(1) Characteristics of Carboniferous-Permian source rocks in Huainan
At present, only Well Heshen 4 drilled in Dingyuan Depression has encountered the Permian Shihezi Formation (not drilled), and the Carboniferous-Permian thickness in Huainan area in the northern margin of the basin is 900 ~ 1800 m, in which the dark mudstone is 4 10~540m ~ 540 m, with 38 ~ 50 layers of coal, and the cumulative thickness reaches 3/kloc.
1. Rich organic matter
The organic carbon content of mudstone in Shihezi Formation of Well Heshen 4 is 0.60% ~ 4.64%, with an average of 65,438 0.96%. Chloroform asphalt "A" content is 0.037% ~ 0.363%, with an average of 0.1318%; The total hydrocarbon content is 79 ~ 1000× 10-6, with an average of 363.6×10-6. The hydrocarbon generation potential of S 1+S2 is 0.42 ~ 9.56 mg/g, with an average of 272mg/g, the asphalt conversion rate is 4.63% ~ 7.83%, and the total hydrocarbon conversion rate is 1.28% ~ 2. 17%. According to the abundance of organic matter, the mudstone of Shihezi Formation in Well Heshen 4 belongs to good-good source rock (Table 6-6).
Table 6-6 Statistics of Organic Matter Abundance of Carboniferous-Permian Source Rocks in Hefei Basin and its Northern Margin
The Permian organic matter abundance in holes BU Ⅷ-2 and BU Ⅷ-3 in Huainan coalfield also has similar characteristics (Table 6-6). The organic carbon content of coal is 58.6% ~ 63.05%, and the "A" content of chloroform asphalt is as high as 1.2838%, generally 0.4805% ~ 0.8926. The organic carbon content of dark mudstone is 0.67% ~ 2.03%, the content of chloroform asphalt "A" is 0.04% ~ 0.07%, and the hydrocarbon generation potential of S 1+S2 is 0.1825 ~1.38 mg/g.
From the above geochemical analysis, it can be seen that the organic carbon content of dark mudstone in Huainan-type Carboniferous-Permian coal-bearing source rocks in Hefei Basin is obviously higher than the lower limit of organic carbon of source rocks by 0.5%, so Huainan-type Carboniferous-Permian coal-bearing source rocks are good. In addition, it is worth noting that the average organic carbon content of Carboniferous Taiyuan Formation limestone is 0.5%, which is higher than the lower limit of organic carbon of marine limestone source rocks of 0. 1%, so the hydrocarbon generation capacity of Carboniferous Taiyuan Formation limestone is also worthy of attention.
2. Types of organic matter
Characteristics of (1) chloroform asphalt "A" and its family components. The content of soluble organic matter in Permian mudstone in Well Heshun 4 is generally high (Table 6-7), generally above 50%, as high as 78.63%, with saturated hydrocarbon content ranging from 6.89%- 1 1.52% and aromatic hydrocarbon content ranging from10.
Table 6-7 Statistical Table of Chloroform Asphalt "A" and Its Group Composition Characteristics of Permian Source Rocks in Hefei Basin and Northern Margin
(Data quoted from former Anhui Petroleum Exploration and Development Company)
The analysis of the "A" group composition of the Permian coal chloroform asphalt in BU Ⅷ-2 and BU Ⅷ-3 holes in Huainan Coalfield shows (Table 6-7) that the average content of non-hydrocarbon+asphaltene is 69.98% ~ 82.9 1%, and the content of saturated hydrocarbon+aromatic hydrocarbon is17.09% ~ 30%. Dark mudstone has similar characteristics, the ratio of saturated hydrocarbon to aromatic hydrocarbon is between 0.54 and 0.98, and it also shows the characteristics of type ⅲ kerogen.
