Deep Burial Research Articles

Adapting an X-Ray/Debris Shield to the Cascade ICF Power Plant: Neutronics Issues

A neutronics analysis has been carried out to determine the effects on the Cascade ICF reactor concept of adding a solid-lithium x-ray and debris shield to each ICF capsule. Results indicate that tritium breeding in LiAlO{sub 2} is possible with a modest isotopic enhancement in {sup 6}Li (to 15%). The shallow-burial index is greater than 1 (indicating that deep burial may be required) if the blanket is kept in the reactor for more than 2.5 yr. Nine percent of the total thermal power is unrecoverable. Parts of the chamber wall may require replacement once during the reactor life due to radiation damage. Part of the SiC chamber end cap must be replaced annually. The reactor may not require any nuclear-grade construction. 20 refs., 4 figs., 3 tabs.

Optimal temperature conditions for deep hole burial of heat-emitting wastes

Optimal temperature conditions for deep hole burial of heat-emitting wastes

Deep burial of liquid radioactive waste in porous geologic formations

Deep burial of liquid radioactive waste in porous geologic formations

Dolomitization of the Devonian Swan Hills Formation, Rosevear Field, Alberta, Canada

ABSTRACTThe Swan Hills Formation (Middle‐Upper Devonian) of the Western Canada Basin is host to several NW‐SE‐trending gas fields developed in massive replacement dolostone. One of these, the Rosevear Field, contains two major dolostone trends along opposing margins of a marine channel that penetrates into a platform‐reef complex. Dolostones consist predominantly of branching and bulbous strdmatoporoid floatstones and rudstones with well‐developed moldic and vuggy porosity. Replacement dolomite is coarsely crystalline (100‐600 μm), inclusion‐rich, composed of euhedral through anhedral crystals and has a blotchy to homogeneous red cathodoluminescence. Geochemically, replacement dolomite is characterized by (i) nearly stoichiometric composition (50.1‐51.1 mol% CaCO3), (ii) negative δ18O values (mean=‐7.5‰, PDB) and (iii) variable 87Sr/86Sr ratios ranging from values similar to Late Devonian‐Early Mississippian seawater (∼0.7082) to radiogenic compositions comparable to saddle dolomite cements (>0.7100).Dolomitization began after widespread precipitation of early, equant calcite spar and after the onset of pressure solution, implying that replacement dolomite formed in a burial environment. Oxygen isotope data suggest that dolomite formed at 35‐75°C, temperatures reached during burial in Late Devonian through Jurassic time, at minimum depths of 450 m. The linear NW‐SE orientation of most dolomite fields in the Swan Hills Formation is suggestive of fault control on fluid circulation. Two models are proposed for fault‐controlled circulation of dolomitizing fluids at the Rosevear Field. In the first, compaction‐driven, updip fluid migration occurred in response to basin tilting commencing in the Late Palaeozoic. Deep basinal fluids migrating updip were focused into channel‐margin sediments along fault conduits. The second model calls upon fault‐controlled convective circulation of (i) warm Devonian‐Mississippian seawater or (ii) Middle Devonian residual evaporitic brines.The overlap in 87Sr/86Sr and δ18O compositions, and similar cathodoluminescence properties between replacement and saddle dolomites provide evidence for neomorphism of some replacement dolomite. Quantitative modelling of Sr and O isotopes and Sr abundances suggests partial equilibration of some replacement dolomite with hot radiogenic brines derived during deep burial of the Swan Hills Formation in the Late Cretaceous‐Palaeocene. Interaction of replacement dolomite with deep brines led to enrichment in 87Sr while leaving δ18O similar to pre‐neomorphism values.

