Taiyuan basin is located in the middle of Shanxi province, also known as Jinzhong fault depression, between east longitude11150' ~1337' and north latitude 37 50' ~ 38 10', including Taiyuan Xishan coalfield. Taiyuan basin is a faulted basin, which starts from Taiyuan in the north and ends at Jiexiu in the south, bounded by Jiaocheng fault in the northwest and Taiping fault in the southeast. It is banded and northeast-trending, with a length of about 130km, a width of about 40km and an area of about 5600km2. In regional structure, this area is a fault depression in the middle of the late Cenozoic fault depression belt of Shanxi platform. The surface is generally covered by modern accumulation, and only Pliocene and Pleistocene are exposed at the edge.
Before the oil survey, the geological research in this area was very low, with only some coalfields, shallow hydrological drilling and sporadic electrical survey work, and little understanding of the geological characteristics of deep basins. 197 1 ~ 1972 Team 904 of Aviation Geophysical Exploration Brigade of State Geological Administration conducted1∶ 200,000 oil aeromagnetic survey in Qinshui Basin, Shanxi Province, and included this area in its work area. 1972 factory 646 of the former Ministry of Fuel and Chemical Industry also studied this area during the petroleum geological survey in Qinshui Basin, and made an electrical sounding profile. In the same year, the Petroleum Geophysical Exploration Bureau of the former Ministry of Combustion and Chemical Industry conducted a gravity survey of1∶ 200,000 in this area.
On the basis of the above work, according to the decision of Shanxi Geological Bureau, the oil survey exploration team of Shanxi Geological Bureau began to carry out a comprehensive and systematic oil survey in Taiyuan Basin from 65438 to 0975. During the period from 1975 to 1979, the following work was carried out: general survey of1∶ 200,000 earthquakes in the whole region, detailed survey of local structures1∶ 50,000 earthquakes, completion of survey line 1047.85km, distance of Qingxu fault depression line 4km, and the line in the basin. Deep well drilling with the main purpose of finding oil is carried out in local structures. In the basin, there are 7 deep wells, 2 medium-deep wells/kloc-0, and the drilling workload is 23907.8+0m. The above work has made rich geological achievements, and on this basis, a lot of comprehensive research work has been carried out. According to the analysis of exploration results, it is considered that Taiyuan basin is a Cenozoic fault depression on the uplift background, with low organic matter abundance and low maturity, and does not have the oil-forming conditions of Paleogene and Paleogene.
The evaluation of Taiyuan Basin in the new round of national oil and gas resources evaluation is jointly completed by China Petroleum Exploration and Development Research Institute and China Petroleum Changqing Oilfield Company.
Second, the geological conditions
(1) Geological survey
Taiyuan basin is one of the fault basins in the late Cenozoic fault zone of Shanxi platform, which is located between the platform and the fault zone. It is embedded in the northwest corner of the northeast Qinshui syncline, with anticlinorium, Lvliang, in the west and Wutai syncline in the north. Different parts of Taiyuan basin have different tectonic development history and sedimentary construction characteristics. According to the fluctuation of basic strata and the difference of Cenozoic strata development, Taiyuan basin is divided into five secondary structural units (figure 10- 13- 1, table 10- 1).
Figure 10- 13- 1 Structural Outline of Taiyuan Basin
Table 10- 13- 1 division table of structural units in Taiyuan basin
According to the lithology, lithofacies characteristics, paleontological fossil assemblage and the structural relationship between strata, the late Cenozoic strata encountered in this area are divided into the following stratigraphic units: Quaternary Fenhe Group; Neogene Pliocene Jinzhong Formation (Yi 'an Formation, Shijiashe Formation, Xigu Formation, Hucun Formation, Chengzi Formation and Wuwang Formation).
(2) Source rocks
The exploration results show that Taiyuan basin has no oil-generating conditions, no oil-gas display and poor oil-bearing prospects, so it is not suitable for finding late Cenozoic oil and gas reservoirs in this area.
