Borfixierung in authigenen und detritischen Tonen

Clay minerals synthesized under surface conditions take up boron in proportion to the concentration of boron in the solution. A synthetic montmorillonite contained 2160 ppm B when precipitated from a solution containing 5 ppm B and having a pH of 8.5. The fixation of B by the synthetic clay mineral is greatly reduced by a high ^?HCO₃ concentration in the solution. Part of the fixed boron is incorporated into the tetrahedral sheet of the clay mineral structure, whereas the remainder is present in the form of a Mg borate-like complex. Mg borate-like complexes similar to those found in the synthetic clay may also occur in detrital clays. This possibility was examined experimentally using natural illite and montmorillonite suspended in solutions containing boron alone and magnesium-boron in combination at pH 8–9 and 10. The results show that more B was sorbed from solutions containing both Mg and B than from solutions containing B alone. For example, at pH 8, montmorillonite takes up 18 ppm B from the combination solution and 11 ppm from a solution containing only B. In a similar experiment, illite takes up 28 and 20 ppm B, respectively. It is suggested that B in natural clays is fixed partly as a Mg-borate-like complex. This complex probably formed as a first step by sorption of B to Mg(OH)₂, which may be present in clays.These results should apply to the interpretation of the boron content of recent clayey sediments     

台湾海峡表层沉积物中黏土矿物特征及物质来源

利用X射线衍射仪,对比分析台湾海峡及周边表层沉积物的黏土矿物特征,讨论长江、福建和台湾河流对海峡黏土矿物的贡献程度。福建河流（闽江）的黏土矿物以高岭石为主（>40％）,其次是伊利石（30％）和绿泥石,不含蒙脱石;长江和浙闽泥质区的黏土矿物特征是伊利石含量最高（61％～66％）,其次是绿泥石（16％～17％）和高岭石（10％～13％）,少量蒙脱石（6％～9％）。台湾河流（浊水溪）黏土矿物组合为伊利石（>70％）和绿泥石,不含高岭石和蒙脱石为特征。台湾海峡高岭石含量由西向东明显减少,说明高岭石主要来自福建的河流。〖JP2〗台湾海峡50 m等深线以东区域的黏土矿物具有高的伊利石和绿泥石含量,低的蒙脱石和高岭石含量,且伊利石的化学指数和结晶度值都明显低于长江和福建的河流,反映台湾河流的物质来源。在台湾暖流作用下,台湾河流为台湾海峡中北部提供了大量的伊利石和绿泥石。福建近海的黏土矿物主要是由福建河流提供的,但在平潭岛周边海域的黏土矿物可能有长江物质的混合。〖JP〗     

松辽盆地扶新隆起带北部扶余油层超低渗储层 粘土矿物特征及其对敏感性的影响

以储层敏感性流动实验为基础,结合X射线衍射、扫描电镜、铸体薄片等方法,对松辽盆地扶新隆起带北部扶余油层超低渗储层粘土矿物特征及其对储层敏感性的影响进行了研究,结果表明:扶余油层主要含有绿泥石、伊利石、高岭石和伊/蒙混层等4种矿物,其中伊利石含量最多,绿泥石和伊/蒙混层次之,高岭石最少。储层速敏性和碱敏性相对较弱,水敏性和盐敏性较强,对储层影响较大,勘探开发时应重点关注。粘土矿物对储层水敏性的影响主要有3个方面,其中粘土矿物绝对总量是扶余油层水敏性强弱的主控因素。     

Characteristics and distribution of clay minerals and their effects on reservoir quality: Huagang Formation in the Xihu Sag,East China Sea Basin

Abstract

The characteristics and distribution of clay minerals and their effects on reservoir quality in the Huagang sandstones in the Xihu Sag, East China Sea Basin were studied by using X-ray diffraction, casting thin-sections, scanning electron microscopy, electron microprobe analysis, fluid inclusion analysis, constant-rate mercury injection and nuclear magnetic resonance. Clay minerals consist of kaolinite, chlorite, illite and illite–smectite mixed layer (I/S); kaolinite forms from dissolved feldspars, chlorite occurs as clay coatings that are transformed from clay precursors owing to the flocculation of suspended detrital clays or the crystallisation of pore fluids, and illite forms from the illitisation of detrital smectite, authigenic kaolinite and K-feldspars. Clay distribution is controlled by sedimentary environments, burial history and lithologies. Typical reservoirs in the western sub-sag are thin and developed in braided river facies at relatively shallow burial depths with clays dominated by kaolinite. However, typical reservoirs in the central inversion tectonic zone are thicker and developed in a braided delta front facies at deeper burial depths with clays mainly consisting of chlorite, illite and I/S. High-quality reservoirs are characterised by coarse granularity, high quartz content and low clay content with widespread development of chlorite coatings that inhibit quartz cements at low temperatures. At higher temperatures, the high-quality reservoirs develop more pores providing growth space for quartz cements and result in the coexistence of chlorite coatings and quartz cements. The high-quality reservoirs are controlled by their lithological characteristics rather than chlorite coatings. Illite and I/S clays create severe damage to reservoirs by reducing the size and connectivity of pore-throats.     

