Analysis of the Genesis of Shale Gas in the Southern Ordos Basin: Based on Geochemical Evidence

1. Introduction

As the second-largest sedimentary basin in China, the Ordos Basin hosts abundant hydrocarbon resources. In the basin’s Yanchang exploration area, Shaanxi Yanchang Petroleum Group has identified shale gas occurrences within the Mesozoic Yanchang Formation in recent years. The genetic origin of this shale gas is still poorly constrained, and whether it originates from coalbed methane, biogenic gas, or shallow crude oil-associated gas remains unresolved. Such uncertainty impedes the evaluation of favourable exploration targets and limits further development of unconventional energy resources in the region.

The genesis and origin of conventional oil and gas in the Ordos Basin have been studied extensively [1,2]. With recent progress in molecular and isotopic identification of natural gas origins, more reliable diagnostic frameworks are now available for distinguishing thermogenic, biogenic, and mixed gases [3,4]. Systematic studies of shale gas sources, however, are still largely lacking, especially with respect to the genetic characterization of Mesozoic lacustrine shale gas in the southern Yanchang exploration area. General formation rules of lacustrine shale gas in the Ordos Basin were summarized by Wang (2017), whereas the present work concentrates on the geochemical discrimination of shale gas origin in the southern Xiaoshiwan sub-block using newly measured well data [5]. In response to this gap, the Mesozoic Yanchang Formation shale gas in this region is examined here, and geochemical methods are applied to establish its genetic origin. By comparing the geochemical signatures of the target shale gas with those of shallow crude oil-associated gas from the Ordos Basin, Upper Paleozoic coal-derived gas, and biogenic gas from adjacent basins, a geochemical framework useful for subsequent geological studies is developed. It is worth noting that the southern Ordos Basin is among the earliest onshore regions in China where shale gas was identified [6]; despite this, no commercial exploration breakthrough has yet been achieved [5].

2. Geological Setting

Multiple sedimentary types, multiple tectonic systems, and multiple cycles of evolution characterize the Ordos Basin. Its petroleum resources are concentrated chiefly in the Mesozoic, where the Triassic Yanchang Formation and the Jurassic Yan’an Formation are the principal units, while natural gas is mainly hosted in the Paleozoic, occurring in the Upper Paleozoic marine-continental transitional strata as well as the Lower Paleozoic carbonate rock strata [5].

The southern extension exploration area of the Ordos Basin lies in the central-eastern portion of the basin [6] and belongs structurally to the Yishan slope tectonic unit, where the fault tectonic system is poorly developed. In the Ganquan area of the basin, Mesozoic continental shale gas resources have been discovered in recent years [5]. The geological background, reservoir-forming conditions, and enrichment patterns of this shale gas set have been investigated by scholars both in China and abroad [6,7], yielding a series of results. These studies indicate that the gas-rich areas are located mainly in the Zhiluo-Xiasiwan area of the middle and eastern Yishan slope, and are developed chiefly in the Chang 7 and Chang 9 shale, mud shale, and their interlayers [8]. Shale of the Chang 7 section has a thickness of 10 to 70 m and an organic matter abundance of 2.0% to 5.0%. Its kerogen is dominated by the humic-sapropelic type, and the micro-components are mostly humic-sapropelic amorphous. Organic carbon content ranges from 1.76% to 6.28%, chloroform asphalt “A” from 0.25% to 0.67%, and Rₒ from 0.7% to 1.5%, placing the organic matter in the mature stage [5]. The shale gas resources of the Mesozoic strata in this section are estimated at (626.4 to 6037.2) × 10⁸ m³ [8]. More recent petrological and organic-geochemical work has further clarified the sedimentary environments and organic matter enrichment of the Chang 7 lacustrine shale and mudstone in this formation [14].

3. Datasets and Methods

Geochemical analysis data and indicators are used in this study, including gas component data such as methane content and heavy hydrocarbon content. The data originate from test results of multiple wells in the Xiashiwan area, from composition analysis of shale gas in the Chang 7–9 section of the Xiashiwan area, from composition analysis of natural gas in the eastern Ordos Basin, and from the composition of shallow gas in some wells of the Yanchang exploration area. Carbon isotope data, covering methane carbon isotope values, ethane carbon isotopes, propane carbon isotopes, and related parameters, come mainly from carbon isotope analysis of shale gas in the Chang 7–9 section of the Xiashiwan area, natural gas in the eastern Ordos Basin, and shallow gas in some oil fields of northern Shaanxi. To determine the genesis of the Yanchang Formation shale gas, its geochemical characteristics were compared with those of the Upper Paleozoic coal-shaped gas of the Ordos Basin, biogenic gas in surrounding basins, and shallow crude oil-associated gas within the basin.

