Cement Stratigraphy Research Articles

Structural and diagenetic signatures of a very shallow sub-volcanic intrusion into a carbonate platform (Albian, Basque-Cantabrian Basin, Northern Spain)

In several tectonic contexts and especially in hyper-extensive basins, magmatic rise may crosscut shallow marine carbonate series, forming either volcanic systems or only igneous intrusions at various depths. Impacts of these magmatic events on the surrounding carbonate sediments can be significant and are relatively well documented from a geometrical, sedimentological and hydrochemical point of view. Their diagenetic signature (e.g., early diagenetic phases in cases where magma reaches or approaches the surface) still remains poorly investigated. In this paper the authors use a multi-proxy approach combining morpho-structural, diagenetic (cement stratigraphy), geochemical (carbon and oxygen isotope, U–Pb dating) and sedimentological study to investigate the Larrano igneous body contemporaneous with the Urgonian limestones (Early Albian) of the Duranguesado platform, in the well-preserved and hyper-extended Basque-Cantabrian Basin. This investigative work produced several results: the first result, combining morpho-structural and sedimentological data, is the model of a basaltic magma rising along normal faults but stopping about 50 m below the paleo-seafloor; another is the detailed description the “Larrano eogenetic anomaly” that includes cataclastic fractures, dissolution, Fe oxy-hydroxides, ferroan saddle dolomite and magnesian calcite. Such phases form an early diagenetic overprint around the igneous body and testify to the thermal shock and warm seawater circulation occurring at the time of and just after the intrusion. Pre-intrusive and post-intrusive hydrothermally-influenced cements and facies are also recorded and are perfectly calibrated by a detailed cement stratigraphy associated with a layering model of the sedimentary units. The role of magmatic CO2 contribution is not quantified in the present study. Furthermore, the δ13C signature of various cements, micrites and bioclasts suggests no thermogenic methane contribution, unlike other Albian magmatic intrusions hosted by different lithologies and recorded at various depths below the paleo-seafloor.

Differentiating early from later diagenesis in a Cretaceous sandstone and petroleum reservoir of the Cedar Mountain Formation, Utah

ABSTRACT Previously published anomalous whole-rock stable isotopic values from the Poison Strip Sandstone Member of the Cretaceous Cedar Mountain Formation (CMF) of eastern Utah are of uncertain origins. This study investigated the diagenetic history and the processes responsible for these anomalous data. Accordingly, we integrated photomicroscopic techniques including polarized light microscopy, epifluorescence and cathodoluminescence (CL) imaging, micromilling of stable isotope samples, and fluid-inclusion heating and freezing measurements to this end. The key observations involved the microscopic mapping of calcite cement stratigraphy using CL imaging to permit the analysis of stable isotopes of calcite cements that crystallized during early and late diagenesis. The mapping of calcite cement zones of sufficient submillimeter size to mill out and isolate microgram-sized stable isotope samples enabled this discrimination. Early diagenetic calcite cements have the most positive δ18O values (-10 to -8.5‰ Vienna Pee Dee Belemnite [VPDB]) of all components. The pattern of δ13C and δ18O variation in this early diagenetic cement indicates affinities with early meteoric diagenesis previously documented in published literature on the CFM. The late diagenetic calcite cements yield the most negative δ18O values (-18 to -16‰ VPDB). We interpret the late diagenetic cements to be responsible for the anomalously low whole-rock δ18O values previously reported from the Poison Strip Sandstone Member. Our discoveries of bitumen in late-stage pore fillings and liquid petroleum in the fluid inclusions of late diagenetic calcite cements of the Poison Strip Sandstone Member explain the lower whole-rock organic matter δ13C values and anomalous Δ13C values reported from the unit. Comparatively lower carbonate δ18O and organic δ13C values originally derived from whole-rock analyses of samples from the Poison Strip Sandstone Member resulted from high-temperature basinal diagenesis (hydrothermal circulation and/or petroleum migration), rather than the alternative interpretation of early diagenesis related to a Cretaceous paleoclimatic perturbation. Our results are illustrative of methods to resolve the long-standing geologic problem of discriminating and characterizing products of early vs. late diagenesis in terrigenous clastic sedimentary strata.

