Quantitative analysis of the main controlling factors for coal-formed gas accumulation in Dongpu Depression

Key words:  coal-formed gas reservoir  accumulation factors  enrichment scale  control factors  quantitative evaluation  Dongpu Depression

Abstract:

Based on exploration results and previous studies, the differences in the coal-formed gas accumulation conditions among different coal-formed gas areas of Dongpu Depression were compared, and the main controlling factors of coal-formed gas accumulation were determined. The coefficients of fault transport efficiency and preservation condition coefficients were proposed quantitatively in this paper. The quantitative analysis of controlling factors for the scale of coal-formed gas reservoirs were carried out based on weights assignment method, fuzzy evaluation method and multiple linear regression method, and a quantitative prediction model for the enrichment scale of coal-formed gas reservoirs in Dongpu Depression was established based on multiple factors. Finally, the exploration potential areas were optimized. The results show that the macro distribution of coal-formed gas in the study area is controlled by the hydrocarbon generation capacity, and all the coal-formed gas reservoirs are distributed around the area where the gas generation intensity is greater than 20×10⁸ m³/km². The relationship between the fault activity and the hydrocarbon generation period of gas source rocks controls the effectivity of gas migration. The combination between fault and cap rocks controls the vertical enrichment of natural gas, and the coal-formed gas is distributed in the stratigraphic level which the faulted thickness of cap rock is more than 40 m. Sufficient residual pressure D-value between source and reservoir is a necessary condition for efficient coal-formed gas accumulation. The enrichment degree of coal-formed gas reservoirs is multi-controlled by "gas supply-dynamics-transportation-preservation" conditions, among which, the gas supply and preservation condition are the main controlling factors, while the dynamic and transportation conditions are less important. Combining results from quantitative forecasting model, it is concluded that the central uplift zone and the southwestern depression are the key potential areas.