鄂尔多斯盆地中东部奥陶系碳酸盐岩-膏盐岩体系沉积特征与天然气成藏潜力

鄂尔多斯中东部地区奥陶系马家沟组发育巨厚的碳酸盐岩与膏盐岩交互的沉积体系,但对其沉积发育的规律性及其内部天然气成藏聚集的基本地质特征尚未形成明确的认识。通过对奥陶系碳酸盐岩-膏盐岩体系形成时的构造与古地理背景、沉积特征、储层发育、烃源供给及圈闭聚集等方面的分析,形成新的观点与地质认识:(1)受盐洼周边古隆起影响,奥陶系碳酸盐岩-膏盐岩共生体系形成于与外部广海周期性隔绝的局限海蒸发环境,导致其岩性“相控”与“层控”分布特征极为显著;(2)首次提出“非层状沉积分异”的概念,并认为它是控制奥陶系内幕“溶孔型”储层发育的决定性因素;(3)受区域岩性相变及后期构造反转的控制,奥陶系盐下层系具有形成大区域分布的岩性圈闭体系的有利条件;(4)盐下层系虽自身的海相烃源岩生烃潜力有限,但由于燕山期东部构造抬升与西侧上古生界煤系烃源“供烃窗口”及膏盐封盖层区域性连续分布等要素密切配合,仍具有规模供烃、大区带成藏的潜力。

Sedimentary characteristics and gas accumulation potential of the Ordovician carbonate-evaporite paragenesis system in central and eastern Ordos Basin

A sedimentary system consisting of interactive ultra-thick carbonate rock and gypsum-salt rock developed in the Ordovician Majiagou Formation in the central and eastern Ordos Basin, however, its development process and natural gas accumulation characteristics are not clear. By studying its tectonic and paleogeographic background, sedimentary features, reservoir development, hydrocarbon source supply and trap accumulation, we gain new geological insights into the carbonate-evaporite paragenesis system, which can be summarized as follows: 1) Affected by its surrounding ancient uplifts, the carbonate-evaporite paragenesis system is formed in a restricted sea with periodic isolation from the open sea, therefore its lithologic distribution has obvious “facies-control” and “strata-bound” characteristics. 2) The key factor controlling the development of the Ordovician “interior dissolved pore” type reservoir is “nonstratiform sedimentary differentiation”—a novel concept, proposed here for the first time. 3) Controlled by regional lithologic transition and later structural inversion, the Ordovician sub-salt stratum has favorable conditions for forming lithologic trap system on a large scale. 4) Although the sub-salt stratum has limited hydrocarbon-generation capacity from its own marine source rocks, it still has the potential of large-scale hydrocarbon supply/accumulation due jointly to the Yanshanian tectonic uplift in its eastern part, hydrocarbon supply from Upper-Paleozoic coal-bearing source-rock on its west side, and regional continuous distribution of gypsum-salt capping strata.