Sequence-paleogeography and coal accumulation of the Late Carboniferous – Early Permian paralic successions in western Shandong Province, northern China

Abstract

The coal-bearing strata of the Late Carboniferous to the Early Permian in western Shandong Province, northern China, were deposited within a range of paralic facies associations spanning from marine offshore carbonate shelf to fluvial and delta, which constituted a general paralic cratonic setting. In this paper, the sedimentary characteristics, sequence stratigraphy, paleogeography, and coal accumulation of the Benxi, Taiyuan and Shanxi formations of the Moscovian to Sakmarian age are studied by using data of outcrops, borehole cores, and geophysical loggings. A total of 14 different lithofacies are identified, with lithologies including sandstones, siltstones, mudstones, and bioclastic limestones, and grouped into fluvial, delta, tidal bar-lagoon, and offshore shelf facies associations. Furthermore, the key sequence boundaries represented by regional unconformity, incised valley erosional surface, switching surface of transgressive direction, stratigraphic color change surface marking the abrupt change of environment, and paleosol horizon have been recognized. Based on these sequence boundaries, three third-order sequences (S1, S2, S3 in ascending order) and corresponding lowstand, transgressive, and highstand systems tracts are subdivided. S1 corresponds to the Benxi Formation and the lower part of Taiyuan Formation, S2 corresponds to the middle and upper parts of Taiyuan Formation, and S3 corresponds to the Shanxi Formation. The lithofacies paleogeography maps of three sequences are reconstructed based on the contours of a variety of lithological parameters including strata thickness, limestone thickness, limestone percentage content, coarse (sandstone)/fine (siltstone and mudstone) ratio, and total coal thickness. S1 and S2 were developed with the offshore carbonate shelf paleogeographic units and the tidal bar – lagoon paleogeographic units, and S3 was developed with delta and fluvial units. In general, the western Shandong experienced transgression in the S1, transgression direction switching between S1 and S2, transgression again in S2, and then regression in the S3 during the Late Carboniferous to the Early Permian. Most significant coal accumulation occurred in the S3, followed by S1 and then S2. The coal accumulation centers of S1 were mainly distributed in Liaocheng, Feicheng, Jining regions in the west of the western Shandong where the tidal bar and lagoon were developed, those of S2 were mainly distributed in Tai'an, Jining and Tengzhou regions in the central part of the western Shandong, where the lagoon and tidal flat were developed, and those of S3 were mainly distributed in Heze, Jining, Tengzhou and Zaozhuang regions of the southern part of the western Shandong where the delta environment was developed. Overall, the high accommodation setting of S1 and S2, represented by the Benxi and Taiyuan formations, favors thick coal accumulation in the early TST, while the low accommodation setting S3, represented by the Shanxi Formation, favors thick coal accumulation in the middle and late TST. The research results of this study can be used in the evaluation of the favorable areas for the exploration and development of the deep-buried coal and CBM resources in western Shandong Province.