The palaeoweathering crust-type karst reservoir rocks in carbonate rocks

The giant oil and gas fields are totally hosted in marine basins in the world.The reservoir spaces mainly include pore spaces and fissures of carbonate karsts.The palaeoweathering crust-type karsts tend to be developed on the composite surfaces between sequence unconformity and regional stratigraphic unconformity in the epidiagenetic environments.The breakthrough in the studies of the Ordovician palaeokarst oil and gas accumulations in the Tarim Basin,Xinjiang and Ordos Basin,Nei Mongol is circumstantial evidence in favour of special significance of carbonate karst reservoir rocks.The formation and evolution of the carbonate palaeoweathering crust-type karsts in the marine basins in China are all coupled with episodic paleo-uplifts and sea-level falls in the Caledonian tectonic cycles.The burial stages and hydrothermal corrosion may improve the reservoir capacity of carbonate karsts.The prediction of the palaeweathering crust-type karst rerservoir rocks in large-scale oil and gas fields may be implemented by tracing the regional stratigraphic unconformity and sequence boundary with the aid of seismic profiles,delineating the spatio-temporal distribution of palaeokarsts,and reconstructing the palaeokarst geomorphology and its relations with different lithofacies.The oxygen and carbon isotopes are different for pore space-filled calcite and carbonate rocks as wall rocks:Z values are less than 120 for the former,which may be an important indicator for fresh-water karst environments.