An Integrated Core and Wireline Log Approach to Understand the Depositional Environment of the Mid Birkhead Formation, Eromanga Basin, South Australia
Geoscience Honours Degree, 2012
University of Adelaide
This study focuses on the development of a depositional model for the Mid Birkhead through the integrated approach of utilizing core and wireline log data from the Growler and Charo/Snatcher Fields. Core taken from the wells Growler 2, Growler 6, and Snatcher 2 were analysed with sedimentary structures, lithology and grain sizes defined and characterised, resulting in the identification of thirteen lithofacies and four facies associations. The core was then calibrated to the wireline logs leading to the recognition of three electrofacies and allowing interpretation to take place across the fields where there was no core available and a depositional model to be built.
Previous work suggests that the Jurassic Birkhead Formation in the Eromanga Basin marks a change in provenance from the cratonic derived underlying Hutton Sandstone to the lithic sediments of the Whitsunday Volcanic Province. Contrasting the Hutton Sandstone’s high energy braided fluvial system, the Birkhead Formation is identified as having three phases of deposition from a low energy high sinuosity fluvial facies, to a lacustrine, coal swamp facies and finally a high energy fluvial facies. The changes in depositional energies are likely brought about by drainage network blockages, tilting of the basin or possible short lived glacial events.
The Birkhead Formation in the Growler and Charo/Snatcher field regions has been interpreted in this study as a fluvial lacustrine deltaic environment with the results showing that over the course of its deposition Lake Birkhead fluctuated quite dramatically, experiencing periods of transgression and regression. During the regressive periods the underlying strata was incised with palaeocurrent data suggesting that the resultant valleys were filled with cratonic sourced sediments from the south. A lack of accommodation space being created ensured the further reworking of the channels, the erosion of fine grained sediments and the development of multistorey channel fill. Rising lake levels then initiated transgressive periods which generated more accommodation space and the establishment of complete fining upward isolated channels. Continued lake level rises then saw the drowning of the system and the inundation of the basin.