Geological Characteristics and Exploration Practices of Multilayer Shale Oil and Gas in the Yanchang Formation, Fuxian–Ganquan Area, Ordos Basin

Abstract

The Chang 7, Chang 8, and Chang 9 members of the Triassic Yanchang Formation in the Fuxian–Ganquan area of the Ordos Basin all develop lacustrine shales. However, current geological research and shale oil and gas exploration mainly focus on Chang 7 shale, with little attention given to Chang 8 and Chang 9 shale formations. Based on the experimental data from whole-rock mineral analysis, organic geochemical analysis, field-emission scanning electron microscopy analysis, and hydrocarbon generation simulation experiments, combined with well-logging data, the shale distribution, mineral composition, source rock characteristics, reservoir properties, and oil and gas contents of Chang 7, Chang 8, and Chang 9 shales were comprehensively analyzed. Moreover, the effect of integrated exploration of multilayer shales was evaluated based on a specific example. The results indicate that three sets of shales are extensively developed in the Yanchang Formation in the study area, but their thicknesses and distribution ranges vary greatly, and Chang 7 shale has the largest thickness and distribution range. Their clay mineral contents are relatively high, reaching an average of 46.7%. Also, the types of their organic matter are mainly Type I-II₁, with high abundance and an average organic carbon content of 4.7%. Their vitrinite reflectance is between 0.7% and 1.3%, indicating that they are in the oil–gas symbiosis stage. Furthermore, they develop various types of nanoscale pores, such as intergranular pores, intragranular pores, and organic pores, and their porosity has an average value of 2.51% and increases significantly after crude oil is extracted. Oil and gas coexist in these three sets of shales, with an average free hydrocarbon content of 3.9 mg/g and an average gas content of 2.6 m³/t. Finally, in order to explore the integrated exploration and development of multilayer shale oil and gas formations, multilayer staged fracturing tests were carried out on six vertical wells for three sets of shales; the production results show that the gas production rate significantly increased by threefold, with a daily oil production rate of more than 1 ton.

1. Introduction

In recent years, the exploration and development of unconventional oil and gas resources has been continuously conducted [1,2,3,4]. In 2023, China’s shale gas production reached 2.50 × 10¹⁰ m³ [1,2], accounting for more than 10% of national total natural gas production and becoming an important part of the growth of natural gas production. Moreover, China’s shale oil production exceeded 4 × 10⁶ t, and this is considered to be a crucial substitute for stable crude oil production [4]. Ordos Basin is an important oil and gas basin in China, and the Triassic Yanchang Formation is rich in unconventional oil and gas resources. Currently, the Changqing Oilfield has found and proven a 1-billion-ton oilfield, the Qingcheng Oilfield, in the Chang 7 member of the Yanchang Formation in the Longdong area of the Ordos Basin [5,6]. In recent years, the Shaanxi Yanchang Petroleum Group has also made significant theoretical advances in lacustrine shale gas exploration in the Ordos Basin, with several wells in the Chang 7 shale yielding shale gas flows [7,8,9].

In addition to the Chang 7 shale in the Yanchang Formation of the Ordos Basin, shale layers are also developed in the Chang 8 and Chang 9 members. However, current shale oil and gas exploration and geological research are mainly concentrated on the Chang 7 member [10,11]. Due to the limited distribution range and depositional thickness of Chang 8 and Chang 9 shales, only a few studies on source rock evaluation have been conducted [12,13,14]; thus, the systematic assessments of shale reservoirs are very rare. Furthermore, current geological research and exploration practices often tend to only focus on shale oil or shale gas. However, the Yanchang Formation shales exhibit the characteristics of oil and gas symbiosis and sequential accumulation [15,16]. The state of occurrence of shale oil and gas, and their respective processes of enrichment and accumulation, influence each other. Therefore, conducting a comprehensive evaluation of the geological characteristics of multilayered shale oil and gas in the Yanchang Formation is of great significance to the later comprehensive exploration of shale oil and gas.

