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Insights Into Reservoir Development Based on Integrated Studies of Organic Matter and Pore Size in Lacustrine Shale: Triassic Yanchang Formation, Ordos Basin, China

Abstract

The Triassic Yanchang Formation was deposited in a fluvio-lacustrine environment and is dominantly distributed on the Shanbei Slope in the southeast Ordos Basin. The current burial depth ranges from 500m to 1800m. Three organic-rich layers (the Chang 7, 8, 9 members) are considered to be the main sources of oil accumulation in the sandstone reservoirs which are interbedded with clay mineral and organic matter-rich shale layers. The deliverability of shale gas/oil from a lacustrine shale system containing high clay mineral content remains a contentious issue. Geochemical characterization, including TOC and Rock-Eval analysis, was conducted for 40 core samples recovered from three organic-rich layers of the Triassic Yanchang Formation. TOC content ranges from 0.4 - 8.2% (average 4.2%). The thickness of layer with TOC values greater than 4% is about 30m (Chang 7 member). Organic matter is dominantly Type I and Type II kerogen, as shown by HI vs. OI plots. Tmax values range from 440°C to 455°C, indicating a thermal maturity in the oil generation window. The Chang 7 member is thus considered a potential target for lacustrine shale gas and oil resource exploration. The production index (S1/(S1+S2)) in the lower part of the Chang 7 interval is higher than in the upper part, whereas the potential hydrocarbon generation (S2) in the lower Chang 7 interval is apparently smaller than the upper, suggesting that generated oil in the upper part partially migrated into the lower part of organic-rich interval of Chang 7 member. However, retained gas yield from rock crushing, a proxy for pore connectivity, is low in the lower Chang 7 member, and the lesser amount of retained gas is an indication of better pore connectivity. Pore characterization of these organic-rich shales using N2 adsorption/desorption isotherms demonstrates that 2–10nm size pores represent more than 60% of the total pore volume for samples with TOC values greater than 4%. The higher TOC directly corresponds to larger BET surface area, indicating that 2–10nm size pores are dominated by organic matter-hosted pores. Development of organic matter-hosted pores in lacustrine shale system may be crucial to gas storage, migration, and deliverability.