2018 Vol. 36, No. 4
Display Method:
2018, 36(4): 639-650.
doi: 10.14027/j.issn.1000-0550.2018.134
Abstract:
The 33rd International Sedimentological Congress was held in Toulouse, France from 9th to 12th, October, 2017, continuously concerning on the topics of microbial carbonate rocks. Based on the summarizing outcome of the conference combined with related domestic and international literatures in recent years and also field investigations on modern microbial carbonate sediments, some research progress and hot spots were investigated and discussed for microbial carbonate sedimentology in this paper. The primarily concerned topics include the following four fields:1) Microbial mat mineralization products and sequences, of which the mineralized products include calcite (low Mg, high Mg, ultra high Mg), aragonite, carbonates containing iron, manganese and calcium, products of magnesium-silicon and pyrite. The mineralization sequences are diverse. 2) Elemental records of the microbial carbonate, in which carbon, oxygen, nitrogen, phosphorus, sulfur, iron, silicon and arsenic may retain the information about the microbial sedimentary carbonate and early diagenesis. 3) The structures mechanism and their significance for the microorganism carbonate rocks such as laminates and clots. 4) the sedimentary environments of microbial carbonates. It is considered that the stromatolite, clot rock and other microbial carbonate rocks are mostly developed in the tidal flat environment, especially the shallow subtidal zone and the upper layers of stromatolitic limestone and clotting rocks. According to the internationally developing characteristics of microbial carbonate sedimentology, the research trend in related fields in China is further discussed in this paper. It is necessary to strengthen the research on the current microbial carbonate sediments and the diagenesis of ancient microbial carbonate rocks.
The 33rd International Sedimentological Congress was held in Toulouse, France from 9th to 12th, October, 2017, continuously concerning on the topics of microbial carbonate rocks. Based on the summarizing outcome of the conference combined with related domestic and international literatures in recent years and also field investigations on modern microbial carbonate sediments, some research progress and hot spots were investigated and discussed for microbial carbonate sedimentology in this paper. The primarily concerned topics include the following four fields:1) Microbial mat mineralization products and sequences, of which the mineralized products include calcite (low Mg, high Mg, ultra high Mg), aragonite, carbonates containing iron, manganese and calcium, products of magnesium-silicon and pyrite. The mineralization sequences are diverse. 2) Elemental records of the microbial carbonate, in which carbon, oxygen, nitrogen, phosphorus, sulfur, iron, silicon and arsenic may retain the information about the microbial sedimentary carbonate and early diagenesis. 3) The structures mechanism and their significance for the microorganism carbonate rocks such as laminates and clots. 4) the sedimentary environments of microbial carbonates. It is considered that the stromatolite, clot rock and other microbial carbonate rocks are mostly developed in the tidal flat environment, especially the shallow subtidal zone and the upper layers of stromatolitic limestone and clotting rocks. According to the internationally developing characteristics of microbial carbonate sedimentology, the research trend in related fields in China is further discussed in this paper. It is necessary to strengthen the research on the current microbial carbonate sediments and the diagenesis of ancient microbial carbonate rocks.
2018, 36(4): 651-663.
doi: 10.14027/j.issn.1000-0550.2018.055
Abstract:
Microbial carbonates are not only important sedimentary records for paleo environments, paleoclimate and geological events, but also significant hydrocarbon reservoirs. Physiological characteristics of diverse microbial communities in part control the formation and preservation of primary pores, microbial depositional fabrics in various scales affect the diameter and distribution of pores. Combination patterns of microbial carbonates and associated facies have a great impact on usefulness and scale of pores. Different sedimentary processes and depositional environments of microbial carbonates affect sequences and degrees of cementation and dissolution during meteoric diagenesis, resulting in heterogeneity of pores. Microbe-related diageneses such as dolomitization, cementation, and karstification may form primary pores, protect residual pores and form secondary pores during various stages. Pore diameters of microbial carbonates are in range from nanometer-submicron level to centimeter and even meter level, which may correspond to multi-scale sedimentary structures to some extent. Therefore, it's necessary to analyze and characterize samples in various scales. In addition, the combination of geology and geophysics should be enhanced, being an important way to facilitate research of hydrocarbon reservoirs of microbial carbonates also benefit for the exploration and exploitation of oil and gas resources.
Microbial carbonates are not only important sedimentary records for paleo environments, paleoclimate and geological events, but also significant hydrocarbon reservoirs. Physiological characteristics of diverse microbial communities in part control the formation and preservation of primary pores, microbial depositional fabrics in various scales affect the diameter and distribution of pores. Combination patterns of microbial carbonates and associated facies have a great impact on usefulness and scale of pores. Different sedimentary processes and depositional environments of microbial carbonates affect sequences and degrees of cementation and dissolution during meteoric diagenesis, resulting in heterogeneity of pores. Microbe-related diageneses such as dolomitization, cementation, and karstification may form primary pores, protect residual pores and form secondary pores during various stages. Pore diameters of microbial carbonates are in range from nanometer-submicron level to centimeter and even meter level, which may correspond to multi-scale sedimentary structures to some extent. Therefore, it's necessary to analyze and characterize samples in various scales. In addition, the combination of geology and geophysics should be enhanced, being an important way to facilitate research of hydrocarbon reservoirs of microbial carbonates also benefit for the exploration and exploitation of oil and gas resources.
2018, 36(4): 664-673.
doi: 10.14027/j.issn.1000-0550.2018.053
Abstract:
Aboundant authigene carbonate minerals and iron sulfide in sea-bed methane cold seeps are important geologic markers to prospect natural gas hydrate(NGH). However, there are very limited experimental researches on their formation conditions. The aquantic chemical changes of water systems with sulfate reducing bacteria and anaerobic methanotrophic archaea under various temperatures and pressures were simulated and investigated as the first time by using a home-made sealed bioreactor system. Minerals morphology, carbonate species and iron sulfide under different temperatures and pressures were checked using scanning electron microscope (SEM) and X-ray diffraction (XRD) to study on their precipitation regulation. For the experiments under 5℃, 8℃ and 10℃, the pH values and concentrations of HCO3- were in ranges of 6.5-7.4 and 202.55-639.93 mg/L, respectively. The oxidation-reduction potential (ORP) stayed near -100.0 mV. For the experiments under 2.5 MPa, 5.0 MPa and 7.5 MPa, the pH values and concentrations of HCO3- were in range of 6.1-7.2 and 324.08-789.95 mg/L, respectively. The oxidation-reduction potential (ORP) was remained from -100 mV to -50.0 mV. These experimental results demonstrated that temperature and pressure might control authigene carbonate minerals formation by influence of microbial metabolism to provide the source for S2- and HCO3-, which were able to form the minerals combined with cations. Calcite, siderite and iron sulfide were precipitated in all experiments whereas the HCO3- concentration was diminishing, the S2- concentration was rised and ORP decreased when the temperature was increased. The distribution of siderite and iron sulfide was increasing with temperature and S2- concentration, probably indicating that increasing temperature could facilitate the formation of iron sulfide and siderite. The S2- produced by microbes metabolism and negative values of ORP(below 0) were decreasing along with increasing pressure. Simultaneously, carbonate formation was increased and the distribution range of iron sulfide became wider and also better in morphology. The decrease of calcium carbonate precipitation might be controlled by the concentration of S2-. These results could be significant to provide insights into the storage and cycle of carbon and sulfur, and the formation mechanism of authigene minerals in global ocean.
