Episodic migration of natural gas: A new concept of dynamic filling of oil and gas fields

Journal Article · Tue Aug 01 04:00:00 EDT 1995 · AAPG Bulletin

OSTI ID:127699

Schoell, M ^[1]

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1. Chevron Petroleum Technology Co., La Habra, CA (United States)

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. Episodic migration and multiple filling of reservoirs is a ubiquitous phenomenon observed in small and giant gas fields throughout the world. 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. 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 log, 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 CO{sub 2} rich gas into the structure.

• OSTI ID: 127699
• Report Number(s): CONF-950995--
• Journal Information: AAPG Bulletin, Journal Name: AAPG Bulletin Journal Issue: 8 Vol. 79; ISSN 0149-1423; ISSN AABUD2
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
CHEMICAL ANALYSIS
ISOTOPE RATIO
MIGRATION
NATURAL GAS
NATURAL GAS FIELDS
ORIGIN
PRODUCTION