Potential field interpretation and modelling in the deep-water Otway Basin

Keywords: deep-water, gravity, high-pass filter, hydrocarbon prospectivity, igneous rocks, magnetic modelling, magnetics, Otway Basin, tilt angle, wells.

The Otway Basin is a broadly NW–SE trending basin and is part of a rift system that developed along Australia’s southern margin during the Late Jurassic to Late Cretaceous (Moore et al. 2000). Unlike the onshore and shallow offshore parts, with hydrocarbon discoveries, the deep-water part is underexplored with sparse 2D seismic lines and no wells. The 2020 Otway Basin Seismic Program (OBSP) (Karvelas et al. 2021; Fig. 1) acquired 8200 line-km of 2D seismic, gravity and magnetic data, and had the key objective of improving the quantity and quality of data over the deep-water parts of the offshore basin for regional interpretation and basin analysis.

Fig. 1.  Map of the Otway Basin showing the location of petroleum wells and the Otway Basin Seismic Program seismic lines along which gravity and magnetic data were also acquired. The background image is the swath bathymetry (from the OBSP) and public-domain topography.

Geoscience Australia has examined the deep-water Otway Basin as a potential area for hydrocarbon exploration. The work involves an integrated approach to build a better understanding of the deep-water part of the basin, its prospectivity and the identification of new hydrocarbon plays.

As part of this basin study, the newly acquired magnetic and gravity data have been processed and integrated with existing open-file data. Moho and lithosphere surfaces have been derived from the data. Comparisons of basement interpretations derived from the potential field data, including major structures, with those derived from the seismic data (Nicholson et al. 2022) have been made. The data have also been used to guide the mapping of igneous rocks and their extent using seismic data. The outcomes of this work are expected to have a significant impact on updated petroleum systems modelling results later in the study.

Potential field data is a cost-effective means to providing additional constraints on important characteristics of sedimentary basins, including: sediment thickness, fault architecture, distribution of igneous rocks and basement composition. Tilt-angle filtering of magnetic data (Fig. 2) shows narrow subtle anomalies that correlate with igneous rocks in the onshore Otway Basin. This correlation suggests that the tilt angle of magnetic data may help map the unexposed igneous features offshore. Positive gravity anomalies, as seen on the high-pass 150 km filtered data (Fig. 3), correlate well with basement highs identified through seismic interpretation in the outer margins of the basin. This correlation suggests that the enhanced gravity data is a useful tool to map structural elements within the deep-water area.

Fig. 2.  Tilt derivative image of the merged public-domain and magnetic data acquired during the Otway Basin Seismic Program. The coastline is shown in black for reference. The cyan line is the outline of the Otway Basin. Note the location of the onshore Casterton 1 well which correlates with a positive magnetic anomaly and where igneous rocks were intercepted at 2400 m depth. This correlation onshore can be used to guide exploration wells offshore (on the flanks of magnetic highs – yellow ovals) and help map more igneous rocks.

Fig. 3.  This image shows the 150 km high-pass filter of the merged gravity grid overlain by polygons representing outer margin basement highs (large yellow polygons) derived from the interpretation of seismic data. Note the correlation between the outer margin basement highs and the large positive gravity anomalies. The brown labelled polygons are the major structural elements of the Otway Basin (Nicholson et al. 2022).

As part of the OBSP survey, gravity and magnetic data were acquired along all 2D seismic lines (Fig. 1) by PartnerPlast on behalf of SeaBird Exploration for Schlumberger Australia Pty Ltd. These data were processed to produce the potential field final grids provided with the seismic survey, both of which are available from www.nopims.gov.au under ENO0603786. The magnetic data were merged with public-domain survey data (Meyer et al. 2017) to produce seamless magnetic maps of the combined onshore and offshore parts of the Otway Basin. Similarly, the gravity data were merged with public-domain satellite data (Sandwell and Smith 2009) to produce gravity images. The integration of data from the nearshore to the deep-water basin is crucial as resolution of interpretation and well control inboard are necessary constraining factors for basin interpretation.

