中上扬子地区燕山运动及其对油气保存的影响

通过对中上扬子地区不同部位侏罗—白垩系中不整合面、构造活动定年及构造变形的系统分析,将中上扬子地区的燕山运动分为4幕,分别发生在中侏罗世晚期、晚侏罗世末—早白垩世初、早白垩世末与晚白垩世。发生在晚侏罗世末-早白垩世初的燕山Ⅱ幕是该期构造变形的主变形期。燕山期构造运动是中上扬子地区的重要构造转换事件,燕山早期的构造运动(燕山Ⅰ,Ⅱ幕)表现为多向挤压、块体旋转、多向冲断走滑和复杂的联合复合关系等特点,而燕山晚期的构造运动(燕山Ⅲ,Ⅳ幕)则呈现"西挤东张"的构造格局,与之相应的盆山关系、变形样式及其动力学机制均有较大差异。同时,发生在晚侏罗世末—早白垩世初的燕山运动Ⅱ幕亦是影响中上扬子地区油气保存的最重要构造事件,它决定了中上扬子区隆升与沉降、以及海相油气保存的区域性差异,从而决定了不同构造带油气勘探前景的优劣。具体表现为,四川盆地持续沉降,油气保存条件好,经历了早期油藏、油气转化、晚期调整等3个成藏阶段;上扬子区周缘褶皱—冲断带,表现为持续抬升剥蚀,油气保存条件较差,只经历了早期油藏、晚期改造与破坏等2个阶段;中扬子地区在该期构造运动后转入陆内断陷沉积体系,油气保存条件的优劣取决于重建型陆相盖层的封盖性能,油气成藏受限于二次生烃的有效性。四川盆地内,上、下组合均为良好的勘探领域;在黔中隆起周缘及齐岳山断裂以东的湘鄂西地区,寒武系膏岩层之下的下组合可能有一定的勘探前景;江汉盆地南部坳陷区陆相盖层之下可能有较好的二次生烃、晚期成藏及保存条件。

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.