Classification and Diagenetic Characteristics of the Cretaceous Sandstones in the Southern Bredasdorp Basin,Offshore South Africa

A systematic petrographic and geochemical studies of 92 representative sandstone samples from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1 and E-D3 in the southern part of the Bredasdorp Basin was undertaken to classify the sandstones as well as unravel the main diagenetic processes and their time relations. Petrographic study shows that the sandstones are largely subarkosic arenite and arkosic litharenite, which have underwent series of diagenetic processes as a result burial, rifting and subsequent uplift. The main diagenetic processes that have affected the reservoir properties of the sandstones are cementation by authigenic clay, carbonate and silica, growth of authigenic glauconite, dissolution of minerals and load compaction. The major diagenetic processes reducing the porosity are calcite cementation in the subarkosic arenite, and compaction and quartz cementation in arkosic litharenite. On the other hand, the formation of secondary porosity due to the partial to complete dissolution of early calcite cement, feldspars and minor grain fracturing has improved the reservoir property of the sandstone to some extent. The clay minerals in the sandstones commonly acts as pore choking cement, which reduces porosity. In general, there is no particular diagenetic process that exclusively controls the type or form of porosity evolution in the sandstones.     

鄂尔多斯盆地华庆地区长6油层组砂岩成岩相及储层质量评价

本文借助于薄片分析、扫描电镜分析、阴极发光分析和物性分析等研究手段,以鄂尔多斯盆地华庆地区长6油层组低渗透砂岩的形成机制为解剖对象,通过对砂岩埋藏前组成、化学成岩作用和物理成岩作用的相对重要性、自生矿物组成、孔隙构成、物性与孔隙结构等研究,明确了主要成岩作用对储层的影响。在此基础上,结合沉积相的研究,选择骨架颗粒构成、自生矿物组成、物性、孔隙构成以及粒间体积等有关的15个成岩相定量评价参数,对华庆地区长6油层组低渗透砂岩进行成岩相的定量评价。结果表明：① 华庆地区长6油层组砂岩经历了较强的物理成岩作用和较弱的化学成岩作用,原生孔隙是主要储集空间,压实作用是孔隙度降低的最主要因素,其次为方解石的胶结作用;② 成岩相定量评价表明,弱胶结、强压实成岩相以及强方解石胶结、中-强压实成岩相的储层质量均较差,受北部物源区控制的绿泥石胶结、弱-中等压实成岩相具有较好的储层质量,建议作为今后的优选勘探开发区。     

四川盆地川中地区中下侏罗统砂岩储层异常致密成因机理

四川盆地川中地区中下侏罗统砂岩储层现今埋藏深度一般在1 500～3 000 m,但砂岩致密化程度异常偏高,其平均孔隙度小于5%,平均渗透率小于1×10-3μm2,为典型的超低孔、超低渗储层。岩石显微镜下成岩作用特征及压实与胶结作用的减孔强度统计结果显示,强压实作用（包括压溶作用）是导致砂岩储层异常致密的主要原因。利用砂...     

塔里木盆地西北地区震旦系苏盖特布拉克组潮坪相碎屑岩成岩作用与成岩演化研究

塔里木盆地西北地区苏盖特布拉克组发育一套潮坪相沉积,是该层系油气勘探的主要目标。基于什艾日克剖面、奇格布拉克剖面、肖尔布拉克剖面的地质测量,采用薄片鉴定、阴极发光、扫描电镜、流体包裹体测温及黏土矿物X衍射等分析手段开展海相碎屑岩成岩作用和成岩演化研究,为塔里木盆地苏盖特布拉克组进一步开展油气勘探部署和甜点储层预测提供地质依据。结果表明：（1）研究区苏盖特布拉克组发育潮坪沉积环境的砂体,砂岩类型以岩屑石英砂岩、岩屑砂岩为主,偶见长石岩屑砂岩,成分成熟度和结构成熟度中等;（2）苏盖特布拉克组砂岩经历了压实（溶）作用、胶结作用、溶蚀作用及交代作用等成岩作用,压实作用是造成储层致密的直接原因,钙质胶结和硅质胶结是储层致密化的根本因素;（3）苏盖特布拉克组砂岩的成岩演化阶段已达到中成岩B期,成岩演化序列为：压实作用/自生黏土矿物（绿泥石）环边—第一期长石、岩屑溶蚀作用—第一期硅质胶结—第一期泥微晶方解石胶结/压溶作用/黏土矿物的伊利化—第二次硅质胶结/第二期铁方解石胶结—钙质胶结物溶蚀。据此建立的成岩演化模式为塔里木盆地超深层致密砂岩中甜点储层预测提供可靠的地质依据。     

