Abstract: The Cambrian explosion has always been a hotspot of international research interest. As one of the key indispensable regions for global geological events and evolution, the fundamental geological research of the Cambrian in the Ordos Basin has yet to be further understood, and the oil and gas exploration potentials in this layer is yet to be revealed. In order to enrich the basic geological data in this area further, this study selected nine field profiles of the Middle Cambrian Xuzhuang Formation in the periphery of the Ordos Basin. A total of 192 rock samples were systematically sampled, sliced, and identified. The rock types studied include terrestrial clastic, carbonate, and mixed rocks. The dataset shows the mineral composition and content of the rock slices of the Xuzhuang Formation in detail, including the structures; observations; and descriptions of biological fossils and diagenetic processes, while providing systematic polarized photomicrographs. This dataset does not only provide supporting data for basic Cambrian geological research on a global level but also fundamental geological data that can be used for the oil and gas explorations of the Middle Cambrian strata in the Ordos Basin.
Keywords: micro atlas; thin section; Middle Cambrian Xuzhuang Formation; Ordos Basin
|Title||Dataset of microscopic images of rock slices from the Xuzhuang Formation in Ordos Basin|
|Corresponding author||Xing Fengcun (email@example.com)|
|Data authors||Qian Hongshan, Xing Fengcun, Zhang Chunlin, Gu Qiang, Chen Xiaoquan, Fu Hui|
|Time range||The stratigraphic age of samples is Middle Cambrian; rock samples were collected from 2017 to 2019; polarizing microscope photos of rock slices were taken in 2020.|
|Geographical scope||Luliang City, Yuncheng City, Xinzhou City, Shuozhou City, Shanxi Province, Xianyang City, Shaanxi Province, Wuzhong City, Ningxia Hui Autonomous Region.|
|Polarized microscope resolution||1280 × 960 pixels|
|Data volume||1.98 GB|
|Data format||*.png, *.xlsx|
|Data service system||<http://dx.doi.org/10.11922/sciencedb.j00001.00102>|
|Sources of funding||National Natural Science Foundation of China (Grant No. 41672103, 41302089)|
|Dataset composition||1. This dataset consists of two parts: (1) Thin-film photos and polarized micrographs of 192 rock samples collected from 9 field profiles, totaling 836 that are stored in three folders of clastic, carbonate, and mixed rock slice photos. The photos are in PNG format, and the data volume is 1.98 GB. (2) The thin-slice information table shows the basic information of rock thin slices. The identification data table, including the clastic and carbonate rock thin-slice identification tables, and the information of the thin slices of mixed rocks have been written into the carbonate rock identification table, and the data volume is 68.5 KB.|
The Cambrian explosion and plate movement during that period are the hotspots of international research. The proposal of the Cambrian extensional trough theory in the Sichuan Basin in China and the gradual attention of other basins [1-5] have also increased the exploration of a series of earth science issues, such as the plate movement process and mechanisms and sedimentation responses that took place during this period. Currently, comprehensive research on the Cambrian geology and geological events of the Yangtze and Tarim plates has been continuously investigated on a deeper level, and numerous major oil and gas explorations breakthroughs have been made [4-5]. By comparison, the Cambrian strata in the Ordos Basin have large thickness and contain wide range of deposits. The Cambrian lithostratigraphic units have unique characteristics that record rich geological information. Even so, basic geological research in this area has been relatively weak, and breakthroughs have not been made in the oil and gas explorations. Whether for the purposes of researching geological systems or oil and gas exploration, it is necessary to systematically study the comprehensive geological information of the Cambrian strata in the Ordos Basin. Currently, there are few basic petrographic materials that can be publicly accessed for the Middle Cambrian Xuzhuang Formation. Microscopic image data in open-access (public) publications and other published literature is also quite scarce, which directly affects the development of basic geological research.
2.1 Data collection
Based on previous literature research and field exploration work, this paper selected 9 sections for systematic sample collection, while the total number of samples obtained is 192. The samples collected in the field were firstly pre-processed. In order to avoid the difficulties and errors in the later identification work that could be caused by the contamination of rock samples, the authors screened the field samples, carefully selected fresh samples and assigned ordinary thin slices in accordance with the national standard. Optical rock flakes had a thickness of 0.03 mm.
2.2 Processing method
Observations under the microscope were completed in the Sedimentary Geology Laboratory of the Institute of Sedimentary Geology, Chengdu University of Technology. A polarizing microscope with the instrument model WieNikon LV100POL was used to collect thin-slice images. The test conditions were at a temperature of 25°C with humidity levels at 50%. The methods of photographing and information collection of the thin slices were uniformly implemented in accordance with the standards of “Special Topics on Rock Microscopic Images” . The system collects rock microscopic images and at the same time, obtains relevant information about the thin slices. On this basis, systematic observations, analyses, and identifications were carried out. The thin-slice descriptions and sedimentary rock names were based on the standards of “Special Topics on Rock Microscopic Images” .