A comparative study of two Mississippian dolostone reservoirs in the Volga-Ural Basin, Russia

A significant proportion of the remaining oil reserves worldwide is concentrated in dolostone rock bodies. The present paper focuses on Tournaisian and Visean limestones and geologically younger diagenetic dolomite units in two mature oil fields of the Volga-Ural Basin, Russia. In contrast to many dolostone-hosted hydrocarbon reservoirs worldwide, the Tournaisian and Visean dolostones in the Demkinskoye and the Onbiyskoye oil fields plot near the low end of the porosity–permeability range. Replacive and porosity-destructive burial dolomitization has deteriorated the reservoir properties of these units. Dolomitization is related to the ascent of hot (170–215 °C) and saline (22–24 wt%) fluids as based on fluid inclusion data. This temperature range points to an abnormally high temperature background thermal regime of the Volga-Ural basin. The onset of burial dolomitization predates oil charge as documented by the fact that oil-bearing inclusions are only present in the outermost zones of porosity-occluding dolomite cements. Isotope geochemical data, dolomite petrography and fluid inclusion data reveal important similarities between hydrothermal dolostone units in the Demkinskoye and the Onbiyskoye oil fields. Based on these data and the detailed cement stratigraphy presented here, it is argued that dolomite bodies in both oil fields formed as the result of one main, Alpine tectonic event pushing hot and saline dolomitizing brines ahead of ascending hydrocarbons.

Characterization of ferruginous cements related with weathering of slag in a temperate anthropogenic beachrock.

This work outlines a temperate latitude beachrock occurrence, which represents the legacy of heavy anthropogenic environmental disturbance. The units contain high amounts of slag and iron-rich wastes derived from metallurgical activities that attest the impact of the past industrial development on such coastal systems. The exposition of the anthropogenic wastes to weathering processes, such as the influence of marine aerosols and the chemical attack of acid gases like the SOx coming from the nearby urban-industrial atmosphere, gave rise to the formation of early diagenetic ferruginous cements. A new analytical methodology based on the combination of micro-Raman spectroscopy (MRS), Raman chemical imaging, SEM-EDS and the Structural and Chemical Analyzer (SCA, an emerging system that hyphenates micro-Raman and SEM-EDS), was applied for the first time to characterize the ferruginous cements. The MRS analyses revealed Fe2+/Fe3+ oxides and oxyhydroxides, CaCO3 polymorphs and less frequently silicates. The Fe mineral species detected were hydrated goethite, hematite, magnetite, magnesioferrite, lepidocrocite and goethite. Complementary Raman imaging, SEM-EDS and SCA analyses unraveled the preferential distribution of hydrated goethite. The identified iron mineral phases are weathering sub-products of hematite commonly derived from atmospheric/aqueous leaching processes triggered by the chemical attack of the acid gases. EDS showed the existence of other elements such as Si, Mg, Cl, Na, Al, K and sporadically S that indicated the importance of permeability, atmospheric deposition and the acid attack. Additionally, calcite and gypsum minerals also evidenced the action of meteoric waters, dry deposition processes or the attack of SOx acid gases. The presence of such compounds is modifying the cement stratigraphy and suggests that the dissolution of carbonates is currently taking place. Those facts influence the erosive susceptibility and the release of the anthropogenic materials trapped originally in the beachrocks, which could act as potential secondary sources of contaminants to the coastal environment.

Diagenetic controls on the location of reservoir sweet spots relative to palaeotopographical and structural highs

Abstract Many carbonate reservoirs are located on top, or down the flanks, of extant structural highs or syndepositional palaeo-highs. This study examines diagenesis in Pennsylvanian oolitic reservoirs close to the crest and down the flank of a long-lived anticline. It illustrates that the position of the best reservoir quality shifted back and forth during successive diagenetic events. Cement stratigraphy shows that early diagenesis did not enhance reservoir character significantly. Most oomoldic porosity formed penecontemporaneously with compaction. Fluid-inclusion and stable isotope data indicate that late cements precipitated during burial conditions by refluxing brines and later hydrothermal fluids. After initial burial, greater permeability existed downdip, where smaller amounts of early meteoric cement allowed for compaction. Subsequent reflux cementation initially degraded downdip reservoirs preferentially and then progressed updip, resulting in relatively uniform reservoir porosity. Later hydrothermal events are most important in affecting the distribution of the highest quality present-day reservoir. Highest porosity is preserved in wells down the flanks of the structure, where hydrothermal cements are not as prevalent. Understanding the effect of diagenesis on location of the best reservoir in relation to palaeotopographical and structural highs allows for the prediction of reservoir quality using seismic and mapping data typically available in the subsurface.

