Abstract:

Guided by coal-derived gas theories， a batch of coalbed methane fields with external sources have been discovered in China. However， “internal-sourced” coalbed methane fields are barely found in the country despite many studies indicating great potential of internal-sourced coalbed methane from deep layers （more than 2 000 m deep）. This study carries out experiments on the core samples taken from deep Carboniferous-Permian coal-bearing strata in the Daniudi gas field， Ordos Basin， and finds that all coal measures in the field generally contain gas and sometimes free gas and that the coalbed methane abundance can be as high as 3.86 bcm/km². Combined with stratigraphic burial history and thermal evolution， it is suggested that these layered heterogeneous strata like mudstone and limestone contain mostly small-sized pore throats and thus form the lithological traps dominated by sandstone and coal rocks due to capillary resistance sealing. The coalbed lithological traps control the accumulation of free gas within. Low water saturation values （6.5 %-30 %） of the cores may explain the phenomenon that the depressurization via dewatering in conventional coalbed methane extraction often ends with depressurization via de-gassing， which is conducive to the producing of deep coalbed methane. The primary-cataclastic coal structure is also a favorable feature for fracturing stimulation. The study concludes that deep coalbed methane can be commercially extracted with suitable techniques and thus is worthy of attention.

Key words: source rock, free gas, lithological trap, deep coalbed methane, unconventional gas, Upper Paleozoic, Daniudi gas field, Ordos Basin