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1.
The aim of the present work is to define the geo-microbial signatures along with trace metals, and to investigate whether the geo-microbial anomalies have correlation with trace metal anomalies in Ahmedabad block of Cambay basin. The surface geochemical techniques are based on seepage of light hydrocarbon gases from the oil and gas pools to the shallow surface and can bring up near-surface oxidation reduction zones that favor the development of a diverse array of chemical and mineralogical changes. The paper reports the role of hydrocarbon microseepage in surface alterations of trace metal concentrations and hydrocarbon oxidizing bacteria and its interrelationship. For the purpose a total of 90 soil samples are collected in grid pattern of 2 x 2 km interval. The paper reports the chemical alterations associated with trace metals in soils that are related to hydrocarbon microseepages above some of the major oil and gas fields of this petroliferous region. The concentrations of V (0 to149 ppm), Cr (2 to 192 ppm), Cu (4 to 171 ppm), Se (98 to 440 ppm), Zn (56 to 1215 ppm) are obtained. It is observed that the concentrations of trace elements are tremendously increased when they are compared with their normal concentrations in soils. In this study the hydrocarbon oxidizing bacterial counts ranged between 1.0 x 103 and 1.59 x 106 cfu/g of soil sample respectively. The attempt has made for the first time, which revealed good correlation as both these anomalies are found as apical in relation. Integrated studies between trace elements and hydrocarbon oxidizing bacterial anomalies showed positive correlation with existing oil and gas wells in the study area.  相似文献   

2.
Direct correlation have been observed between certain trace element and hydrocarbon anomalies in the near subsurface soils of Vindhyan basin, India. This relationship with hydrocarbon is very useful in hydrocarbon exploration. 52 soil samples from Vindhyan basin were collected from a depth of 2.5m. All the soil samples were analyzed for light hydrocarbon, isotope and trace element concentrations. The adsorbed light gaseous hydrocarbon analyses show the presence of methane (8–328 ppb), ethane (0–27 ppb) and propane (0–11 ppb) respectively and these values indicate the presence of hydrocarbon micro-seepage in the study area. The carbon isotopic values determined for methane and ethane for these soil samples are (?26.41 to ?47.70 ‰ PDB) and (?20.07 to ?35.30 ‰ PDB) respectively and they are thermogenic in nature. The trace element concentrations of nickel (33–220 ppm), vanadium (72–226 ppm), copper (20–131 ppm), chromium (94–205 ppm), zinc (66–561 ppm) and cobalt (9–39 ppm) have higher than the normal concentrations in soils. Trace element concentrations are used to plot with the data obtained from light gaseous hydrocarbon concentrations and carbon isotopic values of soil samples of the Vindhyan basin. Trace element anomalies have been observed around the hydrocarbon anomalies in the study area.  相似文献   

3.
The stable carbon isotopic compositions of light hydrocarbon gases adsorbed in near-surface soil and sediments from the Saurashtra basin were characterized for their origin and maturity. Saurashtra is considered geologically prospective for oil and gas reserves; however, a major part of the basin is covered by the Deccan Traps, hindering the exploration of Mesozoic hydrocarbon targets. Surface geochemical prospecting, based on micro-seepage of hydrocarbons from subsurface accumulations, could be advantageous in such areas. In light of this, 150 soil samples were collected from the northwestern part of Saurashtra, around the Jamnagar area, where a thick sedimentary sequence of about 2–3 km exists under 1–1.5 km of Deccan basalt. The concentration of acid desorbed alkane gases from soil samples was found to vary (in ppb) as: methane (C1) = 3–518; ethane (C2) = 0–430; propane (C3) = 0–331; i-butane (iC4) = 0–297; n-butane (nC4) = 2–116; i-pentane (iC5) = 0–31 and n-pentane (nC5) = 0–23, respectively.Fifteen samples with high concentrations of alkane gases were measured for their δ13C1; δ13C2 and δ13C3 compositions using gas chromatography–combustion-isotope ratio mass spectrometry (GC–C-IRMS). The values for methane varied from ? 27 to ? 45.4‰, ethane from ? 20.9 to ? 27.6‰, and propane from ? 20.4 to ? 29.1‰ versus the Vienna PeeDee Belemnite (VPDB). The carbon isotope ratio distribution pattern represents isotopic characteristics pertaining to hydrocarbon gases derived from thermogenic sources. Comparisons of carbon isotopic signatures and compositional variations with the standard carbon isotopic models suggest that hydrocarbon gases found in the shallow depths of the study area are not of bacterial origin but are formed thermally from deeply buried organic matter, likely to be mainly a terrestrial source rock with a partial contribution from a marine source. These gases may have migrated to the near-surface environment, where they represent an admixture of thermally generated hydrocarbon gases from mixed sources and maturity. The maturity scale (δ13C versus Log Ro %) applied to the surface sediment samples of the Jamnagar area indicated the source material to be capable of generating oil and gas. The detection of thermogenic alkane gases in near-surface sediments offers the possibility of hydrocarbons at depth in Saurashtra.  相似文献   