In this study, three sets of shales in the Yanchang Formation in the Fuxian–Ganquan area of the Ordos Basin were investigated. Cores and outcrop samples were utilized for X-ray diffraction mineral analysis, organic geochemical analysis, field-emission scanning electron microscope analysis, and hydrocarbon generation thermal simulation experiments. By integrating logging data and core test data, a comprehensive examination was conducted on the shale distribution characteristics, mineral petrology characteristics, organic geochemical characteristics, reservoir characteristics, and oil and gas contents of the Chang 7, Chang 8, and Chang 9 shales. Based on the experimental results and the exploration practice of multiple vertical wells, the feasibility of the integrated exploration and development of multilayer shale oil and gas formations was evaluated to provide a valuable reference for the exploration of similar continental shale oil and gas formations.

2. Geological Background

The Ordos Basin is located in Central China, with an area of about 25 × 10⁴ km². The faults and folds at its edges are relatively developed, and its internal structure is relatively simple. According to its current tectonic morphology, basement properties, and structural characteristics, the Ordos Basin can be divided into six first-order structural units: Yimeng uplift, Weibei uplift, Jinxi flexural fold belt, Yishaan slope, Tianhuan depression, and western margin thrust tectonic belt.

The study area is located in the southern part of the Yishaan slope in the Fuxian–Ganquan region of the Ordos Basin (Figure 1). Structurally, it is characterized by a broad westward-dipping monocline with underdeveloped faults and gentle slopes [17]. The Triassic Yanchang Formation comprises an inland delta-lacustrine clastic sedimentary system, divided into ten members from Chang 10 to Chang 1 from bottom to top. The Chang 10 to Chang 1 members represent the complete cycle of lake formation to decline. The expansion of the lake basin began during the sedimentation period of Chang 10, reaching its maximum extent during the deposition of Chang 7, with a semi-deep–deep lake area of approximately 6.5 × 10⁴ km². The formation of this large inland sag lake basin laid the foundation for the extensive development of black shales and dark mudstones in the lower part of the Yanchang Formation, making these layers the primary targets for shale oil and gas exploration, with significant potential [17,18,19].

3. Geological Characteristics of Shales in the Yanchang Formation

3.1. Characteristics of Shale Distribution

The development of shales in the Yanchang Formation is closely related to lacustrine depositional systems, including semi-deep–deep lacustrine and nearshore shallow lacustrine depositional systems. During the Chang 9 period of the Yanchang Formation, the main process was lake transgression [20]. The Ganquan area was situated at the center of the lake basin, characterized mainly by argillaceous deposits. At the top of the Chang 9 member, a set of oil shales, commonly known as the “Lijiapan Shale”, developed with a maximum thickness of about 25 m in the Xiashiwan–Shimen area of Ganquan (Figure 2). During the Chang 8 period, the lake basin continued to expand, with the central part of the basin primarily featuring shallow lacustrine subfacies. The shale in the Chang 8 member in the Zhangjiawan–Zhiluo area of Fuxian exhibits the greatest thickness, exceeding 30 m, and gradually thins towards the northeast, with relatively poor development in the Ganquan area (Figure 3). The Chang 7 period represents the peak development phase of the lake basin in the Yanchang Formation, with the largest lake area. The study area during this period was mainly dominated by deep–semi-deep lacustrine subfacies, with the Chang 7 member shale being the most extensive and thickest in the Yanchang Formation, reaching up to 100 m in thickness in the Zhangjiawan area of Fuxian (Figure 4). Overall, the thickness of these three sets of shales in the Yanchang Formation correlates well with the distribution of depositional facies, though their distribution range and thickness vary significantly.

3.2. Lithology and Mineral Compositions

The results of field profiles and core observations showed that the lithology of the Yanchang Formation mainly includes mud shale, silt-fine sandstone and thin-layer tuff [19]. The lower part of Chang 7, the middle part of Chang 8, and the top part of Chang 9 have each developed a set of shale formations. Their lithology is mainly black shale, mixed with dark gray sandy laminated shale and siltstone. Shales in the Yanchang Formation are easily identifiable on well-logging curves, exhibiting obvious electrical characteristics such as high acoustic time difference, high resistivity, low density, and high gamma values (Figure 5). The mineral composition of Yanchang Formation shales predominantly includes quartz, feldspar, carbonate minerals, and clay minerals, with minor amounts of pyrite. The average clay mineral content is 46.7%, the quartz content averages 27.8%, the feldspar content averages 16.8%, the carbonate content averages 5.9%, and the pyrite content averages 2.5% (Figure 5 and Figure 6). The clay minerals are primarily mixed-layer illite–smectite, illite, and chlorite, with an average mixed-layer illite–smectite content of about 46.8%. Compared to marine shales in Southern China, the Yanchang Formation shales have higher clay mineral contents and lower brittle mineral contents.

