Characteristics of tight oil in Triassic Yanchang Formation, Ordos Basin

TLDR

The Triassic Yanchang Formation contains low‑permeability reservoirs and is considered an important potential resource for future oil exploration and development. The study analyzed reservoir and source rock distribution, petrology, and geochemistry, concluding that large‑scale superimposed tight oil reservoirs form from interbedded lithologic combinations of extensive source rocks and reservoirs coupled with strong hydrocarbon generation and expulsion during geological history. Proved tight‑oil reserves in the Triassic Yanchang Formation total about two billion tons, with permeability below 2 × 10⁻³ µm²; the oil is confined to tight sandstone reservoirs of the Chang6–Chang8 members interbedded with oil shale, with especially low permeability (<0.3 × 10⁻³ µm²) in the large‑scale gravity‑flow sandstone reservoirs of Chang7 and Chang6, and the reservoirs exhibit complex pore structures, high rigid component content, abundant fractures, good crude properties, low fluid pressure, and low oil yield.

Abstract

By comprehensive study of reservoir and source rock distribution, petrology and geochemistry, the tight oil and its exploration potential was analyzed in the Triassic Yanchang Formation, Ordos Basin. The Triassic Yanchang Formation is rich in low permeability reservoirs. The proved geological reserves of tight oil, with the permeability less than 2×10−3 μm2, is about two billion tons by now. The tight oil mainly occurs in tight sandstone reservoirs of Chang6-Chang8 oil-bearing members which are close to or interbedded with the oil shale layers, without long-distance migration. The large-scale gravity flow sandstone reservoirs of Chang7 and Chang6 oil-bearing members in the center of the lacustrine basin are particularly tight, with the permeability less than 0.3×10−3 μm2 in general. The tight oil in the Yanchang Formation features large scale in sand body complex, tight reservoir, complicated pore throat structure, high content of rigid components, abundant fractures and saturation, good crude property, low fluid pressure and low oil yield. The formation of large-scale superimposed tight oil reservoirs is controlled by the interbeded lithologic combination of extensive source rocks and reservoirs and the strong hydrocarbon generation and expulsion during geological history. This type of pools is an important potential resource for future oil exploration and development.