Shallow Burial Depths Research Articles

Genesis of Siderite in the Upper Miocene, Offshore Sarawak: Constraints on Pore Fluid Chemistry and Diagenetic History: ABSTRACT

The cored sequence of the Upper Miocene reservoir of Baram field, offshore Sarawak, consists of cyclic deltaic-related quartz-rich sandstones and mudstones interpreted to have been deposited during storm events in shallow to midshelf water depths. The sequence is intercalated with minor tidal intervals. Authigenic siderite is common in sandstones throughout the sequence. Siderite cemented zones are up to 2 m thick. The cement is found in four different sandstone types: laminated sandstone, massive sandstone, bioturbated sandstone, and in association with shell fragments. Whole-rock XRD gives estimates of 20 to 40% siderite in bioturbated sandstones and 10 to 25% for the others. Petrographic analysis reveals that diagenetic siderite occurs in four different crystal morphologies: rhombic, [open quotes]bundles,[close quotes] acicular, and cylindrical. The rhombic siderite, which commonly occurs in bioturbated sandstone, has the most adverse effect on the poroperm characteristics of the sandstones, reducing porosity to 10% and permeability to 2 md. [sigma][sup 13]C and [sigma][sup 18]O plots show groupings based on morphology. Bundled and acicular siderite show ranges of [sigma][sup 13]C[sub PDB] of [minus]15 to [minus]25 and [sigma][sup 18]O[sub PDB] of 0 to [minus]1. The late Miocene seawater [sigma][sup 18]O estimate for the region is [minus]0.8 PDB. This would give themore » bundles/acicular siderite a temperature of formation range of 26[degrees] to 30[degrees]C. The [sigma][sup 18]O values are compatible with precipitation at shallow burial depth from unaltered seawater; [sigma][sup 13]C values are inherited from sulfate reduction horizons. Rhombic siderite has ranges of [sigma][sup 13]C[sub PDB] of [minus]5 to [minus]15 and [sigma][sup 18]O[sub PDB] of [minus]3 to [minus]4. The range of [sigma][sup 13]C indicates that siderite diagenesis occurred within both the shallow sulfate reduction zone and at deeper levels within the zone of decarboxylation.« less

Controls on Clay Mineral Distribution and Chemistry in the Early Permian Rotliegend of Germany

AbstractAuthigenic clay minerals in the German Rotliegend formed mainly at burial depths >1.5 km. Illite is the dominant cement, although kaolinite, dickite and early radial chlorite are abundant locally. Illites contain more AI and late chlorites more AI and Fe in sequences showing extensive grain dissolution. This relationship between clay chemistry and grain dissolution suggests that clay cementation is linked to grain dissolution. Sequences at relatively shallow burial depths (<3-5 km) contain less clay cement. In the more deeply buried sections, increased illite and kandite cementation, together with extensive grain dissolution, is evident where the Rotliegend is juxtaposed against Carboniferous Coal Measures. Faults also acted as important conduits for acidic fluids. Illite and kandite growth occurred at similar depths and from waters of similar isotopic composition (618O = 1-6‰ SMOW) throughout most of the Southern Permian Basin. However, the timing of illite growth varied between areas and corresponded to periods to tectonic activity.

Basement heat flow in the Western Canada Sedimentary Basin

The effective thermal conductivity of the entire sedimentary succession is calculated at 487 locations in the Western Canada Sedimentary Basin, based on net-rock analyses and thermal conductivity values corrected for temperature and porosity effects. An easterly trend of increasing values is caused by shallower burial depth and by a reduced proportion of low conductivity shales. Geothermal gradients, calculated previously and corrected for Pleistocene glaciations, are used in estimating the basement heat flow after subtracting the heat generated by radioactive decay in the sedimentary strata. The distribution of basement heat flow, with values between 30 and 80 mW/m 2, is linked with the complex structure of the basement, which is comprised of Archean blocks, accreted terranes, orogenic belts and magmatic arcs. Heat flow-heat generation relations in Alberta show that the reduced heat flow and the characteristic length scale for the uppercrust thermal layer are of the order of 35 mW/m 2 and 6 km, respectively.

