The Place of Faults in Petroleum Traps

Abstract

Ever since Frederick Clapp included fault structures as significant petroleum traps in his landmark paper in 1910, the myriad function of faults in petroleum migration and accumulation in sedimentary basins has drawn increasing attention. Fault analyses in petroleum traps have grown along two distinct and successive lines of thought: (1) fault closures and (2) fault-rock seals. Through most of the last century, geometric closure of fault traps and reservoir seal juxtaposition by faults were the focus of research and industrial application. These research and applications were made as structural geology developed quantitative methods for geometric and kinematic analyses of sedimentary basins, and plate tectonics offered a unified tool to correlate faults and basins on the basis of the nature of plate boundaries to produce stress. Over the last two decades, compartmentalization of reservoirs by fault seals has been more intensively investigated as three-dimensional seismic images better resolve fault structures. Geometric characterization of fault architecture, identification of various sealing processes in fault zones, and quantitative appraisal of petrophysical properties of fault rocks have significantly advanced in recent decades. Fault-seal analyses have shifted from two-dimensional fault juxtapositions to three-dimensional models encompassing fault surfaces, fault transmissibility, and juxtaposed reservoir units. Current methodologies for fault-seal assessment mostly address normal faults in clastic reservoirs. Fault sealing processes in thrust faults and in carbonate reservoirs represent important blind spots in our knowledge. Shale smear has been effectively applied for sealing assessment of syndepositional faults in sandstone-claystone successions. However, fault-seal analyses based merely on shale smear ignore other important sealing processes, notably cataclasis and cementation in fault zones. During their active stages, faults are conduits of subsurface fluids, irrespective of any sealing mechanism that operated before fault rupture. Therefore, a comprehensive fault-seal assessment needs to be a four-dimensional model integrating fault motions, fault-zone processes, and fluid flow. This remains a major challenge. However, integration of in-situ fault stress analysis and fault-seal analysis has provided a technological breakthrough. The realization that fault rocks are low-permeability and high-capillarity features in sedimentary basins has given an economic impetus for exploration of fault traps. The shift from modeling of single-phase fluid flow to multiphase or even mixed-phase fluid flow along and across fault zones will be of more value to these exploration efforts. Recent studies have transformed the old polarized view of faults as either leaks or seals into realistic notions of more complex fault-fluid flow behavior. Current shortcomings in fault-seal assessment are largely caused by the scarcity of detailed data and the need for robust calibration of numerical models. This implies that empirical data will form the cornerstone of near-future advances in fault-seal methodologies.