An Investigation Of Reservoir/Seal Couplets In The Eromnga Basin; Implications For Petroleum Entrapment And Production. Development Of Secondary Migration And Seal Potential Theory And Investigation Techniques
Boult, Peter J.
Doctor of Philosophy, 1996
University of South Australia
Studies of producing oil pools of the predominantly fluvial to shallow lacustrine Eromanga Basin in central Australia indicate that capillary seals have a significant control on hydrocarbon occurrence and production. Oil pools sealed by the Callovian Birkhead Formation at the Jackson and Gidgealpa fields and the Berriasian Murta Formation at the Murteree Ridge fields are filled to capillary seal capacity. Resultant upward leakage has taken place filling oil pools higher up. Oil pools sealed by the Birkhead Formation at the Bodalla South field are filled close to capillary seal capacity but due to lack of charge leakage has been minimal. The oil pool at Moorari is not filled to capillary seal capacity.
The major producing reservoir of the Eromanga Basin is the braided fluvial Hutton Sandstone. The transition from this to the Birkhead Formation seal is marked by two features; firstly a change from braided fluvial through to deltaic-lacustrine facies and secondly, a change from craton-derived to volcanic-arc-derived sediment. The craton- derived and volcanic-arc-derived sediments have Sm / Nd model ages of approximately 1500 and 800 million years respectively, which suggests a major provenance change. In the most prospective part of the basin, this provenance change and the resultant switch from quartz to lithic arenites controls the location of the effective intra-Birkhead Formation seal. These lithic arenites are sometimes the same grain size as the underlying reservoir quality quartz arenites, however, their pore-throat sizes have been reduced by diagenesis and the precipitation of pore-filling authigenic clays.
Maturation of organic matter may have changed the wettability of the Birkhead and Murta Formations. Palaeo-oil accumulations, which were possibly full to structural spill point, have been recorded beneath both formations which act as the seals to the Hutton and Namur Sandstones respectively. These larger accumulations probably relate to a time when oil was trapped beneath an immature water wet seal. When organic matter in the seal started to produce its own oil, but only in small quantities, it changed the wettability of the seal thus allowing much of the previously trapped oil to leak away.
Lateral migration at the gradational bases of the Birkhead and Murta Formations within the Eromanga Basin would have been very inefficient compared with that in the Cooper Basin where well defined reservoir/ seal boundaries occur. This could be a major contributing factor to the lack of oil accumulations in the Eromanga Basin away from the edge of the underlying Cooper Basin. Owing to the low seal potential of major seals in the Eromanga Basin, vertical migration pathways link multiple stacked oil fields and pathways between these are likely to be locations of migration related diagenetic zones (MRDZs).
This thesis also contains new secondary migration and seal mechanism theory along with improvements in investigative methodology.