Structural characterization of the Phitsanulok basin, onshore Thailand, and regional tectonic implications

Abstract

The Phitsanulok basin is the largest onshore rift basin in Thailand and has been a major hydrocarbon producer for over 30 years. The basin formed in the mid-Cenozoic and was episodically reactivated after the early Miocene from nearby active tectonics, including the India-Asia collision. The basin is formed near a key tectonic transition between north and south Thailand between 16°N and 18°N. Previous studies recognized a north-south variation in structural styles across the Phitsanulok basin that may express deeper basement structures. The northern Phitsanulok basin is predominantly extensional whereas the southern basin shows unconformities, folding, and reverse faults from stronger inversion. Here we systematically characterize structural domains within the Phitsanulok basin from a basin-wide 2D and 3D petroleum industry seismic dataset and examine our observations within the context of regional tectonics. Our analysis includes time structure and isochron maps, spatial fault patterns, fault dip angles, fault throws, and fault azimuths. We find the most dramatic structural changes occur between the northern and southern Phitsanulok basin across 16°50′N latitudes. North of 16°50′N, the Phitsanulok basin has near-planar, steeply-dipping normal faults; arrays of low displacement, cross-cutting synthetic and antithetic extensional faults; and, a thickened and relatively undeformed Sukhothai depression that had up to 1700 ms two-way time (TWT) thickness (~2.3 km) between the Pliocene to Upper Early Miocene strata. In contrast, south of 16°50′N the basin has listric extensional faults; an abruptly thinner stratigraphic section (~400 ms TWT between the Pliocene to Upper Early Miocene strata; ~0.4 km), more gently-dipping faults, and larger fault throws. Our interpreted 16°50′N structural transition occurs near the southern limit of major earthquakes, changes in regional fault trends, and ENE-WSW magnetic lineaments and may highlight deep basement fabrics.