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Sequence Stratigraphic Interpretation of the Cape Range Group in the Dampier Sub-Basin:An Investigation of Low Sonic Velocity Intervals

Mohammed Al-Ojeh
Bachelor of Science-Honours in Petroleum Geology and Geophysics - 2009
Australian School of Petroleum
University of Adelaide


The Oligo-Miocene Mandu Limestone within the Cape Range Group forms part of the Tertiary progradational succession of the Dampier-Sub-Basin. It is represented by prominent sigmoidal clinoforms on seismic. A sequence chronostratigraphic model was developed for the Cape Range Group based on seismic and sequence stratigraphic interpretation of the Mutineer-Exeter 3D seismic volume. A depositional sequence template based on the development of a succession of distally steepened ramps linked to heterozoan mound development within the Mandu Limestone was used to interpret the distribution of carbonate depositional systems that would enable delineation and prediction of low sonic velocity anomalies within the Oligo-Miocene wedge required to address problems associated with time to depth conversion at top reservoir level within the underlying Angel Formation reservoirs of the Mutineer-Exeter Field.

Based on seismic facies interpretation of the interpreted depositional sequence framework (in part based on 2D Wheeler transforms of the seismic data using OpendTect Sequence Stratigraphic Interpretation Software), the Oligo-Miocene architectural evolution of the Cape Range Group (Lower and Upper Mandu Limestone) and overlying Yardie Group (Trealla Limestone and Bare Formation) broadly displays three main phases of carbonate ramp to rimmed carbonate platform evolution:

• Initial development of a homoclinal carbonate ramp within the basal portion of the Mandu Formation during the Early Oligocene,

• formation of a distally steepened carbonate ramp in the lower part of the Mandu Limestone by lateral accretion of heterozoan mound communities formed during multiple lowstand episodes connected with third order sequence development during the Late Oligocene and Early Miocene in the Lower and Upper Mandu Limestone

• development of a rimmed carbonate platform phase during the late Early Miocene associated with development of patch reef communities in the Trealla Limestone and dominance of Photozoan communities associated with the Mid Miocene Climatic Optimum

Clustering of lowstand heterozoan mounds with primary depositional textures preserved is interpreted to be linked to the development of uncemented zones recorded from within carbonate microfacies along the strike of the clinoform rollover and may exert a control on the mapped distribution of low sonic velocity zones within the Upper Mandu Formation. Preparation of regional palaeogeographic maps that display the distribution of heterozoan mounds within each lowstand phase of the Upper Mandu may then delineate the low velocity zone that results in a push-down of the Mutineer-Exeter reservoir target in two -way time.

The low velocity zone of the T30, T31 and T32 sequences overlie the depositional topography of an older Oligocene strike fed sub-marine anastomosing slope channel complex. The complex is oriented parallel to the shelf slope break formed by the ramp complex of the Mandu Limestone. Low velocity is interpreted to be caused by alignment of poorly cemented heterozoan carbonate microfacies and incision fill facies.

Australian School of Petroleum



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