Microscopic occurrence of oil in tight sandstones and its relation with oil sources - A case study from the Upper Triassic Yanchang Formation, Ordos Basin

Identifying pore distribution of tight reservoirs and the microscopic occurrence of oil in pores of different scales is critical to understand the characteristics of tight oil. In this study, tight sandstone samples from Yanchang Formation, Ordos Basin, were investigated through Nuclear Magnetic Resonance (NMR) experiment. Nano-scale pores are common in the tight sand of Yanchang Fm and macro-scale, medium-scale, small-scale, micro-scale and nano-scale pores co-exist in the samples. The microscopic occurrence of oil in multiple-scale pores are compared between near-source reservoir and far-source reservoir. In near-source reservoirs, oil exists in all kinds of pores and the micro-pores and nano-pores are saturated with oil. In contrast, in far-source reservoirs, oil selectively occur in macro-scale and medium-scale pores and nano-pores contains no oil. The abnormal pressure caused by hydrocarbon-generation in Chang 7 layer is believed to be the main controlling factor for different occurrences of oil in multi-scale pores in tight reservoir. This provides a theoretical basis for understanding the distribution of tight oil reservoir. It is concluded that the upper Chang 8, the Chang 7 and lower Chang 6 reservoirs within the distribution area of Chang 7 source rock are the main potential targets for tight oil exploration in the future.