Siliciclastic grain breakage and displacement due to carbonate crystal growth: an example from the Lueders Formation (Permian) of north-central Texas, U.S.A.

The Lueders Formation (mid-Permian) in Baylor County, Texas, is an intercalated suite of fluvial siliciclastic, shallow marine siliciclastic, and shallow marine carbonate strata. There are at least two generations of carbonate cements (probably originally composed of calcite) in the fluvial sandstones where fractured grains are observed. These cements represent the initial stages of caliche formation. Cementation is envisioned as a two step process. In the first step, calcite cements form from supersaturated fluids in a freshwater, vadose environment as a meniscus cement at grain contacts. Areas of cement formation are restricted to these sites because fluid distribution is restricted to these sites. Stresses generated by the growth of cements at grain contacts are transmitted through and concentrated at quartz/quartz grain point-contacts until the stress is sufficient to fracture quartz grains, even though the ultimate strength of calcite is less than that of quartz, per unit area. This process occurs too rapidly to be accommodated by pressure solution. In the second phase of cementation, cement nucleation is no longer restricted by vadose conditions. In this phase, calcite growth can no longer result in quartz grain breakage; rather, the quartz grains are dispersed in poikilotopic calcite cement.     

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。     

Diagenetic features of Jurassic Fort Member sandstone,Jaisalmer formation,western Rajasthan

Jaisalmer Formation consists of 360m thick succession of medium to coarse grained sandstones with interbeds of shale, claystone and occasional lignite that rest over Lathi Formation, is the basal part of the Jaisalmer basin. The rocks are exposed amidst desert, low mounds and shallow stone quarries. Sandstones were deposited in shallow marine to deltaic environments. The studied sandstones consist of abundant quartz followed by feldspar, mica, chert, rock fragments and heavy minerals. The study mainly deals with identification of various diagenetic features such as compaction, cementation and porosity evolution. During mechanical compaction rearrangement of grains took place and point, long and suture contacts were formed. The sandstones are cemented by iron oxide, silica overgrowth, carbonate and clay. Porosity has developed due to dissolution of iron, carbonate cement and feldspar grains. Dissolution and alteration of feldspar, lithic fragments and pressure solution were the main source of quartz cements. The sandstones show good amount of existing optical porosity with an average of 7.19%. Porosity reduction is mainly due to early stage of mechanical compaction and subsequent pervasive calcite and iron oxide cementation. Further, porosity reduced due to deposition of clay cement.     

Genesis and diagenetic evolution of Habo formation,Kachchh, Gujarat

The Kachchh Basin is a pericratonic rift basin situated at the western margin of the Indian plate. The Habo Dome embodies an important exposure of Bathonian to Kimmergian sediments among the Kachchh Mainland exposures. Based on vertical facies transitions, facies associations were documented: mixed shallow marine (Facies association 1), shoreface and lagoon deposits (Facies association II) and subtidal innershelf below fair weather wave base (Facies association III). The documented facies associations reflect that Habo Dome sediments deposited in a variety of environments from shallow marine to fluvio-deltaic and were strongly influenced by fluctuation of relative sea level. The dominance of floating grains and point contacts in the sandstone indicate that detrital grains do not show much pressure effects as a result of either shallow burial or early cementation. The sandstones were cemented by iron oxide, carbonate and silica in order of abundance. Three types of cements, blocky, rim and fibrous cement occur in the studied limestone representing phreatic, fresh water phreatic and deep burial diagenetic stages. Neomorphism and micritization are common. Both primary and secondary porosity exists in these sediments. Different graphs of porosity versus depth suggest a depth of burial in the range of 615–769 m.     

Facies controlled diagenetic evolution of the Delhi Group Sandstones,Bayana Basin,Rajasthan

The Bayana Basin forms the eastern most limit of north Delhi fold belt covering parts of northeastern Rajasthan. The deposition of sediments took place during middle Proterozoic in fluvial and shallow marine environments. The rocks are mainly clastics and include conglomerate, sandstone and shale. During mechanical compaction rearrangement of grains took place and point and long contacts were formed. The sandstones are cemented by iron oxide, silica, carbonate and barite. The porosity has developed due to dissolution of iron, carbonate cement and feldspar grains. Dissolution of quartz along grain boundaries and silica rich compaction pore water seems to be the main source of silica. These observations suggest progressive compaction which initiated at the sediment water interface and continued till deep burial diagenesis in a rapidly subsiding basin.     

