Abstract: A Reassessment of the Economic Potential of Bacterial Gas
It is a meanwhile common belief that appr. 20% of the world's gas reserves, or ~1000 TCF, are of bacterial origin (Rice and Claypool 1991, AAPG Bull. 65, 5-25). I will show that this is an overly optimistic assessment. A critical evaluation reveals that only about 8% of all conventional gas reserves in the world are of bacterial origin and outside of NW Siberia possibly only about 2-3% of all conventional gas reserves are of bacterial origin.
Bacterial gas formation is important - and can even be the predominant pathway of gas formation - under special geological conditions i.e. depocenters of young Tertiary basins that filled with high sedimentation rates. High sedimentation rates are conducive to preservation of small amounts of disseminated organic matter. Turbiditic sedimentation of shale sand sequences and early subtle tectonic deformation allow accumulation of initially disseminated methane in typically multilayer gas fields. Bacterial gas in commercial quantities forms almost exclusively in these areas of optimal bacterial gas formation. The "motherloads" for bacterial gas occurrence are the Apennine foredeep in Italy, with up to 4000 m Pliocene sediments, the Gulf of Mexico and the Niger and McKenzie Delta.
An understanding of the origin of the NW Siberian gas fields is critical in this discussion as they hold about 1000 TCF of gas representing about 25% of the world's gas reserves. Previously the Urengoy gas field was believed to be of bacterial origin, but Galimov (Chem. Geol. 71, 77-95, 1988) suggested a non-bacterial early thermogenic formation from Cenomanian coaly measures of the Urengoy gas. An alternative, based on the methane-ethane carbon isotope relationship, could be that Urengoy is a mixture of thermogenic with appr. 30% bacterial gas.
The major "bacterial" gas provinces in the world, Niger Delta, Gulf of Mexico, Italy, McKenzie Delta and NW Siberia (assuming 25% being bacterial), total to 365 TCF or 7.6% of world gas reserves (4,800 TCF). If we exclude NW Siberia, the bacterial gas in the world accounts for only 2-3%. Outside the geologically favorable areas the chance to find fields of more than 1TCF is very small. After all, the largest bacterial gas accumulation in an optimal area for bacterial gas formation (not counting NW Siberia) is Santa Barbara Field in Northern Italy which contains about 1 TCF. Explorationists, therefore, should keep these facts in mind when dealing with plays that only contain bacterial gas.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France