(West New Area Research Center of China Petrochemical Shengli Oilfield Company, Dongying, Shandong 257000)

Fund Project: Part of the achievement of the project "Exploration Target and Technology Research of Fault Depression in Southern Songliao Basin" (P06022) of China Petrochemical Science and Technology Department.

Author: Gao Xiangcheng, male, engineer, mainly engaged in comprehensive evaluation of petroleum geology and oil and gas exploration. Email: gaoxch.slyt @ sinopec.com.

Abstract: Changling fault depression is located in the south of Songliao basin. With the deepening of exploration and research, a major breakthrough has been made in the drilling of volcanic oil and gas reservoirs in Changling area in recent years. Many exploratory wells, such as Shen Yao 1 and Shen Yao 2, have successively obtained high-yield hydrocarbon gas flow in the volcanic rocks of Yingcheng Formation, revealing that the volcanic rocks of Yingcheng Formation in this area will be the main field of deep natural gas exploration in Changling fault depression. This paper takes the volcanic reservoir of Yingcheng Formation in Changling fault depression as the research object. On the basis of volcanic mechanism and volcanic facies division, the characteristics and influencing factors of volcanic reservoirs are systematically analyzed by comprehensively using drilling and logging data, which lays the foundation for fine evaluation and optimization of trap targets and effective reservoir prediction in this area. The analysis shows that reservoir physical properties are an important factor affecting gas accumulation in volcanic gas field in Changling fault depression, and it will be helpful to guide the exploration and deployment of deep volcanic oil and gas in this area to understand the reservoir physical properties and influencing factors. Based on drilling and logging data, the physical characteristics and influencing factors of volcanic reservoirs in this area are systematically analyzed. The results show that the volcanic reservoirs in the study area are mainly tuff and rhyolite, and the reservoir space types are primary gas pores, secondary dissolved pores and fractured dual pore media. Volcanic facies belt and burial depth are two key factors affecting the physical properties of volcanic reservoirs, especially pyroclastic rocks (tuff) which are greatly affected by depth. In addition, the distance from oil and gas source is another important factor affecting oil and gas filling in this kind of reservoir.

Key words: volcanic rocks; Reservoir characteristics; Reservoir space; Influencing factors; Songliao basin

Characteristics and influencing factors of volcanic reservoir in Changling fault depression of Songliao basin

Gao Xiangcheng

(West New Area Research Center of China Petrochemical Shengli Oilfield Company, Dongying, Shandong 257000)

Reservoir physical property is an important factor affecting natural gas accumulation in volcanic gas field in Changling fault depression of Songliao Basin. Identifying the characteristics and influencing factors of volcanic reservoirs is helpful to guide the exploration and deployment of deep volcanic reservoirs. Combined with drilling and logging data, the characteristics and influencing factors of volcanic reservoir are systematically analyzed. The results show that the lithology of volcanic reservoirs is mainly tuff and rhyolite, and the reservoir space types are mainly primary gas pores, secondary dissolved pores and fractured dual pore media. Phase zone and depth are the key factors affecting reservoir physical properties, especially pyroclastic rocks (tuff) are greatly affected by depth.

Key words: volcanic rocks; Reservoir characteristics; Reservoir space; Influencing factors; Songliao basin

introduce

Changling fault depression, located in the south of Songliao Basin, is one of the larger fault depressions in Songliao Basin, with an area of about 7000km2. Large-scale oil and gas exploration in Changling area began in 1980s, and the oil and gas exploration process in this area can be roughly divided into four stages: 1980 ~ 1995 oil and gas exploration stage, 1996 ~ 2000 comprehensive exploration and key area evaluation stage, and 200 1 ~ 2005 target evaluation stage. With the deepening of exploration research [1 ~ 12], a major breakthrough has been made in the drilling of volcanic oil and gas reservoirs in Changling area, and many exploration wells have successively obtained high-yield hydrocarbon gas flow in Yingcheng Formation volcanic rocks, revealing that Yingcheng Formation volcanic rocks will be the main field of deep natural gas exploration in Changling fault depression. In this paper, the volcanic reservoir of Yingcheng Formation in Changling fault depression is taken as the research object, and the reservoir characteristics and influencing factors are systematically analyzed by comprehensively using drilling and logging data, which lays the foundation for fine evaluation of trap targets and effective reservoir prediction in this area.