Pressure solution in sandstones: influence of clays and dependence on temperature and stress

The enhancement of dissolution of quartz under the influence of clays has been recognized in sandstones for many years. It is well known that a grain of quartz in contact with a clay flake dissolves faster than when in contact with another grain of quartz. This phenomenon promotes silica transfer during the diagenesis of sandstones and is responsible of deformation and porosity variations. Here we make an attempt to explain the process of this rock deformation using a pressure solution mechanism.

The model of water film diffusion assumes that matter is dissolved inside the contact between two grains. The resulting solutes are transported to the pore fluid through diffusion along an adsorbed water film. Between two micas, this trapped film is thicker than between two grains of quartz. As a consequence diffusion is easier and the rate of pressure solution faster.

Experiments on pressure solution show that diffusion controls the mechanism at great depth whereas a model based on natural mica indentation indicates that kinetics is the limiting process through the precipitation rate of quartz at low depth, thus temperature is a crucial parameter. There should be a transition between thermally controlled rate and diffusion limited evolution.     

鄂尔多斯盆地上三叠统延长组机械压实作用与 砂岩致密过程及对致密化影响程度

近年来,随着致密砂岩油气藏研究的不断深入,亟需明确致密砂岩储层致密化机理,从而更好地利于致密砂岩油气藏的勘探与开发。机械压实作用通常被理解为中浅层砂岩所经历的主要成岩作用,而深层的机械压实作用长期以来被人们所忽视。为探讨机械压实作用在砂岩致密过程中的作用及对致密化的影响程度,本次研究以鄂尔多斯盆地上三叠统延长组致密砂岩为例,通过井孔浅层与深层孔隙度变化趋势分析、镜下岩石结构的证据和砂岩压实模拟实验3个方面对延长组致密砂岩储层机械压实作用特征进行了分析。研究发现砂岩在深层与浅层压实减孔趋势一致或平行;随着上覆压力的增加,砂岩一直表现为减孔效应且机械压实作用强度随埋深的增加而变大;薄片特征显示延长组致密砂岩机械压实作用存在分级特征,不同级别的压实强度对应一定的镜下压实特征,通过统计薄片数据发现样品都处于压实作用与胶结作用对孔隙度影响评价图(Houseknecht图版)的左下区域。结果表明砂岩在埋藏致密化过程中自始至终存在机械压实作用,并且机械压实作用是导致砂岩致密减孔最重要的原因,远远超过胶结作用产生的减孔效应。由于砂岩孔隙度演化不仅与埋深有关,同样受到地质时间的影响,本次研究通过数学推导建立了砂岩孔隙度时间埋深双元函数模型,进而通过孔隙度双元函数可以对压实减孔进行定量分析,表征机械压实作用对砂岩致密化过程的影响程度。     

石英砂岩的硅质胶结作用及其对储集性的影响

作者应用阴极发光显微镜、扫描电镜、电子探针及包体测温等手段,对华北太原组石英砂岩的硅质胶结现象、形成温度、物质来源及其对储集性的影响作了研究。结果表明:太原组石英砂岩有两期硅质胶结,第二期规模大;其形成温度为130°-140℃;砂岩处于中成岩阶段晚期;硅质胶结作用是使该砂岩丧失良好储集性能的主要因素。     

A new mechanism for pressure solution in porous quartzose sandstone

The mechanism of pressure solution, a source of controversy for years, must be understood before we can evaluate the effectiveness of pressure solution during geological processes. The water film diffusion (WFD) mechanism proposed by Weyl (1959) and Rutter (1976, 1983) is believed by many to be the primary mechanism responsible for intergranular pressure solution (IPS) in non-porous metamorphic rocks as well as porous sedimentary rocks. Tada and Siever (1986), experimenting with halite single crystals, suggested the new plastic deformation plus free-face pressure solution (PD + FFPS) mechanism.The effectiveness of PD + FFPS as an IPS mechanism is theoretically evaluated for porous quartzose sandstone and compared with WFD. The result suggests that, though the driving force of the reaction (relative activity increase) is 4 to 5 orders of magnitude larger in WFD, the ease of diffusion (diffusion path width times the diffusion coefficient) is 7 to 9 orders of magnitude larger in PD + FFPS. Consequently. PD + FFPS yields diffusion rates 2 to 5 orders of magnitude faster than WFD.In WFD, diffusion is always the rate-controlling process, whereas either dissolution at IPS contacts or precipitation on free grain surfaces may be the rate-controlling process in PD + FFPS, when temperatures are low and/or grain sizes are small. The dissolution or precipitation rate of PD + FFPS is faster than the diffusion rate of WFD except when the total free grain surface area is very small. In final stages of compaction, when the total free grain surface area has become very small, WFD replaces PD + FFPS.     