1. Fault depression does not have oil generation conditions.
At the beginning of the general survey, Shijiashe Formation and Xigu Formation of Jinzhong Group were selected as possible source rocks in this area. First of all, because there are multiple layers of oil shale (with a total thickness of 5. In the Pliocene Zhangcun Formation in Yushe and Wuxiang areas, which are only separated from Taiyuan Basin by a mountain, oil shale is used to build factories and refine oil locally. Xiaobai Formation, underground Shijiashe Formation and Xigu Formation on the edge of Taiyuan Basin are similar to Zhangcun Formation in horizon and lithology and lithofacies, so it is speculated that Shijiashe Formation and Xigu Formation may also have similar oil generation conditions (Table 10- 13-2). Secondly, from the analysis of lithology and oil generation index, these two groups of strata are relatively good compared with other strata in Jinzhong Formation. The lower three groups of Jinzhong Formation are mainly red coarse-grained sediments, reflecting the rapid deposition under strong oxidation conditions, and the average organic carbon content of Hucun Formation is less than 0. 2%, which is extremely unfavorable to oil generation. Xigu Formation and Shijiashe Formation are sediments in the heyday of lake basin development, and lacustrine strata with a thickness of more than 1000 m are deposited in the center of sediments, which contain more ostracods, mollusks, fish fossils and pyrite.
Table 10- 13-2 Gold 1 Oil Production Index of Well Jin 2
The thickness of Shangxigu Formation and Shijiashe Formation in the uplift and fault steps of Taiyuan Basin becomes thinner, only 200 ~ 400 m, the lithology turns red and thicker, and the organic carbon content is very low, such as platform 1 well and small 1 well Xigu Formation is 0. 27%. Therefore, the abundance of organic matter in these areas is very poor, and there are no conditions for oil generation. The abundance of organic matter in Jiaocheng sag is also insufficient. Even in the deep wells of Qingxu fault depression, the average organic carbon content of Xigu Formation is 0. The coal ratio of Shijiashe Formation is generally 0. 3%. 27-0. 4 1%. Only Xigu Formation of Jin 1 well and Shijiashe Formation of Jin 2 well have higher content. 5 1% and 0. 2% respectively. In particular, the lithology of Xigu Formation changes greatly, and the oil generation performance is also very different. Only in the sedimentary center of the fault depression (such as gold 1 well), dark argillaceous rocks are dominant, and the organic matter abundance is relatively good. However, in slightly higher structural parts (such as well Jin 4 in Duancun structure), the lithology becomes coarse rapidly, and the organic matter abundance becomes worse (Table 10- 13-3
Table 10- 13-3 Gold 1 Well Jin 4 Source Rock Parameter Table
The above data show that the organic carbon content of Xigu Formation and Shijiashe Formation in Taiyuan Basin only meets or approaches the minimum requirement of organic carbon content of source rocks in some areas, but fails to meet the minimum requirement in most areas. Therefore, Xigu Formation and Shijiashe Formation lack the necessary material basis for oil generation-rich primitive organic matter, and do not have the conditions for oil generation.
In Taiyuan Basin, Xigu Formation and Shijiashe Formation were sampled and analyzed by gas chromatography. However, because the sample extracts can't reach the detection amount of the instrument, most samples don't get the spectrogram. The characteristics of some spectrograms obtained are as follows (Figure 10- 13-2): Generally, there is no peak before C 16; The main peak carbon and high value band of the distribution curve tend to be high carbon, and the main peak carbon in the spectrum is generally after C25. Most of the "OEP" values of C25, C27 and C29 are greater than 1.5, and more than half of them are greater than 2.
Figure 10- 13-2 Chromatograms of n-alkanes in Well Jin 5 and Well Jin 7.
The above characteristics show that the normal alkanes in this area are mainly high-carbon long-chain normal alkanes, with less light components and extremely obvious odd-even advantages, so it is possible that the organic matter in source rocks is still in an immature and primitive state.
