Depositional environment of terrestrial petroleum source rocks and geochemical indicators in the Songliao Basin

Abstract

To determine geochemical indicators for depositional environment favored by terrestrial petroleum source rocks, we selected 40 source rock samples from the Late Cretaceous Qingshankou Formation (K2qn) and the first member of Nenjiang Formation (K2n1) in the Songliao Basin to qualify saturate fraction and aromatic fraction using GC-HRT (gas chromatography high resolution time-of-flight mass spectrometry) and quantify important biomarkers using GC-MS. The results reveal that source rocks from the 1st member of Qingshankou Formation (K2qn1) are characterized by not only high contents of terpanes, regular steranes and 4-methylsteranes but also high contents of dinosteranes, C31 steranes and aryl isoprenoids. Presence of specific biomarkers like elementary sulfur and lanostanes indicates a depositional environment of lagoon characterized by water stratification and high salinity. In the 2nd–3rd members of Qingshankou Formation (K2qn2+3), source rocks contain lower contents of biomarkers, indicating a depositional environment of shallow fresh-water lake delta. Source rocks in the K2n1 contain high contents of terpanes, regular steranes and 4-methylsteranes but lower contents of dinosteranes, C31 steranes and aryl isoprenoids, indicating a depositional environment of fresh-brackish open lake characterized by low salinity and poor water stratification, where organic matter is seriously altered by bacteria. Overall analysis shows that primary geochemical indicators for terrestrial petroleum source rocks are as follows: 1) C30 hopanes > 1500 ppm; 2) gammacerane >190 ppm; 3) C27 steranes >200 ppm; 4) 4-methylsteranes >100 ppm; 5) aryl isoprenoids > 3 ppm; 6) dehydroxyl-vitamin E >10 ppm.