TYPES AND RECOGNIZABLE INDICATORS OF ORDOVICIAN CARBONATE ROCK OKARSTIFICATION IN TARIM BASIN

Three types of palaeokarst can be recognized in Ordovician non-marine carbonate in Tarim Basin,which are syndepositional rock-dissolution,weathering karstification and buried rock-dissolution.Their main recognizable indicators are summed up by analyzing the responsive characteristics in the aspects of the geology,well logging,drilling and geophysics.The recognizable indicators of syndepositional rock-dissolution include abundant texture-optimal dissolved-pores,such as the intra-grain,mold and inter-grain dissolved-pores,and non-texture-optimized dissolved-pores,small-scale dissolved-caves,dissolved channels,dissolved-fissures,as well as vadose silts and meteoric freshwater cements.The recognizable indicators of weathering karstification include the Ordovician carbonate weathering out and residue on a regional unconformity,high-angle dissolved channels,dissolved fissures,sinkhole,dissolved pores and caves with different scale and the fillings,the carbonate fillings in the dissolved pores and caves are of the geochemical characteristics that meteoric freshwater is of,the drilling break leakage occures,the well logging response is characteried by high combination gamma ray neutron value and low electrical resistivity value,as well as the low-velocity seismic response characteristics are dominant.The buried rock-dissolution can be recognized by further original pore and cave dissolution and infilling by oil and gas.It is indicated that early development and distribution of carbonate reservoir are controlled by the syndepositional rock-dissolution,the weathering karstification is the key to the formation of carbonate reservoir,the buried rock-dissolution is one of key factors of optimal reworking of carbonate reservoir.