Fault superimposition and linkage resulting from stress changes during rifting: Examples from 3D seismic data, Phitsanulok Basin, Thailand

Abstract

The Phitsanulok basin, Thailand provides examples of changing fault displacement patterns with time associated with faults of different orientations. In the Northern Phitsanulok basin three main stress states have been identified associated with Late Oligocene–Recent fault development: (1) Late Oligocene–Late Miocene approximately E–W extension (N–S Shmax), ‘main rift’ stage, (2) Late Miocene–Pliocene transtension to tranpression (?) (E–W to NE–SW Shmax), ‘late rift’ stage, and (3) Pliocene–Recent very minor faulting, E–W extension, N–S Shmax, ‘post-rift’ stage. Syn-rift faults tend to strike N–S, but also follow NE–SW and NW–SE trends and are basement involved. The Late Miocene deformation produced a distinctly different type of fault population from the main rift fault set, characterized by numerous, small displacement (tens of metres), faults striking predominantly NE–SW. Most of these faults are convergent, conjugate sets aligned in discrete zones and nucleated within the sedimentary basin. Reactivation of main rift faults trends during the late rift stage favoured strike directions between 350° and 50°. The displacement characteristics of three large faults within the basin show variations depending upon fault orientation. The low-angle (23°-30° dip), Western Boundary Fault (∼7 km throw) displays little discernible difference in the distribution of displacement on fault zone during the different stress states other than increases and decreases in displacement amount. Smaller faults exhibit a more selective reactivation history than the Western Boundary fault and are more informative about fault response to a varying stress field. Activation of the (oblique) NE–SW striking NTM-1 initially produced a fault divided into three segments, splaying into N–S trends. Stress reorientation during the late rift stage finally linked NE–SW striking segments. The partial linkage of the fault zone at the time of oil migration resulted in the southwestern part of the NTM-1 fault sealing hydrocarbon bearing reservoirs, whilst late linkage areas along the northeastern part failed to seal hydrocarbons. The N–S striking PTO-1 Fault shows early isolated fault segments, linkage during syn-rift motion, then a more patchy distribution of displacement late in the fault history, perhaps due to non-optimal orientation of the fault to the regional stress field, when compared with the NTM-1 Fault. The fault characteristics described indicate that where strong pre-existing fabrics are present, and varying stress regimes have occurred, fault population characteristics and evolution of fault displacement can depart considerably from extensional fault populations associated with a single phase of extension.