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