THE BIOMARKER COMPOSITION OF SOME CRUDE OILS FROM SUMATRA

Crude oils from the North, Central and South Sumatra basins have been analysed using GC and GC-MS techniques, and the biomarker compositions of the crude oils have been used to classify the samples into three t ypes. One oil from the North Sumatra Basin contained 3-methylhexadecan e > 2-methylhexadecane, pristane and phytane in similar abundances, re gular steranes and 24-n-propylcholestanes indicative of an algal contr ibution, a series of 30-norhopanes and an absence of plant-derived pen tacyclic triterpanes. These features have been interpreted as indicati ng a marine carbonate depositional setting. Two crude oils from the Ce ntral Sumatra Basin are waxy crudes with abundant botryococcane, prist ane > phytane and abundant 4alpha-methyl-24-ethylcholestanes relative to dinosteranes. These features suggest a brackish lacustrine depositi onal setting. One light oil from the North Sumatra Basin and two crude oils from the South Sumatra Basin have abundant pristane compared wit h n-C-17 and a high pristane/phytane value. The saturated polycyclic c ompounds are dominated by bicadinanes and other pentacyclic triterpane s of land-plant origin. The presence of abundant oleananes in these sa mples has been interpreted as evidence for deltaic/near shore depositi onal setting. These results show that crude oils from the Sumatran reg ion have source rocks with a range of depositional environments, howev er, the major oil fields are associated with lacustrine source rocks.