Formation Mechanism and Influencing Factors of Micro-Fractures in Tight Glutenite of Kongdian Formation in Bozhong Sag
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摘要: 渤中凹陷西南部多口井钻遇巨厚孔店组砂砾岩，整体埋深大，储层明显致密化，部分井段产能不足10 m3/d，少部分井产能较高，储层差异演化机制是制约勘探的核心问题.针对储层形成机制，开展了薄片、扫描电镜、压实模拟等实验.研究表明，微裂缝发育程度决定了储层物性差异，储层裂缝发育程度受砾石成分构成及杂基含量的影响.以花岗岩为母岩的低杂基砂砾岩，钾长石裂缝发育，溶蚀作用强，储层物性好；高杂基砂砾岩，裂缝不发育，溶蚀作用弱，测试产能低；混杂了大量碳酸盐岩砾石的砂砾岩，储层早期胶结作用强，整体致密，压裂改造后效果仍较差.压实应力物理模拟实验表明，在模拟埋深2 500 m以下，砾石出现粒内缝，并随着模拟埋深加大不断增加；裂缝发育的类型、特点与研究区微裂缝特征可比对，证实压实成缝是研究区裂缝形成的重要机制.钾长石与斜长石裂缝生成有很大差异性：钾长石更容易发育破裂缝，经后期流体溶蚀改造，形成粒内溶蚀扩大缝；斜长石由于易发生高岭土化、钠黝帘石化等次生改造作用，改变了矿物的力学性质，不易产生裂缝.Abstract: Many deep wells in the southwestern part of the Bozhong sag reveal thick glutenite of Kongdian Formation. Due to the large burial depth, the glutenite reservoir is obviously densified, and the production of some wells is less than 10 m3/d. A part of wells have high productivity, and the differential evolution mechanism of reservoirs is a key issue that restricts exploration. Aiming at the differential evolution mechanism of the reservoir, this study carried out experiments such as thin section, scanning electron microscope, compaction simulation. It is found that the degree of development of micro-fractures determines the physical properties of reservoirs. And the differential evolution of the reservoir is significantly affected by the composition of gravel and the content of matrix. The glutenite whose parent rock comes from granite, with low matrix content, develops potash feldspar fractures, strong dissolution, and good reservoir properties. The glutenite with high matrix is characterized by underdeveloped fractures, weak dissolution, and low productivity. The glutenite mixed with a large amount of carbonate gravel has strong cementation in the early stage, and the productivity is still poor even after fracturing. The physical compaction simulation shows that intragranular fractures appear in the gravel below simulation burial depth 2 500 meters, which increase with the increase of simulation burial depth; the types and characteristics of fractures can be compared with the characteristics of micro-fractures in the study area. This confirms that glutenite with low matrix content can generate a large number of compaction fractures. Potassium feldspar and plagioclase are quite different in their ability to form fractures: potash feldspar is more prone to form fractures, and which is further corroded by later fluids, forming dissolution enlarged fractures. Plagioclase is prone to kaolinization, sodium zoisite and other secondary transformations, which changes the mechanical properties of minerals, and is not prone to form fractures.
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Key words:
- Bozhong sag /
- Kongdian Formation /
- glutenite /
- fracture /
- reservoirs /
- control factors /
- petroleum geology
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图 2 渤中凹陷西环孔店组沉积相图及母岩区岩性分布预测
Fig. 2. Sedimentary facies map of Kongdian Formation in the west ring of Bozhong sag and prediction map of lithology distribution in the parent rock area
