Development Conditions and Factors Controlling the Formation of the Permian Pingdiquan Source Rocks in the Wucaiwan Sag, Junggar Basin, China: A Comprehensively Elemental, Biomarker and Isotopic Perspective

Abstract

This paper is a synthetic use of carbon isotope composition, Rock-Eval data, organic petrology, element composition of kerogen, major and trace elements, and biomarker characteristic of the Permian Pingdiquan (P₂p) source rocks in the Wucaiwan sag, Junggar Basin, China as proxies (1) for evaluations of hydrocarbon potential, organic matter (OM) composition and thermal maturity of the OM in the source rocks, (2) for reconstruction of paleodepositional environment, and (3) for analysis of controlling factor of organic carbon accumulation. The P₂p Formation developed good-excellent source rocks with thermal maturity of OM ranging from low-mature to mature stages. The OM was mainly composed of C₃ terrestrial higher plants and aquatic organisms including aerobic bacteria, green sulfur bacteria, saltwater and fresh algae, Sphagnum moss species, submerged macrophytes, Nymphaea, and aquatic pollen taxa. The proportion of terrestrial higher plants decreased and that of aquatic organisms increased from margin to center of the sag. The benthic water within reducing environment and brackish-water column were superposed by periodic/occasional fresh-water influx (e.g., rainfall and river drain), which led to fresh-water conditions and well oxygenating in the water column during overturn process. The whole study area developed lacustrine source rocks without seawater intrusion. During periodic/occasional fresh-water influx periods with plenty of terrestrial plant inputs, the paleoredox conditions of the sag were relatively oxic in the shallow fresh-water which experienced strong oxidation and decomposition of OM, therefore were not conducive for the OM preservation. However, the overall middle primary productivity made up for this deficiency, and was the main controlling factor on the organic carbon accumulation. A suitable supply from terrestrial inputs can promote biotic paleoproductivity, and a relatively high sedimentation rate can reduce oxidation and decomposition times of OM. On the contrary, during the intervals of the fresh-water influxes, relatively reducing conditions are a more important controlling factor on the OM accumulation in the case that the decrease of the terrestrial biotic source.

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