Influence of tectonics and palaeoenvironment on late cretaceous clay sedimentation in the upper benue trough,Nigeria

The Late Cretaceous sedimentary sequence in the Upper Benue Trough consists of seven formations. Outcrop samples from the type localities of these units were analysed by X-ray diffraction for their day mineral content. The composition and distribution of the clay minerals are influenced by tectonics, relief and water depth. Three main depositional phases are recognizable on the basis of clay mineral assemblages, two of which are associated with tectonic movements. The first phase, characterized by the presence of chlorite, illite, kaolinite and mixed-layer clays is common to Albian-Cenomanian sediments. This phase is succeeded by mixed-layer clays and smectite-rich, Turonian-Early Santonian sediments. The re-occurrence of chlorite and illite in association with kaolinite and mixed-layer clays in the Post-Santonian sediments constitutes the third depositional phase. These clay mineral assemblages are indicative of two periods of tectonism (Albian and Late Santonian) and an intervening Turonian-Early Santonian period of quiescence in the Upper Benue Trough.     

The Speeton Clay of North Yorkshire, England: An investigation of its geotechnical properties

The Lower Cretaceous in North Yorkshire is represented by a marine succession termed the Speeton Clay. The principal minerals in these clays consist of quartz, mica and clay minerals, although pyrite, glauconite and siderite are notable in some beds. The proportions of clay minerals change within the Speeton Clay and generally illite becomes more important in the younger horizons.

The Speeton Clay is a silty clay which generally has a high plasticity, the latter being influenced by the proportion of clay fraction present. All the clays tested were inactive although they would appear to have a potential for expansion. Neither the plasticity nor the moisture content were affected by depth.

In terms of its undrained shear strength, most of the Speeton Clay could be regarded as a stiff clay. It was also insensitive with generally a low slake-durability index. Its coefficients of volume compressibility were characteristic of overconsolidated and heavily overconsolidated days.     

鄂尔多斯盆地东缘C-P煤系致密砂岩储层敏感性分析

鄂尔多斯盆地临兴地区上古生界致密砂岩储集层中矿物组成多样、孔隙结构复杂且黏土矿物含量高,直接影响储层改造和开发效果。文章基于X 衍射、铸体薄片、气测孔渗、压汞和敏感性实验,系统研究了储层敏感性及其影响因素。结果表明研究区砂岩中石英和岩屑含量高,长石含量较低,以岩屑砂岩、岩屑石英砂岩、长石岩屑砂岩和石英砂岩为主。黏土矿物主要为伊利石、高岭石、绿泥石以及伊/蒙混层;储层普遍低孔低渗,孔隙结构较差。速敏以太原组最强,山西组最弱,与伊利石+绿泥石含量正相关,高岭石含量负相关。水敏下石盒子组最强,太原组最弱,与伊/蒙混层含量正相关。盐敏与水敏有类似特点,与伊/蒙混层含量表现出正相关。酸敏山西组最强,下石盒子组最弱,与绿泥石和铁白云石矿物含量正相关。碱敏性山西组最强,太原组最弱,受长石、石英和高岭石含量影响。相关认识有助于指导研究区钻井、压裂等施工工艺选择和排采控制。     

鸡西盆地下白垩统城子河组-穆棱组露头砂岩中的黏土矿物成因探讨

对鸡西盆地下白垩统城子河组—穆棱组露头砂岩中的黏土矿物进行了X-衍射分析。研究结果表明,黏土矿物主要由伊利石,高岭石和伊/蒙混层组成,组合可分为高岭石型与伊/蒙混层和伊利石型两种,根据黏土矿物组合判断露头砂岩处于中成岩阶段A期。下白垩统城子河组—穆棱组煤系地层和泥岩成岩过程中析出的有机酸是高岭石型黏土矿物组合发育的重要原因,而沉积相带水动力条件弱导致砂岩渗滤条件的变差则是伊/蒙混层和伊利石型黏土矿物组合形成的原因。     

