Development Of Stylolites Research Articles

Progressive development of stylolites in cryptocrystalline quartz

Stylolites are common dissolution structures in carbonate rocks in the upper crust and document a coupled physical-chemical feedback during pressure solution in rocks. By applying high-resolution electron-based analysis we investigate a rare example of stylolites in cryptocrystalline quartz (novaculite) from western Sichuan Basin, China. The investigated stylolites exhibit an increased amplitude from the tip toward the center, which correlates with the maturity of stylolites and thus provides an excellent opportunity to infer the dynamic processes operating during stylolite formation. Microstructurally, stylolites are characterized by a concentration of biotite and hematite, dissolving irregular quartz grains, and the occurrence of dissolution porosity. By investigating the progressive co-variations between stylolite amplitude and microstructure characteristics from the stylolite tip toward the center, we demonstrate, for the first time, the evidence of stylolite growth through anticracks. It is inferred that stylolites in cryptocrystalline quartz originate from the propagation of mode I micro-cracks around pre-existing biotite, followed by pore-scale dissolution. Both processes, micro-cracking and dissolution, are interacting and co-evolving via a self-organization fashion during shortening. Our study contributes to the ongoing debate about whether stylolites are conduits or barriers for fluid flow.

PRE‐, SYN‐ AND POST‐TECTONIC DIAGENETIC EVOLUTION OF A CARBONATE RESERVOIR: A CASE STUDY OF THE LOWER CRETACEOUS FAHLIYAN FORMATION IN THE DEZFUL EMBAYMENT, ZAGROS FOLDBELT, SW IRAN

Lower Cretaceous carbonates of the Fahliyan Formation form prolific reservoir rocks at oilfields in the Dezful Embayment, central Zagros fold‐thrust belt, SW Iran. The carbonates have undergone significant diagenetic alteration in phases which can in general be linked to the pre‐, syn‐ and post‐tectonic evolution of the fold‐thrust belt. This paper investigates the impact of diagenetic processes on the reservoir quality of the carbonates using integrated petrographic, geochemical and sedimentological analyses of subsurface and outcrop samples of the formation. Diagenetic alterations include:(i) pre‐tectonic eogenesis in the marine and shallow‐burial realm, which resulted in micritization of allochems and cementation by equant and isopachous calcite rims and framboidal pyrite together with limited dolomitization and dissolution of metastable bioclasts. The isotopic compositions of micrite and early calcite cement depart from postulated values of Lower Cretaceous marine carbonates, signifying early stabilization of precursor metastable carbonate minerals and the possible effects of the incursion of meteoric waters and/or increasing burial temperatures;(ii) mesogenesis during the subsequent syn‐tectonic phase, which included Late Cretaceous ophiolite obduction at the northern margin of the Arabian Plate and the later Zagros orogeny in the Miocene‐Pliocene. Diagenetic modifications included the emplacement of hydrocarbons, the development of stylolites and fractures, and the precipitation of saddle dolomite, replacive rhombic dolomite, discrete pyrite, microcrystalline quartz, kaolin and anhydrite. The average stable isotope compositions of saddle dolomite (δ18O: ‐6.9 ‰ ± .9 and δ13C 0.5 ‰ ± 1.6, respectively) also reflects the influence of high temprature basinal fluids;and (iii) “late” (telogenetic, post‐tectonic) uplift‐related modification starting in the Pliocene, when the incursion of meteoric waters resulted in the formation of vugs, the calcitization of dolomite, and cementation by fracture‐filling blocky calcite. The negative δ18O and δ13C stable isotope values (average: ‐5.5 ‰ ± 1.5; and ‐3.6 ‰ ± 5.9, respectively) of late blocky calcite cement suggest the incursion of meteoric water into the system.This study demonstrates that diagenetic processes in carbonates in the Fahliyan Formation, which exerted a significant control on the distribution of secondary porosity, can be related to the tectonic evolution of the central Zagros fold‐thrust belt. Thus, constraining the diagenetic history of carbonate successions within the context of their wider tectonic evolution is important for the prediction of the spatial and temporal distribution of reservoir quality.

