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Limestone microporosity is ubiquitous and extensively developed in most Phanerozoic limestones. From an economic perspective, microporosity is important because it contributes substantially to the carbonate pore system, which can host significant volumes of water and hydrocarbons. Therefore, determining the presence and distribution of limestone micropores is necessary for accurate hydrocarbon estimations, reservoir characterization, and fluid flow simulations. From an academic standpoint, microporosity is important because its genesis is intimately linked with the mineralogical stabilization of metastable sediments, a fundamental process in carbonate diagenesis. Many types of micropores contribute to what has been referred to as microporosity, but the vast majority is hosted among low-magnesium calcite (LMC) microcrystals that are present in limestone matrix and allochems. Geochemical, textural, and mineralogical data from natural settings and laboratory experiments indicate that LMC microcrystals are diagenetic in origin. More specifically, these data support a diagenetic model of mineralogical stabilization that involves dissolution of precursor sediments dominated by aragonite and high-magnesium calcite (HMC) minerals, and precipitation of LMC microcrystal cements. The stabilization process is inferred to take place in the meteoric, marine, and burial diagenetic realms. Although it has not been directly observed, carbon and oxygen isotopes, as well as trace element data suggest that LMC microcrystals form during burial diagenesis in marine-like fluids. Evidence suggests that porosity is not generated during this dissolution-precipitation process, but rather inherited from the precursor sediments. The final arrangement of the micropores in a limestone, however, depends on the precise diagenetic pathway. LMC microcrystals exhibit a range of microcrystalline textures that are classified on the basis of crystal morphology and size. The three main textural classes - granular (framework), fitted (mosaic), and clustered - have been recognized across a wide range of ages, depositional settings, burial depths, and precursor types, and are characterized by distinct petrophysical properties, such as porosity, permeability, and pore-throat size. Observations from modern sediments also support the hypothesis that LMC microcrystals develop from aragonite and HMC dominated lime mud. The origin of lime mud has been extensively studied but still highly debated. Of particular interest to the discussion of microporosity are proposed secular variations in the dominant mineralogy of carbonate sediments through the Phanerozoic. Microporous limestones comprised of LMC microcrystals are equally abundant during times of aragonite seas and calcite seas, which suggests that no special mineral precursor is required. Microporous textures are also observed in deep marine chalks where micropores are hosted between chalk constituents. Unlike shallow marine limestones, deep marine sediments start out as mostly LMC therefore mineralogical stabilization is not a significant process in chalk diagenesis.
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Mg zonation and heterogeneity in low-Mg calcite microcrystals of a depositional chalk
ABSTRACT Diagenetic low-magnesium calcite (LMC) microcrystals constitute the framework that hosts most micropores in limestone reservoirs and therefore create the storage capacity for hydrocarbons, water, and anthropogenic CO2. Limestones dominated by LMC microcrystals are also commonly used for paleoclimate reconstructions and chemostratigraphic correlations. LMC microcrystals are well known to exhibit a range of textures (e.g., granular, fitted, clustered), but there exists uncertainty with regard to how these textures form. One hypothesis is that during crystal growth, Mg is incorporated into diagenetic overgrowths (cement), where the chemical zonation and microtexture may reflect diagenetic processes. To evaluate small-scale geochemical zonation in LMC microcrystals, this study uses scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) to measure the Mg/Ca ratio across the interiors of LMC granular microcrystals from a Late Cretaceous marine chalk from the Tor Fm. (Norwegian North Sea). Mg/Ca zonation was identified in all LMC microcrystals with a diameter > 5 μm. Generally, the cores of large crystals have lower Mg/Ca (≈ 5.9 mmol/mol) and the rims have elevated Mg/Ca (≈ 13 mmol/mol). Smaller microcrystals (< 5 μm) show no resolvable zonation, but do exhibit a wide range in Mg/Ca content from 2.9 to 32.2 mmol/mol. Measured Mg/Ca values are arbitrarily divided into three populations: low Mg (average ≈ 5.9 mmol/mol), intermediate Mg (average ≈ 13.3 mmol/mol), and high Mg (average ≈ 20 mmol/mol). The observed zonation and Mg enrichment within LMC microcrystals is interpreted to reflect depositional as well as multiple diagenetic signals, such as constructive precipitation through recrystallization and pore-filling cementation. Although chalk from the Tor Fm. is dominated by granular euhedral LMC microcrystals, using SEM-EDS to find Mg/Ca heterogeneity in other LMC microcrystal textures may provide insight into the diagenetic processes that create textural variations in micropore-dominated limestones. The Mg data also more broadly suggest that there is useful, measurable diagenetic information in material that is otherwise considered homogeneous. Distinguishing between possible primary compositions and secondary cementation has implications for studies that rely on the primary chemistry of fine-grained carbonate deposits (e.g., micrite), such as paleoclimatology, Mg paleothermometry, and chemostratigraphy.
