湖北兴山大峡口剖面二叠系岩石特征及沉积环境分析

Petrological characteristics and sedimentary environment analysis of the Permian of Daxiakou section,Xingshan County,Hubei Province

湖北兴山大峡口剖面二叠系自下而上可以分为栖霞组、茅口组、吴家坪组和长兴组,总厚度为433.72m。该剖面岩石以石灰岩为主,含少量的白云岩、硅岩和页岩。石灰岩可进一步分为生屑石灰岩、生屑质石灰岩、含生屑石灰岩、泥(粉)晶石灰岩和眼球状石灰岩。眼球状石灰岩主要分布于栖霞组下部和茅口组中下部,为沉积作用和成岩作用的共同产物。白云岩主要是呈斑块状分布的石灰质白云岩,主要分布于长兴组上部,其可能为石灰岩经埋藏白云化作用而形成。硅岩呈结核状、团块状、条带状,为交代成因。根据岩性、古生物化石、沉积构造等相标志分析,划分出了滨岸、碳酸盐岩台地、盆地边缘和盆地等4种沉积环境,其中碳酸盐岩开阔台地内局部出现浅滩环境。沉积演化表明,该地区二叠纪发生了两次较大规模的海进、海退旋回,第1次海侵始于中二叠世栖霞组沉积初期,结束于茅口组沉积末期;第2次海侵始于吴家坪组沉积初期,直至长兴组沉积期末结束。初步分析表明,研究区二叠系具有良好的烃源岩条件和储集条件。

The Permian of Daxiakou section at Xingshan County, Hubei Province is composed of the Qixia, Maokou, Wujiaping and Changxing Formations, with a thickness of 433.72 m. The rock types of this section are rather simple. There are limestones, dolostones, siliceous rocks and shales. Limestones can be further divided into bioclastic limestones, bioclastics-bearing limestones, microcrystalline-fine crystalline limestones and eyeball shaped limestones. The eyeball shaped limestones were mainly developed in the lower part of the Qixia Formation and the middle to lower parts of Maokou Formation. They were formed by the interaction of sedimentation and diagenesis. The dolostones are dominated by calcitic dolostones that are distributed in patches . They were mainly developed in the upper part of Changxing Forma- tion, and probably formed by burial dolomitization. Siliceous rocks are of nodule-, lump- and band-like types, which were formed by metasomatism. According to the comprehensive analyses of lithology, palaeontology and sedimentary structures, four kinds of sedimentary environments were identified. They include shore zone, carbonate platform, basin margin and basin, and locally there was bank in the platform. Then the sedimentary environment evolution has been analyzed and the sedimentary model has been built. The sedimentary evolution indicates that two sedimentary cycles of transgression and regression occurred in the Permian. The first transgression began in the Early Qiaxia Age, ended in the Late Maokou Age. In the Early Wujiaping Age, the second transgression began and lasted until the Late Changxing Age. The pre- liminary analyses show that the conditions of hydrocarbon source rocks and reservoir rocks of the Permian in the study area are rather favorable.