Abstract: The Bohai Bay Basin is bordered by the Jiaoliao Uplift in the east, Taihangshan Uplift in the west, and limited by the Luxi Uplift and Yanshan Fold Belt in the north and south respectively. Overall, the basin is in the shape of an irregular rhombus, with an approximate area of 20 × 104 km2. The valley basin is a large-scale (residual) basin superimposed by a Cenozoic fracture on a pre-Cenozoic craton basin, where a large number of buried hills are developed. However, there has been a lack of fundamental geological research on the buried hills of the Bohai Sea, resulting in scarce publicly-accessible petrological data. Thus, our study collected 204 Archean buried hill metamorphic granite sections and their overlying glutenite rock sections from the Paleogene Kongdian Formation in Bozhong Sag, Bohai Bay Basin. They were then divided into two parts: 61 granite sections and 143 clastic sections. The deep-buried hill reservoir in the Bozhong 19-6 (i.e., BZ19-6) structural belt is revealed to be a pan-buried hill reservoir system composed of Archean buried hill metamorphic granite and an overlying glutenite from the Paleogene Kongdian Formation, which formed a multilayered reservoir structure comprising a glutenite pore zone, buried hill weathering crust in the dissolution fracture zone, and an inner fracture zone tight zone, with a complex reservoir genesis and diverse reservoir types. The dataset records the sampling locations, stratigraphic ages, and the names of the rock thin sections. The granite part shows that the buried hill mineral assemblage is dominated by dark minerals such as quartz, feldspar, and a small amount of biotite. It also records the fracture period, filling type, and hole ratio. The clastic rock part records the composition of the glutenite from the Kongdian Formation and the content making up its miscellaneous base. A systematic analysis of the microscopic rock thin sections reveals a differential development of fractures caused by lithologic changes in the BZ19-6 reservoir system of the buried hills, together with the impact of lithologic fluids on reservoir corrosion, providing fundamental petrographic data to support the continued oil and gas exploration in the buried hills of Bohai Sea.
Keywords: Bohai Bay Basin; buried hill; granite; glutenite
|Title||A micrograph dataset of buried hills and overlying glutenite in Bozhong Sag, Bohai Bay Basin|
|Data corresponding author||Hou Mingcai (email@example.com)|
|Data authors||Liu Yanpeng, Hou Mingcai, Liu Xiaojian, Qi Zhe|
|Time range||The strata of core sample collection are buried hill. The rock core samples were collected from 2009 to 2019, with a stratigraphic age attributable to Archean, Kongdian, Shahejie, Dongying, and Minghuazhen formations of the Neozoic. The polarized micrographs of the rock sections were taken in 2019|
|Geographical scope||Oil wells near Dagu Lighthouse in Bohai Sea, China|
|Polarized microscope resolution||1280 × 960, 1600 × 1200, and 2560 × 1920 pixels|
|Data volume||4.06 GB|
|Data format||*.png, *.xls|
|Data service system||<http://www.dx.doi.org/10.11922/sciencedb.j00001.00119>|
|Source of funding||National Science and Technology Major Project (Grant No. 2016ZX05024-003-010)|
|Dataset composition||This dataset consists of three parts: A total of 543 photos of 204 thin sections stored in 15 folders, each folder corresponding to an exploratory well, with a data volume of 4.06 GB; An information table for granite micrographs, which records profile information and identification data of the rock thin sections, with a data volume of 21 KB; An information table for clastic rock micrographs, which records profile information and identification data of the rock thin sections, with a data volume of 36 KB|
The study of rock micrographs can offer rapid retrieval of the existing rock data and achieve a comprehensive integration of regional information. A large amount of geological information is contained in thin rock slices as the most fundamental data of geological research. The formation of a systematic, detailed, and accurate rock micrograph collection can improve the efficiency of thin slice research and enhance comparative research compared with the observation of physical thin slices. Such a collection can thereby greatly improve the utilization efficiency of physical thin slices and enhance information exchange between industries . Based on the study of microscopic image data of buried hills and overlying glutenite in Bozhong Sag, Bohai Bay Basin, the collection can systematically combine the petrological characteristics of the study area, perform comprehensive comparisons, and comprehensively summarize the characteristics and differences of rock developments in the region.