(2) Distribution characteristics of saturated hydrocarbons. Chromatographic analysis of alkanes in Permian mudstone in Well Heshen 4 (Table 6-8) shows that most of them are unimodal, and most of the carbon in the main peak is C 15 ~ C 16, which reflects that the low carbon number is dominant and is the characteristic of coal-bearing organic matter. Several of them are bimodal. The carbon of the front peak is C 15 ~ C 17, and the carbon of the rear peak is C27. The ratio of C2 1+C22/C28+C29 mostly varies between 1 ~ 3, and the modulus of Pr/Ph ratio is between 1 ~ 2, indicating the formation of hydrocarbon-forming parent materials.
(3) Composition characteristics of sterterpenes. Steroids around C30 have a large drum-shaped complex and difficult-to-distinguish peak, which indicates that the content of steroids is high. Color analysis shows that the distribution of terpenoids in coal and dark mudstone is very different. There are no tricyclic terpenoids and γ alkanes in coal and rock, and the content of C32 L hopane is high, which indicates that coal and rock were formed in freshwater swamp environment. Dark mudstone is n-sterane, which is basically located in the range of ⅱ-ⅲ kerogen.
Table 6-8 Statistics of Hydrocarbon Chromatographic Data of Permian Source Rock in Huainan, Well Heshen 4, Hefei Basin
(Data quoted from former Anhui Petroleum Exploration and Development Company)
(4) The maceral characteristics of dark mudstone. According to the microscopic analysis of kerogen (Table 6-9), the microscopic components of Permian mudstone in Well Heshun 4 are mainly vitrinite and crustaceous, with a small amount of sapropelic and inertinite, all of which contain early hydrocarbon-generating components, such as resin, matrix and sporophyte. It shows that higher plants are the main source of organic matter, with type ⅲ kerogen as the main source and a small amount of type ⅱ kerogen.
Table 6-9 Statistical Table of Permian Mudstone Micro-composition Characteristics in Well Heshen 4, Hefei Basin
(Data quoted from former Anhui Petroleum Exploration and Development Company)
(5) Rock pyrolysis and kerogen carbon isotope characteristics. The pyrolysis type index of rocks is low (Table 6- 10), the hydrogen index is only 53 ~ 90 mg/g, and the carbon isotope of kerogen varies from-22.9 ‰ to 23.7 ‰. On the element composition range diagram, most sampling points fall within the range of coal and ⅲ organic matter. Therefore, it belongs to type III kerogen in general and contains a small amount of type II kerogen.
To sum up, the organic matter type of Permian coal and rock in Huainan, Hefei Basin belongs to type III, and the dark mudstone is mainly type III, with a small amount of type II kerogen. Therefore, this area should focus on finding coal-formed gas.
Table 6- 10 Statistics of pyrolysis and isotopic composition of Permian source rocks in Huainan, Well Heshen 4, Hefei Basin (data quoted from former Anhui Petroleum Exploration and Development Company)
3. Maturity of Carboniferous-Permian source rocks
Vitrinite reflectance is the most reliable and only comparable international comprehensive index to measure the maturity of organic matter. In the early 1950s, Teichmuller extended this achievement to determine the maturity of dispersed organic matter in sedimentary rocks, which laid a foundation for oil and gas evaluation. The reflectance of vitrinite in organic matter increases with the continuous improvement of thermal evolution, and this process is irreversible, so it can directly reflect the heating history of organic matter.
The evolution of organic matter in coal measures can be divided into three stages (Dai Jinxing, Chen Yunlin, etc. , 1985).
The first stage: immature stage, namely peat-lignite stage (Ro≤0.5%).
The second stage: the mature stage, which is equivalent to the long-flame coal-coking coal stage (0.5 < Ro < 1.6%). When RO is between 0.5% and 0.8%, it enters the early stage of maturity; When Ro is 0.8% ~ 1.2%, it is in the middle of maturity; When Ro is between 1.2%- 1.6%, it is the end of maturity.
The third stage: over-mature stage, that is, lean coal-anthracite stage (RO > 1.6%).
