川中高石1井区灯影组二段早成岩期岩溶特征及其对储集层的影响^*

Characteristics of eogenetic karst and its influence on reservoir of the Member 2 of Dengying Formation from Wellblock GS1, central Sichuan Basin

摘要

四川盆地震旦系灯影组二段储集层成因长期存在风化壳岩溶、准同生期岩溶及埋藏热液岩溶之争,严重制约了储层预测模型的建立和勘探开发效益。鉴于此,以高石1井区灯二段为例,基于钻井岩心、岩石薄片、测井等资料,识别并梳理出灯二段岩溶特征,讨论岩溶差异性以及其对储集层发育和保持的控制作用。结果表明: 灯二段发育高频暴露溶蚀面、组构选择性溶蚀孔洞、示顶底构造、花斑状和海绵状溶蚀构造及岩溶角砾等溶蚀组构,这类溶蚀特征与高频向上变浅沉积序列密切相关,且主要发育在沉积序列的中上部;根据岩溶强度可识别出组构选择性溶蚀、岩溶分带以及整体角砾化3种岩溶序列;高频旋回界面之下的沉积物具有δ¹³C、δ¹⁸O值负偏、⁸⁷Sr/⁸⁶Sr值正偏特征。进一步分析发现,灯二段孔洞型薄储集层与岩溶组构一样,均发育在高频向上变浅沉积序列中上部,且大量充填残余物的岩溶孔洞周缘具有泥晶套、纤维状环边胶接,部分孔洞中可见示顶底构造,指示岩溶孔洞形成后又重新进入海底成岩环境。结合高频旋回界面之下的沉积物具有碳氧同位素负偏、锶同位素正偏等特征,认为这类孔洞型薄储渗体的发育受控于高频海平面变化驱动下的早成岩期岩溶作用,其在纵向上叠置而呈现出“薄层多套”的分布规律,储集层相控特征明显; 不同强度的岩溶作用改变了孔洞的连通性,其中组构选择性溶蚀阶段多形成相对孤立的小型溶孔洞,因海底胶结作用使孔洞更趋孤立而长期保存,而岩溶分带和整体角砾化阶段的溶蚀孔洞进一步连通,后期更易遭受机械充填和化学胶结作用而使储集层非均质性增强,甚至储集性能显著变差。上述研究结果不仅对古老深层优质储集层预测具有重要的指导作用, 同时也揭示出在不同强度岩溶作用改造下岩溶系统连通性对早成岩期溶蚀孔洞保持路径的控制,为古老深层碳酸盐岩储集层的形成和保持提供了新的研究思路。

Abstract

The genesis of reservoirs in Member 2 of the Sinian Dengying Formation in the Sichuan Basin has long been a subject of debate,including weathered crust karst,pene-sedimentary karst and buried hydrothermal karst,which significantly impedes the establishment of reservoir prediction model and the exploration and development benefit. This paper uses Member 2 of the Dengying Formation from Wellblock Gaoshi 1 as a case study. By analyzing drilling core samples,thin sections,well logging data,etc.,the karst characteristics of Member 2 of the Dengying Formation is identified and categorized. Furthermore,the variations in karst features and their impact on reservoir development and maintenance have been discussed. The results indicate that: (1)Member 2 of the Dengying Formation exhibits features such as high-frequency corrosion surface,fabric-selective corrosion cavity,geopetal structure,semi-dissociation zone,graniphyric textures,spongy corrosion structures,and karst breccias;(2)The dissolution characteristics are closely associated with the high-frequency shallower-upward sedimentary sequence,predominantly developing in the middle and upper parts of this sequence;(3)Based on the karst strength,three types of karst sequences can be identified: fabric selective corrosion,karst zonation,and integral brecciation;(4)Additionally,a negative bias in δ¹³C and δ¹⁸O as well as a positive bias in⁸⁷Sr/⁸⁶Sr,are observed at the high-frequency cycle interface. Further analysis indicates that the thin reservoir of Member 2 of the Dengying Formation is also developed in the middle and upper parts of the high-frequency shallower-upward sedimentary sequence. A significant number of residual karst caverns are filled with micritic envelopes and fibrous isopachous cements. Additionally,some of these caverns exhibit geopetal structures,indicating that they re-entered the submarine diagenetic environment after the formation of karst caverns. Considering the characteristics of negative carbon and oxygen isotope biases and positive strontium isotope biases at high-frequency cycle interfaces,it is proposed that the development of this kind of porous thin reservoir is primarily influenced by eogenetic karst processes driven by the high-frequency sea level changes. The vertical superposition exhibits a distribution pattern characterized by “multiple thin layers”,with distinct facies-controlling features. The karstification with different intensities change the connectivity of pores. Relatively isolated small dissolved pores are primarily formed during the fabric selective solution stage,where they remain more isolated and preserved for extended periods due to seabed cements. In contrast,during the karst zonation and integral brecciation stage,these dissolved pores become increasingly interconnected. In the later stages,they are more prone to mechanical filling and chemical bonding,which enhances reservoir heterogeneity and can significantly deteriorate reservoir performance. Furthermore,it has been revealed that the connectivity of the karst system controls the maintenance pathway of dissolution pores during the early diagenetic stage,under different karst strength transformation. This finding offers a new perspective for understanding the formation and preservation of ancient deep carbonate reservoirs.

关键词

高频暴露 / 早成岩期岩溶 / 岩溶强度 / 孔洞型薄储集层 / 相控特征 / 灯影组 / 四川盆地

Key words

high-frequency exposure / eogenetic karst / karst strength / porous-type thin reservoir / facies-controlling feature / Dengying Formation / Sichuan Basin

基金资助

*中国石油—西南石油大学创新联合体科技合作项目(编号: 2020CX010300)和国家自然科学基金青年基金项目(编号: 42302169)联合资助