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Reservoir Charcterization And Depositional Model For The Stag Oil Field, North West Shelf, Australia.

Saraiva, Isabel

Honours Degree, 2001

University of Adelaide


A geological reservoir model for the Stag Oil Field was developed using petrophysical log signatures, core, petrology and production data. The model was developed as an analogue for evaluating the potential of similar reservoirs on the Enderby Terrace. The Stag Oil Field is located near the south-eastern margin of the Dampier Sub-basin, on the North West Shelf, Western Australia. The reservoir at Stag is a poorly consolidated, glauconitic, sandstone member of the Lower Cretaceous Muderong Shale.

At top reservoir level the Stag Field is a broad, low relief structure with four-way dip closure. The reservoir sand pinches out to the east and south of the field. The base seal for the reservoir is an approximately 1m thick claystone/siderite unit and the top seal the regional extensive Muderong Shale.

The reservoir contains two fine-grained, bioturbated glauconitic sandstone units. The upper unit shows good reservoir quality (permeability to air, 1-2 D). The lower unit shows poorer reservoir quality (permeability, 100-300 mD). This is mainly due to increased clay content; but also to mechanical compaction promoting ductile deformation of glaucony. Where siderite is present in pore spaces, porosity and permeability is diminished.

High gamma-ray readings in good porosity reservoir sand are a function of high glaucony and K-feldspar content. A crossplot of gamma-ray, sonic velocity and density best distinguishes between sands and shales.

Production data indicate heterogenity in the field which is most likely due to facies changes.

A relationship showing increased permeability with increased porosity is evident. Samples can be divided into three groups based on their porosity-permeability characteristics. The grouping corresponds to different units in the middle M.australis sequence set.

Three depositional sequence sets are contained within the M.australis palynozone, designated as the lower, middle (containing the Stag reservoir sandstone) and upper M.australis. The whole sequence is an overall transgression.

Results from the study show that the package containing the middle M.australis reservoir interval is made up of several thin coarsening-upwards cycles. This is indicative of cycles of deposition (corresponding to relative sea level changes) rather than one single event. The results suggest that the inner to middle shelfal marine Early Cretaceous middle M.australis sequence set was deposited as a sheet sand sourced from the NE in the vicinity of Wandoo Oil Field.

The package extends across the whole field in a sheet like manner rather than a ridge or a delta. The relationship between subtle lateral thickness variations of the package and sand percent distribution is parallel. Several evidence of a low energy depositional environment are present indicating that there were not enough energy to form sand ridges.

Potential for middle M.australis reservoirs to the south-west of the Stag Oil Field exists. The most hydrocarbon prospective areas are placed along the strike of the anticline west of the Mermaid Fault.

Australian School of Petroleum



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