Greater Burial Depths Research Articles

Submarine weathering of silicate minerals and the extent of pore water freshening at active continental margins

In order to investigate how submarine weathering processes may affect the water balance of sediments at convergent plate margins, six sediment cores were retrieved off Central Chile at water depth between ∼800 and 4000m. The sediment solid phase was analyzed for its major element composition and the pore fluids were analyzed for dissolved sulfate, sulfide, total alkalinity, major cations, chloride, bromide, iodide, hydrocarbons as well as the carbon isotopic composition of methane.Because of negligible weathering on land, surface sediments off Central Chile are rich in reactive silicate minerals and have a bulk composition similar to volcanic rocks in the adjacent Andes. Deep-sourced fluxes of alkalinity, cations and chloride indicate that silicate minerals are subject to weathering in the forearc during burial. Comparison of deep-sourced signals with data from nearby Ocean Drilling Program Sites reveals two different types of weathering processes: In shallow (tens of meters), methanic sediments of slope basins with high organic carbon burial rates, reactive silicate minerals undergo incongruent dissolution through reaction with CO2 from methanogenesis. At greater burial depth (hundreds of meters), silicate weathering is dominated by authigenic smectite formation. This process is accompanied by uptake of water into the clay interlayers thus leading to elevated salinities in the surrounding pore water. Deep-seated smectite formation is more widespread than shallow silicate dissolution, as it is independent from the availability of CO2 from methanogenesis. Although solute transport is not focused enough to form cold seeps in the proper sense, tectonically induced, diffuse fluid flow transfers the deep-seated signal of smectite formation into the shallow sediments.The temperature-controlled conversion of smectite to illite is considered the most important dehydration process in marine forearc environments (depth of kilometers). However, in agreement with other studies at active margins (e.g. Aleutians, Cascadia, Nankai Trough) and despite ubiquitous evidence for smectite formation, little evidence for seafloor seepage of dehydration fluids could be found off Central Chile. We argue that the circular process of pore water uptake during smectite formation and release upon illitization implies a balanced freshwater budget and therefore a rather limited potential for net pore water freshening on a margin-wide scale. According to this rationale, pore water freshening at seafloor seeps preferentially occurs at lower latitudes (Central America, Barbados, Mediterranean Ridge) where terrestrial weathering is more intense thus leading to external (i.e. detrital) smectite and thus freshwater inputs to the subduction system.

Ion microprobe analyses of δ18O in early quartz cements from 1.9 Ga granular iron formations (GIFs): A pilot study

The low δ18O values of Precambrian cherts have been widely used to infer that temperatures were higher and/or seawater δ18O was lower compared to today's oceans. However, the Precambrian cherts presented as evidence for these temperatures are neomorphosed from amorphous precursors that originally precipitated from ocean water, suggesting diagenetic alteration is an important cause of the widespread low-δ18O values. Many pores between sand-size clasts in granular iron formations (GIFs) are filled with coarsely crystalline quartz cements that are texturally primary. In unmetamorphosed GIF samples from the 1.9Ga Gunflint and Sokoman iron formations, synsedimentary clasts of GIF and high minus-cement porosities (the volume of pore space occupied by recognizable cement, inversely proportional to the degree of compaction) indicate such cements precipitated directly from pore water near the depositional interface. Growth bands identified in quartz cement crystals using cathodoluminescence (CL) imaging further confirm their void-filling and unrecrystallized nature. In order to test the effect of neomorphism on the δ18O of silica in these rocks, unrecrystallized primary quartz cements and neomorphic quartz in adjacent clasts were both analyzed by ion microprobe. Values of δ18O range from 23.5 to 26.4‰ VSMOW in cements and 21.3–26.8‰ in clasts in samples from the Gunflint Iron Formation. Samples from the Sokomon Iron Formation record deeper, more evolved pore waters and in some cases contain lower δ18O cements. In individual pores of Gunflint samples, cement values vary only slightly more than analytical uncertainty from edge to center and no consistent increasing or decreasing trends were observed. Overall, the δ18O of the cements are similar to the most elevated δ18O reported from cherts of similar age, but span a substantially smaller range. This suggests most of the lower values reported previously reflect late-diagenetic conditions at greater burial depths. The shallow, possibly restricted seas above the Gunflint and Sokoman GIFs appear to have been warmer and may have been lower in δ18O than present-day seawater, but they may not have been representative of global seawater.

