Sedimentary Characteristics And Resource Potential Analysis Of Mesozoic In Yancheng Basin, Linfen City

Linfen-Yuncheng basin is located in the south of Shanxi Province, which is a Cenozoic down-faulted basin superimposed on the North China Craton (NCC). Previous exploration and studies have shown that this region experienced relative short down-faulted duration in late Cenozoic, just accumulated thin younger strata, and was lack of hydrocarbon source rock in Cenozoic succession, as a result normal oil and gas resources are proved seldom.But whether their are oil and gas resources potential under Cenozoic strata, previous studies and exploration did not concern. In the Middle-Late Triassic and Early-Middle Jurassic strata of Ordos basin and its adjacent area of main hydrocarbon source beds and oil reservoir. Linfen-yuncheng basin is located in the Ordos basin Mesozoic extending hydrocarbon-rich sag in the east part, so the Mesozoic group in the area are of practical significance for oil and gas resources analysis.According to the geological characteristics and the prospecting data of study area, this paper adopted the idea to recover original basin for the basin which is transformation type, focusing on the the systematic observation for peripheral remaining Mesozoic layer of LinfenYuncheng basin, determined the sedimentary facies and the depositional distribution of basin in the main Mesozoic period through the comparison of strata and the analysis between sedimentary facies, sedimentary provenance and later transformation.On this basis, we infer the era of Cenozoic bedrock formations and its distribution by combining with the latest collection of borehole data,and made the initially research for resource potentiality of Linfen-Yuncheng basin in Pre-Cenozoic.By comparing the regional stratigraphic, Middle-Upper Triassic Tongchuan Formation and Yanchang Formation in Shanxi area correspond to the Yanchang Formation of Ordos basin; Early-Middle Jurassic Datong Formation, Yungang Formation and Tianchihe Formation respectively correspond to Yanchang Formation, Zhiluo Formation and Anding Formation in the Ordos basin. The thickness of the Middle-Upper Triassic Tongchuan and Yanchang Formations in Shanxi region is 280-620 m, with the characteristic of layers where are thick in southern region and thick in northern region as well as fined in southern region and coarse in northern region, which is in consistent with the Ordos basin at the same period formation for the diversity of thickness in southern-northern regions and lithology features. Braided river, meandering river, delta, etc sedimentary facies types were identified from bottom to top, no marginal facies can be found.The layers of each Early-Middle Jurassic formations in Shanxi region changes largely on south-north direction. Datong and Ningwu-Jingle basin completely developed in northern region, Yongding zhuang Fm., Datong Fm., Yungang Fm. and Tianchihe Fm. were all deposited with the thickness of 650-950 m. Yungang Fm. and Tianchihe Fm were only residual in middle-southern Yushe and Hongtong areas with the thickness of 250-550 m; Alluvial fan, braided stream and meandering river etc. sedimentary types can be identified. From north to south, Datong Fm. turns from the coal-bearing delta deposit to the poor coal-bearing lake deposition. The alluvial fan deposits turn thin gradually from north to south at the bottom of Yungang Fm. With the decreased of conglomerate grain diameter, indicating the northern area reaches the provenance.Combined with the research of regional geological and the analysis of deformation phenomenon caused by tectonic compression nearby Fen-Wei graben, we consider that Linfen-Yuncheng basin suffered significant regional extrusion and uplifting events in late Jurassic after the deposition of Triassic to middle Jurassic, the deposition of late Jurassic was missing. According to the thickness recovering of Ordos basin the in lower cretaceous,the Linfen-Yuncheng basin may still accepted deposition; but this area and its western Lv-liang mountain as well as Ordos basin all happened the entire uplift, and suffered denudation. In early Cenozoic, the southern area of Yuncheng-Guanzhong region happened the earliest fault depression in the late Paleocene by the NE-SW cut and extensional movement;however, the northern Linfen region happened fault depression until from pliocene, and accepted deposition.With the comprehensive analysis, the sedimentary features of Mesozoic Linfen-Yuncheng basin in its main period was recovered. Linfen-Yuncheng basin and Ordos were unified large scale basin during the deposition of Yanchang Fm.,the north of taer mountain up thrust was mainly fluvial facies, delta facies and lake deposition in its southern area;the study area were mainly river swamp facies, equal delta sedimentary type during the Datong Fm. sedimentary period, the area of deposition was greatly reduced comparing with the period of Yanchang Fm.; during deposition period of Yungang Fm.. and Tianchihe Fm., the sedimentary boundary was at eastern of Linfen-Yuncheng basin and western of the Taihang mountain, the study area was mainly alluvial fan and fluvial facies, and its area was further reduced compared with the Datong Fm. sedimentary period.This research shows that the area of Linfen-Yuncheng basin all suffered deposition in the main period of Mesozoic, but due to the late extrusion and uplifting tectonic transformation, resulting the relict of Mesozoic in the current faulted depression. According to the latest collection of geothermal and hydrological borehole data, compiled the chart of Linfen-Luncheng basin distribution in the pre-Cenozoic, showed the Early-Middle Triassic only in the deep strata of Linfen-Hongtong area and other small number of Jura, the study area generally lack the equivalent Yanchang Fm. and Datong Fm. formation.Therefore, Linfen-Yuncheng basin Mesozoic exploration potential is not optimistic. Although the Mesozoic potential hydrocarbon source layer within the fault depression were almost absent, but saved widely in the deep part in Carboniferous-Permian period, and the coal exploration in recent years has been proved that work of the coal field of commercial exploitation be valued, so the carboniferous-Permian strata of the deep Linfen-Yuncheng basin has the conditions which can formed the material of coal type gas resource, so the further research should be valuable.