Diagenesis, facies and palaeocurrent analysis of Upper Rewa Sandstone around Sagar, Central India

The stratigraphic surface represented by a major contact between the Archean Bundelkhand Granite and the extensive Proterozoic Vindhyan sediments is a regional basement cover unconformity. This crystalline–sedimentary interface reflects an intense weathering of continental crust during marine transgression. Three time-transgressive sand deposition events viz. Kaimur, Rewa and Bhander are mainly witnessed during the entire Vindhyan sedimentation. Stratigraphically, the Upper Rewa Sandstone comprises one of these events in the Vindhyan Basin. Considerable progress has been made in interpreting these sandstones as a function of entirely marine process to a combination of tidal–fluvio–eolian activities. All the results have so far been attained on the basis of sedimentary facies, provenance, palaeocurrent analysis, and some of petrography. A distinct differentiation between marine and fluvial components of the Upper Rewa Sandstone still remains uncertain. Here, we use diagenesis as a parameter for the first time along with facies and palaeocurrent analysis to acquire a clear comprehension of marine- and fluvial-dominated processes. The present study spans 27 square kilometer area covering 15 stratigraphic sections with a collection of 571 directional data from the facies specific sedimentary structures, and 28 samples obtained for the thin section analysis.The lower unit of the Upper Rewa Sandstone shows facies association typical of tidal environment, along with polymodal palaeocurrent. These rocks are well sorted and dominantly lithified by authigenic cement. Profuse development of a complex of syntaxial, passive pore fills, and grain replacive cements reflect normally pressured nature of the lower unit sandstones. The upper unit of the Upper Rewa Sandstone however, reveals a facies pattern resembling fluvial processes and unimodal palaeocurrent plot. They exhibit poor sorting, siliceous, and ferruginous matrix, and development of intense stylolites. Since, pressure solution is a dominant mode of lithification; the upper unit rocks therefore, are overpressured. Sand deposition event pertaining to the Upper Rewa Sandstone is found to comprise both marine and fluvial processes. Such a diagenesis based approach can be applied for marking a possible facies based unconformity between genetically different depositional units.

Bioturbation changing porosity, permeability, and fracturability in chalk? Insights from an Upper Cretaceous chalk reservoir (Buda Formation, Texas, USA)

The continuous rise in hydrocarbon demand, the production decline in conventional oilfields, and the remarkable improvement in extraction methods have allowed the hydrocarbon industry and subsequently, geoscientists, to turn to studies on both unconventional and mature fields with untapped potential. In such reservoirs, the application of advanced drilling, completion, and production techniques must be preceded by the identification of permeable stratigraphic intervals that favour commercial exploitation. Accordingly, bioturbated deposits can play a key role in providing permeable pathways for enhanced production. Herein, we analyse the effects of bioturbation on the porosity and permeability of the Buda Formation, a highly bioturbated, tight (low-porosity matrix) Upper Cretaceous chalk reservoir from the Texas Gulf Coast Basin, which has been commercially exploited because of the occurrence of natural fractures. Our results show that (in addition to the natural fractures) burrows (and borings) substantially increased the porosity and permeability of this formation, thus, potentially contributing to enhanced hydrocarbon (or groundwater) storage and production. These biogenic structures might also have favoured the development of natural fractures and stylolites in the Buda Formation. However, further studies are required to prove this hypothesis.

Formation Timing and Features of Stylolites and Controlling Factors for the Second-Period Stylolites in the Carboniferous KT-I Formation of NT Oilfield

The formation timing of stylolites, which is of great importance for analyzing the controls of stylolites, has nearly never been examined. In this paper, based on the data of cores, imaging logging, conventional logging, and mercury injection, the characteristics of stylolites formed in different stages of tectonic movement were investigated, and the controlling factors of oil-stained stylolites, formed in the second period of tectonic movement, were analyzed in particular. Furthermore, the influence of different controlling factors on the development of stylolites was compared, by using grey correlation analysis. The results show that there are three periods of stylolites in the study area, and all three periods developed both low-angle stylolites and high-angle stylolites. The prominent characteristics of both the low-angle and high-angle stylolites of the second period, are being oil-stained. The higher the structural location, the greater the buried depth, the lower the dolomite content, the higher the calcite content, the higher the clay content, the smaller the rock density, the greater the porosity, the smaller the rock grain size, the easier it is to develop both the low-angle stylolites and the high-angle stylolites. The influence of different controlling factors on the development of low-angle stylolites is given by depth, porosity, curvature, rock density, rock grain size, clay content, dolomite content, and calcite content, in this order. The importance of the influences on the development of high-angle stylolites proceeds as follows: curvature, calcite content, depth, rock particle size, clay content, rock density, dolomite content, and porosity. Tectonism is the most important influencing factor on the development of stylolites.

