The Late Oligocene-Pliocene Wichianburi Sub-basin, Phetchabun Rift, has undergone three phases of development: Late Oligocene-Early Miocene (∼16 Ma) E-W extension on predominantly NNW-SSE to N-S striking faults; 16 Ma-∼3 Ma NE extension on NW-SE extension on NNW-SSE, N-S and NE-SW trending faults, and 3 Ma – Present post-rift subsidence. The basin is also affected by 2-24 Ma intrusive and extrusive igneous activity. Relationships between structure, rift basin stratigraphy, and sheet intrusions, are unusually well documented because of extensive hydrocarbon exploration drilling and 3D seismic data, and production of hydrocarbons from igneous sills. The oldest igneous activity (∼24-13 Ma) lies SW of the Wichianburi Sub-basin where extrusives exposed at the surface have been dated by K-Ar and 40Ar/39Ar methods. Overall igneous activity youngs from south of the basin, to north of the basin (11-9 Ma). Numerous domal folds occur across the basin; sill and laccolith dimensions, rate of fold development, variable timing of fold formation, and fold geometry indicate the folds are related to magma emplacement, not tectonic inversion. Fold formation occurred around 15-16 Ma, and 11-12 Ma, in single or multiple stages. The timing of folding coupled with dating of some sills radiometrically (K-Ar, 39Ar-40Ar) indicates that greatest intrusive activity occurred between 16 and 11 Ma. 16 Ma coincided with the switch from E-W to NW-SE extension, which promoted dilatancy within the upper crust, along a range of syn-rift fault orientations and some pre-rift trends. The burial history of the main depocentre shows Miocene sill emplacement occurred at depths <1km, yet still was able to cause local maturation of hydrocarbon source rocks. Sill emplacement affected the petroleum system by forming migration pathways, important reservoirs, and also creating forced fold traps.

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