Sedimentary environment of the Lower Cambrian Mufushan Formation in the Lower Yangtze region: Evidence from whole-rock geochemistry
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摘要: 下寒武统为扬子地块最重要的烃源岩层之一。然而下扬子地区在该层位尚未取得工业油气突破，开展下扬子地区早寒武世幕府山组沉积环境研究，对比分析扬子地块这一时期不同地区的油气地质条件具有重要的意义。下扬子盱眙地区实施了官地1井，获得了下寒武统幕府山组全取心样品。该井地化分析结果显示，在幕府山组钙质/炭质泥岩样品中，大多数大离子亲石元素展示富集特征，而高场强元素及过渡元素显示出亏损特征；幕府山组泥岩样品中稀土元素总量为14.81～107.47 μg/g；样品的化学风化指数CIA在64.84至78.81之间变化，结合A-CN-K三角图解，表明幕府山组源区受到中等强度的风化作用；微量元素的富集特征表明幕府山组泥岩沉积于潮湿且低盐度环境，氧化还原敏感参数比值指示幕府山组沉积于硫化还原环境。在Th/Sc-Zr/Sc图解中，大多数样品投在玄武岩与长英质火山岩区间，表明并未经历沉积循环。在Al2O3/TiO2 与 TiO2/Zr图解中，幕府山组泥岩样品指示出中酸性岩浆物源。与此同时，Cr/V与La/Th-Hf图解表明幕府山组泥岩物源主体为中酸性岩石。Abstract: The Mufushan Formation of Lower Cambrian is the most significant hydrocarbon source rock for shale gas in the Yangtze Platform. No exploration breakthrough has been achieved so far in the Lower Yangtze area, compared to the Middle and Upper Yangtze areas. Recently, the well of GD-1 has been completed, for which the Early Cambrian Mufushan (MFS) Formation is completelyly cored. Geochemistry of calcareous/carbonaceous mudstone of Early Cambrian Mufushan (MFS) Formation are carefully investigated for paleo-environment, provenance and tectonic settings. The samples of MFS are characterized by enriched large ion lithophile elements and depleted high field strength elements and transition elements. The analysis results show that the total REE concentrations of MFS mudstones vary from 14.81 to 107.47 ug/g. The Chemical Index of Alteration (CIA) ranges from 64.84 to 78.81. And the A-CN-K plot indicate that the source rocks has undergone a moderate weathering. In the Th/Sc versus Zr/Sc plot, most samples are located in the area between basalt and felsic igneous rocks, with negligible sedimentary recycling. Both the Al2O3/TiO2 ratios and TiO2/Zr ratios indicate an intermediate-felsic igneous provenance. The Cr/V ratios and La/Th-Hf diagrams also suggest that most of the materials are derived from intermediate-felsic rocks.
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Keywords:
- sedimentary enviroment /
- element geochemistry /
- Mufushan Formation /
- Lower Yangtze /
- Early Cambrian
