Source rock quality, depositional and geochemical characterization of the Khoot Basin in Mongolia

Abstract

The Khoot Basin is located in the Eastern Mongolian Basinal Province. The basin has long been of interest to international and domestic petroleum and mining companies as the most proliferous oil shale (Eedemt deposits) in the country outcrop at the basin surface. The Khoot Basin is located near the margin of two larger Early Cretaceous sedimentary basins: Choir-Nyalga and Middle Gobi. Like many Mongolian sedimentary basins, the Khoot Basin is relatively less studied. Therefore, for the first time, this study is providing an integrated source rock evaluation, subsurface correlation and geochemical analysis (XRD, XRF, and WOGC) of the Khoot Basin. Additionally, based on the depositional trends and well-log correlation, the sequence stratigraphic framework of the basin was established. Historically, the Khoot oil shales were regarded, based on outcrop samples, as the best quality oil shale in Mongolia. However, the results from this study suggest that the shale in the subsurface is not rich enough in organic matter to be classified as oil shale. The Khoot outcrop oil shales contain 10-25 % organic matter and are a Type I lacustrine oil shale with higher algal input. The Khoot subsurface samples, on the other hand, are a Type I and mixed Type II lacustrine source rock with greater higher-plant input. The terrestrial contents peak near the maximum flooding surfaces and the bottom sequence. The total organic content of the subsurface oil shales is 2-11 %. Based on these results, the Khoot outcrop and subsurface samples are likely different temporal deposits and possibly deposited in different lakes. From the analytical data and subsurface data correlation, the gross rock volume was calculated for the Khoot Basin and the ultimate expellable potential (UEP) was estimated. The KinEx calculation yielded a total UEP of 4.94 bnboe, in which the oil content is 4.07 bnboe and the gas content is 0.87 bnboe. Although the Khoot oil shale is similar to world-class source rocks based on the yield per unit meter (0.66-0.87 mmboe/ m²), it is not a world-class play, because of the relatively small size and thickness of the basin. The ultimate expellable potentials were also estimated for the neighboring Choir-Nyalga and Middle Gobi basins, which ranged from a low of 2,023 (3,515) bnboe to a high of 18,041 (31,352) bnboe depending on assumptions regarding source rock thickness and type.