Open-system chemical behaviour of Wilcox Group mudstones. How is large scale mass transfer at great burial depth in sedimentary basins possible? A discussion

Open-system chemical behaviour of Wilcox Group mudstones. How is large scale mass transfer at great burial depth in sedimentary basins possible? A discussion

Dynamics of cementation in response to oil charge: Evidence from a Cretaceous carbonate field, U.A.E.

Oil charge is thought to inhibit the growth of cements within subsurface pore systems. We explore this phenomenon in a giant Cretaceous carbonate field from U.A.E., where the oil-filled crest porosity ranges from 10 to 50% and permeability from 0.08 to 830 mD but coeval water leg porosity is reduced to 10 to 23% and permeability to 0.1 to 4 mD. Only 5% of primary interparticle pores (> 30 μm diameter) in the crest are fully cemented, compared to 99% of pores in the water leg. Syntaxial calcite burial cements (> 10 μm diameter) in the oil leg show 12 cathodoluminescence zones with oil inclusions ( n = 27) occurring in four of the five final zones. Mean in-situ ion microprobe δ 18O VPDB data from the oil leg cements range from −1.2‰ in the oldest zone decreasing to −10.3‰ in zone 11, returning to −7.7‰ in the final zone. The oldest distinguishable cement zone in the water leg shows highly variable δ 18O from −3.6‰ to −9.3‰ with a mean of −7.3‰, and with subsequent zones decreasing to a mean value of −9.4‰ for the youngest cement zone. Decreasing δ 18O values are interpreted as indicating increasing temperature reflecting burial and the evolution of pore water composition: broadly similar trends in the oil and water legs suggest precipitation under the same general conditions. Unlike the oil leg cements, the final zone in the water leg occludes nearly all remaining pore space. The δ 18O VPDB of bulk micrite from the water leg shows an average of −7.4‰ ( n = 9) compared to −6.2‰ ( n = 10) from the oil leg, suggesting the precipitation of further micrite cement at greater burial depths. We infer that burial cementation slowed in the presence of oil due to a reduction of potential nucleation sites as well as porewater and solute movement within weakly oil-wet pores, whereas continued flow and solute movement through all pores including the micropores (< 10 μm diameter) enabled extensive cementation in the water leg.

Evaluation of the regional top seal in the Gippsland Basin: implications for geological carbon storage and hydrocarbon prospectivity

GeoScience Victoria and partners have undertaken the first detailed basin-wide study of the regional top seal in the Gippsland Basin. The Gippsland Basin is an attractive site for geological carbon storage (GCS) because of the close proximity to emission sources and the potential for large-scale storage projects. This top seal assessment involved the analysis of seal attributes (geometry, capacity and mineralogy) and empirical evidence for seal failure (soil gas geochemical anomalies, gas chimneys, hydrocarbon seepage and oil slicks). These datasets have been integrated to produce a qualitative evaluation of the containment potential for GCS, and also hydrocarbons, across the basin. Mineralogical analysis of the top seal has revealed that the Lakes Entrance Formation is principally a smectite-rich claystone. The geometry of the top seal is consistent with deposition in an early post-rift setting where marine sediments filled palaeo-topographic lows. The seal thickness and depth to seal base are greatest in the Central Deep and decrease toward the margins. There is a strong positive relationship between seal capacity column heights, seal thickness, depth to seal base and smectite content. At greater burial depths (below 700 m) and where smectite content is greater than 70%, seal capacity is increased (supportable column heights above 150 m). Natural hydrocarbon leakage and seepage onshore and offshore is correlated with fault distribution and areas of poor seal capacity. This study provides a framework for qualitatively evaluating seal potential at a basin scale. It has shown that the potential of the regional top seal over the Central Deep, Southern Terrace, central eastern Lake Wellington Depression and the southern to central near shore areas in the Seaspray Depression are most suitable for the containment of supercritical CO2. Further toward the margin of the regional seal in both onshore and offshore areas, containment of supercritical CO2 is less likely.

