Structural compartmentalization and its relationships with gas accumulation and gas production in the Zhengzhuang Field, southern Qinshui Basin

Abstract

The Zhengzhuang Field is a new developing coalbed methane (CBM) field in the southern Qinshui Basin. To disclose the high interwell heterogeneity in production, we provide a detailed investigation of local structural controls on the gas accumulation in the Zhengzhuang Field. The Zhengzhuang Field is a northwest-dipping homocline that contains abundant normal faults, and several secondary folds, karst collapse columns, and thrust faults. These structures divided the study area into three regions (fault-trough, fault-fold, and sub-sag) and 11 sub-regions. The gas content of the No. 3 coal seam is 0 to 35 m3/t, and the gas content exhibits a distinctly positive correlation with the methane δ13C isotopic ratio. Both gas content and methane δ13C isotopic composition are compartmentalized by structural regions or sub-regions, indicating significant effects from structural effects. The general gas-content distribution was separated by low-permeability boundaries created by F1 and F2 faults, resulting in several gas reservoir compartmentalizations. Locally, secondary faulting and small-scale folding resulted in redistributions of gas contents, i.e., relatively high gas content on the downthrow side of secondary faults and in the structural-trough zones of small-scale folds. As a continuous-type gas reservoir, the CBM accumulation mechanism is complex and differs from conventional gas reservoirs. Because coal has an extremely high adsorption capacity, the gravity separation of free gas and water is notably subordinate to sorption on micropore surfaces in CBM reservoirs. It is known that the No. 3 coal seam has extremely low permeability and low–medium gas saturation; thus, the gas isotopic fraction caused by hydrodynamic flushing probably enhanced the gas desorption and resulted in the migration of δ13C-enriched gas within the coal seam and into adjacent locations with high reservoir pressures for gas re-adsorption. This mechanism explains the relatively high gas content and enriched methane isotopic composition in the local structural lows in the Zhengzhuang Field. Structural controls on gas production indicate that the most productive CBM wells are commonly completed in the structural highs of the fault-nose-structure (i.e., the sweet spots), where there is high reservoir permeability resulting from stress relief, and also relatively high gas content induced by the secondary migration of gas from the structural lows. The source driving force for gas migration within the coal seam is the result of gas diffusion fractionation which commonly occurs simultaneously with the slow seepage of groundwater from low to high spots in the Zhengzhuang Field.