西江主洼缓坡带裂陷期三角洲沉积特征对比

为了弄清珠江口盆地西江主洼南部缓坡转换带文昌组和恩平组地层的沉积样式及其发育规律,通过钻井、测井、地震、岩芯和分析化验等资料,从岩性、层理构造、结构特征及地球物理特征等方面总结两种缓坡转换带沉积体系的特征,并剖析研究区断裂特征。研究认为,文昌期(T₈₀—T_g)主要发育NE和NW向控洼大断裂,分布于盆地边缘,断裂组合形态以平行状、阶梯状及斜交状为主;恩平期(T₇₀—T₈₀)主要发育NE向、EW向和SE向断裂,分布于盆地中央,断裂组合形态以同向、反向及斜交状为主;虽然同属缓坡带,但文昌组地形较恩平组陡,湖盆较深,发育辫状河三角洲;恩平组地形宽缓,发育浅水三角洲,湖盆中央断裂的活动及季节性洪水的发生,使得浅水三角洲水下分支河道极为发育。两种不同类型的三角洲具有不同的岩性组合特征:文昌组辫状河三角洲砂岩粒度粗且厚,泥岩层薄,恩平组浅水三角洲砂岩粒度粗且薄,泥岩层厚,含煤,均为有利油气勘探区域。

Comparison of Delta Sedimentary Characteristics During Rift Depression in Gentle Slope Zone of Xijiang Main Depression

In order to find out the sedimentary patterns and development rules of Wenchang Formation and Enping Formation strata in the gentle slope conversion zone in the south of Xijiang main depression in Pearl River Mouth Basin, the characteristics of the sedimentary systems in the two gentle slope conversion zones are summarized from the aspects of lithology, bedding structure, structural characteristics and geophysical characteristics through drilling, logging, earthquake, core analysis and laboratory tests, and the fracture characteristics in the study area are analyzed. The study shows that during Wenchang period (T$_{80}$-T$_{rm g}$), NE and NW trending depression-controlling faults are developed and distributed at the basin edge, and the fault combination forms are parallel, stepped and oblique. During the Enping period (T$_{70}$-T$_{80}$), NE, EW and SE faults were developed and distributed in the center of the basin. The fault combinations were in the same direction, reverse direction and oblique crossing. Although it belongs to gentle slope zone, Wenchang Formation has steeper topography, deeper lake basin and braided river delta than Enping Formation. The Enping Formation has a broad and gentle topography, with shallow water deltas, the activities of central faults in the lake basin and the occurrence of seasonal floods, making the underwater branch channels of the shallow water deltas extremely developed. The two different types of deltas have different lithologic combination characteristics: the braided river delta sandstone of Wenchang Formation has coarse and thick grain size and thin mudstone layer, and the shallow water delta sandstone of Enping Formation has coarse and thin grain size, thick mudstone layer and coal bearing, which are all favorable areas for oil and gas exploration.