Micrite Cement Research Articles

Petrography and Mineral Content of Sea-floor Sediments of the Tigris-Euphrates Delta, North-west Arabian Gulf, Iraq

Sea-floor sediments of the Iraqi territory of the Arabian Gulf have been studied in detail petrographically and mineralogically. The carbonate fraction of these sediments comprises various allochems in order of abundance: skeletal remains, ooids, peloids, and composite grains. On the other hand, the insoluble residues of the sediments studied consist of quartz, feldspars and mica. The matrix is carbonate micrite, containing various percentages of clays.Mineralogically the carbonate fraction consists of calcite, Mg-calcite, aragonite and dolomite. Mg-calcite, forming the radial cortex of the ooids, is the most unexpected and significant of such components. In addition, the Mg-calcite micritic cement is the most commonly observed diagenetic product resulting in the formation of hardgrounds in some areas. The clay minerals of the (<2 μm) grade are: illite, smectite, chlorite, illite-smectite, kaolinite and palygorskite. They are mainly detrital in origin.The fluvial loads of the Shatt Al-Arab (i.e. Tigris and Euphrates rivers) and Karun rivers, sand and dust from north-westerly storm winds (Shamal) are the main potential sources for the detrital sediments accumulating in the area of study. The indigenous marine organisms and some inorganic precipitation (as Mg-calcite), are additional sources of the sediments and increase in percentage toward the open waters.

Evolution of an early Proterozoic foreland basin carbonate platform, lower Pethei Group, Great Slave Lake, north‐west Canada

ABSTRACTThe Taltheilei, Utsingi, McLean and Blanchet formations form a 175–390 m thick carbonate platform‐to‐basin succession in the lower part of the PaleoProterozoic Pethei Group, preserved in the eastern arm of Great Slave Lake. Carbonates accumulated along the south‐east margin of the Slave Craton within a foredeep formed during the collision of the Slave and Churchill Cratons. The rocks include eight, predominantly microbial, carbonate facies that comprise five facies associations representing (1) shallow‐water rimmed shelf, (2) shallow‐water open shelf, (3) shallow‐water ramp, (4) upper slope and deep ramp, and (5) lower slope and basin plain environments. Microbialite facies grew by organically mediated precipitation of spar and micritic cement and trapping and binding of lime mud.These wholly subtidal facies typically reflect progressive shallowing and changing geometry of the lower Pethei sea floor, from ramp, to open shelf, to shallow rimmed shelf, with associated slope and basin plain deposition. Repeated relative sea‐level changes influenced platform growth. This resulted in five shallowing upward packages; each separated by an incipient drowning event of varying magnitude. Antecedent topography and the size of the preceding drowning event strongly influenced the initial growth of each interval. This repeated pattern is attributed to interaction between (a) the inherent tendency of microbial carbonates to aggrade vertically, (b) changing sedimentation rates and (c) readjustments of relative base level.The lower Pethei succession is one of few PaleoProterozoic examples of carbonate platform growth within a foreland basin. It has (1) a low gradient profile, (2) extensive slope and basin plain carbonate production and sedimentation, (3) no ooids, (4) minor terrigenous clastic sediments, and (4) a mobile, submergent shelf rim lacking substantial carbonate sand shoals.

