应用流体包裹体研究致密砂岩储层多期油气充注历史——以鄂尔多斯盆地镇泾地区延长组长8段砂岩为例

本文以鄂尔多斯盆地西南缘镇泾地区延长组长8段致密砂岩储层为例,利用流体包裹体岩相学、微束荧光光谱及显微测温等分析方法, 厘定油气充注期次和时间,同时结合储层成岩演化,探讨储层致密化与油气成藏过程。研究表明,储层孔隙沥青呈黄色和蓝白色荧光, 并以蓝白色为主;石油包裹体呈黄色、黄绿色及蓝白色荧光,并以后两者居多。与黄色荧光石油包裹体同期的盐水包裹体均一温度主峰为 80℃～90℃; 而与黄绿色和蓝白色荧光石油包裹体同期的盐水包裹体均一温度主峰分别为105℃～115℃和110℃～120℃。研究区储层主要经历了3期烃类充注：第一期发生在早白垩世早期（138～130Ma）,表现为少量的低熟油充注;第二期发生在早白垩世晚期（124～105Ma）,为关键成藏期,与大规模成熟油充注相关; 第三期发生在晚白垩世（85～73Ma）,古油藏发生调整,再次成藏。储层早成岩阶段以机械压实、薄膜状绿泥石、高岭石及微晶方解石胶结为主,在中高孔渗条件下发生了早期石油充注;随着继续压实,石英加大、孔隙填充绿泥石、高岭石及亮晶方解石胶结发育,储层逐渐致密化 晚期石油在低渗致密储层内的运聚成藏主要受早期烃类充注路径控制。     

东营凹陷中央隆起带烃类流体活动在成岩作用上的响应

为探讨成岩作用对烃类流体活动的响应,利用显微观察、包裹体测温、扫描电镜、阴极发光等研究方法,系统地研究了东营凹陷中央隆起带沙三段自生矿物形成与烃类流体活动的相关性。研究表明：研究区沙三段储层的成岩序列为：第一期石英次生加大边→第一期碳酸盐胶结物(黄铁矿)→绿泥石薄膜→第二期碳酸盐胶结物→第二期石英次生加大边（第一期高岭石）→第三期碳酸盐胶结物→第三期石英次生加大边（第二期高岭石）→第四期含铁碳酸盐胶结物（黄铁矿）。第一期油气充注发生于第二期碳酸盐胶结物形成的后期,第二、三期油气充注发生于第三期碳酸盐胶结物形成的中后期,第四期碳酸盐胶结物的形成标志大规模油气充注时期的结束。烃类流体活动对研究区的自生高岭石、含铁碳酸盐矿物（第四期碳酸盐胶结物）、黄铁矿等3类自生矿物的形成过程产生重要影响。砂岩储层中,第二、三期碳酸盐胶结物的港湾状溶蚀现象、高岭石大量发育、第四期碳酸盐胶结物和黄铁矿的共生组合等成岩现象可作为微观上识别烃类流体活动的标志。含油性较好的层段,烃类流体活动可导致碳酸盐胶结物产生大规模的溶蚀作用,并促使长石溶蚀产生大量高岭石,砂泥岩界面处可见黄铁矿与含铁碳酸盐胶结物共生;而含油性差及不含油的层段,由于缺少烃类流体提供的大量有机酸,高岭石发育较少,早期碳酸盐胶结物保存较好。     

库车坳陷克深大气田白垩系致密砂岩储层古流体地球化学特征研究

采用包裹体岩相学、均一温度、盐度、含油包裹体丰度、储层颗粒荧光定量实验分析方法,对库车坳陷克深大气田白垩系巴什基奇克组储层致密砂岩样品进行测试,剖析克深大气田深层致密砂岩储层古流体地球化学特征与油气充注。结果表明,克深大气田发育两期油气包裹体,分别为发亮蓝色荧光的轻质油包裹体与灰黑色的气态烃包裹体。克深大气田包裹体均一温度分布存在130~140 ℃与150~160 ℃两个主峰值,相应的古盐度值为16%~10%与12%~4%,表明有两期烃类流体充注,其中天然气高强度充注发生在2.5 Ma以来。含油包裹体丰度分布在2.1%~13.4%之间,QGF指数为2.2~9.6,QGF-E强度为20~38 pc,揭示克深大气田形成过程中曾经存在强度较弱的轻质油充注。     