Low-Temperature Formation of Pyrrhotite from Magnetite Pyrite: Evidence from Low-Grade Metamorphism and Implications for Magnetite Preservation during Deep Burial

Early magnetite in sedimentary rocks must be preserved to retain a paleomagnetic signature; however, reactions such as Fe{sub 3}O{sub 4} + 3 FeS{sub 2} + CH{sub 4}' = (organic matter) = 6 FeS' (pyrr.) + CO{sub 2} + 2 H{sub 2}O tend to the right for temperatures less than {approximately}200C. Such low temperatures strongly imply that magnetite can be destroyed merely by deep burial. Such pyrrhotite formation is illustrated by very low-grade metamorphism in a contact aureole surrounding the Jurassic Notch Peak stock in the central House Range of western Utah. The stock invades a Cambrian sedimentary sequence containing miogeoclinal limestones with intercalated siltstones. Limestones in the aureole yield a scattered, two-polarity remagnetization, residing in pyrrhotite, which extends into rocks that appear unmetamorphosed (temperatures {approximately}250C). The pyrrhotite could not have been precipitated from externally derived fluids because oxygen isotopic data from the limestones show no influence of magmatic or phreatic water; hence, it must have formed in situ, as in the above equation. Outside the aureole, a characteristic magnetization is preserved that apparently reflects late Paleozoic remagnetization and that probably resides in authigenic magnetite. Hence, it appears that this relatively late magnetite was in turn destroyed by modest reheating frommore » the pluton. In general, thermal remagnetization of sedimentary rocks is probably rare: chemical changes probably have a much more profound effect on the paleomagnetic signatures.« less

Deep Burial Dolomitization Driven by Plate Collision: Evidence from Strontium-Isotopes of Jurassic Arab IV Dolomites from Offshore Qatar

The use of strontium-isotope ratios of dolomites to constrain timing and mechanism of diagenesis has been investigated on Jurassic Arab IV dolomites from offshore Qatar. Reservoir quality is determined by two types of dolomites, which were differentiated geochemically (cathodoluminescence, fluid inclusions, and carbon and oxygen stable isotopes): (1) stratigraphically concordant sucrosic dolomites with high porosity formed during early near-surface diagenesis (Jurassic) and (2) stratigraphically discordant cylindrical bodies of massive, porosity-destroying dolomites formed late during deep burial diagenesis (Eocene-Pliocene). Detailed Sr-isotope analysis of dolomites from the Arab IV confirms an Early Jurassic age of the sucrosic, high porosity dolomites ({sup 87}Sr/{sup 86}SR = 0.70707 for NBS 987 = 0.71024) with magnesium and strontium being derived from Jurassic seawater. Late Tertiary compressional orogeny of the Zagros belt to the north is proposed to have caused large-scale squeezing of fluids from the pore system of sedimentary rocks. A regional deep fluid flow system developed dissolving infra-Cambrian evaporites upflow and causing large-scale deep burial dolomitization downflow.

The Barque Field, Blocks 48/13a, 48/14, UK North Sea

Abstract The Barque Field is associated with some of the earliest gas discoveries in the southern North Sea. In the Sole Pit area the reservoir, the Rotliegend Group Leman Sandstone Formation of Lower Permian age, occurs between Carboniferous Coal Measures, which source the gas, and Zechstein evaporites which form an excellent seal. Primarily aeolian, the sandstone has very low permeability resulting from deep burial of the Sole Pit Trough. The deepest burial and hence the maximum diagenetic damage to the reservoir was achieved in the early Late Cretaceous prior to the two-fold main phases of inversion in Late Cretaceous and Mid-Tertiary. Compaction and diagenesis reduced reservoir permeability to such an extent that parts of the field would be non-productive were it not for the presence of effective natural fracture zones and well stimulation by hydraulic fracturing techniques. Though appraisal and evaluation have been relatively extensive, remaining uncertainties dictate a conservative development in conjunction with the adjacent Clipper Field. A selected initial area was developed for first gas in October 1990. Good reservoir performance may lead to later development of the whole field.

The Clipper Field, Blocks 48/19a, 48/19c, UK North Sea

Abstract The Clipper Gas Field is a moderate-sized faulted anticlinal trap located in Blocks 48/19a and 48/19c within the Sole Pit area of the southern North Sea gas basin. The reservoir is formed by the Lower Permian Leman Sandstone Formation, lying between truncated Westphalian Coal Measures and the Upper Permian evaporitic Zechstein Group which form source and seal respectively. Reservoir permeability is very low, mainly as a result of compaction and diagenesis which accompanied deep burial of the Sole Pit Trough, a sub-basin within the main gas basin. The Leman Sandstone Fm. is on average about 715 ft thick, laterally heterogeneous and zoned vertically with the best reservoir properties about the middle of the formation. Porosity is fair with a field average of 11.1%. Matrix permeability, however, is less than 1 millidarcy on average and is so low that some intervals in the field will not flow gas unless stimulated. Steep dipping zones of natural fractures occur in certain areas of the field; these commonly allow high flow rates to be achieved from large blocks of low-permeability matrix. Expected recoverable reserves from the most favourable part of the field are 558 BCF and Clipper Field is now being developed in conjunction with part of the adjacent Barque Gas Field. Later development of the remainder of Clipper Field will depend upon reservoir performance in the initial development area.

