Research on Rock Minerals and IP Response Characteristics of Shale Gas Reservoir in Sichuan Basin

Abstract

As a kind of clean energy, shale gas has attracted much attention, and the exploration and development potential of shale gas resources in the middle and deep layers is huge. However, due to the changeable geological and burial conditions, complex geophysical responses are formed. Therefore, studying the characteristics of reservoir rock minerals and their complex resistivity response characteristics is helpful to deepen the understanding of the electrical characteristics of shale gas reservoirs and provide theoretical basis and physical basis for exploration and development. The study is based on shale samples from the Longmaxi Formation to the Wufeng Formation of a shale gas well in southern Sichuan, China, and the mineral composition and complex resistivity of shale are measured. Through inversion of complex resistivity model, four IP parameters, namely zero-frequency resistivity, polarizability, time constant and frequency correlation coefficient, are extracted, and the relationship between mineral components of rock samples and IP parameters is analyzed. It is found that the polarizability gradually increases and the resistivity gradually decreases with the increase in borehole depth. With the increase in pyrite content, the polarization increases and the resistivity decreases. The corresponding relational model is established, and it is found that the polarizability is highly sensitive to the characteristic mineral pyrite, which provides more effective data support for the subsequent deep shale gas exploration.

1. Introduction

Under the background of global carbon neutrality, optimizing the energy structure is the primary measure of many countries, and shale gas, as a clean unconventional energy, will play an increasingly important role in the structural adjustment, transformation and upgrading of fossil energy. In terms of source and reservoir, shale gas is characterized by in situ reservoir formation [1,2,3]. The exploration and development of shale gas in the United States started early, and it has been relatively mature since the 1980s. In recent years, many achievements have been made in marine shale gas exploration and development [4,5]. The shale gas exploration and development in China started late due to technical restrictions, but in the last decade, shale gas exploration and development in China have developed rapidly [6,7], and commercial exploitation has been realized in the middle and deep shale gas reservoirs in southern China [8,9,10,11,12]. At present, the shale gas reserves in deep reservoirs are more abundant. Scholars have done a lot of research on the shale reservoirs of the Wufeng Formation of the Upper Ordovician and Longmaxi Formation of the Lower Silurian in the Sichuan Basin and its surrounding areas in China [13,14,15,16,17]. Pyrite is a product of organic matter degradation in marine sediments. It is a common mineral in anoxic sedimentary environment and an important indicator of organic matter enrichment in reservoirs [18,19,20], and pyrite is closely related to rock IP effect [21]. The mineral composition of shale is complex, and the content of brittle minerals such as quartz and feldspar affect the fracturing and mining of shale gas reservoirs [22,23].

The controlled source electromagnetic method plays an important role in oil and gas exploration, and it mainly evaluates reservoirs based on the electrical properties of rocks and ores [24]. Some scholars have predicted the sweet spots of organic-rich shale in the study area by time-frequency electromagnetic method, using the comprehensive analysis of multi-polarization parameters such as the polarizability of shale [25]. At present, the relationship between shale mineral components and IP parameters needs further study. This study is based on the organic-rich shale rock samples from a gas well in a marine stratum in South China. The complex resistivity data is measured by complex resistivity experiment, and IP parameters are extracted by inversion according to complex resistivity model. The relationship between the mineral components of rock samples and IP parameters is analyzed, and the electrical sensitive parameters and reference standards for shale gas reservoir evaluation are put forward, which provides a more valuable reference for shale gas exploration by electromagnetic methods.

2. Geological Overview of the Sampling Area and Sample Information

The target horizon of the study area is the southern marine shale, and the main source rock strata are the Silurian Longmaxi Formation, Ordovician Wufeng Formation and Cambrian Niutitang Formation, all of which are black shale with high TOC content and rich brittle minerals, so they are favorable desert areas for shale gas exploration and development [26].

The geographical location of the study area and the distribution of shale gas wells are shown in Figure 1. According to the geological data of the exploration area, the target shale of Longmaxi Formation and Wufeng Formation has a resistivity range of 30–50 Ω·m, which belongs to the low resistivity layer, and the overlying stratum is the Silurian Luolping Formation, mainly composed of sandstone, shale and limestone, with a resistivity of 100–150 Ω·m, which belongs to the second high resistivity layer. The lower part is the Lower Permian and Emeishan basalt, with a resistivity of 1500–2000 Ω·m, which belongs to high-resistivity strata, while the Permian Leping Fm, Triassic Feixianguan Formation and Quaternary strata have a resistivity of 100–200 Ω·m, which belongs to low to secondary high-resistivity strata. The shale formation of Niutitang Formation has a resistivity of 10–50 Ω·m, which belongs to a low resistivity layer. There is a high resistivity layer between it and the shale formation of Wufeng Formation, and the lithology is mainly limestone. On the whole, the resistivity of the target layer is low and the resistivity of the surrounding rock is high.