4. Results

4.1. Characteristics of Natural Gas Components

The composition and carbon isotopes of Mesozoic shale gas from Xiashiwan, southern Ordos Basin, show that methane content of the shale gas ranges from 46.08% to 94.56%, while heavy hydrocarbon content ranges from 2.86% to 37.61% (Table 1).

In the Upper Paleozoic natural gas of the eastern Ordos Basin, methane content rose progressively from the Shanxi Formation to the Shiqianfeng Formation, whereas ethane and heavy hydrocarbon contents declined (Table 2). Methane content was 97.75% in the Qian 5 section of Well Yu 7, 94.4% in the Qian 5 section of Well Yu 17, 94% in the Shan 3 section of Shenhe, 89.72% in the He 7 section of Well Meng 1, and 92.57% and 91.16% respectively in the He 8 sections of Well Yu 7 and Well Yu 17, with an average of 87.78% in the He 8 section of Well Meng 1. The Shan 2 interval contained 75.55% methane. Overall, methane content increased from the Shanxi Formation through the Shihezi Formation to the Shiqianfeng Formation. Ethane and heavy hydrocarbons followed the opposite trend, decreasing gradually from the Shanxi Formation to the Shiqianfeng Formation, accompanied by a rising drying coefficient.

Analysis of Mesozoic shallow gas from 19 oil wells in the Qiaozhen, Xiashiwan, Shangshangyuan, and Huaishuzhuang areas of the southern Yanchang exploration area (Table 3) indicates that the Mesozoic shallow gas of the Ordos Basin is characterized by low heavy hydrocarbon content and small specific gravity. For the base of the Yan’an Formation (Yan 7+8 section, Yan 8 section), the average values were methane content 92.36%/69.71%, heavy hydrocarbon content 8.06%/26.68%, and specific gravity 0.60/0.84; for one interval, methane content was 73.98%, heavy hydrocarbon content 23.20%, and specific gravity 0.84; the averages were methane content 92.54%/67.85%, heavy hydrocarbon content 4.57%/30.26%, and specific gravity 0.60/0.88; and for Section 6 of the extended group, methane content was 86.43%/73.68%, heavy hydrocarbon content 11.0%/23.01%, and specific gravity 0.67/0.78.

4.2. Carbon Isotope Characteristics of Natural Gas

Carbon isotope data for Yanchang Formation shale gas in the southern Ordos Basin (Table 4) reveal that methane carbon isotope values (δ¹³C₁) of natural gas in the Chang 7 shale generally vary between −52.0‰ and −46.4‰, ethane carbon isotope values (δ¹³C₂) between −39.7‰ and −34.4‰, and propane carbon isotope values (δ¹³C₃) between −38.6‰ and −30.7‰. In the Chang 8 shale, δ¹³C₁ values range from −50.9‰ to −47.3‰, δ¹³C₂ is generally between −38.8‰ and −38.2‰, and propane carbon isotope values fall between −34.3‰ and −33.8‰. For the Chang 9 shale, δ¹³C₁ generally ranges from −51.1‰ to −48.1‰, δ¹³C₂ between −38.6‰ and −36.7‰, and propane carbon isotope values between −38.6‰ and −32.4‰. Across the Chang 7, Chang 8, and Chang 9 shale sections, the carbon isotope characteristics of natural gas are essentially consistent [5].

Carbon isotope analysis of Upper Paleozoic natural gas in the Ordos Basin (Table 5) shows δ¹³C₁ between −37.02‰ and −31.24‰ and δ¹³C₂ between −31.37‰ and −23.7‰. From the Shanxi Formation to the Shiqianfeng Formation, methane isotopes of the natural gas become markedly more negative: δ¹³C₁ is −32.24‰ in the Shanxi Formation of Well Shaan 117, −34.24‰ in the He 8 section of Well Yu 17, −34.17‰ in the He 8 section of Well Yu 12, −34.24‰ in the Qian 5 section of Well Yu 17, −37.02‰ in the Qian 5 section of Well Shen 8, and −36.16‰ in the Qian 5 section of Well Meng 5.

According to the carbon isotope analysis of shallow gas in the Ordos Basin (Table 6), apart from Wells Quan 58 and Quan 56, methane carbon isotope values of the remaining wells fall within −55‰ to −25‰, with most clustering near −45‰.