1 volcanic reservoir space type

The distribution area of large volcanic rocks in Yingcheng Formation of Changling fault depression is about 200km2, which is the main target layer of this study. The sedimentary period of Yingcheng Formation is the largest period for the development of Changling faulted lake basin, which is dominated by sandstone and mudstone deposits, and volcanic rocks are developed in fault active parts. The maximum stratum thickness is greater than 1200m, and the unconformity surface at the top is obvious. The tuff of explosive facies and rhyolite of overflow facies are the most important pay lithology in this area (Figure 1).

Figure 1 regional structural location map of Chaganhua area

The types of volcanic reservoir space in the work area can be divided into pores and fractures (Table 1). The pore types of tuff are mainly intergranular dissolution enlarged pores and intergranular pores, followed by pores in lava matrix; Pores in rhyolite are mainly in lava matrix, followed by breccia pores. Fracture types are mainly structural fractures and swelling structural fractures, and the main fracture combination type is dissolved hole+micropore+fracture type.

Table 1 Table of main reservoir space types and fracture combination types of volcanic rocks

Fig. 2 High-angle fractures in Well Shen Yao 2 (3760.78 ~ 3765.438+0m)

Feldspar dissolved pores, intergranular dissolved pores, intergranular pores and ultramicropores are developed in the rhyolite in the upper part of Well Shen Yao 2. The FMI (3185.1~ 4281m) data of imaging logging show that the structural fractures, high conductivity fractures (Figure 2 and Figure 3) and high resistance fractures of volcanic rocks in Yingcheng Formation are well developed, which mainly play the role of connecting vent holes, dissolved holes and karst caves. The physical properties are relatively good at 3740 ~ 4000 m, and the porosity is 3% ~ 5%. According to the interpretation results of Schlumberger imaging logging, the lithology of volcanic rocks in the upper part of Yingcheng Formation is overflow rhyolite and granite porphyry, the lower part is explosive thermal debris flow unit breccia tuff and tuff, and the strata below volcanic rocks are fan delta front glutenite deposits. Through imaging logging and core observation, the volcanic formation fractures in Well Shen Yao 2 are well developed, mainly distributed in the lower parts of rhyolite and breccia tuff at 3750 ~ 39 15m, 3973~3992m, 4218 ~ 4281m. And the vertical porosity is large, mainly concentrated in: 3747~3875m, 3880 ~ 394 1m, 3978~ 4025m and 4 15 ~ 422 1m and 4227~4380m intervals. According to the porosity statistics of the upper igneous rock, there is no obvious double peak in the section from 3747 to 3875 m, and the maximum effective porosity can reach 1 1.3%, with an average of 5.67%, mainly distributed between 4% and 7%. The maximum secondary porosity is 2.2%, with an average of 0.8%. Although there are some fractures in the reservoir, the calculated fracture porosity is very small and its contribution to the reservoir space is quite small. However, cracks have a great influence on permeability. Like the well area with waist depth 1, fracture is the main factor to improve reservoir permeability.

The lithology of volcanic rocks in the upper member of Yingcheng Formation in Shen Yao 3 well area is explosive phase angle gravelly tuff mixed with sedimentary tuff. Through imaging logging and core observation, fractures and dissolution pores in the upper volcanic rock section of Yingcheng Formation in Well Shen Yao 3 are well developed (Figure 4). The dissolution of breccia tuff in Yingcheng Formation is stronger than that in Huoshiling Formation.

