Full
LIAO Zheyuan,born in 1999,is a Ph.D. candidate. He is mainly engaged in reservoir geology. E-mail: 352929250@qq.com.
Outline
The depositional environment in the Member 2 of Xujiahe Formation on eastern slope of Western Sichuan Depression has long been controversial. Using newly identified storm deposits as key evidence,this study aims to clarify the depositional environment of the in the Member 2 of Xujiahe Formation and the development characteristics of storm deposits in this area. Based on core observation,thin-section identification,grain-size analysis,and geochemical testing,the following conclusions are obtained: (1)Sedimentary structures such as bidirectional cross-bedding,wave-generated cross-bedding,and tidal bundles are widely developed in the in the Member 2 of Xujiahe Formation on the eastern slope of the western Sichuan Depression. Grain-size and geochemical data indicate that the study area was a high-energy environment characterized by periodic energy fluctuations,and that the water body was brackish to freshwater in nature. These features suggest a marine shoreline depositional environment,which can be further subdivided into four sedimentary subfacies: foreshore,upper shoreface,middle shoreface,and lower shoreface.(2)Typical storm-related sedimentary structures,including mud-clast layers,gutter casts,and hummocky cross-stratification,are developed in the foreshore to middle shoreface subfacies. At least nine episodes of storm deposition are vertically identified,and the sequences are dominated by incomplete A-B-C types,reflecting high-energy,high-frequency,and proximal storm depositional characteristics.(3)Against a regressive background,the storm-deposit development belt gradually migrated from shoreface to foreshore. The thickness of storm layers was relatively greater in the southwestern part during the early stage,but became thicker in the eastern part during the late stage,indicating a northeastward migration of the depositional center. The results demonstrate that the Xu 2 Member on the eastern slope of the western Sichuan Depression represents a storm-influenced marine shoreline depositional environment. Together with the paleogeographic framework of “deep in the west and shallow in the east,”these findings reveal a hydrocarbon accumulation element assemblage characterized by “provenance supply from the western depression,sand enrichment on the eastern slope,and sealing in the east,”thereby providing a geological basis for the selection of favorable exploration targets for tight sandstone in the area.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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