Abstract:

It is now a hot spot to prospect oil and gas in deep of Ordos basin, North China. Seven new lithofacies palaeogeography maps on Ordos have been sketched out, according to the guideline of which tectonism controls sedimentation, applying the sequence stratigraphy theory, on the ground of field outcrops observations, core observations, logging data and seismic profiles, applying the method of single factor analysis and comprehensive mapping, and taking tectonic-sediment evolution as storyline, and reservoir-accumulation as the goal. The research has shown that there was a dynamic evolution process in Cambrian in Ordos, in which carbonate ramps formed platforms and restricted platforms. The carbonate ramps were characterized by exchanging of clear water and muddy water. The research also shows that drought in early became relative humidity in late. There was a tectonic differentiation in south-north earlier and west-east later. Southern Ordos margin appeared aulacogen. Ancient Ordos Land (AOL) was, fundamentally, covered by seawater except the parts of north and west margin in the Middle-Late Cambrian. A conversion in which land was changed to sea had been completed. The characteristic of palaeogeography was mainly a shallowwater carbonate platform on the continental sea. The forming of “Ancient Ordos Sea” (AOS) was a turning point. The west arises in Late Cambrian，which is a rudiment of “Central Paleouplift”(CP) which mainly had formed in Ordovician. The Late Cambrian uplift ran in north to south and is basically the same range as “Ordovician Central Paleouplift”. Therefore, it has been proved that the earliest age of forming “Central Ordos Paleouplift” is from Cambrian Furongian to the Early Ordovician. It has also been proved that tidal flat is dominating facies in the Early Cambrian, oliticbeachs are distributed around the land in Zhangxia Formation in the Middle Cambrian and porous dolostones which distributed in whole areas mainly occur in the top of Cambrian. Huaiyuan movement is a key reason why weathering and karstification took place on the top of Cambrian in Ordos. The subduction of ancient Qilian Ocean led to the chief geodynamic differentiation of Ordos Ancient Land. The unconformity surface of the top of Cambrian and part Lower Ordovician should be the most important deep exploration target.

Key words: Ordos basin, Cambrian, sedimentary tectonic, lithofacies palaeogeography, deep exploration