After more than half a century of exploration in the Bohai Bay Basin, no large-scale natural gas fields have been discovered. With a late tectonic history, the Bohai Sea area has long been considered to be difficult to form large-scale natural gas fields. The strong tectonic activity in the Bohai Bay Basin has a destructive effect on the caprock and is not conducive to the preservation of natural gas. In recent years, a lot of research has been conducted on the Bohai Oilfield, located in the Bohai Bay Basin, regarding the material basis of gas formation, trap formation and evolution, the genesis of high-quality reservoirs, and natural gas preservation conditions [2-5]. These research results have guided projects on the Bohai Oilfield. The successful discovery of the Bozhong 19-6 (i.e., BZ19-6) large-scale 100 billion cubic meters gas field has opened a new chapter in deep natural gas exploration in the Bohai Sea. However, a lack of current research work regarding the buried hills still persists in the Bohai Sea, particularly concerned to understanding the law of buried hill hydrocarbon accumulation and enrichment, which needs to be studied in more detail . Basic petrological work is important to clarify the new replacement sea area for oil and gas exploration in the Bohai Sea and effectively guide the oil and gas exploration of buried hills in the Bohai Sea area. The identification and statistics of rock flakes can further provide information regarding exploration and development, including buried hill lithology distribution characteristics, buried hill stratigraphic structures, buried hill fault system characteristics, reservoir space characteristics, reservoir fluid characteristics [7-9] The statistics and collection of various petrological information, including the porosity of granite slices and clastic rock slices calculated in this atlas, clearly reflect the proportion of fractures and pores and provide a basis for the identification of high-quality reservoirs. Clarifying the differences in the lithology and spatial distributions characteristics of reservoirs, particularly the study of the reservoir fluid properties and development of the fracture network system in the metamorphic buried hills, provides a basis for qualitative understanding. The microscopic atlas established for Archean buried hills in the Bohai Sea not only provides the basic petrological data for buried hill oil and gas exploration and summarize and analyze buried hill oil and gas geology, but also provides valuable information regarding the Bohai Sea area based on the in-depth study of the destruction of the North China Craton Rock flake material.
The BZ19-6 structural zone is located on the deep structural ridge in the southwestern part of the Bozhong Sag in the Bohai Bay Basin. The southeast, west, and south are adjacent to Bonan Low Uplift, Chengbei Low Uplift, and Yellow River Mouth Sag, respectively, and Bohai lies in the north. The main depression in the middle sag is an anticline structural belt with the characteristics of a central depression between the southwest and south subsags of the Bozhong Sag (Figure 1).
After analyzing a large number of core observations and consulting data, the main observations under the microscope are thin slices of the target horizon with hydrocarbon display. Observations under the microscope were conducted in the Sedimentary Geology Laboratory of the Institute of Sedimentary Geology, Chengdu University of Technology and the Laboratory of the Research Institute of CNOOC (China) Co., Ltd. Tianjin Branch. A polarizing microscope was the instrument used, the model was Nikon LV100 P0L, and the ambient temperature was measured at 25°C with a humidity of 45%. The methods of photographing and information collection of the thin slices are uniformly implemented according to the standards of “Rock Microscopic Image Topics” . The system collects rock microscopic images and also obtains information regarding thin slices. The description of thin slices and the naming of the sedimentary rocks are based on the “Rock Requirements of Special Topics on Microscopic Images” [10-11].
This rock microscopic image dataset consists of polarized light micrographs of 204 rock flakes. At least one field of view was taken for each rock flake, and each field of view contained at least one cross-polarized photomicrograph and one single-polarized photomicrograph. The shooting process adopted automatic exposure and automatic white balance, which guaranteed maximum shooting capacities. The color of the photomicrograph appears consistent via naked eye observations under a polarized light microscope, and the composition in the microscope image is same as the description in the identification report.