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致密油因其平面分布范围广、资源储量大而成为现今全球非常规石油勘探开发的重要领域[1-5]。致密油的概念最早是用以描述致密砂岩中的石油[6]。随着勘探技术的进步，在很长一段时间内致密油被定义为以吸附或游离状态赋存于生油岩中，或与生油岩互层、紧邻的致密砂岩、致密碳酸盐岩等储集岩中，未经过大规模长距离运移的石油聚集[1]。这一定义包含了页岩油与致密岩油的含义，近几年随着对页岩油的成功勘探与开发，为了区别页岩油与致密油，逐渐将致密油定义为“以吸附或游离状态赋存于紧邻优质生油层系的致密储层中，经短距离运移而形成的石油聚集”[7]。致密油储层是指孔隙度一般小于10%、渗透率小于1×10−3 µm2的致密砂岩、碳酸盐岩等。
鄂尔多斯盆地上三叠统延长组长7段沉积时期发育大型内陆凹陷湖盆[8-10]，沉积了一套以黑色页岩和暗色泥岩为主的富有机质生油岩系，为盆地延长组油藏最主要的烃源岩[11-13]。长期以来，研究区延长组的石油勘探主要集中于长8段、长6段等地层，长7段的勘探程度较低。随着近几年非常规油气勘探的不断投入和页岩油的突破，长7段逐渐成为勘探重点[14-18]。目前，研究区长7段主要集中于沉积相和烃源岩研究，对致密油储层的研究相对较少，特别是对源内薄层致密砂岩储层微观特征研究不足，对致密油储层发育的控制因素认识不清，制约着致密油的进一步勘探与开发。本文综合利用铸体薄片、扫面电镜观察和能谱分析以及高压压汞孔喉结构分析、XRD黏土矿物分析等方法，分析了长7段薄层致密砂岩储层特征及储层发育的主控因素，为鄂尔多斯盆地致密油的勘探开发提供依据。
1. 区域地质特征
鄂尔多斯盆地为准克拉通盆地，可划分为伊盟隆起、西缘逆冲带、天环坳陷、伊陕斜坡、晋西挠褶带和渭北隆起6个一级构造单元，陕北地区位于伊陕斜坡中东部（图1）。晚三叠世开始，鄂尔多斯盆地沉积演化进入内陆差异沉降盆地的形成和发展时期[19]，到晚三叠世末期，盆地整体沉降，整体构造活动微弱，地层产状平缓，研究区内为一向西微倾的单斜构造。
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图 1 鄂尔多斯盆地构造特征与研究区位置Figure 1. Structural characteristics of the Ordos Basin and location map of the study area上三叠统延长组为河流–湖泊相沉积，发育一套中厚层的中细砂岩、粉砂岩和深灰色、灰黑色泥页岩。根据岩电特征及含油性差异，延长组自下而上被划分为10个油层组（长1—长10），其中长7油层组沉积时期为湖盆鼎盛时期，发育一套厚度相对稳定、富含有机质的泥页岩层，为延长组油藏的主力烃源岩，也是鄂尔多斯盆地页岩油富集的最主要层段。
2. 致密油储层特征
岩石薄片鉴定结果显示，陕北地区长7段致密砂岩碎屑以石英、长石为主，少量岩屑，云母较发育（图2）。镜下观察石英表面光洁，部分晶面具波状消光。长石以斜长石、钾长石为主，斜长石聚片双晶发育，蚀变深，泥化、绢云母化。岩屑以酸性喷出岩、石英岩、花岗质岩、泥化碎屑为主。云母以黑云母为主，部分蚀变深，绿泥石化。粒间泥质以绿泥石、伊利石为主，重结晶，绢云母化，部分呈条带状分布。少量泥铁质，局部富集。经X射线衍射全岩分析，陕北地区长7段致密砂岩储层矿物组成主要为长石、石英、黏土矿物、方解石、白云石以及少量的黄铁矿、红金石和菱铁矿，其中长石含量最高（斜长石平均含量为45.92%，钾长石平均含量为13%），其次为石英（平均含量为18.7%）与黏土矿物（平均含量为15.9%），方解石平均含量为4.21%，白云石平均含量为1.67%。通过薄片鉴定和XRD全岩分析，明确研究区长7段致密砂岩类型主要为长石砂岩与岩屑长石砂岩。
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图 2 陕北地区长7段致密砂岩岩石学特征a. 岩石薄片特征（桥136井，1583.5 m，+），b. 岩石类型三角图，c. XRD全岩矿物分析（桥136井，1583.5 m）。Figure 2. Petrological characteristics of tight sandstone in Chang 7 member in northern Shaanxia. Characteristics of rock slices (Well Qiao 136, 1583.5 m, +), b. Triangle map of rock types, c. XRD analysis of whole rock minerals (Well Qiao 136, 1583.5 m).3. 储集特征
3.1 储层物性特征
本次研究对陕北地区长7段砂岩储层样品进行了物性测试，选取了15口井184件柱塞样。岩心取样的孔隙度、渗透率测定结果显示，陕北地区长7段致密砂岩储层较为致密，物性较差，其中孔隙度为2%～17%，平均9.98%，集中分布于8%～14%；渗透率分布范围（0.001～1.486）×10−3 µm2，平均0.54×10−3 µm2，主要分布于（0.01～0.5）×10−3 µm2（图3）。其中孔隙度2%～6%的主要为粉砂岩，占7.8%，孔隙度6%～14%的主要为中细砂岩，占82%；剩余部分样品存在微裂缝，孔隙度较大，占10.8%。
