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

AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

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.