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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