In a sedimentary basin , the lithostatic gradient is I psi/ ft and hydrostatic gradient is 0. 454 psi/ft . Calculate the lithostatic stress and hydrostatic pressure at the depth of 1 km and 4km , respectively , assuming the pressures at the surface is O . If the measured reservoir pressure at this depth is 90 MPa, is the reservoir over - pressured ? ( I MPa = 145 psi ; assume that the Biot parameter is 1 . )