Depositional Surroundings

A depositional surroundings is defined as a site where sediments (due east.g. detrital, chemical) accumulated, governed by concrete, biological, and chemical processes related to mod and applied to aboriginal environments, and lithified into sedimentary rock units.

From: Coal and Coalbed Gas , 2014

PETROLEUM GEOLOGY | The Petroleum System

C. Cornford , in Encyclopedia of Geology, 2005

Source Rock Deposition

Depositional environments favouring the aggregating of petroleum source rocks are illustrated in Figure 4 in the context of a department through a tectonic plate. The corporeality of kerogen nowadays in sediments is a balance between bioproductivity, survival, and dilution by inorganic grains. To take some extremes:

Figure 4. Environments where bioproductivity and depositional environment favour the accumulation of organic-rich sediments.

Low productivity—Aeolian (wind-blown) desert sandstones where land plant growth is restricted due to the extremely arid environment.

Loftier productivity—Areas of ocean upwelling where the enhanced supply of nutrients fuels explosive growth of phyto- and zooplankton.

Poor survival—Chalk composed almost exclusively of coccolith skeletal debris (loftier bioproductivity) only where all the coccolith body tissues are destroyed by bacterial activity under strongly oxic open marine weather.

First-class survival—Early rifting phases of oceans where both optimum sedimentation rates and anoxic conditions of the lakes and enclosed seaways promote high rates of organic affair preservation.

Strong dilution—The prolific sediment supply of major deltas produces organic lean delta-front and pro-delta sediments despite high bioproductivity.

Minimal dilution—Coals where the lignocellulosic and other tissues of high-productivity land plants are well preserved in a delta-top environment starved of (or bypassed by) mineral grains.

Every bit illustrated to a higher place, the aspects of the depositional environment favouring organic preservation are anoxia and elevated sedimentation rates, though excessive sedimentation rates will eventually atomic number 82 to dilution.

As implied in Figure 4, organic affair falling to the ocean or lake floor has to pass through various zones of bacterial degradation (Figure 5). The normal open-water oxic bacterial customs is highly constructive at destroying oil-prone organic matter, then fourth dimension spent in this zone has disastrous effects on potential oil source rocks. These conditions are institute in the water column and surface sediments in the open up ocean, but in strongly stratified basins (e.g., the present-day Blackness Bounding main) only the top of the water column may be oxic.

Figure 5. Processes affecting organic matter degradation and preservation.

Thus to summarize (Figure 6), the optimum oil-decumbent source rocks are deposited where anoxia develops in an aqueous environment enjoying high rates of sedimentation. The combination of high sedimentation rates and oxic environments favours the aggregating of gas-prone source rocks such as dress-down. The combination of depositional weather condition and kerogen amount and type is termed 'organofacies', equally discussed in the next department.

Figure half dozen. Summary of major processes decision-making petroleum source rock accumulation (red   =   gas-prone; green   =   oil-prone).

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Fluvial-Tidal Sedimentology

Northward.D. Webb , ... J.P. Grube , in Developments in Sedimentology, 2015

Abstract

Depositional environment controls the architecture, heterogeneity, and ultimately the quality of oil reservoirs and is therefore ane of the about important considerations in the evolution of whatsoever enhanced oil recovery plan (EOR). Detailed characterization of the Pennsylvanian Bridgeport sandstone reservoirs in Lawrence Field, Illinois Bowl, USA, has revealed juxtaposed deltaic and incised valley fill sediments that were deposited in the fluvial–tidal transition zone. Fluvial–tidal transition zone sediments are some of the near complex deposits known, simply it has been shown through detailed facies assay how two seemingly stratigraphically equivalent reservoirs can exhibit different reservoir properties depending on their position inside the fluvial–tidal transition zone with important implications for operators considering EOR techniques, including chemical or carbon dioxide (CO two) EOR and geologic storage. The thinner, effectively grained, and more than compartmentalized Griggs sandstone was deposited in a low accommodation tidally influenced deltaic setting located in a seaward position within the fluvial–tidal transition zone. The thicker, coarser grained, and younger Robins sandstone was deposited as a fluvial system in an incised valley system that transitioned upwardly to estuarine weather during transgression but remained in the landward portion of the fluvial–tidal transition zone. In the Robins sandstone, preserved primary intergranular porosity in the largely fluvial sandstone has resulted in high-quality, largely homogeneous reservoirs. Whereas in the Griggs sandstone, porosity and permeability are lower considering of finer grained, more heterolithic deltaic reservoir facies that have had porosity and permeability reduced to a greater degree by diagenetic alteration. Only through detailed reservoir label were the differences in these reservoirs realized.