安塞油田长2油层成岩作用及其对储层物性的影响

安塞地区长2油层储层以中细粒长石砂岩为主，主要成岩矿物为绿泥石环边、方解石胶结物、石英和长石加大生长、伊利石、伊一蒙混层、钠长石和高岭石等。绿泥石环边的发育对原生粒间孔的保存起到有利的作用。石英加大级别达Ⅲ级，加大边含有丰富的有机包体，均一温度范围48．1～76．5℃，烃类进入储层的时间为早白垩世中期至晚白垩世。储层发育多种孔隙类型，面孔率平均为11．60％．主要类型有粒间孔、骨架颗粒溶孔和微裂缝等。残余粒间孔和骨架颗粒溶孔是本区长2储层的主要孔隙类型。根据成岩作用的矿物岩石学标志、有机质成熟度及古温度，将长2油层砂岩的成岩作用划分为3个成岩阶段：早成岩阶段、晚成岩阶段和表生成岩阶段。储层性质明显地受到沉积微相和成岩作用的影响，沉积物粒度较粗、厚度较大的分流河道储集物性明显优于各种粒度较细、厚度较薄的分流间席状砂体。     

Diagenesis of the Lower Cretaceous Kanmon Group sandstones, SW Japan

The Kanmon Group (Lower Cretaceous) is a non-marine sequence in the Inner Zone of southwest Japan and is divided into the lower Wakino (lacustrine) and the upper Shimonoseki (fluvial) subgroups. Major diagenetic changes in this group are compaction, iron-oxide cementation, calcite cementation and grain replacement, quartz overgrowth and pore-fill cementation, illite authigenesis, chlorite pore-fill cementation and grain replacement, albitization of feldspar, and grain replacement by pyrite. Two subgroups of the Kanmon Group present no significant differences in general diagenetic features, paragenetic sequence, or the degree of diagenetic changes despite differences in depositional environments (lacustrine vs. fluvial) and stratigraphic positions. However, some differences are recognized in the content and chemistry of authigenic minerals caused by different sandstone framework compositions. The content of authigenic clay minerals is higher in sandstones of the Shimonoseki Subgroup containing abundant volcanic rock fragments. In addition, the composition of chlorite, the most abundant authigenic clay mineral in Kanmon sandstones, is Mg-rich in the volcanoclastic Shimonoseki sandstones, compared to an Fe-rich variety in Wakino sandstones. The original sandstone composition played a significant role in pore-water composition and diagenetic reactions.The Wakino sandstones lost most of its porosity by compaction, whereas Shimonoseki sandstones are only compacted in the vicinity of the basin-bounding fault. The weakly compacted Shimonoseki sandstones, instead, were largely cemented by pore-filling calcite during early diagenesis; cementation prevented compaction during further burial. The Kanmon Group sediments were heated to about 300 °C based on illite crystallinity values.     

Petrofacies, provenance and diagenesis of the dhosa sandstone member (Chari Formation) at Ler, Kachchh sub-basin, Western India

The sandstones of the Dhosa Sandstone Member of Late Callovian and Early Oxfordian age exposed at Ler have been analyzed for their petrofacies, provenance, tectonic setting and diagenetic history. These sandstones are fine to medium grained and poorly- to well sorted. The constituent mineral grains are subangular to subrounded. These sandstones were derived from a mixed provenance including granites, granite–gneisses, low- and high-grade metamorphic and some basic rocks of the Aravalli Range and Nagarparkar Massif. The petrofacies analysis reveals that these sandstones belong to the continental block-, recycled orogen- and rifted continental margin tectonic regime.The imprints of early and deep burial diagenesis of these sandstones include different stages of compaction, cementation, change in crystal boundaries, cement–cement boundaries, chertification and neomorphism. The sequence of cementation includes precipitation of calcite and its subsequent replacement by Fe calcite and silica cements. The typical intermediate burial (2–3 km depth) diagenetic signatures of these sandstones are reflected in the formation of suture and straight-line boundaries, and triple junctions with straight-line boundaries. The depositional environment, relatively low-energy environment that was below storm wave base but subjected to gentle currents, of the Dhosa Sandstone Member controlled the early diagenesis, which in turn influenced the burial diagenesis of these sandstones.     

