S. J. Mazzullo

Organogenic Dolomitization in Peritidal to Deep-Sea Sediments

Presumed barriers to early dolomitization in normal seawater-derived pore fluids at earth-surface temperatures appear to be overcome within some anoxic, organic-rich sediments as a result of bacterial sulfate reduction and methanogenesis. These processes may promote early dolomitization, particularly during methanogenesis and late stages of sulfate reduction, by concurrently raising and sustaining high pH and high total alkalinity and CO 3 2- concentrations in pore fluids, and by simultaneously either decreasing Mg and Ca hydration or by promoting crystal surface reactions with less hydrated Mg-Ca neutral ion pairs. Volumetrically significant quantities of dolomite are associated with sulfate reduction and/or methanogenesis within peritidal, shallow-marine, and deep-sea deposits. Average concentrations of organogenic dolomite are as much as 70% in some Holocene peritidal deposits, and 28% in Mesozoic to Quaternary deep-sea sediments. Organogenic dolomites are mainly cements, and contain relatively low concentrations of Sr and Mn. Sulfate-reduction dolomites generally are Fe-deficient because of concurrent pyrite precipitation, whereas methanogenetic dolomites may be somewhat more ferroan as Fe substitutes for depleting Mg. Sources of Mg and Ca for dolomite are diffusion from overlying seawater and/or dissolution of precursor carbonate sediments. Dolomites are characterized by a wide range in δ 13 C values wherein those of sulfate reduction versus methanogenetic origin typically are 13 C-depleted and 13 C-enriched, respectively. The extent of 13 C depletion or enrichment, however, depends on the extent of organodiagenetic reactions and amount of 13 C contributed by seawater diffusion, and commonly results in overlap of δ 13 C dolomite values. The range of δ 18 O dolomite values is somewhat more restricted, and generally reflects differences in pore-fluid temperature and salinity. Some of the unresolved issues in organogenic dolomitization are: the relative efficiencies of sulfate reduction versus methanogenesis in promoting dolomitization, depths of dolomite formation inferred on the basis of δ 18 O dolomite values and probable sources of Mg and Ca, and the mode of dolomitization with progressive burial into methanogenetic zones.

Geochemical and neomorphic alteration of dolomite; a review

Many ancient dolomites are suspected of being alteration products of preexisting dolomite phases rather than being originally formed, unaltered dolomites. Such diagenetic alteration commonly results in changes in geochemistry and/or neomorphic changes in dolomite crystal sizes and textures. Hence, previous studies that have interpreted environments of initial dolomitization based on presumed preservation of diagnostic geochemical compositions and textures must be reevaluated because these parameters ar known to reequilibrate during later diagenesis. The principal driving forces for neomorphism are the inherent thermodynamic instability of preexisting, non-stoichiometric dolomites, and to an unknown extent, the surface free energy-driven recrystallization of fine crystalline mosaics to coarser crystalline textures. The four inter-related criteria that are used commonly as collective evidence of alternation of preexisting dolomites are: (1)) non-stoichiometric and poorly-ordered dolomites have an inherent tendency to transform to the more stoichiometric and better ordere phase typical of many ancient dolomites, a process that commonly is concurrent with (2) the neomorphic change from fine crystalline to coarse crystalline mosaics of either planar or nonplanar texture; (3) based on comparison to modern dolomites, depletion in18O isotopic composition and Sr and Na concentrations relative to ppresumed preexisting phases; and (4) homogenization of primary cathodoluminescent zonation that may have been present in the preexisting phase. Although certainly not unequivocal, the inference or recognition of such changes suggest the complexity of diagenetic modifications that have affected many ancient dolomites.

Mesogenetic dissolution: Its role in porosity development in carbonate reservoirs

Models of porosity formation in carbonate rocks have stressed subaerial exposure and attendant shallow meteoric diagenesis. Porosity formation also occurs in deep-burial, or mesogenetic, settings as a result of dissolution enlargement of preexisting pores (porosity enhancement) and creation of new pore systems. Brines charged with organic acids, carbon dioxide, and/or hydrogen sulfide derived from organic matter diagenesis and thermochemical sulfate reduction are the likely fluids causing significant mesogenetic dissolution. Enhanced and newly created mesogenetic pore types can mimic pore types formed in shallow meteoric environments, and therefore, the mesogenetic origin of some porosity may go unrecognized.

