台内滩型薄层状白云岩储集层特征及主控因素——以四川盆地高石梯—磨溪地区中二叠统栖霞组为例

Characteristics and main controlling factors of intra-platform shoal thin-layer dolomite reservoirs:A case study of Middle Permian Qixia Formation in Gaoshiti–Moxi area of Sichuan Basin,SW China

通过对四川盆地高石梯—磨溪地区中二叠统栖霞组台内滩分布及白云岩储集层特征的研究,分析储集层发育控制因素,建立“台内滩型薄层状白云岩储集层”成储模式。研究表明:栖霞组自下而上为海退旋回,栖一段发育低能开阔海亚相,栖二段海平面下降演化为台内滩及滩间海,台内滩主要分布于栖二段两个次级变浅旋回近顶部。栖霞组主要的储集层岩性为薄层状裂缝-孔洞型白云岩,其次为孔洞型白云岩,缝洞中常见鞍状白云石半充填,晶间孔、残余溶蚀孔洞叠加裂缝形成了有效的孔渗网络。基于沉积及成岩特征耦合分析,系统建立了“沉积前微古地貌控滩、沉积期滩体控制白云岩展布、准同生白云石化有利于孔渗网格保存,晚期热液作用有效改善储集物性”的成储模式。栖霞组沉积前的“一级高带”微古地貌控制栖二段沉积期高石梯地区台内滩的大面积发育;滩相是早期云化的基础条件,台内滩与白云岩储集层分布范围的吻合程度较高;滩相颗粒灰岩受两期白云石化叠加改造,准同生期白云石化有助于原生孔隙和次生溶孔保存,埋藏期热液沿峨眉山玄武岩事件伴生的裂缝进入早期白云岩体,使其重结晶为中—粗晶白云岩,晶间孔叠加顺缝热液溶蚀的残余孔洞,形成了台内滩型薄层状优质白云岩储集层。该成储模式的建立可为四川盆地二叠系气藏的可持续开发提供地质理论支撑。

Based on the study of the distribution of intra-platform shoals and the characteristics of dolomite reservoirs in the Middle Permian Qixia Formation in the Gaoshiti–Moxi area of the Sichuan Basin,the controlling factors of reservoir development were analyzed,and the formation model of“intra-platform shoal thin-layer dolomite reservoir”was established.The Qixia Formation is a regressive cycle from bottom to top,in which the first member(Qi1 Member)develops low-energy open sea microfacies,and the second member(Qi2 Member)evolves into intra-platform shoal and inter-shoal sea with decreases in sea level.The intra-platform shoal is mainly distributed near the top of two secondary shallowing cycles of the Qi2 Member.The most important reservoir rock of the Qixia Formation is thin-layer fractured-vuggy dolomite,followed by vuggy dolomite.The semi-filled saddle dolomite is common in fracture-vug,and intercrystalline pores and residual dissolution pores combined with fractures to form the effective pore-fracture network.Based on the coupling analysis of sedimentary and diagenesis characteristics,the reservoir formation model of“pre-depositional micro-palaeogeomorphology controlling shoal,sedimentary shoal controlling dolomite,penecontemporaneous dolomite laying the foundation,and late hydrothermal action effectively improving reservoir quality”was systematically established.The“first-order high zone”micro-paleogeomorphology before the deposition of the Qixia Formation controlled the development of large area of intra-platform shoals in Gaoshiti area during the deposition of the Qi2 Member.Shoal facies is the basic condition of early dolomitization,and the distribution range of intra-platform shoal and dolomite reservoir is highly consistent.The grain limestone of shoal facies is transformed by two stages of dolomitization.The penecontemporaneous dolomitization is conducive to the preservation of primary pores and secondary dissolved pores.The burial hydrothermal fluid enters the early dolomite body along the fractures associated with the Emeishan basalt event,makes it recrystallized into medium–coarse crystal dolomite.With the intercrystalline pores and the residual vugs after the hydrothermal dissolution along the fractures,the high-quality intraplatform shoal-type thin-layer dolomite reservoirs are formed.The establishment of this reservoir formation model can provide important theoretical support for the sustainable development of Permian gas reservoirs in the Sichuan Basin.