Anta¡ctica with rifting commencing in the late Jurassic. In the northern part of the study area, the Penola Trough was the initial deposition centre which was filled by the Early Cretaceous Crayfuh Group. During the Aptian/Albian, volcanogenic sediments of the Eumeralla Formation covered both the Penola Trough and adjacent basement higts. In the late Cretaceous the active spreading centre moved to the south with the result that the Sherbrook Group and overlying Tertiary sequences are much thicker in the south than in the Penola Trough. Volcanism occurred during the initial rifting stage, during the Cretaceous and early Tertiary, and during the PleistoceneÆIolocene. Theoretical studies in part of the Penola Trough between Penola 1 and the Katnook gas field were based on models developed from seismic evidence and susceptibility measurements from cores. From these studies, it was found that only faults or offset magnetic horizons within the Sherbrook Group and, especially, the Eumeralla Formation are likely to produce detectable anomalies at the flight altitude used in the survey. The main magnetic features of the survey area result from bodies within or at the top of the Palaeozoic basement, with the exception of the Mt. Garnbier and Mt. Schank volcanic cones as well as an anomaly near Hungerford 1 believed to be a shallow, blind basaltic plug. The depths to the top of the basement bodies, both within the survey area and adjacent to it, were derived using spectral analysis. There are a number of subtle linear magnetic features in the Penola Trough and offshore that are the result of sources within the sedimentary section. In the Penola Trough, and especially southeast of Penola, there are a series of low amplitude, linear anomalies (1 to 3 nT) striking west-northwest to east-southeast which are the result of magnetic sources within the Eumeralla Formation (200-600rn deep). The offshore anomalies appear to be produced by magnetic sources located mostly within Early Tertiary sediments (500 m to 1 km deep). Three models were created from the seismic line OK90-414 which is approximately perpendicular to the East Penola Trough linear anomalies discussed above. These models consisted ol (1) Thin igneous dykes mostly emplaced along the prominent fault planes within the Eumeralla Formation, (2) Thin horizontal layers or shallow charurel fill at or near the top of the Eumeralla Formation with some layers in the overþing Sherbrook Group and Tertiary sediments, and (3) Tnnes of variable magnetisation associated with the major Eumeralla Formation faults. The igneous dyke model seeÍrs to be geologically less likely than the other two models. Measurements from cores have revealed a moderate vertical variation in susceptibility throughout the Eumeralla Formation but the layer and zone models require a similar lateral variation. This study shows that an aeromagnetic survey with these specifications can distinguish the relatively small magnetisation contrasts found in sedimentary basins. Some of the magnetic lineations studied a¡e the result of magnetic horizons being offset by faults and shear zones, but most appea.r to result from demagnetisation of sediments in the vicinity of fault zones. There are two examples of magnetic highs over fault zones suggesting that there may be some process generating magnetic minerals in these areas.