In the Permian 2306 ~ 250 1m interval of Well Heshen 4, the vitrinite reflectance Ro is 0.9% ~ 1.0% and TMAX is 450, indicating that the source rocks are in the middle mature stage.
The buried depth of Permian in Buⅷ hole in Huainan coalfield is 450 ~ 700 m, and the vitrinite reflectance Ro is similar to that of Well Heshen 4, with Tmax of 430 ~ 455. According to the Maturity Study of Carboniferous-Permian Coal Measures Source Rocks in Shengli Oilfield (Xiang Kui et al.,1996; Xu Chunhua, 1996), the Carboniferous-Permian source rocks with a buried depth of less than 3,000 m are still in the primary hydrocarbon generation stage, and the secondary buried depth of source rocks must be greater than 3,000 m to enter the secondary gas generation stage. It is speculated that the Permian in Well Heshen 4 is still in the stage of primary burial depth evolution and has not yet reached the stage of secondary hydrocarbon generation. Therefore, the key to find the Carboniferous-Permian distribution area with secondary hydrocarbon generation ability in Hefei Basin is to find coal-formed gas reservoirs with this system as source rocks.
(2) Geochemical characteristics of Carboniferous source rocks in the northern margin of Dabie Mountains.
Up to now, the Carboniferous Meishan Group in the northern margin of Dabie Mountain has not been drilled in Hefei Basin, and only shallow metamorphic rocks are exposed in Jinzhai area in the southern margin of the basin. The Carboniferous strata exposed in Shangcheng-Gushi area in the southwest margin of the basin are relatively complete, with a cumulative thickness of more than 5,000 meters, of which the dark mudstone is 5,000 meters thick and the coal seam is 20-40 meters thick (Table 6- 1 1). Dark mudstone is mainly developed in Xiahuayuanwang Formation (C 1h) and Shan Yang Formation (C65438).
Table 6- 1 1 Development Statistics of Carboniferous Meishan Formation Source Rock in Shangcheng-Gushi Area, Henan Province
(Data quoted from former Anhui Petroleum Exploration and Development Company)
1. Rich organic matter
Shan Yang Formation is the main coal-bearing series of Meishan Group. The organic carbon content of coal and rock is generally 50.6% ~ 70.2%, with an average of 66%. The organic carbon content of coal and rock gradually decreases from old to new, reflecting the gradual deterioration of coal-forming conditions. The organic carbon content of carbonaceous mudstone is 1.68% ~ 9.47%, and that of mudstone is 1.54% ~ 2.64%. The organic carbon content of the three types of source rocks is high, and the organic carbon content of mudstone in other horizons is 0.55% ~ 18.47%, all of which meet the standard of source rocks (Table 6- 12).
Table 6- 12 Statistics of Organic Matter Abundance of Carboniferous Meishan Formation in Shangcheng-Gushi Area, Henan Province
Note: Maximum value to minimum value (average value).
(Data quoted from former Anhui Petroleum Exploration and Development Company)
The content of chloroform asphalt "A" in mudstone and coal seam of Meishan Formation is generally less than 0.0 1%. The highest value is only 0.0362%, the total hydrocarbon content is generally less than 100× 10-6, and the highest value is 320.8× 10-6. The hydrocarbon generation potential of S 1+S2 is generally around 0. 1mg/g, and that of Shan Yang Formation (C 1y) is slightly higher, reaching about 2 mg/g. ..
Therefore, the organic matter of Meishan Formation has the characteristics of three lows and one high, that is, high organic carbon, low chloroform asphalt "A", low total hydrocarbon and low hydrocarbon generation potential, which shows the characteristics of coal-bearing source rocks.
