鄂西鹤峰地区牛蹄塘组沉积特征及沉积环境分析——以鹤地3井为例

Analysis of Niutitang Formation Sedimentary Features and Sedimentary Environment in Hefeng Area,Western Hubei—A Case Study of Well HD-3

鄂西下寒武统牛蹄塘组是中上扬子地区页岩气勘探的重要层系之一,在对鹤地3井钻井岩心描述、测井曲线特征以及系统的地球化学采样测试分析基础上,对鹤峰地区鹤地3井牛蹄塘组沉积特征、沉积相和沉积氧化还原环境进行了研究,建立了牛蹄塘组垂向沉积相序列,讨论了沉积环境演化的规律。研究认为,鹤峰地区鹤地3井牛蹄塘组垂向上从下至上发育深水陆棚、深水-浅水陆棚过渡相和斜坡相三种类型,其中深水陆棚相中发育深水硅泥质陆棚和深水泥质陆棚,垂向演化序列指示水体向上逐渐变浅。MgO/Al 2 O 3比值结果指示鹤峰地区牛蹄塘组沉积时期整体水体盐度正常,为稳定海相沉积环境;V/(V+Ni)与V/Cr以及Th/U指标分析得到的深水-浅水陆棚过渡相以及斜坡相沉积中富氧环境不同,但是综合岩心、有机碳含量以及GR曲线变化显示整个牛蹄塘组沉积时期,沉积环境是从下到上缺氧-贫氧-富氧状态演化的;V/Cr与Th/U指标与TOC含量变化具有较高的一致性,尤其是在牛蹄塘组下段及深水陆棚段,反映氧化-还原条件的指标越大,则TOC值越大,该段沉积环境水体相对深,有利于有机质的保存。而到中段深水-浅水陆棚过渡相,陆源输入增强,有机碳含量剧减。总体表明氧化-还原环境控制着有机质的丰度。

The lower Cambrian Niutitang Formation in western Hubei is one of shale gas exploration major measures in the middle and upper Yangtze area.Based on well HD-3 core description,well logging trace features and systematic geochemical sampling,testing and analysis carried out study on the Hefeng area well HD-3 Niutitang Formation sedimentary features,sedimentary facies and sedimentary redox environment,established Niutitang Formation vertical sedimentary facies sequence and discussed sedimentary environmental evolution.The study has considered that Niutitang Formation in well HD-3 vertically from bottom to top developed deep water shelf facies,deep-shallow water shelf transitional facies and continental slope facies three types.In which,deep water shelf developed siliceous pelitic and pelitic shelves,the vertical evolutional sequence indicated water mass has a gradual shallowing upward process.The MgO/Al 2 O 3 ratio has indicated during Niutitang Formation sedimentation period,water mass salinity normal,under a stable marine sedimentary environment,different from the V/(V+Ni),V/Cr and Th/U ratios analyzed deep-shallow transitional and continental slope facies oxygen enriched environment.But based on integrated core data,organic carbon content and GR traces variation,during whole Niutitang Formation sedimentation period,sedimentary environment evolution has been under oxygen deficient-poor-enriched situations.While V/Cr,Th/U ratios and TOC content variation has higher consistency,especially within lower member of Niutitang Formation and deep water shelf sector,reflected the large the indicators of redox conditions,the large the TOC contents.During this period,water mass was relatively deep,in favor of organic matter preservation.While in middle member deep-shallow water transitional facies,terrigenous supply increasing,organic carbon content decreased sharply.As a whole,the redox environment has controlled organic matter abundance.