鄂尔多斯盆地东南部下二叠统山西组二段物源体系及沉积演化模式

早二叠世是鄂尔多斯盆地海陆变迁的关键时期,其中下二叠统太原组沉积期由于发育稳定的碳酸盐岩地层,其陆表海沉积环境得到了大多数学者的认可,而对于下二叠统山西组的沉积环境与沉积体系的认识则存在着较大的争议。为此,在研究该盆地东南部区域构造背景与周缘古陆特征的基础上,结合岩石、重矿物与稀土元素的组成特征,分析了该区山西组二段(以下简称山2段)的物源体系,并依据地层、古生物、沉积构造、古盐度等分析结果,判别其沉积环境并建立了沉积演化模式。结果表明:(1)山2段沉积期发育南、北两大物源体系,其中北物源主要来自古阴山褶皱造山带,南部物源则来自祁连—北秦岭造山带,延安南部宜川—富县一带是上述两大物源的交汇区;(2)该区自早二叠世太原末期—山西早期发生区域性海退,海水自东南方向退出,山2段沉积期仍受到海侵作用的影响;(3)延安以北地区发育三角洲沉积体系,其以南地区则发育障壁岛海岸沉积体系;(4)该区山西组沉积在平面上具有"岸线频繁迁移,水体影响范围广"的特点,垂向上多套储集砂岩体与泥岩、煤层频繁交替转换,形成了良好的源储配置关系。

Provenance system and sedimentary evolution model of the second Member of Lower Permian Shanxi Formation in southeastern Ordos Basin

The Early Permian is the critical period of land-sea changes in the Ordos Basin. Carbonate strata were developed stably during the deposition of Lower Permian Taiyuan Fm, so it is recognized by most scholars that the Taiyuan Fm is deposited in the environment of epeiric sea. However, it is controversial about the sedimentary environment and sedimentary system of the Lower Permian Shanxi Fm. In this paper, therefore, the regional tectonic setting in the southeastern Ordos Basin and the ancient land characteristics around it were investigated. Then, combined with the compositional characteristics of rocks, heavy minerals and rare earth elements, the provenance systems of the second Member of Shanxi Fm (hereinafter the Shan 2 Member for short) in this area were analyzed. Besides, its sedimentary environment was discriminated based on the analysis results of strata, palaeontology, sedimentary structure and paleo-salinity. The following results were obtained. First, two major provenance systems of north and south were developed during the deposition of Shan 2 Member. The northern provenance system is mainly originated from Paleo-Yinshan folded orogenic belt and the southern one is sourced from Qilian–North Qinling orogenic belt. Both provenances are intersected with each other in Yichuan–Fuxian area in southern Yan'an. Second, during Late Taiyuan Stage to Early Shanxi Stage of Early Permian, regional regression occurred in the study area with seawater regressing from the southeast. The study area was still affected by the transgression during the deposition of Shan 2 Member. Third, a delta system is developed to the north of Yan'an and a barrier island coast system is developed to the south. Fourth, in southeastern Ordos Basin, the sedimentation of Shanxi Fm is areally plane characterized by "frequent migration of coastline and wide influence of water", and multiple reservoir sand bodies have a frequent mutual conversion with mudstones and coal beds in the vertical direction, so a good source-reservoir relationship is formed.