Relationship between reservoir diagenetic evolution and petroleum emplacement in the Ula Field, North Sea

The diagenetic and petroleum filling histories of the Ula Field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements. This study shows that diagenesis continues actively after the arrival of petroleum in the sandstones, although the reaction rates and petroleum saturation remain obscure. Microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110°C (i.e. ≈ 2.5 km) and that this cementation has continued to the present day. Synchronously with quartz cementation, authigenic albite was formed as inter- and intragranular cements. Large amounts of petroleum inclusions locally occur in the albite and quartz cements. These inclusions were trapped in the cements as the main charge of petroleum arrived at the structure. Diffusion, through thin water layers between the petroleum and the mineral surface, would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite. It is evident from analysis of the petroleum inclusions in early formed K-feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth, before the rapid subsidence in the last 25 my. Compared with petroleum in the free porosity of the Ula Formation, the petroleum in the K-feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature. Later trapped petroleum inclusions, in quartz and albite, have characteristics found both in K-feldspar and in the Ula Formation DST oil and, thus, is likely to reflect the progressive change in the Ula Field petroleum charge which occurred during the time period of quartz diagenesis.

A preliminary study on the distribution and genesis of underground brine in the littoral plain of Laizhou Bay in the Bohai Sea

Quaternary underground brine in the littoral plain region of the Bohai coast is new type littoral facies evaporation ore deposit in liquid state. This model of “littorally-forming brine” is based on study of the distribution, depositional environment, forming/storing process, and genesis of underground brine in the coast of Laizhou Bay. Essential conditions for genesis of brine are arid and semiarid climate, abundant sources of ancient seawater, and geological and geomorphological environment of wide tidal beach along a coast. The process of underground brine genesis may be: seawater/tidal beach » evaporation and concentration » seeping and accumulation » brine/tidal beach » burial (evolution of the sea-land)» underground brine. Underground brine is characterized by high content of chemical elements, shallow burial depth and easy exploitation. The process of extracting salt from underground brine present in the tidal flat is incomparably superior to that of producing salt from seawater, and has brough great economic benefits to the coastal areas of Laizhou Bay, Shandong Province.

Secondary macropores and mosaic structure in a telemagmatic metamorphosed coal and their significance in exploration for natural gas, Henan, China

Secondary macropores and mosaic structure, common characteristics in metallurgical and natural cokes, are considered to be the solid residue after the release of liquid and gas during the heating process. In north-central Henan province, People's Republic of China, secondary macropores and “mosaic” structure are found in semi-anthracite and anthracite of lower Permian telemagmatic (not in contact with intrusion) metamorphic coal. Coal rank exhibits a zoned distribution cored by a northwest-southeast trending zone of anthracite. Coal rank decreases from this central zone toward both the northeast and southwest margins from low-volatile bituminous through medium-volatile bituminous to high-volatile bituminous. The zoned rank distribution of coal in this area is interpreted as being the result of regional metamorphism overprinted by telemagmatic metamorphism. Coal metamorphism in this area occurred in three stages: (1) pre-orogenic (late Carboniferous-late Triassic). Regional metamorphism produced coals of subbituminous to high volatile bituminous rank. (2) Orogenic (early Jurassic-late Cretaceous). Telemagmatic metamorphism resulted in zones of higher rank coal (medium-volatile bituminous through anthracite rank). (3) Post-orogenic (Tertiary-Quaternary). Shallow burial depth due to the tectonic uplift followed by erosion had a negligible effect on coal rank. Secondary macropores and “mosaic” structure found in semi-anthracite and anthracite in the research area are similar in form, by microscopic and SEM observation, to those of metallurgical cokes and other natural cokes, indicating extraordinary heating effect on the coal. High temperature, high-pressure experiments on high volatile bituminous coal from the marginal area show that the pores develop in the 250–300°C range and “mosaic” structure develops in the 350–500°C range. It is therefore postulated that the high rank coal in this area may have been subjected to temperatures similar to carbonization temperatures during the orogenic period. Because the heat sources for telemagmatic metamorphism are deep-seated plutons, the rate of heating on this coal might be relatively slow, the time involved in the metamorphism was considerably longer, and the affected area might be extensive. During the heating process, bituminous coal near the center of high heat flow region was heated to a thermoplastic temperature range and coal underwent a soft stage (plastic). Gases evolved in this phase and left the pore system in their wake. Mesophase growth units may also develop in this phase, which could get compressed into each other to form mosaic structure. The existence of secondary macropores and mosaic structure in coal can be used as an indicator of heating effect and gas generation. Ubiquitous macropores developed in the coal in a considerable area indicate that a regional gas-generation process occurred. The gases generated by thermal cracking of the coal during the heating process could be captured later by the macropores, migrate through the network of pores, and form the underground gas reservoirs where the storage conditions are favorable for gas retention. Therefore secondary macropores and mosaic structure in telemagmatic metamorphic coal have potential significance in natural gas exploration.