Aboundant authigene carbonate minerals and iron sulfide in sea-bed methane cold seeps are important geologic markers to prospect natural gas hydrate(NGH). However, there are very limited experimental researches on their formation conditions. The aquantic chemical changes of water systems with sulfate reducing bacteria and anaerobic methanotrophic archaea under various temperatures and pressures were simulated and investigated as the first time by using a home-made sealed bioreactor system. Minerals morphology, carbonate species and iron sulfide under different temperatures and pressures were checked using scanning electron microscope (SEM) and X-ray diffraction (XRD) to study on their precipitation regulation. For the experiments under 5℃, 8℃ and 10℃, the pH values and concentrations of HCO3- were in ranges of 6.5-7.4 and 202.55-639.93 mg/L, respectively. The oxidation-reduction potential (ORP) stayed near -100.0 mV. For the experiments under 2.5 MPa, 5.0 MPa and 7.5 MPa, the pH values and concentrations of HCO3- were in range of 6.1-7.2 and 324.08-789.95 mg/L, respectively. The oxidation-reduction potential (ORP) was remained from -100 mV to -50.0 mV. These experimental results demonstrated that temperature and pressure might control authigene carbonate minerals formation by influence of microbial metabolism to provide the source for S2- and HCO3-, which were able to form the minerals combined with cations. Calcite, siderite and iron sulfide were precipitated in all experiments whereas the HCO3- concentration was diminishing, the S2- concentration was rised and ORP decreased when the temperature was increased. The distribution of siderite and iron sulfide was increasing with temperature and S2- concentration, probably indicating that increasing temperature could facilitate the formation of iron sulfide and siderite. The S2- produced by microbes metabolism and negative values of ORP(below 0) were decreasing along with increasing pressure. Simultaneously, carbonate formation was increased and the distribution range of iron sulfide became wider and also better in morphology. The decrease of calcium carbonate precipitation might be controlled by the concentration of S2-. These results could be significant to provide insights into the storage and cycle of carbon and sulfur, and the formation mechanism of authigene minerals in global ocean.
2018, 36(4): 674-683.
doi: 10.14027/j.issn.1000-0550.2018.051
Abstract:
It helps further understand and test forcing mechanisms behind millennial-scale events to decipher their regional responses and structures during the periods of large ice sheets. Using 323 stable oxygen isotope ratios and 5 230Th ages of a stalagmite in Yongxing Cave, central China, we reconstruct a detailed history of the East Asian hydrological cycle during the time span surrounding the Dansgaard-Oeschger (DO) 18 event. The calcite δ18O profile, together with another two stalagmite records from the same cave, shows a rapid decrease of 1.5‰ at~64.7 ka, confirming the occurrence of DO18 in the East Asian monsoon (EAM) region. The ending limb of DO18 in the EAM regime is characterized by five centennial-scale monsoon enhancing events, and finally culminates at~63.0 ka. Divergent from a rapid temperature increasing followed by a similarly rapid cooling (~60 a) back to stadial environments of the Greenland DO18 event, the DO18 event in the monsoon region exhibits a significant asymmetric feature with a sharp EAM strengthening but then a smoothing, lasting 1 500 a, decrease. Nonetheless, our stalagmite δ18O records mirror, in shape and timing, the variations of Atlantic meridional overturning circulation (AMOC) estimated by the ratio of 231Pa/230Th and recovered from the deep western North Atlantic. This similarity suggests that the AMOC played an important role in regulating the DO18 event in the EAM region. On the other hand, the discrepancy in the pattern of DO18 between the EAM and the North Atlantic climate systems can be attributed to the different regional response to the AMOC change in the high-and low-latitude climate systems. Spectral analysis of the stalagmite δ18O record denotes significant cycles at the 90% confidence level at 208, 150, 52 and 14-12 a, close to the previously reported solar cycles at 207, 149, 57 and 11 a. This consistency of cyclical variabilities between the EASM intensity and solar activity might indicate that solar forcing of centennial-and decadal-scale EASM variation still existed during the MIS4 interval with larger ice volumes.
It helps further understand and test forcing mechanisms behind millennial-scale events to decipher their regional responses and structures during the periods of large ice sheets. Using 323 stable oxygen isotope ratios and 5 230Th ages of a stalagmite in Yongxing Cave, central China, we reconstruct a detailed history of the East Asian hydrological cycle during the time span surrounding the Dansgaard-Oeschger (DO) 18 event. The calcite δ18O profile, together with another two stalagmite records from the same cave, shows a rapid decrease of 1.5‰ at~64.7 ka, confirming the occurrence of DO18 in the East Asian monsoon (EAM) region. The ending limb of DO18 in the EAM regime is characterized by five centennial-scale monsoon enhancing events, and finally culminates at~63.0 ka. Divergent from a rapid temperature increasing followed by a similarly rapid cooling (~60 a) back to stadial environments of the Greenland DO18 event, the DO18 event in the monsoon region exhibits a significant asymmetric feature with a sharp EAM strengthening but then a smoothing, lasting 1 500 a, decrease. Nonetheless, our stalagmite δ18O records mirror, in shape and timing, the variations of Atlantic meridional overturning circulation (AMOC) estimated by the ratio of 231Pa/230Th and recovered from the deep western North Atlantic. This similarity suggests that the AMOC played an important role in regulating the DO18 event in the EAM region. On the other hand, the discrepancy in the pattern of DO18 between the EAM and the North Atlantic climate systems can be attributed to the different regional response to the AMOC change in the high-and low-latitude climate systems. Spectral analysis of the stalagmite δ18O record denotes significant cycles at the 90% confidence level at 208, 150, 52 and 14-12 a, close to the previously reported solar cycles at 207, 149, 57 and 11 a. This consistency of cyclical variabilities between the EASM intensity and solar activity might indicate that solar forcing of centennial-and decadal-scale EASM variation still existed during the MIS4 interval with larger ice volumes.
