Geo-chromatographic fractionation effect of methyldibenzofuran in dolomite reservoirs and its application in tracing oil filling pathways in the Sichuan Basin

Abstract

Abstract Dibenzofuran and its methyl substituted homologues are the important oxygen-containing polycyclic aromatic compounds in crude oils. The ratio of 1-/4-methyldibenzofuran has been successfully used in sandstone reservoir tracing oil migration orientation. Here we focus on the geo-chromatographic fractionation effect of methyldibenzofuran (MDBF) in dolomite reservoirs. We calculated the charge density differences between 1-methyldibenzofuran (1-MDBF) and 4-methyldibenzofuran (4-MDBF) based on density functional theory. The E I n between MDBF isomers and dolomite (104) surface was also calculated according to molecular dynamics simulation. The result show that comparing with the 4-MDBF, there is strong interaction between 1-MDBF and dolomite (104) surface leading the ratio of 1-MDBF and 4-MDBF increase with the migration distance. There is no significant correlation between bitumen reflectance and the ratio of 1-MDBF and 4-MDBF in our samples. In addition, molecular simulation results show that the MDBF isomers are thermodynamically stable and dissociation of Ph-CH3 is difficult to occur under geological condition. All these demonstrate that 1-MDBF and 4-MDBF can be survived after oil-cracking to gas and pyrobitumen. Therefore, in theory, the ratio of 1-/4-methyldibenzofuran is also regarded as an index for oil migration orientation in dolomite reservoir. Gas chromatography-mass spectrometry results show that MDBFs occur at relatively higher abundance in the bitumens of the Ediacaran Dengying Formation (Z2dn4) and Cambrian Longwangmiao Formation (Є1l) in the Sichuan Basin. The ratio of 1-/4-methyldibenzofuran has been successfully used to trace the crude oil filling pathway in the Z2dn4 and Є1l dolomite reservoir in the Sichuan Basin in this study. Two preferential filling orientations (i.e. from west to east and from north to south) for paleo-oil have been identified in the contour maps of 1-/4-methyldibenzofuran in the Z2dn4 and Є1l, which is consistent with the geological background.