鄂尔多斯盆地中西部长8砂岩的流体包裹体特征与油气成藏期次分析

利用流体包裹体的分析测试技术与研究方法,对鄂尔多斯盆地中西部长8油层组的成藏期次进行了研究。烃类包裹
体及其伴生的盐水包裹体的岩相学特征、均一温度、盐度、密度及成分分析显示,长8储层主要发育两期烃类包裹体,第
一期分布在早期方解石胶结物、石英颗粒表面及其连生的石英加大边中以及未切穿石英颗粒的早期愈合裂缝中,其伴生盐
水包裹体的均一温度峰值为80~90℃;第二期烃类包裹体分布在石英颗粒表面和切穿石英颗粒的裂缝中,其伴生盐水包裹
体的均一温度峰值为100~120℃。储层成岩作用与油气充注微观分析、成岩-烃类充注演化时间序列研究,结合地层埋藏史
和热史分析表明,研究区经历了三期烃类充注事件：第一期（约169~161 Ma） 充注规模小,砂岩中未捕获到这一期适合进
行均一温度与成分测定的烃类包裹体,因此本研究缺乏这期包裹体的温度与成分数据;第二期（约148~135 Ma） 烃类充注
对应前述获得均一温度与成分数据的第一期烃类包裹体;第三期烃类充注规模最大,发生在早白垩世晚期的125.2~105.7 Ma
之间,为主成藏期,对应前述获得均一温度与成分数据的第二期烃类包裹体。     

应用有机包裹体研究天然气成藏特征——以鄂尔多斯盆地苏里格气田西部山1段为例

通过对苏里格气田西部山1段成岩作用及成岩序列研究,利用有机包裹体岩相学观察和流体包裹体测试析技术,探讨苏里格气田西部山1段天然气成藏特征。研究表明苏里格气田西部山1段成岩作用主要有压实和压溶作用、胶结作用、石英次生加大作用、溶蚀和裂隙作用等。依据有机包裹体分布特征和包裹体物理相态特征可以识别出特征明显的2期包裹体。第一期油气包裹体主要相态为气态烃+盐水、气态烃+液态烃+盐水和气态烃+液态烃,气/液比较小,多呈串珠状定向分布在裂隙中,均一温度相对较低,温度主峰为120℃,代表了早期天然气充注作用,天然气开始形成并初次运移进入储层。第二期油气包裹体主要为纯气相和气态烃+少量盐水,气/液比较大,主要分布在溶蚀孔隙和晚期裂隙中,均一温度相对高,温度主峰为150℃,该期包裹体代表了生气高峰期天然气大规模运移成藏事件,也是研究区最重要的成藏事件。利用Tt曲线方法研究表明,苏里格西部地区山1段天然气开始运移进入储层的时间大约为170Ma(相当于中晚侏罗世),天然气规模成藏时间约为120Ma(相当于早白垩世晚期)。     