This dataset is mainly composed of two parts: the slice photo dataset and the slice identification table. The location of the field profile where the sample is located and the number of corresponding photos are presented in Table 1, and the corresponding geographic locations are shown in Figure 1.
|Era||Formation name||Code profile||Profile name||Thin section number||Number of micrographs|
|Middle Cambrian||Xuzhuang Formation||LHT||Ehutan, Xing County, Luliang City, Shanxi Province||36||160|
|SHC||Liquan Shanghan, Xianyang City, Shaanxi Province||34||152|
|XWK||Hejin West Gate, Yuncheng City, Shanxi Province||31||112|
|D04||Kelan Shijiahui, Xinzhou City, Shanxi Province||31||127|
|D05||Xiaokouzi, Wuzhai County, Xinzhou City, Shanxi Province||18||54|
|D06||Luya Mountain, Ningwu County, Xinzhou City, Shanxi Province||13||44|
|BYG||Baiyanggou, Pingliang City, Gansu Province||13||44|
|D09||Xiashuitou Township, Shuozhou City, Shanxi Province||10||26|
|D11||Yanjiapu, Shuozhou City, Shanxi Province||6||16|
3.1 Thin slice photo data set
The entire photo dataset consists of 192 polarized light micrographs of rock slices. Each rock slice contains one cross-polarized light micrograph and one single-polarized light micrograph with the same field of view. For the single-polarized light microscopy, the photo color is consistent with the naked eye.
Magnification and scale position: the appropriate magnification was selected according to the particle size, based on the principle of clear image recognition. During shooting, the objective lens magnification was mainly 2, 4, and 10 times. The scale was placed in the lower right corner of each picture, with units in μm, where there is a red solid line and white background. The resolution of the photomicrographs is 1280×960 pixels and the saving format is PNG.
Numbering principle: slice number + m + digital serial number of the camera field of view + “+ or -,” if multiple slices of XWK-D25-27 are taken, they will be marked as XWK-D25-27 m1- and XWK-D25-27 m1+; XWK-D25-27 m2-, XWK-D25-27 m2+, XWK-D25-27 m3-, XWK-D25-27 m3+. The last three m is the abbreviation of micrograph, “-” is single-polarized light, and “+” is positive cross-polarized light (Figure 2).
3.2 Rock slice appraisal report
The thin section appraisal table consists of two parts: a carbonate rock appraisal table and a terrigenous clastic rock appraisal table. The contents of the appraisal table mainly include the limestone, dolomite, sandstone, and sandstone in the nine measurement sections mentioned above, with basic information and details about lithological features in the siltstone, shale, and other rock flakes.
Table 2 and Table 3 are the headers of the carbonate and sandstone thin-slice identification reports. The observations and descriptions of carbonate rocks mainly include the observations and descriptions of the mineral composition and content, structure (grain structure, recrystallization structure), structure (bird’s eye structure, bottom structure, biological drilling and suture structure, etc.), observation of biological fossils, along with the description, diagenetic, and epigenetic changes. The description of clastic rocks mainly focuses on particle composition (particle type, relative content, etc.), interstitial characteristics (terrigenous base, stucco, sprite calcite, etc.), special structure, diagenesis, and so on. Also, both types of tables have supplementary descriptions and sample information, showing the analyses of the rock slice information.
|Thin sections describe||Sample information|
|Serial number||Original sheet number||Rocks||Rock name||Image number||Description of particle composition||Content of mixed base||supplimental description||Publication ID||Location||Sample/profile latitude||Longitude of sample/section||Group/Formation||Era||Flake owner|
|More than 10% of the main particles||QFL content relationship||Cutting type & condition||Type of cement||diagenesis||else||country||province/State||City/County-Village/Mountain/River/Lake|
|Limestone||108||Microcrystalline limestone 16, ultrafine crystal-microcrystalline limestone 2, mud-microcrystalline limestone 7, micrite limestone 15, oolitic limestone 29, residual oolitic limestone 5, bioclastic limestone 7, sand Detrital limestone 12, dung spherulitic limestone 1, dolomite-bearing limestone 4, dolomite limestone 10|
|Dolomites||34||Coarse-medium crystalline dolomite 1, fine-medium crystalline dolomite 5, medium crystalline dolomite 1, fine crystalline dolomite 8, ultrafine crystalline dolomite 10, microcrystalline dolomite 3, ultrafine crystalline-microcrystalline dolomite 1, Micro-micrite dolomite 1, micrite dolomite 1, residual oolitic dolomite 1, lime dolomite 2|
|Hybrid sedimentary||12||Argillaceous microcrystalline limestone 5, sandy alienated granular limestone 2, argillaceous alienated granular limestone 5|
|Sandstone||18||Quartz sandstone 6, feldspar quartz sandstone 5, lithic quartz sandstone 2, quartzite sandstone 1, quartz complex sandstone 2, feldspar quartz complex sandstone 2|
|Siltstone||10||Siltstone 4, argillaceous siltstone 5, calcareous fine sandstone 1|
|Mud shale||10||Calcareous shale 2, silty shale 4, silty mudstone 4|
The identification results show that the Xuzhuang Formation in the Ordos Basin is dominated by carbonate rocks, and terrigenous clastic and mixed rocks are also developed. Table 4 is a summary table of the rock types and shows the quantitative information of different rock types included in the dataset.