The Formation of Septarian Concretions in Queen Charlotte Islands, B.C.: Evidence for Microbially and Hydrothermally Mediated Reactions at Shallow Burial Depth

Petrographic, chemical and isotopic studies of marine sediment-hosted septarian concretions from Lower Jurassic Whiteaves Formation in the Queen Charlotte Islands, B.C., Canada, reveal a complex diagenetic history that has resulted from bacterially mediated reactions at shallow burial depth as well as hydrothermally modified meteoric waters. The general diagenetic sequence in cement-filled septaria cracks includes: 1) fibrous calcite, 2a) bladed calcite or 2b) ferroan saddle-like calcite, 3) blocky calcite with early ferroan and late non-ferroan stages, and 4) barite. Each cement generation has a characteristic chemical and isotopic signature. Oxidation of upward-diffused methane contributed to an extremely negative value ofdelta C (ca. -35 per thousand ) in the early generation of fibrous calcite. The delta O (ca. -3.0 per thousand ) and 87 Sr/ 86 Sr (0.70683) values of these calcites, however, reflect Jurassic seawater composition. Plutonism during Late Jurassic to Early Cretaceous time provided hydrothermal discharge that altered meteoric waters. This process became the main factor for the precipitation of saddle-like and bladed calcites. These two generations of carbonate cement are characterized by relative depletion in delta 18 O (-11.5 to -17.4 per thousand ) and less radiogenic 87 Sr/ 86 Sr (0.70532 to 0.70575) signatures. Progressive modification in pore water chemistry and increasing depth of burial occurred during the formation of late ferroan blocky calcite.