3.3. Organic Geochemical Characteristics of Shales

3.3.1. Types of Organic Matter

According to the rock pyrolysis data shown in the T_max-HI identification diagram (Figure 7), it can be seen that the types of organic matter of shales in the Yanchang Formation are mainly Type I and Type II₁, with a small amount of Type II₂ [21]. According to maceral analysis, shales in the Yanchang Formation exhibit characteristics of sapropelic and mixed-type kerogen. The sapropelic group content ranges from 46% to 91%, averaging about 65% of the total kerogen content. The vitrinite group contents range from 2% to 39%, averaging 20%, while the inertinite group contents range from 5% to 34%, averaging 15%. The types of organic matter identified by maceral analysis are essentially consistent with the results of the T_max-HI identification diagram.

3.3.2. Abundance of Organic Matter

The total organic carbon (TOC) content in the Yanchang Formation shales mainly ranges from 2% to 8%, with an average of 4.7% (Figure 8). The average TOC content of the Chang 8 shale is slightly lower than that of the Chang 7 and Chang 9 shales, which is related to the more developed interlayers of the sandy laminated shale and aleurolite in the Chang 8 shale.

3.3.3. Maturity of Organic Matter

Vitrinite reflectance (Ro) is the main indicator to characterize the maturity of organic matter in source rocks. The vitrinite reflectance of shales in the Yanchang Formation ranges from 0.7% to 1.3% and generally increases slowly with depth. Peak pyrolysis temperature (Tmax) refers to the maximum pyrolysis temperature of kerogen. The peak pyrolysis temperature of shales in the Yanchang Formation ranges from 445 °C to 460 °C. Shales in the Yanchang Formation predominantly stay in the late stage of the oil generation window and the peak period of the wet gas generation window, and they have not yet reached the over-mature dry gas generation stage.

3.4. Reservoir Pore Volume

The types of reserving space of shales in the Yanchang Formation include pores and microfractures. Compared with microfractures, the pores of shales in the Yanchang Formation are more developed. The pore types of shales are divided into inorganic intergranular pores, inorganic intragranular pores, and organic pores [22,23]. Inorganic intergranular pores include rigid particle intergranular pores, clay debris intergranular pores, intergranular dissolution pores, and authigenic mineral intercrystalline pores. Inorganic intragranular pores include intragranular dissolution pores, fossil cavity pores, authigenic clay mineral aggregates, and pyrite aggregate intragranular pores. Organic pores are classified into kerogen organic pores and migrated solid bitumen organic pores (Figure 9). The contents of rigid clastic particles, migrated solid bitumen, carbonate cement, dissolution effects, and the development of siltstone laminations are the main factors influencing shale’s pore structure and physical properties.

Considering the applicability of different pore size testing methods, this study used both carbon dioxide and nitrogen gas adsorption methods, as well as mercury intrusion porosimetry, to test the pore structure. The obtained data were integrated to establish a comprehensive pore size distribution testing method for the Yanchang Formation shales. The experimental results show differences in pore structure among different lithofacies [24,25,26]. According to the pore classification of the International Union of Pure and Applied Chemistry (IUPAC), pores with a diameter less than 2 nm are micropores, pores with a diameter between 2 and 50 nm are mesopores, and pores with a diameter greater than 50 nm are collectively called macropores. Micropores are relatively well developed in the black shale, accounting for 26.1% of the total pore volume, with mesopores and macropores making up 45.8% and 27.1% of the total pore volume, respectively. In aleurolite, mesopores and macropores are more developed, with micropores accounting for only 3.8% of the total pore volume and macropores exceeding 50%. The micropore development in laminated shale is intermediate between the other two lithofacies, with mesopores accounting for up to 60% of the total pore volume (Figure 10) [27].