Relationship of diagenetic chlorite rims to depositional facies in Lower Cretaceous reservoir sandstones of the Scotian Basin

The relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the Lower Cretaceous Missisauga Formation was investigated using a combination of electron microprobe, bulk geochemistry and X‐ray diffraction data. The succession studied comprises several stacked parasequences. The delta progradational facies association includes: (i) fluvial or distributary channel sandstones (some with tidal influence); (ii) thick‐bedded delta‐front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms; and (iii) more distal muddier delta‐front and prodeltaic facies. The transgressive facies association includes lag conglomerate, siderite‐cemented muddy sandstone and mudstone, and bioclastic sandy limestone. Chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles. Good quality chlorite rims are well correlated with Ti in bulk geochemistry. Ti is a proxy for Fe availability, principally from the breakdown of abundant detrital ilmenite (FeTiO₃). Under conditions of sea floor diagenesis, the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate‐depletion level and of the overlying Eh‐controlled diagenetic zones, resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones. This early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite. Thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation. This study thus shows that the preservation of porosity by chlorite rims is a two‐stage process. Rapidly deposited delta‐front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims, but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis.     

Diagenetic Evolution and Formation Mechanisms of High-Quality Reservoirs under Multiple Diagenetic Environmental Constraints: An Example from the Paleogene Beach-Bar Sandstone Reservoirs in the Dongying Depression, Bohai Bay Basin

The diagenetic environment, diagenetic responses, diagenetic transformation model and formation mechanisms of high-quality reservoirs (beach-bar sandstones of the Paleogene fourth member) in the Dongying depression were studied through the analysis of fluid inclusions, thin section and burial evolution history. The diagenetic fluids of the beach-bar sandstone reservoirs evolved from early high salinity and weak alkalinity to low salinity and strong acidity, late high salinity and strong alkalinity and late low salinity and acidity, which were accompanied by two stages of oil and gas filling. The fluids at the margins of the sandbodies were continuously highly saline and strongly alkaline. The western (eastern) reservoirs experienced early open (closed), middle open, and late closed diagenetic environments during their burial history. The flow pattern was characterized by upwelling during the majority of the diagenesis (in the east, a non-circulating pattern transitioned into an upwelling current). Due to the evolution of the diagenetic fluids, the diagenetic sequence of the beach-bar reservoirs was as follows: early weak carbonate cementation; feldspar and carbonate cement dissolution and authigenic quartz cementation; late carbonate and anhydrite cementation, authigenic feldspar cementation, and late quartz dissolution; and late carbonate cementation, feldspar dissolution, and authigenic quartz cementation. The diagenetic strength during these stages varied or was absent altogether in different parts of the reservoirs. Due to the closeness of the diagenetic environment and the flow pattern of the diagenetic fluids, the diagenetic products are variably distributed in the sandstones interbedded with mudstones and in the fault blocks. The evolution of multiple alternating alkaline and acidic diagenetic environments controlled the distribution patterns of the reservoir diagenesis and reservoir space, and the reservoir quality index, RQI, increased gradually from the margins to the centers of the sandstones. The closeness of the diagenetic environment and the flow patterns of the diagenetic fluids controlled the differences in the reservoir properties among the fault blocks. With increasing distance from the oil-source faults, the RQI values in the west gradually decreased and in the east initially increased and then decreased.     

基于星点状方解石胶结与溶解的识别查明砂岩粒间孔隙类型——以雅布赖盆地新河组砂岩为例

目前含油气砂岩中粒间孔隙是原生孔隙还是次生孔隙的认识仍不一致,而对星点状方解石胶结与溶解的识别能够有效地查明粒间孔隙的类型。文章通过铸体薄片细致地观察雅布赖盆地新河组砂岩中的微观现象,以成岩环境演化和成岩序列分析为主线,重视方解石胶结物的赋存状态与物质来源和溶蚀流体来源的配置关系,精细解剖微观现象,从而弄清楚星点状方解石的成因,进而查明砂岩的粒间孔隙类型和储集空间类型。结果表明,粒间孔隙中的星点状方解石是成岩早期浸染状方解石胶结物的溶蚀残余,溶蚀流体为成岩中期的有机酸流体,溶蚀类型为一致性溶解,形成的粒间孔隙为次生孔隙。鉴于此,雅布赖盆地新河组砂岩的储集空间由次生粒间孔隙和次生粒内孔隙（长石、岩屑、方解石胶结物的溶蚀孔隙）组成。      

