The geochemistry and petrography of oil inclusions in the ca. 1.4 Ga Bessie Creek Sandstone in the Roper Superbasin, Australia, provide insights into poorly understood Middle Proterozoic petroleum systems. Oil inclusions were trapped after the intrusion of ca. 1.3 Ga dolerite sills during basin reactivation and peak maturation. Biomarkers reflect the predominance of prokaryotes in the Mid-Proterozoic biosphere and the most likely source rock is the directly overlying Velkerri Formation. (C) 2003 Published by Elsevier Science B.V.