Introduction
In 1885, the "anticline theory" by White published in the journal of Science marked the world’s oil and gas exploration towards the "theoretical guiding" stage [1]. Since then, geological theories of oil and gas accumulation have been continuously developed and improved, including trap theory [2], hydrocarbon generation from organic matter and oil and gas system theory [3], continental oil and gas geology theory [4-5], offshore deep water oil and gas geology theory [6⇓-8], continuous oil and gas accumulation and unconventional oil and gas geology theory [9], etc. In a sense, the proposal of a theory is not only a new understanding of the accumulation mechanism and distribution of a certain type of oil and gas, but also a new progress in understanding the overall distribution rules of underground oil and gas resources. In recent years, the rapid development of unconventional oil and gas exploration, represented by tight oil and gas and shale oil and gas [9⇓⇓⇓⇓⇓-15], makes scholars rethink the geological problems of the underground generation, migra-tion, enrichment and preservation of special fluid minerals such as oil and gas. In addition, it is more urgent to systematically reveal the geological process of hydrocarbon generation, expulsion, migration and accumulation from a certain point or a certain controlling factor, as well as the enrichment mechanisms of different types of oil and gas during this process, so as to fundamentally solve the problem that conventional and unconventional oil and gas cannot be comprehensively studied. In this circumstance, Jia et al. proposed the basic concept of the total petroleum system [16⇓-18].
The Ordos Basin is one of the important petroliferous basins in China [19-20], and the Upper Paleozoic shows great potential of natural gas resources, with proven natural gas reserves of 3.5×1012 m3 in Sulige gas field [19]. In addition, the cumulative proven natural gas reserves of Yulin, Shenmu, Wushenqi, Daniudi and Zizhou gas fields all exceed 1000×108 m3 [19]. Moreover, in recent years, new discoveries of tight gas and shale gas in the Upper Paleozoic in the eastern margin of the basin have continually made [21⇓-23]. Therefore, it is urgent to further explore the relationship between the natural gas resources at different structural locations, different burial depths and of different types, to analyze whether they have internal genetic consistency, and choose future exploration direction. Based on the analysis of the characteristics of the discovered natural gas reservoirs in the Upper Paleozoic of the Ordos Basin, this paper establishes a total petroleum system model for natural gas enrichment in the Upper Paleozoic of the Ordos Basin from the perspective of total petroleum system.
1. Regional geological overview
1.1. Basin tectonic pattern
Located in the west of the North China continental plate, the Ordos Basin is a typical large craton basin and the second largest sedimentary basin in China [24], with an area of about 25×104 km2. The Ordos Basin fell into the basin patten since the early Paleozoic Ordovician and began to undergo sedimentation. It has successively experienced multiple tectonic movement cycles such as Lüliang, Jinning, Caledonian, Hercynian, Indosinian, Yanshan and Himalayan, and has formed the current structural framework that’s characterized by internal stability, active margin, north-south uplift, west low and east high [25] (Fig. 1 ).
Fig. 1. Basin scope, geological section and Paleozoic gas reservoir distribution map of Ordos Basin at different ages (modified according to Reference [20]). |
1.2. Sedimentary strata
The Ordos Basin experienced the evolution from marine facies, to marine-continental transitional facies, and to continental facies during the Paleozoic. The Upper Paleozoic Carboniferous to the Permian is under a typical marine-continental transitional facies where the Benxi and Taiyuan formations were developed with mainly delta and barrier coast sediments. The sedimentary environment evolved into offshore delta when transgression started during the sedimentary period of the Shanxi Formation, and large sets of fluvial sediments were developed in the upper and lower Shihezi Formation and Shiqianfeng Formation [25-26] (Fig. 2 ).
