The Deformation History of the Palaeozoic and Mesozoic of the Khorat Plateau, North Eastern Thailand and Southern Laos

Booth, John E.¹; Comrie-Smith, Nick²
¹Exploration, Twinza Oil Limited, Bangkok, Thailand.
²Exploration, Salamander Energy, Bangkok, Thailand

The deformational history of the Palaeozoic and Mesozoic basins of the Khorat Plateau is documented by a series of seismic lines from both the Thai and Laos portions of the Khorat Plateau. Age constrains on the major unconformities are provided by samples from most of the 50 exploration and production wells that have been drilled over the last 40 years.

The stratigraphic succession in the region is divided into four episodes of basin subsidence separated by major regional unconformities related to compressional tectonic events.

Fragments of a series of Lower Carboniferous - Devonian basins containing marine sediments are preserved under the Mid-Carboniferous Unconformity.

Following the Mid-Carboniferous Event a series of large Upper Carboniferous - Permian rift basins developed, filled with marine sediments which contain the main source rocks and carbonate platform reservoir plays. The rifting of these basins was terminated by the first major event of the Indosinian Orogeny in the Early - Middle Triassic. The Permo-Carboniferous basins were shortened and inverted, with their fill being folded, thrust and deeply eroded to form the regionally important Indosinian I Unconformity. This event marks the end of marine deposition in the region.

A series of large half-graben developed in the Late Triassic, filled with terrestrial and lacustrine sediments. The rifting of these basins ceased in the latest Late Triassic, when another episode of basin inversion occurred, with the formation of the regional Indosinian II Unconformity.

The alluvial - fluvial Jurassic - Cretaceous Khorat Group was deposited in a regional sag basin, with almost no associated faulting. A major hiatus occurred in the Early - Middle Jurassic, although in only a few places do seismic lines show an associated low angular unconformity. A relatively minor compressional event occurred in the Mid-Cretaceous, with the creation of two separate internally draining basins in which a thick succession of evaporites were deposited.

Starting In the early Tertiary the whole of the region was uplifted, with an average of 1 - 1.5 km of section eroded. The Permo-Carboniferous and Late Triassic basins underlying the Khorat Group were inverted, producing the series of very large wavelength folds in the overlying Khorat Group, which dominate the surface topography.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012