2018, 36(4): 684-694.
doi: 10.14027/j.issn.1000-0550.2018.057
Abstract:
The Triassic Baikouquan Formation in Junggar Basin has not been clear about the characteristics of relative lake level changes, which restricts the analysis of their distribution and evolution of the sediments. This formation can be divided into one long-term sequence cycle, four mid-sequence cycles, 16 short-term sequence cycles, 126 super short-term sequence cycles on the high-resolution sequence stratigraphic division method combined with traditional qualitative analysis and quantitative logging wavelet transform analysis,under the guidance of high-resolution sequence stratigraphy theory, comprehensive drilling, logging and other information. The 126 super short-term sequence cycles were Fisher plots. Analysis of the Fisher plots shows the characteristics of relative lacustrine level changes. The Baikouquan Formation relative lacustrine level change is a continuous lacustrine transgression process.T1b1, T1b2 and T1b3, respectively, corresponds to a large-scale lacustrine transgression process, and each the large lacustrine transgression process contains several times of the lake level rise and fall cycle, and at T1b3 lake level reached the largest area. Compared with previous results and drilling geological information, we find that the lake curve has high reliability.
The Triassic Baikouquan Formation in Junggar Basin has not been clear about the characteristics of relative lake level changes, which restricts the analysis of their distribution and evolution of the sediments. This formation can be divided into one long-term sequence cycle, four mid-sequence cycles, 16 short-term sequence cycles, 126 super short-term sequence cycles on the high-resolution sequence stratigraphic division method combined with traditional qualitative analysis and quantitative logging wavelet transform analysis,under the guidance of high-resolution sequence stratigraphy theory, comprehensive drilling, logging and other information. The 126 super short-term sequence cycles were Fisher plots. Analysis of the Fisher plots shows the characteristics of relative lacustrine level changes. The Baikouquan Formation relative lacustrine level change is a continuous lacustrine transgression process.T1b1, T1b2 and T1b3, respectively, corresponds to a large-scale lacustrine transgression process, and each the large lacustrine transgression process contains several times of the lake level rise and fall cycle, and at T1b3 lake level reached the largest area. Compared with previous results and drilling geological information, we find that the lake curve has high reliability.
2018, 36(4): 695-705.
doi: 10.14027/j.issn.1000-0550.2018.124
Abstract:
Well CSDP-2 is the first overall coring well implemented by the Continental Shelf Drilling Program (CSDP) in the Central Uplift of the South Yellow Sea Basin, eastern China, whose completion depth is 2 843.18 m and mean drill-core rate is up to 97.7%. Based on the property of cored rocks, logging data, rock thin sections and other test data of Well CSDP-2, it is accurately distinguished in this paper that the rock types, sedimentary structures and lithological association sequences of Upper Paleozoic rocks in the Central Uplift of the South Yellow Sea Basin and then discuss their related sedimentary environments and vertical variations. The Devonian-Carboniferous-Permian strata drilled by Well CSDP-2 contain not only clastic rocks but also carbonate rocks and develop many sedimentary facies including delta, shore, tidal flat, carbonate platform and shallow shelf. In addition, the South Yellow Sea Basin has been experienced 2 completed transgressive-regressive cycles. Controlled by this mechanism, this region developed a clastic depositional system of sea-tidal flat, a carbonate depositional system of tidal flat-lagoon-grain beach, and a delta depositional system from bottom to top, indicating a sedimentary filling process from marine facies to transitional facies.
Well CSDP-2 is the first overall coring well implemented by the Continental Shelf Drilling Program (CSDP) in the Central Uplift of the South Yellow Sea Basin, eastern China, whose completion depth is 2 843.18 m and mean drill-core rate is up to 97.7%. Based on the property of cored rocks, logging data, rock thin sections and other test data of Well CSDP-2, it is accurately distinguished in this paper that the rock types, sedimentary structures and lithological association sequences of Upper Paleozoic rocks in the Central Uplift of the South Yellow Sea Basin and then discuss their related sedimentary environments and vertical variations. The Devonian-Carboniferous-Permian strata drilled by Well CSDP-2 contain not only clastic rocks but also carbonate rocks and develop many sedimentary facies including delta, shore, tidal flat, carbonate platform and shallow shelf. In addition, the South Yellow Sea Basin has been experienced 2 completed transgressive-regressive cycles. Controlled by this mechanism, this region developed a clastic depositional system of sea-tidal flat, a carbonate depositional system of tidal flat-lagoon-grain beach, and a delta depositional system from bottom to top, indicating a sedimentary filling process from marine facies to transitional facies.
2018, 36(4): 706-721.
doi: 10.14027/j.issn.1000-0550.2018.059
Abstract:
The theoretical study on the sedimentary texture of Cambrian platform margin is still rather weak at present, which was formed merely under the participation of microorganism due to the lack of reef-building organism. The platform marginal stratum of Lower Cambrian system in the Kalpin outcrop area of Tarim Basin was investigated and studied in this paper, using systematic field survey, field profile's measurement and indoor thin section analysis,to characterize the sedimentary texture of the platform marginal zone, establish a model of the sedimentary texture, and expound the formation process under the participation of microorganism. All study results above showed that the Lower Cambrian platform marginal sediments of the Tarim Basin had a width of about 30 km from south to north and a thickness of 110-167 m, which were composed of micritic deposition, laminated microbialite, microbial mound, granule deposition and stromatolite bottomup. These features indicated an upward shallowing sedimentary sequence from deep sea to intertidal zone, and hence the forming process of the platform margin could be divided into five stages. Micritic deposition, laminated microbialite, granule deposition and stromatolite showed stratiform feature on the whole platform margin, while the microbial mound only had a width of about 10 km from south to north on the platform margin and a maximal thickness of nearly 30 m. During this period, a upwarping region may occure locally on the platform margin and lead to the change of platform margin property before and after the formation of microbial mound. The platform margin showed some property of gentle slope before the formation of microbial mound but then transferred to a platform margin with slight rim after its formation. Such researches could provide sedimentary foundation to confirm the platform structure style and look for high-quality platform margin reservoirs in the Tarim Basin.
