Geophysical Prospecting, 2001, 49, 523±539 The seismic response to overpressure: a modelling study based on laboratory, well and seismic data Jose  M. Carcione and Umberta Tinivella* Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Osservatorio Geofisico Sperimentale, Borgo Grotta Gigante 42C, 34010 Sgonico (Trieste), Italy Received January 2000, revision accepted March 2001 ABSTRACT We investigate the seismic detectability of an overpressured reservoir in the North Sea by computing synthetic seismograms for different pore-pressure conditions. The modelling procedure requires the construction of a geological model from seismic, well and laboratory data. Seismic inversion and AVO techniques are used to obtain the P-wave velocity with higher reliability than conventional velocity analysis. From laboratory experiments, we obtain the wave velocities of the reservoir units versus confining and pore pressures. Laboratory experiments yield an estimate of the relationship between wave velocities and effective pressure under in situ conditions. These measurements provide the basis for calibrating the pressure model. Over- pressures are caused by different mechanisms. We do not consider processes such as gas generation and diagenesis, which imply changes in phase composition, but focus on the effects of pure pore-pressure variations. The results indicate that changes in pore pressure can be detected with seismic methods under circumstances such as those of moderately deep North Sea reservoirs. INTRODUCTION Drilling of deep gas resources is hampered by the high risk associated with unexpected overpressure zones. Knowl- edge of pore pressure obtained using seismic data from, for instance, seismic-while-drilling techniques (Seisbitw, DBSeisw, Tomexw) will help in planning the drilling process so as to control potentially dangerous abnormal pressures. In general, prediction of overpressure has been based on conventional velocity analysis (e.g. Bilgeri and Ademeno 1982) and empirical models relating pore pressure to seismic properties. Recently, Louis and Asad (1994) used a modelling technique to analyse the amplitude variations with offset (AVO) of pressure seals, and Pigott and Tadepalli (1996) estimated porosities and pore pressures in clastic rocks using AVO methods. Acoustic synthetic seismograms based on well logs showed that a strong AVO effect is associated with steep pressure and velocity gradients. We study the seismic visibility of overpressure using seismic, well and laboratory data. The analysis is intended to provide a procedure for overpressure detection from seismic data. We consider an area in the North Sea sedimentary basin. This basin is 170±200 km wide and represents a fault-bounded north-trending zone of extended crust, flanked by the mainland of western Norway and the Shetland Platform. The area is characterized by large normal faults with north, northeast and northwest orien- tations which define tilted blocks. Those flanking the Viking Graben are shown in Fig. 1. Jurassic and older sediments are present in the well used for this study. The main reason for selecting this area is the fact that high overpressure compartments were encountered during drilling, and that even higher overpressures are expected in future wells, down flank, towards the central Viking Graben. The well under study is an exploration well drilled to a total depth of 5149 mRKB (4767.4 mTVD) to test the hydrocarbon potential of the Jurassic Brent Group, which was encountered at about 3300 m depth. Figure 2 shows q 2001 European Association of Geoscientists & Engineers 523 *E-mail: utinivella@ogs.trieste.it