Regional Study Of The Nappamerri Group In The Northern Part Of The South Australian Sector Of The Cooper Basin.
Honours Degree, 1996
University of Adelaide
This thesis is a regional study of the Nappamerri Group in the northern part of the South Australian sector of the Cooper Basin. The study area covers three farmin blocks: Patchawarra Central, Patchawarra East and Merrimelia-Innamincka ie parts of the Patchawarra Trough and the Gidgealpa-Merrimelia-Innamincka Trend.
The Nappamerri Group is the uppermost widespread unit in the Cooper Basin section and is overlain by either the Triassic Cuddapan Formation or Jurassic sediments of the Eromanga Basin. The more prolific reservoirs are of greater importance, and the Nappamerri Group has been largely ignored as an exploration target. However recent hydrocarbon discoveries have enhanced the prospectivity of the Nappamerri Group and further drilling is being considered by the operator, Santos.
Fifty nine wells were selected from the study area for log correlation. Tops of the Nappamerri units were interpreted in each of these wells. A set of regional seismic lines was interpreted in terms of the Nappamerri subdivision. The seismic interpretation was tied with the well data using synthetic seismograms. Isopach and isochron maps were constructed from the well picks and the seismic interpretation to depict regional thickness variations in each of the Nappamerri units.
The project has revealed the close relationship between structural development and sedimentation during the Triassic in the study area. The boundaries between Nappamerri units are unconformities. They indicate periods of tectonic movements.
Northeast-trending anticlines associated with thrusting were uplifted during post-Callamurra Formation movement. Subsequent erosion, led to flattening of structural relief. This is evident in the isopach pattern. The overlying Paning and Wimma Members have more uniform thicknesses over the study area compared to the Callamurra Formation.
Northward tilting of the Cooper Basin, which commenced after deposition of the Paning Member, caused wide subcrop of the Wimma Member and particularly the Tinchoo Formation to the north of the Patchawarra Trough. The northwest-trending Wackett-Naccowlah-Keleary Trend ceased to act as an effective hinge line, as a result of this northward tilting. This is evident from the isopach pattern of the Tinchoo Formation.
The main depocentre in the study area was located around Wimma #1 and Beanbush #1. Development of a local depocentre to the northwest of the Merrimelia structure correlates with an abrupt change in Wimma Member facies.
The observed isopach pattern suggest connection between the Patchawarra and Nappamerri Troughs along lows developed between the Merrimelia and Packsaddle-Pondrinie structures.
The long period of erosion, which followed deposition of the Tinchoo Formation marks the end of the Cooper Basin deposition.