THE IMPACT OF STRUCTURAL AND STRATIGRAPHIC UNCERTAINTY IN FAULT SEAL ANALYSIS

Prediction of single and multi-phase fluid flow in the subsurface is complicated by the presence of structural and stratigraphic geomentric elements. In the sedimentary section of the earth faults appear to behave as both as facilitators and barriers to fluid flow on both geologic and human time-scales. Uncertainty in fault seal analysis, deriving from imperfect stratigraphic and structural input models, commonly results in misprediction of the basic geometric arrangement of permeable layers and sealing layers juxtaposed across faults. In the petroleum industry misprediction of juxtapositions leads to erroneous interpretations and predictions of hydrocarbon distributions, fluid pressures, and flow behaviors in the subsurface, which in turn lead to over or under-assessment of discovered resources, poorly designed field development plans, and poor reservoir management in producing fields.

Fault seal analyses start with predictions of cross-fault juxtapositions by combining structural geometry with stratigraphic architecture. In our experience, starting with carefully constructed geometric and stratigraphic models to evaluate juxtapositions of low and high capillary entry pressure rocks across faults obviates the need to introduce higher uncertainty predictions regarding the presence, distribution, and quality of capillary sealing material in a fault zone. For example, two commonly observed phenomena, offset hydrocarbon contacts and different fluid pressures across faults, lead to hypotheses about sealing materials in fault zones. Often we find that, within the range of structural uncertainty and assumptions, juxtapostion models can be built that satisfy the observations and are testable within the seismic data from which they are derived. Further, recognition that common geometric elements associated with juxtapostion windows (e.g., breakover at the base of a juxtaposition window) can yield both offset contacts and different fluid pressure regimes across faults, reduces the need for ad hoc prediction of fault zone sealing materials. The juxtaposition approach to fault seal analysis frequently guides revised geologic interpretations that can be tested by drilling and examination of seismic data. The distribution and quality of fault zone materials is rarely directly tested.