4.
The Deccan Syneclise is considered to have significant hydrocarbon potential.However,significant hydrocarbon discoveries,particularly for Mesozoic sequences,have not been established through conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks.In this study,near-surface geochemical data are used to understand the petroleum system and also investigate type of source for hydrocarbons generation of the study area.Soil samples were collected from favorable areas identified by integrated geophysical studies.The compositional and isotopic signatures of adsorbed gaseous hydrocarbons(methane through butane) were used as surface indicators of petroleum micro-seepages.An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons(C1-C4) and their carbon isotopes from the western part of Tapti graben,Deccan Syneclise,India.The geochemical results reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases.High concentrations of adsorbed thermogenic methane(C_1 = 518 ppb) and ethane plus higher hydrocarbons(ΣC_(2+) = 977 ppb) were observed.Statistical analysis shows that samples from 13% of the samples contain anomalously high concentrations of light hydrocarbons in the soil-gas constituents.This seepage suggests largest magnitude of soil gas anomalies might be generated/source from Mesozoic sedimentary rocks,beneath Deccan Traps.The carbon isotopic composition of methane,ethane and propane ranges are from-22.5‰ to-30.2‰ PDB,-18.0‰to 27.1‰ PDB and 16.9‰-32.1‰ PDB respectively,which are in thermogenic source.Surface soil sample represents the intersection of a migration conduit from the deep subsurface to the surface connected to sub-trappean Mesozoic sedimentary rocks.Prominent hydrocarbon concentrations were associated with dykes,lineaments and presented on thinner basaltic cover in the study area,which probably acts as channel for the micro-seepage of hydrocarbons.  相似文献   

5.
Reconnaissance surface geochemical survey for adsorbed soil gas analysis conducted in Proterozoic Bhima and Kaladgi Basins, have revealed occurrence of anomalous concentrations of light gaseous hydrocarbons i.e. C1 to C4 (CH4, C2H6, C3H8, i-C4H10 and n-C4H10) in the near surface soils. The concentrations of C1 and ΣC2+(C2H6+C3H8+ i-C4H10+ n-C4H10) in Bhima and Kaladgi Basins are in the range of 1–2594 ppb and 1 to 57 ppb and 1–1142 ppb and 1–490 ppb, respectively. The carbon isotopic data of adsorbed soil gas methane in few selected samples are in the range of −29.9 to −39‰ (PDB). The evaluation of adsorbed soil gas data indicates that all the gas components are cogenetic and hydrocarbon ratios of C1/(C2+C3) < 10 and C3/C1*1000 between 60–500 and 20–60 suggest that the adsorbed gases are derived from oil and gas-condensate zones. The carbon isotopic values of methane further support thermogenic origin of these migrated gases. The concentration distribution of C1 and ΣC2+ in the study areas illustrate C1 and ΣC2+ anomalies near Katamadevarhalli, Andola and Talikota in Bhima Basin and near Kaladgi, Lokapur and north of Mudhol in Kaladgi Basin. The hydrocarbon anomalies near the surface coincide with the favourable subsurface structural features and correlate with existing geochemical and geophysical data in these basins suggesting seepage related anomalies.  相似文献   

