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

摘要

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 C_(29)TS/C_(29) hopane ratios (< 0.6), low gammacerane/C_(30) hopane ratios (< 0.4) and low C_(30) diahopane/C_(30) 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 "C_(29) hopane >C_(29)TS >C_(30) diahopane" and T_(27)ααα20R>C_(28)ααα20R4), high C_(29)TS/C_(29) hopane ratios (>1), high gammacerane/C_(30) hopane ratios (>0.5) and high C_(30) diahopane/C_(30) hopane ratios (>0.5), suggesting an anoxic and brackish-saline water depositional environment and have "C_(29) hopane < C_(29)Ts < C_(30) diahopane" and "C_(27)ααα20R ≤ C_(28)ααα20R < < C_(29)ααα20R 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.