Abstract

The Pattani Basin in the Gulf of Thailand contains a 10-km-thick succession of synrift upper Eocene-lower Miocene, mainly nonmarine clastics and lower Miocene-Holocene postrift marine and nonmarine clastics. Rock-Eval (R) pyrolysis, organic petrography, and thermal history analyses are used to evaluate the petroleum source rock potential and generative history of the strata. The organic matter is dispersed, terrestrially derived, type III kerogen with minor type II kerogen, and consists primarily of vitrinite. The average total organic carbon (TOC) contents are generally low to moderate (0.2-1.4 wt. %). The hydrogen index (HI) averages 100 mg HC/g TOC, the hydrocarbon potential ranges from 0.01 to 2.5 mg HC/g rock, and the quality of the organic matter ranges from 0.13 to 1.9 mg HC/g TOC. Abundance and character of organic matter vary both laterally and vertically across stratigraphic units. Within stratigraphic units, the lowest TOC and HI values occur in high-energy sediments, such as alluvial-fan and braided-stream deposits, likely reflecting both low organic input and a low degree of preservation. Higher TOC and HI values occur in low-energy flood plain, interdistributary bay, and shallow-marine deposits due to the proximity of these sediments to marshes and swamps containing abundant organic matter, and to a greater degree of preservation. Negative correlation between TOC and sedimentation rate in some units reflects the effect of clastic dilution. Mean TOC increases in progressively younger strata, possibly due to changes in depositional environments, plant evolution, or paleoclimate. All strata in the Pattani Basin, except the uppermost (unit 1), are either mature or overmature with respect to the oil window. The current depth to the top of the oil window ranges from 1.4 to 1.7 km, and the base of the oil window ranges from about 2.5 to 3.0 km. Combined geohistory and basin modeling indicates that the main phase of hydrocarbon generation began between about 33 and 35 Ma. A relatively early generation (approximately 5-7 m.y. after deposition) is attributed to high geothermal gradients and rapid burial, which is consistent with the rift origin of the basin. The presence of numerous commercial gas fields suggests that the source rocks are effective in producing and liberating hydrocarbons, despite their very low hydrocarbon potential. The presence of good, although laterally discontinuous, reservoirs; the adjacent occurrences of source and reservoir rocks; and structural and stratigraphic traps are factors that made the Pattani Basin an important hydrocarbon basin.

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