Fig. 2. Stratigraphic histogram of the Ordos Basin (modified after Reference [25]). |
2. Basic concept and geological model of total petroleum system
2.1. Basic concept of total petroleum system
A total petroleum system refers to the natural system including all oil and gas, oil and gas pools, oil and gas resources, their formation and evolution process and distribution characteristics formed by the interrelated source rocks in an oil and gas basin [9]. Total petroleum system breaks through the limitation of the traditional oil and gas system "from source rocks to traps", and analyzes the conventional-unconventional oil-gas accumulation mechanism from a new perspective of "source-reservoir coupling and ordered accumulation", to guide the discovery of all types of oil -gas resources [16].
2.2. Geological model of total petroleum system
According to the concept and basic connotation of total petroleum system, whether for unconventional or conventional oil and gas, the study of source rock is still significant, and even more important than before. From the perspective of total petroleum system, we should not only pay attention to geological elements and geological processes of oil and gas from source rocks to traps, but also to the retention of hydrocarbons in the source rocks and their petroliferous properties, that is, we should study the whole-chain geological process from hydrocarbon generation, retention, expulsion, migration to accumulation in the petroliferous basin. Therefore, the geological model of hydrocarbon generation, expulsion, migration and accumulation in an ideal petroliferous basin can be understood as follows: (1) Source rock as the core, the generated oil and gas first fills its own pore space, including adsorption on the rock particles, or retention and accumulation of gaseous hydrocarbons dissolved in pore water and liquid hydrocarbons in various forms [27-28]. (2) With more and more hydrocarbon generated, and exceeding the accommodating capacity of the source rock, some hydrocarbon begins to be expelled out, migrates and accumulates in two ways. First, if the porosity and permeability of the reservoir are relatively high, and the pore throat radius is large at millimeter-micron scale, hydrocarbon will migrate upward under the action of buoyancy, and accumulate into conventional oil and gas reservoirs. Second, if the porosity and permeability of the reservoir are relatively low, and the pore throat radius is small at micron-nano scale, hydrocarbon can only move like displacing water under the action of its own expansive force, with no buoyancy effect [29⇓-31]. In this case, because of the self-sealing performance formed by capillary force and interfacial tension, large-scale and continuous accumulation of oil and gas can be formed (Fig. 3 ).
Fig. 3. Schematic diagram of the conceptual model and key geological problems of the total petroleum system. |
In fact, if the hydrocarbon generation capacity of source rocks in a petroliferous basin is strong enough, oil and gas can fill into all tight reservoirs, and buoyancy dominated accumulation above the critical sealing position may occur (Fig. 4 ). But it is important to note that this process may be influenced by tectonic movements, especially fault development which makes the process complicated. In addition, whether the above process can occur completely depends on the hydrocarbon generation capacity of the source rock, the coupling relationship between the hydrocarbon generation process and the tight reservoir process, and the development scale of the tight reservoir.
Fig. 4. Schematic diagram of hydrocarbon charging and migration process and critical conditions of different types of hydrocarbon resources in petroliferous basins. (a) Integral hydrocarbon charging in tight reservoir pore- throat system; (b) Oil and gas flow transformation at the interface between tight reservoir and conventional reservoir; (c) Buoyancy-dominated oil and gas migration and accumulation in conventional reservoirs. |
3. Upper Paleozoic total petroleum system in Ordos Basin
3.1. Source rocks
The Upper Paleozoic source rocks in the Ordos Basin are mainly coal measures of the Benxi Formation, Taiyuan Formation and Shanxi Formation [32⇓-34], which have stronger hydrocarbon generation capacity than mudstone and carbonaceous mudstone [35]. According to the comparative analysis of gas sources and data from drilled wells, the Upper Paleozoic natural gas is self-source gas [36], mainly from No.8 + No. 9 coal seams of the Benxi Formation, No.4 + No. 5 of the Shanxi Formation and No. 7 of the Taiyuan Formation. In addition, mud shale and carbonaceous mudstone also have certain gas supply capacity [37⇓⇓-40].
