Abstract:

A series of breakthroughs in shale oil exploration have been made in several onshore non-marine basins in China showing good exploration and production potentials. Here we summarize our current understandings on the lacustrine shale oil enrichment and distribution characteristics as well as recent research achievements. The main enrichment conditions are the following: (1) Stable and widely distributed shales with high organic content and appropriate thermal maturity act as the material base for shale oil retention. The critical shale parameters are organic carbon (TOC) greater than 2% (optimal 3%-4%); kerogen Ⅰ and Ⅱ₁ as the dominant organic matter type; and vitrinite reflectance (R_o) greater than 0.9% (0.8% for brackish water environment). (2) Diverse reservoir types with brittle accumulation zones and certain volume of micro-nano-scale porous space are critical for shale oil accumulation. The preferred reservoir features are porosity greater than 3%-6%; non-dominating pure shale interval while including tight sandstones and hybrid rocks with clay content less than 20% in the early diagenetic stage; and higher clay content (up to ~40%) in the pure shale interval in the mid-to-late diagenetic stage. (3) High-abundance and good-quality retention hydrocarbons are important for ensuring high-mobility shale oil accumulation. Generally, a S₁ threshold higher than 2 mg/g is needed (optimal 4-6 mg/g), and a GOR threshold greater than 80 m³/m³ (optimal 50-300 m³/m³) is desired. (4) Excellent roof/floor sealing conditions in the shale oil enrichment interval is essential for maintaining overpressure and retaining sufficient amount of good-quality hydrocarbons. The distribution characteristics of lacustrine shale oil can be summarized as the following: (1) Major shale oil enrichment zones are concentrated in the semi-deep to deep lacustrine deposition areas that have external material inputs such as falling volcanic ashes, hydrothermal solutions, radioactive substances, etc. (2) Intervals with the aforementioned enrichment conditions, or the so called “four highs, one retention” condition, control the vertical distribution of the enrichment intervals. (3) Favorable lithofacies and lithologic assemblages also determine the distribution of enrichment areas. According to the preliminary estimates China has a total of (131-163)×10⁸ t medium-high thermal maturity shale oil reserves, among which (67-84)×10⁸ t are economically viable resources, which are distributed mainly in the Chang 7¹⁺² interval of the Ordos Basin, the Qing-1, 2 members of the Gulong sag, Songliao basin, the Cangdong sag, Qikou sag and Kongdian-Shahejie formations of the Jiyang depression, Bohai Bay basin, and the Lucaogou formation, Junggar basin.

Key words: medium-to-high maturity lacustrine shale oil, sweet-spot, enrichment conditions, distributional features, assessment standard, onshore China