How pressure-solution enables the development of deformation bands in low-porosity rocks

Occurrence of deformation bands developing with various orientations in a low-porosity bioclastic calcarenite in the Cotiella Massif (Spanish Pyrenees), questions about the mechanism responsible for their generation in low-porosity carbonates. Our work is based on the combination of a 2D and 3D petrographic and microstructural study, using electron backscattering diffraction microscopy and X-ray microtomography, with a diagenetic study, using cathodoluminescence microscopy and scanning electron microscopy images. Diagenetic and petrographic observations indicate early cementation, which led to significant porosity reduction prior to a sequence of deformation processes comprising the development of tectonic stylolites around which deformation bands eventually formed. Along the latter, quartz grains appear fractured with high strain patterns, showing a different appearance compared to the quartz along the stylolite. Moreover, the ratio of carbonate matrix versus quartz grains shows distinctive variations throughout the different stages of deformation. Based on these observations, we propose a conceptual model for the genesis of deformation bands in low-porosity rocks, accounting for the preferential dissolution of calcite with respect to quartz grains along the stylolite plane that resulted in local porosity development in between quartz grains. Consequently, quartz aggregation led to the classical deformation band mechanism of strain accumulation and breakage.

The Nonuniform Distribution of Stylolite in Bioclastic Limestones and Its Influence on Reservoir Petro-Physical Properties—A Case Study of the Mishrif Formation from the Ah Oilfield

The effect of stylolite caused by the pressure dissolution process on the reservoir petro-physical properties is still controversial. This study aims to reveal the effect of stylolite on the porosity and permeability of packstone and wackestone in the Mishrif Formation from the Ah oilfield in the Middle East. Based on the observation of thin sections and cores, X-ray diffraction analysis and porosity and permeability measurement, the lithofacies, diagenesis and patterns of stylolites have been investigated. There are six lithofacies in the Mi4 member, including bivalve green algae packstone, green algae packstone, pelletoid green algae packstone, echinoderm packstone, rudist packstone, planktonic foraminifera wackestone and bioclastic wackestone. The mechanical compaction and pressure dissolution could be observed in these lithofacies, with the development of dissolution seams and stylolites. The density of stylolite has a relationship with the lithofacies and early cementation. The boundary between the echinoderm packstone and the green algae packstone mostly developed as stylolites. There are four types of stylolite on the cores. Type A is the wave-like stylolite developed at the boundary between the echinoderm packstones and green algae packstones. Type B is the zigzag stylolite with high amplitude in the green algae packstones. Type C is the stylolites with low amplitude in the bioclastic wackestones. Type D is the high-angle stylolite, which is oblique to the bedding plane. The permeability of reservoir rocks could be improved by dissolution along the type B stylolite, while the type A and type C stylolite have little effect on permeability. The permeability of green algae packstone and echinoderm packstone will decrease with the development of stylolites. The porosity and permeability of bivalve green algae packstone will decrease after stylolitization, resulting from the relatively high density of stylolite. The physical properties of bioclastic wackestone could be improved by the bioturbation and formation of stylolite. According to the analysis of production performance in the same lithofacies, the occurrence of stylolites could result in the development of oil baffles. This study could be extended to evaluate the effect of stylolite in carbonate reservoir rocks.

Diagenesis and reservoir characteristics of the Lithocodium–Bacinella facies in a Lower Cretaceous reservoir, eastern Persian Gulf Basin

In this paper, the importance of Lithocodium–Bacinella facies (LB facies) as reservoir rock in the Lower Cretaceous carbonates of the Persian Gulf will be addressed. The studied interval belongs to the reservoir zone of upper Fahliyan Formation, which occurs in one of the giant offshore oilfields. A detailed examination of a continuous core (58 m) from this formation allows clarifying the relationship between facies distribution, diagenetic evolution, and porosity development in function of relative sea level changes. Seven facies types are identified based on core descriptions and microfacies analysis. These facies are interpreted to have been deposited in lagoonal, shoal, and open marine settings during Valanginian–Hauterivian times. Petrographic studies and stable carbon and oxygen isotope analyses indicate that these facies have been affected by different diagenetic processes, starting with early marine diagenesis and followed by meteoric and/or burial diagenesis. Based on facies stacking patterns and evidence of subaerial exposures, four high-frequency sequences are interpreted in terms of variations in accommodation space. Combination of facies analysis, diagenetic studies and poroperm data reveals (with the exception of sequence I) that the lower transgressive and the upper highstand intervals of these sequences show different facies types, diagenetic modifications and reservoir properties. The transgressive system tracts (TST’s) correspond to tight zones (Z-1, 3, 5), with poor reservoir quality (Φ < 15% and K < 1 md). These intervals are composed of an alternation of open to restricted marine facies. The facies are characterized by extensive bioturbation, high clay content (argillaceous limestones), displaying marine (micritization) and shallow to deep burial diagenetic processes (pyrite mineralization and solution seams). The reservoir intervals preferentially occur within the highstand system tracts (HST’s), which are commonly composed of vertically stacked Lithocodium-Bacinella facies. This facies with aggradational patterns is capped by subaerial exposure surfaces at the top of the sequences and is associated with leaching and cementation. Meteoric dissolution during subaerial exposure resulted in the formation of vuggy and moldic pores as the main pore types in these system tracts, which survived in significant amounts until present-day. Therefore, the LB facies usually have greater than 15% porosity and 1 md permeability. Diagenetic studies in these units show that the marine diagenesis was interrupted by subaerial exposure and meteoric diagenesis before burial and development of stylolites. Our findings show that despite the uncertain taxonomy and paleoenvironmental setting of Lithocodium aggregatum and Bacinella irregularis, the LB facies has great reservoir importance.