|Serial number||Original sheet number||Rocks||Rocks (choose one of the two)||Image number||Particle structure or residual particle structure||Recrystallized structure||Fixed biological structure or residual biological structure||Supplementary description||Sample information|
|Carbonate particle type||Type of interstitial||Interstitial content||Particle support relationship||>2 mm Particle support relationship|
|Grain size||Grain structure||Grain self-shape degree||Main sessile organism types||Biological structure||Biological construction type (Field observation)|
|Dunham classification||Zeng Yunfu Classification||Particle type||Particle content||Total particle content||Particle size||Particle sorting||Particle preservation degree||>2 mm particle content||Special particles||Cement fabric||Deposition structure||Sedimentary structure||Diagenesis||other||Publication ID||Location||sample/Profile latitude||sample/Longitude of section||Group||era||Flake owner|
Considering the sample acquisition, thin-slice preparation, shooting, and identification process (Figure 3), the quality of this dataset is guaranteed to a certain extent. The thickness of the rock thin-slice sample is 0.03 mm, which meets the national and international standards. A total of 836 microscopic images of rock slices were obtained in this study, with a total storage capacity of 1.74 GB. The polarized microscopic images captured were clear and devoid of chromatic aberration. While shooting images with the microscope, automatic exposure and white balance levels were used to minimize error. The resolution of the photomicrograph is the highest value achievable by the photographing system used, with a resolution of 1280×960 pixels. For the convenience of computer readability, the image format is uniformly saved in a PNG format.
In order to unify and standardize the collection and identification standards of the dataset further, the identification under the microscope is based on the “Sedimentary Rock Microscopic Digital Image Data Acquisition and Information Collection Standard” . The classification and naming standards for the carbonate rocks identified are shown in Table 5. The sandstone classification and naming method is shown in Figure 4, and the mixed rock adopts the Mount (1985)  classification and naming method (Figure 5). In order to further ensure the quality of the data, after the thin slices were observed and identified, the authors checked and verified the information on the data sheet several times.
|Grain size||Zeng Yunfu （1986）||Mud crystal, microcrystal, very fine crystal, fine crystal, medium crystal, coarse crystal, giant crystal|
|Grain structure||Fridedman （1965）||Isogranular structure, porphyritic structure, embedded crystal structure|
|Grain self-shape degree||Fridedman （1965）||Self-form, semi-self-form, other form|
|Grain preservation degree||Yu Suyu （1992）||Self-shape, semi-self-shape, gravel-level other-shape, powder-level other-shape|
|Sorting||Jerram （2001）||Excellent sorting, good sorting, better sorting, sorting, etc., poor sorting|
|Type of interstitial||Embry and Klovan （1971）||Clay crystal, bright crystal|
|Biological structure||Zeng Yunfu （1986）||Skeleton structure, barrier structure, bonding structure|
The microscopic images of this dataset provide fundamental geological data for research on the sedimentary environment and evolution of the Middle Cambrian in the Ordos Basin, as well as the exploration and development of oil and gas and mineral resources. In addition, this dataset can be utilized as a basic material for teaching geoscience and professional training.
This dataset mainly provides microscopic images of rock slices of the Cambrian Xuzhuang Formation in the Ordos Basin. On the one hand, it can provide course materials to be used for education and training, though may also be used as basic data for related research projects. Recommendations for use are as follows:
(1) All the slices that appear in the dataset are stored in the research group of Associate Professor Xing Fengcun of Chengdu University of Technology. If the micrographs provided in the above dataset cannot meet the needs of further research, you can contact the corresponding authors of this article to apply for further use of these flakes.
(2) If you simply use the image collection, you can download it directly from the database. The thin-slice photos and identification results of this dataset can provide basic data services for research in related fields. If you need to solve additional scientific problems related to geoscience, you must combine the geographical location provided in the data information table, as well as the geological age and tectonic background where the rock was formed.
Li Dongyang, Chai Hanbing, Liu Zhibo, etc. helped in the field survey and sample collection. Sun Hanxiao, Zhao Wenyu, Wang Zhaopeng, and others also helped in the sample sorting and subsequent data sorting. Thank you!
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QIAN HS, XING FC, ZHANG CL, et al. A dataset of microscopic image of rock slices of Xuzhuang Formation in Ordos Basin. Science Data Bank, 2020. (2020-09-21). DOI: 10.11922/sciencedb.j00001.00102.
How to cite this article
QIAN HS, XING FC, ZHANG CL, et al. A dataset of microscopic image of rock slices of Xuzhuang Formation in Ordos Basin. China Scientific Data, 2020, 5(3). (2020-09-29). DOI: 10.11922/csdata.2020.0070.zh.