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Lake systems and their economic importance

Chris Sladen , Domenico Chiarella , in Regional Geology and Tectonics (2d Edition), 2020

Principal depositional environments in lake basins

Clastic depositional environments are past far the most dominant in lakes ( Gierlowski-Kordesch and Kelts, 1994, 2000). The principal depositional environments include lake margin fans and deltas, shorelines, playa lakes, shallow water lake (typically <20   m h2o depth), and deepwater lake (Figs xiii.1–13.3, 13.7, 13.8, thirteen.12, 13.16, and thirteen.eighteen).

Figure xiii.18. Abased river and delta channels commonly form modest, curt-lived floodplain lakes (viewed west, part of alluvial system that drains toward Lake Malawi; width of view c. 10   m).

Shallow lake and shoreline deposits may incorporate multicoloured horizons created by groundwater movements, frequent bioturbation, thin shoreface intervals, wave cross-lamination in sands and silts, coarsening upwardly bar sequences, coquinas, and horizons with evidence of aeolian and subaerial reworking, evaporation cracks and polygons, rootlet horizons, palaeosols, sparse coals, shells, and creature tracks.

The relatively deepwater parts of lakes are unremarkably dominated past suspension fallout comprising mud, fine silt, and organic affair produced in situ in the upper parts of the water cavalcade (Fig. xiii.vi). Millimetre-scale laminations are common (Figs xiii.15 and 13.17). The low sulfate content of most lake waters and lake sediments frequently sets the tone for subsequent diagenesis with muds and silts ofttimes rich in dolomite or siderite, rather than pyrite.

Gravity flows and turbidite deposits frequently develop in the relatively deep areas of lakes distal to fans and deltas. Very large subaqueous deepwater systems coordinating to many submarine fans are unlikely. Instead, gravity flows may spread into and fill the topographic lows on the lake flooring. They may be generated by storm floods, seismic shocks, rapid lake-level fluctuations, increases in discharge, or because the high sedimentation rates create oversteepening and unstable pore pressure regimes that atomic number 82 to slumping and gradient failure.

Carbonate facies may develop, normally in shallower h2o areas that have reduced clastic input and the right balance of water chemical science. Nevertheless, lake carbonates often bear little resemblance to marine carbonates. Facies typically include charophytes, oncolites, shell blankets and coquinas of gastropods and bivalves, ostracods, stromatolites, stromatolite bioherms, and polychaetes such as Serpula, together with opaline silica and chert (Fig. xiii.14). At that place may exist build-ups formed in shoals above structural highs within well-oxygenated parts of alkaline lakes. An case of exceptionally well-developed carbonate facies, which includes thick shell blankets, exists in the Early on Cretaceous rift lake sequences preserved in the Campos Basin in Brazil (de Carvalho et al., 2000).

Depending on the interplay of controls, evaporite facies can develop and lakes tin accept the class of sabkhas (Fig. 13.8). Minerals such as epsomite, bloedite, thenardite, trona, natron, mirabalite, ischelite, and glauberite may form, depending on drainage basin geology, water chemistry, and evaporation weather (Fig. 13.16). Lithium, boron, and rare-earth elements frequently accrue in anonymously high concentrations. Gypsum and halite may be volumetrically important particularly when a relict marine water body has been annexed during creation of the lake basin.

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Varves☆

Saija Saarni , in Reference Module in Globe Systems and Environmental Sciences, 2018

Formation and Preservation of Varves

The depositional environment in which varves class is normally aquatic, and varved sediment records are establish from depression to high latitudes from marine and lacustrine environments from modernistic basins but even from sedimentary sequences representing Paleozoic Era ( Zolitschka et al., 2015; Schimmelmann et al., 2016). Despite the diverseness of environments in which varves are found, the varve formation and preservation is non very common just depends on certain boundary conditions such as seasonal variability of sedimentary material and preservation of the laminated construction after accumulation.