Porosity in chalk – roles of elastic strain and plastic strain

AbstractChalks originate as Cretaceous to Recent pelagic or hemipelagic calcareous ooze, which indurate via burial diagenesis to chalk and limestone. Because they accumulate in pelagic settings with high environmental continuity, chalks may form thick formations and even groups. For this reason, and because chalks have a simple mineralogy (low magnesium calcite, silica and clays), they are ideal for the study of diagenetic processes including the depth‐related decrease of porosity. It is the aim of this study to illustrate how the evaluation of in situ elastic strain can help in understanding these processes including the interplay between stress‐controlled diagenetic processes and processes furthered by thermal energy. Petrophysical core and well data can be used for analyses of how porosity reduction via pore collapse and pressure dissolution is related to in situ elastic strain. The data in question are: depth, density of overburden, pore pressure, ultrasonic P‐wave velocity and dry density/porosity. The analysis reveals that the transition from ooze to chalk is associated with high elastic strain and consequent pressure dissolution at calcite–particle contacts causing contact cementation. The transition from chalk to limestone is also associated with high elastic strain, especially at clay–calcite interphases causing development of stylolites via pressure dissolution, and consequent pore‐filling cementation. Following each transformation the elastic strain drops rapidly. The observation of this diagenesis‐related pattern in elastic strain of the sedimentary rock is novel and should not only be helpful in understanding the porosity development in sedimentary basins, but also add basic scientific insight.

Hydrothermal dolomitization and porosity development: An example from Precambrian dolomitic rocks of Água Clara Formation, Ribeira Belt, southern Brazil

Dolomitic rocks from the Rio Bonito Quarry, located in the Água Clara Formation, record multistage dolomitization processes linked with porosity development in the southern Brazilian Shield. Dolomitic marble and breccia samples from this unit contain three textural types of dolomite that either are fabric destructive or void-filling phases. To study the role of dissolution in the porosity development in these dolomite types - from sedimentary to metamorphic domains - we combine petrographic methods, cathodoluminescence, Raman spectroscopy, micro-computed tomography, stable isotopes of carbon and oxygen, and fluid inclusion analysis. The protoliths of the dolomitic rocks of Rio Bonito Quarry are assumed as Mesoproterozoic (ca. 1.5 Ga) marls presenting shallow shelf to deep water lithofacies. The first dolomitization stage, forming a nonmimic replacement of the primary carbonate (Dol1) is fabric destructive and related to a burial setting (δ18O: -12.42 to - 9.68‰; δ13C: -2.29–0.40‰). The development of bedding-parallel stylolites of low amplitude led to increasing of porosity (up to 0.2%) and promoted the fluid circulation. During the Neoproterozoic Brasiliano tectonic events (ca. 650-550 Ma), the metamorphism of marls into dolomitic marble is intimately connected to the development of second replacement dolomite (Dol2). Penecontemporaneous brecciation is assumed as triggering factor to the fluid change, where Dol2 is akin to dolomites from MVT-deposits (δ18O: -19.12 to −12.03‰; δ13C: -0.07 to −2.17‰), and the total porosity is enhanced to 2%. The ubiquitous tectonic stress boosted the hydraulic fracturing, providing conditions to saddle dolomite (Dol3) precipitation, occurring as a replacement and void-filling phase. Lower isotopic values of Dol3 (δ18O: -14.93 to −10.77‰; δ13C: -3.36 to −1.89‰) suggest disequilibrium between the host-rock and the dolomitization fluid, while the porosity is augmented to 9%. The subsequent dedolomitization, generating void-filling calcite, is associated to meteoric phreatic input (δ18O: -8.31 to −5.56‰; δ13C: -5.59 to −2.56‰), similarly to the development of calcretes; therefore, diminishing the porosity towards 4%. Microthermometric data shows different signatures in the dolomitization fluid, which started at 100–150 °C and moderate-salinity (3–4 wt% eq.NaCl) in a burial setting, reaching up to 270 °C and low-salinity (0-2 wt% eq.NaCl) as a response to a shifting tectonic environment or fluid mixing. Additionally, the temperature shift between the host rock (120–130 °C) and the different dolomitization stages (from 140-150° to 210–270 °C), above 10 °C offset, point to a hydrothermal setting.

