Carbonates and Evaporites最新文献

The research aims to determine climatic cycles and the evolution of the Sebkha El-Guettiate during the last two millenniums of the Holocene by examining the regularity of chemical elements (Na, Ca and K) variables, magnetic susceptibility, Spectral analysis and carbonate percentage on a 100 cm core (SG) from the Sebkha El-Guettiate in the Skhira region of south-eastern Tunisia. The variation in the percentage of calcium carbonates in the various samples analysed along the length of the core sample shows that CaCO₃ contents are low, suggesting a chemical origin through precipitation of these carbonates. The sediments studied contained detrital, carbonate and evaporitic minerals, with a high percentage of quartz (56–98%). The chemical element results (Na, K and Ca) were mainly from halite, carbonates and gypsum, in addition to high magnetic susceptibility (MS) values. The variation in CaCO₃ concentration along the core shows significant variability. This variability is due to a radical change in the geochemistry and composition of the sediments as a result of changing climatic conditions. Spectral analysis results in this work can be performed by magnetic susceptibility and sodium shows a cycle of 1000 years and 1300 years. Significant cycles were not identified through spectral analysis of the potassium data. In addition, carbonate and calcium showed double cycles of 1600 to 860 years and 1700 to 889 years. Climate cycle factors are related to solar activity, the ocean and atmosphere.

Abstract

The distribution of fractures in fractured-vuggy reservoirs of Ordovician carbonate rock in the Tahe oilfield is one of the main controlling factors for oil and gas enrichment in this area. Determining the spatial distribution of fractures is the key to stable production in this area. First, this article uses cores and thin sections to analyse the characteristics of fractures. The area is dominated by structural fractures, with relatively few dissolution fractures and diagenetic fractures. The average length of the fractures is 5.25 cm, the average opening is 0.21 cm, the average angle is 40.33°, and the average line density is 3.05 lines/m. Second, rock mechanical parameters were calculated using dipole shear wave logging curves and conventional logging curves, and fracture development index (FI) curves were constructed. Using the variations in the performance characteristics of the FI curve the spatial distribution patterns of fractures in different fracture development segments are qualitatively analyse. The predicted results are then verified by combining core, thin section, and imaging logging data, and the coincidence rate reached 83%. The following conclusions are drawn: Vertically, fractures are mostly concentrated in the Yijianfang Formation, with the characteristics of “small scale and scattered”. On the plane, the development scale of fractures in the denudation area is good, with great development potential. Combined with the production of each well, it is verified that the fracture development segment identified by the fracture FI curve has guiding significance for oil and gas exploration.

Karst regions represent fragile landscapes that are particularly vulnerable to environmental changes. The study aims to assess the soil quality in the karst basin of Ioannina, which is located in the north-western region of Greece. Factor analysis was employed to evaluate the concentrations of trace elements in the soil. Additionally, Geographical Information Systems (GIS) was utilized to visualize the spatial distribution of these trace elements and their potential sources in relation to the local geology and land use. The study findings underscored that most of the karst landscape in the research area is comprised of Quaternary deposits and it is predominantly occupied by agricultural land. The soil displays substantial levels of clay and silt, with noticeably elevated concentration of iron (Fe), manganese (Mn), nickel (Ni), chromium (Cr), lead (Pb), copper (Cu), vanadium (V), and phosphorous (P) compared to the median concentrations observed in European topsoil. The factor analysis is applied to the dataset of elements content in soil to identify the factors controlling their distribution. Factor 1 involves the geological contribution and the adsorption of Fe–Ni–Cr–Pb–V and lithium (Li) into clay minerals. Factor 1 may be termed as “lithogenic factor”. The cultivated land and road network showed a significant correlation with the higher positive loadings of Fe, Mn, Pb and Cu for Factor 2 which may be termed “agricultural-road network factor”. Agricultural activities and cultivated land presented a significant correlation with the higher positive loadings of nitrate (NO₃⁻), nitrite (NO₂⁻), organic matter (OM), ammonium (NH₄⁺) and P for Factor 3 which may be termed as “agricultural” factor. The higher positive loadings of Factor 4 suggest a variation in the mechanical properties of the Quaternary deposits and may be termed as “soil texture factor”. Quaternary deposits and agricultural land exhibit a strong spatial relationship with factor scores of each factor. Combining factor analysis and GIS proved to be an effective method for identifying and confirming the sources of elements content in soil.

