Abstract: Taking the rock of the sandy conglomerate reservoirs of Baikouquan formation in Mahu sag as the study objects, the mechanical characteristics and fracture propagation mechanism of the sandy conglomerate reservoirs with different lithologies are studied through various rock mechanical experiments. Based on which, a 2D model containing true gravels is established by using numerical modeling method, the dynamic process of fracture propagation in the sandy conglomerate are investigated, and the influence of gravel strength on fracture propagation mechanism is discussed. The results indicate that the sandy conglomerate are seriously heterogeneous, resulting in the obvious differences of mechanical characteristics in sandy conglomerates with different lithologies, among which the mechanical strength of fine conglomerate is higher than that of coarse conglomerate; the differences between gravel and matrix compositions have significant influence on rock mechanical characteristics of the sandy conglomerate; the failure mode of the sandy conglomerate is dominated by splitting failure in uniaxial compression and by splitting failure, single shear failure and dual shear failure etc. in triaxial compression; gravel particles have shielding effect on the extension of fractures and the fracture propagation during rock failure process in the sandy conglomerate mainly contains 3 types such as going-through gravel, bypassing gravel and embedded in gravel. The strength of gravels and the matrix reflects the mechanical properties of glutenite. The larger the strength difference between gravel and matrix is, the stronger the shielding effect of the gravels is, the stronger the received resistance of fracture will be, causing that the fracture propagation would transfer from going-through gravel to bypassing gravel.

Key words: Mahu sag, Baikouquan formation, sandy conglomerate reservoir, rock mechanical characteristic, fracture propagation