In addition, according to the characteristics that the main peak carbon and high-value zone of the distribution curve are biased towards high carbon, it is inferred that terrestrial plants account for a high proportion in the original organic matter, because the wax in plants is composed of heavy fractions. The normal combustion spectrum of well Jin 4 further illustrates this problem. There are two main peak carbons in this well, the main peak carbon is C29 and the secondary peak carbon is C 19. The reason for this phenomenon may be related to the different types of original organic matter contained in rocks. Plankton contains short-chain hydrocarbons, and wax of terrestrial plants contains heavy hydrocarbons. When they are mixed in the immature stage, two main peaks of carbon appear on the spectrum.
We also analyzed the infrared spectra of aromatic hydrocarbons in this area, but the obtained spectra are also different from those of known source rocks. The absorption peaks of 740 cm- 1 and 8 10 cm- 1 cannot be obtained simultaneously on the same aromatic spectrum in this area, so the aromatic structure index cannot be calculated.
To sum up, Xigu Formation and Shijiashe Formation in Taiyuan Basin lack both the material basis (rich organic matter) and the objective conditions (temperature) necessary for oil generation, so they do not have oil generation conditions and are not oil-generating beds.
2. No oil and gas show was found during drilling.
There is no oil and gas display in the outcrop area on the edge of Taiyuan Basin. No oil and gas show was found in geological drilling in the basin. In order to find out the oil-bearing property of local structures and obtain accurate evaluation data, well Jin 7 was drilled in Wuwang structure, and wells Jin 4, Jin 5 and Jin 6 were drilled in different fault blocks and parts of Duancun structure. Only some fluorescence display and gas logging anomalies were found in these four wells, and no oil and gas display was found.
Fluorescence shows that the chloroform asphalt content in Jin 4, Jin 5 and Jin 6 intervals is low, and the highest content is less than 0. 02%. It is speculated that the transformed organic matter contained in rocks is the main reason for fluorescence display. Compared with hydrocarbons in petroleum, this organic matter is immature. Therefore, the fluorescence display in this area has nothing to do with oil and gas reservoirs. In addition, some fluorescence displays may also be related to foreign refined oil pollution.
Gas logging anomaly was found at 1 065 ~ 1 068m in well Jin 5. This member belongs to the lower part of Yi 'an Formation, and its lithology is light gray siltstone and fine sandstone mixed with mudstone. In gas logging, the total hydrocarbon content is 0. 79 1%, and the heavy hydrocarbon content is 0. 686%, which is about 8 ~ 10 times of the background value. At the same time, the bubbles on the surface of the mud tank increased, with a diameter of about 1 ~2mm, and there was a noise but no color film when it burst, and the duration was about10min. Sampling analysis shows that its composition is nitrogen 99. 0022%, methane 0. 99.76% and trace carbon dioxide. Because the amount of gas is small and only contains less than 1% methane, the above gas logging anomaly only shows that there is gas containing a small amount of methane in the formation, which has nothing to do with oil and gas reservoirs.
Wuwang structure and Duancun structure are two structures with good sealing conditions, which are located on both sides of the sedimentary center of the depression. If oil and gas are generated, it is the most favorable oil and gas accumulation area in the fault depression. In these two structures, deep wells were drilled with the main purpose of finding oil, especially three deep wells were dissected in different fault blocks and different parts of Duancun structure, and the observation and analysis of wellhead oil and gas were strengthened in logging work. If there is an oil and gas reservoir in the structure, it should be found, but the drilling practice results still show that there is no oil and gas. This shows that there is no oil and gas reservoir in this structure, so it is speculated that the oil-bearing prospect in Taiyuan basin is not great. In addition, the inference that this area does not have oil-generating conditions has been further confirmed by practice.
Third, oil and gas prospects.
Although Taiyuan basin has a certain area and a set of thick lacustrine sediments, it is not rich in organic matter and mature in oil conversion, so it does not have the conditions for oil generation. There is no oil and gas display in the fault depression, which further shows that there is no oil and gas reservoir without oil source. Therefore, it is difficult to find oil and gas generated in late Cenozoic in Taiyuan basin, and the prospect is not great. Therefore, oil and gas resources are not calculated this time.