senoj.
With regards to Molopo's acreage in South Africa, the source of this biogenic gas is at depths of 4000-5000m. They'll intersect the gas through natural fractures at about 500m.
From memory, there has been little or no production rate declines in the 3 years since commencement of data recording by Molopo. So your statement, "This could amount to many millions of scf per day for a very, very long time.", is so true.