Oil-bearing heterogeneity and threshold of tight sandstone reservoirs: A case study on Triassic Chang7 member, Ordos Basin

Abstract

The Ordos Basin is the foremost tight-oil reservoir in China with respect to location and production, where tight-oil reservoirs are characterized by low porosity, low permeability, low pressure, and high heterogeneity. Until now, the cause of the oil-bearing heterogeneity of such reservoirs has not been reported. In this study, the cause of the oil-bearing heterogeneity and threshold of tight sandstone reservoirs in the seventh member of the Triassic Yanchang Formation (Chang7 Member), Ordos Basin, were investigated by core and thin-section observations using a fluorescence microscope, cathodoluminescence microscope, isotope ratio mass spectrometer, and thermal stage, among other devices. The results revealed that calcitic cementation is the major genetic mechanism for the oil-bearing heterogeneity of tight sandstone reservoirs in the study area, and that it is mainly in Phase 1. The clumped isotope revealed that the calcitic cements in Phase I were formed at 18–42 °C during the Middle–Late Triassic to Middle Jurassic periods; thus, they are the diagenetic products, and may be related to the early small-scale tectonic movements in the basin. The synchronous brine inclusions associated with the hydrocarbon inclusions correspond to a homogenization temperature of 90–120 °C. In conjunction with the basin simulation, this suggests that the hydrocarbons were charged in one phase in the Early Cretaceous period (100–130 Ma). Moreover, the modified cements in the Chang7 oil-free tight sandstone were formed earlier than the period in which the oil was charged in the oil-bearing sandstone. The tight sandstone reservoirs mainly contain nanoscale throats (diameter < 1 μm). With respect to the residual oil saturation, average size of mesopores, and the correlation between the oil saturation and porosity, it was concluded that tight oil may be present above the threshold characterized by a 15 mm mesopore average size and 2% porosity. It is first specified that the formation of oil-free calcitic-cemented sandstone is the diagenetic response of the early tectonic movement, which provides a basis for the determination of the influence of calcitic-cemented sandstone on hydrocarbon accumulation. Moreover, the results of the study on the oil-bearing threshold can be helpful in selection of the exploration zone and the resource estimation of unconventional tight-oil reservoirs.