Precipitation Of Chlorite Research Articles

Differential Mobility of Elements in Burial Diagenesis of Siliciclastic Rocks

ABSTRACT Mass-balance calculations involving smectite-rich siliciclastic mudstones sampled from boreholes show that significant K2O addition and CaO loss correlate strongly with the progress of illitization of smectite and with the dissolution of calcite. The calculations are based on the immobility of Al2O3, and require a chemically homogeneous sediment at deposition, such that the shallower samples represent protolith compositions of deeper samples. Constancy of ratios of TiO2/Al2O3 are used to (1) establish that both TiO2 and Al2O3 have similarly low differential mobilities, and (2) identify stratigraphic sections available in the literature that are sufficiently homogeneous to allow the calcu ations. Our calculations show that most major-oxide components are immobile at the 5% limit of resolution of the calculations. Exceptions are K2O, which is almost universally added to mudstones, and CaO, which is commonly removed from mudstones during diagenesis. The addition of up to 50% K2O correlates with percent illite in mixed-layer illite/smectite, indicating that illitization of smectite involves fixation of K2O from diagenetic fluids. Removal of 40% CaO from some sections correlates with decreasing calcite mode and is attributed to dissolution of calcite. Together these reactions lead to a 10-15% loss of mineral volume, a volume loss that must be added to porosity loss when modeling compaction. Illitization and carbonate dissolution may be linked, because K-fixation during illitization decreases the pH of the local fluid (a reverse weathering reaction), which enhances carbonate dissolution. Addition of up to 20% Na2O in some rocks may be caused by albitization of plagioclase, but Na2O addition is not pervasive. The apparent lack of mass transfer associated with dissolution of kaolinite and feldspars or precipitation of chlorite implies that these reactions are balanced by other reactions such that their oxide components in mudstones are conserved. Limited data from sandstone-mudstone pairs are consistent with transfer of K2O from sandstones to mudstone via rising diagenetic fluids. Except for CaO and the alkalis, diagenesis of siliciclastic mudstones appears isochemical (± 5% relative) for most major oxide components. The much larger changes recorded in some reservoir sandstones are clearly exceptions, and may not reflect changes occurring at the scale of kilometers.

Petrography, diagenesis and pore-water evolution of a shallow marine sandstone (Hasle Formation, Lower Jurassic, Bornholm, Denmark)

Lower Jurassic marine sandstones in the Hasle Formation on Bornholm Island can easily be separated in two different petrographic types. In the lower part of the type locality, type 1 sandstones are friable with layers and lenses of concretionary siderite. They are separated by a major erosional surface from the overlying type 2 well-cemented sandstones.All parts of the sandstones were subjected to chloritization of original grain coatings, neomorphism of chlorite and local precipitation of authigenic pyrite. During early diagenesis, diagenetic pathways differed between the two sandstone types. Type 1 sandstones were subjected to compaction before kaolinitization and some quartz authigenesis occurred. Type 2 sandstones immediately above the major erosional surface were cemented by concretionary siderite at a very early stage, and thus further diagenetic modifications were prevented. Above this zone, sandstones are tightly cemented by various carbonate minerals. Diagenetic modifications mainly involved alternating events of carbonate precipitation and dissolution. The sequence of carbonate precipitation was: (1) zoned stable/metastable siderite; (2) siderite, gradually with significant Ca- and Mg-substitution; (3) ankerite; and (4) calcite, reflecting a sequence of precipitating carbonates containing progressively lower amounts of iron. A period of kaolinitization predated calcite cementation.Six diagenetic stages were recognized: (1) early burial reducing conditions resulting in precipitation of pyrite; (2) precipitation of chlorite and siderite caused from compactional flow from underlying brackish and fresh-water sediments; (3) dissolution of carbonate and partly oxidation of siderite caused by fresh-water flushing related to uplift; (4) precipitation of siderite with gradually increasing substitution by CaCO3 and MgCO3 and eventually precipitation of ankerite caused by renewed burial; (5) fresh-water diagenesis with precipitation of kaolinite and quartz; (6) precipitation of calcite in remnant porosity.