The seasonal variability in temperature and precipitation causes alter in physical and chemic processes in water body and in the catchment controlling the supply of different sedimentary fabric. Sediment constituents are formed at the catchment (allochthonous components) and in the water body (autochthonous components). Allochthonous components are transported in the water body by h2o or winds. These components include eroded and weathered minerogenic matter besides equally terrigenous organic matter. Autochthonous components include biogenic matter derived from the productivity in the water body and the chemical atmospheric precipitation of authigenic minerals.

The preservation of the newly formed varve requires a lack of bioturbation and resuspension of the sediments. Burrowing activities of the benthic organisms are restricted in the anoxic bottom waters. Resuspension caused by for example, sediment remobilization as a consequence of wave activity or bottom currents are restricted in the deep lakes or lakes with great relative depth compared to the small surface expanse (Ojala et al., 2000; Tylmann et al., 2013a; Zillén et al., 2003). Potent stratification formed in such basins restricts ventilation of the bottom waters at to the lowest degree seasonally and favors the development of seasonal anoxia (Zolitschka et al., 2015).

In marine environments the bottom waters are isolated due to chemical and thermal processes and varves are found for example in tectonic basins such as Cariaco Basin off Venezuela (Hughen et al., 1996) and Santa Barbara Basin off California (Fig. 1G Schimmelmann et al., 2013), restricted lagoons or silled fjord systems such as Saanich inlet in British Columbia, North America (Dean and Kemp, 2004). Generally marine varves occur in coastal regions or proximity of the shore because of the necessity of sufficient sedimentation charge per unit (Schimmelmann et al., 2016). At present, the expansion of benthic hypoxia due to eutrophication, non just in lacustrine, just too marine environments accept led to formation of "expressionless zones" with no macroturbation below stratified water cavalcade that can potentially induce varve germination (Diaz and Rosenberg, 2008; Jokinen et al., 2018; Lui et al., 2014; Rabalais et al., 2014; Schimmelmann et al., 2016).

The lakes and their catchments are influenced by local geology, hydrology, climate, and country use and thus varve records from different lakes are non straight compared. In contrast, some marine varved sites are successfully connected over distances of more than yard   km (Schimmelmann et al., 2016).

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Geochemistry, Organic

Jürgen Rullkötter , in Encyclopedia of Concrete Science and Technology (Third Edition), 2003

Two.D.two Geological Factors Influencing Crude Oil Composition

The depositional environment of the source stone, its thermal evolution, and secondary amending processes are the most of import factors determining the composition of rough oils. Amidst the ecology factors, those that influence the nature of the organic thing in the source rock and its mineral composition are of primary significance.

Although hydrocarbon source rocks are deposited under aquatic conditions, they may contain varying amounts of land-derived organic matter. The terrestrial contribution can exist significant, particularly in intracontinental basins and in the deltas of big rivers, which may extend far into the open up sea. Continental organic matter (type III kerogen) is rich in cellulose and lignin which, due to their oxygen content, are not considered to contribute much to oil germination. The subordinate lipid fraction together with the biomass of sedimentary microorganisms incorporated into the source rock yields crude oils which are rich in aliphatic units (from wax esters, fats, etc.), i.east., straight-concatenation and branched alkanes (paraffins). Polycyclic naphthenes, particularly steranes, are present in very low concentration. Full aromatic hydrocarbons are as well significantly less abundant than in crude oil derived from marine organic thing, as is the sulfur content.

Marine organic matter (usually type 2 kerogen) produces oils of paraffinic naphthenic or effluvious intermediate type (Fig. vii). The amount of saturated hydrocarbons is moderate, but isoprenoid and polycyclic alkanes, such every bit steranes (from algal steroids) and hopanes (from membranes of eubacteria), are relatively more than abundant than in oils from terrigenous organic matter. Kerogen derived from marine organic matter, particularly when it is very rich in sulfur, is particularly suited to release resin- and asphaltene-rich heavy rough oils at a very early stage of catagenesis. Type II kerogens are preferentially deposited where the environmental weather are favorable for organic matter preservation (anoxic water column in silled basins or in areas of coastal upwelling) and where the continental runoff is express for physiographical or climatic reasons.