The theoretical study on the sedimentary texture of Cambrian platform margin is still rather weak at present, which was formed merely under the participation of microorganism due to the lack of reef-building organism. The platform marginal stratum of Lower Cambrian system in the Kalpin outcrop area of Tarim Basin was investigated and studied in this paper, using systematic field survey, field profile's measurement and indoor thin section analysis,to characterize the sedimentary texture of the platform marginal zone, establish a model of the sedimentary texture, and expound the formation process under the participation of microorganism. All study results above showed that the Lower Cambrian platform marginal sediments of the Tarim Basin had a width of about 30 km from south to north and a thickness of 110-167 m, which were composed of micritic deposition, laminated microbialite, microbial mound, granule deposition and stromatolite bottomup. These features indicated an upward shallowing sedimentary sequence from deep sea to intertidal zone, and hence the forming process of the platform margin could be divided into five stages. Micritic deposition, laminated microbialite, granule deposition and stromatolite showed stratiform feature on the whole platform margin, while the microbial mound only had a width of about 10 km from south to north on the platform margin and a maximal thickness of nearly 30 m. During this period, a upwarping region may occure locally on the platform margin and lead to the change of platform margin property before and after the formation of microbial mound. The platform margin showed some property of gentle slope before the formation of microbial mound but then transferred to a platform margin with slight rim after its formation. Such researches could provide sedimentary foundation to confirm the platform structure style and look for high-quality platform margin reservoirs in the Tarim Basin.
2018, 36(4): 722-730.
doi: 10.14027/j.issn.1000-0550.2018.060
Abstract:
To verify the controversial issue of provenance in the Dongtai depression, Subei Basin, several groups of single factor comparisonal modelings are carried out based mainly on the cores, logging and seismic data by utilizing the most internationally advanced three-dimensional stratigraphic forward modeling. The sedimentary evolution characteristics of stratigraphy are simulated under different provenance data from different viewpoints, and either the Jianhu uplift as the materials source for the Funing Formation in the Dongtai depression is also discussed. A well geologically actual matched simulation result is obtained from a three-dimensional stratigraphic forward modeling with two separated sources, of which one is from north by east and another from west by south. The rationality of the final model is verified through lithology and stratigraphic thickness of multiple logs. Combined with multigroup simulation results, it is showed that the Funing Formation sediments in the Dongtai depression, Subei Basin are mainly originated from the Jianhu uplift and Zhangbaling uplift.
To verify the controversial issue of provenance in the Dongtai depression, Subei Basin, several groups of single factor comparisonal modelings are carried out based mainly on the cores, logging and seismic data by utilizing the most internationally advanced three-dimensional stratigraphic forward modeling. The sedimentary evolution characteristics of stratigraphy are simulated under different provenance data from different viewpoints, and either the Jianhu uplift as the materials source for the Funing Formation in the Dongtai depression is also discussed. A well geologically actual matched simulation result is obtained from a three-dimensional stratigraphic forward modeling with two separated sources, of which one is from north by east and another from west by south. The rationality of the final model is verified through lithology and stratigraphic thickness of multiple logs. Combined with multigroup simulation results, it is showed that the Funing Formation sediments in the Dongtai depression, Subei Basin are mainly originated from the Jianhu uplift and Zhangbaling uplift.
2018, 36(4): 731-742.
doi: 10.14027/j.issn.1000-0550.2018.052
Abstract:
Along with oil and gas exploration of middle-shallow layers entering into medium-high maturity stage, the Upper Paleozoic of North China has become an important area for petroleum exploration and reserves replacement. The study of sedimentary facies and sedimentary evolution process in the Upper Paleozoic had a guiding role for oil and gas exploration. This research was guided by sedimentology theory, based on the outcrop section in the Upper Paleozoic of Zibo Boshan area, central-southern of North China, identified the sedimentary facies types and sedimentary sequence, and ascertained the sedimentary evolution process through the detailed field reconnaissance. Research showed that:it can be divided into Benxi Formation, Taiyuan Formation, Shanxi Formation, lower and upper Shihezi Formation and Shiqianfeng Formation from bottom to top of Zibo Boshan area in the Upper Paleozoic, identified 5 type of sedimentary facies(barrier coast, tide, carbonate platform, delta, meandering river) and 8 type of typical sedimentary sequences. Benxi and Taiyuan formation develop barrier coast and carbonate platform facies; Shanxi Formation develops delta facies;Lower and upper Shihezi Formation and Shiqianfeng Formation develop fluvial facies and tide. Upper Paleozoic of Zibo Boshan area experienced a sedimentary evolution process transition from the barrier coast-platform complex sedimentary system to delta facies, and finally turn into fluvial facies.
Along with oil and gas exploration of middle-shallow layers entering into medium-high maturity stage, the Upper Paleozoic of North China has become an important area for petroleum exploration and reserves replacement. The study of sedimentary facies and sedimentary evolution process in the Upper Paleozoic had a guiding role for oil and gas exploration. This research was guided by sedimentology theory, based on the outcrop section in the Upper Paleozoic of Zibo Boshan area, central-southern of North China, identified the sedimentary facies types and sedimentary sequence, and ascertained the sedimentary evolution process through the detailed field reconnaissance. Research showed that:it can be divided into Benxi Formation, Taiyuan Formation, Shanxi Formation, lower and upper Shihezi Formation and Shiqianfeng Formation from bottom to top of Zibo Boshan area in the Upper Paleozoic, identified 5 type of sedimentary facies(barrier coast, tide, carbonate platform, delta, meandering river) and 8 type of typical sedimentary sequences. Benxi and Taiyuan formation develop barrier coast and carbonate platform facies; Shanxi Formation develops delta facies;Lower and upper Shihezi Formation and Shiqianfeng Formation develop fluvial facies and tide. Upper Paleozoic of Zibo Boshan area experienced a sedimentary evolution process transition from the barrier coast-platform complex sedimentary system to delta facies, and finally turn into fluvial facies.
2018, 36(4): 743-755.
doi: 10.14027/j.issn.1000-0550.2018.066
Abstract:
Multiple sets of marine shales sediment in the Dianqianbei depression lead to a meaningful study area for marine stratum in southern China. The Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation, especially the Wufeng-Lower Longmaxi Formation considered as high-quality source rock has been raised much attention. We classified the shale lithofacies and analyzed the environment by cores observation, thin section identification, XRD and geochemical indicators of trace elements. Six types of shale lithofacies are classified, including siliceous shale, clayey shale, calcareous shale, clayey and siliceous shale, clayey and calcareous shale, calcareous and siliceous shale. Among the six types of shale lithofacies, TOC are richer in the siliceous shale and clayey shale than other four types. V/(V+Ni), V/Cr, Ni/Co and U/Th show Wufeng-Lower Longmaxi Formation sediments in the conditions of low energy, sluggish water flow and constrained sea water, which matches deep shelf. Mo content in Dianqianbei depression is near to the content in Sichuan Basin. Transgressions happen in early stage of Wufeng and Longmaxi, which lead deep shelf plays a vital role in the sedimentary environments. Anaerobic environment of low energy, sluggish water flow, constrained water condition and rapid transgression are essential in the richness and preservation of organic matter.
