Reservoir research is an important part in the petroleum exploration and development,and unraveling the formation mechanism of the reservoir space as well as determining the dominant controlling factors (such as structure,deposition,diagenesis,etc.) is essential.For the formation of reservoir space,most of the previous studies have focused on the mechanisms in terms of deposition and diagenesis,while the understanding to the mechanisms in terms of faulting mainly stays at a simple level of "fracture",lacking a further discussion on the detailed processes.This paper focuses on the sandy conglomerates of fan-delta facies in the Linhe depression of Hetao Basin,west China,and aims at exploring a reservoir space formation mechanism related to fractures.Methods: Based on the seismic profile analysis,through the drilling core and microscopic petrographic observation under polarizing microscope of the fan-delta facies sandy conglomerates.Results: The results indicate that the fan-delta facies sandy conglomerates consist of broken and intact sections vertically alternative.The sandy conglomerates in the broken sections are broken severely with a small amount of loose and fragile rock fragments remained,among which the debris particles are mostly crazed,and the original argillaceous interstitial materials among the particles are mostly disappeared.The sandy conglomerates in the intact sections have no significant fracture,but the ones near the broken sections unevenly have intergranular pores and clastic fracture pores,which are present locally in rocks in various forms (e.g.,strips,irregular patches,subcircular or elliptic spots).Conclusions: Based on the systematic study of macroscopic and microscopic petrology,this paper gives a genesis mechanism of "fault—earthquake—high-pressure water jet process" as follows.① The extremely thick sandy conglomerates of fan-delta facies are originally cemented by argillaceous materials (or micrite calcalites) and are dense with no or rare pores.They are fractured due to the cutting of faults within the basin,forming the most important reservoir spaces, i.e.,fracture zones.② With the occurrence of seismic activities,the faulting will cause rapid structural stress action,and result in brittle fractures within the debris particles of the rocks.At the same time,the earthquakes will cause agitation of groundwater in the fault zones,thus forming high-pressure fluid jet in and to both sides of the fault zones.The impingement of liquid columns can form pore areas in the intact sandy conglomerates near the broken sections,and,at the same time, cause the particles to break by the impact between them.③ The brittle fractures of particles will be preserved by high pressure fluid injection.