Jurassic oils in the central Sichuan basin, southwest China: Unusual biomarker distribution and possible origin

Crude oils in the Jurassic reservoirs of the Shilongchang oilfield in central Sichuan basin are characterized by a high abundance of methylcyclohexane relative to n-heptane in the C-7 light hydrocarbons, high pristane/phytane ratio (Pr/Ph 1.81-2.45), a C-29 sterane dominance and a tendency towards lower delta C-13 values with increasing chain length for C-12-C-19 n-alkanes, reflecting a dominantly terrigenous source deposited in a sub-oxic to oxic environment. The oils are extremely enriched in rearranged hopanes, including 17 alpha(H)-diahopanes, 18 alpha(H)-neohopanes and an unidentified rearranged hopane series with relatively short gas chromatography (GC) retention times. The 17 alpha(H)-diahopanes are present in highest abundance among the hopane classes and exhibit similar carbon number distributions to those of the 17 alpha(H)hopanes, including a range from C-27 to C-35, a dominance of the C-30 isomer and presence of 22S and 22R epimers for C-31-C-35 homologues. The 18 alpha(H)-neohopanes are less abundant than the 17 alpha(H)-diahopanes, with only Ts, C29Ts and a possible C-30 member detected. The early eluting rearranged hopanes extend from C-29 to C-35 and display distributions similar to the 17 alpha(H)-hopanes and 17 alpha(H)-diahopanes. As the origin of rearranged hopanes is relatively well understood, their unusually high relative abundance in the studied Jurassic lacustrine oils from the central Sichuan basin provides strong evidence for the presence of effective highly mature and clay-rich source rocks formed under a sub-oxic depositional setting. (C) 2007 Elsevier Ltd. All rights reserved.