Characteristics and geological significance of fluorescence for sandstone grains and hydrocarbon extracts from low permeability reservoirs in the Ordos Basin, China

The Triassic Chang-3 and Jurassic Yanan low-permeability reservoirs in the southern part of the Tianhuan Depression of the Ordos Basin, north-central China, were investigated using the quantitative fluorescence of reservoir sandstone grains and their hydrocarbon extracts. Values of quantitative grain fluorescence on extract intensity (QGF-E) are generally higher than 150 pc, indicating that oil and gas migration has taken place in the low-permeability tight sand bodies in the study area, so these sandstone strata were the main transport system of oil and gas migration. The values of QGF index and QGF-E in the Chang-3 subsection profile above 2168 m are higher than those below 2168 m; similarly, the two values in the Yanan formation profile above 1849 m are higher than those below 1849 m, indicating that the oil–water contacts of the Chang-3 subsection and Yanan Formation in the study area are 2168.0 and 1849.0 m, respectively. The results of total scanning fluorescence for hydrocarbon extracts from the Chang-3 subsection and Yanan Formation show that the extracts contain low aromatic hydrocarbon and are mature. Our results demonstrate that QGF and QGF-E methods are very effective in understanding the properties and accumulation process of hydrocarbon in low-permeability tight reservoir.