Petrophysical heterogeneity in a Lower Cretaceous limestone reservoir, onshore Abu Dhabi, United Arab Emirates

The upper zone of the Lower Cretaceous Kharaib Formation (151–177 ft [46–54 m] thick in the studied wells) is a major oil reservoir in several giant oil fields. Wide variations in porosity and permeability of this zone have been shown to result from both the inhibition of burial cementation by oil in the crest of each field and localized cementation adjacent to stylolites, combined with the more subtle influence of widely varying depositional mud content and grain size. The present study examines these relationships in closer detail, using core and petrographic observations from two wells on the oil-filled crest and two wells on the water-filled flanks of a giant domal oil field. Although porosities are higher overall in the crestal cores, each well shows wide variations within each of seven main groupings of the samples by depositional texture. This heterogeneity results mainly from the distribution of clay, which is concentrated along depositional laminations and causes widely varying porosity losses in all textures by promoting stylolite development and associated calcite cementation. Higher clay abundance (and lower porosity) within the upper and lower 12–17 ft (4–5 m) of the reservoir reflects increased influx of siliciclastic fines across the epeiric Barremian carbonate platform immediately following and preceding, respectively, third-order falls in global sea level. Most (95%) of porosity-permeability data from the studied wells lie within Lucia rock-fabric class 3, showing distinct but relatively subtle differences between texture groups, whereas a subordinate part of the data from the upper, relatively mud-poor third of the reservoir plot at higher permeabilities. Development of a predictive model for the petrophysical heterogeneity of this example requires a combination of the following: (1) a diagenetic model for porosity controls; (2) the use of a modestly higher porosity-permeability transform (upper class 3) in the upper part of the reservoir than in the lower reservoir (lower class 3); and (3) a recognition of the scattered and widely varying occurrences of exceptionally high permeabilities in the upper reservoir.

Diagenesis and its impact on a microbially derived carbonate reservoir from the Middle Triassic Leikoupo Formation, Sichuan Basin, China

The uppermost Middle Triassic Leikoupo Formation in the western Sichuan Basin of China has recently been shown to host as much as 5.3 tcf (1.5 × 1012 m3) of natural gas resources. The reservoir rocks, composed mainly of microbially derived dolomudstone (e.g., thrombolites and stromatolites), are characterized by low porosity (<8%) and permeability (<0.001 to 10 md). The limestone is commonly tight and not of reservoir quality because of abundant meteoric calcite cementation, whereas the dolostone has various types of pores dominated by solution-enlarged pores and vugs, microbial framework pores, and micropores. Breccias are well developed in places, probably because of dissolution of underlying evaporites (e.g., anhydrite) by an influx of low-salinity fluids (e.g., freshwater and seawater) during an early burial stage. Early dolomitization created micropores in the dolomudstone, and subsequent diagenetic events were dominated by calcite, dolomite, quartz cementation, pyrite replacement, compaction, fracturing, and development of stylolites. Localized hydrothermal activity has been evidenced by high homogenization temperatures (∼160°C–200°C) obtained from fluid inclusions in fracture-filling cements. Bacterial sulfate reduction probably resulted in H2S generation, pyrite precipitation, and solution-enlarged pore and vug formation, whereas part of the current H2S in these reservoirs may have been sourced from thermochemical sulfate reduction or an underlying formation (e.g., the Feixiangguan Formation). Development of microfractures and associated micropores was probably the final diagenetic event, which improved pore interconnectivity. This study confirms the effect of diagenesis on the development of a microbial dolomudstone reservoir, which may be applicable to other similar microbial carbonate reservoirs elsewhere, for example, Middle Triassic sections of the Tethys region and offshore Brazil.

Measurement of geometry and linkage in vein arrays

Veins in an array are characterised by measurement of their length (L), width (W) and orientation (θ), and by the number (0, 1 or 2) of adjacent veins to which they are linked. Data from a single array (129 veins), and two regions (comprising 146 and 64 arrays) have been measured and analysed. The system for measurement provides a sound basis for the quantitative assessment of both the array geometry and the effects of interaction of adjacent veins.Linkage is achieved by a breaching of the bridges of wall rock between veins, either by extension fractures, shear failure or the development of stylolites. It is primarily controlled by the strain, as measured by vein width. The size and shape of the veins is found to be controlled by two factors: the degree of linkage and the kinematics of the array (transtension/transpression). Interplay of these produces variability both within and between arrays, and this is examined for two large datasets from the Lower Jurassic of Somerset and the Lower Carboniferous of the Gower Peninsula, South Wales.