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图 3 长7段致密储层孔隙度、渗透率柱状图Figure 3. Histogram of porosity and permeability of tight reservoirs in Chang 7 Member3.2 储集空间特征
通过30件铸体薄片和24件扫描电镜样品分析，陕北地区长7段致密砂岩主要发育次生孔隙及部分原生剩余孔隙。次生孔隙类型主要是粒间溶蚀孔隙、粒内溶蚀孔隙、填隙物晶间微孔隙及微裂缝（图4、5）。其中以长石溶蚀孔隙（图4a）、晶间微孔和微裂缝（图4c、5d）最为发育。长石溶蚀孔隙多呈狭长状（图5a），部分长石溶蚀孔隙呈圆状（图4b），通过图像测量长石溶蚀孔径分布范围为6～180 µm。粒间溶蚀孔隙主要是长石边界及填隙物溶蚀孔隙，孔隙多呈不规则形（图4a、5b）。扫描电镜图像中粒间溶蚀孔隙多呈不规则圆形（图5b），孔径较大，分布范围为10～240 µm。由于晶间微孔隙非常小，铸体薄片中较难分辨，主要通过扫描电镜分析来研究。晶间微孔类型主要为黏土矿物晶间孔（图5c），包括绿泥石晶间孔、伊利石晶间孔及高岭石晶间孔，孔径多小于20 µm。
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图 4 铸体薄片显示延长组7段致密储层储集空间特征a. 高135井，1783.6 m，长石溶蚀孔隙，粒间溶蚀孔隙（−）；b. 高193井，2117 m，长石溶蚀孔隙，粒间溶蚀孔隙，铸膜孔（−）；c. 丹288井，1106.8 m，微裂缝（+）；d. 丹228井，1143.14 m，微裂缝（−）。Figure 4. Cast thin sections show the reservoir space characteristics of tight reservoirs in the 7th member of the Yanchang Formationa. Well Gao 135, 1783.6 m, feldspar dissolution pores, intergranular dissolution pores (−); b. Gao 193 well, 2117 m, feldspar dissolution pores, intergranular dissolution pores, cast film pores (−); c. Dan Well 288, 1106.8 m, micro-fractures (+); d. Dan 228 well, 1143.14 m, micro-fractures (−).![]()
图 5 扫描电镜显示延长组7段致密储层储集空间特征a. 午100井，1937.5 m，长石粒内溶蚀孔隙，原生剩余粒间孔隙；b. 桥136井，1578.25 m，粒间溶蚀孔隙；c. 顺37井，1915.25 m，晶间微孔；d. 顺37井，1919.25 m，微裂缝。Figure 5. SEM shows the reservoir space characteristics of tight reservoirs in the 7th member of the Yanchang Formationa. Well Wu 100, 1937.5 m, intragranular dissolution pores of feldspar, primary remaining intergranular pores; b. Qiao 136 well, 1578.25 m, intergranular dissolution pores; c. Shun 37 well, 1915.25 m, intergranular micropores; d. Well Shun 37, 1919.25 m, micro-fractures.3.3 孔喉结构特征
通过10口井不同深度的致密砂岩样品进行高压压汞测试，根据毛管压力曲线特征、孔喉分布特征将长7段致密砂岩储层孔喉结构划分为4类：
Ⅰ类孔喉结构：砂岩类型主要为中砂岩与细砂岩，毛管曲线多出现左下凹的平台。排驱压力<1 MPa，图6a为高135井1783.6 m最大连通孔喉半径为0.756 µm，平均孔喉半径为0.05 µm，最大汞饱和度为98.26%，歪度为0.73，为粗歪度，表明孔喉以相对较大孔喉为主。