Structural style of the White Pointer and Hammerhead Delta—deepwater fold-thrust belts, Bight Basin, Australia

GeoScience Victoria and partners have undertaken the first detailed basin-wide study of the regional top seal in the Gippsland Basin. The Gippsland Basin is an attractive site for geological carbon storage (GCS) because of the close proximity to emission sources and the potential for large-scale storage projects. This top seal assessment involved the analysis of seal attributes (geometry, capacity and mineralogy) and empirical evidence for seal failure (soil gas geochemical anomalies, gas chimneys, hydrocarbon seepage and oil slicks). These datasets have been integrated to produce a qualitative evaluation of the containment potential for GCS, and also hydrocarbons, across the basin. Mineralogical analysis of the top seal has revealed that the Lakes Entrance Formation is principally a smectite-rich claystone. The geometry of the top seal is consistent with deposition in an early post-rift setting where marine sediments filled palaeo-topographic lows. The seal thickness and depth to seal base are greatest in the Central Deep and decrease toward the margins. There is a strong positive relationship between seal capacity column heights, seal thickness, depth to seal base and smectite content. At greater burial depths (below 700 m) and where smectite content is greater than 70%, seal capacity is increased (supportable column heights above 150 m). Natural hydrocarbon leakage and seepage onshore and offshore is correlated with fault distribution and areas of poor seal capacity. This study provides a framework for qualitatively evaluating seal potential at a basin scale. It has shown that the potential of the regional top seal over the Central Deep, Southern Terrace, central eastern Lake Wellington Depression and the southern to central near shore areas in the Seaspray Depression are most suitable for the containment of supercritical CO2. Further toward the margin of the regional seal in both onshore and offshore areas, containment of supercritical CO2 is less likely.

Porosity-preserving chlorite cements in shallow-marine volcaniclastic sandstones: Evidence from Cretaceous sandstones of the Sawan gas field, Pakistan

Sandstones that have high porosity and permeability at great burial depth and high temperatures are of economic importance because a significant amount of hydrocarbons have been discovered in such reservoirs. The Sawan gas field, with an expected ultimate recovery of more than 1 tcf, lies in the middle Indus Basin. The reservoir rocks, Cretaceous volcaniclastic sandstones of the lower Goru Formation, show very high porosities at a reservoir temperature of 175C and depths of 3000 to 3500 m (9842 to 11,483 ft). The sandstones are mostly feldspathic litharenites. Strongly altered volcanic rock fragments are the most important lithic component. The clay fraction consists of Fe-rich chlorite (chamosite) and illite. Diagenetic features such as compaction, quartz overgrowths, carbonate cements, and feldspar dissolution are observed. The most distinguishing feature is a double layer of authigenic chlorite, lining the pores of the sandstones. Chlorite additionally occurs as a pore-filling cement and as chloritized detrital components, all having similar chemical composition. The pore-lining cement clearly developed in two stages: an earlier, poorly crystallized, and a later, better crystallized growth. Missing rims at grain contacts show that precipitation occurred after an initial stage of compaction but early relative to other diagenetic phases. Both chlorite rims grew by direct precipitation from pore waters, using products derived from volcanic rock fragments. In areas with no, thin, or discontinuous chlorite rims, quartz cementation is common. Well-developed chlorite rims inhibited quartz cementation, preserved porosities of up to 20%, and good permeabilities. Porosity-preserving chlorite cementation in Sawan is restricted to sediments of a shallow-marine environment.

