Yanshan Movement and its influence on petroleum preservation in middle-upper Yangtze region

Based on systemic researches on unconformity,tectonic dating and deformation of Jurassic and Cretaceous in different locations of the middle-upper Yangtze region,the Yanshan Movement was divided into 4 stages:the end of Middle Jurassic(Ⅰ),the end of Late Jurassic to the beginning of Early Cretaceous(Ⅱ),the end of Early Cretaceous(Ⅲ)and the Late Cretaceous(Ⅳ),among which the stage Ⅱ was the main deformation stage.The Yanshan Movement was significant for tectonic transformations in the middle-upper Yangtze region.Tectonic movements during the early stages(Ⅰ and Ⅱ)were characterized by multi-direction compression and thrusting,while those of the late stages(Ⅲ and Ⅳ)held the structural features of 'western compressing vs.eastern extending' with differences in basin-mountain relationship,deformation pattern and geodynamic mechanism.Meanwhile,the stage Ⅱ had controlling effects on hydrocarbon preservation in the study area.It caused differences in uplifting-subsiding and marine facies hydrocarbon preservation,and further influenced exploration potential in diffe-rent structural belts.For example,the Sichuan Basin which continued subsiding was favorable for hydrocarbon preservation,and had experienced 3 accumulation stages:early reservoir,conversion from oil to gas,and late adjustment.The fold-thrust belt on the upper Yangtze margin which continued uplifting and eroding was unfavorable for hydrocarbon preservation,and had experienced 2 accumulation stages:early reservoir as well as late reconstruction and destruction.The middle Yangtze region developed to depositional system of tectonic rift within mainland after the stage Ⅱ.Its hydrocarbon preservation conditions lay in the sealing capacity of re-type continental caps and the accumulation was limited by the-effectiveness of secondary hydrocarbon generation.The upper and lower combinations in the Sichuan Basin are the most potential exploration targets.On the margin of Central Guizhou Uplift and in the western Hunan and Hubei region to the east of Qiyueshan Fault,the lower combination below the Cambrian gypsum layers may have some exploration prospects.In the southern depression of the Jianghan Basin,conditions for secondary hydrocarbon generation,late accumulation and preservation may be good under continental covers.