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.     

Effects of phosphate ions on intergranular pressure solution in calcite: An experimental study

Intergranular pressure solution (IPS) is a coupled chemical-mechanical process of widespread importance that occurs during diagenesis and low-temperature deformation of sedimentary rocks. Laboratory experiments on IPS in halite, quartz, and calcite have largely concentrated on the mechanical aspects of the process. In this study, we report the effects of pore fluid chemistry, specifically varying phosphate ion concentration, on the mechanical compaction by IPS of fine-grained calcite powders at room temperature and 1 to 4 MPa applied effective stress. Phosphate was investigated because of its importance as a biogenic constituent of sea and pore waters. Increasing the pore fluid phosphate concentration from 0 to 10⁻³ mol/L systematically reduced compaction strain rates by up to two orders of magnitude. The sensitivity of the compaction strain rate to phosphate concentration was the same as the sensitivity of calcite precipitation rates to the addition of phosphate ions reported in the literature, suggesting that the rate of IPS in phosphate-bearing samples was controlled by calcite precipitation on pore walls. The results imply that IPS and associated porosity/permeability reduction rates in calcite sediments may be strongly reduced when pore fluids are enriched in phosphates, for example, through high biologic productivity or a seawater origin. Future modeling of IPS-related processes in carbonates must therefore take into account the effects of pore fluid chemistry, specifically the inhibition of interfacial reactions.     

Fluid overpressure as a control on sandstone reservoir quality in a mechanical compaction dominated setting: Magnolia Field,Gulf of Mexico

The reservoir quality (porosity and permeability) of deeply buried hydrocarbon reservoir sandstones in sedimentary basins is significantly affected by burial diagenesis. Many deep reservoirs develop anomalous fluid overpressures during burial. Previous studies on the effect of fluid overpressure on reservoir quality in these deep reservoirs have been inconclusive because of the difficulty in constraining the individual contributions of various porosity preserving factors that are simultaneously active in these reservoirs. Owing to its rapid burial and low burial temperatures, the Neogene gravity‐flow sandstone reservoirs from the Magnolia Field, Gulf of Mexico, offers a unique opportunity to investigate in isolation the effect of fluid overpressure on reservoir quality. Examination of petrography, pore pressure and routine core analysis datasets showed a positive correlation between high fluid overpressure and enhanced reservoir quality. This study confirms that fluid overpressure preserves reservoir quality in deeply buried sandstone reservoirs in compaction dominated, high sedimentation basin settings.     

Quartz Cement in Middle Jurassic Reservoir Sandstones in North Sea A Review. Part II： Duration and Silica Sources

The timing and duration of quartz cementation in sandstones have been mainly inferred from diagenetic texture, relationship between pore filling minerals, fluid inclusions and isotopic data. Fluid inclusion temperatures from North Sea reservoir sandstones indicate that most of the quartz cement forms at temperature exceeding 90℃ and is continually proceeding after oil emplacement, based on the fluid inclusion temperatures in quartz overgrowth which is approaching the bottom-hole temperatures. The duration of quartz cement after oil emplacement depends upon the saturation of porewater and the distribution of pore water film and the property of water-wet or oil-wet of the reactants. The leaching of K-feldspar by meteoric water requires pore water flow to move the released potassium and sodium and silica out the solution, which suggests the mechanism does not appear to be a major source of silica for quartz cementation. The quartz cementation coincidence with the compaction and pressure solution suggests the major source of silica. The alteration of feldspar by illitization of kaolinite may serve as another important source of silica at deep burial depth. External sources are not need to call on for illustrating the quartz cementation, because there is no evidences for large scale convection of pore water flow occurred in the burial history of reservoir sandstones of middle Jurassic in the North Sea.     

