ON THE POSSIBLE FORMATION MECHANISM OF THE OPEN FRACTURING IN SEDIMENTARY BASINS

Abstract

The paper shows that the formation of orthogonal systems of open macrofractures (cleavages) of sedimentary basins can occur at the stage of diagenesis during the first hundreds of years for a small range of shelf depths (up to 100 m) during sedimentation. During this period, sediments are compacted and the excess water is slowly squeezed out, which determines the decrease in the Poisson’s ratio from values close to 0.5 to values 0.3–0.2. Because of this, in sediments, the stress state of which is almost completely determined only by the gravity of the overlying rocks, is reduced by 50 % or more of the horizontal compression stress level from the initial lithostatic pressure level. On the other hand, if the limit of pseudoplastic flow is reached in the rock, vertical compaction occurs, accompanied by an increase in horizontal compression. These two competing processes together with the factor of fluid pressure in fractures and pores determine the possibility of brittle fracture formation. It is shown that for rocks with a low level of yield strength (clay), compaction leads to an increase in the level of horizontal compression stresses, which makes brittle fracture in such rocks at the stage of diagenesis impossible. In rocks with a high level of strength (sandstone, limestone), brittle fracture due to excess water pressure in the sub­vertical microcracks starts earlier than the shear yield limit is reached.