Early Cretaceous Sequence Stratigraphic Architecture, Enderby Terrace,Carnarvon Basin, Western Australia
Honours Degree, 2003
University of Adelaide
Two reservoir sands have been identified in the Lower Cretaceous section of the Enderby Terrace using analogues from the Barrow Sub-basin and the Lewis Trough. The Middle M. australis Sandstone is known as the reservoir for the Stag and Wandoo Oilfields while the Berriasian Sandstone is a frontier reservoir sand on the Enderby Terrace. The Enderby Terrace located in the southeastern margin of the Dampier Sub-basin is characterised by its abundant glauconite.
Lithological evaluation is difficult because of the effect of glauconite on wireline logs. No common shale baseline can be shown in wells on the Enderby Terrace, therefore each well had to be analysed separately. The optimum method to distinguish lithologies was by means of a modified crossplot using DT on the X-axis, GR on the Y-axis and RHOB on the Z-axis. Sand and shale were identified on each crossplot and siderite was identified by visual inspection of the wireline logs.
Nine sequence boundaries and eight maximum flooding surfaces have been identified on the Enderby Terrace from the Base Cretaceous Unconformity to the top of the Muderong Shale.
The Middle M. australis Sandstone was deposited in a quiescent broad marine shelf and has occurred as a transgressive sand sheet created by the reworking of local sand deposits sourced from the palaeo-Yule River/delta to the northeast of the Enderby Terrace. The extensively bioturbated, glauconitic sandstones with a characteristic coarsening upward profile were deposited during a relative highstand.
The Berriasian Sandstone was most likely deposited as shelf sand ridges created by the gradual reworking of previous shoreline sand deposits. The sand appears to have been sourced from the Talisman Delta in the northeast. There is no laterally extensive seal above the Berriasian Sandstone deposits making this reservoir a high-risk venture.
A potential stratigraphic trap for both reservoir sands has been identified at the eastern edge of the Enderby Terrace where the Lower Cretaceous rocks hit impermeable basement. Other stratigraphic traps identified in this study are sand pinch-out traps identified on well cross sections in both strike and dip directions.