Abstract: Fractures and vugs are the major reservoir spaces in Tahe oilfield,but more than 70% of their spaces were filled by underground river sediments and cavern-roof collapse breccias,of which the latter accounts for over 30% in the cavern fillings.It is of great significant for reservoir geology of karst carbonates to study the formation and distribution of the collapse breccias.Combined with investigations on the Ordovician karst outcrops,observations of core samples and interpretations on well logging and seismic data,it was found that the collapse breccias are various in sizes and shapes in which small size of the breccias are transported by underground river and interbedded with sedimentary debris layers in caverns.The larger the caverns were(such as chamber caverns and main channel caverns)and the closer to active faults and epi-karst zone,the larger the thickness of the collapse breccias are.Porosities and permeabilities of the collapse breccias are low when the grain size of the breccias is smaller than 2cm.The breccias with bigger grain size would have better reservoir physical property and oil-bearing property,and thus should be one part of reservoir rocks in Tahe oilfield.

Key words: fracture-vug, collapse breccias, runoff zone, Ordovician, Tahe oilfield