The sulfur content of rough oils shows a close relationship to the type of mineral matrix in the source rocks. Organic matter in sediments consisting of calcareous (eastward.g., from coccolithophores or foraminifera) or siliceous shell fragments (due east.g., from diatoms or radiolaria) of decayed planktonic organisms and at the same fourth dimension containing abundant organic matter is enriched in sulfur. The reason for this is that nether the anoxic weather which are required to preserve organic thing, sulfate-reducing bacteria grade hydrogen sulfide (HtwoS). This may react with the organic thing, and the sulfur volition become incorporated into the kerogen. Examples are the Monterey Germination with the related crude oils produced onshore and offshore southern California and many of the carbonate source rocks of the Middle East rough oils.

In clastic rocks containing an abundance of detrital clay minerals, the iron content usually is loftier enough to remove most of the HiiS generated by the sulfate-reducing bacteria through formation of iron sulfides. Because terrigenous organic matter is ordinarily deposited together with detrital mineral matter (e.one thousand., in deltas), waxy crude oils derived from type 3 kerogen usually are depleted in sulfur.

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Volume 5

Bruce Levell , in Encyclopedia of Geology (2d Edition), 2021

Stratigraphic Traps

All depositional environments are capable of producing a juxtaposition of permeable and impermeable sediments which have the potential to form stratigraphic traps. In such cases the reservoir geometry becomes part of the trap geometry, normally with a component of simple dip, in which instance the impermeable rocks are up-dip. Examples include eolian sand dunes encased in lacustrine mudstone, sand-filled fluvial channels cut into mud-rich overbank deposits, shallow marine bar sandstones surrounded by marine shales, carbonate reefs isolated past enclosing marls, and submarine fan sands trapped inside pelagic mud.

The Paradox Basin (Colorado and Utah, USA) contains a big array of small oil and gas fields in stratigraphic compression-out traps. Devonian reservoirs occur within shallow marine bar sandstones and Carboniferous reservoirs within carbonate mounds produced largely by algae. Pinch-out traps formed in "paralic", or about-shore, deltaic settings are oft much more than complex in outline resulting in discontinuous reservoir sandstones and formation of multiple fields (Fig. 16).

Fig. 16

Fig. 16. Paralic field outlines commonly have circuitous shapes because of the interaction between structure and sediment bodies. This complexity is multiplied because individual paralic sandstones tend to be stacked. The 4 examples prove: (A) field shape on a simple faulted anticline for which the reservoir interval is much larger than the anticline; (B) the aforementioned structure as in (A), but with the reservoirs adult in aqueduct and crevasse splay sandstones that are smaller in area than the structure; (C) the aforementioned construction as in (A), but with mouthbar sandstones which are also smaller than the structure; (D) a combination of channel and mouthbar sandstones at unlike levels.

A. Reynolds, personal communication, 1994. Reproduced courtesy of BP.

Thinning to zippo in the up-dip portions of a potential reservoir because of erosion below an unconformity can create large traps with enormous petroleum catchment (i.e. drainage from the source rock up-dip to the trap) areas. The largest oilfield in Northward America, Alaska's Prudhoe Bay, is an unconformity trap. Information technology contains about 25   billion barrels of oil and more 20   trillion cubic feet of gas. E Texas, the largest oilfield in the Lower 48 states of the United States, is too a stratigraphic trap.

Both unconformity traps described above are formed by a combination of trapping mechanisms, comprising a gently folded unconformity above, and a mudrock "seat seal" beneath and lateral to the reservoir. Unconformities have a variety of shapes. The most spectacular of the unconformity-bounded traps are those referred to as "buried hills." Such hills are the topography of a previous land surface and it is the unconformity surface itself that provides the trapping geometry (Fig. 17). Buried hill traps are common in karstified areas, such as northern China.

Fig. 17

Fig. 17. Sub-unconformity trap beneath the base Cretaceous unconformity, Buchan Field, Britain Northward Bounding main (fractured Devonian sandstone reservoir).

Reproduced from Abbots IL (1991) U.k. Oil and Gas Fields, 25   Years Commemorative Volume, Geological Lodge Memoir No. fourteen. London: Geological Society.

