Palaeogeomorphology Restoring of Ordovician weathering crust in Yan’an area, Ordos Basin

Key words: 
• Ordos Basin /
• Yan&prime /
• an area /
• palaeogeomorphic feature /
• the method of remnant thickness /
• thickness of bauxite

Abstract: The karst reservoir rock of Ordovician weathering crust is developed on the top of Majiagou Formation, Ordovician system, in Yan′an area, southeastern Ordos Basin. Because of the palaeogeomorphology stand for background of sedimentation and diagenesis in formations, so it is important in formation and evolutionary process of reservoir rocks. On the basis of regional tectonic setting and formation distribution, utilizing comprehensively "methods of residual thickness" and "methods of moulage", combining with thickness distribution of bauxite and identifing results of ancient erosion grooves, the spatial pattern of palaeogeomorphic features from Ordovician weathering crust in Yan'an area is restored correctly. It is showed from the results that it displays for karst highland in southwest, mostly of karst slope in middle parts, and intermediate zone between karst slope and karst basin in east of Yan'an area.The formation and development of reservoir rocks and accumulation of gas are influenced by the spatial pattern of palaeogeomorphic features and distribution of ancient erosion grooves. There are better gas display in karst paleo-highland, as well as in slope adjacent to eroded ancient erosion grooves, responsible for development degree of reservoir rock, physical property and sealing nature of caprock upon unconformity surface, caused probably by learching of meteoric water.