Development Of Secondary Porosity Research Articles

Diagenesis and reservoir quality analysis in the Lower Cretaceous Qishn sandstones from Masila oilfields in the Sayun–Masila Basin, eastern Yemen

Lower Cretaceous sandstones of the Qishn Formation have been studied by integrating sedimentological, petrological and petrophysical analyses from wells in the Masila oilfields of eastern Yemen. These analyses were used to define the origin, type of diagenesis and its relation to reservoir quality. The sandstones of the Qishn Formation are predominately quartz arenite to subarkose arenite with sublitharenite and quartz wackes displaying a range of porosities, averaging 22.33%. Permeability is likewise variable with an average of 2844.2 mD. Cementation coupled with compaction had an important effect on porosity destruction after sedimentation and burial. The widespread occurrence of early calcite cement suggests that the sandstones of the Qishn Formation lost significant primary porosity at an early stage of its diagenetic history. In addition to poikilotopic calcite, several different cements including kaolinite, illite, chlorite and minor illite–smectite occur as pore‐filling and pore‐lining cements, which were either accompanied by or followed the development of the early calcite cement. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar grains. The new data presented in this paper suggest the reservoir quality of Qishn sandstones is strongly linked to their diagenetic history; hence, the reservoir quality is reduced by clay minerals, calcite and silica cements but is enhanced by the dissolution of the unstable grains, in addition to partial or complete dissolution of calcite cements and unstable grains. Copyright © 2015 John Wiley & Sons, Ltd.

Deep-burial dissolution and brecciation in dolostone reservoirs: Striking similarities between the Devonian Keg River Formation of western Canada and the Ordovician Ellenburger group of west Texas

Deeper burial dissolution of dolomitized fabrics created most of the reservoir quality in several Devonian Keg River oil pools on the Comet Platform in northwestern Alberta, Canada. This process also created reservoir quality in two Ordovician Ellenburger dolostone oil pools on the Eastern Shelf of the Midland Basin in west Texas, and provided an alternative explanation for reservoir development unrelated to the subaerial karstification model. Several observations confirmed deeper burial dissolution of Keg River dolostones. First, dolomites replaced, or cemented, grains previously sutured by pressure solution. Their subsequent dissolution postdated this pressure solution, confirming burial dissolution. Second, late-forming saddle dolomites dissolved. Third, secondary porosity developed along, or cross cut, stylolites, often in zebra dolostone fabrics, or formed along fracture planes that cut stylolites. These burial fractures often intersected secondary pores, implying a causal relationship. Fourth, dolostone breccia clasts contained stylolites rotated at different angles to each other and the horizon, confirming deeper burial brecciation. In the Keg River Formation on Comet Platform, proximity to conjugate reactivated basement faults that developed off of a master wrench fault, the Hay River Shear Zone, controlled deeper burial dissolution and pool entrapment. Hydrothermal fluids, either calcium rich or acidic in composition, were moved along these faults and promoted extensive dissolution of completely dolomitized fabrics. Ellenburger dolostone reservoirs at Suggs and Withers Fields on the Eastern Shelf of Texas also resulted from deeper burial dissolution of completely dolomitized facies. The Keg River dissolution fabrics were replicated in Ellenburger samples, with the exception of zebra dolostones. Ellenburger reservoir development was closely aligned to faults and fractures associated with a master wrench fault, the Ft. Chadbourne Fault. These faults were inferred to have moved hot (possibly hydrothermal?), deeper burial diagenetic fluids that were calcium-rich, or periodically acidic, promoting dolomite dissolution and secondary porosity development.