Patterns of reef sedimentation and diagenesis in the Miocene of Cyprus

An Early Tertiary regressive sedimentary succession developed along an active plate margin in Cyprus. A varied clastic and carbonate succession of the Miocene Pakhna Formation and then Messinian evaporites culminate the regression. Within the Pakhna Formation, two phases of reef growth were developed: the Aquitanian—Burdigalian, Terra Member, and the Tortonian, Koronia Member. The Terra Member is exposed only in west and southeast Cyprus, mainly between pelagic carbonates. The Koronia Member is exposed around the margins of the Troodos Massif in north, south and west Cyprus and on the Akamas Peninsula in northwest Cyprus. There was little tectonic influence on the growth pattern of the Terra Member reefs. The Koronia Member reefs, however, are developed on up-faulted blocks. Most of the Koronia Member fore-reef facies and associated clastic input was shed into downfaulted basinal depocentres. There are differences between the first and second Miocene reef phases in Cyprus in primary reef construction and in the level of background carbonate pelagic sedimentation. The Terra Member reefs are dominantly framestones, comprising faviids and domal poritids with several types of secondary reef-dwelling corals. There is no indication of a connection with the Indo-Pacific coral faunal province. Benthonic foraminifera and fragments of crustose coralline algae are abundant in the off-reef. By contrast, the Koronia Member is a bindstone comprising virtually monospecific, laminar poritid corals but with coralline algae also playing a role in encrustation. The Koronia Member off-reef facies comprises decimetre-thick beds of bioclastic reef detritus. In north Cyprus however, beds of similar detritus (centimetres in thickness), are overlain by progressively thicker (up to several metres) debris-flow sheets. The Terra Member reefs have limited acicular fringe cement (probably aragonitic). This is overlain by an abundant, marine, bladed, formerly high-Mg calcite cement. Later cements present in the reef facies include micritised calcite spars, perhaps indicating microboring in cements during their precipitation. Neomorphism was extensive, converting most cements to low-Mg calcite. Stable isotopic data for all the Cyprus calcites exhibit depletion in the heavier isotopes, 18O and 13C, indicative of meteoric alteration. The Koronia Member limestones show abundant indication of early lithification with fringing acicular aragonite and micrite cements. Stable isotopic data of the botryoidal aragonite cements indicate a marine origin. The dolomite is thought to result from marine—freshwater mixing and to be of Pliocene age, on the basis of stratigraphical, solution-replacement fabric and stable isotopic data. Sr isotopic data do not contradict a Pliocene age. Dolomite characteristically replaces the Pliocene fissure fill contents in southeast Cyprus and off-reef horizons in the Terra Member of west Cyprus and Koronia Member of north Cyprus. The growth and burial history of Miocene reefs contribute to the understanding of the tectonic history and palaeogeographical evolution of Cyprus.

Multiple Phase Fracture and Fault-Controlled Burial Dolomitization, Upper Devonian Wabamun Group, Alberta

ABSTRACT Recent data from the subsurface Tangent, Eaglesham and Normandville fields, located east of the Peace River Arch in northern Alberta, indicate that these Wabamun Group subtidal carbonates have undergone a complex diagenetic history related to multiple stages of fracturing and faulting. Burial dolomitization, which generated mainly matrix dolomites, was responsible for the development of porous reservoir rocks which are spatially highly variable. Primary porosity was reduced by marine and early burial diagenesis, which formed micrite, syntaxial overgrowths, granular, blocky and coarse fracture-filling calcite cements. Minor amounts of microdolomite and floating dolomite rhombs formed during shallow burial. Further porosity reduction by mechanical compaction and cementation, except for minor dissolution, and initial fracturing, resulted in predominantly tight limestones. Matrix dolomitization is associated with the second, most extensive phase of fracturing, which greatly increased both the porosity and permeability of the tightly cemented Wabamun limestones. Dolostones, which comprise 85% matrix dolomite, are the principal reservoir rock but have highly variable porosity. Large dissolution cavities developed locally after formation of the matrix dolomites. The third phase of fracturing crosscuts the matrix dolomites. In the Tangent field, these fractures locally form irregular cavity systems which are partly filled with finely crystalline and saddle dolomites. These fine-crystalline dolomites probably represent dolomite silt and sand derived from brecciation of the earlier matrix dolomite during faulting. Renewed faulting was responsible for rotation of breccia clasts. Saddle dolomite was precipitated during at least two phases, on the sides and undersides of some clasts, and in fractures. Late-stage bladed anhydrites and fracture-filling coarse calcites precipitated during hydrocarbon generation, presumably in the Late Cretaceous, except locally where source rocks may have been heated by ascending hot brines. The timing of dolomitization and fracturing is difficult to determine. Early fractures most likely occurred in the Late Devonian to Early Carboniferous during early burial and mechanical compaction. The second phase of fracturing probably occurred during the Mississippian to Pennsylvanian when the Normandville and Dunvegan faults underwent movement during the foundering of the Peace River Arch. The third phase of fracturing is linked to the more rapid subsidence and burial by the Cretaceous and Early Tertiary clastic wedge, which presumably reactivated some of the earlier faults. The combined stratigraphic, petrophysical and geochemical data support the following interpretations. 1) Early dolomite rhombs and patches resulted from the movement of seawater through semi-lithified sediment near the sediment-water interface or during early compaction. 2) Matrix dolomites formed at temperatures of between 40 and 65°C and depths of about 500 to 1500 m. Probable fluid sources for matrix dolomites were Devonian brines buried with these and underlying sediments and expelled by mechanical compaction, possibly aided by seismic pumping, along stage II fracture-fault conduits. 3) Sucrosic, saddle and reworked dolomites postdate stage III fractures and occurred during deep burial. Most saddle dolomites formed at temperatures around 100°C, but a few formed near 200dC. These dolomites formed from warm, highly saline, basin brines and/or from hot brines that ascended along fractures and faults and other stratigraphic conduits from deeper parts of the basin and/or the crystalline basement.