Kaolinitic meniscus bridges as an indicator of early diagenesis in Nubian sandstones, Sinai, Egypt

Characteristic meniscus geometries formed by kaolinitic infiltration are observed in Nubian sandstones collected on the western coast of the Sinai Peninsula (Egypt). Based on petrographic and scanning electron microscope (SEM) observations, the kaolinite forming the menisci consists of mixed-size discrete platelets that gather into the corners around the framework grain-to-grain contacts. Specifically, the internal fabric of menisci indicates a general organizing trend from (1) the centre, where the platelets coat the framework grains; (2) to the peripheral zone, where they are oriented tangentially to grain surfaces; (3) to the pore linings with curvatures that are consistent with theoretical considerations of air–water interfaces. This typical arrangement suggests a detrital origin of kaolinite platelets by mechanical infiltration into sediments lying above the tablewater, in vadose conditions. This type of clay cementation occurring during early diagenesis can prevent (delay) deep burial diagenetic processes and therefore preserve excellent reservoir properties.     

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.     

Very low temperature,natural deformation of fine grained limestone: a case study from the Lucania region,southern Apennines,Italy

Abstract

The deformation behavior of fine grained limestones from the Monte Sirino area (Lucania region) of the southern Apennines has been analysed by constraining microstructural observations and crystallographic fabrics with data on the metamorphic conditions of deformation. X-ray and infrared analysis of clay minerals, together with illite ‘crystallinity’ data, suggest that the studied rocks underwent very low grade metamorphism in the deep diagenetic zone. The limestones consist of very fine grained (<10 μm) aggregates of micrite. Elliptically-shaped radiolarians, preserved as moulds with coarser (>20 μm) crystalline fillings, provide common strain markers. Optical microstructures and strain analysis indicate heterogeneous intracrystalline strain in the coarser (>50 μm) calcite. On the other hand, SEM and TEM observations, and crystallographic fabrics determined by X-ray texture goniometry, indicate a deformation involving not only intracrystalline slip, but also an important component of grain boundary sliding in the fine grained matrix. The inferred microscopic deformation mechanisms are compared with constitutive flow laws derived from experimental studies. For the maximum inferred temperature of deformation of 250 °C and geologic strain rates of 10^?13?10^?15 s^?1, deformation mechanism maps for calcite suggest twinning and other glide mechanisms to be active in grains larger than about 5?10 μm. Smaller grains would be mostly deformed by grain size sensitive creep mechanisms, which include both diffusion mass transfer processes and grain boundary sliding. Deformation features observed in the study limestones are compatible with the prediction of such temperature-dependent mechanism maps. © 2001 Éditions scientifiques et médicales Elsevier SAS     

Disseminated ‘jigsaw piece’ dolomite in Upper Jurassic shelf sandstones,Central North Sea: an example of cement growth during bioturbation?

Unusual textural and chemical characteristics of disseminated dolomite in Upper Jurassic shelf sediments of the North Sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones. The dolomite is present throughout the Franklin Sandstone Formation of the Franklin and Elgin Fields as discrete, non‐ferroan, generally unzoned, subhedral to highly anhedral ‘jigsaw piece’ crystals. These are of a similar size to the detrital silicate grains and typically account for ≈5% of the rock volume. The dolomite crystals are never seen to form polycrystalline aggregates or concretions, or ever to envelop the adjacent silicate grains. They are uniformly dispersed throughout the sandstones, irrespective of detrital grain size or clay content. Dolomite authigenesis predated all the other significant diagenetic events visible in thin section. The dolomite is overgrown by late diagenetic ankerite, and bulk samples display stable isotope compositions that lie on a mixing trend between these components. Extrapolation of this trend suggests that the dolomite has near‐marine δ¹⁸O values and low, positive δ¹³C values. The unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near‐surface zone of active bioturbation. Sea water provided a plentiful reservoir of Mg and a pore fluid of regionally consistent δ¹⁸O. Labile bioclastic debris (e.g. aragonite, Mg‐calcite) supplied isotopically positive carbon to the pore fluids during shallow‐burial dissolution. Such dissolution took place in response to the ambient ‘calcite sea’ conditions, but may have been catalysed by organic matter oxidation reactions. Bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones, but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework. Continued exchange of Mg²⁺ and Ca²⁺ with the sea‐water reservoir maintained a sufficient Mg/Ca ratio for dolomite (rather than calcite) to form. Irregular crystal shapes resulted from dissolution, of both the dolomite and the enclosed fine calcitic shell debris, before ankerite precipitation during deep‐burial diagenesis.     