Physical properties of Cenozoic mudstones from the northern North Sea: Impact of clay mineralogy on compaction trends

Vertical and lateral changes in physical properties in Cenozoic mudstones from the northern North Sea Basin reflect differences in the primary mineralogical composition and burial history, which provides information about sedimentary facies and provenance. Integration of well-log data with mineralogical information shows the effect of varying clay mineralogy on compaction curves in mudstones. The main controlling factor for the compaction of Eocene to early Miocene mudstones within the North Sea is the smectite content, which is derived from volcanic sources located northwest of the North Sea. Mudstones with high smectite content are characterized by low P-wave velocities and bulk densities compared to mudstones with other clay mineral assemblages at the same burial depths. Smectitic clays are important during mechanical compaction because they are less compressible than other types of clay minerals. A comparison between well-log data and experimental work also shows that smectite may be a controlling factor for overpressure generation in the smectite-rich Eocene and Oligocene sediments. At greater burial depths and temperatures (70–80C), the dissolution of smectite and precipitation of illite and quartz significantly increases velocities and densities. Miocene and younger mudstones from the northern North Sea have generally low smectite contents and as a result have higher velocities and densities than Eocene and Oligocene mudstones. Lateral differences in the compaction trends between the north and south for these sediments also exist, which may be related to two different source areas in the Pliocene. The log-derived petrophysical data from the northern North Sea Basin show that mudstone lithologies have very different compaction trends depending on the primary composition. Simplified compaction curves may therefore affect the outcomes from basin modeling. The amplitude-versus-offset response of hydrocarbon sands and the seismic signature on seismic sections are also dependent on the petrophysical properties of mudstones and will vary depending on the mineralogical composition.

Martian high latitude permafrost depth and surface cover thermal inertia distributions

Martian high latitudes have thermal properties consistent with an extensive high thermal inertia permafrost layer near the surface. Surface cover thermal inertias and permafrost depths at Martian high latitudes (50°–80°N/S) are derived from Thermal Emission Spectrometer (TES) data and compared with previously published water ice depths determined from the Mars Odyssey Neutron Spectrometer (MONS). The depth to the permafrost layer is correlated with surface cover thermal inertia, albedo, and latitude in general agreement with predicted trends of water ice stability. Comparison of permafrost depths with water ice rich layer depths derived from MONS data displays good qualitative agreement, although a divergence is present at greater burial depths. This disparity may be due to the presence of hydrated minerals at shallow depths or a lower than expected permafrost thermal inertia corresponding with low water ice concentrations at greater depths. Surface cover thermal inertias are greater in the northern high latitudes than in the south and differences between these and previous results will have significant effects on the predicted depth of Martian water ice stability. Several regions in the northern hemisphere display high surface cover thermal inertia associated with possible receding water ice deposits that are shallow enough to influence diurnal surface temperatures. Significant lateral and vertical heterogeneity in water ice distributions are present and the Martian regolith is likely more complicated than can be described by simple two layered models and a single mode of water ice emplacement.

A simplified approach for rockfall ground penetration and impact stress calculations

This paper presents a simple procedure for calculating rockfall penetration and ground impact stress for design of pipeline burial depth. Rockfall may damage pipelines by penetrating through cover soils and damaging the pipeline by puncture or excessive impact stress. Design engineers may face a challenging question regarding what burial depth is required for adequate pipe protection. Greater burial depth may provide better protection; however, there are usually site and economic constraints. This paper discusses a simple procedure for calculating the maximum rock penetration and ground impact stress due to rockfall. The procedure may be used to estimate the minimum burial depth required to protect pipelines from rockfall damage. A case study involving a large boulder that fell down a slope and landed on a buried high-pressure gas pipeline is presented to demonstrate and verify the procedure. A calculation was performed before the removal of the boulder to evaluate whether any damage was made to the pipeline. The calculated results agreed well with the actual conditions observed after the boulder was removed.

Thermal history of the Rio Muni (West Africa)–NE Brazil margins during continental breakup