Carbonate Cementation and Dissolution in Miocene Sandstones from Corsair Trend, Offshore, Texas: ABSTRACT

Middle Miocene sandstones occur in Picaroon field (Corsair trend; offshore Texas Gulf Coast) at depths of approximately 13,000-17,000 ft (3.9-5.1 km). These deltaic sandstones contain evidence of the following sequence of diagenetic events: (a) precipitation of chlorite coatings on detrital grains; (b) partial dissolution of feldspar; (c) quartz cementation; (d) calcite cementation; (e) dissolution of calcite cement; (f) ankerite cementation. The reservoir quality of the sandstones is largely a function of porosity enhancement due to calcite dissolution. Calcite cement was emplaced at depths of approximately 3,000 to 8,500 ft. The calcite has /sup 87/86/Sr values of 0.70833 to 0.70865, eliminating coeval (/approximately/15 m.y.) seawater and marine carbonate (0.70873-0.70885) as a potential source. The /sup 87/86/Sr composition of calcite cement is consistent with mass transfer of calcite from older marine sources to younger sediments. Fluid inclusion measurements indicate that ankerite cement formed at temperatures of approximately 120/degree/-145/degree/C (11,000 to 14,000 ft). Oxygen isotope modeling predicts that at these depths shales would expel waters with /delta//sup 18/O/sub SMOW/ of +5 to +8 during smectite-illite conversion. Ankerite (/delta//sup 18/O/sub PDB/ = /minus/7.8) would be in isotopic equilibrium with the predicted waters at the temperatures derived from fluid inclusions. Ankerite cements have relatively radiogenicmore » /sup 87/86/Sr ratios (/approximately/0.7097) which are consistent with their formation from shale-derived fluids. Calcite dissolution occurs between the precipitation of calcite and ankerite. It is therefore concluded that calcite cement dissolution occurred at burial depths of 8,500 to 11,000 ft (95/degree/-120/degree/C).« less

Sandstone Diagenesis and its Variation with Deltaic Depositional Environments, Upper Cretaceous, Southern Rio Escondido Basin, Coahuila, Mexico: ABSTRACT

Prodelta, delta-front, and delta-plain sandstones and shales were deposited in a wave-dominated lobate delta in the Rio Escondido basin during Campanian and Maestrichtian time. The End_Page 529------------------------------ composite diagenetic sequence based on petrographic analyses of over 300 thin sections from sandstone samples from these environments consists of: (1) bioturbation of sediment and compaction of ductile grains before cementation; (2) infiltration of minor amounts of pedogenic illite and precipitation of chlorite and minor amounts of smectite; (3) cementation by euhedral, syntaxial quartz overgrowths; (4) extensive poikilotopic calcite cementation; (5) widespread dissolution of calcite; (6) precipitation of kaolinite as patches in some of the dissolution pores; (7) replacement of calcite and framework grains by iron-rich carbonates (ankerite-ferroan dolomite, ferroan calcite, siderite). The relative intensities of these diagenetic events, and thus porosity and permeability, are controll d by and vary with the environment of deposition of the sandstones. The two main factors controlling the differences in diagenetic features within the environments are (1) original composition of the sands, and (2) geometry of the sandstone bodies. Prodelta siltstone and very fine-grained sandstone, deposited as thin frontal-splays, are enclosed in thicker sequences of marine shale. These rocks (dominantly feldspathic litharenite and lithic arkose) typically have the lowest porosity (x = 4%) and permeability. (x = 1 md) of the three environments. Primary porosity was destroyed early by extensive bioturbation and by widespread calcite cementation (x = 23%). The thicker, impermeable shale above and below the thin, lenticular sandstone bodies served as barriers to subsequent fluid migration, thus inhibiting any creation of secondary porosity by dissolution and any later diagenetic changes. Channel-mouth-bar and sheet-sand deposits of the delta-front facies are generally more quartz-rich and thus, due to a greater stability of the quartz grains, underwent the least amount of compaction. Consequently, calcite cementation was widespread; later dissolution of calcite produced secondary porosity up to 28% and permeability up to 557 md. These fine to medium-grained sandstones (dominantly feldspathic litharenite, lithic arkose, and subarkose) have the highest porosity (x = 18%) and permeability (x = 81 md) of rocks from the three facies. Delta-plain fine to medium-grained sandstone has intermediate porosity (x = 8%), but low permeability (x = 1 md). These sandstones (primarily feldspathic litharenite) have the highest concentration of sedimentary and volcanic rock fragments, which were squashed during compaction and altered during burial. Thus, only a relatively small amount of pore space remained for calcite cementation. Kaolinite cement filled in most of the secondary porosity created by dissolution of early formed calcite. End_of_Article - Last_Page 530------------