Fracture Development Characteristics And Cause Analysis Of The Yanchang Formation In The Southeastern Part Of North Shaanxi Slope

Fractures are pathways for oil and gas migration,at the same time they are responses to regional tectonic stress.Ordos basin is a polycyclic superimposed basin located on the North China Craton,which has experienced long-term structural evolution.Due to the influence of regional tectonic stress field,regional tectonic fractures are more developed in sedimentary cover.The upper Triassic Yanchang Formation is the main oil-bearing system in the basin,which is characterized by low porosity,low permeability and extra-low permeability.The development and distribution of structural fractures have an important influence on the development of oil and gas,and can also provide a basic geological basis for the study of tectonic stress evolution in the basin.Through the outcrop observation,combined with core description,microscopic rock thin section analysis,referring to previous research results,and comprehensively using of sedimentology,structural geology,this article carry out the study of sedimentary characteristics of the upper Triassic Yanchang Formation and fracture development characteristics in the southeast of Shanbei Slope of ordos basin,following results and conclusions:The lithology of Yanchang Formation in the study area is gray,gray-green medium thick layer of massive fine sandstone,siltstone,dark gray,gray black mudstone,with a thickness of about 1100m?1400m.The sedimentary facies are mainly lacustrine facies and delta facies.Fractures developed in Yanchang Formation in the study area,give priority to with high angle shear fractures.According to the direction of fracture development and cutting restrictions relationship,four groups of fractures are divided into two periods: the early for E-W direction and nearly NW-SE direction crack,late for S-N direction and nearly NE-SW direction cracks.The former is formed under the Yanshanian tectonic stress field,the latter is formed under the Himalayan tectonic stress field,and the E-W direction fracture is the most developed in the study area.The Yanshanian period was mainly affected by the remote tectonic effect of subduction collision between the Paleo-Pacific plate and the Eurasian plate,and the fractures formed in the direction of E-W and near NW-SE.The Himalayan period was mainly affected by the tectonic effect of subduction and collision process between Indian plate and Eurasian plate,which was NE-SW direction extrusion,corresponding to fractures in S-N direction and near NE-SW direction.In addition to paleotectonic stress,the development of fractures is also affected by lithology,thickness and heterogeneity of rock strata.The fractures in sandstone are relatively developed,but not in mudstone.The thicker the strata,the less developed fractures.Affected by the heterogeneity of rock strata,fracture development is not uniform,and different positions and layers in the study area have different levels of fracture development during the same period.