Fabric and Origin of Gypsum Sand Crystals, Laguna Madre, Texas

ABSTRACT Gypsum sand crystals of Laguna Madre are crystals of selenite of lens-like, discoidal shape that poikilotopically enclose dominantly terrigenous sand. They occur over an 85-km length of the lagoon as inferred from their presence in spoil islands formed during the excavation of the intracoastal waterway. These crystals are most abundant around the region known as the sand bulge. Gypsum sand crystals show a spectrum from two intergrown crystals to clusters of more than 100 crystals, the latter of which may have either a random orientation of crystals or an irregular radial array. Clusters reach 50 cm in length and weigh up to 11 kg. Single crystals and small clusters commonly grew with the largest crystal perpendicular to or at a high angle to bedding, whereas large crystal clusters grew with their long dimension in the plane of bedding. In most sand crystals gypsum grew passively and merely filled pores: these samples have from 40 to 54 percent cement. Where gypsum precipitated rapidly it was displacive: these samples have from 60 to 70 percent cement, and some cm-thick domains are completely free of sand. Growth of gypsum displaced rinds of micrite cement from their sand grain nuclei, sericite flakes from rock fragments, and fractured mollusc shells. SEM examination of sand grains and mollusc fragments from leached sand crystals reveals no evidence of grain corrosion or replacement. If the gypsum cement were to dissolve during further burial, no textural clue to its original presence will exist. Sr isotopic values (< 0.7084) of gypsum cement are below that expected for modern sea water, suggesting the release of non-radiogenic Sr from volcanic rock or carbonate rock detritus in the sediments. 18O values of Laguna Madre gypsum is heavier than expected for modern gypsum probably because of the fractionation effect of sulfate-reducing bacteria. Laguna Madre became hypersaline only 210 years ago, so the gypsum sand crystals are very young, and they must have grown in years to tens of years. The presence of sand crystals to a depth of at least 4.5 m indicates the gypsum formed in the phreatic zone, and their increase in size with depth suggests they formed, in part, by seepage refluxion of lagoonal brine.

The Role of Carbonate Diagenesis in Exploration and Production from Devonian Pinnacle Reefs, Alberta, Canada

Carbonate diagenesis plays an important role in the exploration for, and hydrocarbon recovery from, pinnacle reefs of the Upper Devonian Nisku Formation of Alberta. In the West Pembina area, the Nisku Formlltion occurs as a series of carbonate ramps prograding into the Winterburn shale basin. Discovered on a regional seismic line in 1977 by Chevron Standard Ltd., the play has matured into a pinnacle reef fairway 65 km by 180 km containing over 50 produc­ tive reefs with total reserves of approximately 325 MMstb oil and 500 BCF gas. Texaco's discovery, alone or jointly of27 of these reef pools for a 30% net share in reserves has made understanding reservoir quality of importance to maximizing reserves on the reef trend Carbonate diagenesis exerts a major influence on the development of porosity and permeability in the reefs, and accounts for areal variability among different reefs in the trend. On an exploration basis, knowledge of diagenesis highlights areas for preferred drilling. For development, diagenesis and its effects upon reef petrophysics affects the selection of pools for water versus miscible flood enlulnced recovery schemes. The Nisku pinnacle reefs are built by rugose corals, stromatoporoids and algae with interbedded and matrix lime mudstones and wackestones. Three diagenetic processes affecting porosity and per­ meability within the reefs are early submarine cementation, dolomitization, and pressure solution. The early submarine cements include spectacular multiple fibrous cements together with stroma­ tolitic crusts and micritic cements. While these cements occluded some primary porosity, they provided a net beneficial effect by stabilizing the reef framework and muddy matrix and preserving intergranu­ lar and large interfossil pores. Dolomite is the major determinant of reservoir quality. Fully dolomitized reefs average 10-20% porosity and up to 5 darcies permeability whereas partly dolomitized reefs have porosities of2-5% and approximately 500 millidarcies permeability. Two dolomite types are present and both are believed to result from burial processes. Microdolomite rhombs 10 to 200 J.1.1D in size selectively replace micrite matrix in all reefs and are considered a shallow burial product which increases matrix permeability somewhat. Coarse sucrosic dolomite, with crystals from 50 to 400 J.1.1D in size, is not fabric selective and pervasively obliterates primary rock textures. This dolomite is a deep burial product which greatly increases intercrystalline porosity and permeability of the reefs, and accounts for reservoir variations along the reef trend. Pressure solution has occurred throughout the burial history of the reefs. It is most noticeable as high amplitude stylolites concentrated along facies contacts and testifies to significant removal of potential reservoir section by solution. Additional leaching of skeletal fragments has created secondary porosity.

