A Systematic Fault Seal Evaluation of the Ladbroke Grove and Pyrus Traps of the Penola Trough, Otway Basin

Abstract

Juxtaposition mapping of lithology onto the Ladbroke Grove Fault plane shows that the Pretty Hill Sandstone reservoir, which hosts a 90 m gas column, juxtaposes massive shale units in the hangingwall. Retention of the column at Ladbroke Grove can thus be attributed to favourable across-fault, reservoir-seal juxtaposition. The free water level (FWL) of the Ladbroke Grove column coincides with an abrupt change in strike of the fault from east–west to northwest–southeast. Fault re-activation risking using the FAST (Fault Analysis Seals Technology) technique indicates that the northwest–southeast striking segment of the fault is critically oriented within the in-situ stress field for reactivation, whereas the more east–west trending segment is associated with a relatively lower risk of fault re-activation. Hence recent slip along the northwest–southeast segment may have created permeable fracture networks along this part of the fault plane and thus limited the extent of the column to that bounded by the east–west trending fault segment. This hypothesis is supported by data on soil gases acquired across the fault which suggest that the fault is leaking CO2 across its northwest–southeast striking segment, but not across its east–west striking segment.The Pyrus Fault is not presently sealing by across-fault, reservoir-seal juxtaposition. The throw on the fault plane is sufficient to juxtapose the Katnook Sandstone in the hangingwall against the Pretty Hill Sandstone reservoir in the footwall, providing a sand-on-sand juxtaposition leak point at the structural apex of the trap. Fault re-activation along this fault is likely to have caused fracturing of any shale gouge veneer that may have been present along this sand-on-sand contact resulting in across-fault leakage of hydrocarbons into the Katnook Sandstone and leakage up the fault along permeable fracture networks. FAST predictions of fault re-activation show that the fault is critically oriented within the in-situ stress field for re-activation and soil gas measurements at the surface suggest the fault is leaking CO2.

Remote sensing of subsurface fractures in the Otway Basin, South Australia

“…A total of 470 BBOs and 53 DITFs were identified and ranked according to the World Stress Map (WSM) Project quality criteria (Table ) [ Heidbach et al ., ]. This is consistent with previous studies that have reported a σ H orientation of ~125°N [ Hillis et al ., ; Lyon et al ., , ; Nelson et al ., ]. Previous studies have also placed the Otway Basin within a strike‐slip stress regime [ Jones et al ., ; Nelson et al ., ; Tassone et al ., 2011] (Table ) and have suggested that this stress regime is broadly consistent over both the Victorian and South Australian parts of the Otway Basin [ Nelson et al ., ].…”

Fault reactivation in the Port Campbell Embayment with respect to carbon dioxide sequestration, Otway Basin, Australia

“…Previous soil-gas studies over hydraulically conductive faults above CO 2 sources in both the western Otway Basin and along the San Andreas fault have detected elevated levels of CO 2 in the soil gasses (e.g. Lewicki et al 2003;Lyon et al 2005). Hence elevated levels of CO 2 over the Boggy Creek fault can verify whether the fault is acting as a conduit for fluid flow and thus the applicability of geomechanical risking for prediction of fault leakage.…”

“…The Naylor structure is a fault-bound trap formed during the development of the passive margin of southeastern Australia. Previous studies show that the present-day stress field in the Otway Basin has evolved from the normal faulting regime associated with passive margin development during the Jurassic to Cretaceous to a strike-slip or reverse regime (Jones et al 2000;Lyon et al 2005;Nelson et al 2006).…”

Fault seal modelling – the influence of fluid properties on fault sealing capacity in hydrocarbon and CO 2 systems

“…The Katnook field is stratigraphically lower, located in 194 Pretty Hill Formation of 2 -4.5 km thickness, within the Pretty Hill Sandstone. The 195 main target reservoir is the Pretty Hill Sandstone member at the top of the sequence 196 (Lyon et al, 2005). The formation consists of massive, slumped and cross-bedded 197 sand packages, classified as lith-arenites to feldspathic lith-arenites (Little and 198 Phillips, 1995).…”

“…2d). 234 235 based on thermal maturation modelling and the relationship between GWC positions 251 above spill points and known gas diffusion rates (Lyon et al, 2005). Early oil/wet gas 252 charge was flushed or diluted by later dry gas charge (Boreham et al, 2004).…”