An initial discussion on major controlling factors on formation of coal-formed large-medium gas fields

The so-called “large-medium gas field” means a gas field with proven reserves equal to or larger than 100 × 108 m3. Up to 1997, 23 coal-formed great-medium fields are discovered in China, excluding two in Taiwan Province. Their proven reserves take 50.9% in total natural gas reserves. Therefore, the coal-formed great-medium gas field plays a decisive role in natural gas reserves and the study of their controlling factors is very important both in theory and in practice. There are six major controlling factors on the formation of coal-formed large-medium gas fields: (i) The generating center of coal-formed gas and its surrounding area; (ii) low potential area; (iii) late pool-forming period; (iv) traps related to the paleo-uplifits in coal-formed gas area; (v) above or between abnormally pressured compartments; (vi) faulted traps related to coal-bed.