The Transformation of Sediment Into Rock: Insights From IODP Site U1352, Canterbury Basin, New Zealand

Abstract: At Integrated Ocean Drilling Program (IODP) Expedition 317 Site U1352, east of the South Island New Zealand, we continuously cored a 1927-m-thick Holocene-to-Eocene section where we can uniquely document downhole changes in induration and lithification in siliciclastic to calcareous fine-grained sediment using a wide range of petrological, physical-property, and geochemical data sets. Porosity decreases from around 50% at the surface to 5–10% at the base of the deepest hole, with a corresponding increase in density from ∼ 2 to ∼ 2.5 g cm 3 . There are progressive bulk mineral changes with depth, including an increase in carbonate and decrease in quartz and clay content. Grain compaction is first seen in thin section at 347 m below sea floor and intensifies downhole. Pressure solution (chemical compaction) begins at 380 m and is common below 1440 m, with stylolite development below 1600 m, and sediment injection features below 1680 m. Porewater geochemistry and petrographic observations document two active zones of cementation, one shallow (eogenetic) down to ∼ 50 m, as evidenced by micritic nodules and pore-water geochemistry driven by methane oxidation by sulfate, and another burial-related cementation zone (mesogenetic) starting at ∼ 300 m. A transitional zone occurs between 50 and 300 m. Our results quantify downhole diagenetic changes and verify depth estimates for these processes inferred from outcrop studies, and provide an actualistic example of cementation and compaction trends in a slope setting.

Stylolites and Porosity In A Lower Cretaceous Limestone Reservoir, Onshore Abu Dhabi, U.A.E.

Abstract Abstract The vertical distribution of bedding-parallel stylolites in cores from the approximately 50-m-thick Thamama-B reservoir zone of the Lower Cretaceous Kharaib Formation is compared between two wells on the water-filled flanks and two wells on the crest of a giant oilfield where oil has previously been interpreted as having preserved higher porosity on the crest. Stylolite abundances indicate division of the reservoir zone into three intervals: The top 12 ft (3.7 m) of the zone contains abundant low-amplitude stylolites in both crest and flank locations. The main, central part of the zone has abundant high-amplitude stylolites in the flank wells but nearly no stylolites in the crest. The basal 17 ft (5.2 m) of the zone contains high-amplitude stylolites in all wells, but these are somewhat more abundant in the cores from the crest of the field. The contrast in stylolite abundance in the center interval is consistent with oil having inhibited chemical compaction on the crest of the field. Minor dissolution along wispy seams in the center interval of the crest is interpreted as representing an incipient stage of stylolite development. The similarity in stylolite abundance in the top interval between crest and flank wells is interpreted as reflecting stylolite formation pre-dating oil emplacement because of greater depositional concentration of clay in thin laminations resembling flaser bedding. Stylolite growth in the basal interval may also partly predate oil filling, but the evidence for timing is unclear because stylolites in the heavily bitumen-stained basal interval of the crestal wells have thick bitumen coatings that may be residues from dissolution of the surrounding limestone. Negative correlation between porosity and both amplitude and proximity of stylolites supports the model of porosity loss by calcite cementation derived from stylolites. The overall porosity difference between crest (22%) and flanks (12–16%) approximately matches the volume of cement that would have been supplied by the thickness of strata dissolved on the flanks. The match between porosity difference and thickness variation supports a diagenetic system closed to significant calcite import or export. Profiles of bulk-chemical analyses reveal how clay (proportional to aluminum) and dolomite (proportional to magnesium) vary both vertically in the formation and laterally between the crest and flank locations. Higher clay values, suggested to facilitate stylolite development, occur mainly in the top and basal parts of the zone, reflecting episodic deposition of siliciclastic fines. Higher dolomite contents throughout the wackestone-dominated lower two thirds of the reservoir zone (around 5–10 wt. %) than in the grain-supported upper one third (little or no dolomite) are suggested to reflect differences in early dolomitization by seawater attending slower sedimentation rates in the lower part of the zone. Like the variations in chemical compaction, dolomitization is thus a manifestation of the layer-cake geometry of this reservoir. Detailed core descriptions from each well and tables of bulk-rock chemical analyses and stylolite data are available as supplemental material.

