Migration of immiscible hydrocarbons recorded in calcite-hosted fluid inclusions, Ordos Basin: a case study from Northern China

In this study one- (hydrocarbon liquid), two- (hydrocarbon liquid and vapor) and three- (hydrocarbon liquid and vapor, and aqueous liquid) phase fluid inclusions have been investigated in calcites occurring in veinlets in fine grained sandstones of Chang 7 source rock and Chang 8 reservoir rock. These rocks are parts of the Upper Triassic Yangchang Formation in Ordos Basin, Northern China. The hydrocarbon inclusions studied show high variance in vapor-liquid volume ratio at ambient temperature, suggesting a heterogeneous entrapment of vapor and liquid phases those existed before or during formation of the calcite. This is supported by homogenizations happened into both liquid and vapor phase. Homogenization temperatures are ranging between 70 and 120 °C. We consider that this wide range are rather resulted in heterogeneous entrapment of immiscible liquid and vapor phases than high temperature (e.g., 120 °C) formation of the hydrocarbon inclusions. Therefore, the lower range of homogenization temperatures (around 80 °C) seem to be probable for formation of the hydrocarbon inclusions studied. Fluorescent microscopic results on the hydrocarbon inclusions show bright yellow color for the oil. Fluorescence spectra indexes indicate that oil in the inclusions is quite mature, similarly to the oil extract from Chang 7 source rock. The differences in fluorescent spectra among the hydrocarbon inclusions suggest their chemical modification during evolution, which might be determined by continuous separation of vapor phase (gas condensate) from the system. On the basis of organic geochemistry of the studied samples (e.g., akin distribution of normal-alkanes and similar isoprenoid ratios), it can be concluded that the oil in the inclusions and the crude oil from the reservoir rock Chang 8 are actually derived from the Chang 7 source rock. During tectonic evolution, the hydrocarbon might suffer phase separation (boiling) forming two phases (a liquid and a vapor phase) due to intensive uplifting of Ordos Basin after the Late Cretaceous. The boiling in the accumulated hydrocarbon could result in overpressure in the source rock. This phenomenon might favor formation of fractures and migration of hydrocarbons from their source toward into sandstone reservoirs. Our study demonstrates the significance of detailed analytical investigation of hydrocarbon inclusions in correlation with their potential source rock.