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Provenance of late cretaceous Waare formation sediments, Central and Western Otway Basin.

Frears, Brendan

Honours Degree 2006

University of Adelaide

Abstract

The Late Cretaceous Waarre Formation of the central and western Otway Basin is extensive in areal distribution and variable in terms of lithological composition, gross thickness, burial depth, diagenetic alteration and reservoir potential.

Waarre Formation sediments of the eastern Otway Basin host significant quantities of commercial gas in several onshore and offshore fields. This area represents a major Late Cretaceous depocentre. This study aims to analyse other depocentres of the central and western areas of the Otway Basin in terms of composition, sediment provenance and their likely reservoir quality away from existing well control.

Wireline log data, biostratigraphy, petrology data and lithological descriptions from cuttings, sidewall core and conventional core were collated into a dataset. These data were used to interpret Waarre Formation thickness, composition and reservoir potential. Petrographical data from 40 wells within the study area were collected and their mean QFR distributions were plotted on ternary diagrams. Combining the isopach and QFR data gave geographical distributions of QFR components over the study area, from which interpretations regarding sediment provenance (source areas) could be made. With the addition of log motif and palynological data, inferred depositional environment figures were established for the Waarre Formation (in general) and each of its stratigraphic members (A, B, Ca and Cb).

The Waarre Formation is predominantly characterised by very nearshore marine to fluvial to restricted marine facies comprised primarily of mature sandstones, with lesser siltstone and mudstone interbeds. Thin coal measures associated with floodplain swamps are occasionally found within Waarre sediments in onshore areas. Further basinward, sediments become less quartz-rich, reflecting the inferred distal regions of an extensive deltaic complex, within a restricted marine environment. Sediment input is likely to have been derived from the Mid-Late Cretaceous sediments of the underlying Eumeralla Formation, with some input from a mixture of Palaeozoic igneous and metamorphic terranes to the north and east of the study area.

Degrees of diagenetic alteration of sediments associated with burial were recorded. Primary porosities of sediments were best preserved at depth where sediments were highly chemically mature (quartz rich).

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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