6.
The organic geochemical methods of hydrocarbon prospecting involve the characterization of sedimentary organic matter in terms of its abundance, source and thermal maturity, which are essential prerequisites for a hydrocarbon source rock. In the present study, evaluation of organic matter in the outcrop shale samples from the Semri and Kaimur Groups of Vindhyan basin was carried out using Rock Eval pyrolysis. Also, the adsorbed low molecular weight hydrocarbons, methane, ethane, propane and butane, were investigated in the near surface soils to infer the generation of hydrocarbons in the Vindhyan basin. The Total Organic Carbon (TOC) content in shales ranges between 0.04% and 1.43%. The S1 (thermally liberated free hydrocarbons) values range between 0.01–0.09 mgHC/gRock (milligram hydrocarbon per gram of rock sample), whereas the S2 (hydrocarbons from cracking of kerogen) show the values between 0.01 and 0.14 mgHC/gRock. Based on the Tmax (temperature at highest yield of S2) and the hydrogen index (HI) correlations, the organic matter is characterized by Type III kerogen. The adsorbed soil gas, CH4 (C1), C2H6 (C2), C3H8 (C3) and nC4H10, (nC4), concentrations measured in the soil samples from the eastern part of Vindhyan basin (Son Valley) vary from 0 to 186 ppb, 0 to 4 ppb, 0 to 5 ppb, and 0 to 1 ppb, respectively. The stable carbon isotope values for the desorbed methane (δ13C1) and ethane (δ13C2) range between −45.7‰ to −25.2‰ and −35.3‰ to −20.19‰ (VPDB), respectively suggesting a thermogenic source for these hydrocarbons. High concentrations of thermogenic hydrocarbons are characteristic of areas around Sagar, Narsinghpur, Katni and Satna in the Son Valley. The light hydrocarbon concentrations (C1–C4) in near surface soils of the western Vindhyan basin around Chambal Valley have been reported to vary between 1–2547 ppb, 1–558 ppb, 1–181 ppb, 1–37 ppb and 1–32 ppb, respectively with high concentrations around Baran-Jhalawar-Bhanpur-Garot regions (Kumar et al., 2006). The light gaseous hydrocarbon anomalies are coincident with the wrench faults (Kota – Dholpur, Ratlam – Shivpuri, Kannod – Damoh, Son Banspur – Rewa wrench) in the Vindhyan basin, which may provide conducive pathways for the migration of the hydrocarbons towards the near surface soils.  相似文献   

7.
In 2001 a surface geochemical survey was carried out in the Carpathian Foredeep, in the area between Jaros?aw and Radymno (SE Poland) where multihorizon gas deposits were discovered. These deposits accumulate microbial CH4 with small amounts of N2 and higher molecular weight gaseous hydrocarbons. Soil–gas composition in the hydrocarbon fields in the study area is relatively different from the original composition of natural gas occurring in the subsurface reservoir. In 449 analyzed soil gas samples collected from 1.2 m depth relatively low concentrations were found for CH4 (median value 2.2 ppm) and its homologues (median value of total alkanes C2–C4 – 0.02 ppm). Alkenes were encountered in 36.3% of the analyzed samples (mean value of total alkenes C2–C4 – 0.015 ppm) together with distinctly higher concentrations of H2 (maximum value – 544 ppm, mean value – 42 ppm) and CO2 (maximum value – 10.26 vol.%, mean value – 2.27 vol.%). Individual, very high concentrations of CH4 (up to about 35 vol.%) resulted from sub-surface biochemical reactions whereas higher alkanes detected in soil gases (up to about 68 ppm) originated from deep gas accumulations. Both the H2 and alkenes may be indirect indicators of deep hydrocarbon accumulations. Carbon dioxide may also be useful for hydrocarbon exploration, revealing increased concentrations in those sampling sites where CH4 concentrations are strongly depleted, presumably due to bacterial oxidation. These relationships are valid only for the study area and should not be extended as an universal principle.  相似文献   

8.
Surface adsorbed gas surveys and geo-microbiological surveys are known techniques of petroleum exploration and aim towards risk reduction in exploration by way of identifying the areas warm with hydrocarbons and to establish intense exploration priorities amongst the identified warm areas. The present investigation aims to explore correlation between the adsorbed gas distribution pattern with the distribution of the counts of methane, ethane, propane and butane microbial oxidizers in the sub soil samples to establish the role of the latter in identifying the upward migration of hydrocarbons especially in the known petroliferous Krishna-Godavari Basin, India. A total of 135 soil samples were collected near oil and gas fields of Tatipaka, Pasarlapudi areas of Krishna Godavari Basin, Andhra Pradesh. The soil samples were collected from a depth of 2?C2.5 m. The samples collected, were analyzed for indicator hydrocarbon oxidizing bacteria, adsorbed light gaseous hydrocarbons and carbon isotopes (??13Cmethane). The microbial prospecting studies showed the presence of high bacterial population for methane (3.94 × 105 cfu/gm), ethane (3.85 × 105 cfu/gm), propane (4.85 × 105 cfu/gm) and butane oxidizing bacteria (3.63 × 105 cfu/gm) in soil samples indicating microseepage of hydrocarbons. The light gaseous hydrocarbon analysis showed 83 ppb, 92 ppb, 134 ppb, 187 ppb and 316 ppb of C1, C2, C3, nC4 and nC5, respectively, and the carbon isotopic composition of ??13C1 of the samples ranged between ? 36.6 ?? to ?22.7?? (Pee Dee Belemnite) values, which presents convincing evidence that the adsorbed soil gases collected from these sediments are of thermogenic origin. Geo-microbial prospecting method and adsorbed soil gas and carbon isotope studies have shown good correlation with existing oil/ gas fields of K.G basin. Microbial surveys indicating microseepage of hydrocarbons can, therefore, independently precede other geochemical and geophysical surveys to delineate areas warm with hydrocarbons and mapped microbiological anomalies may provide focus for locales of hydrocarbon accumulation in the K.G basin.  相似文献   