利用天然气包裹体方法划分天然气成藏期次--以鄂尔多斯盆地东南部上古生界储层为例

鄂尔多斯盆地东南部上古生界天然气储层主要为太原组、山西组、下石盒子组和上石盒子组,岩性以石英砂岩为主。储层的主要成岩作用有压实和压溶作用、胶结和交代作用、裂隙和溶蚀作用等,依据显微镜下岩相学研究,在研究区可以识别出四个大的成岩阶段,分别对应早成岩阶段的早期和晚期、晚成岩阶段的早期和晚期。识别出两期天然气包裹体,第一期天然气包裹体形成于早成岩阶段的晚期,分布在早期裂隙及与其相关的溶蚀孔隙中,形态不规则,气/液比较高,偶尔可见包裹体发荧光。第二期天然气包裹体形成于晚成岩阶段晚期,分布在晚期裂隙中和晚期亮晶方解石和石英胶结物中,包裹体气/液比小,大部分为有机气态和盐水组成的气液两相有机包裹体,包裹体壁薄色浅。研究认为第一期代表了早期天然气初次运移和注入储层事件,而第二期代表了天然气大规模二次运移聚集成藏事件。     

库车凹陷克拉2气田深层优质储层成因及成岩作用模式

基于成岩作用的影响因素及库车盆地的实际,深入研究了克拉2气田次生孔隙发育段储层岩石成分特征及早期古气候条件下胶结物对于成岩作用及次生孔隙的影响,分析了地层压力系统中超高压带、构造裂缝、自生矿物分带的成因、作用、特点及其与储层物性、成岩环境的相关关系,评价了储层中长石、方解石的溶解,白云石对方解石的交代,压实、胶结等主要成岩作用类型对次生孔隙的具体影响和贡献,认为该区早期碱性成岩环境下方解石胶结物的溶解是形成该区次生孔隙发育带的主导因素,并根据该区自生矿物流体包裹体及埋藏史特征,建立了该区成岩作用及孔隙演化的模式。     

川中太和气区震旦系灯影组二段储层成岩流体演化及油气成藏史——来自岩石学、原位地球化学、流体包裹体及年代学的证据

川中地区震旦系灯影组四段已获得巨大勘探突破,太和含气区灯二段目前也已展示出较大勘探潜力。但灯二段储层时代老、埋深大、成岩演化历史长,经历了多期次构造运动,储层中矿物胶结充填期次复杂,成岩演化与油气充注关系不清,一定程度上制约了灯二段油气勘探。本文基于岩石组构分析,利用阴极发光、原位微区分析、流体包裹体及白云石U- Pb年代学等方法厘定了川中太和气区灯二段储层成岩序列,明确了不同期次白云石胶结充填物的成岩环境、成岩流体特征、形成时间及代表的油气充注事件,明确了其油气地质意义及灯二段油气成藏历史。研究表明区内灯二段储层孔洞中沥青及胶结充填物的成岩序列为：第一世代海底环境纤维状白云石→第二世代表生成岩环境大气淡水白云石→第三世代浅埋藏环境粉—细晶粒状白云石→第Ⅰ期氧化降解沥青→第四世代中埋藏环境中晶白云石→第五世代深埋藏环境粗晶白云石→第Ⅱ期热裂解沥青→第六世代深埋藏环境巨晶- 鞍状白云石→第七世代抬升埋藏环境石英、萤石等矿物。主要反映了四种不同类型的成岩流体特点：① 具有高Na、K含量,低Fe、Mn含量,负Ce异常,Y正异常特征的高盐度海源流体;② 具有较低Fe、Mn含量,平缓REE模式,无Eu异常特征的淡水与残余海水的混合流体;③ 具有高Fe、Mn含量,Eu明显正异常特征的高温热液流体与早期海水的混合流体;④ 具有高Mn含量,Fe/Mn＜1,HREE较富集,Eu正异常特征且自生石英、萤石矿物沉淀的深部与高温热液相关的酸性、还原性流体。不同世代矿物的充填关系、流体包裹体相态、均一温度以及白云石U- Pb年代学分析表明,开始发育含烃包裹体的第三世代粉—细晶白云石与形成于加里东运动导致的构造隆升期的第Ⅰ期沥青分别记录了加里东期古油藏的形成及破坏;第四世代中晶白云石及第五世代粗晶白云石记录了印支期古油藏的形成,部分第五世代粗晶白云石及第Ⅱ期沥青记录了印支期古油藏裂解事件;第六世代巨晶- 鞍形白云石及第七世代石英、萤石等矿物记录了古干气藏形成及气藏形成、调整和定型的成藏事件。     

