Abstract
The genetic type of the Bayanerhet Formation oil shale in the Bayanjargalan mine area is an inland lacustrine oil shale deposit. Inorganic element analysis and organic geochemical testing of oil shale samples collected in three boreholes show that the Bayanerhet Formation oil shale has relatively high organic contents, e.g., average TOC values of 6.53, 7.32 and 8.84 (corresponding to oil contents of 5.49%, 6.07% and 7.50%) in boreholes BJ3807, BJ3405 and BJ3005, respectively. Analysis of organic matter sources with biomarkers indicates that lower aquatic organisms such as algae contribute more to the organic matter than higher plants do. According to research on the values of Fe2O3/FeO, Rb/Sr and w (La) n/w (Yb)n in cores from the three boreholes, the Bayanjargalan oil shale is inferred to have formed in a humid paleoclimate with a relatively high sedimentation rate. In research on the evolution of the paleoaquifer in which the oil shale formed, the values of Fe3+/Fe2+, V/V + Ni, Ni/V, Ceanom and δCe are applied as sensitive indicators of the redox conditions in the aqueous medium. These values indicate that the Bayanjargalan oil shale formed in a water body with a weak redox environment. Moreover, the values of Ca/(Ca + Fe) and Sr/Ba and the values of gammacerane/αβC30 hopane in biomarkers show that the oil shale was formed in a saltwater environment. Analysis of Mo and U shows high endogenous lake productivity, corresponding to high TOC, which suggests that the lacustrine productivity played an important role in organic matter enrichment. The Lower Cretaceous Bayanerhet Formation (K1bt) in the Bayanjargalan mine area encompasses a complete sequence and was formed during lowstand, transgression, highstand and regression periods. The dominant oil shale deposits were formed in the transgression system tract and high stand system tract, and these oil shales have a high oil content and stable occurrence. A large set of thick, high-TOC and high-oil-content oil shales in the second member of the Bayanerhet Formation was deposited under such conditions. The abundant terrigenous supply under warm and humid conditions significantly promoted the primitive biological productivity, and the weak redox saltwater environment had relatively high productivity. All the favorable conditions promoted the formation of high-quality oil shale.
Highlights
The genetic type of the Bayanerhet Formation oil shale in the Bayanjargalan mine area is an inland lacustrine oil shale deposit
Affected by Late Jurassic-Early Cretaceous faulting in north central Mongolia, a series of secondary graben and half-graben basins formed inside the Bayanjargalan Basin, which is composed of small-scale faulted lake basin groups in terms of geologic characteristics
Samples were collected from the oil shale in the Bayanjargalan mine area, which formed in the Mesozoic Lower Cretaceous Bayanerhet Formation
Summary
The genetic type of the Bayanerhet Formation oil shale in the Bayanjargalan mine area is an inland lacustrine oil shale deposit. Abundant oil shale developed in the Lower Cretaceous Bayanerhet Formation in this basin. Previous research on the sedimentary geology of oil shale in the Bayanjargalan Basin has determined that the Bayanerhet Formation (K1bt) was deposited during the Early Cretaceous faulting period in the Bayanjargalan B asin[1]. Affected by Late Jurassic-Early Cretaceous faulting in north central Mongolia, a series of secondary graben and half-graben basins formed inside the Bayanjargalan Basin, which is composed of small-scale faulted lake basin groups in terms of geologic characteristics. These basins were developed on the basement of the Hercynian fold belt. The formation has an average thickness of 45 m and is in unconformable contact with the underlying strata
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