Characteristics of Coal-Measure Gas Reservoirs in Thin Interbedded Marine-Continental Transitional Facies and Optimization of Combined Production: Examples from the Tucheng Syncline in Western Guizhou

Thin interbedded coal-measure gas (CMG) reservoirs with marine-continental transitional facies have particularities, and the optimization of the production layer is a critical component of their successful development. Parameters from seven Late Permian CMG wells (i.e., geochemical, reservoir physical characteristics, and gas-bearing test data) in the north end of Tucheng Syncline in western Guizhou were collected. Combined with regional geological data, the development characteristics, geochemical characteristics, reservoir porosity, permeability characteristics, and gas-bearing characteristics of CMG in the thin interbedded Longtan Formation of marine-continental transitional facies were analyzed systematically. Then, four geological characteristics of the thin interbedded CMG reservoir of the Longtan Formation of the marine-continental transitional facies were defined as follows. (1) The sedimentary environment of coal measures in marine-continental transitional facies changes frequently; coal, shale and sandstone are interbedded frequently; and the thickness of each type of reservoir varies greatly, but the cumulative result is thick. (2) Mud shale and sandstone in coal-measure strata have high TOC and similar R-O, and shale and sandstone have similar inorganic content, showing the characteristics of high clay and low brittleness, and sandstone showing obvious shale characteristics. (3) The coal seams have a significant gas concentration and are the foundation of CMG resources. The gas content in shale is unevenly distributed. Some carbonaceous mudstone is close to the coal seam, and the gas content of sandstone is the lowest. The gas content of sandstone and mudstone has a certain correlation with the distance from the coal seam. Within 1 m from the coal seam, the gas content of sandstone and mudstone in coal measures is generally higher than 1 m(3)/t. (4) The coal measure reservoir has low porosity and permeability, so industrial gas flow can be obtained only through engineering transformation. Based on these geological characteristics and on the "three-step" optimization combination of multicoal seam production layers and principal components analysis of non-coal measure reservoir physical properties, a "four-step" optimization of coal measure reservoir combined production sections was established. Taking well PC-1 as representative of the spatial relationship between the target coal seam and sandstone layer, three sets of CMG combined production combinations were optimized.