THE PROTOTYPE BASIN OF CAMBRIAN ZHANGXIA PERIOD AND OIL—GAS EXPLORATION PROSPECT IN THE ORDOS BASIN

The substantially distributed Middle Cambrian oolitic limestone and oolitic dolomite in Ordos Basin are potential targets for petroleum and gas explorations.The appearance of alternative pattern of depressions and uplifts of Zhangxia Stage,together with the north—south trending of central paleouplift actually played the role of separating the Qilian Sea on the west margin of Ordos Basin and the Qinling Sea on the south margin of Ordos Basin.The Zhangxia Formation is composed mainly of complex oolitic structures with dolomitization and burial erosion as major diagenetic processes.The Zhangxia Formation is predominately low porosity and low permeability carbonate reservoirs.The reservoir capacity,cap assemblage and tectonic setting are among those favorable conditions for the preservation of petroleum or gas while the poorly developed source rock for hydrocarbon in Cambrian was the disadvantage of further exploration.