湘中坳陷邵阳凹陷佘田桥组泥岩岩相及其成因演化

Lithofacies and Origin Evolution of Mudstone of Shetianqiao Formation in Shaoyang Sag of Xiangzhong Depression

泥盆系佘田桥组泥岩是南方页岩气勘探重点层位之一.为揭示湘中坳陷邵阳凹陷佘田桥组泥岩岩相特征及其发育控制因素,基于系统岩心描述、矿物组分和地球化学采样及测试,对邵阳凹陷佘田桥组泥岩岩相类型、沉积环境特征及其对岩相发育的控制进行了研究,探讨沉积环境演化对岩相类型发育的控制作用.结果表明,佘田桥组泥岩依据矿物组分含量主要识别出硅质泥岩、混合质泥岩、硅质岩、钙质泥岩4种岩相类型,进一步垂向上可划分为5个岩相组合段.岩相组合段Ⅰ和Ⅲ相对富硅质,而组合段Ⅱ、Ⅳ和Ⅴ钙质含量相对较高.沉积环境分析表明佘田桥组泥岩形成于大陆边缘背景;主要处于相对干热气候条件,其中在早期和中期发育相对温湿气候,中期时相对最为温暖潮湿;海平面先上升后下降;中段泥岩中具有过量硅富集特征,主要为生物成因,而受热液作用影响较小;陆源碎屑输入相对较为稳定,在中期时相对最低.沉积环境演化和岩相发育之间的响应关系表明,岩相的发育主要受到古气候、陆源碎屑供给、海平面变化和生物作用等的综合控制,泥岩硅质组分主要来自陆源碎屑输入,生物作用富硅造成了中段硅质富集,而后期当气候向干热转化时,钙质组分增加,泥岩岩相向富钙方向演化.

The mudstone of the Devonian Shetianqiao Formation is one of the key strata for shale gas exploration in South China.In order to reveal the lithofacies characteristics and controlling factors of Shetianqiao Formation in Shaoyang sag of Xiangzhong depression, the lithofacies, sedimentary environment and its control on lithofacies development of mudstone were studied on the basis of systematical core description and test results of mineralogy and geochemistry, and the control of sedimentary environment evolution on lithofacies type is discussed in this study. Four lithofacies are identified, including siliceous mudstone, mixed mudstone, chert and calcareous mudstone, according to the differences of mineral component content. The Shetianqiao Formation can be divided into 5 lithofacies assemblages. Lithofacies assemblages Ⅰ and Ⅲ are relatively rich in silica, while the content of calcite in assemblages Ⅱ,Ⅳand Ⅴ is relatively high. The sedimentary environment analysis indicates that the mudstone of Shetianqiao Formation was formed at the continental margin setting. It was mainly developed in a relatively hot and dry climate,but the climate was relatively warm and humid in the early and middle stages. The middle stage developed the warmest and wettest climate. And the sea levels rose first and then fell in this period. The middle section is characterized by excess silica attributed to the biological origin and little influence by hydrothermal fluids. Terrestrial input was relatively stable and was the lowest in the middle stage. The coupling relationship between sedimentary environment and lithofacies types indicates that the lithofacies are mainly controlled by paleoclimate, terrestrial input, change in sea level, and biological origin. The siliceous components of mudstone are mainly contributed by the terrestrial input. The enriched silica in the middle section is mainly of biological origin.When the climate was hot and dry, the mudstone lithofacies was dominated by the calcium-riched lithofacies in the later period.