超深层砂岩粘土矿物特征及其对储层物性的定量控制作用——以库车前陆冲断带白垩系巴什基奇克组为例

库车前陆冲断带超深层储层是当前天然气勘探开发的重要领域, 具有特低孔、特低渗-低渗, 裂缝-孔隙型, 测试高产储集特征。 为揭示储层中粘土矿物的特征及其对储层演化、储层性质的控制作用, 本文以大量微观实验分析（X 衍射、扫描电镜、激光共聚焦扫描、 铸体薄片、高压压汞）为依据, 研究表明：主要的粘土矿物含量为 5%～12%, 矿物类型以伊利石、伊蒙混层为主, 其次为绿泥石, 成因以成岩早期沉积水体继承性沉淀及成岩中晚期长石质次生溶蚀产物为主;砂岩中早期沉积型粘土矿物主要富集于三角洲前缘水下河道砂体的侧缘、水道砂体的顶底面附近, 其形成粘土薄膜可有效抑制超深层储层早中成岩期硅质、长石质次生加大, 保存基质孔隙, 不同期粘土杂基也可提供大量矿物晶间微孔隙, 微孔隙度可达 1%～4%, 是天然气的主要储集空间之一;粘土矿物是降低储层渗透率的主控因素, 6%～ 9%的粘土矿物含量（尤其是伊利石及伊蒙混层）可降低超深层储层渗透率 10～100 倍, 使渗透率总体为 0.01×10 ^-3 μm ² ～0.1×10 ^-3 μm ² 。 粘土矿物含量及赋存特征：早期长期（130～23 Ma）浅 埋（< 3 000 m）期储集性能保存良好, 晚期（23 Ma 左右）深埋（> 3 000 m）期基质渗透性急剧降低, 总体致密化（< 0.1×10 ^-3 μm ² ）, 这是研究区的关键因素。 粘土矿物的特征及其分布对评价砂岩储集性质, 预测超深层天然气有利储层的分布具有重要地质意义。     

Transport of U in the Opalinus Clay on centimetre to decimetre scales

The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of ²³⁸U, ²³⁴U and ²³⁰Th was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim was to assess the in situ, long-term migration behaviour of ²³⁴U in this rock. Very low hydraulic conductivities of the Opalinus Clay, reducing potential of the pore water and its chemical equilibrium with the host rock are expected to render both ²³⁸U and ²³⁰Th immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of ²³⁴U from the rock matrix to the pore water by the decay of ²³⁸U will be established. Diffusive redistribution separates ²³⁴U from its immobile parent ²³⁸U resulting in bulk rock ²³⁴U/²³⁸U activity disequilibria. These may provide a means of estimating the mobility of ²³⁴U in the rock if the diffusion rate of ²³⁴U is significant compared to its decay rate. Sampling was carried out on two scales. Drilling of cm-spaced samples from the drill-core was done to study mobility over short distances and elucidate possible small-scale lithological control. Homogenized 25-cm-long portions of a 2-m-long drill-core section were prepared to provide information on transport over a longer distance. Variations in U and/or Th content on the cm-scale between clays and carbonate-sandy layers are revealed by β-scanning, which shows that the (dominant) clay is richer in both elements.     

兰州盆地永登剖面晚古新世—早渐新世沉积物中粘土矿物的特征及其古气候指示意义

采用X射线衍射、扫描电镜分析方法,对兰州盆地永登剖面晚古新世—早渐新世沉积物中粘土矿物的相对含量、伊利石结晶度、显微形貌及其古气候意义进行深入研究。结果表明:晚古新世—早渐新世沉积物中粘土矿物主要为伊利石、蒙脱石、绿泥石和少量的坡缕石,其中伊利石的含量最高,而且从晚古新世到早渐新世,沉积物中伊利石的含量呈明显升高的趋势,蒙脱石则呈明显减少的趋势;在大约55 Ma处伊利石含量急剧减小,而蒙脱石含量急剧增大。此外,可见到毛发状的自生坡缕石沿蒙脱石晶片边缘交代生长,而碎屑成因的坡缕石则主要呈破碎的短棒状。粘土矿物分布特征表明,兰州盆地晚古新世—早渐新世的古气候以干旱为主导,并且经历了由相对温暖湿润到半干旱半湿润或干旱炎热的转变,这种气候转型主要受全球气候的影响,即受行星风系的控制;而在大约55 Ma处伊利石、蒙脱石的含量变化则记录了全球性的PETM事件。     