Counterclockwise P-T-t Paths from Amphibolites, Franciscan Complex, California: Relics from the Early Stages of Subduction Zone Metamorphism

Overprinting mineral relationships in amphibolites from the Franciscan Complex, California suggest metamorphic evolution from high-temperature amphibolite facies to the blueschist facies with increasing P/T ratio, a counterclockwise P-T-t path. Early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are, in turn, cut or rimmed by sodic amphiboles. Some amphibolites are overprinted successively by eclogite and blueschist facies assemblages. Clinopyroxenes are zoned with increasing jadeite component from core to rim. Multiple generations of phengites are present in the same sample and are more Si-rich with each successive generation of growth. Garnets in eclogite-overprinted amphibolites are zoned with an initial MgO increase away from the core to a maximum MgO zone followed by MgO decrease toward the rim. Garnets in other amphibolites show continually increasing MgO from core to rim. Several samples, representative of the variety of Franciscan amphibolites, are described in detail. The best-constrained sample exhibits a calculated P-T trajectory from 626-664°C at 9.2-10 kb for early amphibolite metamorphism to 496-537°C at 11.2-11.8 kb eclogite metamorphism to 300-350°C at >6.5-7 kb for final blueschist overprint. Overprinting mineral assemblages in other samples, as well as most Franciscan amphibolites described by other workers, are qualitatively, and in some cases quantitatively, suggestive of the same type of P-T trajectory. Geochronologic, textural, and petrologic data indicate that the overprinted amphibolites are probably the product of a single metamorphic event rather than a consequence of separate metamorphic episodes, but additional age dating is desirable for better confirmation. Amphibolite to blueschist facies metamorphic evolution may have taken place within 5 Ma. The following model is presented for the evolution of Franciscan amphibolites: (1) The amphibolites were metamorphosed as a dynamothermal aureole underneath the hanging wall of a subduction zone at the inception of subduction. (2) The amphibolites were then accreted to the upper plate. (3) Subsequent subduction and underplating of cold material insulated the hanging wall and allowed the upper plate and amphibolite to cool at depth, with the development of metamorphic assemblages of increasing P/T ratio. (4) Subduction beneath the accreted amphibolite may have broken up the amphibolite sheet, dragging blocks deeper down the subduction zone, leading to the pressure increase with cooling that is recorded by some of the samples. Deeper burial by thrusting within the upper plate after amphibolite metamorphism may also explain the pressure increase. The P-T trajectory of these rocks is consistent with P-T paths derived from published thermal models for similar tectonic settings.

Rates of fluid expulsion across the Northern Cascadia Accretionary Prism: Constraints from new heat row and multichannel seismic reflection data

