Abstract: Evaluation of Fault-Related
Hydrocarbon Migration
Pathways
Huang, Jie and James Holl - Exxon Production Research Company
Faults represent potential migration
pathways in many hydrocarbon systems, particularly those in which thick,
seal-prone stratigraphic intervals separate mature source kitchens from
reservoirs. Diverse evidence collected from outcrop studies, reflection-seismic
data, and high resolution seafloor bathymetric surveys suggests that faults
commonly focus fluids and represent preferential flow pathways through
low permeability rock. The same data, however, also indicate that along-fault
flow is heterogeneous in both space and time. Fluid flow is commonly episodic
and restricted to isolated segments of specific faults in a regional fault
network. As a result of this complex behavior, along-fault hydrocarbon
migration
and charge adequacy represent key risks in many fault-dependent
plays. In order to best evaluate the effectiveness of fault
migration
pathways,
Exxon has developed a multi-disciplinary approach that integrates quantitative
hydrocarbon system analysis with geophysical and geochemical constraints.
In this presentation, we summarize our approach and apply several types
of risking criteria to evaluate relative
migration
and charge adequacy
risks for multiple prospects.
First, we examine the relationship between faults
and other elements of the hydrocarbon system through map-based, multi-surface
hydrocarbon migration
modeling. Effective pathway risking is best achieved
by considering the fault as one leg of the total
migration
pathway from
source to reservoir. For effective secondary
migration
to occur, hydrocarbons
must travel along the source-focusing horizon to the fault, up the fault,
and then move laterally along carrier horizons to the trap. Exxon?s multi-surface
migration
modeling technology provides key insights that can be used to
establish this 3-D ?plumbing? system. The spatial relationships between
fault network characteristics, reservoir facies distributions, and source
focus areas across multiple horizons are fundamental constraints on the
effectiveness of fault
migration
pathways. Based on this framework, the
interaction of several highly-coupled timing issues important to cross-stratal
migration
, including fault-timing, trap forma tion, and yield timing, can
be investigated in a quantitative and integrated fashion.
A second and equally critical component of fault
migration
pathway evaluation is the integration of geophysical and geochemical
constraints. By mapping the distribution of surficial features such as
hydrocarbon seeps and fluid expulsion sites and subsurface features such
as fault-localized amplitude flags, we can identify the most likely, fault-related
migration
conduits. Analysis of drop cores collected from fluid expulsion
sites on the seafloor permits identification of active hydrocarbon systems.
More detailed interpretation of complex pathways is possible when gravity
core data is integrated with the fault framework established from side-scan
sonar as well as closely spaced 2D or 3D seismic data. Where faults tip
out immediately below or cut the seafloor, seeps and expulsion vents can
be tied to specific faults and fault segments.
Effective pathways are most evident where those
faults also link downwards to connect reservoir and source. Caution is
required along faults that lack obvious evidence of fluid flow, since seafloor
data record only the most recent flow episodes and source to reservoir
charge may occur without continued seepage to the surface. This multi-disciplinary
approach has been applied in several major petroleum provinces (e.g. Gulf
of Mexico, West Africa, and North Sea) and has substantially improved our
understanding of pathway effectiveness and reduced the hydrocarbon migration
and charge risk for fault-related prospects.
AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil