Understanding the critical success factors determining prospectivity—Otway Basin, Victoria

Abstract

The critical success factors that control hydrocarbon prospectivity in the Otway Basin have been investigated using petroleum systems approaches. It have revealed that greater than 99% of the discovered hydrocarbons in the Victorian Otway Basin have been sourced from Austral 2 (Albian-Aptian) source rocks and that these Austral 2-sourced hydrocarbon accumulations either directly overlie—or are located within 3,000 m—of actively generating Austral 2 source rock kitchens. Importantly, the zones of greatest prospectivity are located where these source rocks have been actively generating and expelling hydrocarbons throughout the Late Paleogene, primarily as a result of sediment loading associated with progradation of the Heytesbury shelfal carbonates. This peak generation window occurs at an average depth of approximately 2,500–3,500 m sub-mud across much of the basin, which has allowed prospective hydrocarbon fairways to be mapped out, thereby highlighting areas of greatest prospectivity. The close spatial proximity of the actively generating source rocks to the accumulations is due to several factors, which include overall poor fault seal in the basin (success cases occur where charge rate exceeds leakage rate) and relatively complex and tortuous migration fairways (which means that large volumes of hydrocarbons are only focussed and migrate for relatively short distances). In areas within which the Austral 2 system comprises the sole hydrocarbon charge—such as across the inner Mussel Platform—the reservoired gas compositions are typically very dry. In contrast, the gas compositions in accumulations sited along or immediately inboard of the Mussel-Tartwaup Fault Zone (La Bella, Geographe and Thylacine) are significantly wetter and also have higher CO2 contents. Throughout this area, the wetter components of the reservoired hydrocarbon inventory may have a source contribution from within the basal (Turonian) part of the younger Austral 3 system, in sequences that have been confirmed by δTLogR analysis to be significantly enriched in total organic carbon content. This observation has significantly upgraded the potential of the upper shelf areas, where a relatively more liquids-rich hydrocarbon inventory might be expected. The CO2 in accumulations located along the Mussel-Tartwaup Fault Zone is interpreted, based upon new helium isotope data, to be of mixed deep crustal-magmatic origin. This CO2 is believed to have migrated from great depth up the crustal-scale fault arrays into the shallower Late Cretaceous reservoirs. Here, the CO2 mixed with crustal gases, typified by helium with a mixed magmatic-crustal isotopic signature. Throughout this area, the traps tend to be large and hence—even though their CO2 contents are only 8–12%—the total CO2 volumes contained in these accumulations are much greater than those in the very CO2-rich—but volumetrically small traps—located onshore (e.g. Boggy Creek). Hydrocarbon accumulations located on the inner shelf, such as Minerva and Casino, have distinctly lower CO2 contents, perhaps because large displacement, through-going faults are lacking in this area. These observations collectively provide a predictive regional framework for understanding the likely distribution of commercial hydrocarbon accumulations in the offshore Otway Basin, as well as for forecasting the gas wetnesses and CO2 contents of undrilled exploration targets in both well-explored and frontier parts of the basin.

Overpressures in the central Otway Basin: the result of rapid Pliocene–Recent sedimentation?

“…maturation of hydrocarbons) or vertical transfer, rather than, or in combination with, disequilibrium compaction (Bowers, 1995;Teige et al, 2007). This conclusion is further supported by the oil leg interpreted to exist within Breaksea Reef-1 and a weak hydrocarbon (gas) show from the Waarre Formation in Normanby-1 (Geary & Read, 1998;O'Brien et al, 2009). Hydrocarbons in both cases are thought to be sourced from Aptian to Albian fluvial and coaly facies of the Eumeralla Formation, that are assigned to the Austral 2 petroleum sub-system (Bradshaw, 1993;Summons et al, 1998).…”

“…the Flaxman and Waarre formations) contains significant enrichment in total organic carbon (TOC) compared to that observed from conventional down-hole analytical TOC data. This has led to the suggestion that a late-Cretaceous fluvio-deltaic facies from the Austral 3 petroleum sub-system (Bradshaw, 1993;Summons et al, 1998) has the potential to generate hydrocarbons in the Otway Basin (O'Brien et al, 2009). No previous discoveries in the Otway Basin have been attributed to an Austral 3 source, though temporally equivalent stratigraphic intervals in the Gippsland Basin (i.e.…”

“…Recent geochemical analyses have found that the gas composition in wells along, or immediately inboard of the Mussel-Tartwaup Fault Zone (Fig. 1), such as La Bella-1, contain significantly wetter components to the dominant Austral 2 reservoired hydrocarbon inventory in comparison to the drier, completely Austral 2 hydrocarbon accumulations in the Port Campbell region (O'Brien et al, 2009). Preliminary results from δTLogR analyses in La Bella-1 have shown that the Turonian interval (i.e.…”

“…No previous discoveries in the Otway Basin have been attributed to an Austral 3 source, though temporally equivalent stratigraphic intervals in the Gippsland Basin (i.e. the Upper Cretaceous to Lower Palaeocene Latrobe Group) are widely recognised as the major source interval in that region (O'Brien et al, 2009). O'Brien et al (2009) suggest that this wetter, geochemical distinction in the La Bella-1 reservoired hydrocarbon inventory, may indicate a source within the basal (Turonian) part of the Austral 3 sub-system.…”

“…the Upper Cretaceous to Lower Palaeocene Latrobe Group) are widely recognised as the major source interval in that region (O'Brien et al, 2009). O'Brien et al (2009) suggest that this wetter, geochemical distinction in the La Bella-1 reservoired hydrocarbon inventory, may indicate a source within the basal (Turonian) part of the Austral 3 sub-system. If TOC enriched, low permeability shales in the Waarre Formation are indeed making some contribution to the hydrocarbon accumulation at La Bella-1, this may further support the notion that the Eaton (1972) method over-predicts pore pressure in the low permeability, unloaded, Waarre Formation shale units, but not the sandstones.…”