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图 6 长7段压汞曲线特征及孔喉半径分布a. 高135井，1783.6 m；b. 午230井，2061.3 m；c. 新140井，2080.4 m；d. 塞544井，2147.85 m。Figure 6. Characteristics of mercury intrusion curve and pore throat radius distribution in Chang 7 sectiona. Gao 135 well, 1783.6 m; b. Wu 230 well, 2061.3 m; c. Xin 140 well, 2080.4 m; d. Sai 544 well, 2147.85 m.Ⅱ类孔喉结构：砂岩类型主要为细砂岩，与Ⅰ类毛管压力曲线不同，没出现平台。排驱压力较Ⅰ类大，集中于1～3 MPa。图6b为午230井2061.3 m致密砂岩高压压汞曲线与孔喉半径分布图，其排驱压力为1.24 MPa，最大连通孔喉半径为0.593 µm，平均孔喉半径为0.015 µm，最大汞饱和度为91.01%，歪度为−0.59，为细歪度，表明孔喉以相对较小孔喉为主。
Ⅲ类孔喉结构：砂岩类型主要为细砂岩，曲线形态出现较明显的右上凸的形态。图6c为新140井2080.4 m深度砂岩样品毛管压力曲线与孔喉半径分布图，其排驱压力为4.71 MPa，最大连通孔喉半径为0.058 µm，平均孔喉半径为0.01 µm，最大汞饱和度为80.98%，歪度为−0.88，为细歪度，表明孔喉以相对较小孔喉为主。
Ⅳ类孔喉结构：砂岩类型多为细砂岩与粉砂岩。图6d为塞544井2147.85 m深度致密砂岩样品毛管压力曲线与孔喉半径分布图。其排驱压力为4.95 MPa，最大连通孔喉半径为0.148 µm，平均孔喉半径为0.03 µm，最大汞饱和度为70.15%，歪度为−0.49，为细歪度，表明孔喉以相对较小孔喉为主。
4. 砂岩储层成岩作用
综合利用铸体薄片、扫描电镜、XRD黏土矿物分析等资料，明确研究区长7段致密砂岩储层地质历史时期埋深较大，并且经历了复杂的成岩作用，主要有压实作用、胶结作用、溶蚀作用等。
4.1 压实作用
研究区长7段现今埋藏深度为1500～2100 m，通过埋藏史研究，长7段砂岩经历过3000 m的埋深，压实作用较为发育。在整个成岩过程中，随着埋深的增加，压实作用变强，碎屑矿物颗粒接触从点接触向点–线接触、线–线接触及缝合线接触过渡，可见碎屑颗粒接触关系有点–线接触、线–线接触和凹凸接触（图7a），以线接触为主，少见缝合线接触，云母等塑性矿物发生挤压变形（图7b）。
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图 7 长7段致密砂岩储层压实作用典型特征Figure 7. Typical characteristics of tight sandstone reservoir compaction in Chang 7 Member4.2 胶结作用
通过扫描电镜观察和XRD矿物分析，研究区长7段致密砂岩储层中胶结作用主要为黏土矿物胶结、碳酸盐胶结和硅质胶结。其中以黏土矿物胶结和碳酸盐胶结为主，硅质胶结作用相对较弱。
（1）黏土胶结
黏土矿物XRD分析，长7段致密砂岩中自生黏土矿物主要为绿泥石（平均含量为43.54%），其次为伊利石（平均含量20.42%）、高岭石（18.17%）及伊蒙混层（17.88%）（图8）。
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图 8 长7段致密砂岩储层黏土矿物相对含量Figure 8. Relative content of clay minerals in tight sandstone reservoirs of Chang 7 Member绿泥石胶结物多为树叶状和针叶状，主要有两种赋存方式：一种是孔隙充填式产出（图9a），一种是围绕颗粒成薄膜式（图9b）。高岭石通常呈假六边形树叶状，伊利石多呈不规则片状或网状集合体产出（图9c），多以孔隙式充填为主。
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图 9 午230井长7段致密砂岩黏土矿物和硅质胶结物特征a. 塞544井，2142.83 m，绿泥石薄膜，孔隙填充绿泥石；b. 塞544井，孔隙充填高岭石；c. 午230井，2018.39 m，碳酸盐胶结物，伊利石；d. 高135井，1971.60 m，石英加大（Ⅱ-Ⅲ级）。Figure 9. Characteristics of clay minerals and siliceous cements of tight sandstone in Chang 7 Member of Well Wu 230a. Well Sai 544, 2142.83 m, chlorite film, pores filled with chlorite; b. Well Sai 544, pores filled with kaolinite; c. Well Wu 230, 2018.39 m, carbonate cement, illite; d. Well Gao 135, 1971.60 m, increased quartz (grade II-III).（2）碳酸盐胶结
