前陆盆地的类型及油气远景

根据前陆盆地形成的大地构造背景,前陆盆地可分为5类,即周缘前陆盆地。弧后前陆盆地。破裂前陆盆地。陆内前陆盆地和走滑前陆盆地。前陆盆地形成于挤压构造环境,具有一些相似的成因特征,如沉降中心与沉积中心不一致;沉降曲线呈陡、缓、陡3段;沉积物遭受挤压变形.根据变形强度的差异.可分出3到5个变形带等。形成于不同大地构造背景中的前陆盆地具有不同的充填特点和盆地结构,也具有不同的油气远景。周缘前陆盆地是由于陆-陆碰撞,在俯冲板块上产生的挠曲盆地,常由被动大陆边缘裂谷盆地转化而成。盆地充填具有双层结构,早期发有复理石前陆盆地,堆积厚度巨大,发育厚度较大的生油岩系;后期发育磨拉石前陆盆地,为向上变粗的序列,发育有较厚的储集岩系。这类前陆盆地具有极好的油气远景。孤后前陆盆地与A型俯冲作用有关,可以由弧后裂谷盆地转化而成。盆地充填可具双层结构或仅有单后结构,早期发育火山复理石前陆盆地,堆积厚度大,发育有较厚的生油岩,后期为火山磨拉石盆地。这类前陆盆地具有较好的油气远景。破裂前陆盆地的形成是因基底卷入前陆变形作用,造成了块状隆起和基底褶皱所分隔的孤立盆地。这类盆地中沉积岩层厚度不大,油气远景欠佳。陆内前陆盆地形成于板内,远离碰撞造山带,可能与碰撞造山带的远程效应有关。盆地充填具双层或单层结构,以发育磨拉石前陆盆地为主,早期复理石不发育。这类前陆地具有一定的油气远景。走滑前陆盆地发育于大型陆内走滑系的两侧,伴有拉分盆地,盆地充填具单层结构,发育磨拉石前陆盆地,沉积中心明显呈雁行排列。这类盆地油气远景有限。前陆盆地的结构具有不对称的特点。自造山带向盆地方向发育山前冲断带、前陆坳陷、前缘斜坡和前缘隆起等构造单元,相应地具有各自的油气藏分布模式。

CLASSIFICATION OF FORELAND BASINS AND ASPECTS OF HYDROCARBON RESOURCES

Foreland basins can be classified into 5 types in accordance with their tectonic settings: periland foreland basin, retroarc foreland basin, broken foreland basin, hinterland foreland basin and strike slip foreland basin. Foreland basins are formed in compressional tectonic environment and show some similar characteristics of formation. Such as discordance of depocentre and subsidence centre; 3 segment of subsidence curve showing steep, gentle and steep; and 3 to 5 deformation zones as a result of compression in sediments of foreland basins, are the common characters. The filling patterns and basinal structures are different in foreland basins formed in different tectonic settings and showed different hydrocarbon potential. Periland foreland basin is flextural basin formed in subducting plate resulted from continental-continental collision. It is generally transformed from rifting basin of passive continental margin. The basin showed dual-layer structure of filling. The flysch foreland basin with great thickness was developed in the early stage and thick source rocks were deposited; and molasse foreland basin with coarsing-upward sequence was developed in the late stage and thick reserviors was formed. This kind of foreland basin is the most favorable petroliferous basins. Retroarc foreland basin was related to A-type subduction and could be transformed from retroarc rif-ting basin. Basin filling can be in dual-layer or single layer patterns. Volcanic-flysch forelang basin with thick sediments was developed in the early and source rocks were relatively thick; and volcanic-molasse foreland basin was developed in the late. This kind of foreland basin showed favorable hydrocarbon potential. The formation of broken foreland basin is due to basement involved foreland deformation, which resulted in isolated basins separated by block uplift and basement fold. The sediments is thin and hydrocarbon potential is poor in the basin. Hinterland foreland basin was formed in the interior of a plate and was far away from collisional orogenic belt. It may be related to long distance effect of collisional orogenic zone. The basin was filled in dual-or single-layer patterns and dominated by molasse foreland basin with little development of early flysch. This kind of foreland basin showed certain hydrocarbon potential.Strike-slip foreland basin was developed on both sides of main continental strike-slip system, accompanying with pull-apart basin. The basin filling pattern was single layer structured and molasse foreland basin was developed.The depocentre were obviously arranged in en echelon. Hydrocarbon potential in the basin was limited.The structures of foreland basin was asymmetric.Piedmont thrust, foreland depression, front slope and fron uplift were successively developed from orogenic ane to basin, in which distribution of hydrocarbon accu-mulation was different: a) hydrocarbon was rnainly distributed in fods of hanging wall of piedmont thrust; b)hydrocarbon was entrapped in stratigraphic and diagenetic traps in foreland depression; c) sedimentary wedge thenned or pinch out towards the front slope, so accurnulation in sand driies sealed by upper-dip pinch out orstraopaphic onlap can be formed; and d) large scale of anticline can be developed in front uplift, such as triangeare, drape anticline, and faulted fold structures and so on are habitat of hydrocarbon.