The shale gas gas-producing horizon has a depth of 2340.0–2515.0 m and a thickness of about 170 m, as shown in Figure 2. The mineral components are mainly clay and quartz, with relatively little calcite. The mineral components of the formation above the gas-producing layer are mainly calcite, clay and quartz. Compared with the gas-producing layer, the calcite content is large, exceeding 50%, and the content of clay and quartz minerals is relatively small. Among them, in the depth range of 2460.0–2515.0 m, clay and quartz minerals are relatively high, calcite is relatively low, especially pyrite, and the total organic carbon contents are higher than that of the overlying strata.

According to the logging curve and lithologic profile results, coring is carried out in shale gas producing horizon, and the coring depth is between 2382.43 and 2513.45 m. A total of 44 cores were taken. Quantitative analysis of mineral components was completed by X-ray diffraction analysis. The core number was from 1 to 44, and the depth gradually increased. The thickness of the formation was about 131.0 m. The types and contents of minerals are shown in Figure 3.

The measurement results show that the mineral composition of shale rock samples includes clay minerals, quartz, feldspar, calcite, dolomite and generally contains pyrite. The mineral components are divided into three categories: clay, carbonate minerals, quartz and feldspar (brittle minerals), and a triangular diagram of mineral components is drawn, as shown in Figure 4. The results show that the content of clay minerals is 10–50%, with an average of 33.2%; quartz and feldspar (brittle minerals) are mainly in 30–60%, with an average of 43.7%; and carbonate minerals are less than 40%, with an average of 19.3%. Brittle minerals represented by quartz and feldspar are rich in content, and clay content is relatively small, with typical mineral characteristics of shale gas reservoirs [28].

3. Measurement and Analysis Method of Shale Complex Resistivity

3.1. Complex Resistivity Measurement Method

According to the relevant data of the study area, the formation water resistivity saturated core is simulated, and the salinity is 40,000 mg/L. The measuring instrument is impedance analyzer, and the complex resistivity of rock samples is measured by quadrupole method. The schematic diagram is shown in Figure 5. A and B are the power supply electrodes, and the impedance analyzer provides alternating current with different frequencies (0.01 Hz~10 kHz in this experiment). M and N are the measurement electrodes, which measure the potential difference between the two ends of the rock sample. After the calculation of the built-in algorithm, the impedance analyzer can measure the complex impedance of the rock sample and then calculate the required complex resistivity according to the resistivity formula in combination with the length and cross-sectional area of the rock sample.

3.2. Complex Resistivity Inversion Model

According to the theory of equivalent circuit and equivalent medium of underground stratum, scholars put forward that the mathematical and physical model expressed by finite parameters can better fit the induced polarization field of underground medium. The single Cole–Cole model is not very effective in practical applications due to its single situation. Combined with the experimental data, the double Cole–Cole complex resistivity model is used in this study to invert the low frequency part of the complex resistivity measurement data from 0.01 Hz to 100 Hz to extract IP parameters [29]. The expression is:

$$\rho \left(\omega \right)={\rho }_0\left\{2-m_1\left[1-\frac{1}{1+{\left(i\omega {\tau }_1\right)}^{c_1}}\right]-m_2\left[1-\frac{1}{1+{\left(i\omega {\tau }_2\right)}^{c_2}}\right]\right\}$$

(1)

where ρ(ω) is the complex resistivity (Ω·m), ρ₀ is the DC resistivity (Ω·m), m is the polarizability (dimensionless), τ is the time constant (s) and c is the frequency correlation coefficient (dimensionless). m₁, τ₁, c₁ and m₂, τ₂, c₂ are the spectral parameters of the IP effect and the electromagnetic effect, respectively. In this study, the inversion and extraction of the m₁, τ₁ and c₁ IP parameters are obtained.

4. Complex Resistivity and IP Parameter Results

According to the experimental measurement of complex resistivity, the amplitude of complex resistivity of 44 samples taken from shale gas producing horizon ranges from 10 to 115 Ω·m, with low resistivity, and the phase amplitude of some samples changes obviously. Part of the measured complex resistivity data of the rock sample is presented in a map, as shown in Figure 6.

The experimental results of complex resistivity show that the amplitude of complex resistivity increases with the decrease of frequency, but the amplitude of resistivity of shallow stratum rock sample changes more at high frequency, while that of deep stratum rock sample changes more obviously at low frequency, and the amplitude of resistivity decreases with the increase of rock sample depth. In the low frequency band, the absolute value of phase increases with the decrease of frequency, and the amplitude of phase change increases significantly with the increase of depth.

