Abstract: Pore volume in closed formation reduces when it is squeezed, and the compressed fluid that cannot be discharged in the pores causes the pressure to rise. A number of previous studies have argued that the tectonic compression effect is one of the main factors that lead to abnormal pressure. However, instead of a quantitative analysis, current studies have paid attention solely to qualitative analysis. In this study, stratum elastic compression caused by extrusion effect was investigated by close reasoning based on the double effective stress theory and the poroelasticity theory, by which the theoretical calculation formula of pore pressure coefficient increment was calculated. Quantitative calculation suggest that, under certain geological conditions, squeezed closed formation leads to a small increase in pore pressure and a slight pressure abnormality, with a limited increment of pore pressure coefficient. It was possible to cause abnormal high pressure by the extrusion effect only when the following occurred simultaneously:stratigraphic burial was sufficiently deep, the strength was sufficiently high, the sealing property of the surrounding rocks was excellent, extrusion occurred in the direction of minimum horizontal stress, and the external extrusion was sufficiently high. The extrusion effect could not generate abnormal high pressure, nor did it serve as the main cause.

Key words: extrusion effect, pore pressure, abnormal high pressure, crustal stress, effective stress