Definition, Deposition and Distribution of the Poolowanna Formation in the Southwestern Patchawarra Trough of the Eromanga Basin
Lahra Lanigan, 2016
Bachelor of Science (Petroleum Geology and Geophysics)
Australian School of Petroleum
The University of Adelaide
The Early Jurassic Poolowanna Formation was deposited on the regional Base Jurassic Unconformity (BJU) and is the oldest stratigraphic unit preserved in the Eromanga Basin. The formation is commonly described as being deposited in a meandering to anastomosing fluvial system, but no detailed depositional model has been published. Further, there is a paucity of publicly available studies dedicated to, or even substantially focused on, the Poolowanna Formation. As a result there is no clear and consistent suite of petrophysical wireline log, core and palynological characteristics to define it. A systematic review of the unit’s basic attributes, inferred depositional history and petroleum prospectivity in the Patchawarra Trough is also lacking. This is likely to be due, at least in part, to the sparse and varied nature of hydrocarbon shows encountered thus far. Only around 3.5 million barrels of oil have been identified in the southwestern Patchawarra Trough to date.
This study investigates well penetrations of the Poolowanna Formation in the southwestern Patchawarra Trough with the aims of;
Establishing wireline log and palynological definitions of the Poolowanna Formation;
Determining depositional environment using palynological data, wireline logs and full core data; and
Mapping the distribution of the formation and its changing thickness, and suggesting controlling factors.
The culmination of these components will facilitate analysis of hydrocarbon shows and reasons for their presence or absence and distribution.
Wireline log definitions of the Poolowanna Formation were developed by comparing log characters across 27 key wells. Gamma ray responses for the Poolowanna Formation are most commonly upward-fining, with some minor serrated, blocky intervals. Palynological criteria were obtained by isolating taxa identified solely within Poolowanna Formation sidewall core samples. Classopollis classoides, Podosporites tripakshii, Retitriletes austroclavatidites, and Schizocysta sp. are the four taxa comprising the suggested palynological definition. The Poolowanna Formation can most confidently be placed within the PJ2.1-PJ3.2 biostratigraphic interval.
Integrated analysis of wireline log, core (including ichnology) and palynological datasets clearly indicate a fluvial depositional environment. However, the commonly ascribed meandering to anastomosing channel patterns cannot be conclusively established. This is because the datasets analysed are from widely-spaced vertical penetrations. The Poolowanna Formation is frequently below seismic resolution, and so cannot be mapped across the whole study area to assess the lateral geometry of channels.
Analysis of thickness changes in the Poolowanna Formation across the study area revealed a link between interval thickness and Early Jurassic topography along the BJU. Intervals are thinner and commonly condensed with clear upward-fining where deposited on Early Jurassic highs. The unit is thicker where deposited in palaeo-depressions.
Accommodation space was identified as a principal control on the thickness of Poolowanna Formation successions. Erosive processes associated with deposition of the Hutton Sandstone were identified as a secondary control that affected preservation. Preliminary analysis of likely structural and stratigraphic controls on Poolowanna Formation hydrocarbon occurrences was undertaken. This revealed that such hydrocarbon accumulations are most likely to occur on Early Jurassic structural highs where condensed intervals dominate.