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Kovack, Gillian E.1, David N. Dewhurst2, Mark Raven3, John G. Kaldi1 
(1) University of Adelaide, Adelaide, Australia 
(2) CSIRO Petroleum, Australian Co-operative Research Centre, Kensington, Australia 
(3) CSIRO Land and Water, Mineralogical Service, Urrbrae, Australia

ABSTRACT: Mineralogical Controls on Seal Capacity in the Muderong Shale, Carnarvon Basin, North West Shelf, Australia

The Muderong Shale is the regional seal in the Carnarvon Basin on the North West Shelf of Australia and varies in both burial depth (<1-4km) and thickness (<15m->900m). Over 90% of commercial discoveries so far in the Carnarvon Basin have been discovered at base Muderong level. However, seal quality is variable in the Muderong Shale with mercury injection capillary pressure (MICP) results indicating that seal capacities vary from 70m of gas in the southern, inboard regions, to over 500m of gas in the major Barrow depocentre and to the north on the Rankin Platform. 
Changes in pore throat radii of shales influence seal capacity and this is likely to be one of the more variable parameters within a sedimentary basin. This study investigates the factors that control pore throat geometry and their predictability throughout a regional seal. These include: 1) stress (mechanical compaction), which causes grain re-alignment and pore collapse; 2) mineralogy, notably the variations in relative abundances of smectite, illite, kaolin and quartz; and 3) diagenesis (chemical compaction), as this changes the mineralogy. 
Variations in seal capacity are inconsistent with changes in grain size (i.e. finer grain size alone does not enhance seal capacity). This suggests that the mineralogy and orientation of the clay fraction (particles < 2ìm) has a direct effect on seal capacity. Compositionally, the Muderong Shale is dominated by interstratified illite-smectite, illite, kaolin and quartz, with actual abundances of each mineral varying across the basin and with depth. A detailed study of one well in the Barrow Sub-basin showed that smectite-to-illite diagenesis occurs with increasing depth. This reaction has a positive correlation with mercury threshold pressure from MICP analyses. Increasing total illite content (discrete illite + illite interstratification) enhances seal capacity in the Muderong Shale.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.