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Sequence Stratigraphy Of The Late Cretaceous Sherbrook Group, Shipwreck Trough, Otway Basin.

Faulkner, Tony

Honours Degree, 2000

University of Adelaide

Abstract

The lowstand systems tract of the Late Cretaceous Waare Formation forms the main reservoir interval in the Shipwreck Trough area of the Otway Basin. The north-south trending Shipwreck Trough formed a prominent structural low during Late Cretaceous times into which a post-rift succession was deposited. Reservoirs have developed along the axis of the trough during lowstand systems tract deposition. The reservoirs are sealed by overlying transgressive systems tract marine shales. Hydrocarbons accumulated via fault migration into fault block and faulted anticline structural traps.

This study integrates wireline logs, biostratigraphy, reflection seismic data and aeromagnetic data with the concepts and principles of sequence stratigraphy to identify the spatial and temporal distribution of reservoir, seal and source facies. The Late Cretaceous sediments of the Shipwreck Trough can be sub-divided into four sequences ranging from Turonian (91 Ma) to Maastrichtian (65 Ma) in age. The sediments were deposited in a tidally influenced, fluvial to deltaic depositional environment. Marine incursions into the restricted area began at the onset of continental break-up between Australia and Antarctica in the Mid Cretaceous (99 Ma).

The positions of drainage systems and depocentres during the Turonian to Santonian (91Ma- 82 Ma) in the Shipwreck Trough were largely controlled by fault block movement. Structural controls preferentially focused the deposition of potential reservoir intervals into the axes of half-graben within the length of the trough. Slow sea floor spreading at this time led to slow subsidence with eustasy and fault activity being the main influence on accommodation space change. The influence of structure on deposition became less pronounced during the Campanian to Maastrichtian (82 Ma-65 Ma). During this time, thermal subsidence was the major influence on relative sea level change and thus the formation of sequence boundaries as opposed to eustatic sea level change. Episodic uplift and erosion in southeastern Australia during the Cretaceous provided the local sediment-source mechanisms. A gradual change in provenance occurred during deposition of Late Cretaceous sediments in the Shipwreck Trough. Sediments were initially sourced from the volcanolithic Eumeralla Formation during uplift of the Otway Ranges. Continued uplift of the ranges and the northern margin of the Otway Basin resulted in erosion and reworking of the more mature Pretty Hill Formation.

Key to petroleum exploration is the delineation of potential untested reservoir and seal intervals above the main Waare Formation play. Additionally, potential exists for stratigraphic trapping of hydrocarbons in the Shipwreck Trough.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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