Estimating pore pressure in the Cooper Basin, South Australia: sonic log method in an uplifted basin

Abstract

The use of sonic log data to analyse overpressure in the Cooper Basin, South Australia is complicated by the occurrence of Tertiary uplift in the basin. Uplift and overpressure are both associated with anomalous porosity/depth relationships: the former being witnessed by overcompaction and the later by undercompaction. Hence uplift may mask overpressure effects on log data. A normal compaction trend was determined for the Cooper Basin by averaging sonic log data from 29 wells. The Maree Subgroup was omitted from the normal compaction trend because it has a markedly different sonic log signature. Shale sequences were isolated for study by applying a gamma ray filter (API > 100). In order to remove the effects of uplift, the normal compaction trend was adjusted to fit the trend of the upper, normally pressured part of the sequence in each well. Quantitative pore pressure analysis was undertaken on shale sequences from 8 wells using the Eaton (1972) method once the effect of uplift had been removed. The predictions of the Eaton (1972) method are consistent with pressure measurements (DSTs and mud weights) in sandstones in 7 of the 8 wells, suggesting that both uplift and overpressure have been successfully determined. Assuming that the shales at Moomba 55 are overpressured, as suggested by the Eaton (1972) method, these shales must be isolated from adjacent near normally pressured sandstones. Furthermore mud weights, which are only at best an approximation of formation pressure, are especially unreliable in overpressured shales adjacent to near-normally pressured sandstones.