9.
This study aims to assess the hydrocarbon potential of Ganga basin utilizing the near surface geochemical prospecting techniques. It is based on the concept that the light gaseous hydrocarbons from the oil and gas reservoirs reach the surface through micro seepage, gets adsorbed to soil matrix and leave their signatures in soils and sediments, which can be quantified. The study showed an increased occurrence of methane (C1), ethane (C2) and propane (C3) in the soil samples. The concentrations of light gaseous hydrocarbons determined by Gas Chromatograph ranged (in ppb) as follows, C1: 0–519, C2: 0–7 and C3: 0–2. The carbon isotopic (VPDB) values of methane varied between ?52.2 to ?27.1‰, indicating thermogenic origin of the desorbed hydrocarbons. High concentrations of hydrocarbon were found to be characteristic of the Muzaffarpur region and the Gandak depression in the basin, signifying the migration of light hydrocarbon gases from subsurface to the surface and the area’s potential for hydrocarbon resources.  相似文献   

10.
《Applied Geochemistry》2006,21(9):1498-1521
A baseline determination of CO2 and CH4 fluxes and soil gas concentrations of CO2 and CH4 was made over the Teapot Dome oil field in the Naval Petroleum Reserve No. 3 (NPR-3) in Wyoming, USA. This was done in anticipation of experimentation with CO2 sequestration in the Pennsylvanian Tensleep Sandstone underlying the field at a depth of 1680 m.The baseline data were collected during the winter, 2004 in order to minimize near-surface biological activity in the soil profile. The baseline data were used to select anomalous locations that may be the result of seeping thermogenic gas, along with background locations. Five 10-m holes were drilled, 3 of which had anomalous gas microseepage, and 2 were characterized as “background.” These were equipped for nested gas sampling at depths of 10-, 5-, 3-, 2-, and 1-m depths. Methane concentrations as high as 170,000 ppmv (17%) were found, along with high concentrations of C2H6, C3H8, n-C4H10, and i-C4H10. Much smaller concentrations of C2H4 and C3H6 were observed indicating the beginning of hydrocarbon oxidation in the anomalous holes. The anomalous 10-m holes also had high concentrations of isotopically enriched CO2, indicating the oxidation of hydrocarbons. Concentrations of the gases decreased upward, as expected, indicating oxidation and transport into the atmosphere. The ancient source of the gases was confirmed by 14C determinations on CO2, with radiocarbon ages approaching 38 ka within 5 m of the surface.Modeling was used to analyze the distribution of hydrocarbons in the anomalous and background 10-m holes. Diffusion alone was not sufficient to account for the hydrocarbon concentration distributions, however the data could be fit with the addition of a consumptive reaction. First-order rate constants for methanotrophic oxidation were obtained by inverse modeling. High rates of oxidation were found, particularly near the surface in the anomalous 10-m holes, demonstrating the effectiveness of the process in the attenuation of CH4 microseepage. The results also demonstrate the importance of CH4 measurements in the planning of a monitoring and verification program for geological CO2 sequestration in sites with significant remaining hydrocarbons (i.e. spent oil reservoirs).  相似文献   

