Occurrence and recoverability of tight oil in Paleogene Funing Formation, Subei Basin

The shale, interlayers and adjacent carbonate rocks and siltstones in the second and fourth members of the Paleogene Funing Formation in the Subei Basin were studied using multi-temperature pyrolysis and NMR. The oil-bearing capacity of shale, interlayers and adjacent tight reservoirs in the Funing Formation and the movability of tight oil were evaluated in combination with with shale oil production data. In the second and fourth members of the Funing Formation, the content of tight oil is generally less than 5.0 mg/g in shale. Adsorption-miscible oil is dominant, and the ratio of which to total retained oil decreases with the increase of burial depth. Light free oil generally has a low content, and the actual movable oil ratio is less than 3%. However, the tight oil content is generally greater than 5 mg/g in mud shale interlayers and adjacent sandstones (oily), dominated by free oil. The light free oil content is generally more than 0.25 mg/g, and the actual movable oil ratio is generally between 4% and 7%, which is close to the 5% recovery ratio in the tight sandstone layers before reconstruction in the middle of Bakken Formation in North America. The movable tight oil mainly exists in fractures in the Funing Formation in the Subei Basin. That is, tectonic fractures are the main host of movable shale oil, and some micro pores (with ramp radius greater than 0.18 μm) work as secondary occurrence porosity. Free oil exceeds adsorbed oil. The initial production of tight oil is high in fissured lacustrine carbonates in the first and third members of the Funing Formation, but the production decreases rapidly with a short production cycle, and the economic recoverability is limited. In contrast, the initial production is lower in the tight interlayer sandstones of the source in the second and fourth members of the Funing Formation, which has a slower decline rate, longer production cycle and better economical recoverability.