Numbering principle: slice number + m + digital serial number of the camera field of view + “+ or -,” if the slice numbered BZ19-6-3-4142 shoots multiple field of view images, they are marked as BZ19-6-3-4142 m1+, BZ19-6-3-4142 m1-; BZ19-6-3-4142 m2+, BZ19-6-3-4142 m2-; BZ19-6-3-4142 m3+, BZ19-6-3-4142 m3-...... The third-to-last m is the abbreviation of micrograph (microscopic image), “+” means cross-polarized light, and “-“ means single-polarized light (Figure 2).
|BZ19-6-1-3585 m1-||BZ19-6-1-3585 m1+|
|BZ19-6-3-4142 m1-||BZ19-6-3-4142 m1+|
|BZ19-6-3-4158.5 m1-||BZ19-6-3-4158.5 m1+|
BZ19-6-1-3585 m feldspar quartz sandstone. Detrital structure: mainly quartz 70%, plagioclase 25%; particle diameter ranging from 0.2 to 2 mm, poor sorting, subangular; contact cementation, the cement is calcareous; there are a few intergranular pores, with an infill of organic matter. Two developed cracks were cutting through the feldspar particles. Two stages of cracks were visible, and no filling was observed in the cracks.
BZ19-6-3-4142 m1 granite gneiss. Granite gneiss structure: mainly quartz 30%, feldspar 65%; feldspar mainly plagioclase, sericitization of plagioclase occurs; feldspar is mostly mechanically broken, passing through two mineral particles, and the dissolution phenomenon is not developed.
BZ19-6-3-4158.5 m1 cuttings feldspar glutenite. Clastic structure: 30% rock cuttings, 30% quartz, 40% feldspar; particle size 1–5 mm, sorting etc., subangular shape; porous contact, cement is siliceous, matrix content is about 8%, main It is a clay mineral, feldspar is plagioclase and micro-plagioclase; the quartz surface is wavy and extinct, polycrystalline quartz is seen, the cuttings are metamorphic granite, the cracks are not developed, the intergranular pores are relatively developed, and the filling is quartz.
The scale bar in the rock photomicrograph is placed in the lower right or upper left corner of the picture, and the units used are mm or μm. When shooting, the appropriate magnification was selected according to the size of the rock’s clastic particles to ensure that the rock micrograph could intuitively display the microscopic features of the thin slice while ensuring image clarity. The resolutions of the photomicrographs are 1280 × 960, 1600 × 1200, and 2560 × 1980 pixels, and png is the image format.
The identified content of granite flakes and detailed information regarding the rock samples are stored in the information table of the excel granite microscopic image database. The appraisal content mainly includes the rock structure, lithology, main minerals, storage space type, filling, and face ratio. The rock sample information includes the detailed geographic location, longitude, latitude, age, and slice owner of the sampling site.
The identified content of the clastic rock slices and detailed information regarding the rock samples are stored in the information table of the excel clastic rock microscope image database. The identified content mainly includes the particle composition, heterogeneous content, cement type, diagenesis. Among them, the particle composition only applies to particle types of over 10% of the three kinds of particles: quartz (Q), feldspar (F), and cuttings (L), as well as the size relationship and proportion of the three kinds of particles. The miscellaneous base content only needs to be judged to determine whether it exists and exceeds 15%; the rock is regarded as miscellaneous sandstone if it exceeds 15%. The rock sample information mainly includes the information from the relevant published literature, detailed geographical location of the sampling site, name of the profile, longitude and latitude of the sample/profile, age, and owner of the slice.
