Difference of lithofacies mechanical properties of the fourth member of Shahejie Formation in the Bonan Subsag, Bohai Bay Basin

Based on the data of well cores in low permeability reservoirs in the fourth member of Shahejie Formation in the Bonan Oilfield, the rock facies types were studied. Using conventional three axis compression test data of different lithofacies, restored static rock mechanical parameters were established under reservoir confining pressure conditions. The differences of static mechanical parameters and stress-strain relation between lithofacies were analyzed. The low permeability reservoirs in the Bonan Oilfield can be divided into 5 types: coarse grained sandstones, fine sandstones, siltstones, mudstones and carbonate sandstones. In the same rock, with the increasing confining pressure, the Young’s modulus becomes larger, and the Poisson’s ratio increases slightly. With the increase of rock gain size, the Young’s modulus increases and the Poisson’s ratio decreases. However, the Poisson’s ratio of coarse grained sandstones does not follow the overall trend, and is larger than that of fine sandstones and smaller than that of siltstones. As the grain size of rock becomes coarser, the stress-strain relation becomes more linear. And the initial grain rearrangement plastic section and the high stress plastic section become smaller.