The volcanic rocks in Well Shen Yao 4 are mainly volcanic breccia and rhyolite, and fractures are generally developed, with dissolved pores, and dissolution and expansion along fractures can be seen in some sections. The microcracks in the volcanic rocks of Yingcheng Formation and the metamorphic rocks of Huoshiling Formation in Beijing are in the same direction, mainly NNE-NNW direction. Pores are generally developed, mostly matrix pores, and dissolution pores are generally developed; The reservoir type is mainly fracture-pore type. The pores in Well Shen Yao 4 mainly include pores (including almond pores), secondary pores (intergranular dissolved pores, matrix dissolved pores and spotted dissolved pores) and intergranular micropores. Pores are mainly in the slurry of pyroclastic flow breccia tuff, and there are also a lot of residual pores. Dissolved pores are mainly developed in explosive phase angle breccia tuff and tuff (Figures 5 and 6).

The volcanic rock interval of Shen Yao 30 1 Well is 3716 ~ 3925m. The TAS classification results calculated by ECS show that the lithology is mainly acidic volcanic rocks, and some reservoirs are neutral volcanic rocks, which basically corresponds to the characteristics of logging and conventional curves. The volcanic rock section of this well has good porosity, which is basically distributed between 3% and 20%, especially between 3860 and 3880 meters. The reservoir has large porosity and developed dissolution. The porosity of sandstone and mudstone reservoirs is poor, mainly distributed between 2% and 5%, mainly between 2% and 3.5%.

Fig. 3 Comprehensive histogram of fault strata in Chaganhua area

Fig. 4 The core of Well Shen Yao 3 shows the characteristics of fractures and holes.

Fig. 5 FMI logging of Well Shen Yao 4 shows the characteristics of fractures and holes.

Fig. 6 Common dissolution pores and fractures (4340 ~ 4342m) in core slices of Well Shen Yao 4.

Well Shen Yao 30 1 has a large number of residual pores in the mud of pyroclastic flow breccia tuff, and in addition, the well is mainly developed with dissolved pores at 3860 ~ 3880m (Figure 7), which is a good reservoir for this well.

Fig. 7 FMI image characteristics of Shen Yao 30 1 well solution hole.

2 Physical characteristics of volcanic reservoirs

According to the statistics of physical properties of volcanic rocks in the work area, volcanic rocks are mainly developed in Yingcheng Formation, with the maximum porosity of 25%, the minimum porosity of 0.44% and the average porosity of 5.8%. The maximum permeability is 23.9 1× 10-3μm2, the minimum permeability is 0.1×13 μ m2, and the average permeability is1.1×10. Rhyolite has the best physical porosity of 25% and permeability of 6.08× 10-3μ m2. The optimum physical porosity of tuff is 2 1.35%, and the permeability is 73.72× 10-3μm2.

The reservoir porosity of Well Shen Yao 2 is 2.6% ~ 4.6%, with an average of 4. 1%, and the permeability is (0.027 ~ 0.036) × 10-3 μ m2, with an average of 0.031×/kloc-0-3 μ m. The reservoir of Well Shen Yao 3 is most developed in the phase angle gravelly tuff in the upper part of Yingcheng Formation. ELAN program of volcanic rock profile explains that the effective porosity is 2.7% ~ 9.2% and the weighted average porosity is 5.4%, which is a favorable reservoir. Its maximum value appears around 3600m, and combined with FMI image, it is considered that the high porosity value is caused by dissolution. Generally speaking, it belongs to low porosity and permeability reservoir.

According to the analysis data of volcanic rocks in the same horizon in Xujiaweizi area in the north of the basin, the porosity of rhyolite is generally 0.9% ~ 17.5%, with an average of 4. 1%, and the permeability is (0.001~ 0.263) ×10-3 μ m2, with an average of. The porosity of andesite is generally 1.2% ~ 10.2%, with an average of 5.4%, and the permeability is (0.02 ~12.8) ×10-3 μ m2, with an average of 0.825×/kloc-0. Basalt is dense, mostly isolated pores, with porosity of 0.3% ~ 4.7%, with an average of 3.2%, and permeability of (0.02 ~10.1) ×10-3 μ m2, with an average of1.495×/.