Multiple sets of marine shales sediment in the Dianqianbei depression lead to a meaningful study area for marine stratum in southern China. The Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation, especially the Wufeng-Lower Longmaxi Formation considered as high-quality source rock has been raised much attention. We classified the shale lithofacies and analyzed the environment by cores observation, thin section identification, XRD and geochemical indicators of trace elements. Six types of shale lithofacies are classified, including siliceous shale, clayey shale, calcareous shale, clayey and siliceous shale, clayey and calcareous shale, calcareous and siliceous shale. Among the six types of shale lithofacies, TOC are richer in the siliceous shale and clayey shale than other four types. V/(V+Ni), V/Cr, Ni/Co and U/Th show Wufeng-Lower Longmaxi Formation sediments in the conditions of low energy, sluggish water flow and constrained sea water, which matches deep shelf. Mo content in Dianqianbei depression is near to the content in Sichuan Basin. Transgressions happen in early stage of Wufeng and Longmaxi, which lead deep shelf plays a vital role in the sedimentary environments. Anaerobic environment of low energy, sluggish water flow, constrained water condition and rapid transgression are essential in the richness and preservation of organic matter.
2018, 36(4): 756-767.
doi: 10.14027/j.issn.1000-0550.2018.050
Abstract:
To do a fine description of paleo-provenance system has great significance for the restoration of sedimentary system, searching for dominant sandstone zone and also prediction of target area. In this study, various data including ancient landform, seismic attribute, heavy minerals, sand ratio of formation and other aspects of information were used to do a fine description and research for the provenance system of Nantun Formation in the Beier depression from different aspects and various levels. The results showed that the source was mainly from the surrounding slope zones and low uplitting regions in the basin, which converged from the edge to the basin center during the Nantun Period in the Beier depression. The main source direction of the two Nantun sub-groups had obvious inheritance. Nine provenance systems in the Beier depression can be defined as mainly developed from the sedimentary environment of fan delta, sub-lacustrine fan and so on. According to the differences in developing locations, construction styles and tectonic activity, six types of materials provenace can be divided as step slope-fracture slope. All above research showed that the coupling system composed of the source and the slope and the gully can fundamentally control and determine the types of sedimentary system and their spreading regularity.
To do a fine description of paleo-provenance system has great significance for the restoration of sedimentary system, searching for dominant sandstone zone and also prediction of target area. In this study, various data including ancient landform, seismic attribute, heavy minerals, sand ratio of formation and other aspects of information were used to do a fine description and research for the provenance system of Nantun Formation in the Beier depression from different aspects and various levels. The results showed that the source was mainly from the surrounding slope zones and low uplitting regions in the basin, which converged from the edge to the basin center during the Nantun Period in the Beier depression. The main source direction of the two Nantun sub-groups had obvious inheritance. Nine provenance systems in the Beier depression can be defined as mainly developed from the sedimentary environment of fan delta, sub-lacustrine fan and so on. According to the differences in developing locations, construction styles and tectonic activity, six types of materials provenace can be divided as step slope-fracture slope. All above research showed that the coupling system composed of the source and the slope and the gully can fundamentally control and determine the types of sedimentary system and their spreading regularity.
2018, 36(4): 768-776.
doi: 10.14027/j.issn.1000-0550.2018.056
Abstract:
The Pingdiquan Formation (also known as Lucaogou Formation) of middle Permian in Huoshaoshan oilfield, Junggar Basin, Xinjiang, is a well-known black oil-bearing series that has been considered as a set of deep, hydrocarbon-rich lacustrine mudstone in a rift basin. Comprehensive mineralogy and petrology analysis indicate that abundant rock fragments are from the explosion of alkaline-peralkaline ultrabasic magmatic rocks (mainly of carbonatite) and intermediate-acid magmatic rocks (mainly rhyolite and andesite). This is characterized by the fragments of bimodal volcanic rocks such as alkaline-peralkaline ultrabasic magmatic rocks (including alkaline feldspar, aluminum-rich quart, magma calcite, aegirine, lithium mica titanium, batisite, serpentine, etc.) and fragments of intermediate-acidic magmatic rocks (including alkaline feldspar, quart). The system of lake-swamp-hot spring is rich in mantle fluids contributing to hydrothermal alternation of above explosive matters and matrix, which was eventually deposited with normal sedimentary rock layers, and formed a set of tuff material-hydrothermal explosive rocks which are rich in mantle-derived fluids. The alternation also leads to hydrocarbon generated in deposited period. Tuff material provided heat and nutrients contributed the algae, microorganisms to promote hydrocarbons generation in lakeshore, marsh, and hydrothermal field. Abundant primary intergranular pores of tuff and analcite dissolution pores are the main reservoir space of hydrocarbons. The thickness of tuff layers varies greatly and they are intebeted in lacustrine sedimentary layers.
The Pingdiquan Formation (also known as Lucaogou Formation) of middle Permian in Huoshaoshan oilfield, Junggar Basin, Xinjiang, is a well-known black oil-bearing series that has been considered as a set of deep, hydrocarbon-rich lacustrine mudstone in a rift basin. Comprehensive mineralogy and petrology analysis indicate that abundant rock fragments are from the explosion of alkaline-peralkaline ultrabasic magmatic rocks (mainly of carbonatite) and intermediate-acid magmatic rocks (mainly rhyolite and andesite). This is characterized by the fragments of bimodal volcanic rocks such as alkaline-peralkaline ultrabasic magmatic rocks (including alkaline feldspar, aluminum-rich quart, magma calcite, aegirine, lithium mica titanium, batisite, serpentine, etc.) and fragments of intermediate-acidic magmatic rocks (including alkaline feldspar, quart). The system of lake-swamp-hot spring is rich in mantle fluids contributing to hydrothermal alternation of above explosive matters and matrix, which was eventually deposited with normal sedimentary rock layers, and formed a set of tuff material-hydrothermal explosive rocks which are rich in mantle-derived fluids. The alternation also leads to hydrocarbon generated in deposited period. Tuff material provided heat and nutrients contributed the algae, microorganisms to promote hydrocarbons generation in lakeshore, marsh, and hydrothermal field. Abundant primary intergranular pores of tuff and analcite dissolution pores are the main reservoir space of hydrocarbons. The thickness of tuff layers varies greatly and they are intebeted in lacustrine sedimentary layers.