塔中地区志留系沥青砂岩成岩作用及其对储层性质的影响

运用沉积学原理对塔中地区志留系经历复杂成岩演化的沥青砂岩进行研究分析,了解到该岩石主要经过压实作用、胶结作用和溶蚀作用等成岩作用。其胶结物的主要类型有绿泥石、伊利石、高岭石、伊-蒙混层等粘土矿物以及碳酸盐矿物、石英、长石、黄铁矿等。孔隙类型以粒间孔隙、粒内孔、铸模孔、特大孔、裂缝孔隙等为主,其中以粒间孔隙最为发育,低地温梯度有利于在深层形成次生孔隙。志留系沥青砂岩储层物性受沉积和成岩双重因素的影响,成岩演化阶段处于晚成岩A期。根据影响储层物性的储层成岩作用,可将成岩相类型划分为不稳定碎屑溶蚀成岩相、压实-弱溶蚀成岩相、压实充填成岩相、压实压溶成岩相、碳酸盐胶结成岩相等几种主要类型,成岩相类型决定了储层性质,不稳定碎屑溶蚀成岩相和压实-弱溶蚀成岩相是本区较好的油气储层。     

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.     

鄂尔多斯盆地彭滩-杨井地区长2储层的成岩作用

彭滩-杨井地区长2主力层砂岩岩石学、成岩作用、孔隙类型研究表明：该区长2储层岩石类型以长石砂岩和岩屑长石砂岩为主,成分和结构成熟度均较低,主要经历的成岩作用有压实作用、胶结作用、交代作用和溶解作用。根据砂岩成岩作用特征,结合其埋藏深度、岩石古地温、有机质成熟度等分析资料及岩石结构、孔隙类型特征,长2主力层主要处于晚成岩A期成岩阶段。     

塔里木盆地大宛齐油田康村组成岩作用及其控制因素

大宛齐油田是迄今为止在塔里木盆地发现的产层最浅的油田 ,主力产油层段为第四系和上第三系的康村组。通过岩心和薄片的观察、X衍射分析、扫描电镜分析等资料 ,对砂岩的岩石学特征进行了探讨 ,深入研究了储层的成岩作用及其控制因素。大宛齐地区康村组砂岩主要由一套不同粒级的岩屑砂岩组成 ,总体上砂岩的成分成熟度较低 ,结构成熟度也不高。主成岩作用是机械压实和胶结作用 ,成岩环境为略偏碱性—中性的干旱环境 ,埋藏深度浅 ,地温梯度较低 ,现今储层正处于早成岩阶段A亚期 ,孔隙度的演化也相应地可分为快速压实、孔隙度快速降低阶段和稳定压实、方解石胶结物快速增加阶段。控制成岩作用的主因素是储层的埋深、储层岩石成分等。     

江汉盆地白垩系渔洋组砂岩的成岩作用及其热力学分析

江汉盆地白垩系渔洋组砂岩存在五种类型的成岩作用。其中,与油气运移有关并对储层孔隙演化产生重要影响的是硬石膏的胶结与交代作用和后期方解石再次胶结作用。为了更好的研究成岩作用发生的控制因素以及成岩环境的变化,我们通过细致的热力学推导,绘制出包含方解石、白云石和硬石膏等矿物相在内的五组分相图。通过相图分析可以看出,pH 值在成岩作用过程中有着重要的作用。方解石和白云石的胶结及交代作用代表了偏中性或碱性的成岩环境;而硬石膏的胶结及交代作用则表明成岩环境由中性或碱性向酸性转变,例如酸性的油气前锋流体的注入。另外,成岩环境中阳离子(Ca~(2 )、Mg~(2 ))和阴离子(CO_3~(2-)、SO_4~(2-))的浓度对方解石、白云石和硬石膏的胶结、溶解及交代作用也有重要影响。     

鄂尔多斯盆地直罗油田长6油层组成岩作用及孔隙演化研究

位于鄂尔多斯盆地的直罗油田三叠系延长组长6油层组为近年来新开发的一个主力油气产层。岩石学和成岩作用特征研究表明,直罗油田长6油层组以细粒长石砂岩和岩屑长石砂岩为主,砂岩经历了机械压实作用、胶结作用、交代作用和溶蚀作用等成岩作用,成岩作用处于晚成岩阶段A期。成岩作用控制了储层砂岩孔隙发育特征,其中压实作用与胶结作用是导致储层超低渗的主导因素,而次生溶蚀孔隙的形成对储层砂岩物性具有一定改造作用,并控制了该区长6油层组相对高孔高渗储层的展布特征。     

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.     

