Organic matter abundance

Whether a sedimentary basin can generate oil and gas, the abundance of organic matter is very important, which is the material basis of oil and gas generation and an important basis for measuring and evaluating the hydrocarbon generation potential of source rocks. At present, the commonly used indicators of organic matter abundance include organic carbon content (TOC), soluble hydrocarbon (S 1), pyrolyzed hydrocarbon (S2), oil production potential (Pg), chloroform pitch A and total hydrocarbon content (HC) in the pyrolysis parameters of source rocks. In addition, there is an obvious positive correlation between the maceral content of source rock TMCS3 and organic carbon, oil generation potential, chloroform pitch A and total hydrocarbon content, indicating that TMCS3 can also be used as an indicator of organic matter abundance.

1. Organic carbon content of immature source rocks in salt lake basin

Organic carbon content is the most commonly used index of organic matter abundance. The organic carbon content of immature source rocks in salt lake basin of Jianghan Basin ranges from 0.24% to 6.19%, with an average of 1.70%, of which 1% accounts for more than 60% of the total samples, and 1% accounts for 21of the total samples. This shows that most of the Paleogene low-mature source rocks (mainly Qianjiang Formation) in Jianghan Basin have reached the standard of "good-good" argillaceous source rocks in China. Therefore, the material basis of hydrocarbon generation is relatively strong.

Table 4- 1 Organic Carbon Content of Low Mature Source Rock in Jianghan Basin

sequential

Figure 4- 1 Frequency Distribution Histogram of Organic Carbon Content in Paleogene Source Rocks in Jianghan Basin

Due to the complex structural and sedimentary characteristics of the sedimentary basin, the organic heterogeneity of source rocks in the salt lake basin is obvious, as shown in Figure 4- 1. Organic carbon content in source rocks of Qianjiang Formation in Jianghan Basin is widely distributed. Generally speaking, the organic carbon content of source rocks in Xingouzui Formation is the highest, but the number of samples is small and the representativeness is poor. The organic carbon content of source rocks in Qianjiang Formation is the highest in the first member of the Company, with TOC ranging from 0.42% to 6.19%, arithmetic average of 1.78%, weighted average of 1.69%, and most samples ranging from 0.6% to 2.0%. Secondly, the source rocks of the second member of Qianlie Formation, with TOC ranging from 0.62% to 4.4%, arithmetic average of 65,438 0.77%, weighted average of 65,438 0.94% and main frequency distribution ranging from 0.6% to 65,438 0.0%; Followed by the third member, the TOC is 0.77% ~ 4.3%, and the arithmetic average is 1.64%. The organic carbon content of source rocks in the first member of Qian Formation is low, with an arithmetic average of 1.60%. This shows that the organic carbon content of Paleogene source rocks in Jianghan Basin is generally high, and its hydrocarbon generation potential cannot be underestimated.

2. Pyrolysis parameters S 1, S2 and Pg of low-mature source rocks in the salt lake basin.

Rock pyrolysis analysis is a method to quickly obtain the information of organic matter abundance, type and maturity of source rocks. The indexes of organic matter abundance are soluble hydrocarbon content (S 1), pyrolytic hydrocarbon content (S2) and oil production potential (Pg). Table 4-2 shows the pyrolysis analysis results of source rocks in Jianghan Basin.

Table 4-2 Pyrolysis Parameters of Source Rocks in Jianghan Basin

Among the salt lake source rocks in Jianghan Basin, the soluble hydrocarbon content of the first two member source rocks is the highest, with an average of 65438 0.36 mg/g, followed by the first four members, the first three members and the Xingouzui Formation, with an average of 65438±0.65438±0.08mg/g/g, 65438 0.02mg/g and 0.02 mg/g respectively. The content of pyrolytic hydrocarbons is the highest in the source rocks of Qian Yi Member, with an average of 15.45mg/g, followed by the first two members, the first three members, the first four members and the Xingouzui Formation, with 14.25mg/g, 12.49mg/g, 7.27mg/g and 2.57 mg/g respectively.

Figure 4-2 shows the distribution histogram of oil production potential of source rocks in Jianghan Basin. As can be seen from the figure, the oil production potential of source rocks is widely distributed. Generally speaking, the first member of source rocks has the highest oil generation potential. 1 1 The distribution range of oil production potential of the sample is 0.44 ~ 64.03 mg/g, the arithmetic average is 16.47mg/g, the weighted average is 17.27mg/g, and the main frequency distribution range is 65438. The second is the second member source rock. The distribution range of oil production potential of 1 1 sample is 1.74 ~ 73.04 mg/g, with an arithmetic average of 15.6 1mg/g and a weighted average of17. Thirdly, it is the source rock of the third member of Qian Formation. The distribution range of oil-generating potential of 7 samples is 0.5 1 ~ 53.40 mg/g, the arithmetic average is 13. 19mg/g, and the weighted average is 14.43mg/g/g, and the source rocks greater than 6mg/g account for 30% of the total samples. The source rocks of Si Qian Formation and Xingouzui Formation have low oil-generating potential, with distribution ranges of 0.6 1 ~ 28.36 mg/g and 0.99 ~ 4.55 mg/g, respectively, with arithmetic mean values of 8.45mg/g and 2.77mg/g and weighted mean values of 8.77mg/g and 2.63 mg/g, respectively. ..

