Investigations of the karst during the Episode III of the mid-Caledonian in the Tahe Oilfield

Researches on the karst stage of Ordovician carbonates in the Tahe Oilfield have been ceaselessly renewed.Predecessors considered that there were mainly mid-Caledonian(Episode Ⅰ,Episode Ⅱ) karst and early-Hercynian karst in the Tahe Oilfield.However,through the study of petrology,sedimentology and seismology,this paper concluded that the Episode Ⅲ karst of mid-Caledonian existed in the Tahe Oilfield.The evidence indicated a series of recognition markers,such as there developed a corroded valley of a NS-trend paleo-river at the top of the Sangtamu Formation and the residual thickness of the formation within the Silurian coverage became thinner from southeast to northwest,etc.The main layer and area affected by the Episode Ⅲ were in the middle-lower Ordovician pure limestone stratum and the northwestern paleo-geographic high,respectively.Restricted by an aquifuge of the Sangtamu Formation and the global ice age from the end of Ordovician to early Silurian,this karst was represented by an exposed weathering crust and the karstification occurring along fractures,respectively in the north and the south to the wedge out of the Sangtamu Formation.