2018, 36(4): 777-786.
doi: 10.14027/j.issn.1000-0550.2018.112
Abstract:
According to drilling data, core samples and casting thin-sections, and combined with data from scanning electron microscopy, X-diffraction, physical property statistics and well logging, the physical properties and main controlling factors are discussed in this paper for the anomalously high porosity zones in the deeply buried reservoirs in west part of northern edge of the Qiadam Basin, northwetern China. The reservoirs buried over 3 000 m in the study area were mainly formed in the Paleogene, and there were a suite of detrital deposits of braided river delta-shallow shore. The grain sizes are small, mainly in a range of silt-fine sand. The lithologies are mainly feldspar lithic sandstone and lithic feldspathic sandstone. The pore types are dominated by intergranular pore, followed by secondary pore, and a few cracks. The pores and throat paths are well matched. The sandstone formed in the distributary channel of braided river delta front and shore-shallow lake have less argillaceous matrix with better compositional and structural maturity, which are the fundamental conditions for the high quality reservoirs formation. The carbonate cements formed in the early digenetic stage play a supporting role among particles, which effectively resist to the compaction. The thick layers of under-compacted mudstone on and below the reservoirs are key factors for the preservation of primary pores, the discharge of fluids in the pore was hampered. So the fluids bear parts of the loads and lead to the preservation of the pores.
According to drilling data, core samples and casting thin-sections, and combined with data from scanning electron microscopy, X-diffraction, physical property statistics and well logging, the physical properties and main controlling factors are discussed in this paper for the anomalously high porosity zones in the deeply buried reservoirs in west part of northern edge of the Qiadam Basin, northwetern China. The reservoirs buried over 3 000 m in the study area were mainly formed in the Paleogene, and there were a suite of detrital deposits of braided river delta-shallow shore. The grain sizes are small, mainly in a range of silt-fine sand. The lithologies are mainly feldspar lithic sandstone and lithic feldspathic sandstone. The pore types are dominated by intergranular pore, followed by secondary pore, and a few cracks. The pores and throat paths are well matched. The sandstone formed in the distributary channel of braided river delta front and shore-shallow lake have less argillaceous matrix with better compositional and structural maturity, which are the fundamental conditions for the high quality reservoirs formation. The carbonate cements formed in the early digenetic stage play a supporting role among particles, which effectively resist to the compaction. The thick layers of under-compacted mudstone on and below the reservoirs are key factors for the preservation of primary pores, the discharge of fluids in the pore was hampered. So the fluids bear parts of the loads and lead to the preservation of the pores.
2018, 36(4): 787-795.
doi: 10.14027/j.issn.1000-0550.2018.048
Abstract:
The central canyon channel of the Xisha trough basin is in large scale, great depth and low sinuosity, showing great restriction and east-west segmentation. By taking advantage of the two-dimensional seismic data, the seismic facies features identification and its segmentation features are investigated in this paper. From the seismic profile in the research area, five seismic facies types are identified. They are:bottom sands and gravel, turbidite deposits, mass transport complex, muddy sediments, and slumps. The internal filling sequence correlates with the restriction level. According to the paleo-geomorphology map, seismic and sedimentary faces maps, the central canyon of the Xisha trough is distributed in the east and west direction. It is low-lying and wider in west, high and narrow in east. Since segmentation shows good consistency and parallelism to the structural fault zone, a turning point is defined to divide the two sections in the Xisha secondary recession. The western section is strongly restricted while the eastern section is weakly restricted. Totally 13 types of segmentation features are summarized from the aspects of distribution, shape, filling etc. of the channel system. And assume that the restriction and segmentation are mainly controlled by tectonic activity, topography and provenance supply.
The central canyon channel of the Xisha trough basin is in large scale, great depth and low sinuosity, showing great restriction and east-west segmentation. By taking advantage of the two-dimensional seismic data, the seismic facies features identification and its segmentation features are investigated in this paper. From the seismic profile in the research area, five seismic facies types are identified. They are:bottom sands and gravel, turbidite deposits, mass transport complex, muddy sediments, and slumps. The internal filling sequence correlates with the restriction level. According to the paleo-geomorphology map, seismic and sedimentary faces maps, the central canyon of the Xisha trough is distributed in the east and west direction. It is low-lying and wider in west, high and narrow in east. Since segmentation shows good consistency and parallelism to the structural fault zone, a turning point is defined to divide the two sections in the Xisha secondary recession. The western section is strongly restricted while the eastern section is weakly restricted. Totally 13 types of segmentation features are summarized from the aspects of distribution, shape, filling etc. of the channel system. And assume that the restriction and segmentation are mainly controlled by tectonic activity, topography and provenance supply.
2018, 36(4): 796-806.
doi: 10.14027/j.issn.1000-0550.2018.044
Abstract:
Plentiful oil and gas resources are contained in the Majiagou Formation of Ordovician system in the northeastern Ordos Basin, particully the recent oil and gas exploration has made important breakthroughs in the subsalt of carbonate rocks of Ma57 sub-member as the main reservoir. Based on wells drilling data, logging, drilling cores, thin section and other related information, the Ma57 sub-member of Ordovician system in the central Ordos Basin has been comprehensively analyzed for sedimentary characteristics of the grain banks and their vertical and lateral distribution patterns, in oder to search for the paleogeography for sedimentary patterns as well as the controlling factors on the development of grain banks in the study area. The grain banks of Ma57 sub-member are mainly consisted of fine crystal grain dolostone and other kinds of grain dolostone. Three types of upward sedimentary sequences were developed including lagoon-grain bank-mounds-platform inner flat, lagoon-grain bank-interact bank sea-grain bank and lagoon-grain bank-mounds. The grain bank formed at the upper part with upward shallowing cycle and became mound-shoal complex with microbes for the most time, showing important reservoir significance. In the lateral direction, the Ma57 sub-member had developed two larger grain bank layers with stable location and quite good continuity, being the east thick and midwest thin for the overall performance characteristics, and for the plane performance characteristics on north-south direction and circularly distribution along eastern depression. Further analysis considering the sedimentary regular patterns indicated the Ma57 sub-member was relatively lacked with tidal flat face, thus the suspected restricted-evaporate platform was much more reasonable. The frequently changed sea levels in depositional stage controlled the development scale of grain bank and its placed order, as well as the ancient tectonic framework with lower west and higher east and its inner micro-topography fluctuation might influence the plane distribution of grain bank of Ma57 sub-member.
Plentiful oil and gas resources are contained in the Majiagou Formation of Ordovician system in the northeastern Ordos Basin, particully the recent oil and gas exploration has made important breakthroughs in the subsalt of carbonate rocks of Ma57 sub-member as the main reservoir. Based on wells drilling data, logging, drilling cores, thin section and other related information, the Ma57 sub-member of Ordovician system in the central Ordos Basin has been comprehensively analyzed for sedimentary characteristics of the grain banks and their vertical and lateral distribution patterns, in oder to search for the paleogeography for sedimentary patterns as well as the controlling factors on the development of grain banks in the study area. The grain banks of Ma57 sub-member are mainly consisted of fine crystal grain dolostone and other kinds of grain dolostone. Three types of upward sedimentary sequences were developed including lagoon-grain bank-mounds-platform inner flat, lagoon-grain bank-interact bank sea-grain bank and lagoon-grain bank-mounds. The grain bank formed at the upper part with upward shallowing cycle and became mound-shoal complex with microbes for the most time, showing important reservoir significance. In the lateral direction, the Ma57 sub-member had developed two larger grain bank layers with stable location and quite good continuity, being the east thick and midwest thin for the overall performance characteristics, and for the plane performance characteristics on north-south direction and circularly distribution along eastern depression. Further analysis considering the sedimentary regular patterns indicated the Ma57 sub-member was relatively lacked with tidal flat face, thus the suspected restricted-evaporate platform was much more reasonable. The frequently changed sea levels in depositional stage controlled the development scale of grain bank and its placed order, as well as the ancient tectonic framework with lower west and higher east and its inner micro-topography fluctuation might influence the plane distribution of grain bank of Ma57 sub-member.
