Quantifying Cretaceous–Cenozoic exhumation in the Otway Basin, southeastern Australia, using sonic transit time data: Implications for conventional and unconventional hydrocarbon prospectivity

Abstract

In prospective basins affected by exhumation, uncertainty commonly exists regarding the maximum burial depths of source, reservoir, and seal horizons. One such basin is the Otway Basin, an important gas province in southeastern Australia, which has witnessed several exhumation events. Here, we present estimates of net exhumation magnitudes for 110 onshore and offshore petroleum wells based on the sonic transit time analyses of Lower Cretaceous fluvial shales. Our results show significant post-Albian net exhumation in the eastern onshore Otway Basin (>1500 m [∼4920 ft]) and a generally minor net exhumation (<200 m [∼655 ft]) elsewhere in the Otway Basin, consistent with estimates based on thermal history data. The distribution of net exhumation magnitudes in relation to mid-Cretaceous and Neogene compressional structures indicates that exhumation was dominantly controlled by short-wavelength basin inversion driven by plate-boundary forces.Deeper burial coupled with high geothermal gradients in the onshore eastern Otway Basin and along the northern basin margin during the early Cretaceous have rendered Lower Cretaceous source rocks mostly overmature, with any remaining

Cenozoic deformation in the Otway Basin, southern Australian margin: implications for the origin and nature of post‐breakup compression at rifted margins

“…In comparison with the Gippsland Basin, the timing and distribution of Cenozoic compressional deformation in the Otway Basin is far less well‐understood. The most significant compressional structures occur onshore in the eastern Otway Basin, in and around the Otway Ranges, and in the adjacent Torquay sub‐basin (Hill et al ., ; Dickinson et al ., ; Sandiford et al ., ; Clark et al ., ; Tassone et al ., , ). Similar to the Gippsland Basin, these structures comprise a combination of ~NE–SW trending high and low‐amplitude anticlines resulting from reactivation of syn‐rift normal faults.…”

Constraining Cenozoic exhumation in the Faroe‐Shetland region using sonic transit time data

“…A thorough exhumation study ideally integrates two or more of these methods to reduce uncertainties and to better constrain exhumation magnitudes (e.g. Japsen & Chalmers, ; Corcoran & Mecklenburgh, ; Japsen et al ., ; Holford et al ., ; Tassone et al ., ).…”

“…anomalously low porosities) with respect to empirically derived baseline trends (e.g. Hillis et al ., ; Hillis, , b; Hansen, ; Japsen, , , ; Ware & Turner, ; Storvoll et al ., ; Japsen et al ., ; Holford et al ., ; Tassone et al ., ). To date, there has been no systematic application of this technique to quantify exhumation magnitudes in the FSR, despite its widespread and successful application in other basins around the British Isles (Hillis, ; Mackay & White, ) such as the North Sea Basin (Hillis, ; Japsen, , , ), Inner Moray Firth (Hillis et al ., ), West Orkney Basin (Evans, ), East Irish Sea Basin (Ware & Turner, ; Holford et al ., ) and Slyne Basin (Corcoran & Mecklenburgh, ).…”

Spatial distribution of micrometre‐scale porosity and permeability across the damage zone of a reverse‐reactivated normal fault in a tight sandstone: Insights from the Otway Basin, SE Australia

“…Moreover, although the Castle Cove Fault is not exposed at the surface, an investigation into the internal structure of the fault, for example, fault core and peripheral damage zone, would result in a better understanding of the permeability structure of the Castle Cove Fault. It is also important to note that the Eumeralla Formation is lithologically variable and is characterised by different zones of diagenetic alteration (Duddy, ) and porosity reduction as a result of over‐compaction during burial (Tassone et al, ), and therefore the permeability structure may vary considerably depending on position in stratigraphy. While this study only focusses on the upper interval of the Eumeralla Formation, our results can be used as an analogue for sandstones that are characterised by high porosities, low permeabilities, and abundant clays.…”

“…The formation is lithologically variable and is characterised by different diagenetic zones of alteration (Duddy, ). Pervasive diagenesis within the Eumeralla Formation has resulted in the significant destruction of porosity and permeability under moderate burial depths of <1,500 m (Tassone, Holford, Duddy, Green, & Hillis, ). Intraformational seals and interbedded coal seams within the lower section of the formation are the source of hydrocarbon accumulations discovered within the eastern Otway Basin (Boreham et al, ; Edwards, Struckmeyer, Bradshaw, & Skinner, ), and therefore this is an important interval for petroleum exploration.…”