Abstract: Large-scale tight sandstone gas reservoirs developed from the Upper to the Lower Paleozoic in the Linxing block on the eastern edge of the Ordos Basin, with Benxi, Taiyuan, Shanxi, Xiashihezi, Shangshihezi, and Shiqianfeng Formations. Tight sandstone reservoirs have low porosity, low permeability, strong heterogeneity, and the distribution of sand bodies varies greatly on the plane. Domestic research on unconventional tight sandstone reservoirs has entered a critical stage, and research concerning tight sandstone petrological data in the Linxing block is particularly important. To this end, 305 Upper Paleozoic thin rock sections were collected from the Benxi, Taiyuan, Shanxi, Xiashihezi, Shangshihezi, and Shiqianfeng Formations, which were photographed using polarized light microscopy. The major rock type is arkose quartz sandstone, followed by lithic quartz sandstone and quartz sandstone. This dataset covers 24 wells in the Linxing block and records the location, stratigraphic age, and rock characteristics of the samples. The rock microscopic image data can be widely used not only in basic geological research but also in interdisciplinary research, such as cooperative research integrating computer artificial intelligence. It also provides data support for unconventional oil and gas exploration, as well as for development and research.
Keywords: thin section; tight sandstone; Linxing block; Upper Paleozoic; Ordos Basin
|Title||A photomicrograph dataset of Upper Paleozoic tight sandstone from the Linxing block on the eastern margin of the Ordos Basin|
|Data corresponding author||Li Panpan, Li Yangbing, Chen Xin, Wang Yuesheng, Li Chenchen, Liu Zaizhen|
|Data authors||Li Panpan (email@example.com)|
|Time range||The rock samples were collected during 2014 and 2019 and can be attributed to the stratigraphic age of Carboniferous and Permian (about 250–300 Ma); polarized photomicrographs were taken in 2020.|
|Geographical scope||The sampling areas include: Linxian & Xinxian in Shanxi Province, located on the eastern margin of the Ordos Basin.|
|Polarized microscope resolution||2,560 \(×\) 1,920 pixels|
|Data volume||5.67 GB|
|Data format||*.png, *.xls|
|Data service system||<https://dx.doi.org/10.11922/sciencedb.j00001.00048>|
|Source of funding||Science and Technology Program of CNOOC LTD (Grant No. CNOOC-KJ 135KJXM NFGJ2020-05).|
|Dataset composition||The dataset includes two data files, “Photomicrographs.zip” and “Information table of database.xls”. (1) “Photomicrographs.zip” is a dataset of polarized photomicrographs of thin rock sections, including 305 thin sandstone rock sections from 24 wells, with a data volume of 5.67 GB; (2) “Information table of database.xls” consists of data sheets for thin rock section identification, with a data volume of 57.3 KB.|
The Ordos Basin is the second largest sedimentary basin in China, spanning from Gansu, to Shaanxi, to Shanxi. The tectonic location is a part of the North China Craton, which is a typical foreland basin. The strata above the Craton basement are well-developed, and the basin is rich in mineral resources due to the abundant sediment source and stable tectonic subsidence. The basin is known as “China’s golden triangle of energy.” Basin evolution has gone through the following five stages: Middle and Late Proterozoic Aulacogen -laying the basic pattern of the basin, Early Paleozoic shallow Haitian platform-formed rich oil layer in Lower Paleozoic, Late Paleozoic coastal plain-coal measure hydrocarbon source rock widely developed and formed Upper Paleozoic gas layer, Mesozoic inland lake basin developed Triassic and Jurassic reservoirs, Cenozoic Himalayan movement-Perfection of the present tectonic framework[1-2].
The Linxing block is in the Linxian and Xingxian districts of the Shanxi Province in the north–east margin of the Ordos Basin. Large-scale tight sandstone gas reservoirs developed in the Upper Paleozoic, and the Benxi Formations, Taiyuan Formations, Shanxi Formation, Lower Shihezi Formation, Upper Shihezi Formation, and Shiqianfeng Formation are developed from bottom to top. Regarding the sedimentary facies, the sedimentary characteristics are coastal facies, sea–land transition facies, and lake, river, and land facies, which have been accumulating since the Paleozoic. The overall stratigraphic characteristics are represented by the lower coal-measure source rock strata, i.e., Benxi, Taiyuan, and Shanxi Formations. The Lower Shihezi and Upper Shihezi Formations are dominated by interbedded mud sandstone, but the Upper Shihezi Formation interbedded mudstone content is relatively scarce. The Shiqianfeng Formation is characterized by the development of conglomerate and coarse-grained sandstone. The specific stratum characteristics are shown in Fig. 1. The experimental data of porosity, permeability, mercury injection, and cast thin section show that the distribution interval of reservoir porosity and permeability in this area are 4%–10% and 0.1–1 × 10-3 m2, respectively. The pore throat mainly consists of very fine pores, and the main reservoir space type consists of intergranular dissolution pores and dissolved particle pores. It belongs to the tight sandstone reservoir with an extremely small pore number and extremely low permeability.
The Upper Paleozoic tight sandstone reservoirs in the Ordos Basin are characterized by low porosity, low permeability, and strong heterogeneity. Moreover, the sand bodies are widely distributed, and the plane distribution varies greatly [3-4]. The research on tight reservoirs in the United States and Canada started in the early 1970s, and the technology has always been in the leading position. Although research on tight reservoirs started late in China, recent achievements have been made in the gas-producing areas of tight reservoirs, such as the Changqing Oilfield. Compared with conventional reservoirs, tight reservoirs face more difficulties with respect to geological understanding as well as exploration and development. In the study of tight sandstone gas reservoir types, the key problems to be solved are the tight evolution history of sandstone reservoirs, reservoir diagenesis, the tight period of sandstone reservoirs, and the influence of tight-period sandstone diagenesis on large-scale natural gas filling[5-6].