Mechanism of the Mudstone Tunnel Failures Induced by Expansive Clay Minerals

Expansive clay minerals can be a serious threat to underground rock structure because of their swelling behavior when absorbing water. Roof and wall collapse as well as large deformation were observed in Xiaotun Coal Mine, Guizhou Province, China. This paper studies the characteristics of expansive clays in order to understand the mechanism of the mudstone tunnel failures in the mine. The physical and mechanical properties of the tunnel surrounding rock samples, including mudstone, silty mudstone, argillaceous siltstone and fine sandstone were determined. X-ray diffraction analysis was conducted to determine the mineralogical composition while the scanning electron microscope was used to examine the internal structure of the different samples. The results indicate that the illite–smectite and the montmorillonite are the main minerals composing the rock samples. A series of micro-cracks and pores occur in the samples which suggest a good hydraulic conductivity. The results indicate that the mudstone has poorer mechanical properties in comparison to the fine sandstone. Also, the Flac3D numerical simulations were conducted and it was shown that the large deformations were consistent with the field observation due to weak mechanical properties of the surrounding rock under seepage action especially with the increase of water head and porosity. It is concluded that internal structure and mechanical strength of the mudstone is weakened by the illite–smectite and the montmorillonite mineral content as well as the combined action of underground water causing physical disintegration.     

The impact of diagenetic fluid–rock reactions on Rotliegend sandstone composition and petrophysical properties (Altmark area, central Germany)

In the framework of the German R&D joint project CLEAN (CO₂ large-scale enhanced gas recovery in the Altmark natural gas field), Rotliegend reservoir sandstones of the Altensalzwedel block in the Altmark area (Saxony-Anhalt, central Germany) have been studied to characterise litho- and diagenetic facies, mineral content, geochemical composition, and petrophysical properties. These sands have been deposited in a playa environment dominated by aeolian dunes, dry to wet sand flats and fluvial channel fills. The sediments exhibit distinct mineralogical, geochemical, and petrophysical features related to litho- and diagenetic facies types. In sandstones of the damp to wet sandflats, their pristine red colours are preserved and porosity and permeability are only low. Rocks of the aeolian environment and most of the channel fill deposits are preferentially bleached and exhibit moderate to high porosity and permeability. Although geochemical element whole rock content in these rocks is very similar, element correlations are different. Variations in porosity and permeability are mainly due to calcite and anhydrite dissolution and differences in clay coatings with Fe-bearing illitic-chloritic composition exposed to the pore space. Moreover, mineral dissolution patterns as well as compositions (of clays and carbonate) and morphotypes of authigenic minerals (chlorite, illite) are different in red and bleached rocks. Comparison of the geochemical composition and mineralogical features of diagenetically altered sandstones and samples exposed to CO₂-bearing fluids in laboratory batch experiments exhibit similar character. Experiments prove an increase in wettability and water binding capacity during CO₂ impact.     

准噶尔盆地南缘新生界粘土矿物分布及影响因素

根据粘土矿物的相对含量研究了准噶尔盆地南缘新生界砂岩粘土矿物类型、组合特征及纵、横向分布规律及其主要影响因素。划分出无序伊/蒙混层型、部分有序伊/蒙混层型、伊利石+伊/蒙混层型、蒙皂石型以及含坡缕石型等5类粘土矿物组合。按照伊/蒙混层相对含量的变化,粘土矿物纵向上演化呈正常转化型(伊/蒙混层相对含量降低)、反向转化型(伊/蒙混层相对含量增加)和“S”型(伊/蒙混层相对含量呈曲线变化)3种形式。平面上,伊/蒙混层和伊利石这两类主要粘土矿物从湖盆的边缘向中心分别呈现减少和增多的趋势,湖盆边缘相带以无序伊/蒙混层型、蒙皂石型和含坡缕石型为主,湖盆中心则为部分有序伊/蒙混层型和伊利石+伊/蒙混层型组合等类型。上述分布规律的控制因素主要有沉积环境、构造运动及层序发育等。     

