The influence of geological structure on stratigraphic development during multiphase rifting in the Songkhla basin, Gulf of Thailand

Abstract

The Cenozoic N-S trending Songkhla rift basin, located in the southwest Gulf of Thailand, was developed as a predominantly orthogonal extended half graben structure. The half graben is controlled by a boundary fault zone on the western side of the basin, which forms a series of N-S to NNE-SSW, east-dipping normal faults. The rift spans two phases of rifting; the Eocene-Oligocene and the Oligocene-Miocene phases, the late syn-rift to post-rift transition occurred from the late Early Miocene to Middle Miocene. The main clastic hydrocarbon reservoirs in the basin are located in the Early Miocene to Middle Miocene section. These reservoirs are predominantly fluvial channels with complex lateral and vertical geometries and distributions particularly as point bars, overbank deposits, and sand- or mud-filled channels. This study characterizes the reservoir architecture and depositional environment of the Songkhla Basin from the late syn-rift to post-rift by integrating the seismic attributes (RMS, similarity, and spectral decomposition), well log interpretations, and well biostratigraphic data. Two distinctive channel characteristics and patterns were interpreted: (A) N-S oriented meandering channel complexes, aligned sub-parallel to the western boundary faults, developed during the late syn-rift (upper Early Miocene) and (B) NE-SW oriented long, narrow fluvial channels with tidal and marine influence, developed in the post-rift phase I (Middle Miocene). These channel orientations reflect changes in the basin architecture passing from the late syn-rift stage to the post-rift. Syn-rift channel orientation was controlled by topography created by motion on the western boundary fault system, while the post-rift the channel orientations were affected by mild inversion on the boundary fault system and thermal subsidence, which resulted in an eastwards shift in the basin depocentre towards the flexural margin.