2. Types of organic matter
The maceral of organic kerogen in Meishan Formation is mainly vitrinite, with the content of 53.5% ~ 89.5%. The inertinite group is10.5% ~ 39.8%; The content of shell components is only 0 ~ 6.7%. The carbon isotope δ 13C of kerogen is-19.89 ‰ ~-24.27 ‰, all of which show the characteristics of type III kerogen (Table 6- 13).
Table 6- 13 Statistical Table of Organic Matter Type Indicators of Carboniferous Meishan Formation in Shangcheng-Gushi Area, Henan Province
(Data quoted from former Anhui Petroleum Exploration and Development Company)
According to the composition of chloroform asphalt "A" n-alkanes, the main peak carbons are generally C 16 and C 17, and C27, c 2 1-/c 2 1+ are found in Shan Yang Formation (C 1y) coal measures.
The pyrolysis type index lacks data. According to the restored original degradation potential (D0) of coal and rock, D0 is generally less than 10%. Mudstone is not high, showing the characteristics of type ⅲ kerogen. Only the D0 value of some samples of Huyoufang Formation (C2h) reached 45.9%, showing the characteristics of type II kerogen.
Saturated hydrocarbons in biomarkers are composed of steranes, the dark mudstone is C29 > C27 > C28, and the coal rock is C29>C27>C28 > C27. The C27/(C27+C28+C29) sterane ratio of AAAR configuration is 0.22 ~ 0.26, indicating that the organic matter is mainly terrestrial higher plants, and the ratio of γ alkane to C3 1 hopane is less than 0.46.
To sum up, the organic matter type of Meishan Formation in Carboniferous is mainly kerogen type III, and only part of mudstone kerogen type II in Huyoufang Formation (C2h) has obvious characteristics.
3. Maturity of organic matter
The vitrinite reflectance Ro value of Meishan Formation coal and mudstone is between 2.35% and 4.37%, with an average of 3.44%, all of which are in the over-mature stage.
From the composition of terpanes (Table 6- 14), the ratio of hopane C3 122S/(22S+22R) is 0.56 ~ 0.60, which has reached equilibrium, which also reflects the high evolution.
Table 6- 14 Statistics of Steroid Biomarkers Composition of Carboniferous Meishan Formation in Shangcheng-Gushi Area, Henan Province
(Data quoted from former Anhui Petroleum Exploration and Development Company)
The thermal evolution of Meishan Group is very high, which may be related to the regional dynamic metamorphism formed by Dabie Mountain thrust. The degree of internal thermal evolution in Hefei basin is still inconclusive, but according to the current burial depth of nearly 10000m, it should also enter a high-over mature stage, and a large number of oil and gas have been generated.
In a word, the source rocks of Meishan Formation are rich in organic matter, mainly kerogen type III, with high degree of thermal evolution, and have generated a large number of oil and gas. Therefore, the most important thing in this area is to study the relationship between hydrocarbon generation process and trap matching history.
(3) Geochemical characteristics of dark carboniferous mudstone in Ansan 1 well type.
The Carboniferous-Permian system drilled by Ansan 1 well is a set of large dark mudstone and calcareous mudstone, which has been transformed into slate or even phyllite through low-grade metamorphism. Geochemical analysis of cuttings shows that the organic carbon content is only 0.03% ~ 0. 13%, with an average of 0.078%, which is far below the lower limit of the evaluation standard of 0.5% for source rocks. The hydrocarbon generation potential of S 1+S2 is only 0.0 1 ~ 0.02 mg/g, which is lower than the hydrocarbon generation potential of traditional source rock evaluation standard 1.0mg/g and belongs to poor source rock. Therefore, the Carboniferous-Permian dark mudstone drilled in this well basically has no hydrocarbon generation capacity (Xu et al., 2000). Its Tmax is higher than 490℃, and it is in an over-mature stage, even undergoing low greenschist facies metamorphism, with the metamorphic temperature ranging from 300℃ to 350℃. It is speculated that large-scale northward thrust nappe occurred in Indosinian strata, resulting in dynamic metamorphism.