四川含油气叠合盆地基本特征

Basic geological features of superimposed basin and hydrocarbon accumulation in Sichuan Basin,China

随着近年来四川盆地油气勘探的不断突破,重新审视其基本地质特征和油气成藏特点变得迫切而必要。四川盆地是典型的叠合盆地,显生宙以来经历了震旦纪—中三叠世伸展体制下的差异升降和被动大陆边缘(海相碳酸盐岩台地)、晚三叠世—始新世挤压体制下的褶皱冲断和复合前陆盆地(陆相碎屑岩盆地)、渐新世以来的褶皱隆升改造(构造盆地)3大演化阶段以及晚三叠世(被动大陆边缘→前陆盆地、海相碳酸盐岩—海相碎屑岩、海相碎屑岩→陆相碎屑岩)、晚白垩世(前陆盆地沉降中心的迁移、秦岭构造域→青藏构造域、沉积→部分隆升剥蚀)、始新世(外流盆地→内流瓮地、沉积盆地→地貌盆地、沉积→整体隆升剥蚀)3大关键构造变革/沉积转换期。印支期以来,四川盆地受周边多个方向造山带(北缘秦岭造山带、东缘雪峰陆内构造系统、西南缘青藏高原)多期活动影响,形成多组、多期构造的复合—联合叠加。现今盆山构造格局呈现明显的三分性(地貌、基底和构造形迹),发育突变型和渐变型两类盆山边界。按盆地不同区域盆山结构特征、定型时间和主控因素,可将四川盆地划分为5大盆山结构区:Ⅰ区:川北突变型盆山结构区(秦岭构造控制域);Ⅱ区:川西突变型盆山结构区(青藏构造控制域);Ⅲ区:川东渐变型盆山结构区(雪峰构造控制域);Ⅳ区:川西南渐变型盆山结构区(青藏—雪峰—基底构造联合控制域)和Ⅴ区:川中原地隆起—盆地区(基底构造控制域)。四川盆地是我国西部重要的含油富气盆地,勘探潜力巨大。这是由充足的烃源和良好的保存条件所决定的。首先,多阶段盆地叠合演化造就了5套重要的烃源层,总厚度可达1500～2500m,有机碳含量高,生烃量大,成气率高。其次,中下三叠统膏盐岩的发育对海相油气起了重要的封闭作用,而冲断带—前陆盆地二元结构和隆升剥蚀作用较弱的特点大大增强了突变型盆山结构区的保存条件。此外,晚白垩世以来的隆升作用使古气藏(储气中心)发生调整或破坏的同时,也为现今气藏(保气中心)的形成创造了条件,隆升作用还造成流体跨层流动和天然气爆发式成藏。叠合盆地演化的多阶段性、多组多期构造的复合—联合作用、储层的非均质性和天然气的活动性决定了四川盆地油气勘探的复杂性、长期性和曲折性,同时说明不能用单一的勘探思路、勘探方法和成藏理论来指导整个盆地的油气勘探,即勘探策略也应多样化。

The recent breakthroughs in natural gas exploration in Sichuan Basin have been making it urgent and indispensable to reinvestigate its basic geological features and characteristics of hydrocarbon accumulation. Being a typical superimposed basin,Sichuan Basin has experienced three major phases of basin evolution during the Phanerozoic time as follows：marine carbonate platform with differential subsidence-uplift in extensional setting from Sinian to Middle Triassic, continental clastic basin with fold-and-thrust deformation under compressive background from Late Triassic to Eocene, and uplift and structural modification since Oligocene. Simultaneously, Sichuan Basin has also gone through three key periods of tectonic transformation. The first tectonic transformation in Late Triassic time is characterized by the changes of tectonic settings from extension to compression, basin property from passive continental margin to foreland basin, deposition from marine carbonate to marine clastic and to continental clastic. The second tectonic transformation in early Late Cretaceous is symbolized by the migration of foredeep, tectonic domain change from Qinling orogenic belt to Tibet Plateau, and the change from deposition to partial uplift and erosion. The last tectonic transformation in Eocene is represented by the changes from exorheic basin to endorheic basin,from sedimentary basin to topographic basin, and from deposition to overall uplift and erosion. Affected jointly by peripheral structural belts since lndosinian movement （the Qinling orogenic belt on the north side, Xuefeng intra- continental tectonic system on the east and southeast sides, and Tibet Plateau on the west and southwest sides）, muhi-stage and multi-direction structures were superimposed in Sichuan Basin. The present-day basin-orogen configuration in Sichuan Basin and adjacent regions exhibits clear trisection in topography, basement and surface structural features, and two types of basin-orogen boundaries, i. e. break and gradual. According to the type of basin-orogen boundary,developing history and controlling factors, the Sichuan Basin can be divided into five basin-orogen structure units： Unit I-North Sichuan break basin-orogen structure area （controlled by Qinling tectonic domain） , Unit lI -West Sichuan break basin-orogen structure area （controlled by Tibet tectonic domain） , Unit llI-East Sichuan gradual basin-orogen structure area（ controlled by Xuefeng tectonic domain） , Unit IV-Southwest Sichuan gradual hasin-orogen structure area（ controlled jointly by Tibet, Xuefeng and basement tectonic domains）and Unit V- Central Sichuan autochthonous uplift-basin area （controlled by basement tectonic domain）. Sichuan Basin is one of major oil-gas-bearing basins in western China,with great natural gas exploration potential owing to abundant hydrocarbon supply and excellent preservation condition. Firstly, five sets of major source rocks developed during the multi-stage basin evolution,with a total thickness of up to 1 500 -2 500 m, and high organic carbon content, hydrocarbon generation capacity and gas productivity. Secondly,the Lower Triassic gypsum-sah rocks played an important role in sealing marine oil and gas, while the binary texture and gentle erosion in thrust belt and adjacent foreland basin greatly enhanced the preservation condition in break basin-orogen structure areas. Further, uplift activities since the Late Cretaceous might cause not only the adjustment or destruction of ancient gas pools（ gas storage center） ,but also the formation of present gas reservoirs （gas preservation centre）, and fluid cross-formation migration and explosive gas accumulation as well. The multi-phased evolution of superimposed basin,composite-association of multi-stage and multi-direction structures, the heterogeneity of reservoirs and the mobility of natural gas decided the complexity,time-consuming and tortuosity of oil-gas exploration in Sichuan Basin. Thus,just a single exploration thinking, approach and hydrocarbon accumulation theory could not be employed to instruct oil-gas exploration; that is, exploration strategies should also be diverse.