5. Discussion

The natural gas resources of the Ordos Basin are distinguished by numerous layers and diverse genetic types, encompassing coal-shaped gas in the coal-bearing strata of the Upper Paleozoic and shallow associated gas. A substantial body of work on natural gas genesis has been carried out by geochemistry scholars at home and abroad [1,3,4,7,11]. Natural gas genesis is generally divided into thermal genesis, biogenesis, and a mixture of the two. Thermal genesis can be further divided into petroleum-associated gas and cracking gas, whereas biogenesis refers to natural gas formed through the chemical action of organic matter by anaerobic bacteria [5].

The geochemical characteristics of the Upper Paleozoic coal-shaped gas of the Ordos Basin, shallow biogenic gas around the basin, and shallow crude oil-associated gas within the basin are analyzed separately below. By comparing these with the geochemical characteristics of shale gas, the genesis of the Mesozoic shale gas in the Ordos Basin is examined.

5.1. The Shale Gas in the Yanchang Formation of the Ordos Basin Is Not Coal-Type Gas and Has a Different Source from the Upper Paleozoic Natural Gas

Carboniferous-Permian coal, dark mudstone, and Carboniferous-marine carbonate gas source rocks were mainly developed in the Upper Paleozoic of the Ordos Basin [1,2,10], and the Upper Paleozoic natural gas was predominantly coal-derived gas [5]. More recent investigations of Upper Paleozoic limestone gas and of the total petroleum system of the basin further support a predominantly coal-derived origin for this gas [10,13,23].

Alkane carbon isotope values of the Upper Paleozoic natural gas in the Ordos Basin are comparatively heavy, displaying the characteristics of coal-based gas, and they differ noticeably from the geochemical characteristics of the shale gas described above [12]. For natural gas from the Shanxi-Taiyuan Formation of the Upper Paleozoic to reach and accumulate in the Mesozoic reservoirs, it would have to migrate through multiple sets of strata from the Upper Permian and Lower Triassic, a process involving long migration distances and considerable dissipation; the likelihood of mixed gas reservoirs forming is therefore very low. On the basis of the above, it is considered that the shale gas of the Mesozoic Yanchang Formation could not have originated from the Upper Paleozoic coal-bearing strata and is not of coal-type gas origin [10,15].

5.2. Shale Gas from the Yanchang Formation in the Ordos Basin Is Not Biogenic Gas

Biogenic natural gas is mainly formed through decomposition of immature to low-mature source rocks by methanogens or anaerobic bacteria [17,18]. Discussions of the geochemical criteria for biogas have continued for many years, and the indicators employed mainly include methane content and proportion, heavy hydrocarbon content, and methane carbon isotope δ¹³C₁, among which δ¹³C₁ is the most commonly used. A single indicator is insufficient to determine whether a natural gas is biogenic; the assessment should instead be made in combination with other parameter standards [16]. In studies of the genesis, composition, and isotopic composition characteristics of biogas in China, δ¹³C₁ and δ¹³C₂ are the common identifying criteria.

In recent years, besides the carbon isotope (δ¹³C), heavy hydrocarbon content (C₂₊) and the methane/(ethane + propane) ratio have emerged as new indicators for determining genesis [5]. Intramolecular and position-specific carbon isotopic approaches have also been introduced to refine the discrimination of gas origins and secondary alteration processes [12,24].

The sources of Mesozoic shale gas in the Ordos Basin were explored by comparing biogenic gas research in the Qaidam Basin and the Hohe Depression of the Hetao Basin, which are adjacent to the Ordos Basin, with the geochemical characteristics of natural gas. The Quaternary gas from the Three Lakes region in the central and eastern Qaidam Basin is biogenic gas, marked by high methane content (above 95%) and heavy hydrocarbon gas content below 0.3% [19,20]. In the Hohhot Depression of the Hetao Basin, adjacent to the Ordos Basin to the south, shallow biogenic gas has been found in the Quaternary and Upper Neogene. Analysis of the Quaternary gas composition reveals the typical features of biogenic gas [5,21].

It was noted that biogenic gas from the Qaidam Basin and the Hetao Basin is characterized by high methane content, low heavy hydrocarbon content, and low δ¹³C₁, which contrasts markedly with the geochemical characteristics of shale gas in the Yanchang Formation, particularly in terms of carbon isotope content. Mesozoic shale gas in the Ordos Basin has methane content of 46.08% to 94.56% and δ¹³C₁ of −52.0‰ to −46.4‰, much heavier than the characteristics of biogenic gas. Consequently, the Mesozoic shale gas in the Ordos Basin cannot be biogenic either [3,19].