图 3 BZ19-6-3井综合柱状图
孔隙度、渗透率为岩心与旋转壁心实测资料；钾长石，斜长石，黏土矿物含量为岩心、岩屑X衍射全岩分析实测资料
Fig. 3. Comprehensive column chart of well BZ19-6-3
图 4 孔店组砂砾岩物性分布特征
Fig. 4. Distribution characteristics of glutenite in Kongdian Formation
图 5 孔店组砂砾岩成岩作用与储层特征
a. 砂砾岩压实作用强烈，石英加大强烈（箭头所示），BZ19-6-1井，3 710 m；b.菱铁矿呈孔隙衬里状充填，BZ19-6-3井，4 047.8 m；c. 颗粒的贴粒缝，斜长石的粘土化（箭头所示），BZ19-6-3井，4 048.57 m；d. 物源区砾石内继承了变质岩的密集微裂缝，微裂缝被云母矿物充填（箭头所示），BZ19-6-1井，3 670 m；e. 钾长石受应力作用粒内发育共轭微裂隙，裂隙溶蚀扩大，BZ19-6-3井，3 851.31 m；f. 贯穿多个颗粒的构造缝（箭头所示），BZ19-6-3井，3 851.16 m
Fig. 5. Diagenesis and reservoir characteristics of glutenite in Kongdian Formation
图 6 不同井区储层储集空间构成（共12件样品）
Fig. 6. Reservoir space composition of different areas (12 samples in total)
图 7 孔店组砂砾岩微裂缝特征及砂砾岩压实模拟实验裂缝特征对比
a.颗粒接触点处钾长石强烈破碎，裂缝发育产状呈放射状（箭头所示），BZ19-6-3井，4 054.4 m；b. 长石颗粒长宽比较高，受到多方向挤压，形成扭张缝（箭头所示），BZ19-6-3井，4 049.70 m；c. 钾长石颗粒被周围多个颗粒围限，挤压作用下颗粒强烈破碎，周围颗粒侵占原颗粒空间（红色箭头所示），破裂的碎块有受压再位移特点（黄色箭头所示），BZ19-6-3井，4 051.35 m；d. 钾长石受压强烈破碎并沿裂缝溶蚀扩大，裂缝共轭发育，以一定角度斜交，BZ19-6-3井，3 850.47 m；e.钾长石挤压破裂发育多组粒内缝，BZ19-6-3井，4 051.92 m；f.片麻岩，长石在接触点部位向粒内分散状破裂（箭头所示），模拟埋深2 500 m；g.片麻岩，长石在接触点强烈破碎（箭头所示），碎裂块发生位移，模拟埋深5 000 m；h.长宽比较高的长石扭张缝，花岗岩，等效埋深4 000 m；i.花岗岩，长石受到强烈挤压破碎，与研究区膨胀缝类似（箭头所示），模拟埋深5 000 m
Fig. 7. Comparison of the micro-fracture characteristics of the Kongdian glutenite and the fracture characteristics of the glutenite compaction simulation experiment
图 8 声发射法测定的古应力值与母岩破裂强度对比
Fig. 8. Comparison of the ancient stress value measured by acoustic emission method and the fracture strength of the parent rock
图 9 受压岩石应力集中区分布（Gallagher et al., 1974）
暖色为高应力值，黄色最高，主要集中于颗粒接触点和颗粒与外壁接触点，冷色为低应力值
Fig. 9. Distribution of stress concentration areas of compressed rocks (Gallagher et al., 1974)
图 10 渤中凹陷渤中19-6构造孔店组成藏史（王清斌等，2019）
星号为包裹体测温投点，箭头为对应的成藏时间
Fig. 10. History map of the pore shop composition of Bozhong 19-6 structure in Bozhong sag
图 11 碳酸盐含量及杂基含量对裂缝发育的影响
a. BZ26-5-1，3 828 m，早期碳酸盐致密胶结，裂缝不发育；b.砾石颗粒由于黏土矿物的“压力缓冲垫”作用（箭头所示），裂缝不发育，BZ19-6-3井，3 858.3 m；c.裂缝发育微观差异明显，有黏土矿物缓冲的裂缝不发育（黄色箭头所示），无黏土矿物缓冲的发育裂缝（红色箭头所示），BZ19-6-3井，4 049.14 m；d.黏土杂基含量较高（30%），砾石不发育裂缝，BZ19-6-5井，3 865 m
Fig. 11. The influence of carbonate content and matrix content on fracture development
图 12 钾长石与斜长石裂缝发育程度差异性对比
a.黄色花岗岩（采样地点鞍山市莘英路路旁），原岩钾长石含量较高，表面洁净不发育裂缝（红色箭头所示）；b.加压后钾长石表面出现较多微裂缝（黄色箭头所示），相邻的斜长石黏土化、绢云母化较明显，未见微裂缝（红色箭头所示），实验条件：应力强度115.19 MPa，围压20 MPa；c.加压后钾长石表面出现大量微裂缝（箭头所示），实验条件：应力强度158.85 MPa，围压40 MPa；d，e.钾长石大量破裂形成共轭粒内缝（黄色箭头所示），相邻的斜长石黏土化蚀变较强（红色箭头所示），部分颗粒绢云母化（红色箭头所示），BZ19-6-3井，4 054.4 m；e：d的同视域正交光；f.斜长石的书斜式错动变形（箭头所示），BZ19-6-3井，4 030 m
Fig. 12. Comparison of the difference in the development of fractures between potash feldspar and plagioclase
表 1 压实模拟实验样品配置方案
样品编号 石英 钾长石 斜长石 黏土矿物 砾石 1 16% 7% 14% 10% 55%（花岗岩） 2 17% 9% 15% 10% 50%（片麻岩） 注：石英、钾长石、斜长石粒度0.25~1 mm, 砾石2~5 mm, 黏土粒度 < 0.062 5 mm. 
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表 2 不同埋藏深度所对应的模拟实验温度和压力
编号 模拟温度
(℃)模拟压力
(MPa)模拟深度
(m)1-6/2-6 125 110 2 000 1-5/2-5 175 137 3 000 1-4/2-4 200 151 3 500 1-3/2-3 220 165 4 000 1-2/2-2 250 178 4 500 1-1/2-1 325 220 6 000
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