2018, 36(4): 807-817.
doi: 10.14027/j.issn.1000-0550.2018.047
Abstract:
Based on a large number of drilling, geophysical and analytical data, the Guantao Formation of Miaoxi'nan Uplift in the Bohai sea area was confirmed to have stable tectonic environments with gentle ancient topography, semiarid-arid climate, sufficient source supply, and shallow ancient water depth. All these conditions were suitable for the formation of a shallow braided river delta. For its special climate environment, the great plains and small front of sedimentary characteristics formed, the water area was small and dominated by river actions. The sand bodies of plain subfacies were characterized by branching channels with double section probability curves of grain size and strong power of sedimentary structures. In addition, under arid climatic conditions, unstable water bodies caused purple red mudstone on behalf of the oxidizing environment to interact with grey green and variegated mudstone on behalf of the weak reducing environment. On the basis of climate change, the relatively lake level change and provenance supply, the shallow braided river delta sand body of the Guantao Formation in the study area can be divided into isolated type, multilateral type and fold type, corresponding to each period of the Guantao Formation. Sand body evolution analysis confirmed that the ancient terrain, climate change and relative lake level variation were the key to influence sand body styles. The multilateral type of reservoir sand body of the middle period of Guantao Formation had good connectivity and flat wide distribution. The top part of the sand body reservoir with plane distribution and thickness could be a key target for the next exploration in this area.
Based on a large number of drilling, geophysical and analytical data, the Guantao Formation of Miaoxi'nan Uplift in the Bohai sea area was confirmed to have stable tectonic environments with gentle ancient topography, semiarid-arid climate, sufficient source supply, and shallow ancient water depth. All these conditions were suitable for the formation of a shallow braided river delta. For its special climate environment, the great plains and small front of sedimentary characteristics formed, the water area was small and dominated by river actions. The sand bodies of plain subfacies were characterized by branching channels with double section probability curves of grain size and strong power of sedimentary structures. In addition, under arid climatic conditions, unstable water bodies caused purple red mudstone on behalf of the oxidizing environment to interact with grey green and variegated mudstone on behalf of the weak reducing environment. On the basis of climate change, the relatively lake level change and provenance supply, the shallow braided river delta sand body of the Guantao Formation in the study area can be divided into isolated type, multilateral type and fold type, corresponding to each period of the Guantao Formation. Sand body evolution analysis confirmed that the ancient terrain, climate change and relative lake level variation were the key to influence sand body styles. The multilateral type of reservoir sand body of the middle period of Guantao Formation had good connectivity and flat wide distribution. The top part of the sand body reservoir with plane distribution and thickness could be a key target for the next exploration in this area.
2018, 36(4): 818-828.
doi: 10.14027/j.issn.1000-0550.2018.093
Abstract:
Based on the GC-MS analytical data of 16 samples of Chang7 source rocks from 13 wells in the Yanchi-Dingbian area, Ordos Basin, the geochemical characteristics of aromatic hydrocarbons of source rocks, the origins and sedimentary environments were investigated in detail, and the validity of maturity parameters of aromatic hydrocarbon biomarkers were discussed. It is shown that the source rocks in the study area are characterized by high abundance of bicyclic and tricyclic aromatic hydrocarbons, low abundance of tetracyclic and pentacyclic aromatic hydrocarbons, relatively high abundance of fluorene, relatively high abundance of 1,2,5-TMN and 1,2,5,6-TeMN in some samples, a certain content of terrigenous biomarkers such as retene, perylene, chrysene and pyrene. All these results indicated the source rocks of Chang7 Formation were mainly developed in slightly oxidizing-reducing shore-shallow lacustrine environment. Besides, the terrestrial higher plants input had certain contribution in the formation of source rocks. The maturity parameters of alkyl naphthalene, alkyl phenanthrene and alkyl dibenzothiophene from aromatic hydrocarbons suggested that the thermal evolution stage of source rocks reached the maturity stage. Although the equivalent vitrinite reflectance calculated by different series of aromatic compounds had some differences, it could qualitatively reflect the stage of thermal evolution of source rocks.
Based on the GC-MS analytical data of 16 samples of Chang7 source rocks from 13 wells in the Yanchi-Dingbian area, Ordos Basin, the geochemical characteristics of aromatic hydrocarbons of source rocks, the origins and sedimentary environments were investigated in detail, and the validity of maturity parameters of aromatic hydrocarbon biomarkers were discussed. It is shown that the source rocks in the study area are characterized by high abundance of bicyclic and tricyclic aromatic hydrocarbons, low abundance of tetracyclic and pentacyclic aromatic hydrocarbons, relatively high abundance of fluorene, relatively high abundance of 1,2,5-TMN and 1,2,5,6-TeMN in some samples, a certain content of terrigenous biomarkers such as retene, perylene, chrysene and pyrene. All these results indicated the source rocks of Chang7 Formation were mainly developed in slightly oxidizing-reducing shore-shallow lacustrine environment. Besides, the terrestrial higher plants input had certain contribution in the formation of source rocks. The maturity parameters of alkyl naphthalene, alkyl phenanthrene and alkyl dibenzothiophene from aromatic hydrocarbons suggested that the thermal evolution stage of source rocks reached the maturity stage. Although the equivalent vitrinite reflectance calculated by different series of aromatic compounds had some differences, it could qualitatively reflect the stage of thermal evolution of source rocks.
