E. Hoskin1, S. O’Connor1, S. Robertson2, J. Streit2, C. Ward1 J. Lee and D. Flett1
The Northern Carnarvon Basin has a complicated geological history, with numerous sub-basins containing varying formation thicknesses, lithology types, and structural histories. These settings make pre-drill pore pressure prediction problematic; the high number of kicks taken in wells shows this.
Kicks suggest unexpected pore pressure was encountered and mudweights used were below formation pressure. The horst block penetrated by the Parker–1 well is focused on in this peer-reviewed paper. This horst is one of many lying along Rankin Trend’s strike. In this well, kicks up to 17.2 ppg (pounds per gallon) were taken in the Mungaroo reservoir. The authors investigate whether the kicks represent shale pressure - or rather, represent pressure transferred into foot-wall sandstones - by using well data from Forrest 1/1A/1AST1 and Withnell–1, and wells located in the Dampier Sub-basin and the hanging-wall to the horst. This anomalous pressure could result from either cross-fault flow from juxtaposed overpressured Dingo Claystone or transfer up faults from a deeper source.
Using a well data derived Vp versus VES trend, the authors establish that the kicks taken in Parker–1 are more likely to result from pressure transfer using faults as conduits. These data lie off a loading trend and appear unloaded but likely represent elevated sand pressures and not in situ shale pressure. Pressure charging up faults in the Northern Carnarvon Basin has been recognised in Venture 1/1ST1, however, this paper presents a focused case study.
Pressure transfer is noted in other basins, notably Brunei. From unpublished data, the authors believe that buried horst blocks, up-fault charging and adjacent overpressured shale may explain high reservoir pressures in other basins, including Nam Con Son in Vietnam.