Holocene shallow-subtidal dolomitization by near-normal seawater, northern Belize

Calcic dolomite cements compose an average of 5% of the upper 4.3 m of subtidal deposits 18 O (+2‰) compositions of the high-Sr dolomites (mean October 1900 ppm), together with near-normal salinity and inherently normal Mg/Ca ratio of pore fluids, suggest precipitation from near-normal seawater. Tidal and wind-driven circulation of seawater through the sediments supplies most of the Mg for dolomitization, which appears to be promoted by elevated pore-water alkalinity resulting from bacterially mediated oxidation of organic matter and, locally, early stages of methanogenesis. Rapid dolomitization here supports the idea that significant quantities of dolomite can form syndepositionally, from normal seawater, in shallow subtidal deposits.

Dolomitization of Holocene Shallow-Marine Deposits Mediated by Sulfate Reduction and Methanogenesis in Normal-Salinity Seawater, Northern Belize

ABSTRACT Dolomite constitutes an average of 12% of the Holocene organic-rich sediments over a 15 km2 area of the Cangrejo Shoals mudbank in northern Belize. Although it defines a laterally persistent stratiform body that averages 3 m thick, it is present throughout the 7.6-m-thick sediment section. These transgressive sediments are less than 6400 years old and were deposited in shallow-marine environments of normal salinity. The dolomite is dominantly cement, and average crystal size is 7 m. There are no significant correlations among amount of dolomite vs. sediment texture, mineralogy, porosity, or mole % MgCO3 in associated particulate high-Mg calcite, depth, or location on the shoals. The dolomites are poorly ordered and calcic (39.5-44.5 mole % MgCO3), with low mean Mn (210 ppm) and relatively high mean Sr (1034 ppm) concentrations. There is no evidence of recrystallization or geochemical alteration of the dolomite. 18O values of the dolomites range from 0.5 to 2.8o/ooPDB, and the mean value (2.1o/oo) suggests that the dolomite precipitated from normal-salinity pore water. Dolomite 13C values range from -5.2o/oo to +11.6o/ooPDB (mean seawater 13C = 0.5o/oo), which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water. Dolomitization attending these organodiagenetic reactions apparently was reversible over time, and episodic rather than continual precipitation is indicated. Requisite Mg and Ca were provided by seawater and by some dissolution of host sediments. The most rapid period of dolomitization may have been during early transgression, when relatively high sedimentation rates sustained high levels of organodiagenesis and pore-water alkalinities.

Dolomitization of Holocene Mg-calcite supratidal deposits, Ambergris Cay, Belize

Dolomitized crusts and associated sediments 845–2925 yr old on Ambergris Cay, Belize, Central America, compose a significant portion of the Holocene sediment section on many supratidal flats. The dolomitic sections are as thick as 0.7 m and contain an average of 70% calcic dolomite. Dolomite occurs as a replacement of high-Mg calcite micrite matrix and allochems, and as passively precipitated cements. Isotopic analyses suggest that dolomitization appears to be promoted primarily by reactions of the magnesian calcite supratidal sediments with essentially near-normal marine waters. The effects of seasonal influxes of meteoric and hybrid fluids, however, are also clearly evident in the crusts as indicated by isotopes, dolomite-crystal etching, the selective leaching of aragonite, and the local loss of Mg 2+ from the host micrite and included skeletal fragments. The amounts, rapid rates of dolomitization, and initial Mg-calcite mineralogy of the sediments make the Ambergris Cay supratidal flats exceptional in the Holocene, and this area may be a model for the genesis of some ancient peritidal dolomites.

Syngenetic Formation of Grainstones and Pisolites from Fenestral Carbonates in Peritidal Settings

ABSTRACT Experiments with unconsolidated muds, and petrofabric studies of some ancient peritidal carbonate rocks, suggest that intense fenestral alteration of micritic sediments can lead to the in situ formation of grainstones of diagenetic origin. Intermediate stages in this process have been recognized in many ancient rocks. The contemporaneous diagenetic modification of such grainstones commonly results in the formation of pisoliths and associated features that mimic those found in marine pisolites as well as in pisolitic caliche deposits of either marine or meteoric origin.

