###### Abstract  
The formation of wellhead blockage increases operating costs and seriously affects the safe production of oil wells. It is crucial to correctly analyze the blockage composition and determine the reasons for wellhead blockage to improve the production efficiency of oil wells. In this study, the material composition and origin of the wellhead blockage in Well DH231 are systematically investigated by means of a thin-section observation, scanning electron microscope, energy spectrum analysis, and molecular geochemical characterization of organic matter. The results show that the wellhead blockage in Well DH231 consists of organic and inorganic materials. The inorganic material was mainly composed of dolomite, fine sand, and unknown black minerals. Four elements, Fe, O, Na, and Cl, could account for 95% of the total elemental content in the unknown minerals. It is speculated that they may have come from rust on the metal parts of the inner wall of the wellbore while being influenced by groundwater during the formation process. The organic matter in the blockage was an oil stain from the geological body. The coexistence of n-alkanes and 25-norhopanes in the oil stain was obvious. It indicated that the oldest crude oil in Well DH231 experienced severe biodegradation, followed by the secondary injection of hydrocarbons. The distribution characteristics of hopanes and steranes in oil stains in Well DH231 were comparable to those of the carboniferous crude oil in neighboring Wells DH4 and DH5. The oil stains in the wellhead blockage may be the product of oil and gas migration in Wells DH4 and DH5. T 2023 13 1504. 2023 1504.  
oil-well production. Therefore, it is crucial to analyze the material composition and source of wellhead blockages to eliminate their formation. To date, there is no unified understanding of the composition of wellhead blockages [6; 7]. In previous studies of wellhead blockages, the method of \"acid dissolution determination and analysis of ionic components in the residual acid\" was used to identify the composition of the blockage [8]. This method is relatively complicated, can only analyze common salt minerals, and the accuracy needs to be improved. With the continuous improvement of equipment for rock and ore testing, many researchers have quantitatively characterized the macroscopic and microscopic compositions of the blockage. [PERSON] et al. (2018) analyzed the blockage in the wellbore of the Yuanba gas field with an X-ray diffractometer and infrared spectrometer. The results showed that the blockage was mainly by pyrolysis products of calcium carbonate, iron sulfide, and a corrosion inhibitor [9]. [PERSON] et al. (2020) conducted X-ray diffraction analysis on blockages in high-pressure gas wells in the Kela 2 gas field, and used a combination of macroscopic and microscopic methods to determine the composition and proportion of substances in the blockages. It was concluded that inorganic substances predominated in the blockage in high-pressure wells in the Clark 2 gas field [10]. [PERSON] et al. (2022) used Fourier transform ion cyclotron resonance mass spectrometry to analyze the differences in the chemical composition of asphaltene in crude oil and wellhead blockage extracts from the Gao Tan 1 Well on the southern edge of the Junggar Basin, and explored the relationship between asphaltene structure and asphaltene deposition [11]. From the previous research methods, the combination of multiple analytical techniques can effectively dissect the material composition of the blockage. However, from the analysis results, there are significant differences in the material composition of the blockage in oil and gas wells drilled in different geological conditions. Therefore, it is difficult to apply the blockage composition research of other wells to the DH231 Well. It is necessary to conduct a more in-depth study on the composition of the wellhead blockage in Well DH231 [12].