Formation conditions and main controlling factors of reservoirs in Jurassic Yan’an Formation in southwestern Ordos Basin

The southwestern part of Ordos Basin is rich in oil and gas resources， and in recent years， a series of lithologic-structural reservoirs with small oil-bearing areas and high reserve abundance have been discovered in the Jurassic Yan’an Formation of the area. The reservoirs are small in scale， and their main controlling factors are unclear. To reveal the formation mechanism and main controlling factors of the Jurassic Yan’an Formation reservoirs in the Ordos Basin， this paper analyzed the characteristics of the reservoirs in the study area in terms of oil source conditions， paleo-geomorphology， structure， and reservoir and summarized the hydrocarbon accumulation modes in the southwestern part of the Ordos Basin under the control of the paleo-geomorphology by means of 3D seismic， microscopic observation， spectral test， and well logging data. At the same time， the main controlling factors of the Jurassic Yan’an Formation reservoir were discussed， including the aspects of oil source filling， unconformity and connected sands， paleo-geomorphology， structures， and reservoirs. The results showed that the source rocks of Chang 7 Member of Yanchang Formation in Ordos Basin generate hydrocarbon and discharge oil and gas. Under the effect of the residual pressure difference， the oil and gas migrate along the faults， fractures， and high-permeability sand bodies laterally and longitudinally， continuously move towards reservoirs in the overlying Jurassic Yan’an Formation， and accumulate at the high-structure position and under conditions of overlying and lateral mudstone occlusion. The Jurassic Yan’an Formation reservoirs in the southwestern Ordos Basin are characterized by late mature to high mature oil source charging， and they are lithological-structural reservoirs dominated by paleo-geomorphology and controlled by faults， fractures， and sedimentary reservoirs. The hydrocarbon accumulation modes are “paleo-river control， sandbody transport， and enrichment at high points，” and the reservoirs are horizontally distributed like a string of beads. The paper proposed a three-dimensional transport network system based on the unconformity， fractures， and high-permeability river sand bodies in the Jurassic Yan’an Formation reservoirs in the Ordos Basin， which provides a theoretical basis for the large-scale exploration and development of the Jurassic reservoirs in the Ordos Basin.