Abstract: Basin Analysis of Tertiary Strata in the Pattani Basin, Gulf of Thailand

Anun Chonchawalit, R. M. Bustin

The stratigraphic and structural evolution of the Pattani basin, the most prolific petroleum basin in Thailand, reflects the extensional tectonics 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 north-south-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 mechanics for the formation of the Pattani basin is confirmed by a reasonably good agreement between modeled and observed vitrinite reflectance at various depths and locations. The amount of stretching and surface heat flow ge erally increases from the basin margin to the basin center. Crustal stretching factor (ß) ranges from 1.3 at the basin margin to 2.8 in the center. Subcrustal stretching factor (^dgr) ranges from 1.3 at the margin to more than 3.0 in the center. The stretching of the lithosphere may have extended basement rocks as much as 45 to 90 km and may have caused the upwelling of asthenosphere, resulting in high heat flow.

The sedimentary succession in the Pattani basin is divisible into synrift and postrift sequences. The synrift sequences comprise (1) late Eocene (?) to early Oligocene alluvial fan, braided river, and flood-plain deposits; (2) late Oligocene to early Miocene flood-plain and channel deposits; and (3) an early Miocene regressive package of marine to nonmarine sediments. Deposition of synrift sequences corresponded to rifting and extension, which included episodic block faulting and rapid subsidence. Postrift succession comprises (1) an early to middle Miocene regressive package of shallow marine to nonmarine sediments, (2) a late early Miocene transgressive package; and (3) a late Miocene to Pleistocene transgressive succession. The postrift phase is characterized by slower subsidence a d decreasing sediment influx. The present-day shallow marine condition in the Gulf of Thailand is the continuation of this latest transgressive phase.

Dispersed organic matter in Tertiary strata is composed mainly of Type 111 and Type IV kerogen, with minor amounts of Type 11 kerogen. The organic matter is predominantly detrital and continental in origin as evident from low HI and high OI values, and maceral composition (mainly vitrinite). The variation in abundance of OM occurs both within and across the stratigraphic units. The lowest TOC and HI occur in high-energy nonmarine deposits such as alluvial fan and braided stream deposits, whereas high 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 fields suggests either the source rocks here, despite very low genetic potentials, are very effective in producing, migrating, and accumulating hydrocarbon or the presence of higher quality source rocks occur within the basin but have not yet been reached by drilling. Mean activation energies (E_o) of prospective source rocks range from 46 to 61 kcal/mol, which agree well with the activation energies required to break down carbon-oxygen and carbon-carbon bonds (4-70) kca/mol). The distribution of activation energies (^sgr_E) varies from 0.3 to 9.3% of mean values (E_o) Analysis of hydrocarbon generation history, using a chemical kinetic model based on this Arrhenius equation indicates, except for the youngest unit, the strata are either mature or overmature with respect to the oil window. The main phase of hydrocarbon generation started at about 33 Ma.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994