Pentamethylnaphthalenes and related compounds in sedimentary organic matter

The isomeric pentamethylnaphthalenes (PMNs) 1,2,3,5,6-PMN, 1,2,3,5,7-PMN, 1,2,3,6,7-PMN and 1,2,4,6,7-PMN have been synthesised, characterised and identified in a number of crude oils and sediments, ranging in age from Proterozoic to Tertiary. We suggest that 1,2,3,5,6-PMN forms predominantly from the aromatisation of drimanoid precursors via 1,2,2,5,6-pentamethyltetralin. When 1,2,3,5,6-PMN was heated in laboratory experiments, the proportions of PMNs changed in a manner which reflected the relative stability of the isomers. The other PMNs in sediments are therefore suggested to arise via acid catalysed isomerisation or transalkylation processes. A conveniently measured pentamethylnaphthalene ratio PNR = 1,2,4,6,7-PMN/(1,2,3,5,6-PMN + 1,2,4,6,7 PMN) has been defined as a maturity parameter. In an Indonesian sedimentary sequence, the Values of PNR changed smoothly with increasing depth with values ranging from 0.1 to 0.4. Values in crude oils range from 0.15 to 0.85, suggesting a value of approximately 0.15 for the beginning of the oil window. (C) 1998 Elsevier Science Ltd. All rights reserved.