Because the physical properties of volcanic rocks are greatly influenced by lithology and lithofacies. According to the statistics of volcanic rocks encountered by the drilled Yingcheng Formation in this area, Yingcheng Formation encountered 9 kinds of lithology (Table 2), namely andesite (waist depth 4 #), porphyry (waist depth 2 #), granite (waist depth 2 #), volcanic breccia (waist depth 620 1 #) and breccia. Microcrystalline tuff (waist depth 20 1, 202 #) rhyolite (waist depth 2, 3, 4, 6, 20 1, 202, 30 1 #), tuff (waist depth 4, 6, 202, 30/kloc)

Table 2 Statistical Table of Physical Property Analysis of Volcanic Reservoir in Yingcheng Formation of Changling Fault Depression

3 Influencing factors of volcanic reservoir physical properties

According to the evaluation criteria of volcanic reservoirs, volcanic reservoirs in this area can be divided into three types, namely, type I reservoirs-with porosity greater than 6% and permeability greater than1.0×10-3 μ m2; Secondary reservoir-porosity is between 3% and 6%, and permeability is greater than 0.1×10-3 μ m2; Class III reservoir-porosity is between 1%-3% and permeability is greater than 0.01×10-3 μ m2; Ⅳ Reservoir-porosity is less than 1% and permeability is less than 0.0 1× 10-3μm2.

From the statistical analysis of volcanic facies and reservoir physical properties (Table 2), overflow rhyolite and explosive tuff are the main reservoirs of deep natural gas in this area. Crater-near-crater facies group is mainly composed of mound lava and breccia (aggregate) lava, mostly belonging to high porosity and high permeability Class I reservoirs, such as Shen Yao 2 well; The near-source facies group is mainly composed of wedge-shaped and massive lava, mostly belonging to high porosity and medium permeability type II reservoirs and medium porosity and medium permeability type III reservoirs, such as the lower volcanic rocks of Shen Yao 3 and Shen Yao 6 #; The far facies formation is mainly composed of layered pyroclastic rocks, sedimentary pyroclastic rocks and gentle layered lava, which mostly belong to Class III reservoirs with medium porosity and low permeability and Class IV reservoirs with medium porosity and low permeability, such as the upper volcanic rocks of Shen Yao 2, Shen Yao 4 and Shen Yao 6 #.

At the same time, from the statistical relationship between volcanic reservoirs and depth (Table 3), generally, I and II reservoirs are dominant over 3500m, and the reservoir physical properties are relatively good; From 3,500 to 3,700 m, the reservoirs of Class II and Class I are dominant, followed by Class III; From 3700 m to 4000 m, the reservoirs are mainly Class II and III, and the reservoirs are mainly Class I.. Below 4000 meters, Class III and IV reservoirs are dominant, with relatively poor physical properties, less Class II reservoirs and rare Class I reservoirs.

According to the spatial types of volcanic reservoirs, reservoir physical properties and their statistical relationship with lithofacies and depth, it is considered that overflow rhyolite and explosive tuff are the main reservoirs of deep natural gas in the study area, and facies zone and depth are two key factors affecting volcanic reservoir physical properties.

Statistics of volcanic rock types and thickness drilled by Chaganhua.

4 conclusion

Volcanic rock reservoirs in Yingcheng Formation of Changling fault depression in Songliao Basin are mainly explosive tuff and overflow rhyolite, and the reservoir space types are primary gas pores, secondary dissolved pores and fractured dual pore media. Phase zone and depth are two key factors affecting the physical properties of volcanic reservoirs, especially pyroclastic rocks (tuffs) which are greatly affected by depth. Crater-near crater facies group is the most favorable reservoir, followed by near source facies group and far source facies group. The reservoirs above 3500m are mainly Ⅰ+Ⅱ type; From 3500 to 3700 m, I+II is the main type, followed by III. From 3700 m to 4000 m, Class II+III is dominant, followed by Class I; Below 4000m: Class III is dominant, followed by Class II.

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