Secondary macropores and mosaic structure in a telemagmatic metamorphosed coal and their significance in exploration for natural gas, Henan, China

Abstract

Secondary macropores and mosaic structure, common characteristics in metallurgical and natural cokes, are considered to be the solid residue after the release of liquid and gas during the heating process. In north-central Henan province, People's Republic of China, secondary macropores and “mosaic” structure are found in semi-anthracite and anthracite of lower Permian telemagmatic (not in contact with intrusion) metamorphic coal. Coal rank exhibits a zoned distribution cored by a northwest-southeast trending zone of anthracite. Coal rank decreases from this central zone toward both the northeast and southwest margins from low-volatile bituminous through medium-volatile bituminous to high-volatile bituminous. The zoned rank distribution of coal in this area is interpreted as being the result of regional metamorphism overprinted by telemagmatic metamorphism. Coal metamorphism in this area occurred in three stages: (1) pre-orogenic (late Carboniferous-late Triassic). Regional metamorphism produced coals of subbituminous to high volatile bituminous rank. (2) Orogenic (early Jurassic-late Cretaceous). Telemagmatic metamorphism resulted in zones of higher rank coal (medium-volatile bituminous through anthracite rank). (3) Post-orogenic (Tertiary-Quaternary). Shallow burial depth due to the tectonic uplift followed by erosion had a negligible effect on coal rank. Secondary macropores and “mosaic” structure found in semi-anthracite and anthracite in the research area are similar in form, by microscopic and SEM observation, to those of metallurgical cokes and other natural cokes, indicating extraordinary heating effect on the coal. High temperature, high-pressure experiments on high volatile bituminous coal from the marginal area show that the pores develop in the 250–300°C range and “mosaic” structure develops in the 350–500°C range. It is therefore postulated that the high rank coal in this area may have been subjected to temperatures similar to carbonization temperatures during the orogenic period. Because the heat sources for telemagmatic metamorphism are deep-seated plutons, the rate of heating on this coal might be relatively slow, the time involved in the metamorphism was considerably longer, and the affected area might be extensive. During the heating process, bituminous coal near the center of high heat flow region was heated to a thermoplastic temperature range and coal underwent a soft stage (plastic). Gases evolved in this phase and left the pore system in their wake. Mesophase growth units may also develop in this phase, which could get compressed into each other to form mosaic structure. The existence of secondary macropores and mosaic structure in coal can be used as an indicator of heating effect and gas generation. Ubiquitous macropores developed in the coal in a considerable area indicate that a regional gas-generation process occurred. The gases generated by thermal cracking of the coal during the heating process could be captured later by the macropores, migrate through the network of pores, and form the underground gas reservoirs where the storage conditions are favorable for gas retention. Therefore secondary macropores and mosaic structure in telemagmatic metamorphic coal have potential significance in natural gas exploration.