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The depositional environment of the Upper Permian quartzose sandstone (Kuishan sandstone in Shihezi Formation of Upper Permian) in the North China epicontinental basin is controversial. In order to test the previous hypotheses, we analyzed sedimentological characteristics of the Kuishan sandstones in outcrops and boreholes, and carried out trace element geochemical analysis by electron probe microanalyzer. Three lithofacies were recognized, including normal-graded conglomerate (Cng), trough and planar cross-bedded coarse sandstone (CStpc), and planar cross-bedded medium sandstone (MSpc). Normal-graded conglomerate (Cng) formed in the meandering river or deltaic distributary channels. Trough and planar cross-bedded coarse sandstone (CStpc) formed in meandering river or distributary channels of near-source deltaic plain. Planar cross-bedded medium sandstone (MSpc) formed in the siliciclastic beach with high- to moderate-energy conditions. By the petrology and trace elements analysis, three relatively large-scale transgressions were revealed. Each transgression was reflected by the lower content of Ba and ratios of Fe/Mn, and the high content of B and ratios of B/Ga. The ratios of Ni/Co of all samples are all lower than 2, suggesting oxygen-enriched shallower water environment during deposition of the Kuishan sandstones.

The Paleogene coal accumulation basins of China are part of the global Tertiary coal-accumulated zone of the Pacific Rim located in the eastern coastal provinces and areas. Although the coal-bearing basins of the China Sea area are faults and depressed basins, they come up in groups. The overall structures are suitable for the development of coal-bearing deposition. The continuity of basin groups are good, and the coal-bearing depositions are thick. For example, the coal-bearing deposition is more than a kilometer thick at the Qiongdongnan Basin and Xihu Sag in the East China Sea, which the continental Paleogene coal basins cannot reach. Research shows that the coal accumulation basins in the sea area consist of many sags. There are two types of coal accumulation sags: half-graben sag and graben sag. In terms of water depth, coal accumulation sags can also be classified as deep-water half-graben sag and shallow-water half-graben sag; the graben sag is the deep-water sag. There are two distinct coal-accumulated zones in the basin: gentle slope and steep slope, with the gentle slope being the dominant one. The marshes of the supratidal zone and intertidal zone in a tidal flat system is favorable for the coal accumulation process widely taking place. There are two types of peat accumulation: autochthonous accumulation and allochthonous accumulation. Because of tectonic activities in the basins, allochthonous accumulations may be the more important form of coal-forming activities in the sea area. The very thick coal-bearing deposition in the sea area provides a rich material base for the assemblage of coal-related gas. Also, as a result of the deep burial depth, the degree of coal metamorphism is relatively high, so the coal-bearing strata become good hydrocarbon source rocks.

The identification of sequence boundaries is the key point for sequence stratigraphic classification. Both the higher-order sequences and the units within the sequences are bounded with the key sediments or isochronous surfaces. Eight sequences can be divided in the whole Permo-Carboniferous strata (the Shiqianfeng Formation is not included), which is from the Benxi Formation, Taiyuan Formation, Shanxi Formation, Xiashihezi Formation, Wanshan Section, and Kuishan Section to the Xiaofuhe Section. Also, different system tracts (lowstand system tract, transgression system tract and highstand system tract) and some parasequences can be recognized in each sequence. Parasequence analysis was on the basis of the division of the base-level cycle. The base-level cycle was mainly identified according to the change of the water area, which was reflected by the depositional sequence. The physical characteristic of the strata was reflected by the well log. It was supplied by the test of the minerals and rocks and the analysis of the microelement in the lab. The paleogeographic characteristic of the lowstand system tract in the sequence VI is that the east-north part takes the river system as its feature; the south part is the lake system, the river channel spreads from north to south, and the area of the flooding plain is great. The paleogeographic characteristic of the water-transgressive system tract is that the range of the lake in the south extended distinctively, the range of the river channel in the east reduced. The coastal shallow lake deposit is the main characteristic in the water-transgressive system tract. The paleogeographic characteristic of the highstand system tract is similar to the one of the lowstand system tract.

The strata of Jurassic was divided into three tectonic sequences and eight sequences of third rank, according to the developing feature of the tectonic inconformity and the transforming feature of the depositional system tracts. Also the identification and the division of the base-level cycle of different period were carried through. Therefore three cycles of super period, eight cycles of long period, twenty-four cycles of middle period and some cycles of short period were identified. From the overall character of the coal-accumulation in the Mesozoic, we can see that the Yan’an formation is of the best nature of coal bearing. When the coal bearing systems of Jurassic were depositing, the Ordos area is the coal accumulating basin of terrene of large scale and located in the same tectonic unit. But the local structure of different part and the paleolandform are different in the basin, which resulted in the difference of the depositional environment. So the layer number and the distribution of the thickness of the coal beds are different in the different part of the basin. The coal-accumulating action migrated regularly along with the development, evolvement and migration of the depositional systems. The layer numbers of the coal beds, which can be mined, are more in the north and west fringe of the basin, whose distributing area is extensive, and they are more steady in the landscape orientation, also the total thickness is great. Therefore the nature of coal bearing and the coal-accumulating action of different part changed obviously in the space in Ordos area.