## 4 Results and Discussion
### Component Identification of Wellhead Blockage
#### 4.1.2 Component Analysis of Oil Stains  
Chloroform bitumen \"A\" was extracted from the blockage with chloroform using the Soxhlet extraction method [22; 23]. The obtained chloroform pitch \"A\" was subjected to group components separation. Studies have shown that the main components of the oil stains were saturated hydrocarbons, aromatic hydrocarbons, resins, and asphalt (Figure 6). The percentages of saturated hydrocarbons, aromatic hydrocarbons, resins, and asphaltenes were 30.03, 13.67, 3.22, and 47.18, respectively. Oil stains have the same material composition as normal crude oil or source rock extracts [24]. It is worth noting that the relative content of asphaltene in oil stains was relatively high, reaching 47.8% (Table 2). The formation of wellhead blockages may be related to the extraction rate of crude oil from the formation. With the continuous extraction of crude oil from the formation, the light and heavy fractions of crude oil are differentiated. Compared to normal crude oil, lighter oil has greater fluidity and a better recovery rate than crude oil. The heavy oil with a high asphaltene content has poor fluidity and is more likely to cause wellhead accumulation. [PERSON] et al. (2022) observed that the degree of condensation of asphaltenes in blockage extracts was significantly higher than that of asphaltenes in crude oil, and the asphaltenes contained more naphthene and aromatic ring structures [11]. This can accelerate the precipitation of other asphaltene components in the crude oil. [PERSON] et al. (2022) further emphasized that the more polar asphaltenes were easier to spontaneously precipitate at the wellhead [25]. High asphaltene content in the oil stains may be the potential reason for wellhead plugging of Well DH231. In the production of heavy oil with high asphaltene content, the viscosity of crude oil can be changed by adding additives to the crude oil to improve the production efficiency of oil wells and reduce wellhead clogging. To verify the secondary impact of acidification during drilling and production on the organic matter in the wellhead blockages, we conducted a comparative test on the contents of each component before and after the acidification of the collected wellhead blockage. The experimental results show that the acid treatment has little effect on the organic matter in the blockage. The composition of the filler obtained after acidification changed by less than 2% compared to before (Table 2). The bitumen \"B\" produced after acidification had a limited impact on the hydrocarbon composition of the blockage. In other words, there is little organic matter hidden in the carbonate rock crystals or lattices [26]. Wellhead blockage occurs during oil and gas production. Due to the particularity of the production process, the organic components in the blockage showed some complexity [26; 27; 28]. Oil stains on the wellhead blockage may be secondary fouling from additives, such as foreign diesel and gasoline in the mining process, or natural organic matter in geological bodies. The extracted chloroform bitumen \"A\" was identified by full oil chromatography, and the characteristics of the oil stains' full oil chromatographic are shown in Figure 7. The carbon number distribution of n-alkanes was very wide, ranging from nC\({}_{13}\)-nC\({}_{25}\). The distribution of n-alkanes was complete, mainly low-carbon compounds,showing a single-peak distribution, with nC\({}_{17}\) as the central peak. High-carbon n-alkanes did not have an odd-even carbon dominance distribution phenomenon. Obviously, the n-alkane components of oil stains were entirely different from refined oils, such as gasoline and diesel. The abovementioned characteristics indicate that the petroleum-like substances in the blockage were the products of geological bodies. It is not an additive used in the drilling process, but a more mature component of crude oil. At the same time, the later acidification process had little effect on the overall distribution characteristics of the chromatogram of the wellhead blockage.