Due to wide application of horizontal well fracturing technology in the Ordovician weathering crust reservoirs of Daniudi gasfield

Ordos Basin

karst reservoirs exhibit its great potential of gas production

regardless of its low porosity

permeability and strong heterogeneity.However

development pattern of reservoir is the key of gas accumulation in karst reservoirs.Karst cycles and reservoirs distribution are studied by using lithofacies based on core observation and well-logging interpretation.The following results were obtained.(1) Three Karst cycles can be identified in the Ordovician Majiagou Formation.The upper two karst cycles formed in the earlier time and are larger in scale than the lower one; therefore the development of karst reservoirs are mainly related to the upper two karst cycles.(2) The long-term dissolution of gypsiferous dolostone and muddy dolostone layers produces fractures in cave-roof zone and finally led to the collapse of overlying strata

forming a thick layer of breccias.(3) Reservoirs are mainly developed in areas with relatively weak karst intensity.Laterally

they mainly occur in monadnock of palaeogeomorphology slope

while vertically

they are distributed in the intact vadose zones of upper two karst cycles.The lithofacies are dominated by crackle-mosaic breccias lithofacies.The research on karst cycles and lithofacies will play significant roles in the reservoir predication for the following gas exploration.