沉积和成岩作用对湖相重力流致密砂岩储层质量的影响——以鄂尔多斯盆地合水地区长7段为例

为揭示深水重力流沉积致密砂岩储层发育特征及分布规律,笔者等以鄂尔多斯盆地合水地区三叠系延长组7段（简称长7段）为例,通过岩芯描述、铸体薄片以及扫描电镜观察,结合物性数据以及分析化验资料对研究区长7段沉积、储层岩石学、孔隙类型以及成岩作用特征进行了分析,在沉积和成岩作用特征研究基础上探讨了优质储层的形成机理,据此建立了湖相深水重力流致密砂岩储层质量的演化模式。结果表明：合水地区长7段储层岩性以长石岩屑砂岩为主,孔隙类型主要为溶蚀孔和残余粒间孔,属于特低孔—超低渗储层,主要成岩作用为压实、胶结和溶蚀作用。重力流致密砂岩储层质量受沉积相和成岩作用共同影响,限制性水道中沉积的砂体由于具有强的抗压实能力、少的伊利石和碳酸盐矿物含量以及较多长石、岩屑溶蚀孔隙,储层质量最好。不同沉积环境中的砂岩在颗粒粒度、分选、沉积厚度方面存在差异,通过影响成岩作用类型与强度综合控制储层质量。研究成果可为相同类型储层的预测和评价提供参考。     

鄂尔多斯盆地东部下二叠统山西组山2段成岩相划分及展布

鄂尔多斯盆地东部下二叠统山西组山2段岩石类型主要为石英砂岩、岩屑质石英砂岩和岩屑砂岩,储集层经历了压实、压溶、硅质胶结、碳酸盐胶结交代、高岭石胶结、杂基蚀变以及溶蚀等多种成岩作用。在成岩作用研究的基础上,通过大量的岩心观察和薄片鉴定,结合阴极发光、探针分析等方法,将研究区划分出7种单因素成岩相,即：强压实、压溶-石英次生加大、蚀变高岭石、沉淀高岭石、杂基充填、碳酸盐胶结交代、不稳定组分溶蚀成岩相,在此基础上对其进行平面叠加,确定了利于储集层发育的优势成岩相。其中压溶-石英次生加大与杂基、假杂基溶蚀作用的综合作用（叠加和改造）决定了山西组山2段有利的成岩相带,在陕141井区、榆37井区、子洲-清涧地区山西组山2段形成了天然气优质储集层。     

Diagenesis impact on a deeply buried sandstone reservoir (Es1 Member) of the Shahejie Formation,Nanpu Sag,Bohai Bay Basin,East China

Diagenesis has a significant impact on reservoir quality in deeply buried formations. Sandstone units of the Shahejie Formation (Es₁ Member) of Nanpu Sag, Bohai Bay Basin, East China is a typical deeply buried sandstone with large hydrocarbon accumulations. The methodology includes core observations and thin section studies, using fluorescence, scanning electron microscope (SEM), cathodoluminescence (CL), fluid inclusion and isotope and electron probing analysis as well as the numerical determination of reservoir characteristics. The sandstones consist of medium to coarse-grained, slight to moderate sorted lithic arkose and feldspathic litharenite. Porosity and permeability values range from 0.5 to 30% and 0.006 to 7000 mD, respectively. The diagenetic history reveals mixed episodes of diagenesis and deep burial followed by uplift. The main diagenetic events include compaction, cementation alteration, dissolution of unstable minerals and grain fracturing. Compaction resulted in densification and significantly reduced the primary porosity. Quartz, calcite and clay are the dominant pore-occluding cement and occur as euhedral to subhedral crystals. Alteration and dissolution of volcanic lithic fragments and pressure solution of feldspar grains were the key sources of quartz cement whereas carbonate cement is derived from an external source. Clay minerals resulted from the alteration of feldspar and volcanic lithic fragments. Porosity and permeability data predict a good inverse relationship with cementation whereas leaching of metastable grains, dissolution of cement and in some places formation of pore-lining chlorite enhanced the reservoir quality. The best reservoir is thicker sandstone bodies that are medium to coarse-grained, well-sorted sandstone with low primary ductile grains with a minor amount of calcite cement. The present study shows several diagenetic stages in the Es₁ Member, but the overall reservoir quality is preserved.