Abstract:

The Ordovician carbonate fracture-cavern reservoir has been the most important oil and gas production interval in Tahe oilfield. Large numbers of karst caverns conduced by the paleo-karstification play an important role in the oil and gas storage. The karst caverns filled with sedimentary deposits are divided into four karst cavern filled facies, which are speleothem facies, transposition lithofacies, collapsebreccia facies, vadose filling facies, in 11 karst caverns filled wells. The transposition lithofacies is the karst cavern fillings at the early stage. When the corrosion of surface karst develops to a certain extent, the rocks break down to form the collapse-breccia facies, and later stages the vadose fillings is the interstitial matrix in collapse-breccia facies.  From the model of well Ta, the authors get a caverns filling order, which has the top cavern disturbed facies,middle collapse facies and the bottom marine influx filled facies. The trace element and rare earth element geochemistry characterization of cavern fillings proves the marine-meteoric water mixed karst, which will guide the production of the oilfield.

Key words:  Tahe oilfield, Ordovician, karst cavern filling, geochemistry