A review of the origin and setting of tepees and their associated fabrics

ABSTRACTCarbonate hardgrounds often occur at the surface of shallow subtidal to supratidal, lacustrine, and subaerial carbonate shelf sediments. These are commonly disrupted and brecciated when the surface area of these crusts increases. In the subtidal environment, megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand. Similar megapolygons form in the supratidal, lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and/or cement also causes the area of the hardgrounds to expand. The arched up antiform margins of expansion megapolygons are known as tepees.The types of tepees found in the geological record include:(1) Submarine tepees which form in shallow carbonate‐saturated waters where fractured and bedded marine grainstones are bound by isopachous marine‐phreatic acicular and micritic cements. The surfaces of these brecciated crusts have undergone diagenesis and are bored. Unlike tepees listed below they contain no vadose pisolites or gravity cements;(2) Peritidal and lacustrine tepees are formed of crusts characterized by fenestral. pisolitic and laminar algal fabrics. This similarity in fabric makes these tepees of different origins difficult to separate. Peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical. They are associated with fractured and bedded tidal flat carbonates. Their fracture fills contain geopetal asymmetric travertines of marine‐vadose origin and/or marine phreatic travertines and/or Terra rossa sediments. The senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material. Lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common. They include groundwater tepees which form over evaporitic ‘boxwork’ carbonates, and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas, but the dominant sediment type is carbonate mud. These latter tepee crusts are coated and crosscut by laminated micrite; the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust.Both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions, in time intervals of days to years. Cement morphologies reflect this and the crusts often contain gravitational, meniscus vadose cements as well as phreatic isopachous cement rinds.(3) Caliche tepees which are developed within soil profiles in a continental setting. They are formed by laminar crusts which contain pisolites, and fractures filled by micritic laminae, microspar, spar and Terra rossa. Most of the cements are gravitational and/or meniscoid.In ancient carbonates, when their cementation and diagenetic fabric can be interpreted, tepee structures can be used as environmental indicators. They can also be used to trace the evolution of the depositional and hydrological setting.

Multi-phase oxygen isotopic analysis as a tracer of diagenesis: The example of the mishash formation, cretaceous of Israel

Isotopic analysis of oxygen in several coexisting phases has two complementary aspects: (a) the analysis of two or more cogenetic phases may yield information about prevailing conditions at definite points in time assuming equilibrium fractionation; and (b) the analysis of paragenetic assemblages of phases unravels the sequence of conditions from sedimentation, through early diagenesis to late diagenesis. δ 18O has been analysed in several coexisting phases from a continuous sedimentary section of the Mishash Formation of Campanian age in central Israel. Three isotopically concordant assemblages were identified: (a) A depositional assemblage, formed in normal seawater at temperatures between 23° and 30°C is reflected by benthic foraminifera ( Nodosaria) skeletons (δ 18O SMOW= +27.5 to +29.5‰) and apatiticbio-detritus (δ 18O= +18 to +19.5‰). (b) An early diagenetic assemblage formed in equilibrium with a marine solution is recorded by calcite of micritic cement ( +26 to +27.5‰), silica in opal-CT (porcellanite) (+31.5 to +33.3‰), silica as quartz in homogeneous cherts ( +29 to +32‰), silica as quartz in fragments of chert breccias ( +31 to +33‰), and some quartzose matrix in silicified phosphorites. (c) A later diagenetic assemblage contains calcitic spar infilling of fossils ( + 20 to + 26.6‰), silica in “matrix” of chert breccias ( +21 to +33‰) and coarse quartz matrix in silicified phosphorites ( +20 to +30‰). The phases of this assemblage can be interpreted as all being deposited in equilibrium with a late freshwater fluid. Comparison of the record of siliceous rocks to that of coexisting phosphates and carbonates suggests a time constant of 10 5–10 6 yr. for the closure of the chertification system in a shallow marine environment.