High resolution cathodoluminescence spectroscopy of carbonate cementation in Khurmala Formation (Paleocene–L. Eocene) from Iraqi Kurdistan Region, Northern Iraq

A combination of high resolution cathodoluminsecnce-spectroscopy (HRS-CL) with spatial electron microprobe analysis and optical microscopy is used to determine paragenesis and history of cementation in the limestones and dolostones of Khurmala Formation which is exposed in many parts of Northern Iraq. Khurmala Formation was subjected to different diagenetic processes such as micritization, compaction, dissolution, neomorphism, pyritization and cementation that occurred during marine to shallow burial stages and culminated during intermediate to deep burial later stages. Five dolomite textures are recognized and classified according to crystal size distribution and crystal-boundary shape. Dolomitization is closely associated with the development of secondary porosity that pre-and postdates dissolution and corrosion; meanwhile such porosity was not noticed in the associated limestones. Microprobe analysis revealed three types of cement, calcite, dolomite and ankerite which range in their luminescence from dull to bright. Cathodoluminescence study indicated four main texture generations. These are (1) unzoned microdolomite of planar and subhedral shape, with syntaxial rim cement of echinoderm that show dull to red luminescence, (2) equant calcite cements filling interparticle pores which shows dull luminescence and weak zonal growth, (3.1) homogenous intrinsic blue stoichiometric calcite with dull luminescence and without activators, (3.2) coarse blocky calcite cement with strong oscillatory zoning and bright orange luminescence which postdates other calcite cements, (4) ankerite cement with red to orange, non-luminescence growth zonation which is the last formed cement.

Clastic Rock Reservoir Physical Characteristics of Southern Songliao Basin Changling Fault Depression Layer

We study the Changling fault depression layer reservoir space type and characteristic of clastic rock reservoirs in the study area, by core observation, thin section analysis, cathodoluminescence analysis, scanning electron microscopy and mercury intrusion method, and discusses the development of secondary porosity and fracture characteristics and formation mechanism.

Diagenetic evolution of the volcaniclastic deposits: an example from Neoproterozoic Dokhan Volcanics in Wadi Queih basin, central Eastern Desert, Egypt

Wadi Queih basin hosts a ∼2,500-m thick Neoproterozoic volcanoclastic successions that unconformably lie over the oldest Precambrian basement. These successions were deposited in alluvial fan, fluviatile, lacustrine, and aeolian depositional environments. Diagenetic minerals from these volcaniclastic successions were studied by X-ray diffractometry, scanning electron microscopy, and analytical electron microscopy. The diagenetic processes recognized include mechanical compaction, cementation, and dissolution. Based on the framework grain–cement relationships, precipitation of the early calcite cement was either accompanied or followed by the development of part of the pore-lining and pore-filling clay cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cement and feldspar grains. In addition to calcite, several different clay minerals including kaolinite, illite, and chlorite with minor smectite occur as pore-filling and pore-lining cements. Chlorite coating grains helps to retain primary porosity by retarding the envelopment of quartz overgrowths. Clay minerals and their diagenetic assemblages has been distinguished between primary volcaniclastics directly produced by pyroclastic eruptions and epiclastic volcaniclastics derived from erosion of the pre-existing volcanic rocks. Phyllosilicates of the epiclastic rocks display wider compositional variations owing to wide variations in the mineralogical and chemical compositions of the parent material. Most of the phyllosilicates (kaolinite, illite, chlorite, mica, and smectite) are inherited minerals derived from the erosion of the volcanic basement complex, which had undergone hydrothermal alteration. Smectites of the epiclastic rocks are beidellite–montmorillonite derived from the altered volcanic materials of the sedimentary environment. Conversely, phyllosilicate minerals of the pyroclastic rocks are dominated by kaolinite, illite, and mica, which were formed by pedogenetic processes through the hydrothermal influence.