3.2. Source-reservoir-cap assemblage
The characteristics of the source-reservoir-cap assemblages are determined by the sedimentary system of marine-continental transitional facies of the Upper Paleozoic in the Ordos Basin. In the Benxi, Taiyuan, and Shanxi formations, the source-reservoir-cap assemblage is mainly composed of self-source, self-reservoir, and self-sealing (an in-source assemblage). In the Upper Shihezi Formation, it is characterized by lower source, upper reservoir, and upper cap (a near-source assemblage). In the east of the Ordos Basin, it is a far-source assemblage which is also characterized by lower source, upper reservoir, and upper cap [41⇓⇓-44]. It should be noted that the tight Upper Paleozoic reservoir weakened the role of the cap rock in the gas accumulation process, but played an important role in protecting the enrichment of natural gas in the Upper Paleozoic while reducing large-scale loss of natural gas [27,45⇓ -47].
3.3. Characteristics and model of gas migration and accumulation
The analysis of typical gas reservoir plays an important role in understanding natural gas accumulation model. From the gas enrichment layers and the distribution characteristics of gas fields, there are obvious differences from the center to the east of the Ordos Basin. The accumulation model of Sulige gas field is as follows: Gas comes from the coal seams of the Shanxi and Taiyuan formations. The reservoirs are the tight sandstone of the inner-source Shan-2 Member and the near-source He-8 Member. After generated, gas migrates into the reservoir and accumulates inside driving by the gas expansion force. The geological model is featured by close source and reservoir, non-buoyancy driving, and large and continuous of tight reservoirs [48] (Fig. 5 ).
Fig. 5. Geological model of the Upper Paleozoic reservoir in Sulige Gas Field (modified after Reference [48]). |
The Yulin gas field has similar forming characteristics with the Sulige gas field, except the main gas producing layer of the Shan 2 Member. The reason for the difference is directly related to the development of coal seams as the main gas source and the distribution of tight reservoirs. On the whole, natural gas accumulation in Yulin gas field follows the model of inner-source and near-source charging, tight reservoir accumulation, and large-scale and continuous distribution [49] (Fig. 6 ).
Fig. 6. Gas accumulation model of the Upper Paleozoic in Yulin Gas Field (modified after Reference [49]). |
Different from Sulige and Yulin gas fields, Shenmu gas field not only has inner-source and near-source reservoirs, but also far-source reservoirs [50] (Fig. 7 ). The development of far-source reservoirs is closely related to the existence of basin margin faults. Taking the Linxing area as an example, it has been demonstrated that faults can be an air pump-type transport channel to communicate deep source rocks and shallow Shiqianfeng Formation reservoirs [34]. In addition, in recent years, shale gas flow has been discovered from exploration of the Permian Taiyuan Formation in the Ordos Basin. Many wells have obtained high production, which shows the resource potential of shale gas, and exhibits the coexistence of multiple gas types including shale gas, tight gas, and coal bed methane [51].
Fig. 7. Shenmu gas field accumulation model (modified according to Reference [50]). |
3.4. Upper Paleozoic total petroleum system in Ordos Basin
Based on the above analysis, natural gas has been widely developed in the Upper Paleozoic of the Ordos Basin, and featured by the coexistence of shale gas, tight gas, and coalbed gas. From the gas-bearing layers, there are mainly inner-source and near-source tight gas, shale gas and coalbed methane in the center of the basin, among which the tight gas can be represented by Sulige gas field. To the margin of the basin, there are inner-source, near-source and far-source tight gas layers, represented by Shenmu gas field. And there are marine-continental transitional shale gas and coalbed methane in the Shanxi Formation. Overall, the Upper Paleozoic of the Ordos Basin shows the characteristics of total petroleum system where the coal seams and coal measures of the Benxi, Taiyuan and Shanxi formations are source rocks, which could retain natural gas inside. Although there is no systematic study, the existence of shale gas has been theoretically confirmed and by the current drilling results at the margin of the basin. For near-source tight reservoirs, natural gas can be preferentially charged in a short distance after generation, forming near-source tight gas accumulation distributed in the center and margin of the basin. However, the gas content is very different, which is mainly controlled by the hydrocarbon supply ability of the source rock. The far-source assemblages are mainly distributed at the margin of the basin and seldom in the central part, which is controlled the development and transport ability of faults (Fig. 8 ).