Diagenesis and reservoir properties of Middle Jurassic sandstones, Traill Ø, East Greenland: The influence of magmatism and faulting

The c. 500 m thick Middle Jurassic sandstones of the fluvial Bristol Elv and marine Pelion Formations of the East Greenland Basin are evaluated here in order to improve the understanding of the processes that influenced the diagenetic evolution. The study may help to predict the reservoir properties of sandstones affected by magmatism and faulting, both in general and specifically in undrilled areas on- and offshore East Greenland and, in the Vøring Basin on the Mid-Norwegian shelf. The study shows a variety of authigenic mineral phases dominated by quartz cement, carbonate cement, illite and iron-oxide. One of the clear differences between the two formations is the presence of early carbonate-cemented horizons in the marine sandstones; these horizons are inferred to reflect a primary concentration of biogenic clasts and fossil shells. Intense quartz cementation occurs primarily in the fluvial sandstones but the marine sandstones are also highly quartz-cemented. Two episodes of burial and uplift are recorded in the diagenetic sequence and widespread grain-crushing in coarse-grained intervals is believed to result from overpressure and subsequent compaction due to sudden pressure release along major faults. Maximum burial depths may only have been around 2000–2500 m. Cathodoluminescence analyses show that grain crushing was followed by intense quartz cementation. The quartz cement is to a great deal believed to have formed due to increased surface area from crushing of detrital quartz grains, creating fresh nucleation sites for the quartz. Cathodoluminescence investigations also show that only minor pressure dissolution has taken place between detrital quartz grains and that the ubiquitous quartz cementation displays several growth zones, and was thus in part the result of the introduction of silica-rich extra-formational fluids related to the flow of hot fluids along reactivated faults and increased heat flow and temperature due to magmatism. This interpretation is supported by fluid inclusion homogenization temperatures between 117 and 158 °C in quartz cements. In one of the two study areas, the development of macroscopic stylolites has significantly enhanced quartz cementation, probably in connection with thermal convection flow. As a result of the magmatic and fault-related quartz cementation and illitization, the reservoir quality of the sandstone formations deteriorated and changed drastically.

Pressure dissolution features in Oxfordian microbial-sponge buildups with pseudonodular texture, Kraków Upland, Poland

Part of the Oxfordian carbonate buildups in the southern part of the Kraków Upland is developed as pseudonodular limestones, which represent segment reefs. These limestones are composed of connected, rounded-oval to subangular carbonate pseudonodules. The pseudonodules, densely packed within the limestone, fall out easily under mechanical stress. The recently observed texture of pseudonodular limestones resulted from two stages of chemical compaction. During the first stage, in the Late Jurassic, high-amplitude and low-amplitude stylolites and dissolution seams were formed. The sites particularly favourable for the development of high-amplitude stylolites were the boundaries between already lithified fragments of the laminar, rigid microbial-sponge framework. The low-amplitude stylolites formed mainly in the intercalated wackestone-packstone, which was lithified somewhat later; hence, the dissolution seams originated at the contacts between the rigid microbial-sponge framework and the wackestone- packstone. After Early Cretaceous erosion, which decreased the burial load, Late Cretaceous sedimentation enabled the renewal of pressure dissolution. Thus, some low-amplitude stylolites evolved into dissolution seams. In stylolites composed of both low- and high-amplitude segments, dissolution proceeded at the bases of interpenetrating high-amplitude stylolite columns, with the simultaneous transformation of low-amplitude stylolite segments into dissolution seams. These seams, which formed at the initial stage of chemical compaction, were subjected in turn to further pressure dissolution, giving rise to the formation of horsetail structures. The vertical stress field, which triggered the pressure dissolution processes, presumably resulted in the formation of high-angle and vertical incipient tension gashes. At the beginning of the processes, these gashes remained closed. In the Cenozoic, under the extensional regime generated by overthrusting Carpathian flysch nappes, some high-angle and vertical dissolution seams and low-amplitude stylolites opened up, forming deformed dissolution seams and deformed stylolites. Under the same conditions, the high-angle and vertical tension gashes opened up as well. Subsequently, during the exposure period, unloading fractures developed, partly as a result of the opening of some subhorizontal and horizontal dissolution seams and stylolites. The unloading fractures, along with the already existing vertical and high-angle tension gashes, formed the network changing the limestone into pseudonodules of various shapes and sizes. The open spaces between the limestone fragments became local conduits for karst waters.