应用流体包裹体研究油气成藏期次——以柴达木盆地南八仙油田第三系储层为例

储层中的流体包裹体是储层成岩过程被自生矿物晶格所捕获而形成的油气水包裹体 ,它们记录了油气水充注储层时的组成、性质以及物理化学条件。不同自生矿物或不同期次的胶结矿物捕获的包裹体 ,其组成和物理化学性质不同 ,这些特征是油气藏形成演化历史的直接标志。柴达木盆地南八仙油田是一个侏罗系生烃、第三系储集的成藏系统。文章通过包裹体类型、成岩矿物期次、烃类包裹体成分以及均一化温度和盐度等分析 ,证明南八仙油田上第三系与下第三系有着不同的油气注入和成藏期次 :下第三系的油气藏为两期充注的结果 ,而上第三系油气藏则主要为一期成藏。结合区域构造演化史以及南八仙第三系埋藏史和古地温资料推断 ,南八仙油田下第三系油气藏形成的地质时期为渐新世和上新世 ,上第三系油气成藏的地质时期为上新世。     

流体包裹体多参数综合划分东营凹陷沙三段油气充注期次及充注时期确定

对采自东营凹陷沙河街组第三段(1沙三段,E§3)223块流体包裹体薄片进行了有机包裹体产状观察、显微荧光光谱分析和显微测温等流体包裹体系统分析.综合分析这些参数表明沙三段发育3期油气充注:第一期发生在成岩早期,主要为发橙黄色荧光的低成熟油气充注;第二期发生在成岩中期,主要为发淡黄色荧光的成熟油气充注;第三期发生在成岩作用晚期,主要为发蓝白色荧光的高成熟油气充注.在此基础上,运用埋藏史-均-温度投影法确定了这3期油气的充注时期:第一期为36～24 Ma;第二期为13.8～8.0 Ma;第三期为8.0～0 Ma.     

松辽盆地钱家店砂岩型铀矿床油气运移与铀成矿的关系:来自流体包裹体的证据

为探究油气运移与铀成矿的关系,笔者以松辽盆地钱家店砂岩型铀矿床为研究对象,系统开展了含矿砂岩薄片鉴定、电子探针研究以及流体包裹体岩相学和显微测温研究.结果表明:①研究区强矿化段砂岩硅质胶结物含量较多,为隐晶质玉髓胶结物;成岩演化序列为,泥晶方解石、自生黏土膜,石英加大边、石英加大边外围黏土膜、颗粒间压实的隐晶质玉髓胶结...     

库车前陆盆地大北地区砂岩储层致密化与油气充注的关系

库车前陆盆地大北地区盐下致密砂岩储层具有埋藏深度大、低孔和低渗的特征,以产气为主并伴生少量的轻质原油.在对大北地区盐下致密砂岩储层特征、油气成熟度、油气充注史研究基础之上,结合致密砂岩孔隙演化特征综合分析油气充注与储层致密化的时间关系.大北地区盐下致密砂岩储层孔隙空间主要以次生溶蚀孔为主,大部分孔隙空间都被沥青充填,由孔隙和沥青组成的空间网络具有较好的连通性,压实作用为储层致密化重要成因,碳酸盐胶结作用应该主要在库车组沉积之前使孔隙度降低.致密砂岩储层中发育沥青,油气地球化学特征以及流体包裹体分析结果均表明库车前陆盆地大北地区致密砂岩储层存在两期油和一期天然气充注.晚期原油充注发生在距今5~4 Ma,对应库车组沉积早期,天然气充注发生在距今3~2 Ma,对应地层抬升剥蚀时期.库车前陆盆地大北地区盐下砂岩储层在油充注时期埋藏深度较浅,具有较高的孔隙度,而天然气充注时期,砂岩储层已经遭受了强烈的压实作用达到了致密化程度.      