Slow advance of the weathering front during deep, supply-limited saprolite formation in the tropical Highlands of Sri Lanka

Silicate weathering – initiated by major mineralogical transformations at the base of ten meters of clay-rich saprolite – generates the exceptionally low weathering flux found in streams draining the crystalline rocks of the mountainous and humid tropical Highlands of Sri Lanka. This conclusion is reached from a thorough investigation of the mineralogical, chemical, and Sr isotope compositions of samples within a regolith profile extending >10m from surface soil through the weathering front in charnockite bedrock (a high-grade metamorphic rock), corestones formed at the weathering front, as well as from the chemical composition of the dissolved loads in nearby streams. Weatherable minerals and soluble elements are fully depleted at the top of the profile, showing that the system is supply-limited, such that weathering fluxes are controlled directly by the supply of fresh minerals. We determine the weathering rates using two independent means: (1) in situ-produced cosmogenic nuclides in surface soil and creek sediments in the close vicinity of the regolith combined with immobile element mass balance across the regolith and (2) river dissolved loads. Silicate weathering rates determined from both approaches range from 16 to 36tkm−2y−1, corresponding to a weathering front advance rate of 6–14mmky−1. These rates agree across the 101 to 104ytime scales over which our rate metrics integrate, suggesting that the weathering system operates at steady state. Within error these rates are furthermore compatible with those obtained by modeling the advance rate of the weathering front from chemical gradients and mineral dissolution rates. The silicate weathering flux out of the weathering profile, measured on small creeks, amounts to 84% of the profile’s export flux; the remaining 16% is contributed by non-silicate, atmospheric-derived input. The silicate weathering flux, as measured by dissolved loads in large catchments, amounts to ca. 50% of the total dissolved flux; the remainder being contributed by dust, rain, and weathering of local marble bands. Spheroidal weathering is the key processes of converting the fresh bedrock into saprolite at the weathering front. The mineralogical composition of weathering rinds shows that the sequence of mineral decomposition is: pyroxene; plagioclase; biotite; K-feldspar. Observable biotite alteration does not appear to initiate spheroidal weathering within corestones; therefore, we infer that other processes than biotite oxidation, like pyroxene oxidation, clay formation from pyroxene and plagioclase decomposition, the development of secondary porosity by plagioclase dissolution, or even microbiologic processes at depth enable the coupling between slow advance of the weathering front and slow erosion at the surface. The comparison to tectonically more active tropical landscapes lets us conclude that the combination of hard rock with tightly interlocked mineral grains and slow erosion in the absence of tectonically-induced landscape rejuvenation lead to these exceptionally low weathering rates.

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.

Petrography and stable isotope geochemistry of cretaceous sandstones, Orange Basin, South Africa

Some siliciclastic rock samples of the Orange Basin were subjected to diagenetic and reservoir quality evaluation; 33 core and side wall core samples from 8 wells within the genetically related packages (systems tracts); lowstand systems tract, transgressive systems tract and highstand systems tract were collected for this study, to determine the prevailing conditions at deposition and post deposition of the sediments within the age bracket of Barremian to Campanian. The samples were subjected to petrographic and mineralogical analyses including bulk stable isotope analysis of oxygen and carbon. From the petrography of the samples, the basin shows complex diagenetic histories which include compaction, cementation/micritization, dissolution, overgrowth of quartz, and fracturing. The Eh–pH shows that the cements in the area of the basin under investigation were precipitated under weak acidic and slightly alkaline conditions. The δ18O isotope values of the samples range from −1.648 to 16.180 % whilst δ13C isotope values range from −25.86 to −6.95 %, which is indicative of terrestrial biomass contribution. The plot of δ18O versus δ13C shows that the sediments were deposited in shallow marine temperate conditions with some Barremian and Aptian samples more marine. There is no significant variation of δ13C isotope values across the stratigraphic sequences, which could be an indication that the burial diagenesis does not have significant effect on geochemical pattern of the δ13C isotope in the sandstones but contribution from land-derived materials. The authigenic minerals precipitated blocked the grain interspaces and interlayers and with continued burial, compaction impeded the development of secondary porosity resulting in the poor reservoir quality. The origins of the cementing materials were both autochthonous and allochthonous.

