Abstract:

The HQ area in the Southwestern Ordos Basin is a transformation block in the Yumen oilfield and a key exploration area for enhancing storage and production.Due to the influence of multi-stage tectonic movements, the source of oil and gas remains controversial.In this study, shale samples from different submembers of the Chang 7 and oil samples from the Chang 8 of the Upper Triassic were analyzed.We conducted various experimental analyses in this study, including total organic carbon analysis, rock pyrolysis, maceral analysis, and saturated hydrocarbon gas chromatography-mass spectrometry.The geochemical characteristics of these samples were studied, and the oil-source correlation was performed using pedigree clustering, multidimensional scaling, and principal component analysis.The rationality of the oil-source correlation results was evaluated based on the availability of source rocks and hydrocarbon accumulation conditions.The results show that: (1) The source rocks of the Chang 7 Member in the eastern part of the study area are generally of good to high quality, whereas those in the western part are of medium to good quality.The source rocks of the Chang 7 Member in the east are primarily composed of Type I-Ⅱ₁ kerogen, while those in the west are predominantly Type Ⅱ₁-Ⅱ₂ kerogen.The degree of thermal evolution of organic matter in the source rocks of the Chang 7 Member is generally at a mature stage.(2) The oil in the Chang 8 Member in the east area of HQ is sourced from the local black shale in the Chang 7₃ submember.In the contrast, the oil in the west Chang 8 Member is a mixture of local dark gray mudstone from the Chang 7₃ submember and black shale from the eastern Chang 7₃ submember.(3) The black shale of the Chang 7₃ submember has a high organic carbon content, served as a high-quality source rock.Additionally, the dark gray mudstone of the Chang 7₃ submember, also with high organic carbon content, can be considered as an effective source rock for hydrocarbon generation.The Late Jurassic was the initial stage of hydrocarbon accumulation in the Chang-8 reservoir, during which low-maturity oil from the eastern source rocks migrated to the higher structural position in the western part of the study area.By the end of the Early Cretaceous, the western source rocks in the study area had reached maturity, and the crude oil sourced from the western source rocks supplied hydrocarbons to the local Chang 8 reservoir.This study provides valuable guidance for exploring oil and gas reservoir in the lower strata of the western margin of the Ordos Basin.

Key words: chemometrics, geochemical characteristics, oil-source correlation, Chang 8 Member oil, Chang 7 Member source rock, Ordos Basin