Basin analysis of Tertiary strata in the Pattani Basin Gulf of Thailand
Creators: name: Chonchawalit, Anun
Publisher: name: University of British Columbia
Year: 2008

description: The stratigraphic and structural evolution of the Pattani Basin, the most prolific petroleum basin in Thailand, reflects the extensional tectonic regime of the Continental Southeast Asia. East-west extension, a product of the northward collision of India with Eurasia since the Early Tertiary, resulted in the formation of a series of N-S trending sedimentary basins including the Pattani Basin. Subsidence and thermal histories of the basin can generally be accounted for by nonuniform lithospheric stretching. The validity of nonuniform lithospheric stretching as a mechanism for the formation of the Pattani Basin is confirmed by a reasonably good agreement between the modeled and observed vitrinite reflectance at various depths and locations. The amount of stretching as well as surface heat flow generally increases from the margin to the basin center. Crustal stretching factor varies from 1.3 at the basin margin to 2.8 in the center. Subcrustal stretching factor (8) ranges from 1.3 at the basin margin to more than 3.0 in the basin center. The stretching of the lithosphere may have extended the basement rocks as much as 45 to 90 km and have caused the upwelling of aesthenosphere resulting in high heat flow. The sedimentary succession in the Pattani Basin is divisible into synrift and post-rift sequences. The synrift sequence comprises three stratigraphic units: 1) Late Eoceneto Early Oligocene alluvial fan, braided river and floodplain deposits; 2) Late Oligocene to Early Miocene floodplain and channel deposits; and 3) an Early Miocene regressive package comprises marine to nonmarine sediments. Deposition of the synrift sequences corresponded to rifting and extension which included episodic block faulting and rapid subsidence. Post-rift succession comprises: 1) an Early to Middle Miocene regressive package of shallow marine sediments through floodplain and channel deposits; 2) a late Early Miocene transgressive package; and 3) a Late Miocene to Pleistocene transgressive succession. The post-rift phase is characterized by slower subsidence and decreased sediment influx. The present-day shallow marine condition in the Gulf of Thailand is the continuation of this latest transgressive phase. The dispersed organic matter in Tertiary strata is composed mainly of Type III and Type IV kerogen with minor amounts of Type II kerogen. The organic matter is predominantly detrital and continental in origin as evident from low HI and high (II values, and maceral composition (mainly vitrinite). The variation in abundance of organic matter occurs both within the stratigraphic units and across the units; the lowest TOC and HI occur in the high energy nonmarine deposits such as alluvial fan and braided stream deposits, whereas higher TOC and HI generally occur in low energy deposits. Prospective petroleum source rocks generally have low TOC and very low hydrocarbon potential as defined by pyrolysis. The presence of numerous commercial gas fields suggests either that the source rocks here, despite very low genetic potentials, are very effective in producing, migrating, and accumulating hydrocarbon or higher quality source rocks occur within the basin but have not yet been reached by drilling. Mean activation energies (E0) of the perspective source rocks range from 46.1 to 60.6 kcal/mol which agree well with the activation energies required to break down carbon-oxygen and carbon-carbon bonds (40-70 kcal/mol).The dispersion of activation energies (crE) varies from 0.26 to 9.30% of the mean values (E0). Analyses of hydrocarbon generation history, using a chemical kinetic model based on the Arrhenius equation, indicates, except for the youngest unit (unit1), the strata are either mature or over mature with respect to the oil window. The main phase of hydrocarbon generation started at about 33-35 Ma.