The characteristics and origins of cleat in coal from Western North China

Cleat in coal influences the occurrence. migration, and production of coalbed methane. Therefore, research on the cleat can provide some important information for coalbed methane exploration and development. The characteristics and origin of cleat in coal of Western North China are discussed with respect to field investigation and laboratory experiments. Cleat in coal of Western North China can be divided into two classes: face cleat and butt cleat. Three network patterns of face and butt cleats are identified: reticular, isolated, and random. When the vitrinite reflectance is less than 1.35%, the cleat frequency increases with the rise of coal rank. When the reflectance is more than 1.35%, there are three relationships between the cleat frequency and coal rank: with the rise of coal rank, (1) the cleat frequency decreases, (2) remains constant. or (3) decreases slowly and then remains constant. The mineral filling in the cleats is mainly calcite, rarely clay minerals and pyrite. Intrinsic tensile forces, fluid pressure. and tectonic stresses are three controls to form cleats. Two mechanisms of cleat annealing, which are the filling of secondary maceral and agglutination, are revealed. Finally, the controls on the cleat annealing are briefly described. (C) 2001 Elsevier Science B.V. All rights reserved.