The mineralogy of some carbonate rocks from the Abakaliki Basin has been investigated to determine the depositional environment classification and assess the suitability of the rocks for application in geotechnical construction and industrial process systems. The method of investigation included field mapping and petrographic and geochemical techniques to characterize their field characteristics, mineral types and texture and chemical contents. Mapping revealed two lithologic units of sandstone and shale that associate with the carbonates in a similar sequence. The carbonates contain fossils and are composed of calcite, quartz and rockclasts. The grains have angular to sub-rounded sphericity and are dispersed in hetereolithic groundmass. The major oxide composition shows average value of 32.30wt% SiO₂, 26.37wt% CaO, 7.23wt% MgO, 6.56wt% Fe₂O₃, 4.38wt% Al₂O₃, and 3.23wt% K₂O + Na₂O in decreasing order of abundance. Concentrations of trace elements are significantly higher (> 100 ppm) for V, Cr, Sr, Zr, Cu, Ba, Pb and Zn and comparatively lower for Sb, Ge, Ce, Y, As, Hg, Tiand Au. The studied rocks qualify as biomicrite or wackestone and range from dolomitic, through carbonaceous to siliceous limestone deposited in a wide spectrum of environment from shallow marine to transitional setting. The rocks are marginally suitable for use as aggregates due to the presence of deleterious contents of quartz and organics. For cement production, their low content of CaO, relatively high SiO₂, comparatively high LOI and elevated abundances of contaminant heavy elements are major limitations. Similarly, in paper, plastic, paint, metallurgy and pharmaceutics, the rocks are unsuitable due to the constituent minerals.

Fractures are widely developed in lacustrine carbonate reservoirs in the Oligocene Lower Ganchaigou Formation of the western Yingxiongling area and their distribution trend is unclear. Based on well logging, formation testing, rock mechanics parameters obtained by logging interpretation, and the paleotectonic stress obtained by acoustic emission experiments, finite element numerical simulation is used to restore the tectonic stress of the typical section and target layer plane models in the late Himalayan period in the study area. The fracture distributions of the section and the plane are quantitatively predicted and their distribution trends are summarized. The results show that the tectonic compression at the end of the Neogene was strong in the study area. Under the effect of the paleotectonic stress field, the fracture development degree of the upper member of the Lower Ganchaigou Formation is generally high. Fractures mainly developed in the fault zones and their surroundings, the hanging wall of the Shizigou fault, and the abnormally high-size area of tectonic stress intensity in the northern and middle tectonic zones of the presalt carbonate reservoirs. The fracture distribution is mainly affected by faults, lithology, structural parts, and tectonic stress intensity. The predicted favorable development areas of fractures can be considered favorable exploration positions in the deep layers of the study area. This study can provide a guide for the deep oil–gas exploration in the study area and a feasible method for predicting the sweet spots of this type of tight reservoir.

Graphical abstract

Using the finite element numerical simulation, the geological models for the lacustrine carbonate oil reservoirs are established. The tectonic stress field of theirs is restored. Combined with the comprehensive fracture index evaluation, the fracture development degree parameters are constructed to realize the quantitative prediction of the fracture distribution.