The evolution of a fractured chalk reservoir: Machar Oilfield, UK North Sea

Machar is the first of the ‘Diapir’ fields discovered in the eastern trough of the Central North Sea, in BP sole interest Block 23/26a. It comprises a fractured Cretaceous chalk and Paleocene sand reservoir overlying a steeply dipping salt structure. The oil column is greater than 1100 m and reserve estimates are in the region of 100 million barrels of oil equivalent. The history of the Machar reservoir represents a complex interplay of salt movement, oil migration and burial diagenesis. Structural studies have shown that the Permian Zechstein salt has been in a state of buoyant equilibrium for most of its geological history, with only one phase of true diapirism identified in the mid-Miocene. The buoyancy resulted in the deposition of a condensed (<300 m) Triassic–Eocene section over the salt high. In the surrounding area, sediment loading and related salt withdrawal caused ‘downbuilding’ and the deposition of a thick (3000 m) sediment package. A 50–300 m section of chalk was thus deposited over the diapir with a mature Jurassic oil source in the adjacent depocentre. Paleocene sands are thin or absent over the crest of the structure, but thicken to about 100 m downflank. Regional subsidence and oil migration commenced in the early Oligocene, the trap filling from the top down. Here, shallow burial depth ( c . 100 m) and early oil emplacement ensured porosity was preserved in the chalk matrix. At the same time, on the flanks of the structure greater depth of burial and diagenesis were destroying chalk porosity from the bottom up. These competing influences resulted in a diagenetic front marking the limit of pay in the chalk. That front lies in the depth range 2000–2500 m. Passive movement of the chalk during periods of buoyancy, plus regional tectonic compression in the mid-Miocene, created a widespread fracture system. Studies have identified a continuous process of fracture, stylolite and cement formation across a variety of brittle and ductile deformation styles. Average spacing of open fractures in 200 m of chalk core from Well 23/26a-13 is 7.5 cm. About a third of these are open. The burial history, oil emplacement and fracturing resulted in a reservoir with a dual porosity system. The fine-grained pelagic chalk matrix has primary porosity in the range 12–35% with permeability generally less than 1 mD. Secondary fracture porosity, although difficult to measure, is probably less than 1%. Production test data over fractured intervals, however, show permeability in excess of 1000 mD. The fracture system is the key to developing the chalk reservoir as it provides effective drainage of the tight matrix. In a wider context the recognition of buoyancy, not piercement, as the dominant structural process provides a model for exploration elsewhere.

Reworked early diagenetic concretions and the bioerosional origin of a regional discontinuity within British Jurassic marine mudstones

ABSTRACTThe Coinstone is a well known hiatus‐concretion level in the Lower Lias (Lower Jurassic, Upper Sinemurian) of Dorset, southern England. It has long been recognized as a layer of bored and encrusted, early diagenetic, clay‐hosted septarian concretions coincident with a biostratigraphic gap of three ammonite subzones. Several different types of concretion of variable complexity can be distinguished, of which two, probably derived from slightly different stratigraphic levels, have been juxtaposed by condensation at the erosion surface. Diagenetic and biological processes occurring before, during and after exhumation on the Jurassic sea‐floor can be recognized. The relative timing of these events can be distinguished, suggesting that initial concretion consolidation, the first generation of septarian cracking, and the precipitation of the first generation of crack‐lining calcite preceded exhumation. These, therefore, probably took place at an early stage, at shallow burial depths within the accumulating sediment pile. The early calcite is brown, UV‐fluorescent and inclusion‐rich, and is similar to the first calcite generations seen in many other clay‐hosted septarian concretions. A generally early diagnetic origin of this material is thus inferred. Observations on crack textures and geometries and the interactions of the post‐exhumation fauna of encrusters, borers and burrowers lend support to previous suggestions that initial cracking in some septarian concretions took place in a stiff rather than a fully rigid concretion body, possibly given coherence by initial growth of some organic substance that was only later replaced by the calcite cements seen in most such concretions today.The burrowing activities of a benthic fauna in muds cause resuspension of sediment and facilitate erosion, even in the absence of high energy physical processes. Regional stratigraphic gaps may be formed as a consequence of sea level rises or falls, or as a response to sediment supply reduction independent of sea level change. Such major episodes of biologically mediated erosion in mudstone sequences may be of more general importance than has hitherto been recognized.