Inversion results of rock IP parameters are shown in

Table 1. Inversion results of shale complex resistivity.

Number	Depth	ρ0 (Ω·m)	m1	τ1 (s)	c1
1	2382.43–2382.72	153.06	0.09	2.02	0.13
2	2384.13–2384.43	162.42	0.07	0.96	0.15
3	2385.46–2385.73	153.72	0.08	0.96	0.16
4	2386.75–2387.05	125.41	0.08	0.96	0.14
5	2388.16–2388.43	184.75	0.08	0.98	0.15
6	2389.48–2389.78	154.23	0.07	0.99	0.13
7	2391.22–2391.49	113.71	0.17	0.92	0.18
8	2393.19–2393.47	156.30	0.09	0.93	0.15
9	2467.47–2467.77	140.45	0.13	0.70	0.20
10	2469.01–2469.30	144.82	0.15	0.59	0.21
11	2469.77–2470.07	136.01	0.13	0.65	0.21
12	2471.00–2471.28	186.98	0.11	0.57	0.24
13	2472.19–2472.45	151.87	0.11	0.81	0.20
14	2472.93–2473.22	165.32	0.13	0.47	0.24
15	2473.72–2473.97	155.21	0.11	0.80	0.21
17	2477.61–2477.91	71.45	0.14	0.89	0.18
18	2479.66–2479.93	189.55	0.14	0.81	0.19
19	2480.34–2480.64	143.16	0.16	0.70	0.19
20	2481.09–2481.39	127.45	0.15	0.69	0.20
21	2481.92–2482.22	134.70	0.17	0.82	0.17
22	2482.96–2483.23	163.70	0.17	0.96	0.19
23	2483.82–2484.10	160.76	0.18	0.75	0.17
24	2485.19–2485.48	96.97	0.15	0.93	0.23
25	2488.52–2488.80	120.32	0.19	3.89	0.50
26	2489.48–2489.77	127.14	0.15	0.81	0.20
27	2490.28–2490.55	107.18	0.16	0.98	0.22
28	2492.71–2492.99	109.32	0.15	0.95	0.20
29	2494.15–2494.45	141.09	0.14	0.75	0.23
30	2496.82–2497.08	45.24	0.18	8.84	0.88
31	2498.44–2498.75	117.28	0.13	0.94	0.17
32	2499.44–2499.71	58.96	0.17	8.69	0.84
33	2500.30–2500.57	52.24	0.22	11.97	0.95
34	2502.86–2503.13	55.65	0.18	11.10	0.96
35	2504.09–2504.32	64.31	0.17	2.36	0.32
36	2504.90–2505.17	64.89	0.14	8.41	0.87
37	2505.89–2506.17	66.85	0.13	8.20	0.83
38	2507.40–2507.67	73.98	0.18	2.26	0.27
39	2508.09–2508.37	60.78	0.18	8.62	0.84
40	2509.22–2509.50	60.45	0.20	3.25	0.45
41	2510.38–2510.65	43.52	0.18	7.19	0.81
42	2511.42–2511.69	33.86	0.16	5.67	0.73
43	2512.15–2512.42	66.67	0.20	5.30	0.67
44	2513.20–2513.45	46.12	0.15	7.50	0.80

.

5. Analysis of Minerals and IP Parameters

The mineral composition of shale is very complex, and previous studies have found that some mineral compositions have a great influence on the development of shale gas reservoirs. The indicative parameter for evaluating shale gas reservoir is the total organic carbon content (TOC) of the reservoir. The larger the TOC, the greater the organic matter abundance of the reservoir, and the greater the gas content of the corresponding reservoir. According to previous scholars’ research, pyrite in shale gas reservoir can well characterize its organic matter deposition, which is also verified by the data in this paper. As shown in Figure 7. With the increase of sampling core depth, the pyrite content gradually increases, and the TOC content of rock samples also increases synchronously, that is, there is a strong correlation between pyrite and TOC content, and the existence of pyrite will also lead to more obvious induced polarization effect of rocks.

Brittle minerals account for a large proportion in the triangle diagram of mineral composition of the above rock samples, and brittleness is one of the most important mechanical properties of rocks. At present, this property has been used in petrophysical reports of unconventional shale gas reservoirs, and is used as an important evaluation index parameter. When the brittleness index is high, it is conducive to the realization of fracturing during shale gas production. Therefore, this study mainly analyzes the relationship between the content of pyrite and brittle minerals in shale and IP parameters, as shown in Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13.

