裂陷盆地初始阶段构造—沉积协同机制——以蒙古塔木察格盆地塔南凹陷铜钵庙组为例

Tectono⁃sedimentary Interaction of Early Syn-rift Successions:A case study from the Tobomiao Formation,Tanan Depression,Tamtsag Basin,Mongolia

裂陷盆地蕴含丰富的油气资源,盆地不同演化阶段发育独特的地层结构样式及其砂体成因类型,形成各具特色的油气藏系统。近年来湖盆初始裂陷层系不断获得油气勘探突破,使之成为石油工业界重要关注对象,其多级次断裂演化、组合关系、地貌特征及其与水系和沉积响应关系已成为当前地质学领域关注的热点科学问题。蒙古塔南凹陷下白垩统铜钵庙组良好记录了一套初始裂陷沉积序列,丰富的钻井及地震资料使之可作为理想的研究对象。综合利用地震、岩心及测录井资料,在构造—沉积学理论指导下重建了塔南凹陷初始裂陷构造—沉积演化及源—汇响应模式。研究表明,塔南凹陷初始裂陷第一阶段以新生的分割型小洼陷群为特征,与前裂陷阶段“高山深谷”地貌背景联控下形成短距离输送且数量众多的小型扇群;初始裂陷第二阶段伴随控洼断层长度的迅速增加发生软联结,形成连通且宽浅的盆地结构,发育多套低坡降构造转换带(面积大于50 km^(2))和西北侧缓坡带(延伸约28 km)供给型大套扇三角洲体系,长轴及陡坡方向水系运输距离较短。实例解析结合调研结果表明,先存水系和盆地地貌结构联合控制初始裂陷盆地源—汇系统,进而形成初始裂陷第一阶段盆地满盆富砂(沟通先存水系)或欠补偿(不沟通先存水系),年轻短程水系主导的孤立小湖盆群则主要发育小规模近源碎屑沉积物;初始裂陷第二阶段源—汇系统则与该时期断裂体系联结方式有关:断裂晚期联结型湖盆主要发育以短程断崖或小型转换带水系为特征,而断裂早期联结型湖盆形成大型构造转换带水系及三角洲体系,其缓坡带长度亦快速增大,盆地整体具有“富砂”特征。本研究为其他裂陷盆地寻找大型优质砂体提供了科学理论依据。

Rift-related basins are rich in petroleum reservoirs.The evolutionary stages of rift systems are diagnosed by their stratigraphic patterns and depositional assemblages,forming distinctive hydrocarbon accumulation systems.Recent discoveries of rich petroleum resources in initial rift-related stratigraphic successions have aroused much inter⁃est in the petroleum industry.Furthermore,multi-stage fault activation and stacking patterns associated with geomor⁃phic drainages networks and depositional responses during the initial rifting phase have become an important scientif⁃ic focus internationally.The Lower Cretaceous Tongbomiao Formation in the Tanan Depression in Mongolia records such initial rift-related stratigraphic successions;its dense well penetration and large-scale seismic data make an ide⁃al study area.Seismic,core and logging data were used to reconstruct the tectono-sedimentary evolution and sourceto-sink system during the initial rifting phase,indicating that the first stage of initial rifting formed isolated smallscale sags,which were filled for a short distance by small-scale alluvial fans/deltaic fans in response to the pre-rift‘high mountain/deep valley’geomorphology.The second stage was accompanied by fault interactions with rapidly increasing fault length;it then transitioned into a wide connected but shallow basin structure,with three low-gradient but large-scale(>50 km^(2))relay ramp zones.The 28 km-long dip-slope allowed long-distance transport to form a fandeltaic sand-rich system,without an axially-sourced or footwall-supplied depositional system.This case study and previous studies show that the pre-existing drainage catchment and the rift-related physiography jointly control the depositional pattern and source-to-sink system in the initial rifted basins.The basins in the first stage of initial rifting were either overfilled with sand-rich sediments(connected with pre-existing drainage)or sediment-starved(not con⁃nected with pre-existing drainage).Small-scale,short-distance transport of sediments was facilitated in the isolated basins dominated by young short-range drainage.However,the second stage of initial rifting is characterized by two distinctive infill patterns in response to the fault linking process.Early fault linkage contributed to rapid fault propaga⁃tion to their full length(L),whereas the displacement(D)remained almost constant before significant basin forma⁃tion.This type of basin benefitted from expanded drainage basins and long dispersal depositional systems of sand-rich sediments.By contrast,other basins are commonly observed with a constant D-L relationship that resulted from later fault linking;these basins are isolated and are characterized by local drainage catchments and relatively small-scale deltaic systems.This study provides a detailed example of reporting tectonic-sedimentary interaction and source-tosink systems for early rift-related successions in rift basins,and has significant implications for good sandstone pre⁃diction and petroleum exploration in other rift basins.