Corrosion of heavy minerals during weathering and diagenesis: A catalog for optical analysis

A practical classification of surface textures observed on detrital grains in sands and sandstones is proposed, in order to enhance data reproducibility among operators and to implement the use of high-resolution heavy-mineral data in studies of sediment-generation, provenance, and diagenesis. Five stages of progressive weathering (unweathered, corroded, etched, deeply etched, skeletal) are recognized for diverse detrital minerals. Archetypal grains displaying increasing degrees of corrosion are illustrated in numerous color tables for visual comparison. This catalog, specifically devised to systematically collect valuable information for paleoclimatic or diagenetic interpretation during routine grain-counting under the microscope, is here shown to represent a useful subsidiary tool to reveal the different degrees of weathering for diverse minerals in modern sands of equatorial Africa, and to identify post-depositional modifications of detrital assemblages in buried orogenic sediments of the Bengal Basin. The data thus obtained need to be interpreted by carefully considering the concentration of heavy minerals in each sample, which provides the fundamental clue to quantify the degree of heavy-mineral depletion caused by either pre-depositional or post-depositional processes. The scrutiny of dissolution effects has applications in the study of the chemical properties of minerals and of diagenetic evolution, helping us to understand the development of secondary porosity and to assess the potential of water and hydrocarbon reservoirs.

Diagenetic characteristics and reservoir quality of the Lower Cretaceous Biyadh sandstones at Kharir oilfield in the western central Masila Basin, Yemen

The Lower Cretaceous Biyadh Formation in the Masila Basin is an important hydrocarbon reservoir. However, in spite of its importance as a reservoir, published studies on the Biyadh Formation more specifically on the diagenesis and relate with reservoir quality, are limited. Based on core samples from one well in the Kharir oilfield, western central Masila Basin, this study reports the lithologic and diagenetic characteristics of this reservoir. The Biyadh sandstones are very fine to very coarse-grained, moderate to well sorted quartzarenite and quartzwacke. The diagenetic processes recognized include mechanical compaction, cementation (carbonate, clay minerals, quartz overgrowths, and a minor amount of pyrite), and dissolution of the calcite cement and feldspar grains. The widespread occurrences of early calcite cement suggest that the Biyadh sandstones lost a significant amount of primary porosity at a very early stage of its diagenetic history. Based on the framework grain–cement relationships, precipitation of the early calcite cement was either accompanied or followed by the development of part of the pore-lining and pore-filling clay cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cement and feldspar grains. In addition to calcite, several different clay minerals including kaolinite and chlorite occur as pore-filling and pore-lining cements. Kaolinite largely occurs as vermiform and accelerated the minor porosity loss due to pore-occlusion. Chlorite coating grains helps to retain primary porosity a by retarding the envelopment of quartz overgrowths. Porosity and permeability data exhibit good inverse correlation with cement. Thus, reservoir quality is controlled by pore occluding cement. Diagenetic history of the Biyadh sandstones as established here is expected to help better understanding and exploitation of this reservoir. The relation between diagenesis and reservoir quality is as follows: the initial porosity was decreased by compaction and cementation and then increased by dissolution of early calcite cement and feldspar grains. The reservoir quality is also affected by depositional environment controls of grain size, sorting and matrix. Thus, best good quality reservoir rocks were deposited in braided river channels environment, where no inhibited authigenic clays and high percentages of detrital quartz.

Facies controlled porosity evolution of the Neoproterozoic Upper Bhander Sandstone of Western India

Abstract The Upper Bhander Sandstone is dominantly composed of quartzarenites. The basal and top portions are sandstones, with the middle section comprising thinly bedded shales with interlayer silt and sandstone units. The sandstone units are composed of several varieties of quartz, feldspar, micas, rock fragments and heavy minerals. The Upper Bhander Sandstone was deposited in a transgressive phase and later modified by tidal processes and wave- and storm-dominated processes in a tide-influenced Barrier Beach Complex of the shallow marine environment. This study reveals that, during mechanical compaction, a rearrangement of grains took place and point and long contacts were formed. The early silica cementation and shallow burial resulted in high primary porosity. This phase was followed by chemical compaction and the replacement of silica cement by iron cement (Fe-cement) under the deep burial phase of these sandstones. Dissolution of Fe-cement and feldspars resulted in secondary porosity development. Quartz overgrowths are better developed on coarse- to medium-sized grains than on fine-sized grains. These observations suggest a progressive compaction, which initiated at the sediment–water interface and continued till deep burial diagenesis in a rapidly subsiding basin. The existing optical porosity of the Upper Bhander Sandstone is 4% and the minus cement porosity is 18%.