11.
The alternative development of coal-bearing hydrocarbon source rocks and low-porosity and low-permeability tight sandstone reservoirs of the Triassic Xujiahe Formation in the Sichuan Basin is favorable for near-source hydrocarbon accumulation. The natural gas composition of the Xujiahe Formation in the Sichuan Basin is dominated by hydrocarbon gases, of which the methane content is80.16%-98.67%. Typically, the C_2~+ content is larger than 5% in main wet gas. The dry gas is mainly distributed in the western and northern regions of the basin. The non-hydrocarbon gases mainly contain nitrogen, carbon dioxide, hydrogen, and helium, with a total content of 2%. The carbon isotope ranges of methane and its homologues in natural gas are: δ~(13)C_1 of-43.8‰ to-29.6‰, δ~(13)C_2 of-35.4‰ to-21.5‰, δ~(13)C_3 of-27.6‰ to-19.8‰,and δ~(13)C_4 of-27.7‰ to-18.8‰. δ~(13)C_3δ~(13)C_4 occurs in some natural gas with a low evolution degree; such gas is mainly coal-related gas from humic-type source rocks of the Xujiahe Formation. As for the natural gas, δ~2 H_(CH4) values ranged from-195‰ to-161‰,δ~2 H_(C2H6) values ranged from-154‰ to-120‰, and δ~2 H_(C3H8) values ranged from-151‰ to-108‰. The dry coefficient,δ~(13)C and δ~2 H_(CH4) are all positively correlated with the maturity of source rocks. The higher the maturity of source rocks is, the larger the natural gas dry coefficient is and the larger the δ~(13)C and δ~2 H_(CH_4) values are, indicative of the characteristic of near-source accumulation. The δ~2 H_(C2H6) value of natural gas is influenced by paleosalinity to a relatively large extent; the higher the paleosalinity is, the larger the δ~2 H_(C2H6) value is. The Pr/Ph value of the condensate oil ranged from 1.60 to 3.43, illustrating light oxidization-light reduction and partial-oxidization characteristics of the depositional environment of coal-bearing source rocks of the Xujiahe Formation. The natural gas light hydrocarbon(C_5-C_7) from the Xujiahe Formation presented two characteristics: the first was the relatively high aromatic hydrocarbon content(19%-32.1%), which reveals the characteristic of natural gas with humic substances of high-maturity; the second was the low content of aromatic hydrocarbon(0.4%-9.3%),reflecting water-washing during the accumulation of the natural gas. The reported research outcomes indicate a potential mechanism for natural gas accumulation in the Xujiahe Formation, which will further guide natural gas exploration in this region.  相似文献   

12.
A geochemical study of surface sediments from Pranhita-Godavari Basin, Andhra Pradesh, India was carried out using light hydrocarbon compounds to assess the hydrocarbon potential of the basin. Suite of 80 soil samples were collected from the depth of 2.5 m and analyzed for adsorbed light gaseous hydrocarbons namely methane (CH4), ethane (C2H6) and propane (C3H8) in Gas chromatograph. Compound specific Carbon isotope ratios for CH4 and C2H6 were also determined using GC-C IRMS (Gas Chromatograph Combustion Isotope Mass Spectrometer). The presence of moderate to low concentrations of methane (CCH4 C_{CH_4 } : 1 to 138 ppb), ethane (H4{H_4 }: 1 to 35 ppb) and propane (CC3 H8 C_{C_3 H_8 } : 1 to 20 ppb) was measured in the soil samples. Carbon isotopic composition of d13 CCH4 \delta ^{13} C_{CH_4 } ranges between −27.9 to −47.1 ‰ and d13 CC2 H6 \delta ^{13} C_{C_2 H_6 } ranged between −36.9 to −37.2 ‰ (V-PDB) indicating that these gases are of thermogenic origin. Study of soil samples suggests the area has good potential for hydrocarbon.  相似文献   

13.
Surface adsorbed gas surveys and geo-microbiological surveys are well known techniques of petroleum exploration and aim towards risk reduction in exploration by way of identifying the areas warm with hydrocarbons and to establish inter-se exploration priorities amongst the identified warm areas. The thermogenic surface adsorbed gaseous hydrocarbons distribution patterns in petroliferous areas are considered to be a credible evidence for the upward migration of hydrocarbons. The present investigation aims to explore correlation between the adsorbed gas distribution pattern and microbial oxidizers in identifying the upward migration of hydrocarbons especially in the tropical black soil terrain of known petroliferous Mehsana Block of North Cambay Basin, India. A set of 135 sub-soil samples collected, were analyzed for indicator hydrocarbon oxidizing bacteria, adsorbed light gaseous hydrocarbons and carbon isotope ratios (13Cmethane and δ13Cethane). The microbial prospecting studies showed the presence of high bacterial population for methane (5.4 × 106 cfu/gm), ethane (5.5 × 106 cfu/gm), propane (4.6 × 106 cfu/gm) and butane oxidizing bacteria (4.6 × 106 cfu/gm) in soil samples. The light gaseous hydrocarbon analysis showed that the concentration ranges of C1, C2, C3, iC4 and nC4 are 402 ppb, 135 ppb, 70 ppb, 9 ppb and 18 ppb, respectively, and the value of carbon isotope ranges of methane ?29.5 to ?43.0‰ (V-PDB) and ethane ?19.1 to ?20.9‰ (V-PDB). The existence of un-altered petroliferous microseep (δ13C, ?43‰) of catagenetic origin is observed in the study area. Geo-microbial prospecting method and adsorbed soil gas and carbon isotope studies have shown good correlation with existing oil/gas fields of Mehsana. Microbial surveys can independently precede other geochemical and geophysical surveys to delineate area warm with hydrocarbons, and mapped microbiological anomalies may provide focus for locales of hydrocarbon accumulation in the Mehsana Block of Cambay Basin.  相似文献   