We document the thermal record of breakup of the conjugate Rio Muni (West Africa) and NE Brazil margins using apatite fission track analysis, vitrinite reflectance data and stratigraphic observations from both margins. These results permit determination of the timing of four cooling episodes, and the temperature of samples at the onset of each episode. All samples are interpreted to have experienced higher temperatures in the geological past due to i) elevated basal heatflow (palaeogeothermal gradient in Rio Muni-1 well decaying from 58 °C/km during the Mid Cretaceous to 21.5 °C/km in the Late Cenozoic) and ii) progressive exhumation from formerly greater burial depth. A well constrained history of changing palaeogeothermal gradient allows for much more precise quantification of the thickness of eroded section (exhumation) than if a constant heatflow is assumed. Cooling episodes identified from the palaeotemperature data at 110–95 Ma (both margins) and 85–70 Ma (Rio Muni only) coincide with major unconformities signifying, respectively, the cessation of rifting (breakup) and compressional shortening that affected the African continent following the establishment of post-rift sedimentation (drift). The interval between these separate unconformities is occupied by allochthonous rafts of shallow-water carbonates recording gravitational collapse of a marginal platform. The rift shoulder uplift that triggered this collapse was enhanced by local transpression associated with the obliquely divergent Ascension Fracture Zone, and thermal doming due to the coeval St Helena and Ascension Plumes. The data also reveal a c.45–35 Ma cooling episode, attributed to deep sea erosion at the onset of Eo-Oligocene ice growth, and a c.15–10 Ma episode interpreted as the record of Miocene exhumation of the West African continental margin related to continent-wide plume development. Integration of thermal history methods with traditional seismic- and stratigraphy-based observations yields a dynamic picture of kilometre-scale fluctuations in base level through the breakup and early drift phases of development of these margins. Major unconformities at ocean margins are likely to represent composite surfaces recording not only eustasy, but also regional plate margin-generated deformation, local ‘intra-basinal’ reorganization, and the amplifying effect of negative feedbacks between these processes.

A Gel Probe Equilibrium Sampler for Measuring Arsenic Porewater Profiles and Sorption Gradients in Sediments: II. Field Application to Haiwee Reservoir Sediment

Arsenic (As) geochemistry and sorption behavior were measured in As- and iron (Fe)-rich sediments of Haiwee Reservoir by deploying undoped (clear) polyacrylamide gels and hydrous ferric oxide (HFO)-doped gels in a gel probe equilibrium sampler, which is a novel technique for directly measuring the effects of porewater composition on As adsorption to Fe oxides phases in situ. Arsenic is deposited atthe sedimentsurface as As(V) and is reduced to As(III) in the upper layers of the sediment (0-8 cm), but the reduction of As(V) does not cause mobilization into the porewater. Dissolved As and Fe concentrations increased at depth in the sediment column driven by the reductive dissolution of amorphous Fe(III) oxyhydroxides and conversion to a mixed Fe(II, III) green rust-type phase. Adsorption of As and phosphorous (P) onto HFO-doped gels was inhibited at intermediate depths (10-20 cm), possibly due to dissolved organic or inorganic carbon, indicating that dissolved As concentrations were at least partially controlled by porewater composition rather than surface site availability. In sediments that had been recently exposed to air, the region of sorption inhibition was not observed, suggesting that prior exposure to air affected the extent of reductive dissolution, porewater chemistry, and As adsorption behavior. Arsenic adsorption onto the HFO-doped gels increased at depths >20 cm, and the extent of adsorption was most likely controlled by the competitive effects of dissolved phosphate. Sediment As adsorption capacity appeared to be controlled by changes in porewater composition and competitive effects at shallower depths, and by reductive dissolution and availability of sorption sites at greater burial depths.

Dolomite formation in the dynamic deep biosphere: results from the Peru Margin

AbstractEarly diagenetic dolomite beds were sampled during the Ocean Drilling Programme (ODP) Leg 201 at four reoccupied ODP Leg 112 sites on the Peru continental margin (Sites 1227/684, 1228/680, 1229/681 and 1230/685) and analysed for petrography, mineralogy, δ13C, δ18O and 87Sr/86Sr values. The results are compared with the chemistry, and δ13C and 87Sr/86Sr values of the associated porewater. Petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate. Dolomite precipitation often pre‐dates the formation of framboidal pyrite. Most dolomite layers show 87Sr/86Sr‐ratios similar to the composition of Quaternary seawater and do not indicate a contribution from the hypersaline brine, which is present at a greater burial depth. Also, the δ13C values of the dolomite are not in equilibrium with the δ13C values of the dissolved inorganic carbon in the associated modern porewater. Both petrography and 87Sr/86Sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment (&lt;30 m below the seafloor). A significant depletion in the dissolved Mg and Ca in the porewater constrains the present site of dolomite precipitation, which co‐occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate–methane interface. It has been hypothesized that microbial ‘hot‐spots’, such as the sulphate–methane interface, may act as focused sites of dolomite precipitation. Varying δ13C values from −15‰ to +15‰ for the dolomite are consistent with precipitation at a dynamic sulphate–methane interface, where δ13C of the dissolved inorganic carbon would likewise be variable. A dynamic deep biosphere with upward and downward migration of the sulphate–methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter. Thus, the study of dolomite layers in ancient organic carbon‐rich sedimentary sequences can provide a useful window into the palaeo‐dynamics of the deep biosphere.

