Publication Name: Eastern Australian Basins Symposium 2001
Authors: M. Lennane, R. Weeden and J. Cant
Date Published: November 2001
Number of Pages: 29
Reference Type: Magazine Article
Abstract:
The Shipwreck Trough (Figure 1) in the Victorian offshore Otway Basin is the southern extension of the onshore Port Campbell Embayment and the fairway for the gas fields occurring in the Late Cretaceous Waarre Formation. The Waarre Formation reservoir sand is predominantly fluvial onshore and marginal marine in a deltaic setting in the offshore well, LaBella-I. The overlying Belfast Mudstone is the regional top and lateral seal. The structural fabric of the Shipwreck Trough is characterised by northwest trending en-echelon faults which appear to extend from two to ten kilometres in length with variable throws up to 700 m. Structural traps are set up where the northwest trending faults are limited by intersecting northeast fault sets and typically have large extensional fault throw in the southwest dip direction with variable rollover or fault throw in the northwest -southeast strike direction.Petrophysical evaluation for seismic attribute prediction was undertaken on three key wells, LaBella-1 and Minerva-2A (both gas discoveries) and Conan-1 (a dry well), to determine which combination of seismic attributes would be most diagnostic for reservoir quality and fluid content prediction away from the well control.
LaBella-1 , drilled in 1993, was the first significant gas discovery in the offshore Victorian Otway Basin where a gross gas column of 68 metres was intersected in the Waarre Formation. LaBella-1 is shown on seismic line OH91-149R (Figure 2) where the top Waarre reservoir is at approximately 1.5 seconds TWT. Synthetic gathers at LaBella-1 (Figure 3) show an amplitude versus offset (AVO) response at both the upper and ower boundaries of the gas-bearing Waarre reservoir. However only the lower ngas-bearing unit appears to give an AVO response when the seismic gathers at LaBella-1 (Figure 3) are correlated with the synthetic gathers. Brine substitution in the petrophysical model did not show an AVO response on the synthetic gathers.
Minerva-2A, drilled on the southernmost fault block in the Minerva gas field intersected a gross gas column of 111 metres in the Waarre Formation. Synthetic gathers generated from petrophysical models at Minerva-2A (Figure 4) clearly indicate a strong AVO response.
Conan-1 was a dry well drilled on a valid structural closure with seismic line OH94-246R (Figure 5) indicating an amplitude response within the Waarre Formation at approximately 1.4 seconds TWT. Conan-1 intersected fifty metres of reservoir quality Waarre Formation sand with two thin calcite cemented beds in the basal section of the unit (Figure 6). The seismic gathers at the Conan-1 well location show an AVO response that is directly comparable to the modelled synthetic gather with an AVO response for the brine-filled reservoir containing the thin cemented beds (Figure 6). Conan-1 shows that an AVO response can be produced from a reservoir with no hydrocarbons.
The problem of differentiation between an AVO response due to gas and AVO response due to lithology is llustrated by evaluation of the Casino Prospect (shown on seismic line OH94-225R, Figure 7) using a modelling approach.
Fluid substitution at Conan-1 shows the brine-filled reservoir model with the cemented beds AVO response has a different character to the AVO response from the same model with gas fill (Figure 8). Synthetic gathers were also generated using the Conan-1 reservoir but without the thin cemented beds. The model with brine fill did not show an AVO response. The same model with gas fill produced a strong AVO response with a different character directly comparable to the AVO response seen in seismic gathers over the Casino Prospect (Figure 8). This suggests gas in the Waarre Formation. The well and prospect AVO responses at the top Waarre reservoir are summarised in Table 1.
Integration of geology, petrophysics and geophysics at the well locations has resulted in a methodology to allow
discrimination between 'true' and 'false' AVO anomalies on seismic away from well control. The use of amplitude versus offset analysis can detect gas in the Shipwreck Trough.