One hundred and ten closely spaced probe heat flow measurements provide new constraints on the thermal regime of the northern Cascadia accretionary sedimentary prism off Vancouver Island. Complementary heat flow values have been obtained from the depth of a bottom‐simulating seismic reflector (BSR) that is interpreted to mark the thermally controlled base of a methane hydrate layer. The only local heat flow variations observed are associated with a sediment slump that is seen in SeaMARC II acoustic images and with the outcrop of several major thrust faults. These anomalies appear to be the result of material displacement only; strong localized fluid flow along faults or elswhere is not evident in the probe or BSR results. Fluid expulsion resulting from the dewatering of the prism sediments appears to occur regionally in the 10–20‐km‐wide zone landward of the deformation front. In this area there is a significant disagreement between the probe and BSR heat flow estimates (roughly 30%) that can be explained by a regionally uniform vertical fluid flow at a rate of about 8×10−10 m s−1. This is in good agreement with the estimated fluid expulsion rate required by the decrease in porosity landward of the deformation front, as estimated from the increase in seismic velocities derived from multichannel reflection data. The heat flow in Cascadia Basin seaward of the deformation front, corrected for the effect of sedimentation, is in excellent agreement with that predicted by cooling plate models. Landward, there is a regional trend of decreasing heat flow across the accretionary prism, which is consistent with a model of simple tectonic thickening. Temperatures at the top of the oceanic crust are estimated to be over 200°C beneath the 2–3 km of sediment in Cascadia Basin. Temperatures at the interface between the prism and the oceanic crust continue to increase landward, and reach 400°–450°C beneath the middle to inner continental shelf. Initiation of megathrust earthquake failure along the main subduction thrust may be thus restricted by the high temperatures to the zone beneath the continental slope and outer shelf. Early and shallow metamorphism of the prism material probably is another important consequence of the early deep burial of the young oceanic crust in this setting.

EXPERIMENTALLY‐SIMULATED STYLOLITIC POROSITY IN CARBONATE ROCKS

An experimental program of stylolitic porosity development in stylolitic Atokan (Middle Pennsylvanian) limestones under simulated conditions of deep burial has shown that experimental dissolution preferentially occured and developed along existing styloties. The dissolution sequence was as follows: incipient pores following the traces of stylolites, grading into stylolitic lamellar pores, which enlarged eventually into non‐fabric‐selective channels. These stylolite‐controlled conduits were surrounded by adjacent incipient‐to‐complete oomolds, enlarged oomolds, incipient‐to‐complete algal bioclast molds, enlarged biomolds, and halos of intercrystalline porosity to microporosity.Trace element analysis (Mg, Sr, and Ca) of pore‐fluid samples during early phases of experimental deep burial dissolution indicated that slightly less stable protions of carbonates enriched in Mg and Sr dissolved preferentially, as observed also in natural cases. Pyrolysis data indicate that stylolites concerntrated organic matter derived from inter‐stylolite areas. This situation demonstrates the pressure‐solution origin of stylolites.Experimental stylolitic pore networks are similar in type, size, and geometry to natural stylolitic porosity. This preferential dissolution suggests that natural stylolites result from late (post pressure‐solution dissolution, are not necessarily mechanically induced during stylolite formation as sometimes postulated, and act as effective conduits for secondary porosity generation in deeply‐buried carbonate rocks

七谷期以降の火山活動と石油・天然ガス鉱床

Deep-seated volcanic reservoirs of oil and gas in the Niigata basin consist of andesites of the Shiiya and Teradomari formations and rhyolite lavas of the Nanatani formation. Among these volcanic rocks the last one is most suitable as potential hydrocarbon reservoir.Volcanism of the Nanatani stage is bimodal and occured in trough of back-arc side after the opening the Japan Sea and is dominated by acidic volcanism in the Niigata basin. Acidic rocks of the latter half of the stage are widely distributed beneath the Niigata and Nagaoka Plains. Although acidic rocks of the Nanatani stage were recently ascertained by drilling near Itoigawa, these rocks may be not found in the Kubiki area where is occupied by so-called Nanbayama facies of submarine fan nature according to tectonic development of hinterland. Volcanic rocks erupted after the Teradomari stage are mostly andesite and dacite and are maldistributed.Recently from geochemical view point, it is stated by many petroleum geologists that maturation of source rocks started after the Teradomari stage and formation of reservoir and migration of oil and/or gas occured during and after the Nishiyama stage in the oil and gas fields having volcanic reservoir. Such views are due to the assumption that temperature capable of maturation and formation of secondary vug was attained by deep burial of thick sediments. However, it will be infered that heating of mudstone around rhyolite body and formation of secondarty vug by post-action alteration of rhyolite took place during and soon after the Nanatani stage.

