Exploring the Potential for Oil Generation, Migration and Accumulation in Cape Sorell–1, Sorell Basin, Offshore West Tasmania

Abstract

Given the underexplored nature of the Sorell Basin, offshore Tasmania, the reported presence of oil stains and shows in the Late Cretaceous sequence below 3,000 m in Cape Sorell–1 is seen as encouraging evidence of an effective petroleum system. To investigate the significance of these shows, an integrated palynological, geochemical and burial history analysis of Cape Sorell–1 has been undertaken. New data have been collected on palynology, potential source rocks (biomarker and chemical kinetics), oil migration indicators (quantitative grain fluorescence—QGF, and grains–with–oil– inclusions—GOI) and thermal history parameters (vitrinite reflectance—VR, vitrinite–inertinite reflectance and fluorescence—VRF® and apatite fission track analysis—AFTA®). A synthesis of these analyses has resulted in a model that suggests that the terrestrial organic–rich potential source rocks in Cape Sorell–1 are very labile for hydrocarbon generation and are presently at the initial phase of oil generation. The model also indicates that increasing hydrocarbon generation with time reflects a progressive increase in temperature reaching maximum temperatures at the present–day. According to the model, accelerated rate of oil generation from the Maastrichtian potential source rock interval at ~3,200 m in the lower Sherbrook Group Equivalent occurred at ~48 Ma and is in response to the maximum burial heating rate in the Early Eocene, during rapid deposition of the thick Wangerrip Group Equivalent. This heating event may have been related to gateway opening along the Otway coast and west Tasmanian margin. Although there was a declining heating rate since the Early Eocene, gas and oil may continue to be generated to the present–day at Cape Sorell–1.The low content of mobile oil below sealing facies higher in the section negates a pervasive oil migration phase sourced down–dip from the basin centre, or from older sedimentary sequences below TD in Cape Sorell–1. However, the possibility that Cape Sorell–1 is in a migration shadow cannot be excluded. The restricted areal extent of the depocentre associated with Cape Sorell–1, together with thin, isolated potential source beds at the well site, would indicate the major risk for hydrocarbon occurrences in the local region is limited source rock volume. However, seismic evidence suggests the possible presence of similar facies within the deeper syn–rift succession below TD at Cape Sorell–1. The labile nature of the organic matter would support oil generation and migration at maturities lower and depths shallower than traditionally viewed. This work provides evidence to support a possible oil play from terrestrial source rocks in the Sorell Basin, and may also provide useful insights into recent large offshore gas discoveries to the north in the adjacent Otway Basin.

Late Eocene–Quaternary Dinoflagellate Cysts from ODP Site 1168, off Western Tasmania

“…Site 1168 Eocene sediments are similar in nature to those at Cape Sorell (Boreham et al, 2002) and to those reported from DSDP Site 282 to the northwest, suggesting widespread late Eocene suboxic to anoxic conditions in the eastern Australo-Antarctic Gulf. Following a transitional phase during the Oligocene, in the Neogene these conditions had been replaced by deposition of carbonate ooze in a welloxygenated open ocean on a passive margin at middle bathyal depths (Shipboard Scientific Party, 2001ab;Boreham et al, 2002). The curves of Figure F2 both confirm the broad trend of initial shallow-marine, nearly continental conditions evolving into an open oceanic environment.…”

“…Palynological studies (MacPhail in Boreham et al, 2002) indicate that the signature of these deposits is very similar to those recovered at Site 1168, with endemic species being relatively rare. This contrasts with results from dredge samples taken slightly farther to the south, along the South Tasman Rise (Truswell, 1997).…”

“…nocysts have been recorded from locations around the world, but notably in central and northern Italy, including the Priabonian-type section (Brinkhuis and Biffi, 1993;Brinkhuis, 1994 (Boreham et al, 2002). Palynological studies (MacPhail in Boreham et al, 2002) indicate that the signature of these deposits is very similar to those recovered at Site 1168, with endemic species being relatively rare.…”

Tectonic, Oceanographic, and Climatic Controls on the Cretaceous‐Cenozoic Sedimentary Record of the Australian‐Antarctic Basin

“…Drill sites in the adjacent Eyre and Inner Duntroon subbasins reveal that sedimentary unit S2 is absent leading to the merging of horizons U1 and U2, corresponding to a break in the stratigraphic record of ~24 Myr (~83–59 Ma, based on spore‐pollen and foraminifera biozonation; Totterdell et al, ; Figure ). In the Otway and Sorell basins, U2 is dated by drill sites to latest Maastrichtian‐early Paleocene (~65 Ma; based on spore‐pollen biozonation; Krassay et al, ; Boreham et al, ; ~67 Ma onshore in the Otway Basin; Frieling et al, ; Figure ). Consequently, sedimentary unit S2 was likely deposited between ~94 and 65 Ma in the Bremer Subbasin, ~83 and 65 Ma in the central Ceduna, eastern Recherche, outer Duntroon subbasins, and offshore the Otway Basin, and ~80 and 65 Ma in the Sorell Basin.…”

A Different Look at Gateways: Drake Passage and Australia/Antarctica

“…By early Eocene, seafloor spreading in the Tasman Sea had ceased [Gaina et al, 1998] and the magnetic anomalies, C24 and younger created at the Australia-Antarctic ridge can be clearly identified; see the picks from Tikku and Cande [2000] shown as the C24 (53.3 Ma) picks in Figure 5. Boreham et al [2002] note that there was an accelerated rate of oil generation at~48 Ma in the Sorell Basin, in response to the maximum burial heating rate in the early Eocene which coincides with the passage of the nearby spreading center as shown in Figure 5. Chron 33/34 is perhaps the only clearly identifiable anomaly on the Australian side older than C24 and is seen on four of the five lines that Tikku and Cande [2000]) highlight in their Figure 4.…”