威远基底结构与龙马溪组地层构造及沉积相关系研究
The Relationship Study between the Basement Structure and the Structure-Sedimentary Facies of Longmaxi Formation in Weiyuan Syncline

威远向斜钻井揭示，龙马溪组I段为主力气藏，其页岩厚度、品质变化较大，与产能关系密切。沉积微相的研究是储层目标评价、气藏描述的关键。但用地震资料精细描述龙马溪组I段优质页岩分布，还有一定难度。为此，通过基底结构分析上覆地层构造、沉积微相变化规律，并分析其形成机理；采用单井旋回划分，结合地震反射结构识别，建立层序地层格架；综合构造、沉积相描述，预测龙马溪I段页岩气藏分布规律。研究表明：下震旦统发育一个大型的洼陷，东南侧发育一个大型的隆起；上覆地层的向斜和背斜，与下震旦统的洼陷－隆起正好对应。其次，威远向斜中部优质页岩厚度较大、品质优，正对下方古洼陷位置；龙马溪组2-3段在东南侧发育由东南向西北进积的斜交反射，正对下震旦统古隆起位置，页岩品质也相对较差。威远向斜的构造、沉积相明显受下震旦统古洼陷–隆起控制。下震旦统大型洼陷上方的志留系洼陷，预测为龙马溪组I段优质页岩、最佳气藏分布带。
In the Weiyuan syncline, the shale of I section of Longmaxi group is the main gas reservoir. The thickness and the quality of the shale changes greatly, which is closed to the capacity. The study of sedimentary microfacies is the key to evaluate the reservoir targets and to describe gas reservoirs. However, it is difficulty using seismic data to describe the shale’s distribution. The structure of overlying strata and sedimentary microfacies are analyzed by studying basement structure and its formation mechanism. The sequence stratigraphic framework is established by single well cycle and seismic reflection structure. The description of structure and the sedimentary facies is synthesized to predict the shale gas reservoir of Longmaxi group. The research shows that: a large depression develops in the Lower Sinian and a large uplift in the southeast. The syncline and anticline of the overlying strata correspond to the depression-uplift of the Lower Sinian. Secondly, the shale of Weiyuan syncline, which is of great thickness and high quality, directly faces the ancient depression. Section 2-3 of the Longmaxi Formation develops oblique reflections from southeast to northwest which is opposite to the ancient uplift of the Lower Sinian, and of relatively poor quality. In Weiyuan syncline, the tectonic and sedimentary facies are obviously controlled by lower Sinian depression-uplift. It is forecasted that the Silurian depression, which is at the top of the sub-sag of the lower Sinian, is the best shale’s gas reservoir distribution zone.