Geochemical signatures of pervasive meteoric diagenesis of Early Miocene syn-rift carbonate platform, Red Sea, NW Saudi Arabia

Different diagenetic environments have been recognized in the Early Miocene carbonate platform of Musayr Formation in the Red Sea rift area. Early marine diagenesis includes micritisation that occurs as thin envelope around skeletal and non-skeletal grains in low-energy mud dominated facies and isopachous fibrous calcite in high-energy grain-dominated facies. Pervasive meteoric water diagenesis resulted in cementation of the carbonates by coarse-crystalline blocky-drusy calcite and meniscus cements. Depletion of oxygen (avg. –9.08‰), carbon (avg. –1.6‰) isotopes and trace elements concentrations (avg. values of Fe: 1387 ppm; Mn: 1444 ppm; Sr: 419 ppm; Na: 1194 ppm) in conjunction with negative correlation between Mn2+ and oxygen isotope data suggest variable degrees of fluid-rock interactions and pervasive meteoric diagenesis. The formation of meteoric diagenesis in the Musayr Formation can be explained by two subsequent mechanisms: (1) the presence of meteoric lenses during the time of deposition might have been associated with active freshwater input from the hinterland (NE) due to fall in the relative sea level; (2) later uplift episode during Plio-Pleistocene may have also contributed to the pervasive meteoric diagenetic alterations of the carbonates of the Musayr Formation. The first mechanism is supported by the cement stratigraphy where the blocky-drusy cements postdate the meniscus cement. The latter mechanism seems to have more pronounced effect on the alteration of Musayr carbonate sequence by observing the occurence of late cements such as blocky calcite in most of the samples. The impact of meteoric diagenesis on the studied samples suggest that dissolution is less severe than cementation, hence the visible porosity is very low. Understanding the timing of meteoric diagenesis provides useful information about the reservoir quality distribution in syn-rift carbonate sequences.

Paleosol Diagenesis and Its Deep-Time Paleoenvironmental Implications, Pennsylvanian–Permian Lodève Basin, France

Abstract Sedimentological, petrographic, and geochemical investigations of the Permo-Carboniferous Lodeve Basin, France show complete diagenetic resetting of soil-formed minerals. Carbonate cements include diagenetic end-member phreatic-zone and burial cements in nodules and veins. Cement stratigraphy reveals multiple generations, including low-Mg calcite, ferroan calcite, dolomite, barite, and silica cements that record the burial and exhumation history of the basin. Stable-isotope analysis of calcite nodules yields uncorrected δ13C values of −5.0‰ and −0.7‰, and between 0.4‰ and −3.8‰ for calcite veins; corresponding δ18O values of the calcite nodules are −3.6‰ and 0.2‰, while veins are between −3.1‰ and −11.0‰. Dolomite cements have uncorrected δ13C values of −0.2‰ to −6.7‰ and δ18O values of 2.0‰ to −6.1‰. These values represent micritic calcite as well as mimetic dolomite, dolomite rhombs, microspar calcite, sparry calcite, and ferroan calcite. Isotopic values record multiple fluids of increasing temperature that flowed through the basin from the Triassic through Cenozoic, and are affiliated with the burial of the basin as well as lithospheric thinning during Pangean breakup. Following careful screening of groundwater nodules, veins and paleosols (n = 46), no carbonate retains its primary pedogenic signal or utility for use in quantitative paleoclimate reconstructions. These data document the importance of careful petrographic and geochemical screening for diagenetic alteration of paleosol carbonates before quantitative paleoclimate proxies are applied.

