Application of fluid inclusions to the charging process of the lacustrine tight oil reservoir in the Triassic Yanchang Formation in the Ordos Basin, China

This study examines the organic geochemistry, physics and accumulation dynamics of the Chang 7 tight oil reservoirs in the Yanchang Formation in the Ordos Basin. We analyse the episodes, timing, and forces of hydrocarbon charging and ascertain the charging process of the lacustrine tight oil in the Chang 7 oil reservoirs by using petrography, micro-beam fluorescence spectra, micro-thermometry, and trapping pressure simulations of fluid inclusions from these reservoir beds. Three conclusions are reached. First, five episodes of brine inclusions and three episodes of hydrocarbon inclusions occurred in the Chang 7 tight reservoir beds. The hydrocarbon inclusions are mainly distributed in fractures that cut across and within quartz grains and have blue-green, green, and yellow-green fluorescence colours. Second, the peak wavelengths, QF535, and Q650/500 of the micro-beam fluorescence spectra indicate three charging episodes of tight oil. According to the burial and geothermal histories, the charging timing were 142-110 Ma, 128-122 Ma and 114-106 Ma, which correspond to the Early Cretaceous. The timing approximately match the main generation and expulsion timing (140-110 Ma) of the Chang 7 source rocks. Third, the pressure differences between the source rocks and reservoir rocks, whose lower limit was 3.97 MPa, served as the primary expulsion and charging forces for crude oil during the charging periods and guaranteed the occurrence of non-buoyancy migration and accumulation.