Pore pressure estimation from mudrock porosities in Tertiary basins, southeast Asia

Porosity reduction during mechanical compaction of a sediment generally has been assumed to be controlled by the increase in vertical effective stress , which is convenient because vertical stress profiles may be readily calcu lated from density logs. Poroelasticity theory shows, however, that mean ef fective stress controls porosity reduction. According to published data, ho rizontal stresses increase with overpressure, as well as with depth, so mea n stress and vertical stress profiles are poorly correlated in overpressure d sections. We have used wireline logs to compare the pore pressures estimated in mudro cks by relating porosity to mean effective stress and to vertical effective stress for overpressured Tertiary sections in southeast Asia. Wells from t hree different basins were studied. Mudrock porosities were estimated from the sonic log response and sorted by lithology according to the natural gam ma-log response. Two sets of normal compaction curves, relating porosity to mean effective stress and to vertical effective stress, were determined em pirically by fitting data points where the pore pressure was thought to be hydrostatic. These curves were then used to estimate the minimum pore press ure corresponding to mudrock porosity values in the overpressured sections. The pore pressures inferred using the mean effective stress are consistent with direct measurements of pore pressure in the adjacent sands. In contras t, pore pressures inferred in mudrocks using the vertical effective stress are significantly lower for the overpressured sections, implying discontinu ities in the pore pressure profiles at lithological boundaries, which canno t readily be explained. We conclude that the pore pressures estimated using the vertical effective stress are wrong and that empirical relationships b etween porosity and vertical effective stress should not be used for estima ting pore pressures: porosity should be empirically related to mean effecti ve stress instead.