鄂尔多斯盆地延长期富烃坳陷形成的动力学环境与构造属性

Geodynamic environment and tectonic attributes of the hydrocarbon-rich sag in Yanchang Period of Middle-Late Triassic,Ordos Basin

鄂尔多斯盆地丰富的石油资源来自盆地南部中晚三叠世延长期富烃坳陷,其中贡献最大的长7段页状优质烃源岩以富铀、夹多层凝灰岩为特征。延长期大型富烃坳陷具南深北浅的坳陷型结构,内部接受统一的湖相沉积。在富烃坳陷的深湖-较深湖区,为优质烃源岩发育区;亦为烃源岩中较高铀含量(测井高伽马异常)和凝灰岩夹层较厚分布区,在位置上三位一体、彼此交互叠置。盆地上古生界-中侏罗统多层系高热演化区上下同位,早-中侏罗世多期持续沉降区先后叠置,均位于延长期富烃坳陷范围内。综合调查研究揭示,在延长期,特别是长7段沉积期间,各类深部物质上拱挤入构造(如砂岩墙(脉)、泥岩脊和热液碳酸盐岩结核)、同沉积变形构造(如断裂、褶皱、揉皱、滑塌、滑坡等(软)地层变形)和事件沉积(如浊流沉积、震积岩及湖底扇等)发育,主要分布在富烃坳陷深湖-较深湖区。在烃源岩等地层中,检测出多种指示深部热流体活动的岩石矿物和地球化学异常。深部探测显示,在富烃坳陷深部400km以上壳幔各界面呈上拱的结构特征。这表明延长期富烃坳陷和其中优质烃源岩是在构造活动明显、深部作用活跃的地球动力学环境中形成演化的,从深层次揭示了其形成条件与发育环境。延长期富烃坳陷与印支期秦岭造山带的形成演化及其岩浆活动,在空间上相邻、发育时限相当、演化阶段响应、活动兴衰同步,是在华北-扬子两大陆块汇聚碰撞,进而向板内构造环境转换的动力学环境中进行的。延长期富烃坳陷两次较长距离的沉积-沉降中心迁移,是对秦岭碰撞造山环境重大变革始发的响应和纪录。在南北两大陆汇聚碰撞过程中,在仰冲华北板块后陆地带深部积聚的巨量俯冲物质发生熔融和热量转换,促使该区深部物质过饱和聚集、热能及压力超常骤增,形成较小尺度地幔对流,熔融物质与热能上拱,引发地壳表浅层拱张破裂和侧向扩展,于是发生沉降,形成延长期具热-张性特性的富烃坳陷,称其为后陆盆地。后陆盆地与前陆盆地分别位于碰撞造山带两侧仰冲和俯冲板块临山一侧,但其构造属性有质的不同,油气赋存和成藏特征也差别较大。

The abundant petroleum resources of Ordos Basin mostly come from the hydrocarbon-rich sag of Middle-Late Triassic Yanchang Formation located at the southern of the basin.The shaly high-quality source rock from Chang 7 Member is the greatest contributor,which has the characteristics of rich uranium and multi-layered tuff.During Yanchang Period,the large hydrocarbon-rich sag had the paleogeographic structure of deep in south and shallow in north,and developed in semi-deep and deep lacustrine facies with high-quality source rock,high uranium content and thick tuff interlayer in trinity and mixed.The high-thermal evolution zones with multi-layers from Upper Paleozoic to Middle Jurassic were basically located at the same area,and the continuous subsidence areas of Early-Middle Jurassic were also superimposed on them.Comprehensive investigations have been revealed that during Yanchang Period,especially in the period of Chang 7 Member,various types of diapir structures(such as sandstone dyke or vein,mud phacolith as well as hydrothermal calcareous concretion),syndepositional deformations(such as faults,folds,crumple deformation,slump and landslide etc.)and event deposits(such as turbidite deposit,seismite and sublacustrine fan)occurred due to activity of the deep crust matters and mainly distributed at semi-deep to deep lacustrine facies of the hydrocarbon-rich sag.A variety of minerals and geochemical anomalies are detected in the source rocks which indicate the activity of deep thermal fluid.The deep geophysical survey shows that all the interfaces of crust-mantle above 400km in deep of the sag are arched.All above information indicates that hydrocarbon-rich sag and the high-quality source rock in Yanchang Formation formed under significant tectonic activity and deep process,which reveal the active geodynamic conditions and environments.The hydrocarbon-rich sag was spatially adjacent to the Qinling Orogenic Belt during Indosinian Period.The formation of the sag was simultaneous and synchronous with the development and magmatism,developing states,evolutionary stages and activity level changes of Qinling Orogenic Belt,which was related to the dynamic environment transition from collision to intraplate tectonic of the North China block-Yangtze block.The twice long-distance migration of the deposit-subsidence centers of the sag in Yanchang Period were the response and record to the initial significant environment changing of the Qinling Orogeny.Massive subduction materials were accumulated,melted and converted at the deep part backland of the North China block,which promoted the supersaturated and accumulation of deep matter,and increased abnormally of the thermal energy and pressure in this area during the convergent and collision of the two blocks.Under this circumstance,small scale mantle convection was formed,and material melted and thermal energy arched,caused the shallow superficial rupture and lateral extension in the crust.This process should be the main reasons to lead to the subsidence occurred,and the appeared of the hydrocarbon-rich sag with thermal-tension characteristics in Yanchang Period.This type of basin is named backland basin.The backland basin and the foreland basin are located at the thrusting block side and subduction block side respectively and very close to the collisional orogenic belt.These two types basin differ greatly in tectonic attributes,hydrocarbon occurrence and accumulation characteristics.