Diagenesis of sandstones in the back-arc basins of the western Pacific Ocean

ABSTRACT Sandstones occur in back-arc basins of the western Pacific at DSDP sites 299 (Sea of Japan), 297 (northern Shikoku Basin), 445 and 446 (Daito-Ridge-and-Basin Province), 453 (Mariana Trough), 286 (New Hebrides Basin) and 285 (South Fiji Basin). These sandstones are dominantly volcaniclastic arenites derived from andesitic island arcs. The degree of sandstone diagenesis is dependent on original composition, burial rate, heat flow history of the basin, and timing of sandstone deposition with respect to rifting processes and associated high heat flow.
Sandstones containing a larger proportion of volcaniclastic components showed more diagenetic effects than sandstones containing a significant volume of other rock fragments and mineral components. Sandstones deposited during early stages of rifting (sites 445, 446) with a slow burial rate and high crustal heat flow showed the greatest degree of downhole diagenetic change. These diagenetic changes include early pore-space reduction and rim cementation by clay minerals followed later by calcite, and subsequent pore-fill cementation by clinoptilolite, heulandite, analcite and later calcite. Replacement of recognizable volcanic rock fragments by chert, calcite and zeolites was observed in the deepest part of the hole. Sandstones deposited after rifting under conditions of associated lower heat flow showed considerably less diagenetic changes, particularly if burial was rapid.
The high heat flow associated with earliest rifting, associated fluid circulation driven by thermal convection, and slow burial rate controlled the diagenetic history of these sandstones. Thus, timing of sandstone deposition with rifting stage and associated burial rates were key factors in controlling sandstone diagenesis in back-arc basins.     

Experimental pressure solution compaction of synthetic halite/calcite aggregates

Experimental observations are reported of weakening of sediment-like aggregates by addition of hard particles. Sieved mixtures of calcite and halite grains are experimentally compacted in drained pressure cells in the presence of a saturated aqueous solution. The individual halite grains deform easily by pressure solution creep whereas calcite grains act as hard objects and resist compaction. The fastest rate of compaction of the mixed aggregate is not obtained for a 100% halite aggregate but for a content of halite grains between 45% and 75%. We propose that this unusual compaction behavior reflects the competition between two mechanisms at the grain scale: intergranular pressure solution at grain contacts and grain boundary healing between halite grains that prevent further compaction.     

The effects of diagenesis on the reservoir characters in sandstones of the Late Cretaceous Pab Formation,Kirthar Fold Belt,southern Pakistan

The Maastrichtian Pab Formation in the southern part of Pakistan is composed of fine- to very coarse-grained texturally mature quartz arenite and subordinate sublitharenite varieties. The sandstones have undergone intense and complex diagenetic episodes due to burial and uplift. Diagenetic modifications were dependent mainly on the clastic composition of sandstone, burial depth and thrust tectonics. Diagenetic events identified include compaction, precipitation of calcite, quartz, clay minerals and iron oxide/hydroxide, dissolution and alteration of unstable clastic grains as feldspar and volcaniclithic fragments as well as tectonically induced grain fracturing. The unstable clastic grains like feldspar and lithic volcanic fragments suffered considerable alteration to kaolinite and chlorite. Dissolution and alteration of feldspar and volcanic lithic fragments and pressure solution were the main sources of quartz cements. Mechanical compaction and authigenic cements like calcite, quartz and iron oxide/hydroxide reduced the primary porosity, whereas dissolution of clastic grains and cements has produced secondary porosity. Chlorite coatings on clastic grains have prevented quartz cementation. Coarse-grained, thick bedded packages of fluviodeltaic, shelf delta lobe and submarine channels facies have higher average porosity than fine-grained, thin bedded and bioturbated sandstone of deeper shelf and abyssal plain environments and these facies are concluded to be possible future hydrocarbon prospects.