The hydrocarbon source potential of Permo-Triassic rocks of the Indochina Block adjacent to the Paleo-Tethys suture in Thailand

Abstract

The main Thailand petroleum systems are found in the syn-rift and post rift sections of Cenozoic rift basins. The Khorat Plateau area is a separate gas province with Permian limestone reservoirs possibly sourced from Triassic lacustrine, and Permo-Carboniferous marine shales. However, these source rocks and those of equivalent age west of the Khorat Plateau are not well described in the literature. This study investigated the source potential of outcropping Permo-Triassic shales from the western margin of the Khorat Plateau and the Khao Khwang Fold and Thrust Belt (KKFTB). The key formations sampled are: Permian: Alum Shale, Nong Pong, Sap Bon, Khao Khad, and Nam Duk formations; Triassic: Pang Asok and Hua Hin Lat Formation. Most potential Permo-Triassic source rocks sampled exhibited little potential due to the dominance of Type IV, non fluorescent amorphous, vitrinite and intertinite mascerals and low <1 % TOC content. Probably the Type IV kerogens are not original, but the product of subsurface alteration of other kerogen types (most likely Type III). The complex burial histories of the formations indicates such alteration could have occurred post-maturation. Probably maximum burial in the KKFTB was attained during the Triassic Indosinian Orogeny when upper diagenetic to anchizone conditions were attained. Significant burial also occurred when the Jurassic-Cretaceous Khorat Group (>2 km thick) was deposited. This later burial could be responsible for later maturation in areas north of the KKFTB. Simply in terms of TOC content >1, the Hua Hin Lat and Khao Khad formations have the most promising source potential.