长7段碳酸盐胶结普遍发育，以方解石为主（最高可达28%），另外发育铁方解石、铁白云石和白云石等碳酸盐胶结物。长7段碳酸盐胶结物主要为早期方解石连晶胶结，充填于孔隙中。晚期碳酸盐胶结物主要呈半自形到自形晶（图9c）充填孔隙，并交代碎屑颗粒。碳酸盐胶结物为方解石胶结，且主要为连晶胶结，表明碳酸盐胶结为早成岩阶段产物。
（3）硅质胶结
长7段致密砂岩硅质胶结物含量平均为1.6%，硅质胶结物多以Ⅰ—Ⅱ级石英的次生加大边和自生石英颗粒两种类型。在铸体薄片下，原生石英颗粒边界清晰（图4c，图7b），与次生石英边界之间通常为绿泥石或高岭石等黏土薄膜，次生石英加大边一般发育在原生颗粒局部，少见环边石英次生加大。
4.3 溶蚀作用
研究区长7段溶蚀作用主要为碎屑颗粒的溶蚀作用和填隙物的溶蚀作用，其中长石溶蚀最为发育。长石溶蚀主要是沿着长石解理缝溶蚀，形成长石粒内溶蚀孔隙（图4a、5a），长石和岩屑被彻底溶蚀后形成铸膜孔（图4b）。粒间不稳定填隙物发生部分溶蚀，从而产生粒间孔（图4a、4b、5b）。
4.4 成岩演化序列
基于陕北地区延长组长7段砂岩的骨架颗粒接触关系、孔隙结构、自生黏土矿物组合及泥岩镜质体反射率特征，划分了研究区长7段砂岩成岩阶段。砂岩骨架颗粒多呈线接触，压实作用较强；孔隙类型以粒间溶孔、粒内溶孔和黏土矿物晶间微孔为主；胶结作用以黏土胶结和碳酸盐胶结为主，黏土矿物以绿泥石为主，黏土矿物分析时伊蒙混层I/S中S层为30%，部分为40%，石英发育级次生加大；泥岩镜质体反射率Ro为0.65%～1.27%。据此判断研究区延长组长7段砂岩储层主要处于中成岩阶段A期（图10）。
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图 10 陕北地区延长组长7段成岩演化序列Figure 10. Diagenetic evolution sequence of Chang 7 member of Yanchang Formation in Northern Shaanxi早成岩A期，古地温较低，有机质未成熟，以机械压实作用为主，伴有早期绿泥石以薄膜状出现，少量方解石胶结物产出。早成岩B期，古地温为65～85 ℃，随着埋深的不断增加，强烈压实使颗粒呈点-线接触，部分亚溶作用为硅质胶结提供物质，石英加大为Ⅰ级加大，此时Ro为0.35%～0.5%，有机质半成熟，流体为弱酸性，长石开始发生溶蚀，孔隙类型为残余粒间孔和次生溶孔。中成岩A期，埋深进一步加大，古地温达到85～130 ℃，有机质处于大量生烃，使孔隙水呈酸性，溶蚀作用强，同时蒙脱石向伊利石快速转化，此时发育Ⅱ级石英加大，孔隙类型主要发育次生孔隙。
5. 储层主控因素
5.1 沉积作用对储层物性的影响
沉积作用对储层的影响主要体现在储层原始矿物组成及储层结构上，不同沉积环境中因水动力条件、搬运距离等的差异，使得沉积的砂岩成分、粒度、分选及磨圆条件存在差异[19-21]。陕北地区长7段主要为三角洲前缘水下分流河道及河口坝砂体，局部地区发育三角洲前缘滑塌形成的浊积岩[8-10]。研究区长7段物源主要受东北物源控制，仅东南局部受东南物源影响，三角洲前缘–滨浅湖沉积环境，水动力较弱，粒度一般为0.02～0.65 mm，分选磨圆较差，杂基含量相对较高。长7段砂岩主要为中砂岩、细砂岩和粉砂岩，其中中细长石砂岩、岩屑长石砂岩粒度相对较粗，分选较好，磨圆度为次棱–次圆，成分成熟度和结构成熟度较高，物性较好，粉砂粒长石砂岩、岩屑长石砂岩粒度较细，分选磨圆较差，杂基含量高，物性较差（图11）。
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图 11 长7段砂岩典型特征a. 陕365井，1893.7 m，灰色细砂岩，交错层理；b. 陕365井，1880.2 m，灰白色细砂岩，沙纹层理；c. 新271井，1991.8 m，灰白色细砂岩，爬升沙纹层理；d. 新283井，1993.8 m，灰白色细砂岩，板状交错层理； e. 新324井，1849.7 m，褐灰色细砂岩，交错层理；f. 新271井，1989.1 m，灰色细砂岩，平行层理；g. 新283井，1990.85 m，冲刷面；h. 午100井，1941.57 m，灰白色细砂岩，平行层理；i. 灰色细砂岩，块状层理，突变接触。Figure 11. Typical sedimentary characteristics of sandstone in Chang 7 Membera. Shan 365 well, 1893.7 m, gray fine sandstone, cross bedding; b. Shan 365 well, 1880.2 m, gray white fine sandstone, sand grain bedding; c. Xin 271 well, 1991.8 m, gray white fine sandstone, climbing sand Laminate bedding; d. Well Xin 283, 1993.8 m, gray-white fine sandstone, plate cross bedding; e. Well Xin 324, 1849.7 m, brown-gray fine sandstone, cross bedding; f. Well Xin 271, 1989.1 m, Gray fine sandstone, parallel bedding; g. Well Xin 283, 1990.85 m, scour surface; h. Wu 100 well, 1941.57 m, gray fine sandstone, parallel bedding; i. Gray fine sandstone, massive bedding, abrupt contact.5.2 成岩作用对储层的控制作用