粘土矿物钝化改性和钝化机理初步研究

在岩土工程中,进行灾害防治和软基处理的一个核心问题是把粘土矿物由亲水性改造成疏水性,抑制和降低其含水量,使得粘土遇水后不再吸水膨胀,从而保持土壤的稳定性。岩土力学工程中以保持土壤稳定性为目的对粘土的改性称为“钝化”。所谓钝化就是用 钝化剂与粘土矿物表面发生反应,改变粘土矿物的表面结构,提高憎水性,增加粘土整体的力学性质,从而达到稳定土体的目的。在粘土矿物钝化中表面活性剂是一个重要的组成部分 ,但是表面活性剂在固液界面的吸附,除决定于其本身的性质外,还决定于固体的性质、溶 液的pH值和温度等。因此,研究粘土矿物表面结构和性质以及与钝化剂的相互作用是提高粘 土矿物钝化效果的重要途经。利用近代分析方法（XRD、IR、Raman、MAS NMR、TEM、AFM和S EM）,对粘土矿物（高岭石、蒙脱石和伊利石）钝化前后的表面结构和性质进行了系统研究 ,并结合相关的土力学、工程力学参数的测定,阐明粘土矿物钝化的机理,剖析和研制粘土 矿物钝化剂,为解决各类工程和灾害问题中的土壤稳定提供理论依据。     

西昆仑山前陆盆地粘土矿物特征及其地质意义

为重建西昆仑山地区晚新生代的地质构造及环境演变过程,本文利用X射线衍射（XRD）及扫描电子显微分析（SEM）方法,对西昆仑山前陆盆地（叶城盆地）该时期沉积的一套磨拉石建造中的粘土矿物进行了研究。结果表明,在整个晚中新世到早更新世的沉积物地层中,粘土矿物的种类始终以伊利石和蒙脱石为主,含少量的绿泥石和高岭石。但伊利石、蒙脱石、高岭石的相对含量,以及伊利石结晶度值等均显示出明显的变化。在8~3.5 Ma B.P.期间,伊利石相对含量值和结晶度值均比较高,而蒙脱石相对含量低,并有少量的高岭石产出;在3.5~3.3 Ma B.P.期间,伊利石相对含量值和伊利石结晶度值均急剧降低,而相反,蒙脱石相对含量值急剧升高,高岭石基本消失;3.3~2.8 Ma B.P.期间沉积物的粘土矿物参数与8~3.5 Ma B.P.时期的粘土矿物参数非常接近;而2.8~1.8 Ma B.P.期间沉积物的粘土矿物参数则与3.5~3.3 Ma B.P.期间的粘土矿物参数相类似。根据沉积物中的粘土矿物特征变化规律可以推断,物源区母岩成分的变化是粘土矿物特征变化的主要原因,古气候古环境变化则是次要影响因素。粘土矿物特征的变化规律显示西昆仑山体在晚新生代具有不均匀的阶段性隆升的特点,在3.5 Ma B.P.、3.3 Ma B.P.、2.8 Ma B.P.左右分别发生了三次明显的构造隆升事件,而且总伴随着风化母岩成分的变化。物源区在早更新世中期（12.5 Ma B.P.)很可能处于干旱的古气候古环境中。     

南阳膨胀土的工程地质特征和填筑适宜性

施工阶段的工程地质调查发现,宁（南京）西（西安）铁路内乡-镇平段南阳膨胀土成因类型上不仅有第四系冲积洪积粘土,而且发育上第三系湖相沉积粘土。不同成因的膨胀土具有不同的空间分布特征和工程性质,X射线衍射测试表明,南阳膨胀土的主要膨胀性粘土矿物是伊利石/蒙脱石混层矿物。物质组成和物理化学性质差异导致击实土在含水量变化条件下膨胀力和膨胀变形有明显的差别;对同一种膨胀土材料,其击实含水量越小,膨胀性越强,甚至含水量降低1 %～2 %,膨胀力就可以增加到200 %以上;击实膨胀土的直剪强度受到钙质结核含量的影响,对于膨胀势为高或强的膨胀土,结核含量小于15 % 时,经过干湿循环后直剪强度衰减量为40 %～65 %,结核含量为25 %～30 % 的强膨胀土直剪强度衰减量则低于30 %;对膨胀土的地质特征、工程性质、击实土的膨胀性和直剪试验结果进行分析,探讨其用于填筑铁路路基的适宜性,进行了填筑适宜性分级。依据研究成果制定的路基施工工艺取得了比较理想的工程效果     

