DEPOSITION AL AND DIAGENETIC CONTROLS ON THE QUALITY OF BARROW GROUP SANDSTONE RESERVOIRS, BARROW SUB-BASIN, NORTH WEST SHELF, WESTERN AUSTRALIA

Abstract

The main controls on Barrow Group reservoir quality are depositional (lithofacies) and post depositional (di- agenetic). Barrow Group sediments were deposited in environments ranging from non-marine to deep marine. The upper sandstones (Flacourt Formation) in the centra] and southern parts of the sub-basin are dominantly of good reservoir quality, consisting mainly of marine- reworked strandplain and shoreface deposits. The lower sandstones (Malouet Formation), however, in the Barrow Sub-basin depocentre, are composed predominantly of slope apron deposits, consisting of tight, matrix-rich, non-productive sandstones. The Flag Sandstone, distributed along the northern part of the sub-basin, is made up mainly of mounded turbidite deposits consisting of excellent reservoir quality, coarse-grained sheet turbid-ites.

Petrographic observations of theliarrow Group show that sands are, in general, extremely well sorted and display a unimodal grain-size distribution. Grain size varies slightly with the depositional environment; the mounded turbidite sandstones average 197 f.im (fine sand) and the slope apron sandstones average 185 pm (fine sand). The grain size in the marine-reworked strandplain sandstones averages 300 j.im (medium sandstones). Detrital depositional matrix, mechanical compaction and cementation are the main causes of poor reservoir quality. Mechanical compaction is responsible for destroying up to 60 per cent of the original porosity. Chemical compaction has had much less effect. Precipitation of authigenic minerals has reduced, on average, the primary intergranular porosity to 12 per cent of the whole rock-volume.

In some cases, secondary porosity development has greatly enhanced the reservoir quality of the Barrow Sub-basin sandstones. It results from dissolution of calcite, dolomite and potassium feldspar. Reservoir quality is most improved along the margins of the sub-basin as this is where carbonate and feldspar were most abundant.