Abundant oil-bearing fluid inclusions are present in four reservoir s
andstone samples from the slope
and fault zone areas in the northwestern margin of the Junggar Basin (NW China). Free oil in intergranular pores, adsorbed oil on grain surfaces,
and petroleum in inclusions hosted by mineral grains of these samples were analyzed by gas chromatography
and gas chromatography–mass spectrometry. Analytical results indicate similar biomarker distributions in the three bitumen fractions of the samples collected from the slope area, which correlate well with characteristic of the source rocks in the Lower Permian Fengcheng Formation. This is in contrast to the fault zone, where the three bitumen fractions vary significantly in their geochemical signatures, suggesting that all of the three Permian source sequences rocks (including the Lower Permian Jiamuhe
and Fengcheng formations,
and the Middle Permian Lower Wuerhe Formation) have been mature
and contributed to the oils in the fault zone. The presence of an unresolved hydrocarbon hump
and a full range of unaltered
n-alkanes in the gas chromatograms of samples collected from the fault zone indicate an earlier hydrocarbon biodegradation
and subsequent injection of fresh oils, distinctly different from that in the slope area.
Petrographic and microthermometric investigations of oil-bearing fluid inclusions suggest a single oil charge in the slope area (to the Lower Triassic reservoir, occurring during the Late Triassic to Early Jurassic time). In the fault zone, oil migration and mixing took place mainly during the Late Triassic to Early Jurassic and Cretaceous, followed by gas migration in the Tertiary period. The different petroleum migration and mixing histories in the slope and fault zones are attributed to the effect of fault-controlled oil and gas migration.