Bedding-parallel stylolites in shallow-water limestone successions of the Apulian Carbonate Platform (central-southern Italy)

Bedding-parallel stylolites typically represent the product of chemical compaction (overburden weight-induced pressure solution) experienced by carbonate successions during their burial history, when bedding is still horizontal. Due to their common occurrence in carbonate rocks, with lateral extents that can exceed 1 km, bedding-parallel stylolites are of special interest for the hydrocarbon industry because they may affect the regional fluid flow in the subsurface. Aimed at assessing the development and distribution of bedding-parallel stylolites in shallow-water, platform limestone successions, field and laboratory studies were carried out on Cretaceous limestones originally pertaining to the Apulian Carbonate Platform realm and now exposed in three distinct Italian locations: Maiella Mountain, Gargano Promontory and Murge Plateau. Results point to a prominent role played by the geological characteristics of limestones on development and localization of bedding-parallel stylolites within shallow-water, platform limestone successions. In particular, bedding-parallel lamination and fine rock grain size, co-occurring in stromatolitic limestones, determined there laterally more extensive and closely spaced stylolites than in the associated calcilutites and calcarenites. Large fenestral pores, which are ubiquitous in stromatolitic limestones, represent rock heterogeneities able to influence the roughness of individual stylolites. Laboratory measurements revealed that the permeability of the studied Cretaceous limestones is very low (<10 μD). Pilot tests suggest that bedding-parallel stylolites in stromatolitic layers are not barrier to fluid flow but may represent pathways through low-permeability, platform limestone successions in the subsurface.

Regional hardening of Upper Cretaceous Chalk in eastern England, UK: trace element and stable isotope patterns in the Upper Cenomanian and Turonian Chalk and their significance

AbstractThe regional hardening of the Late Cenomanian to Early Turonian Chalk of the Northern Province of eastern England has been investigated by examining the pattern of trace elements and stable carbon and oxygen isotopes in the bulk calcite of two extensive and stratigraphically adjacent units each 4 to 5 m thick of hard chalk in Lincolnshire and Yorkshire. These units are separated by a sequence, 0.3-1.3 m thick, of variegated marls and clayey marls. Modelling of the geochemistry of the hard chalk by comparison with the Standard Louth Chalk, combined with associated petrographic and geological evidence, indicates that (1) the hardening is due to the precipitation of a calcite cement, and (2) the regional and stratigraphical patterns of geochemical variation in the cement are largely independent of each other and have been maintained by the impermeable nature of the thin sequence of the clay-rich marls that separate them. Two phases of calcite cementation are recognised. The first phase was microbially influenced and did not lithify the chalk. It took place predominantly in oxic and suboxic conditions under considerable overpressure in which the Chalk pore fluids circulated within the units, driven by variations in compaction, temperature, pore fluid pressure and local tectonics. There is evidence in central and southern Lincolnshire of the loss of Sr and Mgenriched pore fluids to the south during an early part of this phase. The second phase of calcite precipitation was associated with the loss of overpressure in probably Late Cretaceous and in Cenozoic times as the result of fault movement in the basement penetrating the overlying Chalk and damaging the seal between the two chalk units. This greatly enhanced grain pressures, resulting in grain welding and pressure dissolution, causing lithification with the development of stylolites, marl seams, and brittle fractures. Associated with this loss of overpressure was the penetration of the chalk units by allochthonous fluids, rich in sulphate and hydrocarbons, derived probably from the North Sea Basin. Microbial sulphate-reduction under anoxic conditions within these allochthonous fluids has been responsible for dissolving the fine-grained iron and manganese oxides within the chalk, locally enriching the Fe and Mn content of the calcite cement. The possibility is discussed that the pattern of cementation preserved in these regionally hard chalks of Late Cenomanian and Early Turonian age may be different from that preserved in the younger (late Turonian to Campanian) more basinal chalks of eastern England.

The Development of Stylolites in Carbonate Formation: Implication for CO2 Sequestration

AbstractThe impact of CO2 sequestration on the host formation is an issue occurring over geologic time. Laboratory tests can provide important results to investigate this matter but have limitations due to a relatively short timeline. Based on literature review and core sample observation, naturally occurred geological phenomena, stylolites are studied in this paper for understanding CO2 sequestration in deep carbonate formations. Stylolites are distinctive and pervasive structures in carbonates that are related to water‐assisted pressure solution. Pressure solution involving stylolitization is thought to be the main mechanism of compaction and cementation for many carbonates. In parallel, CO2 sequestration in carbonate formation involves extensive chemical reactions among water, CO2 and rock matrix, favoring chemical compaction as a consequence. An analogue between stylolites and CO2 sequestration induced formation heterogeneity exists in the sense of chemical compaction, as both pressure solution in stylolites and CO2 enriched solution in CO3 sequestration in carbonate formations may all introduce abnormal porous regions. The shear and/or tension fractures associated with stylolites zones may develop vertically or sub‐vertically; all these give us alert for long‐term safety of CO2 sequestration. Thus a study of stylolites will help to understand the CO2 sequestration in deep carbonate formation in the long run.