Toledo, Marco A. S.¹, Antonio C Buginga Ramos¹ 
(1) PETROBRAS S/A, Rio de Janeiro, Brazil

ABSTRACT: Overpressure Impacts on AVO Analysis

AVO anomalies are normally associated to changes in the rock’s compressibility and density, which are normally caused by changes in lithology and saturant fluids. Frequently, however, AVO signatures can be quite ambiguous causing the need for well/seismic calibration using rock properties and reflectivity modeling. An important source of ambiguity is the abrupt drop in effective stress, which affects the skeleton bulk moduli of the rock. Effective stress is defined as the difference between the overburden pressure and the pore pressure. In clastic basins pore pressure largely controls the changes in seismic velocities. Different overpressure mechanisms can promote the effects described above, as the disequilibrium compaction that is usually considered a primary cause of abnormal pore pressure in consequence of shale undercompaction. Because water tend to be adsorbed on the clay particles, pore pressure has a much greater influence in the elastic properties of shales than it has in sands. In this paper, we show how the elastic properties of the shale cap rock can be affected by pore pressure causing significant changes in the AVO attributes. The change in elastic properties of the shale section can even induce false AVO anomalies, entirely related to the overpressure phenomenon. AVO pitfalls are particularly important in the contact between overpressured shales and high impedance sands.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.