The Bilong Co oil shale zone is located in the South Qiangtang depression, northern Tibet, southwestern China. This zone, together with the Shengli River-Changshe Mountain oil shale zone in the North Qiangtang depression, northern Tibetan Plateau, potentially represents the largest marine oil shale resource in China. Altogether 18 samples of oil shale and micritic limestone were collected from the Bilong Co area to determine their Sr and Nd isotopic composition, as well as discuss sediment sources and paleoenvironmental changes. The Sm-Nd model ages for the Bilong Co oil shale samples support its derivation from the Nadi Kangra source whose model ages reflect the origin of volcanic rocks from the older continental crust by its melting. The Nd isotopic composition of the Bilong Co oil shale is similar to that of Late Triassic Nadi Kangri Formation volcanic-volcaniclastic rocks, which implies that this oil shale stems from said rocks.
Generally, the vertical variations of εNd(0) values in the Bilong Co oil shale section are relatively homogeneous, suggesting a stable provenance for sediments.
The 87Sr/86Sr ratio of the Bilong Co oil shale is much higher than that of contemporary seawater. Such a high 87Sr/86Sr ratio reflects the original composition of the deposited sediment, both chemical and clastic. In the Bilong Co oil shale area, micritic limestone generally has a slightly lower 87Sr/86Sr ratio than oil shale. The Sr isotopic fluctuation in the Bilong Co oil shale is closely connected with sea level change. Sea level rise and consequent marine incursion may change the aquatic environment of the pre-existing lagoon, lowering the 87Sr/86Sr ratio of the precipitation fluid.
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