Length of the Year during the Silurian and Devonian Periods: New Values

Daily growth increments and monthly markings on Silurian and Devonian corals and brachiopods were counted using a maximum count method. Early and Middle Silurian fossils indicate that the number of days per year during these periods was 421 and 419, respectively; the number of days per year in the early Middle Devonian Period was 410. The number of days per month has decreased from 32.4 in the Early Silurian to 31.5 in the early Middle Devonian. The values obtained for these periods are considered more accurate than the values obtained using average count methods.

Facies and Burial Diagenesis of a Carbonate Reservoir: Chapman Deep (Atoka) Field, Delaware Basin, Texas

Hydrocarbon production at Chapman Deep Atoka field is from a complex microfacies mosaic of shallow-water bank limestones deposited along the northern hingeline of the Delaware basin. Reservoir localization is essentially stratigraphic in terms of depositional and diagenetic facies, although regional draping and a system of vertical fractures are significant structural aspects of the field. The bank facies consist of cyclic alternations of Donezella (algal) bioherms, oolite-biograinstone shoals, and low-energy interbank deposits. Laterally equivalent slope and basinal facies include spiculitic and crinoidal argillaceous limestones and shales, with interbedded lenses of fine-grained carbonate and siliciclastic turbidites. Early diagenetic effects include incipient marine ce entation and the formation of secondary porosity, most of which was occluded by calcite cements, internal sediments, and dolomitization. In contrast, reservoir evolution is principally related to diagenesis in the deep subsurface (mesogenetic) environment. Bulk-volume reduction by chemical and physical compaction was counterbalanced by porosity rejuvenation through the selective dissolution of allochems, cements, and stylolite surfaces, and the formation of open-gash fractures and adjoining stylolites. Although this limited pore system is of inherently low permeability, effective communication within and between individual reservoir lenses was enhanced by later fracturing. Although potential reservoir facies can be mapped regionally and burial diagenetic effects can be recognized petrogr phically, exploration for similar reservoirs in the Delaware basin is hindered by our limited knowledge of mesodiagenesis.

Petroleum reservoirs within a spiculite-dominated depositional sequence: Cowley Formation (Mississippian: Lower Carboniferous), south-central Kansas

Oil and gas reservoirs in the Cowley Formation (upper Osagean to lower Meramecian) are within a thick (up to 400 ft [122 m]) section of spiculite-dominated rocks, derived from demosponges, deposited in a low-latitude setting. These rocks are present in the subsurface for 325 mi (523 km) along paleostrike in southern Kansas and some adjoining states. They represent a stratigraphically significant lithosome that markedly contrasts thin and areally restricted spiculitic rocks present in some Mississippian reservoirs elsewhere in the mid-continent. Cowley lithologies represent a low-gradient ramp, whereon (1) bedded spiculites were deposited in moderate-energy, shallow-water, inner-ramp settings; (2) lenticular-, nodular-, or flaser (L/N/F)-bedded spiculite and shale were moderate- to low-energy, progressively deeper-water medial-ramp deposits; and (3) dark shales are deepest-water, outer-ramp facies. The internal stratigraphic architecture of the unconformity-bounded Cowley identifies it as a depositional sequence with component deepening-upward basal strata (transgressive systems tract) overlain by shallowing-upward, progradational clinoforms (highstand systems tract). Sequence deposition was punctuated by several unconformities attending short periods of subaerial exposure. Suppression of otherwise warm, shallow-water carbonate production, and instead spiculite deposition, in this low-latitude setting was likely a consequence of elevated concentrations of dissolved silica and nutrients in the ambient marine environment. Three successive generations of silicification are recognized in the rocks. Early partial silicification is presumed to have begun in the marine environment, and ensuing silicification and attendant porosity formation were likely coincident with falling sea level as pore fluids evolved from being of mixed marine-meteoric to meteoric composition. Petroleum reservoirs mainly with vuggy porosity are present in relatively high-porosity bedded spiculites and less porous L/N/F-bedded rocks. Traps commonly are developed in structurally modified, subunconformity buried-hills and truncated, gently dipping strata. Reservoirs in the L/N/F-bedded rocks locally extend considerable distances downdip within individual clinoformal parasequences in the section, thereby locally creating thick gas-saturated reservoir columns. Because of its great subsurface extent, the Cowley section, commonly bypassed during drilling, offers considerable potential for as-yet discovered fields in the mid-continent.