THE DISCOVERY AND CHARACTERISTICS OF COVERED-PRESSURIZATION KARST DURING EARLY HERCYNIAN IN SOUTH TAHE OILFIELD

The Ordovician formations of Tahe Oilfield mainly undergo two karsts of mid-Caledonian and earlyHercynian.The predecessors considered that because of the thick aquifuge in Sangtamu Group(O3),the effectingrange of early-Hercynian karst was controlled by the edge line of the Sangtamu Group: The Northern part which lackNo.3 Abstract Vthe Sangtamu Group was the affecting area of early-Hercynian karst,while,the Southern part which was coveredby the Sangtamu Group was the the affecting area of only mid-Caledonian karst.Recent research shows that for thereason of its favourable factors,such as high-uplift amplitude of structure,long time exposure,highly developmentof fractures,fluent hydrodynamic etc,it is extremely beneficial to karst effections.On top of that,the early Hercyniankarstis able to break through the restriction of the Sangtamu aquifuge.The coverage of the Sangtamu group onlychanges the model of early-Hercynian karst:from crust karst to Covered-Pressurization karst.The discovery of thiskarst has a great guiding significance to the exploration for cavity-reservoirs in the southern and southwestern areas.It points out that,only the fault zones which are connected with the main region can be the favourable reservoirzones,otherwise,Reservoir would be less developed.