四川盆地寒武系沉积前震旦系顶界岩溶地貌特征及其地质意义

受桐湾运动的影响,四川盆地震旦系灯影组发育2期大规模风化壳岩溶作用,岩溶识别标志明显。为探寻岩溶作用与震旦系灯影组油气成藏的关系,综合应用野外剖面、新钻井以及地球物理勘探资料,通过分析不整合面上、下地层的补偿关系,根据灯影组"残余厚度"和下寒武统"印模厚度"的特征,依据地层厚度组合关系恢复了该区寒武系沉积前震旦系顶界的岩溶地貌,并将其分为岩溶高地、岩溶坡地、岩溶谷地和岩溶盆地4个二级地貌单元,认为寒武系沉积前四川盆地震旦系顶界具有大型近南北向侵蚀谷地和大面积分布的岩溶坡地并存的地貌特征。在此基础上,研究了桐湾期的表生岩溶作用对灯影组储层形成和油气成藏的重要意义:①大面积分布的岩溶坡地与灯影组丘滩体叠合形成了连片叠置发育的优质储层;②近南北向大型侵蚀谷地充填下寒武统巨厚的优质烃源岩,形成了古隆起高部位下寒武统生烃中心;③优质烃源岩与储层大面积对接有利于油气侧向运聚成藏。

Features and geologic significances of the top Sinian karst landform before the Cambrian deposition in the Sichuan Basin

Large-scale weathering crust karstification of two stages was developed in the Sinian Dengying Fm in the Sichuan Basin due to the Tongwan Movement. The identification marks of these karsts are very evident. In order to study the relationship between the karstification and hydrocarbon accumulation in the Dengying Fm, various data including outcrop observation data, new drilling data and geophysical exploration data were collected and integrated to analyze the compensation relationship between the overlying and underlying strata of the unconformity. According to the residual thickness of the Dengying Fm and the moldic thickness of the Lower Cambrian, we restored the top Sinian karst landform before the Cambrian deposition in the study area and recognized 4 second-order geomorphic units including karst high, karst slope, karst valley and karst basin. It was believed that a near-NS-trending large erosion valley and a large karst slope coexist on in the top Sinian strata in the Sichuan Basin before the Cambrian deposition. The significance of supergenetic karstification during the Tongwan Movement to the formation of hydrocarbon pooling in the Dengying Fm reservoirs was also studied. (1) The superimposition of the large karst slope and deposits of mound-beach facies in the Dengying Fm led to the formation of connected and overlapped quality reservoirs. (2) The super-thick source rocks of the Lower Cambrian filled in the NS trending large scale erosion valley acted as the hydrocarbon kitchen of the Lower Cambrian on the structural high of the Paleohigh. (3) The large area juxtaposition of quality source rocks with reservoirs was favorable for the lateral migration and accumulation of hydrocarbon resources.