The Impact of carbonate cements on the reservoir quality in the Napo Fm sandstones (Cretaceous Oriente Basin, Ecuador)

The Napo Formation of Lower-Middle Cretaceous age in the Oriente basin, Ecuador, is an important sandstone reservoir. The formation is buried at a depth of 1,500 m in the eastern part of the basin and down to 3,100 m in the western part. The sandstones display higher porosity values (av. 20%) than other reservoirs in the region. These sandstones were deposited in fluvial, transitional and marine environments, and they are fine to medium grained quartzarenites and subarkoses. The principal cements are carbonates, quartz overgrowth and kaolin, with scarce amounts of pyrite-pyrrhotite and chlorite. Carbonate cements include: Eogenetic siderite (S1), mesogenetic and post-compactional calcite, Fe-dolomite, ankerite and siderite (S2). Early siderite and chlorite helped to retain porosity by supporting the sandstone framework against compaction. Dissolution of feldspars and carbonate cements are the main mechanism for secondary porosity development during mesodiagenesis. The high intergranular volume (IGV) of the sandstones indicates that cementation is the predominant contributor to porosity loss in the reservoir and that the precipitation of the carbonate cement occurred in early and late diagenetic stages. The stable–isotope composition of the S1 siderite is consistent with precipitation from meteoric waters in fluvial sandstones, and from mixed meteoric and marine waters in transitional sandstones. The low � 18 O‰ values of some of these carbonate phases reflect the replacement and recristalization from S1 to S2 siderite at deep burial and high temperature. Textural evidence, together with a low Sr content, also suggests that siderite (S1) in fluvial environment is an early cement that precipitated from meteoric waters, near the sediment/water interphase, followed by the generation of calcite with a higher Fe and Mg content. However, due to this higher Mg content, siderite S2 could have precipitated as a result of the thermal descarboxilation of the Mg rich organic matter. The progressive decrease in � 18 O values in all carbonate cements could be related to the continued precipitation at different temperatures and burial depth.

Tectonic, sedimentary and metallogenic re-evaluation of basal strata in the Mesoproterozoic Bylot basins, Nunavut, Canada: Are unconformity-type uranium concentrations a realistic expectation?

The Mesoproterozoic Borden, Hunting-Aston, Fury and Hecla, and Thule basins, collectively known as the Bylot basins, are a series of sub-parallel extensional depressions associated with the Mackenzie igneous event at ∼1270Ma. In the Borden basin, basal strata of the Eqalulik Group are dominated by tholeiitic basaltic rocks and siliciclastic sandstones previously interpreted as predominantly terrestrial flows and fluvial deposits respectively. New field data indicate that thick basalt flows in the Nauyat Formation contain cryptic pillow structures and typically have fine-grained, fragmental tops. A subaqueous origin for these flows is supported by the presence of stromatolitic carbonate on the top of several flows, and the presence of peperite and sand-injectite structures at the base of others. Associated quartz arenite units are all of marine origin. Although much of the overlying Adams Sound Formation was previously interpreted as fluvial, there is little evidence for this, except locally at the base of the succession near Paquet Bay. Paleocurrents do not exhibit a strong unimodal pattern to the northwest, as indicated in earlier work, but reflect a less restricted basin, with paleoflow oblique to later graben margins. Most of the Adams Sound Formation was deposited on a tide- and storm-dominated shelf, with local development of large sand-wave complexes. A similar, marine-dominated setting can be demonstrated for basal terrigenous clastic units in both the Hunting-Aston, and Fury and Hecla basins. The latter contains thin sheets of fluvial strata only in the Nyeboe Formation on the south side of the basin, and in the basal 3m of the Whyte Inlet Formation on the northeast margin of the basin. Paleocurrent patterns in all three basins do not reflect confinement between the walls of restricted grabens. This suggests that the early development of the basins may have been related to transtensional extension rather than aulacogen development.Although uranium, present in basement granites north of the Fury and Hecla Strait, may have been scavenged by fluids well after lithification of the Mesoproterozoic basin fill to produce local vein-type uranium deposits, the potential for extensive unconformity-type deposits, analogous those in the Athabasca basin, is exceptionally low. Most of the sandstone units have early siliceous cement, and low initial feldspar content, both of which would have inhibited development of extensive secondary porosity.

