Analysis of Control Factors of Ordovician Karst Reservoir in Ordos Basin

Ordovician karst reservoirs of Ordos Basin experienced several superpositions.According to evolutionary periods of paleokarst,three maps are drawn: Ordovician stratum thickness map,Pre-carboniferous karst landform and Ordovician top structure maps of Ordos Basin.Combined with karst development mechanism,the control factors of karst reservoir formation of each period are systematically analyzed,and then the favorable belts of karst reservoirs are comprehensively forecasted,so an advantageous direction of oil and gas exploration of Ordovician is obtained.In syndeposit peroid,there exited three old uplifts in Ordos Basin which controlled the karst reservoir development: Yimeng uplift,Central uplift and Hancheng uplift.The saddle and slope of the uplifts and micro-uplift of the depression are favorable areas of karst reservoir.In naked weathering period,paleokarst landform and paleo-groove distribution were the main control factors of karst reservoir development.Paleokarst sloping field was more favorable for karst evolution.In burial period,the middle part of the Ordovician top surface was high in the west and low in the east.There was a steep slope between the east uplift and the central depression.Water of compaction migrated and concentrated towards the central uplift and the steep slope was an intensive transition region.This character defined the final distribution of karst reservoirs.As a result,the superimposed position of the uplift slope in syndeposit period,the karst sloping field in naked residue period and the steep slope in burial period should be the favorable areas of karst reservoir development.