Post-Hydrocarbon Migration Deep Burial Diagenesis of Smackover Formation, Black Creek Field, Wiggins Arch, Mississippi Salt Basin: ABSTRACT

The Smackover Formation is buried to over 6 km in Black Creek field and exhibits diagenetic phases unique to deep burial. These include replacement of sulfates by calcite, pore-fill calcite cement, silicification, and elemental sulfur as cement. The above diagenetic products are the result of thermal reduction of sulfates and thermal degradation of hydrocarbons and have formed at temperatures greater than 140/degrees/C. Calcite replaces anhydrite in two textural forms, one of which is pseudomorphic. Pore-fill calcite cement contains inclusions of pyrobitumen and heals cracks formed in the pyrobitumen due to high thermal maturity. This indicates that calcite cement postdates bitumen formation. Silicification is a major deep burial event. Micron to millimetersize crystals of quartz replace the host carbonate rock as well as other deep burial diagenetic phases. Elemental sulfur occurs as inclusions in incompletely destroyed sulfates and as pore-fill cement. In both cases it exhibits extensive bubble-shaped cavities similar to pumiceous texture found in volcanic rocks. This indicates that elemental sulfur was possibly crystallized rapidly from a liquid sulfur while the cores were being taken. Similar diagenetic products are formed by low-temperature (< 80/degrees/C) bacterial reduction of sulfates and bacterial degradation of hydrocarbons. However, carbon isotopic composition of calcite, sulfurmore » isotopic composition of elemental sulfur, and textural characteristics of elemental sulfur produced by the two diagenetic processes are significantly different.« less

Porosity Formation in Deep-Burial Environment: Overview, with Examples, from Permian Basin: ABSTRACT

Porosity formation accompanying deep burial is ubiquitous and widespread in the Permian basin, particularly but not exclusively in offshore platform and resedimented basinal carbonates of Pennsylvanian and Permian age. Hydrocarbon reservoirs in such platform carbonate examples locally contain evidence of subaerial exposure and meteoric diagenesis. Commonly, much of the porosity formed during exposure is ultimately reduced by compaction and cementation during early burial. By contrast, no evidence of meteoric diagenesis is observed in associated basinal carbonates, although compaction and cementation accompanying progressive burial are readily evident. In both cases, however, such early diagenesis is overprinted by late burial dissolution, sometimes coincident with hydrocarbon emplacement, creating rocks of high porosity. The formation of porosity by cement dissolution may exhume occluded pores or enhance relict pores that formed in the eogenetic zone, the result being a preponderance of interparticle and moldic pores and residual cements that mimic vadose and phreatic products. In other cases, nonfabric selective dissolution, locally associated with fractures or stylolites, creates vuggy porosity which may resemble that formed during eodiagenesis. Multiple phases of deep-burial dissolution and partial cementation or replacement (by calcite or dolomite) are indicated for many of these diagenetic systems and result in a complex suite ofmore » different pore types.« less

Estimation of Longevity of Portland Cement Grout Using Chemical Modeling Techniques

ABSTRACTPortland cement has been identified as a likely candidate seal material by programs investigating the deep burial of nuclear waste as a disposal mechanism. The longevity of performance of cement grout is currently being investigated, along with bentonite, under the auspices of the Stripa Project. Coordinated laboratory, field, and modeling studies are underway to produce fundamental data, practical experience, and estimates of long-range performance, respectively. Long-term performance of cement grout is of particular concern. Since most of the solid phases of which grout is comprised are metastable, it is likely that grout performance will decrease with time. The question is whether performance will still be acceptable after this decrease. This issue is being addressed with the coupled use of geochemical and permeability modeling. For a simplified cement system, two mechanisms for chemical degradation have been considered: phase change and dissolution. For dissolution, both equilibrium (slow flow) and open (fast flow) systems have been analyzed as bounding scenarios. Granitic terrain groundwaters ranging from fresh to saline have been used in the analyses. To assess the consequences in terms of flow, an empirical relation between cement permeability and porosity has been developed. Performance changes with time have been predicted by making conservative estimates of local hydraulic head conditions for successive periods of repository history. For the granitic rock environments considered, preliminary results indicate that cement grout performance may be acceptable for tens of thousands to millions of years, providing its initial hydraulic conductivity is on the order of 10−12 m/s. Other conditions favoring long-term performance include minimizing the ettringite content of the grout, and emplacement at a site where the groundwater has an elevated TDS, and where the local hydraulic gradient is flat or repository resaturation times are short.

