Abstract: In tight reservoirs,lime,mica,calcareous and other brittle mineral often exist because the composition of mudstone interbeds is impure.This greatly heightens the brittle degree of mudstone,and makes it liable to form fractures under tectonic stress.So tectonic fractures are relatively developed in mudstone.Different series and different dips of tectonic fractures in mudstone interbeds are interconnected with nontectonic fractures often to form the 3-D network ststem of fractures.This makes the permeability of mudstone interbeds greatly increased and the fractures in mudstone commonly have better oil-bearing features.Thus,the mudstone in tight reservoirs generally cannot perform a true sealing function and can be classified as quasireservoirs or fractures reservoirs of mudstone.Their accumulation patterns are massive.