Characteristics of Jurassic/Triassic Unconformity Structure in Ordos Basin and Its Effect on Oil and Gas

Longdong area is located on the southwest edge of the Ordos basin. As the Indosinian movement at the end of the Triassic led to the uplift and erosion of the Ordos basin, the pre Jurassic ancient landform in Longdong area was formed. The erosion weathering time, erosion depth, late filling, etc. of different geomorphic units are obviously different, forming the unconformity structure of different geomorphic units. After studying the logging curve, element geochemistry, physical property and other data of key well locations in the study area, It is found that the unconformity structure characteristics of different palaeogeomorphic units are different. The results show that the strata above the unconformity are mainly channel filled sandstone, and the logging curves are characterized by high resistivity and low natural gamma; Weathered clay layer is a layer with poor physical properties in unconformity structure. The lithology is mainly mudstone and sandy mudstone. The logging curve is characterized by high natural gamma and low density; The weathering leaching zone has a variety of rock types, and the reservoir connectivity is good. The logging curve is characterized by obvious sawtooth diameter curve and low resistivity, which is mainly developed on the slope of the ancient landform. The impact of unconformity structure on oil and gas accumulation is mainly reflected in that the weathering leaching zone provides a transport system for upward migration of oil. On the other hand, the distribution of weathered clay layers controls the location of oil entering Jurassic reservoirs.