Geochemical characteristics and effectiveness of thick, black shales in southwestern depression, Tarim Basin

An interval of black shales deposited from Precambrian to lower Cambrian from the southwestern depression of the Tarim basin (SDTB) was systematically evaluated in terms of hydrocarbon potential, organic matter, depositional environment and oil-source relationship. The shales have good TOC contents (up to 3.15%) but low S1+S2 values (<0.03 mg/g), indicating the source rocks are currently at high thermal maturities. The relationship between pristane/n-C17 and phytane/n-C18 indicates the kerogen was oil prone and dominant by algal input. Furthermore, the depositional environment had relatively moderate salinity and stratified water environment indicated by G/C30H ratios (0.15–0.24). The high algal input may have enhanced TOC preservation under reducing conditions reflected by high dibenzothiophene content and low pristane/phytane ratios (mean 0.79). Thus, these source rocks may have generated a significant amount of oil during their geological history. Additionally, in view of the molecular geochemistry indicators, the extracts of black shales hold marked differences with the Cambrian-Ordovician end-member oils, but are quite similar to oils in the northeastern portion of the SDTB. It means that the lightly explored SDTB may contain significant hydrocarbon resources, and coeval source rocks in the ultra-deep portion of the basin may have contributed to the prolific petroleum resources in the Tarim Basin.