Characteristics and formation mechanisms of tight oil: A case study of the Huahai Depression, Jiuquan Basin, Northwest China

The exploration of tight oil reservoirs has achieved a big success, especially in the Huahai Depression, Jiuquan Basin, China, but the characteristics and formation mechanisms of the tight oil reservoir are unclear. In this paper, a series of analyses including thin section, scanning electron microscope, gas chromatography–mass spectrometry, isotope, and fluid inclusions were taken to study the tight reservoirs of the Xiagou Formation (K1g) and Zhonggou Formation (K1z) in the Huahai Depression. The results show that the main factors influencing the porosity and permeability of the tight reservoir are the compaction and two periods of carbonate cementation. The second type of carbonate cementation was related to the decarboxylation of organic acids in parts of the study area. The tight oil accumulation periods for different formations are confirmed, during 115–102 Ma in K1g1, 114–97 Ma in K1g2, and 103–85 Ma in K1z1, respectively. The tight oil migrations are executed for only several meters vertically but...