The Northwest Offshore Otway Basin Well Folio

Keywords: biostratigraphy, hydrocarbon prospectivity, lithologies, offshore, organic geochemistry, Otway Basin, petrophysical analysis, well data.

The northwest trending Otway Basin was formed on the southern Australian passive margin as a result of multi-stage rift-sag and inversion phases throughout the Upper Jurassic and Cretaceous, followed by thermal subsidence and marine transgression during the Cenozoic. Exploration is mature onshore and relatively immature offshore. Commercial gas production and exploration drilling are active in the onshore northern part of the basin, and eastern offshore Otway Basin, within the Shipwreck Trough. In the northwest offshore part of the basin, commercial volumes of hydrocarbons have not yet been discovered where, notably, oil and gas shows are reported in some wells. The main exploration targets for the basin are the Upper Cretaceous Shipwreck Supersequence (Turonian Waarre Sandstone and Santonian Thylacine Sandstone Member), Lower Cretaceous Crayfish Supersequence (Valanginian–Hauterivian sandstones of the Pretty Hill Formation) and locally within the basal Eumeralla Supersequence (Berremian Windermere Sandstone Member, Katnook Sandstone) (Geoscience Australia 2021a).

Eleven wells (Fig. 1) are included in the ‘Northwest Offshore Otway Basin Well Folio’ forming part of Geoscience Australia’s regional seismic interpretation and well composite (Fig. 2) summary data release. These new precompetitive data further support the assessment of hydrocarbon prospectivity across the region (Geoscience Australia 2021a, 2021b). In addition to updated stratigraphic well markers and interpreted well lithologies, the Well Folio includes well logs, derivation of shale volume (V_shale), reservoir characterisation, and the integration of historic organic geochemistry and organic petrology results from cutting and core samples.

Fig. 1.  Map location of the studied wells and the 2020 new and reprocessed seismic.

Fig. 2.  Well composite for Argonaut A1, illustrating data collected and interpreted in this study, including; supersequences, well logs, lithology, estimated shale volume and reservoir unit characterisation (using >10% porosity and <40% shale volume cut-off) and organic geochemical data. Complete well composites and data for the studied wells are available in the Well Folio.

Eleven wells in the northwest offshore Otway Basin, in proximity to the regional seismic interpretation study area, are presented in this Well Folio. Furthermore, the well locations, trajectories, total depths and other well header information are summarised in Table 1.

Table 1.  Well header information of the studied wells.

The National Offshore Petroleum Information Management System (NOPIMS, https://www.ga.gov.au/nopims) is the main source for well data. Wireline logs, Logging While Drilling (LWD) logs, sidewall and conventional core lithology, along with significant oil and gas shows, are extracted and presented in this Well Folio. Biostratigraphy datasets were obtained from MGPaleo whose reports include the biostratigraphic zonations and the laboratory analysis of cuttings and sidewall core samples (MGPaleo 2020). To ensure uniformity across wells, geological names from the Australian Stratigraphic Unit Database (Geoscience Australia 2022) were used and geological names from the original data sources were replaced where needed. Organic geochemical data, namely total organic carbon (TOC) and programmed pyrolysis, along with organic petrological data, were compiled by Geoscience Australia using well completion reports and other publicly available laboratory reports.

Wireline log data are the main inputs for the petrophysical analysis in this Well Folio. The primary data were calliper (CAL), gamma ray (GR), spontaneous potential (SP), resistivity (IND, LLD, ILD), sonic (DT), density correction (DRHO), density (RHOB) and neutron porosity (TNPH). Schlumberger’s petrophysical analysis software package Techlog^TM was used to perform petrophysical analyses and to create the well composites. Gamma ray (GR) logs were primarily used to determine the shale volume (V_shale). Clean sand and shale points were picked based on GR values together with other data, where available, such as density-neutron crossplot and spontaneous potential (SP). Lithology facies information from the various well completion reports were used together with well logs for selecting sand/shale points. V_shale was calculated for shale corrections in the subsequent effective porosity estimates. Subsequently, porosity estimates were analysed from both density (RHOB) and sonic logs (DT) and compared with laboratory core porosity measurements where available. The density log porosity was typically considered to be the best fit with core porosity and was selected as the primary source for effective porosity calculations. The exceptions being the Crayfish Supersequence in Crayfish 1A, and the Shipwreck Supersequence in Argonaut A1, where sonic porosity provided better calibration and hence, are represented in the final effective porosity calculation method. Net reservoir properties, including average shale volume and average effective porosity summaries, are estimated for the deeper supersequence(s) in each well (Table 2). The net reservoir cut-off assumptions are shale volume <40% and effective porosity >10%.

