Abstract: Episodic Migration of Natural Gas: A New Concept of Dynamic Filling of Oil and Gas Fields

Martin Schoell

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 maturation and mixing. 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. As a result of episodic migration, gases are often different between individual reservoir sands, which is useful for many development and production applications.

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 phenomenon:

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 continuous reservoir sands suggest a two phase filling of the reservoirs which are in some cases still unmixed.

Gulf of Mexico: Gases in the Gulf of Mexico are in many cases mixtures of thermogenic and bacterial gases. Variations in isotopic signatures in one field often change from reservoir to reservoir as a result of different mixing ratios of bacterial and thermogenic 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 two 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.

Gorgon Field/Australian NW shelf and Sleipner Field/Norway: Thermogenic gases as well as inert gas contents in the Gorgon and Sleipner fields vary vertically in wells and between wells across the fields, suggesting episodic charges to these fields.

The variations in inert components in the Yacheng, Gorgon and Sleipner Fields suggest that the concept of episodic migration applies also to inert gases. Gas Isotope variations in individual gas fields caused by episodic migration can be used for reservoir mapping, production allocation and many other oil and gas field operations.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France