Abyssal recolonization by benthic foraminifera after the Cenomanian/Turonian boundary anoxic event in the North Atlantic

Benthic foraminiferal assemblages of distinctive taxonomic composition occur at the top of benthic fossil-free black shales which correspond to the anoxic event at the Cenomanian/Turonian boundary in the North Atlantic abyssal DSDP/ODP sites 386, 398, 603 and 641. These assemblages consist of minute, thin-walled agglutinated foraminifera with low specific diversity of 2 to 4 species, variable abundance and dominance of few taxa ( Haplophragmoides, Rhizammina and Glomospira). The species are inferred to be opportunistic, able to survive in low-oxygen environments and to be pioneers recolonizing the seafloor after cessation of bottom-water anoxia. Most species are characterized by test morphologies with high surface/volume ratios and single-layered wall structures, with loosely agglutinated grains, and small amounts of organic cement for agglutination. These features are best observed in material from ODP Hole 641 A which has exceptional foraminiferal preservation because of its shallow burial depth. The successive appearance of benthic foraminifera after the anoxic event is probably controlled by the continuous reoccurrence of more oxygenated bottom- and interstitial-water conditions. With the final development of oxic bottom-water conditions in the Turonian, a rapid radiation of deep-water agglutinated foraminifera occurred in the North Atlantic.

Carbonate replacement of lacustrine gypsum deposits in two Neogene continental basins, eastern Spain

Bedded nonmarine gypsum deposits in the Miocene Teruel and Cabriel basins, eastern Spain, are partly replaced by carbonate. The Libros gypsum (Teruel Graben) is associated with fossiliferous carbonate wackestones and finely laminated, organic matter-rich mudstones which accumulated under anoxic conditions in a meromictic, permanent lake. The gypsum is locally pseudomorphed by aragonite or, less commonly, replaced by calcite. Low δ 13C values indicate that sulphate replacement resulted from bacterial sulphate reduction processes that were favoured by anacrobic conditions and abundant labile organic matter in the sediments. Petrographic evidence and oxygen isotopic composition suggest that gypsum replacement by aragonite occurred soon after deposition. A subsequent return to oxidising conditions caused some aragonite to be replaced by diagenetic gypsum. Native sulphur is associated with some of these secondary gypsum occurrences. The Los Ruices sulphate deposits (Cabriel Basin) contain beds of clastic and selenitic gypsum which are associated with limestones and red beds indicating accumulation in a shallow lake. Calcite is the principal replacement mineral. Bacterial sulphate reduction was insignificant in this basin because of a scarcity of organic matter. Stable isotope composition of diagenetic carbonate indicates that gypsum replacement occurred at shallow burial depths due to contact with dilute groundwaters of meteoric origin. Depositional environment evidently has a major influence upon the diagenetic history of primary sulphate deposits. The quantity of preserved organic matter degradable by sulphate-reducing bacteria is of particular importance and, along with groundwater composition, is the main factor controlling the mechanism of gypsum replacement by carbonate.

Mineralization of organogenic ammonium in the Monterey Formation, Santa Maria and San Joaquin basins, California, USA

Inorganic fixed-ammonium (NH 4) contents as high as 0.28 wt% were measured in organicrich, quartz-grade siliceous rocks of the Miocene Monterey Formation from the Santa Maria and San Joaquin basins, California. The greatest amount of fixed-NH 4 was found in rocks associated with hydrocarbons in the Point Arguello and Lost Hills oil fields, where the NH 4 (NH 4 + K) molar ratio of bulk samples ranges from 0.17–0.35. The formation of NH 4-illite is suggested by the parallel increase in the percent of illite in the mixed-layered illite/smectite ( I S ) and in the NH 4 (NH 4 + K) molar ratio of the clay-sized fraction with increasing burial depth. Mineralization of NH 4 appears to be promoted by the coincident timing of the smectite-to-illite clay mineral transformation and the release of NH 4 during catagenesis. NH 4-feldspar may form at shallow burial depths in rocks from the Point Arguello field that contain a greater amount of detrital K-feldspar and in which the I/S contains only 10–20% illite. Quartzgrade siliceous Monterey rocks from coastal outcrops in the Lions Head area lack significant amounts of hydrocarbons and have NH 4 (NH 4 + K) molar ratios of 0.14–0.21. Rocks from the Lions Head area show a strong positive correlation between diagenetic illite and fixed-NH 4 contents, with NH 4 constituting 18–21 Mol% of the fixed interlayer cations in the I/S. Low grade, opal-A and opal-CT siliceous Miocene rocks from coastal outcrops in the Pt. Pedemales area have low fixed-NH 4, contents (≤0.01 wt%) because these rocks contain minor amounts of diagenetic K-bearing minerals (I/S contains ≤ 10% illite) and they lack significant amounts of generated or migrated hydrocarbons. The increase in the organic ( C N ) ratio with burial depth most likely reflects the preferential release of N compared to C and suggests that NH 4 release is greatest during shallow burial bacterial degradation and deep burial catagenesis (oil generation). The results of this study support the suggestion of Williams et al. (1989) that high fixed-NH 4 contents may provide a long-term geologic record of low-temperature (<150°C) NH 4 mineralization associated with hydrocarbon generation and migration.