The term micrite or micritic cement is a contradiction; discussion of micritic cement in microborings is not necessarily a shallow-water indicator; discussion

The term micrite or micritic cement is a contradiction; discussion of micritic cement in microborings is not necessarily a shallow-water indicator; discussion

Calcified Algae as Sediment Contributors to Early Paleozoic Limestones: Evidence from Deep-Water Sediments of the Cow Head Group, Western Newfoundland

ABSTRACT Calcified algae are important elements of deep-water carbonate sediments in the Cow Head Group. Large algal boundstone boulders in debris-flow conglomerates consist of Girvanella sheets and arborescent clusters of Epiphyton. Silt- and sand-sized particles in fine-grained, thin-bedded slope limestones include abundant Girvanella, as single tubules, rafts of intertwined tubules, oncolites, and intraclasts. Integration of petrographic information from the boulders and fine-grained limestones indicates that many micritic peloids of silt to very fine sand size, the single most abundant particle type in the Cow Head Group, are a combination of comminuted Girvanella tubules and broken Epiphyton rods. Much of the structure grumeleuse in intraclasts is also diagenetically modified calcified algae. These findings suggest that many peloids and intraclasts in Early to Middle Paleozoic carbonates, in both shallow and deep water, may have been derived from calcified algae. Calcified algae may have been just as important in producing carbonate sediment during this time as benthic calcified algae are in the modern ocean. The difficulty in recognizing the algal contribution in most ancient sediments is probably caused by the simple microfabric of the resultant grains, filling and obliteration of tubules with micrite cement, or micritization.

Disrupted Carbonate Hardgrounds in Shallow Carbonate-Shelf Sediments: Origin and Setting of Tepees and Their Associated Fabrics: ABSTRACT

Carbonate-sediment surface hardgrounds are commonly disrupted and brecciated. Some of these breccias are the result of repeated episodes of fracturing and fracture fill by sediment and/or cement. Each fracture and fracture-fill phase causes the crust to grow in surface area and crumples it into megapolygons whose anteform margins are called tepees. Fracturing is caused by thermal contraction, water-table movement buoying up the crust, mass movement of sediment, and tectonic events. Tepees are commonly ascribed to tidal flats but, in fact, also occur in many settings that can be determined from their fabric and facies associations. (1) Submarine tepees from shallow, carbonate-saturated water occur in fractured, bedded, marine grainstones with acicular and micritic cements. They contain no vadose pisolites or gravity cements, and the hardground surface is altered and bored. (2) Peritidal tepees occur in fractured, bedded, tidal-flat carbonates characterized by fenestral, pisolitic, and laminar algal fabrics close to the marine water table. Fracture fills include gravity-controlled marine travertine and/or marine and terra rossa sediments. (3) Groundwater tepees form in fractured fenestral, pisolitic, and laminar algal crusts over boxwork carbonates at the margin of shallow salinas where periodic groundwater resurgence is common. (4) Extrusion tepees, which are also from salina margins with periodic groundwater resurgence, form in crusts coated with laminated micrite that extend from the fractures downward into dolomitic micrite. (5) Caliche tepees, from c ntinental settings overlying soil profiles, form in End_Page 272------------------------------ laminar crusts with pisolites. Fractures are filled by micritic laminae, microspar, spar, and terra rossa. End_of_Article - Last_Page 273------------

Preserved Aragonite Cements in Miocene Coral Reefs: a Record of Messinian Salinity Crises in Mediterranean: ABSTRACT

Layers of fibrous aragonite cement up to 2 cm thick, developed on aragonitic corals and micritic cements, occur in outcrops of Miocene coral reefs in western Sicily. These aragonitic fabrics show only minor amounts End_Page 253------------------------------ of corrosion after subaerial exposure for at least 3 m.y. Their preservation is attributed to encasement by subsequent gypsum cements. Although these botryoidal, banded aragonite cements are strontium-rich (7,000 ppm) and resemble modern marine examples, they were precipitated in secondarily enlarged pores that formed during erosional episodes. Multiple cycles of enrichment in oxygen and carbon stable isotopes are recorded in the aragonite cement layers. The ^dgr18O values of these cycles range from -0.9 to + 6.8^pmil, whereas the ^dgr13C values range from +0.6 to + 3.8^pmil (PDB). These cyclic variations, indicated by isotopic data together with the petrology of the cements, are believed to record major changes in salinity, temperature, and organic productivity f the Mediterranean waters during the Miocene-Pliocene transition. These Messinian reefs were subaerially exposed and later onlapped by the upper evaporite unit with multiple cycles of marine hypersaline carbonate and evaporite deposition separated by periods of erosion. Aragonite cements formed in the enlarged cavities of the lower Messinian reefs during time of deposition of the upper evaporite and recorded the changes in Mediterranean water chemistry. This cementation is believed to have continued into the early Pliocene when colder Atlantic waters invaded the Mediterranean, ending reef growth and evaporite deposition. End_of_Article - Last_Page 254------------

