The Muderong Shale is the regional top seal in the Carnarvon Basin on the Northwest Shelf of Australia. Evidence for top seal breach in this area comes from post-Muderong plays and hydrocarbon shows. Capillary pressure tests show this shale has the capability of restraining gas columns in excess of 250 m height, a value which is unlikely to be reached due to the intrinsic weakness of the Muderong Shale. Apart from one example, all hydrocarbon columns so far encountered in the Carnarvon Basin are well below this limit. Geomechanical testing of the Muderong Shale indicates it is a weak rock, commensurate with its high illite–smectite content. Comparison of laboratory-generated failure envelopes with in situ stress conditions suggest that intact Muderong Shale is not generally at risk of hydrofracture but that pre-existing faults and fractures may well be critically stressed in the present-day stress field. It is postulated that such faults and fractures may be responsible for top seal failure in the Muderong Shale and the resultant presence of hydrocarbons in the post-Muderong succession.