The functionality of organic nitrogen in some recent sediments from the Peru upwelling region

Nitrogen in fossil fuels is present in pyrrolic- and pyridinic-functional groups. In contrast, organic nitrogen in living organisms is overwhelmingly found in amino acids in proteins and peptides. Petroleum is the result of the maturation of the organic remains of organisms, so what causes the change in nitrogen functionality between organisms and oil, and when does it occur? Using X-ray photoelectron spectroscopy (XPS) supported by elemental analysis, biochemical analysis, and pyrolysis-gas chromatography with atomic emission detection (py-GC-AED) for nitrogen, surface sediments from the Peru upwelling region were shown to contain at least four different organic nitrogen-containing functional groups: amino, pyrrole, pyridine and (tentatively) quaternary nitrogen. The percent in amino groups is at most 40–45%, and possibly as little as 10–15%. Heterocyclic nitrogen (pyrroles and pyridines) makes up a greater proportion of the total than amino N, with pyrroles more abundant than pyridines. With increasing burial depth, the percentage of total N present as amino nitrogen declines to low levels, whilst the proportion present as (tentatively) quaternary nitrogen increases. Pyrroles are always more abundant than pyridines. These semi-quantitative data suggest that the pyrrolic and pyridinic structures, found in petroleums and coals, are present at a very shallow burial depths in the Peru upwelling sediments.

Diagenetic processes in the Brent Group (Middle Jurassic) reservoirs of the North Sea: an overview

Abstract The Brent Group is extensively cored in the northern Viking Graben and East Shetland Basin in the range between 1.8 and 4.5 km. It therefore offers the opportunity to study diagenetic processes arrested at different burial depths and temperatures. The sandstones of the Brent Group remain poorly indurated and contain only small amounts of quartz cement down to burial depths of about 2.5–3.5 km. The quartz cementation increases with depth reflecting progressive silica cementation sourced by pressure solution. Dissolved feldspar and authigenic kaolinite occur in the shallowest of the Brent Group reservoirs and secondary porosity shows no significant increase with depth. Petrographic analyses show a strong depletion of K-feldspar with depth. This can be explained by albitization of K-feldspar at intermediate burial depths (2.5–3.5 km) and dissolution of K-feldspar during illitization of kaolinite on deeper burial (below 3.7–4.0 km). Extensive quartz cementation and formation of illite are the main causes of an observed rapid permeability reduction below 4 km burial depth. K-Ar dates of authigenic illite indicate that illite formed mostly in Early Tertiary at rather shallow burial depth. Illite formed by alteration of kaolinite and K-feldspar seems, however, to increase very significantly at present burial depths of 3.5–4.0 km, suggesting a more recent formation. Emplacement of oil probably does not stop all diagenetic reactions in the remaining residual pore water, and precipitation of illite and quartz may continue in a closed system.