Geochemistry and episodic accumulation of natural gases from the Ledong gas field in the Yinggehai Basin, offshore South China Sea

The Ledong gas field, consisting of three gas pools in a shale diapir structure zone, is the largest gas discovery in the Yinggehai Basin. The gases produced from the Pliocene and Quaternary marine sandstone reservoirs show a considerable variation in chemical composition, with 5.4–88% CH 4, 0–93% CO 2, and 1–23.7% N 2. The CO 2-enriched gases often display heavier methane δ 13C values than those with low CO 2 contents. The δ 15N values of the gases range from −8 to −2‰, and the N 2 content correlates negatively with the CO 2 content. The high geothermal gradient associated with a relatively great burial depth in this area has led to the generation of hydrocarbon and nitrogen gases from the Lower–Middle Miocene source rocks and the formation of abundant CO 2 from the Tertiary calcareous-shales and pre-Tertiary carbonates. The compositional heterogeneities and stable carbon isotope data of the produced gases indicate that the formation of the LD221 gas field is attributed to three phases of gas migration: initially biogenic gas, followed by thermogenic hydrocarbon gas, and then CO 2-rich gas. The filling processes occurred within a short period approximately from 1.2 to 0.1 Ma based on the results of the kinetics modeling. Geophysical and geochemical data show that the diapiric faults that cut through Miocene sediments act as the main pathways for upward gas migration from the deep overpressured system into the shallow normal pressure reservoirs, and that the deep overpressure is the main driving force for vertical and lateral migration of the gases. This gas migration pattern implies that the transitional pressure zone around the shale diapir structures was on the pathway of upward migrating gases, and is also a favorable place for gas accumulation. The proposed multiple sources and multiple phases of gas migration and accumulation model for the Ledong gas field potentially provide useful information for the future exploration efforts in this area.

Role of within-individual variation in capitulum size and achene mass in the adaptation of the annual Centaurea eriophora to varying water supply in a Mediterranean environment.

To clarify the adaptive value of variation in capitulum size and achene mass, plants of Centaurea eriophora were studied in a glasshouse and in a natural population. C. eriophora plants consist of a basal leaf rosette from which an erect stem grows, with lateral branches of various orders ending in capitula of different orders. Primary, secondary and tertiary capitula are comparable in size and they produce similar numbers of achenes, which are similar in weight (large achenes). These capitula are formed during April, May and early June, and constitute the normal or primary flowering. Following ripening of tertiary capitula, leaves senesce, but, later during June and the first half of July, a secondary flowering of a variable number of smaller capitula may occur if wet conditions persist for longer than usual. Plants that have almost senesced develop small lateral branches 1-2 cm long bearing a few small leaves and ending in a capitulum about half the diameter of capitula from the primary flowering period. The number of achenes produced in these capitula (small achenes) and their weight are 70 and 30% less, respectively, than those of capitula formed during primary flowering. These reductions appear to result from restricted availability of resources. Large and small achenes have similar dispersal characteristics and possess similar germination potential. However, large achenes produce seedlings that are capable of emerging from greater burial depths, providing the resulting plants with a potential advantage. The normal flowering period coincides with the optimum time of year for flowering and fruiting in the south of Spain, and only if rainfall lasts longer than usual does secondary flowering occur. Secondary flowering extends the normal flowering and fruiting periods, thereby providing a supplementary crop of smaller, yet viable, fruits. It can be considered to be an adaptive response to the unpredictable Mediterranean climate, optimizing the use of available resources.

