Bio-precursors of Cretaceous black shales in the Qiangtang Basin, Tibet

A new sequence of hydrocarbon source rock has been discovered recently in the Qiangtang Basin, Tibet, i.e., the Cretaceous black shales. We investigated the bio-precursors of the shales by an integrated method of organic petrology and geochemistry to improve the understanding of their resource potential. Results showed that the bio-precursors of the shales can be classified into four types in general based on their shapes, sizes, accumulation forms and fluorescence. They include marine benthic algae and zooplankton, bacteria, phytoplankton and terrestrial higher plants in decreasing order of abundance. Benthic algae are the dominant precursors. Similar to source rocks of the same type worldwide, these benthic algae dominant source rocks have high organic matter abundance (TOC>2.0%), reaching the level of good quality and belonging to organic matter type Ⅱ₂, indicating that they were deposited in a marine environment. Combined with the regional thermal evolution of the shales, we deduced that the black shales have a good hydrocarbon resource potential and are likely to produce dominantly gas. They may comprise a good exploration target and merit further geological research.