Formation model and special significance of helium-rich coalbed methane reservoirs in Mu’ai Block, Junlian

The Junlian Mu’ai core area of Sichuan province is the first successfully commercialized Coalbed Methane (CBM) field in southern China, which has leading significance in the development of CBM in southern China. However, its rich geological reason remains to be further revealed. Based on the data from a large number of CBM evaluation test wells, CBM test wells and CBM development wells in the study area, combined with testing and assaying analysis, referring to and borrowing from oil and gas research results, and combining with the geological background, the unique CBM enrichment and formation mode has been analyzed. The research shows that the average CBM of the main coal seams in Junlian Mu’ai core area is 15.58 m3/t, which is characterized by “medium high ash coal, high CBM content and high saturation”, and the helium is abnormal, with an average content of 0.074%. The CBM enrichment area is mainly located at the Mu’ai fault side of the syncline axis of Tiechanggou and the western and southern edges of the block. The helium anomaly area and the CBM enrichment area have synergy. The high salinity of groundwater in this area is its distinctive feature. The average TDS is 10 638.3 mg/L, which is far higher than the TDS value of domestic and foreign shallow CBM production areas. And it also has the characteristics of “high chloride ions, high sulfate ions”. This is closely related to its location at the edge of Sichuan Basin, weak tectonic activity, and the formation water is mudstone compaction drainage water mixed with atmospheric precipitation. The area with high salinity has high CBM content, helium anomaly, and overpressure phenomenon. It is speculated that the helium anomaly mainly comes from the degassing of water-soluble helium in deep centrifugal flow. Therefore, a new model of CBM reservoirs is proposed, that is, the structure-mixed hydrodynamic sealing helium-rich CBM accumulation model. The structure mainly reflects two aspects: one is that the reverse fault with good sealing provides a beneficial boundary for helium-rich CBM enrichment; the other is that the structural trace controls the reservoir, including the low part of the wide and gentle syncline axis and the high part of the structure. The mixed hydrodynamic force is reflected in the comprehensive control of mudstone compaction centrifugal flow and atmospheric precipitation centripetal flow in the unique sedimentary basin, which contributes to the formation of helium-rich CBM reservoirs. As a result, the water-CBM and helium gas in the original gas reservoir have synergism, and this dynamic synergism is still consistent in the later drainage and production process. Therefore, the enrichment of helium in CBM reservoirs may be an important indicator for the CBM enrichment.