Louann salt geochemistry (Gulf of Mexico sedimentary basin, U.S.A.): A preliminary synthesis

Samples of Jurassic salt and anhydrite were obtained from throughout the Gulf of Mexico sedimentary basin. Salt samples have been analyzed for bromide content and 87 Sr 86 Sr of anhydrite inclusions, and anhydrite for 87 Sr 86 Sr . Most samples of diapiric salt contain <60 ppm Br, the minimum value expected for first-cycle precipitation of marine halite. In addition, 87 Sr 86 Sr ratios are commonly radiogenic relative to the ratio in presumed coeval Jurassic (Callovian) seawater. Both these observations suggest that during deformation, salt undergoes extensive rock-water interaction, losing bromide to the reacting brines, and incorporating radiogenic Sr. Similar behavior of anhydrite during deformation with respect to Sr is also observed. Samples of deeply buried salt exhibit bromide concentrations consistent with the precipitation of marine halite, but include several samples with extremely high values. Few 87 Sr 86 Sr ratios, however, seem to be characteristic of coeval Callovian seawater. Samples with normal bromide content but radiogenic Sr suggest primary deposition from, or recrystallization in the presence of radiogenic fluids with marine-like bromide content. Samples with extremely high bromide content and radiogenic Sr values suggest recrystallization in fluids with elevated bromide content due to loss of bittern phases or extensive rock-water interaction of bedded salt during deep burial. The data suggest a dynamic system in which physical modification of evaporites during deformation and deep burial is accompanied by chemical modification.

Taphonomic comparison of passive and active continental margins: Neogene shell beds of the Atlantic coastal plain and Northern Gulf of California

To test for differences in the nature of the fossil record as a function of terrigenous sedimentation, macroinvertebrate concentrations in a recently rifted continental margin (Pliocene Imperial Formation, southeastern California) are compared with a mature passive margin (Miocene Calvert and Choptank Formations, Maryland coastal plain). Previous work suggested that shell beds are primarily a product of absolute low net sedimentation rather than high skeletal input; the duration and environment of low sedimentation of (erosion, omission) determine the quality of paleontologic data through time-averaging and selective preservation of assemblages, and the frequency of low sedimentation episodes determine the stratigraphic density of richly fossiliferous horizons for sampling. Settings of rapid subsidence — and potentially high average sedimentation rates — such as active continental margins are thus expected to have fewer and taphonomically less complex skeletal concentrations than slowly subsiding passive margins, and where steep bathymetric gradients co-occur with high subsidence, skeletal concentrations should be diachronous, patchy in development, and highly variable laterally. These predictions are largely borne out by the shallow marine Imperial Formation, whose average sedimentation rate is several orders of magnitude higher than the Maryland Miocene section. In both tectonic settings, major accumulations record relatively prolonged episodes of low terrigenous sedimentation, attesting to the predictive and explanatory power of a sedimentologic model for fossil accumulation. Major complex shell beds in the Imperial Formation (Latrania member) are exceptional deposits, formed under conditions of sediment starvation along the seaward edges of prograding coastal alluvial fans and fandelta complexes and on bedrock highs; whereas major shell beds in passive margin deposits are stratigraphically condensed records of repeated marine transgressions. Although actualistic experiments suggest that rapid, deep burial is essential to the preservation of skeletal fossils, the stratigraphic record demonstrates that periods of sediment starvation and bypassing — and thus intermittent or retarded shallow burial of shells — are fundamental to the formation of many major skeletal concentrations.

The impact of fluvial processes and landscape evolution on archaeological sites and settlement patterns along the San Xavier reach of the Santa Cruz River, Arizona

AbstractThe Santa Cruz arroyo in the Tucson Basin, Arizona, has undergone major environmental changes over the last 8000 years. the Holocene stratigraphy along a 15 km segment of the arroyo, known as the San Xavier reach, which traverses the San Xavier Indian Reservation was investigated in detail. the Holocene alluvial sequence reveals that aggradation occurred until 8000 yr B.P. within a braided stream, was followed by a major period of channel erosion and widening from 8000 to 5500 yr B.P., which, in turn, was followed by vertical aggradation of the floodplain and five short periods of channel cutting. Broad climatic changes are correlated with major changes in the fluvial regime and landscape. However, the cycles of arroyo cutting and filling, during the semiarid climate of the last 2500 years, were probably the result of the creation of unstable internal geomorphic conditions, flooding, and human impacts on the floodplain. the alluvial history of the San Xavier reach has had a pronounced effect on the preservation and completeness of the archaeological record. Periods of erosion have created absences or gaps in the archaeological record and deep burial has removed some of the preserved archaeological remains from view. Environmental changes on the floodplain also influenced late prehistoric Hohokam settlement and subsistence patterns in the San Xavier region.