The rock slices in this dataset include 33 conglomerates, 102 sandstones (96 pure sandstones, 6 complex sandstones), 7 siltstones and 1 mudstone, and 59 granite gneiss and 2 two-long granite flakes gneiss. More details are in Table 3.
|rocks||quantity||type and quantity of rocks|
|sandstone||Net sandstone||102||96||quartz sandstone (7)、arkose quartz sandstone (3)、lithic arkose32)、feldspar sandstone (37)、lithic sandstone (6)、feldspar litharenite (11)|
|graywacke||6||arkose greywacke (5)、lithic arkose sandstone (1)|
|mudstone||1||calcareous mudstone (1)|
|volcanic||granite||61||granite gneiss (59)、two-long granite flakes gneiss (2)|
The thickness of the rock flake samples meets the national and international standards. During the photomicrograph shooting and thin slice identification process, the interference color grades of quartz particles observed in all thin rock slices are grade 1, indicating that the thickness of the thin rock slices meets the national standard of 0.03 mm. All microphotographs are taken with automatic exposure and an automatic white balance, so that the colors perceived by the naked eye and system photos are as consistent as possible; the resolution of the microphotographs is uniformly adopted by the highest values of the camera system, with a resolution of 1280 × 960, 1600 × 1200, and 2560 × 1980 pixels, where the pictures are saved in png format. Therefore, the quality and clarity of the photomicrographs are reliable. To ensure the accuracy of the dataset, the data sheet information was checked and verified several times after the initial identification of all rock flakes. The naming of igneous rock thin slices and the collection of various information are in accordance with “Ignition of Igneous Rock and Its Role in Geology” [12-13]. The naming of sedimentary rock slices and the collection of various information are strictly in accordance with the requirements of the “Sedimentary Rock Microscopic Image Data Collection and Information Collection Standards,” [10-11] which can guarantee the reliability of the information.
This atlas has provided detailed statistics of the mineral composition, fracture period, and face ratio of thin rock slices. The data provides important evidence for sedimentology and reservoir geology research and plays an important role in oil and gas exploration. The Xinglongtai buried hill metamorphic rock oil and gas reservoir in Liaodong Sag are typical examples. The understanding of “dominant lithology” reservoirs has been established (under the same stress conditions, the lithology that forms the reservoir is called the “dominant lithology”[14-16]) through a large number of thin-section studies, having liberated several Archaean basement areas abandoned because of lithological factors and expanded the exploration field of Archaean buried hills in the Liaohe Depression. Similarly, the Archaean deep-buried metamorphic rock buried hill of the BZ19-6 structure in the Bohai Sea has deepened the understanding of the formation mechanism of high-quality reservoirs via the study of atlas rock slices. The research shows that the Archean BZ19-6 structure is based on gneiss and metamorphic rocks. Being mainly made up of granite and mixed rocks, the rock contains high felsic minerals, the formation is brittle, and forming cracks is easy, which creates favorable lithological conditions for the development of high-quality reservoirs . The large-scale fracture development and dynamic fracture zones developed in the inner part of the mountain are the key to the formation of high-quality metamorphic rock reservoirs [18-19]. Under the influence of weathering, a large number of secondary dissolution and dissolution-enlarged pores are formed along the cracks on the top of the buried hill, but the deep burial effect has made the weathering crust at the top relatively dense. Considering the abovementioned controlling factors, a distribution model of large-scale deep-buried metamorphic high-quality reservoirs has been established with “vertical penetration, horizontal continuous.”
The data format of this dataset is simple; the following points must be considered when using:
(1) All slices that appear in the dataset are centrally stored in the Tianjin Branch of CNOOC (China) Co., Ltd. If the micrographs provided in the above dataset cannot meet the needs of further research, you can contact the author of this article, who will assist in applying for further use of these flakes.
(2) If you simply use the image set, you can directly download and use from the database; however, if you need to further solve the scientific problems related to geosciences, you need to combine the geographic location provided in the data information table, as well as the geological age and structural background of the rock formation.
Thanks to the Reservoir Geochemical Laboratory of the Research Institute of CNOOC (China) Co., Ltd. Tianjin Branch for their assistance in the process of collecting thin slice images.
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How to cite this article
LIU Yanpeng, HOU Mingcai, LIU Xiaojian, et al. A micrograph dataset of buried hills and overlying glutenite in Bozhong Sag, Bohai Bay Basin. China Scientific Data, 2020, 5(3). (2020-09-29). DOI: 10.11922/csdata.2020.0087.zh.