川西南下寒武统筇竹寺组页岩旋回地层学研究

Cyclostratigraphy Research on Well-logging of the Lower Cambrian Qiongzhusi Formation in Southwestern Sichuan Basin

[目的]四川盆地下寒武统的泥页岩不仅包含重要的环境生命演化信息,同时也是我国当前页岩气勘探开发的重点层位,对其开展精细地层年代学研究具有生产及科研的双重价值。[方法]为了更准确地探明四川盆地下寒武统筇竹寺组黑色页岩层序沉积特征,对川西南地区金页1井筇竹寺组页岩地层开展了旋回地层学研究。[结果与结论](1)通过分析不同测井曲线测井响应特征,认为钾元素测井序列是最适合古环境、古气候的替代指标。(2)将钾元素深度序列进行频谱分析和滑动窗口频谱分析,识别出偏心率和斜率周期(405 kyr,131 kyr,95 kyr,36.4 kyr,28.2 kyr);并结合相关系数分析,以不同沉积速率为限,将整段曲线划分为四段,分别进行405 kyr长偏心率周期的带通滤波;以周缘地区的年代地层年龄526.86±0.16 Ma为年龄控制点,建立了金页1井筇竹寺组的“浮动”天文年代标尺。(3)筇竹寺组共记录了29个长偏心率周期,四段地层的最优沉积速率分别为2.5,2.9,3.4和4.8 cm/kyr。(4)以405 kyr的轨道调谐为基础的沉积噪音模型DYNOT及ρ1曲线重建了筇竹寺组沉积时期相对海平面变化曲线;以该曲线为理论参照,在筇竹寺组中划分了4个三级层序,识别出4个最大海泛面及与之对应的T-R旋回;4个三级层序的沉积时限从下到上为3.18,2.03,2.97和3.63 Ma,分别包含了8个,5个,7个和9个长偏心率周期。(5)海平面变化曲线的低值对应了全球碳同位素负偏,推测是由于海平面的下降引起的有机质发生氧化,并向海水释放碳同位素偏轻的无机碳,形成碳同位素负偏。

[Objective]The Lower Cambrian in the Sichuan Basin is a key area for understanding the evolution of life and the paleoenvironment,and is a most significant area for future oil and gas exploration.Study of the precise chro-nostratigraphy has both production and scientific value.[Methods]To define the sedimentary sequence in the re-gion,cyclostratigraphical studies were conducted for well Jinye-1.[Results and Conclusions](1)Well-logging re-cords were taken to investigate the effects of content variation in strata components.(2)The K logging series was cho-sen to be the best proxy in the Lower Cambrian Qiongzhusi Formation in the southwestern Sichuan Basin.The K log-ging series was divided into four segments,and spectra for each segment were analyzed in detail.Evolutional spec-trum analysis was conducted for the whole Qiongzhusi Formation.Orbital cycles of 405,131,95,6 and 28 ka were identified in the sedimentary records.A floating astronomical time scale was constructed by applying Gaussian filter-ing of the 405 ka cycle together with the control age of 526.86±0.16 Ma from a related region.(3)The time scale con-tained 29 long-eccentricity cycles,and the optimal sedimentation rate of each logging series segment was found to be 2.5,2.9,3.4 and 4.8 cm/ka.(4)The DYNOT sedimentary noise model andρ1 were built on the 405 ka tuning time series to obtain the relative sea-level change plots in the time domain.Analysis of relative sea-level change recognized four third-order sequences and four maximum flooding surfaces,and the transgressive-regressive(T-R)cycles.The deposition times of each sequence from early to late are 3.18,2.03,2.97 and 3.63 Ma respectively,with each con-taining 8,5,7 and 9 long eccentricity cycles.(5)The relative sea-level decrease corresponds to the global carbon iso-tope negative excursion;it is presumed that the decreasing sea-level led to oxidation of exposed organic matter,releasing the lighter carbon isotopes and resulting in the negative carbon isotope excursion.