Charge Contrast Imaging (CCI): Revealing Enhanced Diagenetic Features of A Coquina Limestone

Abstract: Charge Contrast Imaging (CCI) is a low-vacuum scanning electron microscope (LV-SEM) technique that can be induced through partial surface charge suppression of uncoated nonconductive samples, imaged with a suitable detector such as a gaseous secondary electron detector (GSED). The technique commonly produces results similar in style to that of SEM-cathodoluminescence (SEM-CL), providing information on zoning, twinning, annealed fractures, and subtle chemical changes. The current work outlines an example from a Brazilian Lower Cretaceous coquina limestone, in which both optical and SEM-CL imaging produces a limited response from much of the sample. Backscattered electron (BSE) imaging typically suggests only a hint of the cement present, whereas CCI clearly displays a rich and varied cement stratigraphy. The earliest cement displays strong CCI, but appears mainly dark under CL imaging conditions (SEM-CL and optical CL). Later-stage manganese-“enriched” carbonate cement displays luminescence with both optical and SEM-CL, as well as a CCI response. Therefore CCI can provide additional information on cement zonation in an area where CL cannot.

Repeated injection of hydrothermal fluids into downdip carbonates: a diagenetic and stratigraphic mechanism for localization of reservoir porosity, Indian Basin Field, New Mexico, USA

Abstract A detailed study of the Pennsylvanian Indian Basin Field of New Mexico, USA is used to develop a conceptual model that predicts reservoir porosity in clean down-depositional-dip marine carbonate, where repeated fracturing allows for hydrothermal fluid flow. Strata updip probably experienced repeated events of subaerial exposure, resulting in mineralogical stabilization and extensive calcite cementation that prevented extensive hydrothermal alteration. Clean, carbonate sediment deposited downdip was more prone to alteration by hydrothermal fluids. In the reservoir, cement stratigraphy shows regionally persistent zones closely associated with fracturing. Fluid inclusion data show high temperatures and repeated rises and falls, indicating tectonic valving in a hydrothermal system. Salinity data support this and indicate a late event of meteoric influx. Sr- and O-isotope data indicate higher temperature and less rock–water interaction in fault damage zones, evidence for intensified fluid flow in such areas. Formation of vugs and molds was associated with the hydrothermal fluid flow, which was driven by convection, probably initiated during 40–30 Ma intrusive activity and continuing after the onset of Basin and Range uplift and unroofing.

Diagenetic patterns and pore space distribution along a platform to outer-shelf transect (Urgonian limestone, Barremian–Aptian, SE France)

The Urgonian limestones of Late Barremian/Early Aptian from Provence (SE, France) are characterized by the occurrence of microporous limestones at regional scale alternating with tight carbonates. This study, based on petrographical (sediment texture, facies) and diagenetical analyses (cement stratigraphy, porosity and isotope geochemistry) of more than 800 limestone samples provides insight into the parameters controlling the genesis, preservation or occlusion of microporosity along an inner platform to outer shelf transect. The tight and microporous Urgonian limestones from Provence can be grouped into 5 rock-types based on textures, associated depositional environments, porosity and pore-type, being: (1) tight inner-platform: TIP; (2) porous inner platform: PIP; (3) tight outer platform: TOP; (4) porous outer platform: POP and (5) tight outer shelf: TOS. In tight (TIP, TOP and TOS types) limestones intergranular and intragranular pore spaces were entirely occluded by early marine and/or early meteoric cementation, whereas in microporous (PIP, POP) limestones a significant fraction of the intergranular macroporosity was preserved during early and shallow burial diagenesis. Micrite neomorphism (hybrid Ostwald ripening process) occurred during meteoric shallow burial diagenesis in PIP and POP limestones during the regional Durancian Uplift event (Albian–Lower Cenomanian). This process resulted in microporosity enhancement and preservation. Circulation of meteoric fluids during exhumation produces intercrystalline microporosity enhancement and moldic porosity development. The present study documents the important role that both early diagenetic and depositional cycles and long-term tectonic processes have on pore space evolution and distribution in Mesozoic platform carbonates.

