Coal measures superimposed gas reservoir and its exploration and development technology modes

Coal measures superimposed gas reservoir is a key reservoir type of coal-measure gas in coal bearing formation. The discovery, exploration and development practice of coal measures superimposed gas reservoirs in China have opened up a new chapter for the coal-measure gas exploration. In this study, the concept, main types, development characteristics and occurrence distribution of coal measures superimposed gas reservoir are proposed and expounded. The formation prerequisite, accumulation process and reservoir forming mechanism of coal measure superimposed gas reservoir are analyzed. Specifically, the geological suitability exploration and development technology modes of two typical coal measures superimposed gas reservoirs are emphatically discussed, and the application of coal measure superimposed gas reservoir exploration and exploitation technologies in coal-measure gas reservoirs is prospected. At present, three types of coal measure superimposed gas reservoirs have been found in China, which are: (distributary) channel sandstone-coal syntagmatic relation type, developed in the Shanxi Formation of north China (denoted as the North China type); coal-sandstone-mudstone interbedding type, developed in the Longtan Formation of south China (denoted as the South China type); coal-sandstone-mudstone interbedded conglomerate type, developed in the Chenghe Formation of northeast China (denoted as the Northeast China type). As a crucial controlling geological factor, sedimentary microfacies combination of mire, distributary channel and interdistributary bay in delta depositional system is favorable to form the coal measure superimposed gas reservoir. The superimposed composite reservoir structure and lithologic trap are the important characteristics of coal measure superimposed gas reservoir and the unified gas-bearing system and pressure gradient account for its essential characteristics. Furthermore, the critical formation mechanism of coal measure superimposed gas reservoir involves the transfer and balance of matter and energy among coal seams, sandstones, and shales, the migration and phase transformation among coalbed methane, tight sandstone gas, and shale gas. Himalayan orogeny is the key period for the coal measure superimposed gas reservoir formation in Shanxi formation in north China. On a technical level, high-resolution seismic lithofacies interpretation for gas reservoir identification and virtual gas-producing bedding development have been proven to be powerful means in the exploration and development of North China type coal measure superimposed gas reservoirs. Techniques such as interval optimization, small-layer perforation, staged fracturing, ball-drop pressure separation and combined layer drainage corporately contribute to the adaptive development of Longtan Formation coal measures superimposed reservoir in south China. These technical modes have made breakthroughs in the co-exploration and co-production of coal measures gas, and predictably, they will continually provide strong backing for the high efficiency development of deep buried coalbed methane in China.