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- Award ID(s):
- 1916642
- PAR ID:
- 10292470
- Date Published:
- Journal Name:
- Journal of Sedimentary Research
- Volume:
- 91
- Issue:
- 8
- ISSN:
- 1527-1404
- Page Range / eLocation ID:
- 795 to 811
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Magnesium (Mg) in natural waters plays a critical role in governing carbonate mineral formation, dissolution, and diagenesis. Previous laboratory experiments show that Mg can strongly inhibit direct calcite precipitation as well as aragonite to calcite diagenetic transformation. Data from natural settings, however, suggest that diagenetic calcite in most Phanerozoic limestones has formed in the shallow marine burial realm in the presence of ample Mg. Thus, the diagenetic conditions under which aragonite-rich sediments convert to calcite-rich limestones are poorly understood. Here, we present data from laboratory experiments whereby aragonite is converted to calcite at 70◦C in Mg-bearing solutions to investigate the effects of fluid:solid ratio (F:S), which varies greatly across diagenetic environments, on Mg inhibition and incorporation in calcite. Our data show that not only can the transformation of aragonite to calcite occur in solutions with higher [Mg] than previously shown possible in laboratory experiments, but that progressively lower F:S increase the rate at which aragonite stabilizes to calcite. For example, in experiments with an F:S of 0.3 mL/g, which corresponds to sediments in a closed system with 50% porosity, aragonite stabilizes to calcite in solution with [Mg]=30 mM (Mg/Ca=5.14) when an initial high degree of undersaturation with respect to aragonite is used and in a solution with [Mg]=20 (Mg/Ca=5.14) when a low degree of undersaturation is used. In contrast, aragonite does not stabilize to calcite after nearly 3000 h in experiments with an F:S of 100 mL/g, which is more typical of an open system, even in a solution with [Mg]=5 mM (Mg/Ca=5.14) regardless of the degree of undersaturation. Our results also show that the amount of Mg incorporated into calcite products increases linearly with the increase of F:S. Collectively, these observations further point to F:S as an important factor in carbonate diagenesis with broad implications. First, the observations that transformation of aragonite to calcite is inhibited at high [Mg] and F:S imply that calcite precipitation is unlikely to occur in marine diagenetic environments that are in direct hydrologic contact with seawater. This leaves aragonite dissolution as the dominant diagenetic process in these environments, which may represent an underrated source of alkalinity to the open ocean. Second, transformation from aragonite-rich sediments to the calcite-rich limestones that dominate the rock record is likely promoted by a decrease in the F:S and the development of a closed system during progressive burial.more » « less
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null (Ed.)Phanerozoic limestones are composed of low-Mg calcite microcrystals (i.e., micrite) that typically measure between 1 and 9 lm in diameter. These microcrystals, which host most of the microporosity in subsurface reservoirs, are characterized by a variety of microtextures. Despite the overwhelming consensus that calcite microcrystals are diagenetic, the origin of the various textures is widely debated. The most commonly reported texture is characterized by polyhedral and rounded calcite microcrystals, which are interpreted to form via partial dissolution of rhombic microcrystals during burial diagenesis. A proposed implication of this model is that dissolution during burial is responsible for significant porosity generation. This claim has been previously criticized based on mass balance considerations and geochemical constrains. To explicitly test the dissolution model, a series of laboratory experiments were conducted whereby various types of calcites composed of rhombic and polyhedral microcrystals were partially dissolved under a constant degree of undersaturation, both near and far-from-equilibrium. Our results indicate that calcite crystals dissolved under far-from-equilibrium conditions develop rounded edges and corners, inter-crystal gulfs (narrow grooves or channels between adjacent crystals), and a few etch pits on crystal faces—observations consistent with the burial-dissolution hypothesis. Crystals dissolved under near-equilibrium conditions, in contrast, retain sharp edges and corners and develop ledges and pits—suggesting that dissolution occurs more selectively at high-energy sites. These observations support the longstanding understanding that far-from equilibrium dissolution is transport-controlled, and near-equilibrium dissolution is surface-controlled. Our results also show that while the rhombic calcite crystals may develop rounded edges and corners when dissolved under far from- equilibrium conditions the crystals themselves do not become spherical. By contrast, polyhedral crystals not only develop rounded edges and corners when dissolved under far-from-equilibrium conditions but become nearly spherical with continued dissolution. Collectively, these observations suggest that rounded calcite microcrystals more likely form from a precursor exhibiting an equant polyhedral texture, rather than from a euhedral rhombic precursor as previously proposed. Lastly, the observation that calcite crystals developed rounded edges and corners and intercrystal gulfs after only 5%dissolution indicates that the presence of such features in natural limestones need not imply that significant porosity generation has occurred.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.2110/jsr.2020.173