Impact of fracture stratigraphy on the paleo-hydrogeology of the Madison Limestone in two basement-involved folds in the Bighorn basin, (Wyoming, USA)

Based on the study of the Madison Limestone at Sheep Mountain and Rattlesnake Mountain, a unique outcrop dataset including (1) facies and diagenetic analyses, (2) vertical persistence and cement stratigraphy of vein sets and (3) fluid inclusions thermometry are used to demonstrate the impact of folding and fracturing on paleo-hydrogeology. Quantification of the vertical persistence of fractures shows that Sheep Mountain and Rattlesnake Mountain differ by the vertical persistence of the pre-folding Laramide vein sets, which are strictly bed-confined in Sheep Mountain but cut across bedding at Rattlesnake Mountain, whereas the syn-folding veins are through-going in both. The emplacement chronology and the various sources of the fluids responsible for the paragenetic sequence are based on isotope chemistry and fluid inclusions analysis of the matrix and vein cements. At Sheep Mountain and Rattlesnake Mountain, the cements related to the burial are characterized by isotopic signatures of marine formation waters that were diluted during the karstification of the Madison Platform at the end of Mississippian. Meteoric fluids, presumably migrating during the Cenomanian from Wind River Range and Teton Range, recharge zones located in the south-west of the Bighorn Basin, were remobilized in the early bed-confined and through-going syn-folding veins of the Sheep Mountain Anticline. The former vein set drained only local fluids whose isotopic signature relates to an increase of temperature of the meteoric fluids during their migration, whereas the latter set allowed quick drainage of basinal fluids.

Cement stratigraphy: image probes of cathodoluminescent facies

Cement stratigraphy of carbonates aims to establish the chronology of processes involved in the rock diagenesis. Regional cement stratigraphy allows correlations and understanding of the petrological heterogeneities in reservoirs and aquifers, but is a long and rigorous approach. This article exposes a methodology of image analysis that facilitates the spatial correlation of diagenetic events in carbonate rocks. Based on the statistical comparison of signals extracted from the red spectrum emission of cathodoluminescence digital images, it gives via crosscorrelation a measure of similarity (values scaled from minimum −1 to maximum 1) between two cathodoluminescence facies. Cementation events and diagenetic chronologies can thus be quickly correlated without the support of a full chronology, the model normally established on cement morphologies, petrological analyses and cathodoluminescence zonation sequences. A case study from two Upper Kimmeridgian Mount Salève outcrops (France) illustrates this methodology. Their diagenetic sequences recorded in cathodoluminescent cements are presented and being compared. The final statistical similarity between the two outcrops reaches an index of R = 0.78. This result is sustained by petrological and geochemical analyses such as alizarine–ferricyanure stained thin sections, X microfluorescence mapping of elements, and microthermometry of fluid inclusions.

Progress in application of cathodoluminescence (CL) in sedimentary petrology

¶Significant progress has been made in cathodoluminescence (CL) studies in the last three decades due to the application of enhanced methods such as high-resolution spectroscopy. The luminescence of all detrital and diagenetic minerals such as quartz, feldspar, phyllosilicates, carbonates, apatites, zircons etc. can now be quantitatively characterized by spectroscopy. For siliciclastic sediments, the potential for provenance studies has been explored because varying conditions at the time of formation (e.g. of quartz and zircons) might be encoded within the CL and therefore provide fingerprints for particular grain populations. Different cement generations allow a cement stratigraphy to be established and the deconvolution of the diagenetic history of carbonate and clastic sediments. CL investigations of carbonate, apatite and – with some limitations – siliceous biogenic skeletons provide evidence of growth zonation (ontogenetic cyclicities and changes in spatial distribution) and diagenesis (intra-skeleton cementation and recrystallization). However, further research using CL is required to fully understand detrital components, primary and secondary mineralization in sedimentary deposits and biogenic materials. We present here an, admittedly incomplete, overview of these developments.

Upper Triassic reef facies in the Asher-Atlit-1 borehole, Northern Israel: Microfacies, cement stratigraphy and paleogeographic implications

The late Triassic succession of the Asher-Atlit 1 borehole is over 1000 m thick, and is composed of reefal and associated facies interbedded with volcanics of Norian age. Only borehole cuttings are available. Microfacies classification and cement stratigraphy determined by optical and CL microscopy, allowed discrimination of six episodes of reef establishment, progradation, shallowing, and termination. Organic buildups are constructed of reef-building biota (sponges, possible corals, encrusting organisms) typical for the late Triassic of the Tethys. Reef-associated facies include fore-slope, central reef, ooid shoal, lagoonal, and supratidal environments. Cement zoning patterns trace diagenetic signatures which range from early neomorphic skeletal replacements and original marine cements, via characteristic burial sequences; depositional and diagenetic sequences are terminated by marginal marine intra- or supratidal conditions, and subaerial exposure with pedogenic overprints. Volcanic episodes tend to be associated with termination of carbonate sedimentation episodes, while volcanic quiescence and subsidence permit vertical progradation of reefal and associated facies. The depositional and progradational environment, rapid rate of sedimentation, periodicity, association with volcanics, and regional considerations, suggest a depositional setting on the rifted shelf-margin of the nascent Neo-Tethys, with a possible eustatic overprint.