5.3. The Shale Gas in the Yanchang Formation of the Ordos Basin Is of the Same Origin as the Shallow Gas in the Basin and Is a Natural Gas Formed from Sapropelic Source Rocks

Detailed research on the definition and classification of shallow gas has been conducted by scholars at home and abroad [22], and shallow gas is mainly divided into two types: from the perspective of depth, it refers to natural gas buried relatively shallowly (generally less than a certain specific depth), while from the perspective of oil-bearing strata, it refers to natural gas above a certain oil and gas reservoir. In the former Soviet Union, the depth was defined at 1200 m, whereas in the United States and China it is defined at around 1500 m. In the extension exploration area of the Ordos Basin, the burial depth of strata varies greatly because of the cover of Quaternary loess. The burial depth of the Chang 7–9 section is around 1400–2000 m. If shallow gas were defined by burial depth, the geological problems could not be analyzed well. The shallow gas described in this study is therefore classified from the perspective of oil-bearing strata, that is, natural gas in the Triassic Yanchang Formation to the Jurassic Yan’an Formation. Shallow gas accumulation mechanisms in this part of the basin have been widely documented [21,25], which is consistent with the classification adopted here.

The Mesozoic shallow gas in the Yanchang exploration area of the southern Ordos Basin is characterized by shallow burial [5], low formation pressure, and easy extraction of dissolved gas from the oil layer to form a free-state gas-cap gas reservoir. Analysis of Mesozoic shallow gas from 19 oil wells in the Qiaozhen, Xiashiwan, Shangshangyuan, and Huaishuzhuang areas of the Yanchang exploration area shows that Mesozoic shallow gas has the characteristics of low heavy hydrocarbon content and small specific gravity.

The carbon isotope analysis of shallow gas (Table 6) shows that, apart from Well Quan 58 and Well Quan 56, methane carbon isotope values of natural gas in the other wells are all between −55‰ and −25‰, with most around −45‰. Component characteristics and carbon isotope data suggest that the shallow gas in the Ordos Basin is a typical crude oil-associated gas, and the hydrogen isotope values in Table 6 also support this conclusion.

The natural gas components of the Mesozoic shale production gas and the shallow crude oil-associated gas in the study area differ only slightly. Shale production gas and crude oil-associated gas are mainly hydrocarbon gases, accounting for 85.71% to 97.41% of the total natural gas volume, with an average of 92.87%, while non-hydrocarbon gas content is relatively low. Methane constitutes 51.57% to 96.8% of shale gas and crude oil-associated gas, averaging 79.45%. The average methane content is 69.55% in shale gas and 86.22% in associated gas; shale gas is slightly higher than associated gas, but the difference is not significant. Ethane content ranges from 1.5% to 11.52%, averaging 7.78%, with shale gas averaging 8.0% and associated gas averaging 7.8%, showing little difference. Methane carbon isotopes of both the associated gas and the shale gas range from −55‰ to −40‰ and ethane carbon isotopes from −40‰ to −20‰, indicating that both sets of natural gas are derived from sapropelic source rocks [5,24].

6. Conclusions

• Methane content of shale gas in the Mesozoic Yanchang Formation in the Ordos Basin ranged from 46.08% to 94.56%, heavy hydrocarbon content from 2.86% to 37.61%, and methane carbon isotope values from −52.0‰ to −46.4‰.
• Methane content of the Upper Paleozoic natural gas in the Ordos Basin is between 75.55% and 97.75%, with heavy alkane carbon isotope values, indicating the coal-derived gas type.
• The geochemical characteristics of shale gas in the Yanchang Formation differ markedly from those of biogenic gas, which has high methane content, low heavy hydrocarbon content, and very negative δ¹³C₁; the shale gas therefore cannot be biogenic gas.
• The Mesozoic shale production gas and the shallow crude oil-associated gas in the study area show small differences in natural gas composition, and their carbon isotopes both indicate oil-type gases formed by cracking of sapropelic source rocks, indicating the same genesis.
• The carbon isotopic characteristics of production gas and natural desorption gas in the Chang 7, Chang 8, and Chang 9 shales, and of crude oil-associated gas in the Chang 2, Chang 6, and Chang 8 sections of the southern extension exploration area of the Ordos Basin, are basically the same, indicating the same genetic type. They are neither biogenic nor coalbed methane, but oil gas formed by primary thermal cracking of sapropelic source rocks [3,4]. Moreover, the shale gas of the Yanchang Formation in the Ordos Basin can only come from the Yanchang Formation itself.