Low-temperature opal-CT precipitation in Antarctic deep-sea sediments: evidence from oxygen isotopes

Porcellanite has been found in Antarctic deep-sea sediments of shallow burial depth at four different sites in host sediments younger than 4 Ma. Oxygen isotope analysis shows that the opal-CT samples are extremely rich in 18O ( δ 18O= 41.2to44.7‰ rel. SMOW). According to the quartz/water fractionation the calculated isotopic opal-CT formation temperature is in the range of 0 to 4°C. This agrees well with sediment temperature measurements. The low opal-CT precipitation temperature contrasts with current ideas about later diagenetic formation of opal-CT at higher temperatures of 18 to 56°C.

The concretions of the Bearreraig Sandstone Formation: geometry and geochemistry

ABSTRACTThe sandbodies of the Bearreraig Sandstone Formation (Inner Hebrides, UK) are cemented by two generations of calcite. The first generation, an inhomogeneous ferroan calcite (0.05−3.28 mol% FeCo3) formed during sulphate reduction (δ13C =−24 to −32%o PDB) in marine porewaters (δ18O of cement from −1 to −4%o PDB) at very shallow burial depths (a few centimetres). These cements are rare but form millimetre‐scale clusters of crystals which acted as nuclei to the later, concretionary cements.The second generation of cements are more homogeneous ferroan calcites (mean 1−58% mol% FeCo3) which evolve to progressively higher Fe/Mg ratios. They are sourced by shell dissolution (δ13C of cement from +1 to −3%o PDB) into meteoric (δ18O of cement from −6 to −10%o PDB) or mixed marine meteoric waters (δ18O of cement from −4 to −6%o SMOW). These were introduced into the formation either during Bathonian times as a freshwater lens, or, subsequent to partial inversion, by confined aquifer flow. Corroded feldspars within the concretions suggest that an interval of at least 8 Ma separated the deposition of the sediments from the onset of concretion growth.Abundant concretions are preferentially developed at certain horizons within the sandbodies, where the early generation of ferroan calcite cements provided nuclei. The latter formed close to the sediment‐water interface, the concentration of cement within the sediment being related to sedimentation rate. The relatively high concentrations of the first generation of cement, upon which the concretionary horizons are nucleated, formed during periods of minimal sedimentation.

Relationship between petroleum generation, migration and sandstone diagenesis, Middle Jurassic, Gifhorn Trough, N Germany

The Gifhorn Trough in North Germany contains an interbedded succession of Lower to Middle Jurassic source and reservoir rocks. Regional variations in the burial history made it possible to investigate different stages in the maturation history of source rocks and the diagenetic history of reservoir rocks. This study focuses in particular on the temporal and spatial relation between secondary petroleum migration and diagenetic events in the reservoir sandstones. Reconstruction of the burial and geothermal history indicate the initiation of petroleum generation at the beginning of the Upper Cretaceous. Lower Toarcian shales have been identified as the common source rock of all oils, based on biomarker and stable isotope studies. Differences in the composition and maturity of reservoired oils are interpreted to be related to separated oil kitchens of different maximum source rock maturity, rather than in-reservoir alteration after migration and accumulation. Major diagenetic events in the shallow marine, arenitic reservoir sandstones are linked to various processes operating during burial: (a) Fe-calcite cement formation — isotope analyses indicate that this is facies-related from carbonate redistributed at shallow burial depth (800–1200 m); (b) Fe-dolomite — dolomitization of early formed Fe-calcite is discussed based on pore water and oxygen isotopic composition; (c) late kaolinite — two models are discussed to explain the observed simultaneous formation of kaolinite and the creation of minor amounts of secondary porosity; (d) late quartz overgrowths — diffusive silica transport from quartz grain dissolution in shales into adjacent sandstones is proposed as a formation mechanism. Geochemical analyses of carbonate-cemented intervals suggest that early generated condensate was trapped during carbonate precipitation, prior to the main phase of petroleum migration. These early formed carbonate-cemented horizons probably compartmentalized the reservoir, controlled petroleum migration routes and promoted the formation of diagenetic traps.