2018, 36(4): 829-841.
doi: 10.14027/j.issn.1000-0550.2018.074
Abstract:
Based on compositions of various biomarkers, the crude oils from northern Qaidam Basin have relatively higher Pr/Ph ratios (>2.0), moderate contents of rearranged steranes (diaC27/reC27=0.5-0.7), rearranged hopane (diaC30H/C30H=0.2-0.4) and neohopane (C29Ts/C29H=0.4-0.7), and lower gammacerane (gammacerane index<0.05), consistenting with depositional environments of Lower Jurassic source rocks. However, the crude oils from the north oilfields in western Qaidam Basin have the characteristics of lower Pr/Ph ratios (<0.8), lower contents of rearranged steranes (diaC27/reC27<0.1), rearranged hopane (diaC30H/C30H<0.05) and neohopane (C29Ts/C29H=0.2-0.4), and abundant gammacerane (gammacerane index=0.4-0.8), which may relate to saline source rocks. The crude oils from the Dongping area have a completely different biomarker assemblage from the two kinds of crude oils mentioned above in the study area, that is, abundant gammacerane (gammacerane index=1.2-3.0) coexists with high rearranged steranes (diaC27/reC27>0.4), rearranged hopane (diaC30H/C30H>0.7) and neohopane (C29Ts/C29H>1.5). It is very difficult to be explained by the distributions of gammacerane, rearranged steranes, rearranged hopanes and neohopanes in geological samples, because occurrence and formation of abundant gammacerane, rearranged biomarkers and neohopanes require completely different geological conditions. Furthermore, the relative compositions of various biomarkers in different crude oils from the study area suggest that this unusual biomarker assemblage in crude oils from the Dongping area is not from mixing of different crude oils, but an objective geological phenomenon, although its generation condition and geochemical significance remain unknown at present. Therefore, the crude oils having unusual biomarker assemblage are a new kind of crude oils in the Qaidam Basin, and their source rocks are probably deposited in an acid and saline environment.
Based on compositions of various biomarkers, the crude oils from northern Qaidam Basin have relatively higher Pr/Ph ratios (>2.0), moderate contents of rearranged steranes (diaC27/reC27=0.5-0.7), rearranged hopane (diaC30H/C30H=0.2-0.4) and neohopane (C29Ts/C29H=0.4-0.7), and lower gammacerane (gammacerane index<0.05), consistenting with depositional environments of Lower Jurassic source rocks. However, the crude oils from the north oilfields in western Qaidam Basin have the characteristics of lower Pr/Ph ratios (<0.8), lower contents of rearranged steranes (diaC27/reC27<0.1), rearranged hopane (diaC30H/C30H<0.05) and neohopane (C29Ts/C29H=0.2-0.4), and abundant gammacerane (gammacerane index=0.4-0.8), which may relate to saline source rocks. The crude oils from the Dongping area have a completely different biomarker assemblage from the two kinds of crude oils mentioned above in the study area, that is, abundant gammacerane (gammacerane index=1.2-3.0) coexists with high rearranged steranes (diaC27/reC27>0.4), rearranged hopane (diaC30H/C30H>0.7) and neohopane (C29Ts/C29H>1.5). It is very difficult to be explained by the distributions of gammacerane, rearranged steranes, rearranged hopanes and neohopanes in geological samples, because occurrence and formation of abundant gammacerane, rearranged biomarkers and neohopanes require completely different geological conditions. Furthermore, the relative compositions of various biomarkers in different crude oils from the study area suggest that this unusual biomarker assemblage in crude oils from the Dongping area is not from mixing of different crude oils, but an objective geological phenomenon, although its generation condition and geochemical significance remain unknown at present. Therefore, the crude oils having unusual biomarker assemblage are a new kind of crude oils in the Qaidam Basin, and their source rocks are probably deposited in an acid and saline environment.
2018, 36(4): 842-852.
doi: 10.14027/j.issn.1000-0550.2018.042
Abstract:
Lacustrine carbonates are formed in continental environments, normally containing high content of terrestrial silicate component that may affect their elemental composition and also the measuring results. Thus, many methods commonly used for marine carbonates, such as acid-soluble method, Electron Probe Microanalysis (EPMA) and Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) can not simply be used for lacustrine carbonates. Lacustrine carbonates are widely developed in the Tertiary strata of the Qaidam Basin and provide very suitable materials for the study to select suitable methods. By means of comprehensive geochemical study on the lacustrine carbonates, which spread from Xiaganchaigou Formation to Xiayoushashan formation of the Hongshanliang section in Mangdong area, the south area of western Qaidam Basin, the limitation of each method was investigated and their related influence was also discussed for the geochemical characteristics of lacustrine carbonates by terrestrial silicate components. For examples, the limitation of acid-soluble method is the testing results of alkali metals such as K non-suitable for subsequent analysis, which may come from acid-dissolving process. For the micro-area test method including EPMA and LA, there are two limitations. Firstly, the data by LA and EPMA can be used only when the results using these two methods have some correlation and relatively small errors. Secondly, the testing results can not be used directly for the lacustrine carbonates with muddy components including a large degree of terrestrial silicate component. This limitation can be explained by the relative size of the sampled spot and the mineral particle. When the sampled spot size is larger than the mineral particle, the testing results will be mixed and interfaced with surrounding components. As a result, the muddy portion may represent the characteristics of sedimentary water only by excluding terrestrial interference. Based on the analysis for the effect of terrestrial silicate components on the geochemical characteristics of lacustrine carbonates, we proposed two possible ways to exclude terrestrial effects. The first one is selecting baseline and screening, and the other one is calculating by reverse deduction.
Lacustrine carbonates are formed in continental environments, normally containing high content of terrestrial silicate component that may affect their elemental composition and also the measuring results. Thus, many methods commonly used for marine carbonates, such as acid-soluble method, Electron Probe Microanalysis (EPMA) and Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) can not simply be used for lacustrine carbonates. Lacustrine carbonates are widely developed in the Tertiary strata of the Qaidam Basin and provide very suitable materials for the study to select suitable methods. By means of comprehensive geochemical study on the lacustrine carbonates, which spread from Xiaganchaigou Formation to Xiayoushashan formation of the Hongshanliang section in Mangdong area, the south area of western Qaidam Basin, the limitation of each method was investigated and their related influence was also discussed for the geochemical characteristics of lacustrine carbonates by terrestrial silicate components. For examples, the limitation of acid-soluble method is the testing results of alkali metals such as K non-suitable for subsequent analysis, which may come from acid-dissolving process. For the micro-area test method including EPMA and LA, there are two limitations. Firstly, the data by LA and EPMA can be used only when the results using these two methods have some correlation and relatively small errors. Secondly, the testing results can not be used directly for the lacustrine carbonates with muddy components including a large degree of terrestrial silicate component. This limitation can be explained by the relative size of the sampled spot and the mineral particle. When the sampled spot size is larger than the mineral particle, the testing results will be mixed and interfaced with surrounding components. As a result, the muddy portion may represent the characteristics of sedimentary water only by excluding terrestrial interference. Based on the analysis for the effect of terrestrial silicate components on the geochemical characteristics of lacustrine carbonates, we proposed two possible ways to exclude terrestrial effects. The first one is selecting baseline and screening, and the other one is calculating by reverse deduction.