14.
The formation mechanisms and processes of geochemical anomalies used as proxies in surface geochemistry exploration (SGE) have not been well understood. Previous studies cannot realize 3D measurement of microseeping hydrocarbons from reservoirs to the surface, which made it difficult to understand the features and pathways of deep hydrocarbon microseepages. Understanding the processes of hydrocarbon microseepages will contribute to the acceptance and effectiveness of surface geochemistry. Based on a simplified geological model of hydrocarbon microseepages, including hydrocarbon reservoir, direct caprock, overlying strata and Quaternary sediments, this work established a 3D experimental system to simulate the mechanisms and processes of deep hydrocarbon microseepes extending to the surface. The dispersive halos of microseeping hydrocarbons in the subsurface were adequately described by using this 3D experimental system. Results indicate that different migration patterns of hydrocarbons above the point gas source within the simulated caprock and overlying strata can be reflected by the ratio of i-butane to n-butane (i-C4/n-C4), which follow diffusion and infiltration (buoyancy) mechanisms. This is not the case for vertical measurement lines far from the point gas source. A vertical gas flow in the form of a plume was found during hydrocarbon microseepage. For sampling methods, the high-density grid sampling is favorable for delineating prospecting targets. Hydrocarbon infiltration or buoyancy flow occurs in the zones of infiltration clusters, coupling with a diffusion mechanism at the top of the water table and forming surface geochemical anomalies. These results are significant in understanding hydrocarbon microseepage and interpreting SGE data.  相似文献   

15.

This study evaluated the Cretaceous (Campanian–Maastrichtian) kaolinitic sediments of the Ajali/Mamu and Enugu/Nkporo Formations from the Lower Benue Trough of Nigeria. A combined method of inductively coupled plasma–mass spectrometry and isotope ratio mass spectrometry was used to investigate trace and rare-earth element geochemistry and hydrogen and oxygen isotopic compositions. These data were then used to infer the sediments’ provenance and paleoclimatic conditions during their deposition. The sediments contained low concentrations of most trace elements, with the exceptions of Zr (651–1352 ppm), Ba (56–157 ppm), V (38–90 ppm), and Sr (15.1–59.6 ppm). Average values of Co and Ni were 1.5 and 0.7 ppm, respectively. Trace and rare earth element values were lower than corresponding values for upper continental crust and Post-Archean Australian Shale, with the exception of Zr. The samples showed only slight light rare-earth enrichment and nearly flat heavy rare-earth depletion patterns, with negative Eu and Tm anomalies, typical of felsic sources. Geochemical parameters such as La/Sc, Th/Sc, and Th/Co ratios support that the kaolinitic sediments were derived from a felsic rock source, likely deposited in an oxic environment. 18O values ranged from + 15.4 to + 21.2‰ for the investigated samples, consistent with a residual material derived from chemical weathering of felsic rock and redeposited in a sedimentary basin (typical values of + 19 to + 21.2‰). While in the basin, the sediments experienced extended interactions with meteoric water enriched in δD and δ16O. However, the variation in δD and δ16O values for the investigated samples is attributed to the high temperature of formation (54–91 °C). The δD and δ18O values suggest that the sediments, although obtained from different localities within the Lower Benue Trough, formed under similar hot, tropical climatic conditions.