Mineral cements are known to course top, lateral, and even seat seals to reservoirs. Examples in carbonate systems are more numerous than those in clastic systems. In the Albion-Scipio Field of Michigan (United states), all surrounding stone to the trap is thoroughly cemented limestone and dolomite. A comparable situation exists for many of the carbonate-hosted oilfields of Abu Dhabi - porosity only exists where there is oil. Rock volumes that at one time must have been the aquifers to the oilfields, and through which oil must accept migrated to the traps, accept been thoroughly cemented. For a few fields, such cementation has allowed traps to retain petroleum despite tilting of the field after petroleum accumulated.

Tar-mat seals are common in the shallow subsurface. They human activity equally "cap stone" for the largest unmarried accumulation of heavy (glutinous) oil in the globe, namely, the Faja of southward-eastern Venezuela which contains near one.2   trillion barrels of oil. Tar seals and tar sands are too common inside the Western Canada Basin and Californian basins.

Gas trapped below permafrost forms large fields in the northern part of the W Siberia bowl, side by side to the Kara Sea. In common cold regions, gas (marsh gas) is also trapped as gas hydrate, with gas molecules accumulating within the open molecular structure of the water ice itself. Such hydrates near the sea lesser may themselves grade top seals to free gas beneath.

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PALEOCLIMATOLOGY | Varves

R. Gilbert , in Encyclopedia of Atmospheric Sciences (Second Edition), 2015

Formation of Varves

The depositional surround in which varves form is usually aquatic, although varves may occur subaerially, for instance as a issue of seasonally varying aeolian processes or snowfall (degradation of the crystalline mineral H twoO) in the accumulation areas, specially mid- and depression-latitude alpine glaciers. In the latter case, melt or windblown grit in summer distinguishes depositions in each year as the snowfall transforms into glacial ice.

Aquatic varves have been described from many settings in lakes and the bounding main, but they all fall into one of 3 classes. Clastic varves normally consist of laminae of fine silt and clay-sized sediment deposited from suspension during periods of express inflow of water and sediment, and periods of diminished processes of distribution in the water torso. Coarse silt and sand layers are deposited in response to abundant inflow and vigorous circulation in the h2o body. Usually, these couplets cannot be distinguished in fresh samples of wet sediment, but on drying, the sand and coarse silt have on a light tone, while the hydrophilic clay remains moist and dark. Thus, with careful attention during drying, clastic varves are easily distinguished by middle (Figure 1(a)–one(c)), and photographic records are usually the best for analysis of thick varves, while thin sections are unremarkably used for submillimeter-sized varves.

The deposition of fine- and fibroid-grained layers in a clastic varve is enhanced past processes within the h2o body. In lakes experiencing sediment-laden inflow, turbidity currents efficiently deliver bursts of sediment that are deposited virtually instantaneously as graded beds, and thus distinct layers are associated with each, often daily, result. Complex varves unremarkably form in this style. The silty sand layers within the dark winter clay layers shown in Effigy 1(c) result from the incursion of intense autumn and wintertime storms from the Pacific Ocean into the mountains. Snowmelt augmented by warm rain generates large floods that deliver bursts of coarser sediment to the lake, where turbidity currents efficiently transport them to the lake flooring. The fine-grained laminae are deposited from settling through the water column. According to Stokes' law, a 1 μm diameter particle requires about 3 years to settle through a 100 m deep lake. Contempo studies have demonstrated that the formation of pocket-size flocs in fresh water greatly increases settling velocity and allows the fine-grained sediments to accrue during several months of quiet weather condition, which are appropriate for the formation of the winter cap on a varve. In common salt water, much larger flocs are formed and are removed from the water column in a few days to weeks. This may be a reason why clastic varves are uncommon in marine settings.

Biogenic varves form by the seasonal deposition of organic material derived from country (e.g., pollen production in spring) or originating in the water body itself (about commonly as blooms of diatoms or other aquatic organisms). Degradation of darker terrigenous organic rest and inorganic sediment separates the lighter biogenic laminae and then forms varves. These are the nearly common blazon of marine varves (Figure i(d)); they usually class in basins with anoxic bottom water that prevents the institution of a benthos that would bioturbate and destroy varves as they formed. Biogenic varves are too found in lakes, but here anoxic weather condition are less of import because the lacustrine benthos is commonly sparse or absent. Yet, some of the best varve sequences occur in strongly stratified meromictic lakes that have virtually no currents at depth and where dissolved oxygen is depression or absent. Anoxia too slows the decay of organic carbon that may form the distinctive lamina in some biogenic varves.

