The 3D Reconstruction of the Donkey Bore Syncline, South Australia
Mohamed Ibrahim, Khairul Anwar
Engineering Honours Degree, 2006
University of Adelaide
The use of outcrop analogues in reservoir characterisation has now widely been accepted as it is reliable in providing a generic understanding of reservoir geometry and connectivity. The aim of this project is to resolve the internal geometries of the turbidite system in the Donkey Bore Syncline, Flinders Ranges, South Australia through its 3D reconstruction.
The study area is a syncline with a topographic low in its centre, featuring an excellent outcrop exposure throughout the field. The syncline mainly consists of interbedded sandstone and shale with interbedded muddy limestone and sandy limestone at the flanks. The focus of the study, the Bunkers Sandstone, is overlain by the Oraparinna Shale and underlain by the Mernmerna Formation. The Bunkers Sandstone is made up by seven sandstones units, namely Unit A, B, C, D, E, F and G. All the seven units have an average of approximately 30 % net:gross. The depositional environments of the units are interpreted to be basin floor fan facies for Unit A and turbidite facies for Unit B to G.
The 3D reconstruction of the syncline is possible through the integration of field data as gathered from the fieldwork and geological maps before being integrated in the surface mapping software, Surfer8. Aerial photography is used in interpreting the geological units boundaries (Unit A to G) across the syncline with the help of a stereoscope, a tool that visualises the topography in 3D view. Then, the reconstruction is carried out by constructing cross sections across the syncline to interpret the bed boundaries in the subsurface. The XYZ data is obtained from the geological map and the cross sections according to the northing and easting grids, and the height above sea level. After all data is gathered, they are then incorporated in Surfer8 to create structure contour and isopach maps.
All the seven units of the Bunkers Sandstone are traceable on the aerial photograph. They can be distinguished based on the differences of the units’tone colours between the successions. This method of interpretation is reasonably accurate and was verified further by walking out the units in the field. The cross sections interpret the thickness of the beds in the subsurface and show some variability between the northeastern and southwestern flanks. This might be caused by the complex topography of the syncline and the sparse structural data that is available to interpret the bedding dip angles. Seven surface maps of the units were created subsequently using the Surfer8 software based on the XYZ data and they display the typical shape of a syncline. The isopach maps that are generated from these surfaces can be used to estimate the thickness of each unit.
The reconstruction can be inaccurate due to the uncertainties and errors that are involved in the methodology. The sources of error that have been recognised in this project are the human interpretation errors as well as problems with available data in terms of their quality and quantity. Despite careful interpretation, some interpretation errors are inevitable. Minimising the errors to reduce the uncertainties should be a main concern to ensure the quality of the results can be improved further. Therefore, it is recommended that more field data be acquired and the aerial photographs be subsequently revised and updated.