Silicification and dolomitization of anhydrite nodules in argillaceous terrestrial deposits: an example of meteoric-dominated diagenesis from the Triassic of central Spain

Cauliflower-shaped nodules are widespread in a single red mudstone bed in the Buntsandstein (Triassic) facies of the Iberian Range. They consist mostly of quartz, dolomite and calcite, but other minerals, such as barite, kaolinite and iron oxyhydroxides, are also present. The nodules are spherical, ovoid or elongate in shape and range from 1 to 8 cm across. The surface of the nodules is irregular, and some show a pedogenic coating of microspar. The sedimentological and petrographic data suggest that the initial anhydrite nodules formed through a progressive increase in the porewater concentration of Ca²⁺ and SO₄^2– in a vadose environment, occasionally under the influence of pedogenic processes. Partial replacement of the anhydrite by megaquartz occurred under more dilute conditions in the same type of setting, as indicated by the presence of organic filaments on the quartz crystal surfaces. In type A nodules, the dissolution of the innermost anhydrite was complete, and different types of quartz cement filled the porosity. Fracturing and meteoric cementation by calcite and minor amounts of kaolinite were the latest processes affecting these nodules. In type B nodules, the dissolution of the anhydrite was incomplete, inhibiting quartz cementation and enabling later dolomitization of the anhydrite. Dolomitization appears to have been driven by sulphate reduction, as indicated by the presence of bacterial bodies within the dolomite crystals. Dedolomitization and precipitation of barite, kaolinite and calcite spar cements occurred later under the influence of meteoric solutions. The nodules may mark the former locations of the water table and provide evidence for an episode of highly evaporitic conditions throughout wide areas of the basin. Their occurrence reveals not only a complex diagenetic history but is also indicative of palaeogeographic and palaeoclimatic conditions.     

储层成岩产物对于油气运移的指示意义——以塔里木盆地群苦恰克构造带东河塘组为例

泥盆系东河塘组下段砂岩储层是塔里木盆地群苦恰克构造带内重要的勘探层系,有关储层内油气充注方向与富集规律的研究目前仍未有统一认识。研究层段内碳酸盐胶结物(主要为白云石和铁白云石胶结物)分布较为广泛,其中作为早成岩阶段产物的白云石胶结物的含量在平面上呈现出自东向西依次增加的趋势; 而铁白云石胶结物则以交代先期形成白云石胶结物的形式产出于中成岩阶段,略滞后于储层内的油气充注,其胶结程度在平面上呈现出自东向西依次减弱的分布规律。铁白云石胶结物作为反映储层内较强还原环境的指示性矿物,其在储层内的分布规律也承担着判明油气运移方向或富集规律的"责任",而其上述分布规律为研究区储层内油气自东向西充注提供了全新的判定依据。     

海拉尔盆地乌尔逊凹陷与松辽盆地孤店CO2气田含片钠铝石砂岩的岩石学特征

海拉尔盆地乌尔逊凹陷南屯组与松辽盆地孤店CO₂气田泉头组发育大量含片钠铝石砂岩。通过偏光显微镜、扫描电镜、茜素红-S染色、X射线衍射、电子探针与INCA能谱分析等,对含片钠铝石砂岩的骨架碎屑组分、胶结物与自生矿物、成岩共生序列等岩石学特征进行了系统研究。研究表明,含片钠铝石砂岩的岩石类型为长石砂岩和岩屑长石砂岩,粒度以细粒、细-中粒为主,分选差-中等。砂岩中胶结物主要为次生加大石英、自生石英、片钠铝石、铁白云石和粘土矿物。其中,片钠铝石最高可达砂岩总体积的22％在砂岩中或以放射状、束状、菊花状、杂乱毛发状、毛球状、板状等集合体充填孔隙,或呈束状和板状交代长石和岩屑。电子探针与INCA能谱综合分析表明,片钠铝石主要由Na、Al、O、C等组成。在含片钠铝石砂岩中,成岩共生序列依次为粘土矿物包壳-次生加大石英、自生石英、自生高岭石-油气充注-CO₂充注-片钠铝石-铁白云石。其中,CO₂注入前形成的自生矿物组合主要为次生加大石英、自生石英和自生高岭石,CO₂注入后形成的自生矿物组合主要为片钠铝石和铁白云石。     