Chemogenic varves form in lakes as a upshot of the seasonal atmospheric precipitation of salts, especially calcium carbonate, from a supersaturated solution created during summertime when water temperature is high or when the uptake of carbon dioxide past aquatic vegetation is greatest. Coarse particulate carbonate commonly settles through the water and becomes buried sufficiently quickly that most is non redissolved in the undersaturated cold-h2o deep in these lakes. In wintertime, the whole lake is sufficiently common cold that carbonate does not precipitate, and even a small amount of terrigenous sediment is sufficient to produce a distinctive lamina to course a varve. Biogenic and chemogenic processes are more likely to class simple varves because they ordinarily occur simply one time per year, whereas processes associated with the deposition of clastic sediment are oftentimes of shorter elapsing and higher frequency.

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Classification of Estuarine and Nearshore Coastal Ecosystems

D.M. Kennedy , in Treatise on Estuarine and Littoral Science, 2011

Abstract

Estuaries are depositional environments which receive and shop sediment from both fluvial and marine sources. Their evolution is primarily linked to eustatic sea-level fluctuations, which inundation coastal embayments, creating accommodation space for sediment aggregating. The style of infill within an estuary will depend on the v boundary-condition processes of the system: (1) tidal range; (two) wave climate; (3) tectonic stability; (iv) glacial history; and (5) magnitude of fluvial inflow. The relative authorization of each process will determine the type of landforms that class inside an estuary and resulting sediment facies, which progressively infill the drowned coastal embayment.

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PALEOCEANOGRAPHY, BIOLOGICAL PROXIES | Alkenone Paleothermometry Based on the Haptophyte Algae

S.L. Ho , ... F. Lamy , in Encyclopedia of Quaternary Science (Second Edition), 2013

Stratigraphic Offsets Between the Alkenones and Other Sedimentary Proxies

In some depositional environments, vigorous currents or eddies are able to transport alkenones laterally over corking distances from their source, as seen in the southwestern Atlantic ( Benthien and Müller, 2000) and sediment drifts in the Cape Basin (Sachs and Anderson, 2003). Depending on the source of these advected alkenones, 1 could detect cold or warm biased SSTs in the sediment, or a discrepancy between different paleoceanographic proxies at the same site. Yet, lateral advection at sediment drifts does not necessarily result in erratic downcore SST variation patterns as demonstrated by Sachs and Anderson (2003) using a multiproxy arroyo and thorium-derived focusing factors.

Recent advancements in the radiocarbon technique provide boosted ways of constraining the source origin of alkenones in marine sediments. For instance, at the Bermuda Rise (Ohkouchi et al., 2002) and the Benguella upwelling arrangement off Namibia (Mollenhauer et al., 2003), studies using accelerator mass spectrometry (AMS) 14C revealed discrepancies betwixt the fine organic matter (<63   μm), including the algal C37 alkenones, and planktonic Foraminifera in the marine sediments, which implied that the alkenones were several hundreds or even thousands of years older than the Foraminifera that are taken equally reference materials deposited at the aforementioned depth. This sedimentary asynchrony is explained by the potent differences in size between coccoliths (on the order of 5   μm) and the foraminiferal tests (>150   μm). In cases of resuspension due to intense lesser currents, the finer alkenone-bearing particles are susceptible to easier mobilization than the larger foraminiferal remains (Ohkouchi et al., 2002). The event is that older, previously deposited fabric can be remobilized and deposited in a new environment, leaving the fine fraction signal old with respect to the adjacent fibroid cloth. However, the differences in particle size are non a priori very relevant for sedimentation through the water cavalcade because nigh algal remains make it to the lesser as aggregates, for example, in the course of zooplankton fecal pellets (~50   μm).