Figure 4-2 Distribution Histogram of Oil Production Potential of Source Rocks in Different Layers in Jianghan Basin

Soluble hydrocarbons (S 1) and pyrolytic hydrocarbons (S2) in source rocks are all generated by dispersed organic matter, and they have a good correlation with organic carbon content (Figure 4-3). Soluble hydrocarbons and pyrolytic hydrocarbons in source rocks increase with the increase of organic carbon content, and they are logarithmic. As can also be seen from Figure 4-3, the correlation ratio between S2 and organic carbon is S65433. This situation is not accidental. As far as S2 is concerned, it is the product of thermal degradation of dispersed organic matter in source rocks, which is related to general mature oil and gas accumulation. However, the source of soluble hydrocarbon S 1 is complicated. For mature source rocks, this part of hydrocarbons should be kerogen-generated hydrocarbons, which remain in the source rocks after migration and accumulation. As far as Jianghan Basin is concerned, the vitrinite reflectance Ro of all samples is between 0.30% and 0.60%, which is still in the immature-low mature stage. At this time, kerogen has not undergone obvious thermal degradation, that is, this part of hydrocarbons can not come from thermal degradation of kerogen, nor is it a mature hydrocarbon substance in the general sense. Therefore, the high content of soluble hydrocarbons in source rocks of low evolution stage is an important sign of early hydrocarbon generation.

Fig. 4-3 Relationship between pyrolysis parameter S 1 and S2 and organic carbon content of source rocks in Jianghan Basin.

Three. Soluble organic matter content of source rocks in salt lake basin

Chloroform asphalt A and total hydrocarbon content are one of the most commonly used indexes to measure the abundance of organic matter. Generally speaking, the content of chloroform asphalt A and total hydrocarbon in source rocks of Salt Lake Basin is relatively high, and the content of chloroform asphalt A in almost all samples is higher than the lower limit of 0.0 1%, and the content of total hydrocarbon is higher than 100× 10-6, reaching the lower limit standard of argillaceous source rocks (Table 4-3).

Table 4-3 Soluble Organic Matter Parameters of Source Rocks in Jianghan Basin

The content of chloroform asphalt A in source rocks of Jianghan Basin is the highest in Qian-2 member, with a distribution range of 0. 1372% ~ 2.3460%, with an average of 0.736 1%, followed by Qian-4 member with an average of 0.5394%. The content of chloroform asphalt A in the first section and the first three sections is also high, with an average of 0.4985% respectively. In contrast, the content of chloroform asphalt A in Xingouzui Formation is low, which is 0.2560%, but it also meets the standard of "good" argillaceous source rocks. The total hydrocarbon content is the highest in the source rocks of the first member of the Division, and its distribution range is (431~ 6215 )×10-6, with an average value of 24 19× 10-6, followed by EQ2 > EQ3 >. See Figure 4-4 and Figure 4-5 for the histogram of chloroform asphalt A and total hydrocarbon distribution.

Figure 4-4 Distribution Histogram of Chloroform Asphalt A in Source Rock of Jianghan Basin

Figure 4-5 Frequency Distribution Histogram of Total Hydrocarbon Content in Source Rocks of Different Layers in Jianghan Basin

The conversion degree of organic matter to hydrocarbons is also an important index to measure the hydrocarbon generation potential of source rocks. Commonly used conversion parameters are chloroform asphalt A/TOC and HC/TOC. It is an indisputable fact that the conversion rate of source rocks in salt lake basin is high. Although the evolution of source rocks in Jianghan Basin is very low, and it is still in the immature-low mature stage, its chloroform pitch A/TOC and HC/TOC are both high, indicating that it has great hydrocarbon generation potential.

As described in the second chapter, the content of maceral TMCS3 has a good correlation with the organic carbon content, chloroform asphalt A, total hydrocarbon and oil generation potential Pg of source rocks in Jianghan Basin, so the content of maceral TMCS3 can also indicate the abundance of organic matter. Generally speaking, the maceral content of source rocks in Qian-2 member and Qian-1 member is high, which indicates that their organic matter abundance is high, and the above characteristics are basically consistent with other organic matter abundance indexes.