Thermal maturation pattern in the southern Bowen, northern Gunnedah and Surat Basins, northern New South Wales, Australia

Comprehensive maximum vitrinite (telocollinite) reflectance data have been obtained for more than 260 polished sections from 28 petroleum exploration wells in the southern Bowen and northern Gunnedah Basins, and the overlying Surat Basin, in an area from north of Boggabri to the New South Wales–Queensland border. The samples studied were coal and dispersed organic matter (DOM) taken from ditch cuttings and drill cores of the Permian, Triassic and Jurassic sedimentary successions. Vitrinite reflectance profiles show that the coals and DOM in the Early Permian Back Creek Group of the southern Bowen Basin have anomalously low (suppressed) reflectance values and a higher reflectance gradient (rate of increase in vitrinite reflectance with depth) relative to the overlying Late Permian, Triassic and Jurassic sequences. Suppression of vitrinite reflectance is recorded in the Early Permian Maules Creek and Goonbri Formations in the northernmost part of the Gunnedah Basin, relative to the overlying Triassic and Jurassic sequences. Dispersed organic matter in the marine sediments of the Watermark and Porcupine Formations of the Gunnedah Basin also shows suppression of vitrinite reflectance. The vitrinite in several wells of the study area, however, displays anomalously high reflectance in parts of the sequence due to the influence of igneous intrusions. After allowing for heat effects due to intrusions and anomalies due to marine influence, the reflectance gradient over equivalent intervals in the southern Bowen Basin is higher towards the west, where the Permian sequence pinches out and the Triassic sequence overlies the basement. Vitrinite reflectance also increases at a higher rate with depth close to the Gil Gil Ridge in the east and close to the Moree High in the south. This may reflect additional heat flow associated with the basement high features. The areas between the Gil Gil Ridge and the Goondiwindi Thrust in the east, and to the west of the Gil Gil Ridge, appear to have lower reflectance gradients, and equivalent or lower reflectance values despite the greater burial depths.

Assessing the Validity of Seismo-Acoustic Predictor Equations for Obtaining Seabed and Sub-Surface Sediment Physical Properties

The interdependence between the seismo-acoustic properties of a marine sediment and its geotechnical/physical parameters has been known for many years, and it has been postulated that this should allow the extraction of geotechnical information from seismic data. Though in the literature many correlations have been published for the surficial layer, there is a lack of information for greater sediment depths. In this article, a desktop study on a synthetic seafloor model illustrates how the application of published near-surface prediction equations to subsurface sediments (up to several tens of meters burial depth) can lead to spurious predictions. To test this further, acoustic and geotechnical properties were measured on a number of sediment core samples, some of which were subjected to loading in acoustically-equipped consolidation cells (oedometers) to simulate greater burial depth conditions. For low effective pressures (representing small burial depths extending to around 10 meters subsurface), the general applicability of established relationships was confirmed: the prediction of porosity, bulk density, and mean grain size from acoustic velocity and impedance appears generally possible for the investigated sedimentary environments. As effective pressure increases through, the observed relationships deviate more and more from the established ones for the near-surface area. For the samples tested in this study, in some instances increasing pressure even resulted in decreasing velocities. There are several possible explanations for this abnormal behavior, including the presence of gas, overconsolidation, or bimodal grain size distribution. The results indicate that an appropriate depth correction must be introduced into the published prediction equations in order to obtain reliable estimates of physical sediment properties for greater subsurface depths.

