Occurrence, thermal evolution and primary migration processes derived from studies of organic matter in the Lucaogou source rock at the southern margin of the Junggar Basin, NW China

Abstract

The Lucaogou Formation carbonate-rich oil shale source rock is exposed at the southern margin of the Junggar Basin, Xinjiang, NW China. We have sampled it in detail and conducted microstructural, mineralogical and geochemical studies, including thin section petrography, UV fluorescence petrography, X-ray diffraction, vitrinite reflectance, bitumen reflectance, fluid inclusion analysis and Raman spectroscopy. Organic matter is disseminated through the carbonate-bearing siltstone source rocks and concentrated in numerous bedding parallel stylolites and in two sets of carbonate veins, one along bedding parallel fractures and the other cross-cutting stylolites and bedding. The research about maturity of organic matter finds vitrinite reflectance values increase from the dispersed kerogen (0.64%) to the stylolites (the one of oriented vitrinite is 0.72% and the one of migrated bitumen is 2.38%); Homogenization temperatures of fluid inclusions in veins containing hydrocarbon fluid inclusions show an increase from 178.5℃ in the bedding parallel veins to 222℃ in the cross-cutting veins, confirmed by Raman spectroscopy. These results support a model of progressive heating accompanied by fluid loss during later stages of thermal maturation of source rock and the onset of primary migration. Obviously, the occurrence of organic matter is the trace of hydrocarbon primary migration, and the bedding lamination surfaces and cross-cutting fissures are the principal pathways of hydrocarbon-bearing fluids migration. Bedding lamination surfaces evolved into stylolites along the earliest primary migration pathways, followed by bedding parallel and cross-cutting fissures.hydrocarbon source rock, thermal evolution history, primary migration, migration pathway, Junggar Basin

Intact microbial fossils in the Permian Lucaogou Formation oil shale, Junggar Basin, NW China

“…The Junggar Basin is a large, organic-rich foreland basin in northwest China (Jiao et al, 2007). Bogda Mountain is in the eastern Tianshan Mountain range and is located on the southern margin of the Junggar Basin ( Fig.…”

Characteristics and developmental mechanisms of mud volcanoes on the southern margin of the Junggar Basin, NW China

“…). These events generated a strong N–S‐oriented regional stress field, in which a piedmont nappe belt and a number of concealed, underlying structures began to form (Zhao et al ., ; Jiao et al ., ; Li et al ., ; Charreau et al ., ; Yang et al ., ). During this time, the Mesozoic and Cenozoic formations in the depression gradually shifted northward to create folds in the vicinity of the Yanshan en‐echelon faults, including the Dushanzi, Xihu, and Kayindike anticlines.…”

“…Based on the deformation characteristics from east to west, the southern margin of the Junggar Basin can be divided into three parts: the eastern part, middle part and western part, that is located in the Sikeshu Depression. The deformation can be vertically divided into several layers due to the presence of tectonic slip surfaces (Jiao et al ., ; Qiu et al ., ; Bao et al ., ).…”

“…The carbon isotope ratios (δ 13 C 1 ) of the methane range from −38.92‰ to −42.82‰, with an average of −40.53‰, while that of the ethane (δ 13 C 2 ) ranges from −20.50‰ to −22.95‰, with an average of −21.45‰. There are three sets of source rocks in the southern Junggar Basin: Middle Permian rocks over mature source rocks (253–258 Ma); Middle–Low Jurassic mature–over mature coal‐measure source rocks (163–213 Ma); and Palaeogene immature–low mature gas source rocks (24.6–65 Ma) (Jiao et al ., ; Qiu et al ., ; Bao et al ., ). According to the geochemical data, the released gas is a coal‐type thermogenic gas, derived primarily from a Middle–Lower Jurassic coal‐measure source.…”

Source Rock Potential and Depositional Environment of Upper Permian Oil Shales of the Lucaogou Formation in the Southeastern Junggar Basin, Northwest China

“…These minerals are common in the semi-deep to deep lacustrine environment, and are indicative of the redox condition of bottom waters. The Lucaogou Formation contains abundant fossils of freshwater organisms such as Tienshaniscus longipterus, Turfania taoshuyuanensis, Anthraconauta pseudophilipsii, Anthraconauta ilijinskiensis, Darwinula parallela, Darwinula ornata and Darwinula momitoria [28][29][30][31]. …”