The porosity of the Yanchang Formation’s shale layers ranges from 0.2% to 5.1%, with an average of 2.51%. After oil extraction using the solvent dichloromethane, the porosity increases significantly, and the average porosity can reach 3.18%. The permeability of the Yanchang Formation shales ranges from 0.0043 millidarcy (mD) to 0.239 mD, with an average of 0.0133 mD, indicating that these shales generally belong to low-porosity, ultra-low-permeability reservoirs (Figure 11).

3.5. Temperature and Pressure

The burial depth of the Yanchang Formation shales in the study area ranges from 1200 to 1800 m. Pressure testing results show that the formation pressure coefficient ranges from 0.53 to 0.91, with an average of 0.68, indicating an overall abnormally low-pressure system. On-site formation temperature testing results indicate that the shale layers in the study area have temperatures between 48 °C and 54 °C, with a geothermal gradient of 2.98 to 3.09 °C/100 m. The low formation pressure significantly impacts shale gas extraction, and currently, shale oil and gas production in the study area primarily uses pumping methods.

3.6. Shale Oil and Gas Occurrence Characteristics and Contents

3.6.1. Hydrocarbon Generation Simulation Experiment

To study the coexisting characteristics of shale oil and gas in the Yanchang Formation, a hydrocarbon generation simulation experiment was conducted using outcrop samples (Ro = 0.5%, TOC = 3.3%) of the Chang 7 shale from the Hejiafang area. The experimental pressure was 50 MPa, and the temperature ranged from 336 °C to 600 °C. The experiment showed that, as the maturity increased, the proportion of gaseous hydrocarbons generated by the shale gradually increased. When the maturity was around 0.6%, the shale primarily generated liquid hydrocarbons, with gaseous hydrocarbons accounting for less than 0.4% of the mass. When the maturity increased to 0.8%, the proportion of gaseous hydrocarbons rose to 2.82%, with methane accounting for about 1.26% and C_2–5 components for about 1.56%. The mass percentage of liquid hydrocarbons was about 97.18%, with light hydrocarbons (C_6–14) accounting for about 13.04% and C₁₄₊ components accounting for 84.14% of the total. When the maturity increased to 1.2%, the proportion of gaseous hydrocarbons rose to 20%, with methane accounting for about 5.95% and C_2–5 components for about 14.05%. The mass percentage of liquid hydrocarbons was about 80%, with light hydrocarbons (C_6–14) accounting for about 20.83% and C₁₄₊ components accounting for 59.17% of the total. The hydrocarbon generation simulation experiment indicated that the Yanchang Formation shales are generally in a stage of orderly coexisting oil and gas, with the proportion of gaseous hydrocarbons increasing as maturity increases (Figure 12).

3.6.2. Characteristics and Contents of Shale Oil and Gas

In the Yanchang Formation shales, hydrocarbons exist in diverse states, including adsorbed and free-state shale oil, as well as adsorbed, free-state, and dissolved shale gas [28,29,30,31,32]. Some shale oil and gas exist in a free state within pores and fractures, while others are adsorbed onto the surfaces of organic matter and clay minerals. Additionally, some shale gas dissolves into the shale oil under formation temperature and pressure conditions. When observed under a scanning electron microscope, shale oil can appear as droplets filling pores or existing in microfractures (Figure 13), or as thin films adsorbed onto solid particle surfaces.

The contents of shale oil can be approximated using parameters such as chloroform asphalt “A” and the pyrolysis parameter S₁. Chloroform asphalt “A” refers to the organic matter extracted from shale using chloroform, reflecting the content of soluble organic matter in the shale. S₁ refers to the hydrocarbons volatilized from a rock sample when heated to no more than 300 °C during rock pyrolysis analysis, representing the content of free hydrocarbons that can be extracted from the rock. The chloroform asphalt “A” content in the Yanchang Formation shales mainly ranges from 0.31% to 1.15%, with an average of 0.73%. The S₁ values in the Yanchang Formation primarily range from 1.0 to 7.0 mg/g, with an average of 3.9 mg/g. Among the Yanchang Formation, the shale in the Chang 7 member has the highest S₁, with an average of 4.1 mg/g; the shale in the Chang 8 member is second, with an average of 3.6 mg/g; and the shale in the Chang 9 member has the lowest S₁, with an average of 2.9 mg/g (Figure 14 and Figure 15). The mobility of shale oil is characterized by the oil saturation index (OSI). In the Yanchang Formation shales, the OSI mainly ranges from 25 to 200 mg/g TOC, with an average of 86.13 mg/g TOC. Among the lithofacies, aleurolite has the highest OSI, followed by sandy laminated shale, with black shale having the lowest. The total gas content of the Yanchang Formation shales, as determined through core gas content analysis, ranges from 2.2 to 3.8 m³/t, with an average of 2.6 m³/t [33,34].

