The Yanchang Formation in the Ordos Basin has deposited a set of mudshales and fine-grained sandy rocks

rich in shale oil resources

with an estimated resource potential exceeding billions of tons. However

shale oil reservoirs exhibit poor mobility

shallow burial depths

the development of bedding

fractures

and faults in horizontal sections

and unknown fracture propagation patterns

making volumetric fracturing challenging. To address this

cement-encased cores of full-diameter tight sandstone-mudstone and shale from the sublayer in the seventh member of the Yanchang Formation (Chang 7) were used in actual triaxial hydraulic fracturing physical model experiments. These experiments revealed hydraulic fracture morphologies and the fracture propagation mechanism under weak stress fields in shale oil reservoirs. The experiments found that shale oil reservoirs had tight layered structures and weak bonding between rock grains

causing fracturing fluid to easily infiltrate along bedding planes. When the difference between vertical stress and minimum horizontal principal stress was less than 2 MPa

hydraulic fractures predominantly formed horizontal fractures

with the fluid primarily infiltrating along bedding planes or horizontal natural fractures. When this stress difference increased to 7 MPa

vertical cross-layer fractures appeared

forming localized steps that eventually became captured by weakly bonded bedding planes

propagating horizontally along the layers. For fracturing operations

regions with a larger difference between vertical stress and minimum horizontal principal stress

such as wellheads at hilltops

are preferred. This facilitates vertical fracture propagation

improves volumetric fracturing effectiveness in reservoirs

enhances shale oil production

and increases economic benefits.