CBM sealing system and its relationship with CBM enrichment

Abstract

It is of great significance to study the controlling effect of sealing systems on CBM enrichment in coalbed methane (CBM) exploration and development. In this paper, the relationships between CBM enrichment and geological elements were analyzed. The geological elements include sealing layers (e.g. regional cap rock, regional floor, and immediate roof and floor), later structural adjustment and strata production status. It is shown that CBM tends to enrich in the areas where regional mudstone cap rocks and floors are distributed stably, structures are uplifted and inversed slightly after the hydrocarbon accumulation period and the strata is gentle in a balanced state. Then, the concept of sealing system was put forward based on the worldwide CBM exploration and development practices over the years. A sealing system refers to a geological unit composed of a lateral stable zone and cap rock which prevents gas from migrating upward and downward. In a sealing system, CBM can get enriched and coal-measure gas can also be accumulated. Finally, three gas reservoir types (i.e., sandwich-type CBM reservoir, associated CBM-sandstone gas reservoir and coal-derived sandstone gas reservoir) were identified based on the configuration relationships between elements of the CBM (or coal-measure gas) sealing system. It is recommended to change the exploration ideas from simple CBM exploration to 3D CBM and coal-measure gas exploration. In addition, an evaluation index system of CBM (or coal-measure gas) geological selection was established. It is pointed out that good application effects may be realized if the stereoscopic CBM and coal-measure gas exploration is applied in the Junggar Basin and the eastern margin of the Ordos Basin.