Abstract: Importance of Cement Stratigraphy in Studying Marine Carbonate Cements as Proxies for Paleo-Oceanic Conditions: Insights from the Devonian of the Canning Basin, Australia

Abstract: Importance of Cement Stratigraphy in Studying Marine Carbonate Cements as Proxies for Paleo-Oceanic Conditions: Insights from the Devonian of the Canning Basin, Australia

Suboxic to anoxic diagenesis of platform-marginal ooids and bladed-to-fibrous calcite from the Middle Ordovician Ottosee Formation (east Tennessee)

The oolite member of the Ottosee Formation (early Caradoc) has bladed-to-fibrous calcite cement that formed under oxygen-depleted conditions atypical of most abiotic marine precipitates. The sequence of early diagenetic events was as follows: (1) ooid formation, (2) precipitation of rare turbid marine cement, (3) minor dissolution of ooids and initial marine cement, (4) precipitation of translucent bladed-to-fibrous calcite, and (5) stabilization of metastable components, such as ooids and turbid marine cements. Ooids and turbid marine cements precipitated from oxic fluids. Minor dissolution of ooids and turbid marine cements was associated with aerobic degradation of organic material, which decreased alkalinity through generation of CO 2 . Translucent bladed-to-fibrous calcite precipitated from suboxic to anoxic marine fluids that were modified by bacterially mediated Mn 4+ , Fe 3+ , and SO 4 2− reduction. Translucent bladed-to-fibrous cement was originally low-magnesian calcite and has δ 13 C and δ 18 O values near other Caradoc marine values. Conversely, ooids and turbid marine calcite had a metastable magnesian calcite mineralogy. These components were dramatically altered by stabilization as reflected by negative δ 13 C and δ 18 O ooid values. Mineralogical stabilization of ooids and turbid marine cements mainly postdated bladed-to-fibrous calcite formation, but some stabilization preceded bladed-to-fibrous calcite cementation. Additionally, this study documents locally variable cathodoluminescence zonation patterns, which are at variance with the cement stratigraphy paradigm of regionally correlatable CL zones.

Carbonate cement stratigraphy and timing of diagenesis in a Miocene mixed carbonate-clastic sequence, offshore Sabah, Malaysia: constraints from cathodoluminescence, geochemistry, and isotope studies

A mixed carbonate-clastic sequence is one of several Miocene hydrocarbon-bearing reservoirs in the northwestern Sabah Basin. This sequence consists of bioclast-rich sandstones interpreted as prograding storm shoal deposits. The sequence contains a complex variety of carbonate nodules.The sequence has undergone nine stages of cementation that completely occluded pores. Each stage represents a distinctive cement texture, precipitating at specific temperatures and burial conditions. The diagenetic evolution began with a syndepositional iron-free marine calcite (Ca1), followed by an early methane-derived dolomite (Do1), bladed calcite (Ca2), blocky, vein-filling Fe-calcites (Ca3Ca4, clay mineral-associated dolomite (Do2), Fe-dolomite and ankerite (Do3Do4, and late iron-rich calcite (Ca5). Each stage of cement has distinctive Sr2+, Mn2+, Mg2+, Fe2+, Ca2+, and Na+ contents, reflecting changes in the palaeo-porefluid systems with time. The cements are correlatable over the whole study area.Precipitation occurred from very early to late stages of diagenesis at near surface to 2.0 km depths. With progressive burial and temperature increase, oxygen isotope values become strongly negative (stage Ca4: δ13C = −0.5%. PDB, δ18O = −5.7%. PDB). The cements were precipitated at 20–75°C. The source of carbon varies from marine dissolved carbonates to fermentation of organic matter, methane, meteoric waters, and possibly hydrothermal fluids.Strontium isotope dating of Do1 (with δ13C = −32.5%. PDB, δ18O = +2.60%. PDB) and Do2 (δ13C = 3.0%. PDB; δ18O = −2.5%. PDB) dolomites indicates that these dolomitization events took place at 10.5 Ma and 8.9 Ma, respectively, assuming precipitation within a completely open seawater system. The Do1 dolomite was precipitated from a methane-derived brine at the boundary between the Serravallian and Tortonian stages after major uplift, and induced faulting, at a sea-level lowstand. The Do2 dolomite was precipitated during intermediate to deep burial diagenesis during smectite-illite transformation before hydrocarbon emplacement.

