Characteristics and origin of underpressure system in the Shiwu Fault Depression, south-east Songliao Basin, China

The vertical characteristics of underpressure, pore fluids and sealing cond ition in the Shiwu Fault Depression, south-east Songliao Basin, are investi gated. Based on the pore pressure data from drill stem tests collected from 40 wells, the vertical distribution of pore pressure in this depression co nsists of a shallow hydrostatic pressure system and a deep underpressure sy stem. The observed low pressures range from about 4 to 10 MPa in the depth interval of about 1550-2800 m. The pore water chemistry data document that the ionic evaporite trends of the pore water in the underpressure zone are different from overlying sediments in the hydrostatically pressured section , indicating that the underpressure system is sealed. The study indicates t hat the depression has undergone rapid deposition in a rift period during t he Cretaceous followed by a long-term slow uplift and erosion in the Tertia ry. On the basis of the modelled results of the abnormal pressure evolution in a typical cross-section using a two-dimensional numerical flow model, w e believe that the predecessor of the underpressure system was an overpress ure system. Since the end of the Cretaceous, the observed underpressure sys tem has developed as a result of a geotemperature decrease ranging from 30 to 70 degrees C, owing to palaeoheat flow reduction and long-term uplift an d erosional cooling. The mudstones below a depth of 1550 m in the deep subn ormal pressure system have small measured porosities ranging from 4% to 1.2 % with calculated permeability of about 10(-21)-10(-24) m(2). The observed underpressure can be modelled if we assign permeabilities below 10(-20) m(2 ) with a linear reduction of geotemperature. The geotemperature decrease, i n combination with very low permeability in the Lower Cretaceous mudstones, is therefore a possible origin for the formation of the underpressure syst em in this depression.