Some relationships between lithology, basin form and hydrology: a case study from the Thames basin, UK

AbstractThe role of lithology in influencing basin form and function is explored empirically by investigating correlations between a range of catchment variables, where the spatial unit of analysis is not surface catchments but lithologically coherent groundwater units. Using the Thames basin, UK, as a case study, nine groundwater units have been identified. Values for 11 hydrological and geomorphological variables, including rainfall, drainage density, Baseflow Index, aquifer porosity, storage coefficient and log‐hydraulic conductivity, aquifer and drainage elevation, river incision, and hypsometric integral have been estimated for each of the groundwater units in the basin, and Pearson correlation coefficients calculated for all pairs of variables. Seven of the correlation coefficients are found to be significant at a confidence level of > 99%. Negative correlations between drainage density and log aquifer hydraulic conductivity, and between drainage density and river incision, and positive correlations between log‐hydraulic conductivity and river incision, log‐hydraulic conductivity and Baseflow Index, and between Baseflow Index and river incision are inferred to have consistent causal explanations. For example, incision of rivers into aquifers leads to relative increases in hydraulic gradients in the vicinity of rivers which, in turn, promotes the development of secondary porosity increasing both aquifer hydraulic conductivity and, hence, Baseflow Index. The implication of this interpretation is that the geomorphological evolution of basins is intimately linked to the evolution of hydraulic conductivity of the underlying aquifers. This is consistent with, and supports the notion of a coupled complexly evolving surface water‐groundwater system. Copyright © 2011 John Wiley & Sons, Ltd.

CO 2 sequestration and extreme Mg depletion in serpentinized peridotite clasts from the Devonian Solund basin, SW-Norway

The conglomerates of the Solund Devonian basin of SW-Norway contain numerous (locally up to 20 vol.%) peridotitic clasts with concentric mm- to 10-cm thick zones of varying red to black color. The peridotite clasts show a clear, alteration-related textural evolution. The least-altered rocks are partly serpentinized peridotites, showing a typical mesh texture with veins of serpentine, magnesite and Ni-rich magnetite surrounding olivine (Fo 91) relicts and its Mg-depleted, clay-like alteration product (deweylite assemblage). In the more advanced ophicarbonate stage, the mesh cells contain calcite, silica and are surrounded by talc. In the final stage, quartz, calcite, and hematite dominate the mineralogy and occur together with minor amounts of chromite, talc, Cr-chlorite, and Cr-hydroandradite. In tandem with this textural evolution is a decrease in MgO from 40 to 2.5 wt% and a CaO increase from 1 to 35 wt%. All peridotite clasts are characterized by high Cr and Ni concentrations. The chemistry and the textural evolution show that the clasts formed by an extreme Mg-mobilization from the peridotite, with development of secondary porosity and subsequent precipitation of calcite. MgO removed from the clasts after burial is in part consumed by replacement reactions in the sediment matrix around the clasts where Mg-free minerals (e.g., almandine) are replaced by Mg-bearing minerals (e.g., talc). Calculated apparent 87Sr/ 86Sr ratios of the clasts at 385 Ma (0.7124–0.7139), corresponding to the inferred age of sediment deposition and incipient clast alteration, indicate interaction with diagenetic basinal fluids. We explain the reaction history as a three stage process involving (a) partial serpentinization of olivine in an oceanic environment (b) breakdown of olivine relicts to the deweylite assemblage resulting in mobilization of MgO under (near-) surface conditions in a tropical Devonian climate and (c) further Mg-mobilization and replacement of the deweylite assemblage by calcite and quartz after diagenesis. Sedimentary basins with abundant weathered peridotite represent potential sites for a permanent CO 2 storage by formation of calcite in a low-temperature environment.