The sedimentary sequence, extending from the Maastrichtian to the lower Eocene and exposed in two sections in the easternmost part of the Izeh Zone (Kuh-e-Sefid and Mish anticlines) in the Zagros Basin, has been subjected to a comprehensive analysis of its Planktic foraminiferal assemblage. This time interval encompasses the upper part of the Gurpi Formation and the lower to middle part of the Pabdeh Formation, spanning from the base to the top. The lithology of the Maastrichtian-early Eocene successions comprises calcareous shales, argillaceous limestones, glauconitic and phosphatic limestones, marlstones, purple shales, and cherty argillaceous limestones. A total of 35 genera and 118 species of Planktic foraminifera have been conclusively identified. The identified biozones include the Contusotruncana contusa Interval Zone and the Abathomphalus mayaroensis Interval Zone, indicating an early to late Maastrichtian age for the upper part of the Gurpi Formation. The lower to middle part of the Pabdeh Formation exhibits distinctive biozones, such as Praemurica uncinata (P2), Igorina albeari (subzone P3b), Igorina-Globanomalina pseudomenardii Interval Zone, Globanomalina pseudomenardii (P4), Morozovella velascoensis (P5), Acarinina sibaiyaensis (E1), Globanomalina pseudomenardii-Pseudohastigerina wilcoxensis Interval Zone, Pseudohastigerina wilcoxensis/Morozovella velascoensis (E2), Morozovella marginodentata (E3), Morozovella velascoensis-Morozovella formosa formosa and/or Morozovella lensiformis Interval Zone (subzone P6a), Morozovella formosa (E4), Morozovella aragonensis/Morozovella subbotinae (E5), Acarinina pentacamerata (E6), Acarinina cuneicamerata (subzone E7a), and Turborotalia frontosa (subzone E7b). The boundary between the Pabdeh and Gurpi Formations in the examined sections is demarcated by an unconformity, characterized by a sedimentary hiatus from the early Danian to the early Selandian within biozones P0 to P3a.

Devonian thick–layer dolostone is widely developed in the Shawozi Formation (SWZF) of the Sichuan Basin, however, the properties of the dolomitisation fluid and its genesis require clarification. This study adopted field observations, microscopic identification, and geochemical data analysis methods that major and trace elements, rare–earth element (REE) analyses, carbon and oxygen isotopes based on 31 samples. The results showed that dolomites were divided into crystalline dolomites and calcareous dolomites. Crystalline dolomite, which is the most widely distributed, can be further divided into fine crystalline dolomite (FCD), fine–medium crystalline dolomite (FMCD), medium crystalline dolomite (MCD), and coarse crystalline dolomites (CCD). Calcareous dolomite, fine crystalline dolomite (FCD), and fine–medium crystalline dolomite (FMCD) have higher Fe, higher Mn, and lower Sr, whereas medium crystalline dolomite (MCD) and coarse crystalline dolomite (CCD) have the opposite characteristics. The REE content was lower and weak negative anomalies of Ce and Eu were apparent, indicating that dolomite was formed in a fragile redox environment and at a relatively low temperature. The δ¹³C_PDB value had a small variation range, with an average value of 0.04‰. Although the variation range of δ¹⁸O_PDB value was more extensive, most samples were concentrated in the range of -6.53‰~-2.1‰, indicating that the overall effect of diagenetic alteration was more negligible. Two dolomitisation models were developed: reflux seepage and burial dolomitisation. The reflux seepage model mainly developed calcareous dolomite, whereas the burial model mainly developed crystalline dolomite. The degree of dolomitisation increased with increasing burial depth and formation pressure.

Calculating the volume of hydrocarbons in place in carbonate reservoirs is associated with a high uncertainty due to the sedimentary texture and diagenetic characteristics. The Archie’s equation is one of the most common methods to measure the volume of reservoir fluids. Petrophysical parameters and geological attributes control the exponents of this equation. If Archie’s exponents (m and a) are considered constant, it can have a high uncertainty in the volume of reservoir fluids. This study investigates the effect of petrophysical parameters (pore type, pore size distribution, and pore throat radius) and geological attributes (sedimentary facies and diagenesis) on the Archie’s exponents. Based on petrography results, seven sedimentary facies related to a carbonate ramp were identified, and the diagenesis processes have caused a great heterogeneity of the studied carbonate reservoir. Archie’s exponents were calculated for four models of reservoir rock types. Finally, a total of five rock types were defined based on the study of thin sections, well logs, scanning electron microscopy images, special/conventional core analysis, and mercury injection testing. Combining pore types is influenced by diagenetic processes and sedimentary texture, so the diversity in the composition of pore types reduce the measurement accuracy of Archie’s exponents and resulted water saturation. The results show that pore type, pore size distribution, and fractures significantly affect the cementation exponent and the tortuosity factor. Accordingly, the determination of the rock types in heterogeneous reservoirs based on the pore size distribution, pore type, pore throat radius and fractures has the most significant impact on the accuracy of the Archie exponents.