Meaning and Usage of Micrite Cement and Matrix: REPLY

Meaning and Usage of Micrite Cement and Matrix: REPLY

Micritic Cement in Microborings is Not Necessarily a Shallow-Water Indicator

ABSTRACT Intragranular, micritic cement has been observed in microborings from deep-sea sedimentary particles (pteropod shells), depths 528 m to 871 m. It is particularly conspicuous in depositional environments such as the Blake Plateau. Such cementation may be concurrent and analogous to the formation of intergranular deep-sea cement. It can occur at such energy levels where particle-to-particle contacts do not persist for sufficient time to allow intergranular cementation.

Algal Origin of Peloids, Peloidal Intraclasts, and Structure Grumeleuse in Paleozoic Limestones: Evidence from Cow Head Group, Western Newfoundland: ABSTRACT

Deep-water carbonate strata of Cambro-Ordovician age in western Newfoundland are composed of interstratified conglomerates and thinly bedded calcarenites, lime mudstones, and shales. Early lithification has preserved an abundant but simple algal flora in shallow-water clasts within both conglomerates and deep-water slope sediments. Large boulders contain sheets of Girvanella and arborescent clusters of Epiphyton, but only Girvanella as intraclasts, rafts of intertwined tubules, single tubules, and oncolites can be distinguished in calcarenites. Preservation of Girvanella in boulders varies from distinct tubules to (clotted fabric) in which patches of micrite mask any evidence of former algal structure. Comparison of Girvanella in boulders and calcarenite grains indicates that much carbonate sand consists of broken and abraded Girvanella sheets. In contrast, Epiphyton thalli appear to be comminuted to form silt- and sand-size micrite peloids. An extension of these findings is that many peloids, peloidal intraclasts, and much structure grumeleuse in early Paleozoic carbonates, both shallow and deep water, are derived from calcareous algae. These End_Page 464------------------------------ algae may have been just as important for producing carbonate sediment during the early Paleozoic as codiacean algae are in the modern ocean. This algal contribution is difficult to recognize in most ancient sediments probably because absence of early interparticle cementation, allows compaction, filling of tubules with micrite cement, or micritization. End_of_Article - Last_Page 465------------

Rules of Sandstone Diagenesis Related to Reservoir Quality: ABSTRACT

The reservoir quality of sandstone is almost entirely controlled by diagenetic events. The chemical and physical processes responsible for diagenesis are complex and they influence sand during all stages of burial and, in some basins, during subsequent uplift. Petrographic studies by many workers in the past 10 years provide the basis for formulating rules of sandstone diagenesis that help in predicting reservoir quality in different End_Page 1215------------------------------ formations. Most of the rules listed below are empirical, and causative factors are still poorly understood. The list is also not complete. 1. The detrital mineral composition of a sand predetermines 50-80% of its diagenetic history. 2. Porosity lost by compaction cannot be regenerated during subsequent diagenetic events. Sands with abundant ductile grains (clay clasts, fecal pellets, shale clasts, micaceous rock fragments) can lose much primary porosity from the mashing of these grains during compaction. 3. The loss of primary porosity through compactional deformation of clays and ductile grains takes place during burial of 8,000-10,000 ft (2,450-3,050 m); loss of porosity by compaction at greater depths is by pressure solution of detrital grains. 4. Pressure solution of quartz grains is enhanced by the presence of grain coatings of illite and to a lesser degree by chlorite. 5. Quartz cement has an affinity for the coarser, more permeable sands in a formation. However, it rarely fills all pores in a sandstone except in some coarser grained laminae or in quartzarenites. 6. Carbonate cement may have a patchy distribution in a bed, but it fills all pores to produce a nonporous rock where it is present. 7. Carbonate cement is the dominant or only cement in sands with abundant carbonate fossil fragments or carbonate rock fragments; the carbonate cement is derived from the sand. 8. Poikilotopic calcite cement is the result of cementation that progressed from widely spaced nucleation sites. 9. Kaolinite forms by the replacement of feldspar and to a lesser degree of muscovite and also by free-standing growth in both primary and secondary pores. 10. Kaolinite should be expected as a diagenetic mineral in feldspar-rich sands (arkose and subarkose) that are poor in volcanic rock fragments. 11. Chlorite and/or mixed-layer clay should be expected as a diagenetic mineral in sandstone with more than 10% volcanic rock fragments. 12. Chlorite and mixed-layer clay, because of their tendency to bridge pores and produce baffles in pores, can seriously reduce permeability. 13. Early formed illite, typically present as grain coatings, does not seriously reduce permeability; late-formed illite tends to bridge pores and produce baffles and seriously reduce permeability. 14. Micrite cement is formed in continental sands above the water table. 15. Opal cement is formed in continental sands above the water table in stratigraphic sections where volcanic ash beds are intercalated with sands. 16. Some degree of secondary porosity is to be expected in a sandstone. However, there are few clues at present to predict the degree of secondary porosity or the cleanliness of secondary pores produced. Many secondary pores are micropores within incompletely dissolved feldspar or rock fragments. The best secondary pores are produced by dissolution of carbonate cement and evaporite cement. 17. Secondary porosity develops chiefly at depths greater than 6,000 ft (1,830 m) in the Gulf Coast, but can persist to depths of 20,000 ft (6,100 m). After their generation, secondary pores undergo some destruction during subsequent deeper burial by infilling with late-diagenetic ferroan carbonate and/or kaolinite. 18. Sands that had the greatest permeability at the time of deposition will develop the best secondary porosity. The best permeability will be in the coarsest, well-sorted sandstones (except in quartzarenites, where the coarsest beds selectively may undergo complete cementation by quartz). End_of_Article - Last_Page 1216------------

