The Place of Faults in Petroleum Traps
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 threedimensional 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.
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Japan National Oil Corporation (JNOC) (presently Japan Oil, Gas and Metals National Corporation) launched a multidisciplinary and international project on the Evaluation of Traps and Seals in 1997. The project ended in 2003. This publication resulted from that project and includes JNOC research articles as well as contributions from industry and academia. The 17 papers in this volume cover topics such as a method to estimate the amount of oil/gas accumulation using the concept of equivalent grain size in seal rock, and oil/gas migration to and spill-point geometry of petroleum traps; two case studies of fault seal assessment applied to normal faults in Tertiary clastic reservoirs in offshore Sarawak and offshore Gulf of Thailand; and physical analog studies of the development of extensional faults. This publication also contains a valuable bibliography of nearly 1000 additional articles and books published on fault traps, fault seal processes, and fault-related fluid flow in sedimentary basins, for use as a reference tool to delve into publications preceding this volume.