  相似文献   

16.
The barium deposits in Ankang and Xunyang counties,Shaanxi Province,China,occur in the northernmost part of the world-class barium metallogenic belt in south Qinling.The deposits are hosted by the Lower Silurian carbonaceous siliceous rocks,with a unique combination of barite and witherite.The homogenization temperatures of fluid inclusions in the barite are mainly concentrated between 135 and 155 ℃,whereas those from the witherite have two peaks of 165-175 ℃,and 215-225℃,respectively.Laser Raman analysis of fluid inclusions indicates that the vapor phase of fluid inclusions in barite is dominated by H_2O,although some contains N_2,H_2S,and CH_4.The compositions of the vapor and liquid phases of fluid inclusions in witherite can be divided into two end-members,one dominated by H_2O without other volatiles,and the other containing CH_4,C_2H_6,C_3H_8,C_2H_4,and C_6H_6 in addition to H_2O.CO_2,H_2S,and some CH_4 are interpreted as products of chemical reactions during mineralization.Organic gases(CH_4,C_2H_6,C_3H_8,C_2H_4,and C_6H_6) in the fluids were critical in the formation of barium sulfate versus carbonate.The δ~(34)S values of barite range from 38.26‰ to54.23‰(CDT),the δ~(34)S values of sulfides coexisting with barium minerals vary from 22.44‰ to25.11‰(CDT),and those in the wall rock from 11.60‰ to 19.06‰(CDT).We propose that the SO_4~(2-)generally experienced bacterial sulfate reduction in seawater before mineralization,and some SO_4~(2-)also experienced thermochemical sulfate reduction in hydrothermal system during mineralization.The δ~(13)C values of witherite range from-27.30‰ to-11.80‰(PDB),suggesting that carbon was sourced from organic substances(like CH_4,C_2H_4,and C_2H_6).The formation of witherite was possibly associated with thermochemical sulfate reduction,which caused the consumption of the organic gases and SO_4~(2-) in the hydrothermal solutions,consequently inhibiting barite formation.The important conditions for forming witherite include high fluid temperatures,high Ba~(2+) concentrations,CO_2 in the fluids,low HS~- concentrations,and the subsequent rapid diffusion of H_2S during thermochemical sulfate reduction of the fluids.  相似文献   

17.
Deep fluids in a petroliferous basin generally come from the deep crust or mantle beneath the basin basement, and they transport deep substances(gases and aqueous solutions) as well as heat to sedimentary strata through deep faults. These deep fluids not only lead to large-scale accumulations of CO_2, CH_4, H_2, He and other gases, but also significantly impact hydrocarbon generation and accumulation through organic-inorganic interactions. With the development of deep faults and magmatic-volcanic activities in different periods, most Chinese petroliferous basins have experienced strong impacts associated with deep fluid activity. In the Songliao, Bohai Bay, Northern Jiangsu, Sanshui, Yinggehai and Pearl Mouth Basins in China, a series of CO_2 reservoirs have been discovered. The CO_2 content is up to 99%, with δ~(13)C_(CO2) values ranging from-4.1‰ to-0.37‰ and ~3He/~4He ratios of up to 5.5 Ra. The abiogenic hydrocarbon gas reservoirs with commercial reserves, such as the Changde, Wanjinta, Zhaozhou, and Chaoyanggou reservoirs, are mainly distributed in the Xujiaweizi faulted depression of the Songliao Basin. The δ~(13)CCH4 values of the abiogenic alkane gases are generally -30‰ and exhibit an inverse carbon isotope sequence of δ~(13)C_(CH4)δ~(13)C_(C2H6)δ~(13)C_(C3H8)δ~(13)C_(C4H10). According to laboratory experiments, introducing external H_2 can improve the rate of hydrocarbon generation by up to 147% through the kerogen hydrogenation process. During the migration from deep to shallow depth, CO_2 can significantly alter reservoir rocks. In clastic reservoirs, feldspar is easily altered by CO_2-rich fluids, leading to the formation of dawsonite, a typical mineral in high CO_2 partial pressure environments, as well as the creation of secondary porosity. In carbonate reservoirs, CO_2-rich fluids predominately cause dissolution or precipitation of carbonate minerals. The minerals, e.g., calcite and dolomite, show some typical features, such as higher homogenization temperatures than the burial temperature, relatively high concentrations of Fe and Mn, positive Eu anomalies, depletion of 18 O and enrichment of radiogenic ~(87)Sr. Due to CO_2-rich fluids, the development of high-quality carbonate reservoirs is extended to deep strata. For example, the Well TS1 in the northern Tarim Basin revealed a high-quality Cambrian dolomite reservoir with a porosity of 9.1% at 8408 m, and the Well ZS1 C in the central Tarim Basin revealed a large petroleum reserve in a Cambrian dolomite reservoir at ~6900 m. During the upward migration from deep to shallow basin strata, large volumes of supercritical CO_2 may extract petroleum components from hydrocarbon source rocks or deep reservoirs and facilitate their migration to shallow reservoirs, where the petroleum accumulates with the CO_2. Many reservoirs containing both supercritical CO_2 and petroleum have been discovered in the Songliao, Bohaiwan, Northern Jiangsu, Pearl River Mouth and Yinggehai Basins. The components of the petroleum trapped with CO_2 are dominated by low molecular weight saturated hydrocarbons.  相似文献   