Role of Cementation in Diagenetic History of Devonian Reefs, Western Canada: ABSTRACT

Devonian (Givetian and Frasnian) reef reservoirs in Alberta and British Columbia contain 60% of the conventional recoverable oil and 20% of the recoverable gas in the Western Canada sedimentary basin. Although the depositional history of these reefs is well understood, it is the diagenetic overprint that is often responsible for their reservoir quality. Frasnian (Woodbend and Beaverhill Lake Group) reefs are characterized by stromatoporoid and coral knoll reef belts deposited near moderately sloping bank edges. Bank margin sediments are composed of skeletal lime grainstones, packstones, rudstones, and rare framestones. In contrast, bank interiors are often extensive (e.g., Redwater, Swan Hills) and characterized by cyclic deposition of lagoonal and tidal flat sediments. Certain Givetian reefs found in evaporate basins (e.g., Rainbow or Zama) usually occur as reefs with steep (> 20°) margins and only minor bank interior development. Frasnian reef complexes range in size from 1 km2 (0.4 mi2) to greater than 600 km2 (230 mi2) with thicknesses from 100 to 400 m (330 to 1,300 t). Givetian pinnacle reefs are commonly as much as 300 m (984 ft) thick, but with areal extents of less than 1 km2 (0.4 mi2). Regardless of differences in size, depositional history, and age, most reefs have been subjected to diagenesis in essentially three environments: (1) submarine (marine to hypersaline pore waters), (2) subaerial (fresh to brackish pore waters), and (3) subsurface (below phreatic aquifers, saline to brackish pore waters). Fibrous calcite cements, syndepositional fracturing, displacive calcite cements, micrite cements, and bored hardgrounds are typical submarine diagenetic fabrics, particularly at bank margins in Rainbow reefs and certain Leduc reefs (e.g., Golden Spike, Ricinus). Subaerial disconformities are numerous in most reefs, and associated vadose diagenesis produces localized paleosols, microstalactitic and meniscus cements, and abundant solution porosity. Phreatic or shallow bu ial cements usually include clear, equant calcite or dolomite that vary in Fe++ and Mn++ concentrations. Subsurface cementation produces nonferroan calcites and dolomites which are often related to stylolite formation (e.g., Kaybob, West Pembina D-2, Strachan, Ricinus). Other diagenesis occurring during burial includes dolomite and anhydrite replacement, sulfide mineralization (e.g., Pine Point, Presqu'ile barrier reef), and bitumen formation (e.g., Clarke Lake, Rainbow). Primary porosity and permeability are altered by the overlapping processes of cementation and solution (vadose and/or phreatic) that occur early in the diagenetic history. In reef interiors these subaerial processes produce stratified reservoirs with impermeable barriers (cemented beds) to vertical flow (e.g., Golden Spike, Swan Hills, Judy Creek). Submarine cementation is rare in most reefs but can be locally pervasive resulting in occlusion End_Page 565------------------------------ of bank margin and fore-reef porosity. Absence of significant subsurface cementation in many reefs (e.g., Redwater, Golden Spike) aids in preservation of the reservoirs formed during earlier diagenesis. In summary, it is the early diagenetic history in many Devonian reefs in the Western Canada basin that is responsible for reservoir distribution and quality. Likewise, the knowledge that reef margins and interiors often have different cementation histories is important in both reef exploration and reservoir management. End_of_Article - Last_Page 566------------