Depositional Environments and Hydrocarbon Potential of Copper Ridge Dolomite in Union County, Tennessee: ABSTRACT

The Upper Cambrian Copper Ridge Dolomite in Union County, Tennessee, consists of subtidal, intertidal, and supratidal facies that formed in a hypersaline setting. Subtidal facies are dark and thrombolitic; intertidal facies contain thick, planar to wavy laminations, and may be cross-stratified. Supratidal facies are light colored, mud cracked, and thinly laminated. Algal stromatolites, thought to comprise up to 60% of the unit, were not found to be a major rock type in the area studied. Facies in the unit occur as cyclic, shallowing-upward sequences. The lower member consists of complete cycles, whereas the upper member consists of partial cycles. Cycles are a result of low-amplitude sea level oscillations with long oscillation periods and slow rates of subsidence. Partial cycles lack subtidal facies and were probably caused by tidal flat progradation. Nearly all cement in the unit consists of white sparry (saddle) dolomite, and occurs in vugs, fractures, and interparticle voids. White sparry dolomite displays cathodoluminescence that consists of dark to bright to dull zones. This sequence suggests that cementation occurred during deep burial of the unit. Total organic carbon content of carbonate rocks in the unit indicates that the rocks are not a potential hydrocarbon source. Porous zones with upmore » to 12% porosity occur in thrombolitic rocks and packstones to grainstones.« less

Mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon reservoirs

Secondary porosity created by the post-depositional leaching of detrital grains and cements can be of great economic importance. The term secondary porosity is only descriptive and does not give any information on whether the porosity of the rock has been enhanced. In reality, there are two end member volumetric outcomes to the leaching process. Firstly, the mineral can be completely dissolved and its components transported out of the system. In such an instance the porosity of the rock will be enhanced. Secondly, it is possible that a mineral will decompose and by-product minerals will be precipitated which will occupy approximately the same volume as the original mineral. The latter case is more likely where the pore waters are close to equilibrium with the majority of the minerals of the rock. This will occur at elevated temperatures and/or where the pore waters have been in contact with the rock for a long time. These conditions are commonly met during intermediate to deep burial. Many metastable minerals such as high temperature feldspars, probably decompose in this way. A consequence of the decomposition of such metastable minerals is that primary porosity will be swapped for secondary and that older and more deeply buried rocks will contain a higher proportion of secondary porosity. The mass-transport equation may be used to examine the physico-chemical environment where enhanced porosities most likely occur during leaching. By using a simple non-equilibrium mass-transport model based on the silica system it is possible to demonstrate the relative importance of flow rate, reaction rates, specific surfaces and temperature in controlling the areal extent of leaching. Wide leaching zones are related to high flow rates, which, in turn, promote a high throughput of undersaturated fluid and dispersion. Leaching over a wide area is also favoured by low reaction rates, which will occur at low temperatures. Although it is true to say that less of a given silicate mineral can be dissolved by an undersaturated pore water at low temperature than at high temperature, it is probable that pore fluids from deeply buried sediments are more likely to be close to equilibrium with a given mineral than, for instance, meteoric water. Thus the most likely environment for the production of widespread enhanced porosity is in the shallow subsurface. The lower flow rates and higher temperatures that are encountered during deep burial diagenesis will result in far narrower, more intense leaching zones. As a result, providing sufficient reactive mineral is present any undersaturated pore water will approach equilibrium in a short distance. In order to transport undersaturated pore waters appreciable distances in the deep subsurface, it is necessary to minimize their interaction with the host rock. This could occur if the pore fluid were to move through a fracture system where it would interact with a far smaller surface area of reactive mineral than if it moved through the pore network.