Exploitation and utilization for karst groundwater is an effective way of solving the imbalance between supply and demand of water resources in Guilin City, southwestern China. To better develop and utilize karst groundwater, the development characteristics of underground karst in Guilin City and their influence on the development of karst groundwater were systematically investigated. The results show that the thick limestone formation of D₃r is the most ideal aquifer because it has the highest karst development and the most abundant karst groundwater. The main karst development zone is above 90 m a.s.l., and accounting for 88.5% of the karst caves, 11.5% karst caves are located in the weak karst development zone of 40–90 m a.s.l, and below 40 m a.s.l., the karst is not developed. Thus, the borehole depth for groundwater exploitation is suggested to be approximately 100–120 m owing to the ground-surface elevation of Guilin City being 140–160 m a.s.l. The zone above 120 m a.s.l. is not optimal for karst groundwater exploitation because of its high karst cave filling rate (82.89%) and low water-filling cavity ratio (16.95–30.08%), in which the karst groundwater is easily connected with surface water, therefore, an increase of possibility of collapse when abstracting groundwater. The zone between 120 and 90 m a.s.l. could be more optimal for groundwater extraction due to its medium scale of karst caves, relatively low karst cave filling rate (62.07%), and high water-filling cavity rate (30.43–62.10%) relatively. Meanwhile, a favorable groundwater exploitation and utilization region is located in the eastern study area, which has the highest underground karst development, with the highest percentage of boreholes encountering caverns (80.21%), linear karst rates (8.58%), and the medium karst cave filling rate (65.61%). This study provides a scientific basis for the exploitation of karst groundwater and construction of a backup emergency water sources for Guilin City.

The present study aims to elucidate the possible sources of uranium and thorium content in the Campanian–Maastrichtian phosphorites from the Duwi Formation in the Nile Valley and Red Sea by conducting facies analysis and sequential leaching method. Nile Valley samples were collected from the El-Sibaiya East area, while those of the Red Sea were collected from two locations: Hamadat and Zug El Bahar. The petrographic investigation revealed that the Sibaiya East phosphorites exhibit peloidal bioclastic phospharenite–phospharudite microfacies, while Hamadat and Zug El Bahar phosphorites display peloidal bioclastic phosphalutite and silicified peloidal bioclastic phospharenite microfacies, respectively. Besides, U–Th bearing accessory minerals, such as zircon and monazite occur in Sibaiya East phosphorites. Thorium is present in Zug El Bahar phosphorites as minute accumulations associating apatite and quartz. Moreover, uranium is found with vanadium and iron as fine patches in the Sibaiya East phosphorite, and as small disseminations associated with Ca and Si in the Hamadat phosphorite. The X-ray diffraction shows that the investigated phosphorites are essentially built up of hydroxyl apatite Ca₅(PO₄)₃(OH) and quartz SiO_2. To accurately evaluate the bioavailability and mobility of uranium and thorium in the investigated phosphorites, it was necessary to identify the overall concentration and the various chemical forms of these elements by a five-step sequential leaching technique. The results indicate that Th and U are more abundant in the Red Sea phosphorites than in the Nile Valley phosphorites. Furthermore, Th is not bio-available and it is mostly found in the residue as Th-bearing minerals. Uranium, unlike Th is bio-available and fractionates among all fractions, indicating that U accumulation is the result of various diagenetic processes.