Chemical Geology, 93 ( 1991 ) 129-146 129 Elsevier Science Publishers B.V., Amsterdam Geochemical characteristics of oils from the Sirikit Oilfield, Phisanulok Basin, Thailand Kulwadee Lawwongngam and R. Paul Philp The University of Oklahoma, School of Geology and Geophysics, Energy ('enter Building, 100 East Boyd Street, Norman, OK 73019, USA (Received August 3, 1990; revised and accepted November 6, 1990 ) ABSTRACT Lawwongngam, K. and Philp, R.P., 1991. Geochemical characteristics of oils from the Sirikit Oilfield, Phisanulok Basin, Thailand. In: J.A. Curiate, R. Alexander and P.W. Brooks (Editors), Organic Geochemistry of Hydrocarbon Basins. Chem. Geol., 93: 129-146. Twelve oils from the Sirikit field of the Phisanulok Basin, Thailand, have been examined to determine the nature of their source materials, depositional environment, relative maturity and extent of biodegradation. The geochemical anal- yses performed in this study suggested that all of the oils were derived from similar types of organic source matter consist- ing of a mixture of bacterial, algal and higher plant material. The sterane contents of the oils were relatively low and conventional maturity parameters based on sterane ratios suggested the oils were relatively immature. Extensive strike/ slip faults in the Phisanulok Basin may have led to rapid burial of the sediments during deposition preventing these ratios from having attained their equilibrium ratios. 1. Introduction The Sirikit Oilfield discovered in 1981, is lo- cated in the central plain of Thailand and has an estimated 350 million bbl ofoil in place and a recovery expectation of 41 million bbl. The oilfield lies in the Phisanulok Basin which is one of a series of Tertiary extensional rift bas- ins in the northern part of Thailand that over- lie the suture between the Shan Thai and In- donesia cratons. The basin is 100 km wide, contains up to 8 km of Tertiary age sedimen- tary rocks and is surrounded by complex struc- tures of deformed Paleozoic/Mesozoic rocks (Workman, 1975; Knox and Wakefield, 1983 ). The tectonic history of the basin is compli- cated since it is situated in a triangular zone at the intersection of two regional strike-slip faults, the northwest-southeast-trending Mae Ping Fault Zone and the northeast-southwest- trending Uttraradit Fault Zone (Flint et al., 1988; Fig. 1 ). The Sirikit field is located on the southern flank of the graben on a local base- ment high of the basin. The major petroleum producing units of the basin are the Lan Kxabu Formation (interbed- ded sandstones, siltstones and claystones) and the partially time-equivalent intercalated Chum Saeng Formation which consists of an organic-rich claystone containing a gastropod fauna found only in non-marine sequences. The Lan Krabu Formation, can be divided into three reservoir units (K, L and M) separated and sealed by tongues of open-lacustrine clay- stones of the Chum Saeng Formation (Flint et al., 1988 ). A lacustrine origin for the claystone is supported by palynofacies of detrital land- plant debris plus freshwater algae and an ab- sence of marine indicators. The Chum Saeng Formation acts as both source rock (to the 0009-2541/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.