The effect of high pH alkaline solutions on the mineral stability of the Boom Clay – Batch experiments at 60 °C

Boom Clay is currently viewed as a reference host formation for studies on deep geological disposal of radioactive waste in Belgium. The interactions between bulk rock Boom Clay and 0.1 M KOH, 0.1 M NaOH, 0.1 M Ca(OH)₂, young cement water and evolved cement water solutions, ranging in pH from 12.5 to 13.2, were examined as static batch experiments at 60 °C to simulate alkaline plume perturbations, which are expected to occur in the repository due to the presence of concrete. Both liquids and solids were investigated at specific times between 90 and 510 days in order to control the elemental budget and to search for potential mineralogical alterations. Also, the clay fraction was separated from the whole-rock Boom Clay at the end of each run and characterized for its mineralogical composition. Thereby, the importance of the mineral matrix to buffer the alkaline attack and the role of organic matter to protect clay minerals were also addressed. The results indicate that the degree of geochemical perturbation in Boom Clay is dependent on the initial pH of the applied solution together with the nature of the major cation in the reactant fluids. The higher the initial pH of the media, the stronger its interaction with Boom Clay. No major non-clay mineralogical alteration of the Boom Clay was detected, but dissolution of kaolinite, smectite and illite occurred within the studied experimental conditions. The dissolution of clays is accompanied by the decrease in the layer charge, followed by a decrease in the cation-exchange capacity. The highest TOC values coincide with the highest total elemental concentrations in the leachates, and correspondingly, the highest dissolution degree. However, no quantitative link could be established between the degree of organic matter decomposition and clay dissolution.     

Physical and chemical steady-state compaction in deep-sea sediments: Role of mineral reactions

Marine sediments typically exhibit steep porosity gradients in their uppermost centimeters. Although the decrease in porosity with depth below the sediment-water interface is primarily due to compression arising from the accumulation of overlying sediment, early diagenetic mineral dissolution and precipitation reactions may potentially also affect the porosity gradient. Here, we present a steady state compaction model, based on the mass and momentum conservation of total fluid and solid phases, in order to quantify the relative contributions of mineral reactions and physical compaction on porosity changes. The compaction model is applied to estimate hydraulic conductivity and compressive response coefficients of deep-sea sediments from measured porosity depth profiles. The results suggest an inverse relation between the compressive response coefficient and the lithogenic content of marine sediments. For deep-sea sediments exhibiting high rates of dissolution of siliceous shell fragments, the compaction model ignoring mineral reactions overestimates the hydraulic conductivity and compressive response coefficients. In contrast to non-compacting porous media, mineral dissolution in surficial sediments can lead to lower porosity. However, as illustrated for a deep-sea sediment in the equatorial Atlantic characterized by extensive dissolution of calcareous shell fragments, the effect of mineral dissolution and precipitation reactions on porosity gradients is, in most cases, negligible.     

Illite-smectite Mixed-layer Minerals in the Alteration Volcanic Ashes under Submarine Environment

The clay mineralogy of the clay intervals interbedded with siliceous mudstones across the Permian-Triassic boundary (PTB) in Pengda, Guiyang, Guizhou province, was investigated by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The clay mineral assemblages of the sediments are mainly I/S clays and minor smectite, kaolinite, and illite as reveled by XRD analyses. The peak-shape parameters BB1 and BB2 of I/S clays of the representative clay bed PL-01 are 4.7° and 4.4°, and the peak position of the low angle reflection is at 6.8° 2θ (13.6 ), suggesting that the I/S clays has a IS type of ordering. However, the presence of multi-order reflections and their intensities are different from those of completely ordered 1∶1 mixed-layer I/S clay rectorite, indicating that I/S clays of the Pengda section have partially ordered IS structures. HRTEM observations show that most of the I/S clays exhibit an IS stacking ordering. However, in some areas within a IS particle, smectite layer is observed in doublets, triplets, and quartets, which are interstratified by various amounts of illite layers, suggesting the presence of other irregular stacking in addition to the major 1∶1 IS ordered stacking. Transformation of smectite layer into illite layers is also observed in the I/S clays, suggesting that the Pengda I/S clays are derived from smectite illitization, in good agreement with the clay mineral assemblage. The I/S clays of the Pengda section contain up to 45% to 95% smectite layer, the notably higher contents of smectite layer relative to those of other PTB stratigraphic sets in south China can be attributed to difference in alteration and smectite illitization processes due to different sedimentary environments.