Geochemical characteristics and grouping of the crude oils in the Lishu fault depression, Songliao basin, NE China

Abstract

Samples from a number of oilfields located in the southern part of the Songliao Basin in NE China were analyzed using techniques including gas chromatography and gas chromatography–mass spectrometry. Biomarker compositions and parameters were used to delineate oil families. The samples investigated in this study can be classified into three groups, Groups I, II and III. Group I oils predominately occur in the Denglouku Formation and the Quantou Formation and have relatively low Ts/Tm ratios (<1.8), low C29Ts/C29 hopane ratios (<0.6), low gammacerane/C30 hopane ratios (<0.4) and low C30 diahopane/C30 hopane ratios (<0.2). The Group I results indicate a suboxic and freshwater deposition environment for the respective source rocks, and the oils have a distribution pattern of “C29 hopane>C29Ts>C30 diahopane” and “C27ααα20R>C28ααα20R<C29ααα20R steranes” on the m/z 191 and m/z 217 mass fragmentograms, respectively. Group II oils, by contrast, are characterized by high Ts/Tm ratios (>4), high C29Ts/C29 hopane ratios (>1), high gammacerane/C30 hopane ratios (>0.5) and high C30 diahopane/C30 hopane ratios (>0.5), suggesting an anoxic and brackish-saline water depositional environment and have “C29 hopane<C29Ts<C30 diahopane” and “C27ααα20R≤C28ααα20R<<C29ααα20R steranes” distribution patterns. These oils dominant in the Yingcheng Formation and the Shahezi Formation. Group III oils are a mixture of Group I and Group II oils and show intermediate geochemical compositions and occur in the Denglouku Formation and the Shahezi Formation. Oil groups identified by biomarker compositions and parameters may indicate that at least two or more groups of effective oil source rocks should exist.