The ages of the marine sedimentary sections are currently adamant from the composition of Foraminifera, either by interpretation of the δ eighteenO curve or past measurement of AMS 14C in their carbonate skeleton. The potential temporal get-go betwixt alkenone SST and foraminiferal records may thus jeopardize the possibility of using alkenones for high-resolution studies in sedimentary environments because the betoken that is stored in their composition may be older than the age determined for the sedimentary sections in which they are institute.

Close comparison of loftier-resolution alkenone-derived SST and δ 18O profiles beget a practical way to notice whether this problem is pregnant in the sedimentary core sections under study. When both proxies exhibit synchronous changes, possible temporal offsets due to different sedimentation processes between organic matter and Foraminifera are negligible. This approach has been followed in some studies (e.g., Martrat et al., 2004), and in most cases, no sedimentation asynchrony was observed.

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The Formation of Petroleum Accumulations

Harry Dembicki, Jr. , in Practical Petroleum Geochemistry for Exploration and Production, 2017

Carbon Dioxide

In the depositional surround, carbon dioxide (CO 2) is produced by aerobic respiration in the sediments. If the sediments are anaerobic, CO2 could as well be formed past microbial oxidation of marsh gas. In either case, the carbon dioxide formed during this early stage of sediment diagenesis is unlikely to significantly contribute to reservoired gases.

There are many sources for the carbon dioxide that is found in reservoired gases. It is a past-production of the oil generation process via the decomposition of oxygen begetting function groups such as carboxyl (single bondCOOH), carbonyl (Cdouble bondO), and hydroxyl/phenolic (single bondOH). This occurs mainly in the temperature range from lxxx to 120°C (176–248°F). Types III kerogen produces the well-nigh CO2, followed by Type II, while Types I and II-S kerogens produce the least. Coals can exist a substantial source of carbon dioxide and are capable of producing upward to 75   L of COtwo per kilogram of coal (Karweil, 1969).

Another source of significant carbon dioxide in a reservoir at low temperatures is related to the biodegradation of crude oil. This will be discussed subsequently in the section on biodegradation in Affiliate 4.

At temperatures above 120°C, thermal decomposition of carbonates becomes a more meaning source of carbon dioxide gas. In argillaceous sandstone reservoirs with carbonates cements, Smith and Ehrenberg (1989) attributed observed increases in CO2 with increasing temperature to the interaction of feldspars and clay minerals with the carbonate cements, equally shown in Fig. 2.31. This procedure initiates at temperatures of about 120–140°C (248–284°F) and accelerates equally the temperature increases. The potential for these reactions is not bars to the reservoir rock. Similar interaction may too occur in the source stone during late-phase gas generation (Smith and Ehrenberg, 1989).

Figure 2.31. Mechanism for thermal decomposition of carbonates in argillaceous sandstone and shale.

As proposed past Smith, J.T., Ehrenberg, S.N., 1989. Correlation of carbon dioxide affluence with temperature in clastic hydrocarbon reservoirs: human relationship to inorganic chemic equilibrium. Marine and Petroleum Geology 6, 129–135.

Straight thermal decomposition of carbonates to produce CO2 requires temperatures in excess 300°C. This can occur during contact metamorphism when igneous intrusions penetrate into carbonate rocks. Carbonate minerals in contact with magma can produce large quantities of carbon dioxide that can drift to and accumulate in nearby reservoirs. High concentrations of carbon dioxide from magmatic-induced thermal decomposition of carbonates have been documented in many areas worldwide including the Rockies, West Texas, and Indonesia (Thrasher and Fleet, 1995).

Magmatic degassing may also contribute carbon dioxide to reservoirs. This can happen during exsolution of gases from loftier-volatile magmas. This may occur in tectonically agile areas where deep penetrating faults or fractures can admission the magma bodies. A large igneous trunk is required for meaning CO2 contributions. In these instances, the carbon dioxide is associated with radiogenic sourced gases, such equally helium and argon.

CO2 derived from organic thing is commonly distinguished from CO2 derived carbonate decomposition and magmatic outgassing past its carbon isotope ratio. Carbon dioxide from carbonates typically has δ13C in the range of +iv to −v‰, while magmatic CO2 is in the −4 to −8‰ range. Carbon dioxide derived from thermal maturation of sedimentary organic matter is usually much more than depleted in 13C and falls into the range of −10 to −25‰ δthirteenC (Thrasher and Fleet, 1995).

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