陆表海台地沉积充填模式及内克拉通碳酸盐岩勘探新启示

The sedimentary filling model of epeiric platform and new inspiration of innercratonic carbonate for oil & gas exploration

内克拉通陆表海盆地是古老碳酸盐建造形成的重要场所。除其顶部风化壳岩溶储层外,广泛分布的内幕非岩溶改造的白云岩勘探长期未受到足够的重视。近年来,随着深层、超深层勘探技术的进步,内幕陆表海白云岩陆续获得油气勘探新发现,但其预测难度远比镶边台地中的礁滩"移动靶"更大。为更好地揭示这类未经岩溶改造的白云岩储层的宏观发育分布规律,本文研究了鄂尔多斯盆地奥陶系马家沟期和四川盆地雷口坡期陆表海台地的高精度岩相古地理和沉积充填模式。结果表明:陆表海台地一般形成于温室期低纬度干燥气候下的稳定内克拉通盆地,与盐湖伴生的台地潮坪广泛分布是其标志性的沉积特征,微环境可进一步划分为潮上带泥云坪、上潮间带云坪、上潮间带微生物席、下潮间带席状潮缘滩、上潮下带灰云质澙湖、下潮下带膏云质澙湖;虽然内克拉通陆表海台地是一个相对静态的沉积环境,其沉积建造并非平板一块,海平面的升降变化往往会在相对平坦的陆表海内引起大幅度的相带迁移,造成潮上带、潮间带、潮下带频繁交互;相带频繁往复迁移过程中,潮间带的晶粒/颗粒/微生物白云岩受到盐湖、古隆起或潮坪岛的制约,形成大面积分布的席状富孔白云岩透镜体;陆表海沉积层序结构为典型的向上变浅序列,主要发育潮坪进积楔和潮坪岛拼嵌两种沉积充填模式,这两种模式形成的席状白云岩透镜体在层序结构和厚度都能很好地对比,通常延伸都在100km以上,甚至可跨越整个陆表海台地,是一个能够媲美台缘礁滩相的油气勘探新领域。

The epeiric sea of intracraton is an important basin for the formation of the carbonate rocks. In addition to the karst reservoir occurred in the weathering crust, the non-karst dolomite reservoir which distributed widely hasn＇t got enough attention for a long time. In recent years, with the progress of the deep and ultra deep exploration technology, the non-karst dolomite has been successively awarded hydrocarbon discoveries, but its prediction difficulty is far more than the ＂moving target＂ of rimmed platform reefs. In order to reveal the macro-development distribution of the non-karst dolomite reservoir fatherly, this paper focus on the study of high-precision lithofacies paleogeography and depositional filling model of Majiagou Formation of Ordovician, Ordos Basin, and Leikoupo Formation of Triassic, Sichuan Basin. The results showed that： the epicontinental platform located in stable intrarcraton basin and generally formed in the low latitude dry climate during the geological greenhouse period. The tidal platform distributed widely associated with salt lake is the hallmark characteristics of the epeiric sea. Its micro environment can be further divided into the mud-rich dolomite supratidal flat, dolomite upper intertidal flat, upper intertidal microbial mats, lower intertidal sheet tidal-front beach, upper subtidal calcitedolomite lagoon, lower subtidal gypseous dolomite lagoon. Although intracraton epeiric platform is a relatively static environment, the sedimentary formation is not a homogeneous geologic material. Because the sea-level changes lead to sedimentary facies belt migrate frequently in such a flat intracraton geomorphology, the supratidal facies, intertidal facies and subtidal facies interact and stack frequently. This reciprocating migration process, the flat shaped of grain/particle/microbial dolomite developed broadly, which restricted by lagoon, paleo-uplift or tidal flat island. The sequence of epeiric facies is a typical upward shallowing structure, including two main depositional filling patterns, such as the tidal flat progradation wedge and mosaic of tidal flat islands. The sheet dolomite lens body formed by these two kinds of patterns, usually extending more than 100km, even across the whole epeiric platform, can be contrast in the sequence structure and the thickness, and is a favorable exploration target that can be comparable to the reef-bank complexes of the platform margin.