Diagenesis in the Middle Jurassic Khatatba Formation sandstones in the Shoushan Basin,northern Western Desert,Egypt

The Middle Jurassic Khatatba Formation acts as a hydrocarbon reservoir in the subsurface in the Western Desert, Egypt. This study, which is based on core samples from two exploration boreholes, describes the lithological and diagenetic characteristics of the Khatatba Formation sandstones. The sandstones are fine‐ to coarse‐grained, moderately to well‐sorted quartz arenites, deposited in fluvial channels and in a shallow‐marine setting. Diagenetic components include mechanical and chemical compaction, cementation (calcite, clay minerals, quartz overgrowths, and a minor amount of pyrite), and dissolution of calcite cements and feldspar grains. The widespread occurrence of an early calcite cement suggests that the Khatatba sandstones lost a significant amount of primary porosity at an early stage of its diagenetic history. In addition to calcite, several different cements including kaolinite and syntaxial quartz overgrowth occur as pore‐filling and pore‐lining cements. Kaolinite (largely vermicular) fills pore spaces and causes reduction in the permeability of the reservoir. Based on framework grain–cement relationships, precipitation of the early calcite cement was either accompanied by or followed the development of part of the pore‐lining and pore‐filling cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar. Late kaolinite clay cement occurs due to dissolved feldspar and has an impact on the reservoir quality of the Khatatba sandstones. Open hydraulic fractures also generated significant secondary porosity in sandstone reservoirs, where both fractures and dissolution took place in multiple phases during late diagenetic stages. The diagenesis and sedimentary facies help control the reservoir quality of the Khatatba sandstones. Fluvial channel sandstones have the highest porosities and permeabilities, in part because of calcite cementation, which inhibited authigenic clays or was later dissolved, creating intergranular secondary porosity. Fluvial crevasse‐splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolinite matrix and pervasive, shallow‐burial calcite and quartz overgrowth cements, respectively. Copyright © 2013 John Wiley & Sons, Ltd.     

酒西盆地碎屑岩胶结物的形成问题

酒西盆地下白垩统陆相碎屑岩胶结物的形成关系到油气储集的重要问题,富铁的洪积扇沉积体系和非富铁的河流一湖泊沉积体系形成两种不同的胶结物组合类型,平面上可分为三带胶结物:混合胶结带、铁镁胶结带和浓聚成岩带。盆地边缘带是以铁、泥、钙为主的混合胶结物,中间为以碳酸铁镁矿物的白云石、铁白云石为主的胶结物,盆中带形成胶结物的浓聚成岩层,事实证明过渡的铁镁胶结带是最富于聚集油气的地带。本文还在纵向上探讨了泥质粘土矿物和碳酸盐胶结物的成岩变化。     