Cementation of Upper Miocene Reefs in Western Mediterranean: ABSTRACT

Coral reefs in the western Mediterranean (southeast Spain, Balearic Island, northern Morocco, Sicily, and Italy) show a wide variety of cement types, ranging from completely tight, well-cemented, to poorly cemented reefs with most of the primary porosity still preserved. Cementation processes in those coral reefs appear to be controlled to a great extent by repeated changes of relative sea levels and regional variations of seawater chemistry. Reef progradation occurred during four to six (or more) important sea level changes, resulting in complicated geometric relationships of reef complexes and their freshwater lenses. Progradation occurred during sea level rises and falls and is reflected in abrupt escarpments in some field localities, generally separated by important t rraced erosional surfaces. Various types of aragonitic isopachous cement fringes of marine origin, 0.1 to 1.5 cm (.04 to .6 in.) thick, are well preserved in some localities. This is probably due to subsequent plugging by gypsum cement during the Messinian salinity crises. Another possible effect of salinity fluctuations is the abundance of thick crusts of peletoidal, micrite cement of marine origin, locally forming about three-fourths of the volume of the reef core. End_of_Article - Last_Page 457------------

Structure and Diagenesis of Recent Algal-Foraminifer Reefs, Fernando de Noronha, Brazil

ABSTRACT The volcanic island of Fernando de Noronha, in the South Atlantic, is fringed by an emergent algal ridge rising from a subtidal volcanic or eolianite foundation. The ridge and adjacent reef flat result from aggradation by skeletal growth, mainly crustose coralline algae and foraminifer Homotrema rubrum, and deposition of volcanic gravel extending landward as beachrock. The aggrading units involve sand-filled skeletal and gravel frameworks with the detrital sediment representing about two-thirds of the reef volume. Distribution of radiocarbon dates suggests combined effect of marine abrasion and seaward progradation after an initial upward growth rate of 1 m/500 years prior to 3000 years B.P. At present, the ridge and reef flat are undergoing considerable diagenetic alteration. Pervasive generation of Mg-calcite micrite cement occurring simultaneously with bioerosion and deposition of new carbonate fabric produces dense and texturally complex reefrock. This early lithification is taking place in an extremely turbulent wave-swept environment. and its upper limit extends several meters above the intertidal zone.

Early Carbonate Fabrics in Silurian Reefs of Gotland, Sweden: ABSTRACT

The Hogklint reefs of Gotland occur within a shallowing-upward sequence and are developed in the outer zones of a carbonate wedge adjacent to a shale belt. Shoal and peritidal sequences are the major carbonates which occur in the inner parts of the wedge and are preferentially associated with erosion surfaces. The Hogklint reefs show carbonate fabrics that are inferred to indicate synsedimentary marine cementation of the reefs. These fabrics are: (1) pseudofibrous calcites that are believed to have replaced acicular/fibrous cements with original aragonite and/or Mg-calcite mineralogy, (2) pelmicsparite-pelmicrite areas, (3) stromatolite-like crusts, and (4) early micritic cements. The reefs also show evidence of subaerial exposure by the occurrence of erosion surfaces at the top of the reefs, dissolution cavities, and vadose crystal silt infills in cavities in the algal biofacies which cap the reefs. End_Page 640------------------------------ The distribution of the various cement types within the reefs can be linked with the shallowing-upward nature of the limestone sequence as there is a predominance of early marine cement fabrics in the middle and upper parts of the reefs, particularly in the algal biofacies. The spatial and sequential nature of the various carbonate diagenetic fabrics show that the Hogklint reefs went through four main diagenetic events. Microstalactitic cements, vadose crystal silts, grain-contact cements, and pseudofibrous isopachous cements are developed within cross-bedded peloidal grainstones at the top of the shallowing-upward sequence and indicate cementation within the freshwater vadose zone and intertidal and/or marine phreatic zone. End_of_Article - Last_Page 641------------