Table 2.  Summary of net reservoir, average shale volume and average effective porosity over the intersected supersequences for the studied wells.

Of the three described Austral Petroleum Supersystems (Bradshaw 1993; Edwards et al. 1999; Boreham et al. 2004), the Austral 2 Lower Cretaceous fluvial and coaly facies of the Eumeralla Supersequence, and Austral 3 Upper Cretaceous to lowest Cenozoic fluvial-deltaic and marine facies mostly within the Shipwreck and Sherbrook supersequences are considered to be the most prospective source rocks in deep-water areas (Schenk et al. 2021). The Well Folio displays the pyrolysis data for each well; however, high S1, high production index (PI), and low T_max values portray that much of the data are affected by the presence of free hydrocarbons, as shown by the paler coloured bars in Fig. 2. Taking Argonaut A1 as an example, the sample data for this well may show the presence of either migrated hydrocarbons and/or drilling additives. Bearing this in mind, samples with good source potential are present within these aforementioned Cretaceous supersequences.

From this study of wells in the northwest offshore Otway, beyond the more data-rich Shipwreck Trough, the reservoir quality in the Sherbrook Supersequence appears to be fair to good in Argonaut A1, Breaksea Reef 1 ST3 and Voluta 1; although reservoir facies development and charge remains unknown in the deep-water region. The reservoir units within the Shipwreck Supersequence display fair to good net reservoir thickness (with net to gross ranging from 14.8% to 42.9%) and good average porosities (17.7–24.8%) in Argonaut A1, Breaksea Reef 1 ST3, Copa 1 and Morum 1 (Table 2). Having said this, net reservoir development in the Shipwreck Supersequence is low in Voluta 1 (1% net to gross; Table 2) and non-existent in Triton 1 ST1 which intersected tight sandstones/siltstones at the bottom of the well. Average porosity within sandstones in the Eumeralla Supersequence are relatively high (18.8–21.3%) in Chama 1A, Sophia Jane 1 and Troas 1 ST, but with general low net to gross sandstone development in all but Chama 1A which is high at 45.7% (Table 2). Reservoir development within the Crayfish Supersequence (Pretty Hill Formation) is good in Crayfish 1A, Trumpet 1 and Sophia Jane 1 with 17.8%, 18.3% and 23.1% average porosity, respectively, but poor in Troas 1 ST1 (6.9% net to gross and 13.8% average porosity). Although Chama 1A intersected 37 m of Crayfish Supersequence at the base of the well, the interval is majority siltstone with zero net reservoir.

The Northwest Offshore Otway Well Folio presents data from 11 wells enabling the assessment of hydrocarbon prospectivity across this region. The new sequence picks and better understanding of the lithologies and depositional settings will provide valuable input into constraining the regional mapping. It provides well composite logs, organic geochemical data, sequence stratigraphic markers and petrophysical evaluations that form the basis for further studies into reservoir and seal quality along with source rock potential across the northwest region of the Otway Basin.

The Northwest Offshore Otway Basin Well Folio including well composites and data package for key wells will be available on the Geoscience Australia’s Data & Publications website: http://www.ga.gov.au/data-pubs.

The data that support this study are available in the National Offshore Petroleum Information Management System (NOPIMS) at http://www.ga.gov.au/nopims.

No conflicts of interest exist between these authors and any other person or organisation.

No funding from external organisations was received for this research.