Abstract:

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.

Key words: palaeogeographic environment, storm deposits, shoreline environment, the Member 2 of Xujiahe Formation, Triassic, Western Sichuan Depression

摘要：

川西坳陷东坡三叠系须家河组二段(须二段)沉积环境的认识长期存在争议,本研究以新识别的风暴沉积为关键证据,旨在厘清该区须二段沉积环境及风暴沉积发育特征。通过岩心观察、薄片鉴定、粒度分析及地球化学测试等手段,取得了以下认识: (1)川西坳陷东坡须二段广泛发育双向交错层理、浪成交错层理和潮汐束等沉积构造,粒度及地化数据显示该区为能量周期性波动的高能环境,水体性质为半咸水—淡水,指示该区为海相滨岸沉积环境,可进一步划分为前滨、上临滨、中临滨、下临滨4种沉积亚相; (2)前滨至中临滨亚相中发育泥砾层、渠模和丘状交错层理等典型风暴沉积构造,在垂向上识别出至少9期风暴沉积,其序列以不完整的A-B-C型为主,反映了高能、高频的近源风暴沉积特征; (3)在海退背景下,风暴沉积发育带逐渐由临滨向前滨迁移,风暴层厚度表现为早期西南部较厚、晚期东部增厚,指示沉积中心向北东迁移。本研究明确了川西坳陷东坡须二段为风暴扰动的海相滨岸沉积环境,与“西深东浅”的古地理格局共同揭示了“西坳供源、东坡富砂、东部封堵”的成藏要素组合,为该区致密砂岩勘探区优选提供了地质依据。

关键词: 古地理环境, 风暴沉积, 滨岸环境, 须家河组二段, 三叠系, 川西坳陷

CLC Number: 

• P512.32