AAPG Middle East Region Geoscience Technology Workshop

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Seals, Faults and Fractures in Siliciclastic Reservoirs: Integration of Geology and Geomechanics

Abstract

The best fault and top seals in siliciclastics are ductile, laterally continuous, have low permeability and high capillary entry pressures. A thicker, homogeneous seal provides better lithological continuity. There are many critical parameters that determine fault and top seal performance. Both the MICRO- and MACRO -scale aspects of the system, in Trap Geometry, Fault topology, Capillary resistance, Permeability, Brittleness, Heterogeneity, Effective Stress can be dominant in different settings, and can lead to seals which leak gas and oil at different rates and dynamically maintain a hydrocarbon accumulation. Integrated analyses of Structural geology, Sedimentology, Petrophysics and Geomechanics are the most effective. We discuss these basic controlling mechanisms, and illustrate them with field examples in normally pressured and overpressured plays. In the past decade, the development of unconventional reservoirs and the assurance of subsurface integrity in CO2 storage have provided a wealth of new data and insights in seals, and lead to improvements in our ability to evaluate seals in undrilled prospects. Even in normally pressured systems permeable shear fractures may form, and remain open. This is where the rock's ductility, as measured by volume changes during deformation, is the key parameter. A ductile seal or fault rock is able to undergo plastic deformation without increasing its permeability, whereas a brittle one will increase its permeability when deformed. The main controls on dilatancy during shear failure is a function of (i) mechanical properties of the rock (ii) effective stress tensor and (iii) geometry of the shear zone. At a given effective pressure a stronger (overconsolidated or cemented) rock is more likely to dilate than a weaker one. With increasing strain, the early fault rock fabric is reworked and transformed into a fault gouge. Another resealing mechanism is precipitation of veins from hydrothermal fluids which are common in shales. We illustrate these principles with examples from the Middle East, review new developments and technologies and discuss opportunities for further development.