四川盆地西北部中二叠统栖霞组相控早期高频暴露岩溶特征及启示

doi:10.3969/j.issn.1672-7703.2020.03.007

摘要/Abstract

摘要： 文章介绍了一类准同生期云化叠合早期暴露岩溶成因的深层—超深层孔洞型碳酸盐岩储层。基于区内露头与钻井资料，开展了岩石学、储层地质学及地球化学研究，发现四川盆地中二叠统栖霞组发育泥晶灰岩—生屑泥晶灰岩—泥亮晶颗粒灰岩的向上变粗型和亮晶颗粒云岩—中晶云岩—细粉晶藻云岩的向上变细型两类向上变浅序列。两类序列早期高频暴露岩溶具有如下特征：①旋回顶部存在暴露不整合面，并见下一旋回初始海泛沉积物充填于先期岩溶系统和向微地貌高地超覆现象。②旋回上部的多孔基岩受优势岩溶通道切割形成近原地角砾。③部分旋回顶部发育斑块状岩溶系统，其内为陆源黏土、粉屑和少量角砾充填。④少数旋回内部发育岩溶洞穴角砾岩，角砾间多为碳酸盐岩渗流粉砂充填。⑤暴露面之下稳定碳、氧同位素存在一定负偏现象。进一步开展储层物性分析发现，岩溶作用强度较强时，优质储层主要发育于旋回的下部，储层储集空间主要为基岩保留的粒间孔经过云化与溶扩而形成的小型孔洞；岩溶作用强度较弱时，优质储层主要发育于旋回的中上部，储层储集空间为基岩保留的粒间孔经过云化与溶扩而形成的小型孔洞；岩溶作用强度适中时，优质储层主要发育于旋回的中部，储层储集空间主要为洞穴垮塌角砾间的残余孔洞。这些叠合准同生期云化作用的白云岩为基岩的早期暴露岩溶作用形成的孔洞至今仍然被保留下来，并作为有效储集空间。因此认为不同强度的早期高频暴露岩溶作用是栖霞组储层储集空间形成的主要因素。

关键词: 早期高频暴露岩溶；深层&mdash, 超深层；孔洞成因；四川盆地西北部；中二叠统；栖霞组

Abstract: This paper presents a set of deep to ultra-deep vuggy carbonate reservoirs of penecontemporaneous dolomitization genesis superposed with early-stage exposed karstification. Based on outcrops and drilling data from the study area, research has been carried out on petrology, reservoir geology and geochemistry. Two types of shallowing-upward sequences have been identified in the Qixia Formation of the Middle Permian in the Sichuan Basin. The first is the coarsening-upward type; from micrite to bioclastic micrite to micrite-sparry grain limestone. The second is the fining-upward type; from sparry grain dolomite to medium- crystalline dolomite to fine-powder crystalline algal dolomite. The early high-frequency exposed karstification of both types has the following characteristics: (1) There are exposed unconformities at the tops of the cycles, and in each cycle the initial flooding sediment of the next cycle filled in the earlier karst system and overlapped the micro paleo-geomorphic highs. (2) The porous bedrocks in the upper part of the cycle were cut by dominant karst channels and formed near in-situ breccias. (3) Patch-like karst systems are developed at the top of some cycles, which have been filled with terrigenous clay, silty clasts and some breccias. (4) Karst cave breccias are developed in some cycles, and the inter-breccia caves were filled with carbonate vadose silts. (5) The stable carbon and oxygen isotopes exhibit a certain negative bias under the exposure surface. Further study on reservoir physical properties shows that, when karstification is relatively strong, high-quality reservoirs are developed in the lower part of the cycle, and the reservoir space is primarily small vugs formed by dolomitization and dissolution expansion of the intergranular pores preserved in bedrocks. When karstification is relatively weak, high-quality reservoirs are developed in the middle and upper part of the cycle, and the reservoir space is similar to that produced by strong karstification. When the intensity of karstification is moderate, high-quality reservoirs are developed in the middle of the cycle, and the reservoir space is generally residual vugs from collapse of inter-breccias caves. The vugs formed by the early exposed karstification superimposed penecontemporaneous dolomitization of the dolomite bedrocks are preserved today as the effective reservoir space. Therefore, early high-frequency exposed karstification with different intensities is considered to be the primary factor for the formation of reservoir space in the Qixia Formation.

Key words: early high-frequency exposed karstification, deep to ultra-deep, genesis of vugs, northwestern Sichuan Basin, Middle Permian, Qixia Formation

中图分类号:

李明隆, 谭秀成, 罗冰, 张亚, 张本健, 芦飞凡, 苏成鹏, 肖笛, 钟原. 四川盆地西北部中二叠统栖霞组相控早期高频暴露岩溶特征及启示[J]. 中国石油勘探, 2020, 25(3): 66-82.

Li Minglong, Tan Xiucheng, Luo Bing, Zhang Ya, Zhang Benjian, Lu Feifan, Su Chengpeng, Xiao Di, Zhong Yuan. Characteristics of facies-controlled and early high-frequency exposed karstification in the Qixia Formation of Middle Permian in the northwest of Sichuan Basin and its significance[J]. China Petroleum Exploration, 2020, 25(3): 66-82.

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