神狐海域水合物钻探区第四纪米氏旋回高频层序地层划分

基于高分辨率2D地震资料,建立了珠江口盆地白云凹陷第四纪层序地层格架,将第四纪沉积充填序列划分为3个体系域,即低位体系域(LST)、海侵体系域(TST)、高位体系域(HST)。根据陆坡进积特征和地层叠加样式、垂向上侵蚀特征变化(其上见下切水道)、中频强振幅同相轴的空间可追踪性等地震反射特征,将高位体系域(HST)进一步划分为两个沉积旋回单元HST-I、HST-Ⅱ。在等时地层格架的约束下,以神狐海域水合物钻探区well-I钻井作为研究对象,选择自然伽马曲线(GR)通过频谱分析进行米兰科维奇旋回特征的研究,识别出偏心率周期(95 ka)、斜率周期(40 ka)和岁差周期(22、19ka)。研究区第四纪沉积充填序列主要受偏心率周期(95 ka)的控制,偏心率(95 ka)对应于旋回厚度11.494 m,以此周期和对应的旋回厚度计算出钻探区第四纪沉积速率为12.1 cm/ka。通过构建滤波器对测井曲线进行滤波处理,发现95 ka的偏心率控制的优势旋回个数在20个左右。对斜率周期(40 ka)识别出的高频旋回采用Fischer图解法求取其可容纳空间变化曲线,检验了高精度层序地层划分的正确性。

HIGH-RESOLUTION SEQUENCE STRATIGRAPHY ON MILANKOVITCH CYCLES IN THE GAS HYDRATE DRILLING AREA OF SHENHU WATERS

The Quaternary sequence stratigraphic framework was established for the northern slope of the Baiyun Sag, the Pearl River Mouth Basin, based on high resolution 2D seismic profiles. The deposit is divided into three systems tracts, i.e. the LST (lowstand systems tract, LST), TST (transgressive systems tract, TST) and HST (highstand systems tract, HST). The HST could be further divided into two depositional units, HST-I and HST-Ⅱ, according to the prograding styles of the slope, the vertical stacking patterns, the change in erosion features, and the continuity of high-amplitude seismic reflectors. Under the constraints of stratigraphic framework, the natural gamma data from Well-I of the gas hydrate drilling area of the Shenhu waters have been used for spectral analysis. The Milankovitch cycles are identified by spectrum analysis, including the eccentricity cycles (95 ka), obliquity cycle (40 ka), precession cycles (22 ka, 19ka), and the sequence is mainly controlled by the eccentricity cycle (95 ka), corresponding to a thickness of 11.494 m. It is found that the number of dominant cycles controlled by the eccentricity cycles (95 ka) is about 20 after filtering of logs. The sedimentation rate in the drilling area is 12.1 cm/ka calculated with the eccentricity cycle and the corresponding thickness of the cycle. The accommodation change curves of the sequences obtained using Fischer diagram can verify the high resolution sequence stratigraphic division.