Remanence in authigenic magnetite: Testing the hydrocarbon‐magnetite hypothesis

Paleomagnetic, rock magnetic, petrographic, and geochemical studies of hydrocarbon‐saturated speleothems in southern Oklahoma indicate there is a relation between hydrocarbons and a chemical magnetization that resides in magnetite. The speleothems, which are composed of light and dark calcite bands, occur in caves of karst origin in the Ordovician Kindblade Formation. Vertebrate fossils interbedded with the deposits indicate they are Permian in age. The dark bands contain primary hydrocarbon‐filled fluid inclusions. The dark calcites possess over an order of magnitude stronger natural remanent magnetization than the lighter bands which do not yield stable decay during demagnetization. Alternating field (AF) and thermal demagnetization of specimens of the dark bands reveal a Permian direction of magnetization (declination = 160, inclination = 3, k = 17, α95 = 5, n = 56). The results of rock magnetic experiments, and the fact that most maximum unblocking temperatures are below 580°C, suggest that the dominant component resides in magnetite. In some specimens stable decay to 640°C suggests the presence of a weak component residing in hematite. The presence of authigenic magnetite spheres in magnetic extracts of the dark calcites supports a chemical origin for the magnetization. Shallow burial depths probably eliminate the possibility of a thermoviscous magnetization. The occurrence of primary hydrocarbon inclusions suggests that hydrocarbons seeped into the caves during precipitation of the speleothems and were trapped in the calcite crystals. The relationship between intensity of magnetization and hydrocarbon abundance leads us to propose that chemical conditions created by the hydrocarbons caused precipitation of authigenic magnetite and acquisition of the associated chemical remanence.

Diagenesis and reservoir potential of volcanogenic sandstones—Cretaceous of the Surat Basin, Australia

The Mesozoic Surat Basin is an important hydrocarbon-producing basin in Australia. The principal productive reservoirs there are the quartzose sandstones of Early Jurassic age. Although oil and gas shows have been reported from the Cretaceous part of the succession it does not contain any known economic hydrocarbons. The sandstones of the Cretaceous section are characterized by abundant volcanogenic detritus in the form of rock fragments and feldspars. These compositionally immature sandstones are not regarded as favourable exploration targets for hydrocarbons because of their lithic nature, their shallow burial depths, and hence the low thermal maturity of the intercalated mudrocks which might have constituted hydrocarbon source rocks. However, petrographic and petrophysical examinations show that significant primary and diagenetic secondary dissolution porosity and permeability exist in some of these stratigraphic units which under certain circumstances could be the host for hydrocarbons and may become the future exploration targets. Shallow burial depth and early interstitial precipitation of various authigenic minerals have retarded compaction and thereby contributed to the preservation of moderate to good primary porosity. Additionally, flushing by CO 2-charged meteoric water after the inception of the Great Artesian Basin in the Tertiary has created significant amounts of secondary dissolution porosity in these sandstones.

Sand burial effects on seed germination, seedling emergence and establishment of Panicum virgatum

Studies in the field and in a greenhouse were conducted to examine the effects of sand burial on seed germination, seedling emergence and establishment of Panicum virgatum L. on the foredunes of Lake Erie. Under natural conditions, the seedlings emerged from sand burial depths ranging from 0 to 11 cm, with a mean ± SD of 4.73 ± 1.82 cm. The frequency distribution of the depth of emergence of seedlings in the field was significantly skewed to the right and platykurtic. In the greenhouse, some seedlings emerged from a burial depth of as much as 16 cm. Although percent germination of seeds was not affected by sand burial, the percent emergence and the rate of emergence of seedlings were significantly reduced by excessive sand burial. Seedling mortality was found only among seedlings that emerged from sand burial depths of 10 cm or more. In the field, all the seedlings established in one growing season had originally emerged from sand burial depths of less than 12 cm. Within this burial range, seedlings from shallower burial depths had lower chances of establishment than expected, whereas those from deeper burial depths had higher probabilities of establishment than expected.