Based on the above situation and existing problems, it is of great significance to study the Upper Paleozoic tight gas-bearing sandstone reservoir in the Linxing block. At present, the Linxing block is still in the stage of exploration and development, and only a limited number of pictures of typical rock flakes have been published. The author of this paper has collated the micro-image data of 24 Wells and 305 thin sandstone sections from the Linxing block and shared said dataset for the first time with domestic scholars.
Based on the present situation of exploration and development in the Linxing block, representative exploration wells are selected for the systematic acquisition and description of rock slice microscopic images. The preparation standard of thin rock sections is in accordance with SY/T 5913-2004 “Rock thin section preparation” . The description of thin rock sections and the naming of sedimentary rock are based on the standards outlined by the existing research .
This dataset is mainly composed of two parts: thin-section photo dataset and thin-section identification report.
The geographic information, stratigraphic units, and clastic-rock slices of 24 wells and 305 rock slices are shown in Table 1. The main gas-bearing layers in this area are the Upper Shihezi, Lower Shihezi, and Taiyuan Formations.
|Series||Formation||Sample Depth /m||Well Name||Number|
|Upper Carboniferous||Benxi Formation||1,901.66–2,175.60||X-12、X-16、X-18、X-20、X-21、X-26、X-41、X-44、X-45||14|
The thin-section photo dataset consists of 305 polarized micrographs of rock slices, each of which contains one slice with the same field of view, one orthogonal-polarized micrograph, and one unipolar micrograph. The micrograph color is consistent with naked-eye observations under polarizing microscopy, and the composition in the microscopic image is the same as that described in the identification report. The micrograph resolution is 2,560 × 1,920 pixels, and the format is PNG.
The thin-section identification report is a clastic-rock identification form, which mainly contains the basic information, lithologic characteristics, classification, and nomenclature of the aforementioned 305 tight gas-bearing sandstone reservoir thin sections.
Table 2 summarizes the proposed dataset, from which it is evident that of the 305 thin rock sections, the main rock type is feldspathic quartz sandstone (214 blocks), followed by lithic quartz sandstone (64 blocks), quartz sandstone (26 blocks), and quartz feldspar sandstone (1 block). The rock types of the Shiqianfeng Formation are mainly feldspathic quartz sandstone, and the Upper Shihezi Formation is mainly feldspathic quartz sandstone with a small amount of lithic quartz sandstone and quartz sandstone; the rock type of the Lower Shihezi Formation is mainly feldspathic quartz sandstone, followed by lithic quartz sandstone; the rock types of the Taiyuan Formation are mainly feldspathic quartz sandstone and lithic quartz sandstone, with a small amount of quartz sandstone; the Benxi Formation mainly consists of lithic quartz sandstone, feldspar quartz sandstone, and quartz sandstone.
The statistical results of composition identification for all 305 sandstone samples (Fig. 2) show that the Upper Paleozoic sandstone in the Linxing block is mainly feldspathic quartz lithic sandstone, accounting for 70% of all sandstone types, followed by lithic quartz sandstone and quartz sandstone, accounting for 21% and 9% of all sandstone types, respectively, whereas quartz feldspar sandstone accounts for less than 3%.
The sample production of the thin rock sections conforms to the national industry standards. The interference color of quartz particles is consistent across all samples. The micrograph is high definition and has no color difference. In the microscopy process, automatic exposure and automatic white balance were used to ensure that the color of naked-eye observations and system photos are as consistent as possible. The micrographs have a high resolution at 2,560 × 1,920 pixels, and the pictures are uniformly saved in PNG format; therefore, the quality and clarity of the micrographs are reliable.
The sheet identification report is issued by the laboratory after a three-level audit, which ensures the reliability of the identification results.
This dataset can be applied to the study of unconventional oil and gas exploration in China, especially in the study of tight gas-bearing sandstone reservoirs. The microscopic characteristics of typical low-porosity and low-permeability tight sandstone can also be used in professional teaching and popular science popularization.
These high-definition microscopic images can also be used as training and learning samples for the artificial intelligence-based identification of thin rock sections. They also supplement existing library samples and can be used to improve the identification accuracy of thin sections.
The data form for the proposed dataset is simple, and the points outlined below should be considered when using it:
(1) All slices in the dataset are stored in the Unconventional Experimental Center of the Unconventional Technology Research Institute of the China National Offshore Oil Corporation's energy-development engineering-technology company. If the micrographs provided in the dataset do not meet the needs of future researchers, the corresponding author of this paper can be contacted to apply for further study.
(2) Based on the thin-section identification results of the proposed dataset, a series of academic papers have been published on sedimentary geology research (and related areas), which can be read for further details [9-11].
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 Hu Xiumian;Lai Wen;Xu Yiwei, et al. Standards for digital micrograph of the sedimentary rocks . China Scientific Data, 2020. (2020-03-02). DOI: 10.11922/csdata.2020.0008.zh.
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LI PP, LI YB, CHEN X, et al. A photomicrograph dataset of Upper Paleozoic tight sandstone from Linxing block, eastern margin of Ordos Basin. Science Data Bank, 2020. (2020-07-23). DOI: 10.11922/sciencedb.j00001.00048.
How to cite this article
LI PP, LI YB, CHEN X, et al. A photomicrograph dataset of Upper Paleozoic tight sandstone from Linxing block, eastern margin of Ordos Basin. China Scientific Data, 2020, 5(3). (2020-09-01). DOI: 10.11922/csdata.2020.0055.zh.