The merged gravity and magnetic datasets were further enhanced to better highlight some buried features. Various filters were applied, including tilt angle, first vertical derivatives, upward continuation, analytic signal, high-pass and low-pass filters. Some of these filters and the resultant grid interpretation are described below.

The tilt angle is a quantity derived from the ratio of vertical to total horizontal derivative and is defined by Miller and Singh (1994) as:

The tilt angle tends to normalise anomaly amplitudes and is positive over vertically sided anomaly sources. Therefore, positive tilt angles can be used to help identify source bodies irrespective of magnetisation direction (Morse 2010). Tilt angle for the merged magnetic grid of the Otway Basin is shown in Fig. 2. The image shows some narrow subtle magnetic highs that correlate with the occurrence of igneous rocks identified in wells in the onshore Otway Basin. For example, 25 m of igneous rocks were intersected in Casterton-1 at 2400 m depth. Little can be said about this correlation in the offshore due to the absence of exploration wells. These positive magnetic anomalies in the inshore may not have been drilled deep enough to penetrate any igneous rocks. A similar correlation between the magnetic anomalies and the occurrence of igneous rocks in wells was made in the Browse Basin on Australia’s North West Shelf (Hackney et al. 2015). The correlation between the magnetic tilt anomalies and the igneous rocks onshore suggests that this magnetic filter may be used to map the distribution of such rocks in the deep-water Otway Basin.

Fig. 3 shows a 150 km high-pass filter of the combined gravity data over the Otway Basin. This filter emphasises anomalies with wavelengths less than 150 km and is useful to map the Continent–Ocean Boundary (COB) and areas of basement high or exhumed mantle, within the basin. It should be noted that the new gravity data has higher resolution than the satellite-derived data. Combining the two datasets is useful for a regional study, but there is a risk of losing some valuable information contained in the higher-resolution data.

The interpretation of seismic lines has identified some basement highs in the outer margins of the basin (Nicholson et al. 2022). These highs correlate well with the positive gravity anomalies as seen on the high-pass 150 km filtered data. This correlation suggests that the enhanced gravity data is a good tool to use for mapping major structural elements spatially across the deep-water Otway Basin and is valuable in resolving structural trends across the seismic coverage.

Seismic line P1043 (OBSP) at the southeast end of the Otway Basin (Fig. 1) was selected to test the identification of the extent of igneous rocks. In this area, Schenk et al. (2021) identified some isolated igneous intrusions, less than 5 km in width, in the Upper Cretaceous sequence. Using ModelVision Pro, the magnetic anomalies along this line are broad and suggest deep-seated bodies at depths between 12 and 19 km. Modelling is progressing on seismic lines in other areas of the basin to enable more accurate mapping of the location and extent of igneous features.

The new gravity and magnetic data, acquired during the 2020 Otway Basin Seismic Program were processed and integrated with existing open-file data to produce a merged dataset for the onshore and offshore Otway Basin. Enhancement processing of the data provides valuable information on basement structure and the possible distribution of igneous rocks. For example, tilt angle of the magnetic data shows narrow subtle magnetic highs that correlate with igneous rocks identified in wells in the onshore Otway Basin. This correlation suggests that the tilt filter grid can be used to map the distribution of such rocks in the deep-water areas. The newly acquired seismic data reveal basement highs that correlate well with the positive gravity anomalies evident in the high-pass 150 km filtered data. This correlation suggests that the enhanced gravity data is a good tool to use for mapping structural elements within the deep-water areas. Magnetic data modelling is in progress along the seismic lines to help locate potential intrusive rocks.

All the original seismic, gravity and magnetic data from the 2020 OBSP used in this study are available for download from the National Offshore Petroleum Information System (www.nopims.gov.au) under ENO0603786.

All authors confirm there are no conflicts of interest.

No funding from external organisations was received for this research.