RESERVOIR SANDSTONES OF THE CRETACEOUS NAPO FORMATION U AND T MEMBERS IN THE ORIENTE BASIN, ECUADOR: LINKS BETWEEN DIAGENESIS AND SEQUENCE STRATIGRAPHY

This paper investigates whether diagenetic alterations in sandstones and resulting changes in reservoir quality are influenced by depositional environments and sequence stratigraphy. The study focusses on the Cretaceous U and T sandstone members of the Napo Formation in the Oriente Basin of Ecuador. The sandstones were deposited in fluvial, transitional and marine environments, and comprise Lowstand (LST), Transgressive (TST) and Highstand Systems Tract (HST) deposits. The data were obtained by detailed petrographic observations supported by microprobe, stable isotope, and fluid inclusion analyses. The sandstones consist of fine‐ to medium‐grained quartzarenites and subarkoses. Diagenetic events include cementation by chlorite, early and late kaolinite/dickite, early and late carbonates (siderite, Fe‐dolomite/ankerite), and quartz.Early (eogenetic) processes included formation of chlorite grain coatings, kaolinite pore filling, and siderite (SI) cementation. Chlorite is absent in TST sandstones but was found frequently in LST‐HST sandstones. Early kaolinite is not present in LST sandstones but occurred frequently in LST‐HST sandstones.The distribution of mesogenetic cements relative to sequence stratigraphy is different in the U and T units. In the U sandstones, calcite is frequent in LST deposits and absent in the LST‐HST. Fe‐dolomite/ankerite is abundant only in the TST. S2 siderite is present in the TST and LST, but absent in the LST‐HST. Quartz cement and kaolinite/dickite are equally distributed in all systems tracts. In the T sandstones Fe‐dolomite/ankerite is only abundant in the TST, whilst calcite, quartz and dickite have similar distributions in all the systems tracts.The distribution of kaolinite cement is interpreted to be the result of relatively more intense meteoric‐water flux occurring during sea‐level fall, whereas chlorite cement may have formed through burial diagenetic transformation of precursor clays e.g. berthierine which was precipitated in mixed marine‐meteoric waters in tidal channel and estuarine environments.Chlorite cement in the T and U sandstones appears to have retarded development of quartz overgrowths, and 12–13% primary porosity is retained. The T sandstones (LST‐HST) contain up to 4% chlorite cement. Little evidence for chemical compaction was found with the exception of occasional concave‐convex grain contacts.Eogenetic siderite appears to have helped to preserve reservoir quality through supporting the sandstone framework against further compaction, but mesogenetic calcite has considerably reduced primary porosity. Eogenetic siderite (SI) was partly replaced by later carbonate cements such as late siderite (S2) and Fe‐dolomite. Although there appears to be a relationship in the Napo Formation between the occurrence of siderite SI and sequence stratigraphy, the relationship may change when original volumes of siderite are considered. There is likewise partial replacement of early kaolinite and recrystalization to dickite which masks the amount of original early kaolinite. Since the amount of early kaolinite could not be confirmed, the relationship to sequence stratigraphy is tentative. Only chlorite seems to have a clear relationship to sequence stratigraphic framework in the Napo Formation.The high intergranular volume (IGV) of the sandstones indicates that cementation played a more important role than mechanical and chemical compaction in both Napo Formation sandstone members. Later dissolution of feldspar grains and siderite cements was the main process of secondary porosity development (up to 11% in the U sandstones).

