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Площадь: 167396.2 км²
Erlian Basin
The Erlian Basin, one of the most petroleum-productive basins in Northern China, is situated in the middle northern part of the Inner Mongolia Autonomous Region, covering an area of approximately 10 × 104 km2, which is primarily composed of Mesozoic sedimentary strata developed on the Hercynian folded basement. The basin is divided into nine primary structural units, including five depressions (Manite, Wunite, Ulanchap, Jingchuan, and Tengger) and four uplifts (Bayinbaolige, Daxinganling, Sunite, and Wenduermiao) (Figure 1a). The conventional petroleum geological resources in the Erlian Basin are 11.7 × 108 t, and the cumulative proven petroleum geological reserves are 2.34 × 108 t. The discovered oil reserves are mainly distributed in the positive tectonic and slope zones, and the depressional area’s exploration degree is still low.
Figure 1. (a) Regional geological map of the Wuliyasitai sag (b) distribution of the sampling wells;
The depressional troughs have transformed into a crucial succession area for enhancing reserves and production in the Erlian Basin. The drilling of the Tc1 well in the Wuliyasitai southern sub-sag showed good results, and industrial oil and gas flow of 5.6 t was obtained for the first time in 1989. Before 2020, oil and gas discoveries were concentrated on the two nose-like structures of the eastern slope, namely, the Murige and Subu, with low exploration degrees in the trough zone. The Wuliyasitai southern sag through zone’s high potential for shale oil exploration was confirmed by the oil testing results of the T591X well in 2020. Natural gas has a higher risk of being lost due to its strong fluidity, necessitating preservation conditions a crucial component of natural gas production. Some scholars hold that the preservation conditions are one of the elements determining the scale of natural gas reserves, especially for shallow gas.
Owing to its abundant oil and gas resources, numerous attention has been drawn to the Wuliyasitai region in the past decades. However, these previous studies primarily focus on geological features, such as structural evolution, stratigraphic sequence division, sedimentary facies, and reservoir properties. Scant information about formation fluid investigations has been published despite the access to massive data in the Huabei Oilfield Company archives. The physical properties and geochemical characteristics of crude oil and natural gas are a comprehensive reflection of the process of hydrocarbon accumulation, which can reveal the migration distance and direction, origin, preservation conditions, and other reservoirforming factors. The physical and geochemical characteristics of the crude oil allow identification of the organic matter (OM) type, maturity, formation environment, and possible secondary changes suffered. The fluid inclusion records and preserves the geological fluid composition, temperature, and pressure information of its formation period and is widely used in the study of paleo-fluids, paleo-temperature and -pressure, and hydrocarbon accumulation process in sedimentary basins. Nie et al. used the venous fluid inclusions to evaluate the influence of fractures and fluid activities on the preservation and enrichment of shale gas in the Sichuan basin. Material and energy exchange occurs between the formation water, ambient rocks, and hydrocarbons, which contain considerable information related to the formation and preservation of oil and gas reservoirs in petroliferous basins. Chen et al.established a thorough knowledge of hydrocarbon migration in Ordovician limestone reservoirs from the Tazhong oilfield by combining iodine and 129I concentrations with chemical and isotopic data for carbonate reservoirs in the Ordos basin. Yu et al.20 identified a substantial link between formation water chemistry and gas reserves for carbonate reservoirs in the Ordos basin. Therefore, the environmental information of sedimentary strata in various geological periods can be successively reflected by these three different geological fluids. To reveal the characteristics of formation fluids and reservoir preservation conditions in the Wuliyasitai area, extensive samples of crude oil, formation water, and fluid inclusion were collected and analyzed experimentally with the assistance of the PetroChina Huabei Oilfield Company. This study combined crude oil’s physical properties and molecular geochemistry, formation water chemistry, and the inclusions of hydrocarbon-bearing fluids. The results can contribute to the theory of hydrocarbon accumulation and understanding of the implications of reservoir water chemistry in petroleum preservation conditions in the Wuliyasitai sag and other similar sags in the Erlian Basin. Future oil and gas scientific exploration can be directed by identifying favorable hydrocarbon accumulation zones using the total dissolved solids (TDS) and the formation water characteristic coefficients which indicate the sealing conditions.
GEOLOGICAL AND STRATIGRAPHY
Located in the Erlian Basin’s northeastern region of the Manite Depression, the Wuliayasitia sag is split into three sub-sags from south to north (Figure 1), and the past 3 decades have witnessed main oil discoveries concentrated in the southern sub-sag. The maximum sediment thickness of the southern sub-sag is approximately 4500 m with shape like a dustpan on the geological section (Figure 1a). It is divided into two main tectonic zones in the north−south direction: Murige and Subu structural belts. The western steep slope, the central depocenter, and the gentle eastern slope make up the southern sag, while the eastern slope zone is divided into the inner slope zone, middle slope zone, and outer slope zone, but the zonation is not obvious in the northern area (Figure 1c). The faults are generally consistent with the direction of the depression and are mainly northeast-oriented. The Wuliyasitai sag has experienced three evaluations, forming two significant regional unconformities, which are sandwiched between the Aershan Formation (K1ba) and Tengger Formation (K1bt) and between the K1bt and Saihantala Formation (K1bs), respectively (Figure 1c). The Lower Cretaceous lacustrine clastic rocks are mainly developed in the study area, which can be divided into K1ba, K1bt, and K1bs from the bottom to up.
Figure 1. (c) geological section of the Wuliyasitai southern sub-sag.
The hydrocarbon source rocks are mudstones of the K1ba and lower Tengger Member (K1bt1), and a total of four sets of hydrocarbon-bearing strata are encountered during the drilling process (Figure 2).
Figure 2. Generalized chronostratigraphy and lithostratigraphy for the Wuliyasitai southern sub-sag showing the investigated reservoir beds and sampling positions. Sai Fm = Saihantala Formation.
The research area’s OM is in the immature to mature stage, and it generates hydrocarbons when vitrinite reflectance (Ro) values are >0.6%, with the characteristics of unimodal hydrocarbon generation. The hydrocarbon generation threshold is approximately 1500 m deep. Type II OM makes up the majority of the source rocks. The values of the abundance indicators show a good to very good hydrocarbon generation potential. The lower limit of the TOC value for the K1ba source rocks (1.5 wt%) is lower than that for the K1bt1 source rocks (2.5 wt%). The source rocks are sourced from a mixed input of terrestrial higher plants and lower aquatic organisms.
Data source:Characterization of Crude Oil, Formation Water, and Fluid Inclusions of Hydrocarbon-Bearing Strata and Their Hydrocarbon Geological Significance in the Wuliyasitai Southern Sub-sag of the Erlian Basin, China. Wei Si, Dujie Hou,* and Lanzhu Cao. 2023
Следующий Бассейн: Sivas - Malatya