Cement stratigraphy in Triassic and Jurassic limestones of the Weserbergland (northwestern Germany)

Cement stratigraphy of the Korallenoolith (Oxfordian) and Trochitenkalk (Upper Muschelkalk) Formations, southern Lower Saxony Hills of Germany, is based on investigation of 232 carbonate samples by cathodoluminescence (CL). This enables subdivision of cements into four main generations: Generation 1, consisting of fine columnar, equant and syntaxial cements with blotchy CL and/or microdolomite inclusions, that is interpreted as originally submarine Mg calcitic precipitate. Generation 2, found in Jurassic samples only. These cements show intrinsic to non-luminescent CL with thin bright orange subzones and they are interpreted as meteoric phreatic calcites. Generation 3.1, characterized by zoned calcites with variable CL colours. These cements precipitated under shallow burial conditions, in a phreatic environment during an incipient stage of late diagenesis. Generation 3.2, with relatively uniform orange CL, indicative of precipitation in a stable chemical environment. This cement generation is interpreted as late diagenetic deep burial calcites. Generation 3.3, partially strongly zoned and with a characteristically intense CL contrast. It has been identified in only 17 of the 82 Trochitenkalk samples. Deformation twins end within this generation, indicating a post-tectonic genesis for the subsequent younger cement zones. The cements were probably precipitated in a near-surface environment characterized by reducing or variable Eh conditions. Generation 4, consisting of calcites that show intrinsic CL with bright orange subzones comparable to generation 2, but lacking deformation twins. These cements are interpreted as post-tectonic late diagenetic products of a meteoric phreatic environment. Based on detailed CL petrography, fifteen diagenetic events (three calcite cements, three dolomite generations, three fissure generations, two stages of dedolomitization, aragonite dissolution, matrix recrystallization and two stages of HMC → LMC transformation) can be distinguished and timed relative to cement zonation. The progressively decreasing δ 18O values for cement generations 1 to 3.2 likely reflect increasing temperatures caused by burial. The low δ 18C values of cement generation 4 are probably a reflection of a meteoric diagenetic environment. A time—burial—cementation pathway can be modelled combining CL patterns, cement isotopic data and the subsidence history of the study area. The main features of this model are: 1. (1) Generation 1 cements are submarine precipitates and their age therefore coincides with sedimentation. 2. (2) Cement generation 3.1 of the Trochitenkalk commenced precipitation at a depth of about 120 m, reached during Late Triassic time. In case this minimum depth is also the formation depth of generation 3.1 in the Korallenoolith Formation, then precipitation of these cements started during Late Jurassic time. 3. (3) Cement generation 3.2 of both the Trochitenkalk and the Korallenoolith Formation, precipitated at assumed depths > 1000 m, reached during the Doggerian and at the time of Jurassic/Cretaceous transition, respectively. 4. (4) Cement generation 4 precipitated after the sediments had undergone telogenesis, and must thus be younger than the Intra-Senonian (Sub-Hercynian) tectonic phase and possibly even younger than the Paleocene tectonic pulse (Laramian Phase).

Carbonate cement stratigraphy and timing of hydrocarbon migration: an example from Tigapapan Unit, offshore Sabah

Carbonate cement stratigraphy and timing of hydrocarbon migration: an example from Tigapapan Unit, offshore Sabah

Cement Stratigraphy of the Wahoo Formation, Sadlerochit Mountains, Alaska: ABSTRACT

Cement Stratigraphy of the Wahoo Formation, Sadlerochit Mountains, Alaska: ABSTRACT