Hierarchical ZSM‐5 Zeolites in Shape‐Selective Xylene Isomerization: Role of Mesoporosity and Acid Site Speciation

The isomerization of o-xylene, a prototypical example of shape-selective catalysis by zeolites, was investigated on hierarchical porous ZSM-5. Extensive intracrystalline mesoporosity in ZSM-5 was introduced by controlled silicon leaching with NaOH. In addition to the development of secondary porosity, the treatment also induced substantial aluminum redistribution, increasing the density of Lewis acid sites located at the external surface of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o-xylene conversion than its parent, owing to the reduced diffusion limitations. However, the selectivity to p-xylene decreased, and fast deactivation due to coking occurred. This is mainly due to the deleterious effect of acidity at the substantially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p-xylene selectivity and reduce the deactivation rate. The HCl-washed hierarchical ZSM-5 displayed an approximately twofold increase in p-xylene yield compared to the purely microporous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbonaceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post-synthesis treatments, such as base leaching (introduction of mesoporosity) and acid washing (surface acidity modification).

Host rock dolomitization and secondary porosity development in the Upper Devonian Cairn Formation of the Fairholme carbonate complex (South‐west Alberta, Canadian Rockies): diagenesis and geochemical modelling

AbstractThe Upper Devonian carbonate reefs in West‐central Alberta are important petroleum reservoirs that are well‐known for their extensive secondary porosity. An outcrop analogue study indicates that an early matrix‐selective dolomitization event occurred which is characterized by a major Late Devonian sea water component with increased salinity because of evaporation. It is interpreted that the matrix (replacive) dolomite formed during the Famennian as the result of a combination of both seepage and latent reflux dolomitization, although an additional type or overprinting of later intermediate burial dolomitization cannot be excluded. Formation of the moulds is attributed mainly to the dissolution of undolomitized fossil cores, most typically stromatoporoids. Geochemical modelling indicates that carboxylic acid fluids have the highest potential for dissolving residual calcite in this case. Geochemical models consistent with this analysis and interpretations can reproduce the secondary porosity and suggest a viable dolomitization process for the localities studied.

Diagenesis and reservoir quality of Bhuban sandstones (Neogene), Titas Gas Field, Bengal Basin, Bangladesh

This study deals with the diagenesis and reservoir quality of sandstones of the Bhuban Formation located at the Titas Gas Field of Bengal Basin. Petrographic study including XRD, CL, SEM and BSE image analysis and quantitative determination of reservoir properties were carried out for this study. The sandstones are fine to medium-grained, moderately well to well sorted subfeldspathic arenites with subordinate feldspathic and lithic arenites. The diagenetic processes include clay infiltration, compaction and cementation (quartz overgrowth, chlorite, kaolinite, calcite and minor amount of pyrite, dolomite and K-feldspar overgrowth). Quartz is the dominant pore occluding cement and generally occurred as small euhedral crystals, locally as large pyramidal crystals in the primary pores. Pressure solution derived from grain contact is the main contributor of quartz overgrowths. Chlorite occurs as pore-lining and pore filling cement. In some cases, chlorite helps to retain porosity by preventing quartz overgrowth. In some restricted depth interval, pore-occlusion by calcite cement is very much intense. Kaolinite locally developed as vermiform and accelerated the minor porosity loss due to pore-occlusion. Kaolinite/chlorite enhances ineffective microporosity. Kaolinite is a by-product of feldspar leaching in the presence of acidic fluid produced during the maturation of organic matter in the adjacent Miocene or deeper Oligocene source rocks. The relation between diagenesis and reservoir quality is as follows: the initial porosity was decreased by compaction and cementation and then increased by leaching of the metastable grains and dissolution of cement. Good quality reservoir rocks were deposited in fluvial environment and hence quality of reservoir rocks is also environment selective. Porosity and permeability data exhibit good inverse correlation with cement. However, some data points indicate multiple controls on permeability. Reservoir quality is thus controlled by pore occluding cement, textural parameters (grain size, pore size and sorting) and depositional environment. The reservoir finally resumed partly its pre-cementation quality after development of secondary porosity.

A chemical and thermodynamic model of oil generation in hydrocarbon source rocks

A chemical and thermodynamic model of oil generation in hydrocarbon source rocks