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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Episodic
Migration
of Natural Gases: A Worldwide Phenomenon of
Dynamic Filling of Oil and Gas Reservoirs and Resulting Practical Applications in Exploration and Production
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By
Chevron Petroleum Technology Co.
Natural gases can be ubiquitously detected
in the subsurface, either as traces in sediments
or as accumulations in oil and gas
reservoirs. Hydrocarbon gases form
through bacterial and thermogenic conversion
of organic matter over extended geological
time. The higher molecular compounds
in gases (ethane, propane, and butane
or C2+ hydrocarbons) tend to he retained
in the rock fabric during migration
from source to reservoir (
migration
fractionation)
so that natural gases become enriched
in methane in the reservoir. Isotopic
properties of methane and C2+ hydrocarbons
are controlled by temperature dependent
kinetic fractionation during maturation
and by mixing during
migration
in reservoirs.
Gas fields are actively and repeatedly
charged over extended periods of gas formation
(episodic migration
), during which
gases change isotopically. Isotopic signatures
of gases are, therefore, sensitive tracers
for the filling histories of oil and gas
fields and the dynamic processes that drive
episodic
migration
of oils and gases, a ubiquitous
phenomenon observed in small and
giant gas fields throughout the world.
Gulf of Mexico: The Gulf of Mexico is a
unique natural laboratory for the study of
natural gas formation and migration
. The
dynamics of salt removal and formation of
mini-basins with high sedimentation rates
play together in the formation of bacterial
and thermogenic gases. Pervasive mixing
of bacterial and multiple episodes of thermogenic
gas charging result in large variations
in isotopic signatures on a field and
reservoir scale. Bacterial gases form in mini
basins and migrate against rising salt structures
and active growth faults. Different
episodes of thermogenic gas charging can
be identified from C2+ isotope variations.
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 offshore Cabinda
contain thermogenic gases with different
isotopic signatures in the gas cap and the
oil leg, indicating a multi-phased migration
of thermogenic gases from mature to post-mature
sources. The giant oil fields form
in several
migration
episodes whereby late
charges of deep post-mature gases displace
oils.
Yacheng Gas Field: Gas composition and isotopic signatures of methane change from west to east in this giant field in the South China Sea. Inert gas contents also vary vertically in the reservoir, suggesting a late charge of a CO2 rich gas into the structure.
These worldwide observations help us to
appreciate in oil and gas migration
what is
a commonplace for hydrothermal systems,
i.e. that sedimentary basins release fluids
in a pulsating dynamic fashion, ever changing
in composition and filling reservoirs
with complex mixtures that reflect deep
processes in the basin. With this insight into
the dynamic nature of reservoir filling processes
we will be able to use our geochemical
data with more confidence.
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