The retained shale oil in the Yanchang Formation shales significantly affects the gas content. On the one hand, the retained shale oil restricts the adsorption capacity for methane, reducing the amount of adsorbed shale gas. In isothermal adsorption experiments on shale, comparing methane adsorption before and after Soxhlet extraction using chloroform, the methane adsorption amount before extraction was on average reduced by 45% after extraction. On the other hand, under formation conditions, shale oil can dissolve part of the shale gas, which can account for 15% to 20% of the total gas content.

4. Exploration Practices

Through the above systematic study of the geological characteristics of the Chang 7, Chang 8, and Chang 9 shale layers in the Yanchang Formation in the study area, it was found that Chang 8 and Chang 9 shales are much smaller in thickness and distribution range than Chang 7 shale. However, their organic matter abundance, mineral composition, reservoir physical properties, and shale oil and gas contents are relatively similar to those of Chang 7 shale, so they also have good exploration potential. In addition, through hydrocarbon generation simulation experiments, organic matter maturity, and oil and gas content analysis, it was confirmed that the Chang 7, Chang 8, and Chang 9 shale layers in the Yanchang Formation have the characteristics of oil and gas symbiosis. In the past, the target layer for shale oil and gas exploration in the study area was mainly limited to Chang 7 shale, focused only on one aspect of shale oil or shale gas, and did not involve Chang 8 and Chang 9 shales. In order to achieve the integrated exploitation of multilayer shale oil and gas in the Yanchang Formation and obtain full utilization of the above three sets of shales, multilayer staged fracturing has been carried out on six vertical wells for the Chang 7, Chang 8, and Chang 9 shale layers in the past two years, achieving very high oil and gas production.

Taking Well Y13 as an example, it can be seen from Figure 16 that the thickness of Chang 9 shale is 16 m, and the acoustic and resistivity curves show obvious box-shaped characteristics. The thickness of Chang 8 shale is about 13 m, and the siltstone and mud shale are interbedded in this shale, so the resistivity curve presents a multi-thin-layer finger shape. The thickness of Chang 7 shale is about 62 m. As shown in Figure 16, the above three sets of shales have obvious gasometric anomalies. During the initial exploration in 2019, only single-layer fracturing was carried out in the Chang 7 shale layer, and the average daily shale gas production rate was only 471 m³. Meanwhile, in 2022, multilayer staged fracturing (i.e., re-fracturing) was implemented on the Chang 7, Chang 8, and Chang 9 shale layers (Figure 16); the average daily gas production rate increased significantly to 1574 m³/d, with a maximum value of 3024 m³/d; and the average daily oil production rate also grew to 1.3 t/d, with a maximum value of 2.5 t/d (Figure 17).

As shown in Figure 18, for another example of Well Y27, the thickness of Chang 7 shale is about 55 m. From well-logging interpretation, its average TOC is 5.1%, its average porosity is 3.1%, and its average gas content is 3.3 m³/t. The thickness of Chang 9 shale is about 21 m, its average TOC is about 4.5%, the average porosity is 3.2%, and the average gas content is 3.2 m³/t. Chang 8 shale is not developed in this well. After multilayer staged fracturing was implemented on the Chang 7, Chang 8, and Chang 9 shale layers in 2023, the average daily gas production rate was 4745 m³, with a maximum value of 5886 m³/d, and the average daily oil production rate was 1.6 t, with a maximum value of 3.2 t/d (Figure 19).

Table 1. Comparison of oil and gas production rates between multilayer staged fracturing of shales in the Yanchang Formation and single-layer fracturing of Chang 7 shale.

