Episodic Migration of Natural Gas: A Worldwide Phenomenon of Dynamic Filling of Oil and Gas Fields

M. Schoell and M. A. Beeunas

Natural gases form through bacterial and thermogenic conversion of organic matter over extended geological time. The isotopic properties of methane and C2+hydrocarbons are controlled by temperature dependent kinetic fractionation during maturation and mixing during migration and in reservoirs. Gas fields which are actively and repeatedly charged over extended periods of gas formation will receive compositionally and isotopically variable charges of natural gases that can mix in different proportions in single reservoirs. Episodic migration and multiple filling of reservoirs is a ubiquitous phenomenon observed in small and giant gas fields throughout the world. The following case histories from own research and from the literature will be discussed to illustrate this pheno enon:

Gulf of Mexico: Pervasive mixing of bacterial and thermogenic gas charges results in large variations in isotopic signatures on a field and reservoir scale. Major formation and migration episodes are 1) Bacterial gases from in rapidly subsiding mini basins and migrate and accumulate around rising salt structures and active growth faults and 2) Following multiple charges of thermogenic gases mix in various proportions with bacterial gases.

Angola: Giant oil fields off-shore Cabinda contain thermogenic gases with different isotopic signatures in the gas cap and the oil leg, indicating a multi phase migration of thermogenic gases from mature to post-mature sources. The gas/oil columns are in some cases variable and suggest compartmentalization and/or unmixed oil hydrocarbon columns.

Yacheng Gas Field: Gas composition and isotopic signatures of methane change from west to east in this giant field in the S. China Sea. Inert gas contents also vary vertically in the reservoir suggesting a late charge of a CO2 rich gas into the structure.

South Italy: Pliocene reservoirs in the Apulian Basin are mixtures of thermogenic and bacterial gases. Carbon isotope signatures of methane and nitrogen contents are direct tracers for the thermogenic component in the gas mixtures. Gas composition variations within single reservoir sands suggest a two phase filling of the reservoirs.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California