成岩作用对储层发育的控制作用主要体现在两个方面：一是破坏性成岩作用，主要是压实作用、胶结作用；另一个是建设性成岩作用，主要是溶蚀作用。
（1）压实作用对储层物性的影响
根据Schrer [22]提出的砂岩初始孔隙度恢复方法，估算了研究区长7段初始孔隙度。研究区长7段砂岩分选系数平均为1.64，恢复初始孔隙度平均为35%。长7段砂岩虽然岩屑含量少，但泥质含量高，粒度较细，分选较好，压实作用对孔隙减少作用强。压实作用不仅是颗粒发生旋转排列，同时会引起塑性矿物变形，占据孔隙，堵塞喉道，进一步使储层物性变差。根据镜下观察与压实率计算，压实作用对原始孔隙的平均减孔量约为18%（图12）。
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图 12 压实作用与胶结作用造成孔隙度损失量Figure 12. Calculation of porosity loss due to compaction and cementation（2）胶结作用对储层物性的影响
研究区长7段主要发育黏土矿物胶结和碳酸盐胶结（图9a，9c）。黏土胶结矿物多填充孔隙和堵塞喉道，使储层更加致密。但是早期绿泥石薄膜对孔隙具有保护作用，一方面增加了岩石的抗压实能力，另一方面有效阻止了石英自生加大，对剩余原生粒间孔隙起到了一定程度的保护作用。压溶作用产生的硅质在孔隙内胶结，形成石英加大边，减少孔隙（图9d）。早期方解石胶结和中成岩A阶段溶蚀作用形成的物质重新胶结形成的铁方解石等进一步充填孔隙。研究区长7段早期碳酸盐胶结及晚期原始孔隙和溶蚀孔隙的充填，进一步降低了储层物性，对储层起到破坏作用。黏土矿物胶结与碳酸盐胶结使储层原生孔隙消失殆尽，整体上胶结作用是储层致密的重要原因，对储层起到破坏作用。
（3）溶蚀作用
研究区长7段致密砂岩现今孔隙主要为溶蚀作用形成的粒间溶蚀孔隙、粒内溶蚀孔。研究区长7段长石含量较高，随着地层的埋深，烃源岩逐渐成熟，大量排烃，成岩环境变为酸性，使长石和岩屑发生溶蚀，形成溶蚀孔隙，物性变好，是长7段储层发育最为主要的建设性成岩作用。
6. 结论
（1）鄂尔多斯盆地陕北地区长7段致密砂岩长石含量较高，砂岩以长石砂岩和岩屑长石砂岩为主，砂岩黏土矿物含量高，其中绿泥石含量最高，其次为伊利石、高岭石及伊蒙混层。
（2）长7段致密砂岩储层次生孔隙类型主要是粒间溶蚀孔隙、粒内溶蚀孔隙、填隙物晶间微孔隙及微裂缝。根据毛管压力曲线特征、孔喉分布特征将长7段致密砂岩储层孔喉结构划分为4类，其中Ⅰ类和Ⅱ类储层物性较好。
（3）长7段砂岩储层经历了压实作用、胶结作用（黏土矿物胶结、硅质胶结和碳酸盐胶结）和溶蚀作用等复杂的成岩改造，其中压实作用和胶结作用使储层孔隙减小，降低储层质量，溶蚀作用让储层质量得到改善，是长7段有利储层形成的主要原因。
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图 2 官地1井岩性柱状图、总有机碳含量及典型岩心照片
Figure 2. Lithostratigraphic column, TOC and typical core image of Guandi-1 well
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图 4 幕府山组泥岩样品中微量元素与Al2O3相关性分析
Figure 4. Correlation analysis between trace elements and Al2O3 in the Mufushan mudstones
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图 5 幕府山组泥岩样品典型微量元素相对平均大陆上地壳组成的富集情况
Figure 5. Enrichment of typical trace elements compared to UCC
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图 6 幕府山组泥岩样品稀土元素富集特征（球粒陨石标准化）
Figure 6. Enrichment of REE in the Mufushan mudstones (Chondrite normalization)
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