Petroleum Source Rock Potential and Evolution of Tertiary Strata, Pattani Basin, Gulf of Thailand

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® 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. ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received July 17, 1996; revised manuscript received December 10, 1996; final acceptance July 17, 1997. 2Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. 3Present Address: 39/98 Moo Baan Suanthong, Lad Plakhao Road, Bang Khen, Bangkok, Thailand 10220. We thank UNOCAL Thailand for providing access to samples, well logs, and seismic data. This study was supported financially by the Canadian International Development Agency and a Natural Sciences and Engineering Research Grant (Canada) to Bustin. We thank Michelle Lamberson for her comments on an earlier copy of this paper and Matt Bustin for his editorial assistance. Journal reviewers B. J. Katz and L. Snowdon are thanked for their constructive critique of the manuscript.