Sedimentary coal-controlling effect and coal-forming model of Paleogene coal measures in paralic faulted basin：A case study of Huangxian Basin, Shandong Province

There are abundant coal-type oil and gas resources in Chinese sea area, but few researches have been conducted on the coal-type source rock, which seriously restricts the exploration of coal-type oil and gas resources in the sea area. Herein, this study on sedimentary coal-controlling effect and coal-forming model was carried out based on Huangxian Basin that is a paralic faulted basin with same coal-forming period, similar geological conditions and high degree of research for the coal basins in the sea area. Specifically, the pattern, evolution and sedimentary coal-controlling effect of paleogeography during the coal-forming period were reconstructed with full consideration to the previous research results. The results show that: (1) alluvial fan, fan delta, braided river delta and lacustrine deposit are mainly developed in the Paleogene coal measures in Huangxian Basin, where two third-order sequences can be identified. Definitely, the coal seam is mainly developed in the braided river delta and shore-shallow lacustrine environment during the lake expansion systems tract (EST). (2) The sequence-paleogeographic pattern of the EST and high-stand systems tract (HST) in sequence I, as well as the low-stand systems tract (LST) and EST in sequence II, covering the whole coal-forming process of Paleogene was comprehensively reconstructed. Meanwhile, the sedimentary evolution characteristics and sedimentary coal-controlling effect were analyzed. The coal seam is mainly developed in the late LST-early EST and the lacustrine bog environment in the front of braided river delta with the lake level rising slowly, and gradually expanded to the braided river delta. Besides, the coal seam is mainly developed in the middle and late EST, and the shallow lake sedimentary environment with the lake level frequently changing from shallow to deep. (3) Two typical sedimentary coal-forming models of Paleogene paralic faulted basins were established. Firstly, the coal-forming model of shore-shallow lacustrine bog braided river delta is mainly developed during the late LST to early EST when the lake level rose slowly. In this model, the coal seam thickness is relatively large but its distribution is relatively limited, such as coal 4 and coal 2. Secondly, the coal-forming model of shore-shallow lacustrine bog shore-shallow lacustrine is mainly developed during the descending and ascending of lake level in the mid-late EST. The frequent rise and fall of lake level lead to frequent appearance and disappearance of coal-forming environment, resulting in small thickness and wide distribution of coal seam, such as coal 1, upper coal 1, etc. Generally, the development law of coal seam and the sedimentary coal-forming model established in the paralic faulted basin illustrated in this study would provide some theoretical support for the exploration of coal-type source rocks in the sea area.