From the analysis of the overall change trend, the pyrite content in the strata above 2495.0 m in the well is relatively low. With the increase of depth, the average pyrite content increases, the resistivity decreases gradually, and the polarizability increases as a whole. It can be seen that pyrite is related to resistivity and polarizability.

In this paper, the total content of quartz and feldspar is used to characterize the brittle mineral content of rock samples. From the relationship diagram, it can be seen that with the increase of depth, the brittle mineral content keeps increasing at first and then decreasing. Since the core 38 (depth 2495.0 m), with the increase of depth, the resistivity decreases to below 70 Ω·m as a whole, the time constant value and frequency correlation coefficient value increase, the demarcation line is obvious, and the increase rate of polarizability is not large, all of which remain at about 20%, which may be related to the particle size and connectivity of the electronic conductive minerals.

The relationship between shale mineral components and IP parameters is further analyzed through the intersection diagram of pyrite content, resistivity and polarizability, and a preliminary relationship model is established to discuss the electrically sensitive parameters that are helpful for shale gas reservoir evaluation.

The relationship model between pyrite resistivity and polarizability is as follows:

$$p=-0.566\ln \left(\rho \right)+4.2761$$

(2)

$$p=1.4612\ln \left(m\right)+4.5333$$

(3)

The overall pyrite content is between 0.5% and 3%, the polarizability is basically between 6% and 20% and the corresponding resistivity is between 25 Ω·m and 170 Ω·m, which shows that with the increase in the content of electronic conductive minerals (pyrite) in shale reservoir rocks, the polarizability of rocks increases correspondingly, and the resistivity decreases with the increase in conductivity. From the crossplot, as shown in Figure 14 and Figure 15, it can be seen that although the trend of curve fitting of the crossplot is consistent with the theory, the overall level of curve fitting R² is not high. However, it can be clearly seen that the correlation between polarizability and pyrite is better than that between resistivity and pyrite. The resistivity is comprehensively influenced by the content of electronic conductive minerals, particle size and structure, while the polarizability is mainly influenced by electronic conductive minerals, so its sensitivity is better than the resistivity.

Because pyrite is the product of organic matter degradation in marine sediments and a common mineral in anoxic sedimentary environment, the content of TOC and pyrite is regarded as an important index of organic matter enrichment, and the relationship between IP parameters and the total content of TOC and pyrite is analyzed, as shown in Figure 16 and Figure 17. It can be found that the trend of fitting curve is consistent with the theory, and the fitting degree R² is much higher than that of single comparison of the relationship between pyrite content and IP parameters; that is, the total content of TOC and pyrite has a higher correlation with IP parameters. It is related to the correlation between TOC content and pyrite content [30].

The relationship model of pyrite and TOC content with resistivity and polarizability is:

$$p+\mathrm{TOC}=-3.028\ln \left(\rho \right)+18.314$$

(4)

$$p+\mathrm{TOC}=5.0993\ln \left(m\right)+14.264$$

(5)

There is a good correlation between the total content of pyrite and TOC and IP parameters. With the increase of the total content of pyrite and TOC, the polarizability increases obviously, and the resistivity decreases gradually. Through the analysis of the results, when the polarizability of shale gas reservoir rocks is greater than 15% and the resistivity is less than 120 Ω·m, we judge that the total content of pyrite and TOC in the tested reservoir should not be less than 4%. This result has important reference value for the subsequent evaluation of organic matter abundance in shale gas reservoirs.

6. Conclusions

• Through the analysis of mineral composition and IP parameters, with the increase of well depth, the content of brittle minerals increases at first and then decreases, and the content of pyrite and TOC increases on the whole, while the polarizability is increasing and the resistivity is decreasing. Combined with lithologic histogram, it is found that the polarizability is higher and the resistivity is lower closer to the gas-producing horizon, showing the characteristics of low resistance and high polarization.
• The relationship model was initially established by the cross-plot of pyrite content, polarizability and resistivity. The rock sample polarizability gradually increased with the increase of pyrite content, and the resistivity decreased with the increase of pyrite content. The correlation between pyrite and IP parameters is indicative for the preliminary exploration and development of shale gas reservoirs.
• The content of pyrite and TOC are important parameters for shale gas reservoir evaluation. By analyzing the relationship between the total content of pyrite and TOC and IP parameters, it was found that there is a strong correlation between them, and the sensitivity of polarizability is better than that of resistivity. Polarizability is a parameter found in this study that is extremely sensitive to shale gas reservoir information, which can provide reference for shale gas reservoir electrical exploration, subsequent shale gas development and reservoir reconstruction. However, the research content needs to be further deepened, and the relationship between mineral components and IP parameters needs more experimental data to supplement and improve.