Dolomite: occurrence, evolution and economically important associations

Dolomite is not a simple mineral; it can form as a primary precipitate, a diagenetic replacement, or as a hydrothermal/metamorphic phase, all that it requires is permeability, a mechanism that facilitates fluid flow, and a sufficient supply of magnesium. Dolomite can form in lakes, on or beneath the shallow seafloor, in zones of brine reflux, and in early to late burial settings. It may form from seawater, from continental waters, from the mixing of basinal brines, the mixing of hypersaline brine with seawater, or the mixing of seawater with meteoric water, or via the cooling of basinal brines. Bacterial metabolism may aid the process of precipitation in settings where sulfate-reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings. Dolomite is a metastable mineral, early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism. Each new phase is formed by the partial or complete dissolution of an earlier dolomite. This continual re-equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures. Because subsurface dolomite evolves via dissolution and reprecipitation, a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart. Dolomitization also creates new crystals, with new rhomb growth following the dissolution of less stable precursors. Repetition of this process, without complete pore cementation, can generate intercrystalline porosity a number of times in the rock's burial history. Intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage. The fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings. The same association of evaporites (sulfate source) and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals. This tends to occur at higher temperatures (>60°C–80°C) and so the resulting dolomites tend to be ferroan and consist of saddle-shaped crystals.

Diagenesis of the Purington Shale in the Illinois Basin and Implications for the Diagenetic State of Sedimentary Rocks of Shallow Paleozoic Basins

Abstract The clay minerals, micas, and feldspars of the Pennsylvanian‐age Purington Shale have been more diagenetically active than generally recognized. They have undergone diagenetic changes comparable to those of Cenozoic and Mesozoic sedimentary rocks buried three to four times as deeply and heated to approximately twice the temperature. The Purington Shale on the Western Shelf of the Illinois Basin has experienced maximum burial of ≤1.0 km and was never heated to more than 60°C, except for a very brief time at about 80°C. The illite/smectite (I/S) of the Purington Shale has ≥90% illite. The chemical compositions of the feldspar assemblage in the 0.09–0.063‐mm fraction differ from most modern detrital suites of feldspars but are similar to diagenetic sequences described by others from much greater burial depths and are extensively, but not completely, albitized. The simplest interpretation of the apparently advanced stage of diagenesis is that the diagenetic processes have been operating at less press...

A Petroleum System for the Salina Basin in Kansas Based on Organic Geochemistry and Geologic Analog

The Salina Basin historically has been an “exploration desert”—a home of dryholes. Although this basin, which underlies much of north-central Kansas, may never be a prolific source of hydrocarbons, recent research into the maturation and geochemistry of organic matter and oils in Kansas can provide guidelines for a new exploration strategy. The Salina Basin is similar to the oil-productive Forest City Basin in northeastern Kansas in many ways. Both basins originated as a single large basin (i.e., the North Kansas Basin) prior to the rise of the Nemaha Uplift in Late Mississippian-Early Pennsylvanian time. Their Paleozoic stratigraphy thus is similar and the axes of both basins are presently at approximately the same depth. Thermal maturation modeling and available organic-matter maturation data indicate that the lower Paleozoic rocks in the axes of both basins are in the early stages of oil generation. In the Forest City Basin the Ordovician Simpson Group is the deepest known hydrocarbon source-rock—oil-reservoir interval, and by analogy, exploration tests in the Salina Basin, at a minimum, should penetrate through this stratigraphic interval. Ordovician Simpson Group shales in the Forest City Basin are the source rocks for a geochemically distinct oil, which also occurs in Ordovician reservoirs in the extreme southern end of the Salina Basin. To increase the odds of success in an exploration program in the Salina Basin, wildcat wells should be drilled where thermal maturation is greatest. The broad NW–SE-trending basin axis is the most logical area. Exploration tests along this axis in the northern end of the basin may have an extra advantage as organic matter in the Simpson Group may be more thermally mature because of greater burial depth during the Cretaceous. Along the eastern margin of the nearby Central Kansas Uplift and Pratt Anticline, several Paleozoic geologic structures, some of which contain major oil fields, are attributable to tectonic reactivation along the western margin of the Precambrian Central North American Rift System (CNARS). Prospective structural trends in the Paleozoic section of the Salina Basin are anticipated to be associated with this underlying tectonic boundary. The western margin of the CNARS trends NNE–SSW where it passes under the axis of the Salina Basin in northeastern Lincoln and southeastern Mitchell counties. This area is sparsely drilled, with less than two tests per township. If an exploration program can define lower Paleozoic structural closures in this region, these structures may represent the best chance for future petroleum discoveries.