18.
Carbonate concretions can form as a result of organic matter degradation within sediments. However, the ability to determine specific processes and timing relationships to particular concretions has remained elusive. Previously employed proxies (e.g., carbon and oxygen isotopes) cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen, nitrate, metal oxides, and sulfate as electron acceptors, in addition to degradation by thermal decarboxylation. Here, we employ concentrations of carbonate-associated sulfate (CAS) and δ34SCAS (along with more traditional approaches) to determine the specific nature of concretion authigenesis within the Miocene Monterey Formation.Integrated geochemical analyses reveal that at least three specific organo-diagenetic reaction pathways can be tied to concretion formation and that these reactions are largely sample-site specific. One calcitic concretion from the Phosphatic Shale Member at Naples Beach yields δ34SCAS values near Miocene seawater sulfate (～+22‰ VCDT), abundant CAS (ca. 1000 ppm), depleted δ13Ccarb (～?11‰ VPDB), and very low concentrations of Fe (ca. 700 ppm) and Mn (ca. 15 ppm)—characteristics most consistent with shallow formation in association with organic matter degradation by nitrate, iron-oxides and/or minor sulfate reduction. Cemented concretionary layers of the Phosphatic Shale Member at Shell Beach display elevated δ34SCAS (up to ～+37‰), CAS concentrations of ～600 ppm, mildly depleted δ13Ccarb (～?6‰), moderate amounts of Mn (ca. 250 ppm), and relatively low Fe (ca. 1700 ppm), indicative of formation in sediments dominated by sulfate reduction. Finally, concretions within a siliceous host at Montaña de Oro and Naples Beach show minimal CAS concentrations, positive δ13C values, and the highest concentrations of Fe (ca. 11,300 ppm) and Mn (ca. 440 ppm), consistent with formation in sediments experiencing methanogenesis in a highly reducing environment. This study highlights the promise in combining CAS analysis with more traditional techniques to differentiate among diagenetic reactions as preserved in the geologic record and shows potential for unraveling subsurface biospheric processes in ancient samples with a high degree of specificity.  相似文献   

19.
The average compositions (including H2O, Cl, F, and S contents) and chemical structure of oceanic mantle plumes were estimated on the basis of the ratios of incompatible volatile components, potassium, and some other elements in the basaltic magmas of ocean islands (melt inclusions and quenched glasses). The following average concentrations were estimated for the plume mantle: 510 ppm K2O, 520 ppm H2O, 21 ppm Cl, 55 ppm F, and 83 ppm S; these values are significantly higher than those of the depleted mantle (except for S). The abundances of H2O, Cl, and S are lower than in the primitive mantle. The normalized H2O content in the plume mantle is similar to the concentrations of similarly incompatible La and Ce but lower than the concentrations of K2O, Cl, and Sr. This is at odds with the idea of wet mantle plumes. Three types of basaltic magmas corresponding to three types of plume sources (M1, M2, and M3) were distinguished. The concentrations of incompatible elements in these reservoirs were estimated using two models, assuming either an isochemical mantle or a moderately enriched composition of plume material. The latter model gave the following average concentrations of H2O, Cl, F, and S: 130, 33, 11, and 110 ppm for M1, 110, 12, 65, and 45 ppm for M2; 530, 29, 49, and 110 ppm for M3. The plume mantle is not homogeneous, and its heterogeneity is related to the existence of three main compositions, one of which (M1) is similar to the mantle of mid-ocean ridges, and two others (M2 and M3) are moderately enriched in K2O, TiO2, P2O5, F, and incompatible trace elements. The compositions of M2 and M3 are strongly different in H2O, Cl, and S contents. The M2 mantle reservoir is significantly poorer in these components and richer in incompatible trace elements than M3. The plume mantle was formed mainly by the mixing of three sources: ultradepleted mantle, moderately enriched relatively dry mantle, and moderately enriched H2O-rich mantle. In addition to the three main components of the plume mantle, there are probably minor components enriched in chlorine and depleted in fluorine. It is supposed that all these components are entrained into the plume mantle through the mantle recycling of components of the oceanic and continental crust. The established relationships are in agreement with the zonal model of a mantle plume, which includes a hot central part poor in H2O, Cl, and S; an outer part enriched in volatile and nonvolatile incompatible elements; and enclosing mantle material interacting with the plume.  相似文献   

20.
Based on the pyrolysis products for the Jurassic low-mature coal under programmed temperature,and chemical and carbon isotopic compositions of natural gas from the Kuqa Depression, the genetic origin of natural gas was determined,and then a gas filling model was established,in combination with the geological background of the Kuqa Depression.The active energy of CH_4,C_2H_6 and C_3H_8 was gotten after the data of pyrolysis gas products under different heating rates(2℃/h and 20℃/h)were fitted by the Gas O...  相似文献   

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