The Carnarvon Basin on the Northwest Shelf of Australia is one of the region's most productive hydrocarbon provinces. The Muderong Shale forms the regional top seal over much of this province and indeed in excess of 90% of the commercial discoveries so far made in the Carnarvon Basin lie at base Muderong level. However, top seal breach is an exploration issue in the area, illustrated by the presence of seismic leakage indicators, palaeocolumns, post-Muderong shows and occasional post-Muderong fields. Some of the seismic leakage indicators suggest trap breach is very recent. Seal capacities to gas vary from ~70m to >600m and generally suggest good capillary sealing properties. Geomechanical testing of Muderong Shale core reveal it is a weak rock, commensurate with its high clay (particularly mixed-layer illite-smectite) content. Laboratory generated failure envelopes were compared with in situ stress and pore pressure conditions, suggesting that intact Muderong Shale top seal is not generally at risk of hydrofracture in the current day stress field, but that pre-existing faults in certain orientations are likely to be critically stressed, which fits with seismic observations of recent leakage. However, diagenetic changes such as the smectite-illite transformation occur at depth below ~1.5 km and are shown to increase capillary seal capacity and are likely also to affect geomechanical properties, strengthening the top seal through increasing both friction coefficient and cohesive strength.
1. INTRODUCTION
The northern Carnarvon Basin is the southernmost of a series of sedimentary basins that make up the Australian North West Shelf. It is Australia's largest oil and gas producing region, hosting world-class giant gas fields in the outboard regions of the basin [1]. Although many traps contain more than one phase of hydrocarbon, in general, the area may be divided into two main geographical provinces: an inboard oil province that occupies the southern and eastern parts of the basin, and an outboard, gas/condensate province lying along the western margin [2]. Almost all (97%) of the hydrocarbon resources on the North West Shelf are sealed beneath the regionally extensive Muderong Shale [1]. This is a laterally extensive and continuous sealing unit, occurring between depths of ~0.5 km and 3.5 km with thickness varying from 5m to >800m. It is extremely fine-grained and comprises dominantly mixed layer illite-smectite, quartz and kaolinite. These depth and hence temperature variations indicate that compaction and diagenesis are likely to affect both physical and mechanical properties of this top seal. While mechanical compaction reduces pore throat size [3] and strengthens the rock, the effects of the smectite-illite transformation, are as yet unclear. This reaction is controlled by temperature, both rock and solution chemistry as well as time [4]. Previous studies [5,6,7] have suggested a number of mechanisms for which pore-size reduction has occurred as a result of diagenetic mineral growth and alteration. However, the role of the smectite-to-illite transformation on seal capacity and integrity is not well constrained, although it has been suggested to play a role in overpressure development [6].
