Geochemical Characteristics  and Origins of the Lower Cambrian Source Rocks from the Southern Guizhou Depression, SW China

Key words: 
• source rock evaluation /
• marine source rock /
• biomarker /
• productivity /
• anoxic environment /
• Niutitang Formation /
• southern Guizhou Depression /
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Abstract: The Lower Cambrian source rock samples from three typical profiles in the southern Guizhou Depression were analyzed for TOC values, δ13C values,  Tmax temperatures, potential generation, and biomarkers using RockEval, GC, GCMS to elucidate the geochemical characteristics and its origin. The results show that the Lower Cambrian black shales in the southern Guizhou Depression are regional marine excellent source rocks with depth of 50150m, and have the TOC values of 0.13%15.40%, average 3.31%(n=169); the kerogens of the source rocks have the δ13C values of 35.79‰-29.88‰, average 32.85‰(n=35), and the kerogen macerals are mainly sapropelinites; the values of the marine vitrinite reflectance of the kerogens are 1.95%3.96%, and the values of the equivalent vitrinite reflectance are 2.02%3.47%; the distribution shapes of the nalkanes are mainly single peak, and the maximum peak carbon are mainly C22, the C30 hopane and C27ααα20R regular sterane are abundant, the regular steranes are mainly C27>C29>C28, the Pr/Ph values are 0.270.62, the source rocks have relative high abundance of gammacerane and dibenzothiophene, the OEP values are 0.821.11,C31 homohopane 22S/(22S+22R) have values of 0.560.63,average 0.59; the source rocks are generally characterized by enrichment in light REE, Ba, Mo, V and U element, relative strong negative anomaly of Ce element , relative strong positive anomaly of Eu element and relative high ratio of U/Th. In summary, the Lower Cambrian marine source rocks in the southern Guizhou Depression have a wide distribution with a thickness of from tens of meters to hundreds of meters, the source rocks have high TOC values, are excellent for organic matter type, and have a postmaturity. It is the product which is jointly controlled by high paleoproductivity and anoxic environment. It can provide abundant organic matter for hydrocarbon generation in the southern Guizhou Depression. 