准噶尔盆地吉木萨尔凹陷梧桐沟组层序地层划分

准噶尔盆地吉木萨尔凹陷二叠系梧桐沟组作为一套重要的储集岩,具有良好的油气成藏前景,但对其层序划分及地层沉积样式的认识一直存在争议。在构造背景分析的基础上,结合岩芯、测井及地震资料,对研究区梧桐沟组层序地层划分及地层沉积样式研究表明,梧桐沟组沉积时期构造强度较弱,地形较为平缓,地层在全区稳定分布;中晚二叠世盆地发生造山运动,吉木萨尔凹陷东南边缘区域经过构造抬升,上部地层遭受不同程度的剥蚀,形成了现今"底平顶削"的地层样式。以凹陷中部少数地层保存较全的井的地层叠加样式分析为基础,通过井-震结合识别不同类型界面,特别是最大湖泛面,建立了区域层序地层格架,即梧桐沟组为一个完整的长期基准面旋回(三级层序),并在其内部识别出5个中期旋回。梧桐沟组地层在不同的层序发育时期表现出不同的旋回叠加样式:下段沉积时期,即最大湖泛面以下,随着可容纳空间的增大,地层表现为明显的退积叠加样式;上段沉积时期,即最大湖泛面以上,随着可容纳空间的减小,地层表现为明显的进积叠加样式,符合沉积物体积分配原理。

Sequence stratigraphic division of the Wutonggou Formation in the Jimsar sag of Junggar Basin

The Permian Wutonggou Formation is a set of important reservoir rock in the Jimsar sag, with a good prospect of oil and gas accumulation. However, sequence stratigraphic division and stratigraphic sedimentary pattern have long been controversial. Based on the analysis of tectonic setting, cores, well logging and seismic data, this work studied the sequence stratigraphic division and stratigraphic sedimentary pattern of the Wutonggou Formation, and obtained some new understanding. The study shows that during the depositional period of the Wutonggou Formation, the tectonic activity was relatively weak and the original geomorphy of Jimsar was very flat, and the Wutonggou Formation distributed steadily in the sag. As the Mid-Late Permian orogeny caused the uplift and erosion of the southeastern marginal area, the present stratigraphic depositional style becomes flat at the bottom and erosion at the top. Based on the analysis of stratigraphic superposition pattern of wells in the middle of the sag, by using the method of well-seismic combination, the sequence boundary especially the MFS was identified, and the sequence stratigraphy framework was established. The Wutonggou Formation was formed in a complete long-term base-level cycle (third-order sequence), and five mid-term cycles were identified internally. The Wutonggou Formation shows different cycle superimposed patterns in different sequence development stages, which is conformed to the principle of sediment volume distribution. The lower part of Wutonggou Formation, which is below the MFS, shows a retrograding superimposed pattern with the increase of accommodating space, while the upper part shows a prograding superimposed pattern with the decrease of accommodating space.