Application of chemometrics to quantitative source assessment of crude oils from the Zhanhua Depression, Bohai Bay Basin, northeast China

Abstract

The chemometric methods alternating least squares and multidimensional scaling were employed in this study to quantify the relative contribution of the sources and identify the affinities of sixty–five crude oil samples collected in the Zhanhua Depression. The circos diagram was used to reflect the relative contributions of different source rocks. Based on these techniques, two end–members (EMs) were identified which represent the geochemical characteristics of two regional source–rocks (Es3L and Es4U). The EM1 composition assigned to Es3L is characterized by relatively low C35/C34 homohopane ratio and relatively low abundance of gammacerane. This EM is consistent with a source facies that was deposited in freshwater lacustrine environment that contained a non–stratified water column and suboxic to dysoxic bottom waters. Distributions of the regular steranes and tricyclic terpanes, the relatively low steranes/hopanes ratio suggest the organic matter input into this source facies were combined microalgal material and terrestrial organic matter. Oils from the Bonan Sag and Gubei Sag were derived mainly from this source facies. In contrast, EM2 has been assigned to the Es4U source rock and is characterized by relatively high abundance of gammacerane, C35/C34 homohopane ratio greater than one, suggesting deposited in a saline to hypersaline lacustrine environment that contained a stratified water column with strongly reducing bottom waters. Distributions of the regular steranes and tricyclic terpanes as well as relatively high steranes/hopanes ratio suggest the organic matter deposited in this setting was mainly derived from microalgal/bacterial sources. Oils from the Luojia Nose and Chanjiazhang High were derived mainly from this source facies. Mixed contributions of these two source facies (end–members) account for the compositions of crudes produced in the Yihazhuang Arch and Gudao High. In the case of the Yihezhuang Arch the crudes appear to have near equal contributions from the two source faces, whereas crudes from the Gudao High appear to have slightly higher contributions from the Es3L source facies than from the Es4U source facies.