The Origin And Distribution Of Siderite Layers Within The Wandoo Field, Dampier Sub-Basin, Australia.

Brown, Robert

Honours Degree, 1998

University of Adelaide

Abstract

The Wandoo Oil Field is located on the eastern margin of the Dampier Sub-basin on the North West Shelf of Australia. The "M. australis Sandstone" is the reservoir for the Wandoo Oil Field. The aim of this project was to determine the origin, distribution and influence on reservoir performance of the siderite-cemented layers in the "Unit A" sands of the "M. australis Sandstone" using petrographic analysis, carbon and oxygen isotopic analysis, XRD analysis and cathodoluminescence.

The results of the study show that the siderite cement forms in restricted chemical conditions; a reducing environment with high levels of bicarbonate, high Fe²⁺/Ca²⁺ ratio, low dissolved sulphides and relatively high pH. The siderite cement occurs early in the diagenetic history of the reservoir in a modified sulphate reduction zone where the rate of ferric iron reduction is greater than the rate of reduction of sulphate. The controlling factor for this reaction is how much oxidized glauconite is present as this is the source of the ferric iron.

The siderite-cemented layers at Wandoo are stratigraphically significant and are associated with both erosional surfaces and condensed sections. It is during these times that glauconite can be oxidized to goethite. Therefore sequence stratigraphy can be used as a tool in glauconite-rich sediments to predict where siderite-cemented layers will form.

The submarine channel near the eastern and southern margin of the Wandoo Oil Field washed out some of the goethite present around the wells lower on the Wandoo structure. This is the likely cause of the lateral variation seen in the composition of the siderite-cemented layers across the Wandoo Field.