元坝气田长兴组生物礁层序沉积及其对储层发育的控制

四川盆地元坝气田上二叠统长兴组气藏目前已全面转向开发,随着开发工作的有序推进,迫切需要精细描述储层结构,总结储层发育的控制因素。为此,基于岩心观察、薄片鉴定结果,结合物性数据、测井曲线等分析成果,重点研究了该区长兴组生物礁的层序地层、沉积相、储层特征以及储层结构。结果表明:①该区长兴组可划分为2个三级层序和6个四级层序,高频层序界面上、下电测曲线突变响应特征明显,电测曲线组合变化特征与高频层序所控制的沉积旋回具有较好的一致性;②长兴组生物礁主要发育在上部三级层序(SQ2)中的台地边缘外侧并呈条带状分布,纵向上由2个明显的成礁旋回构成,在横向上具有"东早西晚"不对称规模发育的分布特征;③生物礁(滩)型储层总体以低孔隙度、中低渗透率溶孔白云岩储层为主,主要发育于2个四级层序中部上部生物礁礁盖部位,具有"层数多、单层薄、不同类型储层呈不等厚互层、非均质性强"和"双层储层结构"的特征。结论认为.高频层序控制了该区生物礁沉积微相的分布,是影响储层发育与空间展布的关键因素。

Sequence and sedimentary characteristics of Changxing Formation organic reefs in the Yuanba Gasfield and their controlling effects on reservoir development in the Sichuan Basin

In the Yuanba area, Sichuan Basin, the gas reservoirs in the Upper Permian Changxing Formation are now at development stage. With the smooth progress of development, it is urgent to characterize the reservoir architectures accurately and summarize the controlling factors of reservoir development. In this paper, research was mainly performed on the Changxing Formation organic reefs in terms of their sequence stratigraphy, sedimentary facies, and reservoir characteristics and architectures based on core observation and thin section analysis, combined with physical property data and logging curves analysis results. It is shown that the Changing Formation can be divided into two third-order sequences and six fourth-order sequences, whose electric logs are characterized by sudden change above and below the high-frequency sequence boundary and by better accordance with the sedimentary cycles controlled by high-frequency sequences. Besides, the Changxing Formation organic reefs are mainly distributed in the form of ribbons outside SQ2 platform margin, and they are vertically composed of two obvious two reef sedimentary cycles and laterally developed in asymmetric patterns (early in the east and late in the west). Finally, in general, organic reef (bank) reservoirs are mainly composed of low-porosity and middle-low-permeability dissolved dolomite reservoirs, and they are mostly distributed at reef caps in the upper-middle parts of the two fourth-order sequences, with the characteristics of multiple beds, thin single beds, interbedded strata of different types of reservoirs with different thickness, strong heterogeneity and double-layer reservoir architectures. It is concluded that the distribution of organic reef microfacies in this area is controlled by high-frequency sequence which is the key factor controlling the development and spatial distribution of reservoirs.