Preliminary Evaluation Of Gas Enrichment And Exploitation Potential Of Coalbed Methane In Sanjing-Mulinghe Coal-bearing Area

Sanjiang-Mulinghe coal-bearing zone is rich in coalbed methane (CBM) resource, and isone of the most prospective low-rank CBM development districts in China. However, it isdifficult to ascertain CBM enrichment and its geological controls due to complicated tectonicevolution, various depositional environments of the coal-bearing strata, multiple and thin coalseams in the zone. In this paper,4basins (Hegang, Jixi, Hulin and Boli) and1depression(Suibin) were studied on geological setting, depositional environment, coal reservoir propertyand CBM enrichment. They were also evaluated comprehensively for favorable CBMdevelopment district. Results are as follows.(1) The study area underwent multiple tectonic uplift and subsidence. At ChengziheFormation stage, Boli and Jixi basins and Suibin depression underwent intensely tectonicsubsidence leading to the deposition of thick coal-bearing sedimentary system. In Hulin basin,thick coal-bearing strata were formed resulting from the Paleogene taphrogeny. At ChengziheFormation stage, the depositional environment is dominated by lacustrine facies in Suibindepression. Favorable coal-accumulation zone is located at delta front and shore-shallow lakeenvironment. Coal-accumulated environment is mainly composed of fluvial facies andfluvial-dominated delta in Boli basin, while delta plain and fluvial alluvial plain in Jixi basin.For Hulin basin, Paleogene Hulin Formation is the main coal-bearing strata, and itsdepositional environments are dominated by fluvial facies and lacustrine facies.(2) Many methods (mercury porosimetry, low-temperature N2, nuclear magneticresonance, X-CT, isothermal adsorption, proximate and ultimate analyses, etc.) were used forinvestigating coal reservoir physical properties. Results show that:(a) Coal macerals arecomposed of abundant vitrinite (34.0-95.1%), moderate inertinite (0.6-62.7%) and a few ofexinite (0.7-18.2%);(b) Coal pores are mainly dominated by micropore (mean55.9%),secondly by mesopore and macrpore (mean22.3%and21.8%). Volume content is higher forthe micropores (<10nm, in diameter) than that for the micropores with a diameter of10-100nm. Additionally, structural morphology of micropores is usually like ink-bottle andparallel-plate;(c) Types C and D are main microfractures in coals, while Type A is notexistent in coals;(d) Langmuir volume is relatively high in the zone ranging from14.32to21.79m3/t (mean17.18m3/t, in air-drying basis).(3) Based on mercury porosimetry analyses of18coal samples, micro-, meso-,macropores and fractures were distinguished by the4"turning points" of mercury intrusioncurves. The method was verified by the fractal geometry theory used for calculating fractal dimensions of differently sized pore-fractures. Thus, the pore-fractures of coals werecharacterizated quantitively by mercury intrusion curves. Furthermore, concerning CBMexploitation, a new method evaluating favorable coal reservoirs was established by usingmercury porosimetry based on achievements above.(4) The pore-fractures of coals were studied by low-field nuclear magnetic resonance(NMR) and CT techniques in this paper. The porosity, effective porosity (mobile fluid) andpermeability of coals were evaluated by using NMR technique. T₂spectral distribution ofcoal will change resulting from the change of pore structure with increasing temperature. Thechange is different for coals with different rank. Coal pore-fractures and mineral matterdistribution can be quantitively analyzed by CT technique. Based on that, planar porosity andmineral matter content of coals were evaluated.(5) In the study area, the Mesozoic-Cenozoic coals are characterized by gas-lean coalswith relatively high metamorphic grade. Coalbed methane is mainly thermogenic, secondlybiogenic. However, thermogenic methane scattered and escaped seriously during multipletaphrogeny in the study area. Thus, favorable CBM development districts are alwayscharacterized by thick coalbed, good sealing-capping of roof and effective tectonic orhydrodynamic sealing.(6) Based on above-mentioned achievements, geological structure, coal rank, sedimentaryfacies, accumulated coal thickness, gas content, burial depth and permeability were selectedfor evaluating favorable CBM development districts by combining fuzzy mathematic theorywith GIS software on five primary coal-bearing basins. Results show that favorable districtsprimarily locate at southern Jidong depression, and Nanshan and Junde mines in Hegangbasin. Additionally, CBM resource and its abundance were calculated using volumetricmethod, which providing the foundation of the exploration and development of CBM.