Applied Geochemistry, Vol. 6, pp, 143-157, 1991 0883-2927/91 $3.139+ .00 Printed in Great Britain © 1991 Pergamon Press plc Biomarker characterisation of an oil and its possible source rock from offshore Korea Bay Basin S. D. KILLOPS Chemistry Department, Royal Holloway and Bedford New College, University of London, Egham Hill, Egham, Surrey TW20 0EX, U.K. and M. S. MASSOUD and A. C. Scot Geology Department, Royal Holloway and Bedford New College, University of London, Egham Hill, Egham, Surrey TW20 0EX, U.K. (Received 16 March 1990; accepted in revised form 12 September 1990) Abstract--Comparison of biomarker distributions in an oil and in sedimentary rock samples from an offshore well in Korea Bay Basin indicates that an Upper Jurassic, -500 m thick, mudstone zone could form the source rock for the oil. The organic material in this zone appears to be H-rich and of sufficient abundance, from pyrolysis measurements, to generate oil. It seems to derive primarily from bacterial lipids, as reflected in high levels of bacterial hopanoids and predominance of amorphous kerogen macerals. A higher plant wax contribution to the organic matter is indicated by an OEP in n-alkanes >C20, and there may also be an autochthonous algal contribution. Sedimentological and biomarker evidence is consistent with input from a river delta prograding into a deep freshwater lake. Biomarker distributions suggest that pronounced bacterial reworking of allochthonous higher plant material had occurred in probably anoxic conditions. High heat flows in a rapidly subsiding extensional basin probably account for the observed extensive aromatisation of monoaromatic steroidal hydrocarbons at a relatively early stage of sterane isomerisation. INTRODUCTION THE KOREA Bay Basin lies off the west coast of the Democratic People's Republic of Korea in the north- ern part of the Yellow Sea, adjacent to the petrolifer- ous Bohai Basin (Fig. 1). Korea Bay covers an area of -35,000 km e, with water depth ~<75 m. The edges of the Basin appear to extend into the Chinese offshore area in the west and into the Korean onshore area in the east. E 123"5° 124"0° , & N 39.5°- 390 o- 38.50- CH I NA , ",,...,~ ~ ~ YELLOW SEA ¢z hanghai. 606® o.5 JAPAN 30 mites 1o ,2,o ,. o I'o 2'0 30 ~o 50km 124.50 .O' 125.0 ° 125.5 ° E I I I IN KOREA BAY DEMOCRATIC PEOPLES REPUBLIC OF KOREA N ,,a ~" I i I I I E 123.5 ° 12/,.0 o 12t,.5o 125.0° 125.5 ° FIG. 1. Location map for well 606 in Korea Bay. 143 -39.5 o -38.5 ° IN E ¸