蒙古国塔木察格盆地塔南凹陷铜钵庙组火山碎屑岩中片钠铝石胶结物的成因

塔木察格盆地塔南凹陷铜钵庙组火山碎屑岩中发育大量的片钠铝石自生矿物。通过偏光显微镜、扫描电镜及X射线衍射分析等,查明了火山碎屑岩中的胶结物与自生矿物类型和成岩共生序列。含片钠铝石火山碎屑岩的成岩共生序列为粘土包壳-方解石-一期微晶石英-一期高岭石-二期微晶石英、二期高岭石、片钠铝石-铁白云石;通过MAT253稳定同位素质谱仪确定了片钠铝石和铁白云石的稳定同位素组成为片钠铝石的δ¹³C_PDB为-8.15‰~-2.81‰,与片钠铝石平衡的CO₂的δ¹³C_CO2为-12.91‰~-7.55‰,通过与海拉尔盆地及BGS盆地系碳氧同位素数据的类比,表明片钠铝石中的碳主要为幔源岩浆来源,形成片钠铝石的钠和铝主要来源于安山岩岩屑、长石晶屑碎屑、玻屑和火山灰等;铁白云石的δ¹³C_PDB为-6.76‰~-2.65‰,与片钠铝石的碳氧同位素数据相似,结合相关地质实例及地球化学模拟实验,可以推断铁白云石中的碳也是幔源岩浆来源,铁白云石中的铁离子主要来源于安山岩岩屑、玻屑、火山灰和蒙脱石等矿物。     

Isotopic evidence on the diagenetic evolution of coastal sabkha reservoirs from the Solimões Basin, northern Brazil

The isotopic composition of diagenetic carbonates, sulfates, illites and associated diagenetic constituents of the Carboniferous Juruá sandstones, Solimões Basin, largest hydrocarbon reservoirs of northern Brazil, was analyzed in conjunction with petrographic characterization and chemical composition. Eolian Juruá sandstones, deposited in a coastal sabkha of a wide cratonic sag, are the best reservoirs, but diagenetic processes introduced strong and complex heterogeneities, which affect hydrocarbon production. Dolomite and anhydrite cementation, together with compaction, exerts the main control on the quality of the reservoirs. Early dolomite cements precipitated under strong evaporation, while late dolomites are related to thermal decarboxylation. Late anhydrite cements are related to the interbedded evaporites and possibly to the oxidation of dissolved sulfide. Authigenic illite K–Ar ages are related to the voluminous Triassic basic magmatism (around 200 Ma), and to the Jurassic–Cretaceous tectonism (around 150 Ma), which may have also affected the late anhydrite and dolomite–ankerite cementation. This isotopic study revealed the major conditions of the diagenetic processes that strongly affected the Juruá reservoirs, which are essential for the development of models that will optimize the production, as well as decrease the risks in the exploration for new Juruá reservoirs.     

Diagenetic processes in cretaceous sandstones from occidental Brazilian Equatorial Margin

Despite a great interest in Brazilian Equatorial Margin exploration, very little was published on the diagenesis of sandstones from that area. A wide recognition petrographic study was performed to identify the major diagenetic processes that impacted the porosity of Lower Cretaceous sandstones of the Pará-Maranhão, São Luís, Bragança-Viseu and Barreirinhas basins. Arkoses from the Pará-Maranhão Basin show neoformed or infiltrated clay coatings, mica replacement and expansion by kaolinite and vermiculite, and precipitation of grain-replacive and pore-filling quartz, kaolinite, albite, chlorite, calcite, dolomite, siderite, pyrite and titanium oxides. Compaction, quartz and calcite cementation were the main porosity-reducing processes. Barreirinhas Basin lithic arkoses and subarkoses display clay coatings, compaction of metamorphic fragments into pseudomatrix, and precipitation of grain-replacive and pore-filling kaolinite, quartz, albite, chlorite, calcite, dolomite, TiO₂ and pyrite. The main porosity-reducing processes were calcite cementation in the subarkoses, and compaction and quartz cementation in lithic arkoses. Quartzarenites from this basin were early- and pervasively cemented by dolomite. Arkoses and lithic arkoses of the São Luís and Bragança-Viseu basins show clay coatings, pseudomatrix from mud intraclasts compaction, and precipitation of pore-filling and grain-replacive kaolinite, vermiculite, smectite, quartz, albite, chlorite, illite, calcite, dolomite, hematite, TiO₂ and pyrite. Compaction of mud intraclasts and dissolution of feldspars and heavy minerals were the main porosity-modification processes. These preliminary results may contribute to the understanding of the spatial and temporal distribution of the diagenetic processes and their impacts on the porosity of the sandstones from these basins.