Well Name	Single-Layer Fracturing	Multilayer Staged Fracturing
	Average Daily Gas Production Rate (m3/d)	Average Daily Gas Production Rate (m3/d)	Average Daily Oil Production Rate (t/d)
Y27	/	4745	1.6
Y13	471	1574	1.3
Y11	348	1091	2.1
Y1	/	1364	1.0
Y16	/	1200	1.1
Y8	/	501	1.8

compares the oil and gas production rates between multilayer staged fracturing of shales in the Yanchang Formation and single-layer fracturing of Chang 7 shale. It can be seen that the overall exploration of these three sets of shale formations in six vertical wells in the study area has achieved significantly increased oil and gas production rates. Compared with single-layer fracturing in Chang 7 shale, multilayer staged fracturing of shales in the Yanchang Formation successfully increased the gas production rate by more than threefold, with a daily oil production rate of more than 1 ton. The main reasons for the above phenomenon are as follows: First, the fracturing layers were expanded from the Chang 7 shale to three sets of shale layers, where Chang 8 and Chang 9 shales contributed to the increased oil and gas production. Secondly, the perforation clusters in Chang 7 shale are much denser than the initial fracturing (i.e., single-layer fracturing), which helps to enhance oil and gas production. Finally, the initial fracturing was only carried out in Chang 7 black shale, where shale oil has extremely poor mobility and is difficult to extract directly, so there is no shale oil production. Meanwhile, for multilayer staged fracturing, in addition to the black Chang 7 shale, the siltstone interlayer in Chang 8 and Chang 9 shales is also fractured. As the siltstone interlayer has high porosity and permeability, the shale oil inside it has good mobility, which, in turn, leads to a remarkable increase in oil and gas production.

Although the overall exploration of the three sets of shale formations in the study area has achieved good results in increasing oil and gas production, there are still many difficulties and challenges in the exploration and development. First of all, Yanchang Formation shale has high clay mineral contents, low brittle mineral contents, poor physical properties, a high adsorbed gas ratio, and a low pressure coefficient. Also, during the stage of oil and gas production, a pumping unit is required to simultaneously produce gas and water. Compared with marine shales in Southern China, the oil and gas production rate of single wells is still low, so further increasing production is a great challenge. Secondly, Yanchang Formation shale is generally in the stage of oil and gas symbiosis. Even if the static reservoir parameters are similar, oil and gas production varies greatly between different wells. Thus, the main factors controlling the enrichment and production of shale oil and gas still require further research.

5. Conclusions

Based on the results of analytical studies, the following conclusions can be drawn:

(1)

In the Fuxian–Ganquan area of the Ordos Basin, three sets of shales are developed in the Yanchang Formation, but they vary significantly in thickness and distribution. The Chang 7 shale is extensively developed throughout the study area, with a maximum thickness exceeding 100 m, thinning gradually from southwest to northeast. The thickest area of the Chang 8 shale is located in the Zhangjiawan–Zhiluo area of Fuxian, with a maximum thickness of about 30 m, and it is not developed in the northeastern part of the study area. The thickest area of the Chang 9 shale is in the northwest of the study area, in the Xiashiwan–Shimen area of Ganquan, with a maximum thickness of about 25 m, and it is relatively undeveloped in the southwest.

(2)

The Yanchang Formation shales in the study area have typical geological characteristics of “high organic content, high clay mineral content, low thermal maturity, low pressure coefficient, and coexisting shale oil and gas”. The average TOC of the Yanchang Formation shales is 4.7%, the average clay mineral content is 46.7%, the vitrinite reflectance ranges from 0.7% to 1.3%, the organic matter type is primarily Type I to II₁, and the average formation pressure coefficient is 0.68.

(3)

The pore structure of the Yanchang Formation shales is closely related to the lithofacies types, with an average porosity of 2.5% and an average permeability of 0.0133 mD, classifying them as low-porosity and ultra-low-permeability shales. The Yanchang Formation’s shale layers exhibit coexisting oil and gas, with multiple fluid types, including liquid hydrocarbons and free, adsorbed, and dissolved shale gas. The average total gas content of the shale is 2.6 m³/t, and the average oil saturation index is 86.1 mg/g TOC.

(4)

According to the geological characteristics of multilayer shales in the Yanchang Formation in the study area, the Chang 7–Chang 9 shale formations were developed using multilayer staged fracturing of vertical wells. Compared with previous single-layer fracturing, the average daily gas production rate significantly increased by 3-fold, with daily oil production of more than 1 ton, achieving the combined exploration and production of shale oil and gas, as well as comprehensive utilization of shale resources.