Fig. 8. Comprehensive geological model of gas accumulation in the Upper Paleozoic in the Ordos Basin. |
4. Controlling factors and geological models of the Upper Paleozoic total petroleum system in the Ordos Basin
4.1. Controlling factors of hydrocarbon accumulation in the total petroleum system
4.1.1. The source-reservoir coupling relationship controls the distribution and enrichment of tight gas
According to the drilling results from exploratory wells in the Upper Paleozoic in the Ordos Basin, the distribution of gas flow wells has a certain correlation with the gas generation intensity of source rocks. Generally, when the hydrocarbon generation intensity is greater than 10×108 m3/km2 or the vitrinite reflectance of the source rock is greater than 1.5%, the number of industrial gas flow wells significantly increases and with dense distribution [35]. As mentioned above, the Upper Paleozoic in Ordos Basin has the characteristics of total petroleum system, and has the geological characteristics of controlling the distribution of natural gas from the perspective of source rock. Therefore, in the total petroleum system, the action mechanism of source rocks on hydrocarbon distribution mainly lies in the coupling mechanism between hydrocarbon charging intensity and reservoir pore-throat system.
As shown in Fig. 4 , the primary migration of natural gas into tight reservoirs is different from that of conventional reservoirs. The main difference is that the tight reservoirs have low porosity and low permeability, relatively small pore space and narrow throat. It is difficult for natural gas to form a migration mode with relatively large volume, and buoyance dominated. It needs to rely on the continuous accumulation of its own molecular weight to form expansive force and overcome the overburden water column pressure and capillary force to migrate upward in tight reservoirs. Therefore, for tight reservoirs, not all gas source rocks play a role in the formation of tight gas reservoirs when the pore throat is fixed. It needs to reach certain gas generation intensity, that is, the amount of natural gas generated in the tight reservoir space is enough to reach a certain expansion force, and then natural gas can enter the tight reservoir to accumulate (Fig. 6 ). Therefore, it is necessary to establish the balance equation between natural gas charging power and resistance. Based on the derivation of the force balance equation, the authors established a critical condition discrimination model for natural gas charging in tight reservoirs [52]:
\[{{q}_{\text{e}}}=\left( M{{H}_{\text{s}}}\phi /Z{{\rho }_{\text{gl}}}RT \right)\left( {{\rho }_{\text{w}}}gH+2\sigma \cos \theta /r \right)\]
Then the critical charging intensity and critical porosity coupling discrimination equation is established, and the pore-throat radius is taken as the lower limit to judge whether natural gas can be charged into tight reservoirs:
$r=\frac{2\sigma M{{H}_{\text{s}}}\phi }{Z{{q}_{\text{e}}}{{\rho }_{\text{gl}}}RT-M{{H}_{\text{s}}}{{\rho }_{\text{w}}}gh\phi }$
Finally, the correlation between pore-throat radius and porosity is used to calculate the critical porosity for gas charging into tight reservoirs. We applied the above formula to the Benxi Formation of Ordos Basin for verification, and the critical charging intensity and critical porosity of natural gas in tight reservoirs were identified. It is found that when the expulsion intensity of source rocks in the Benxi Formation is 1.5×108 m3/km2, the required lower limit of porosity is 10.2%; when the expulsion intensity is 6× 108 m3/km2, the corresponding critical porosity is 4% (Fig. 9 ).
Fig. 9. Expulsion intensity - porosity composite map of the Benxi Formation in the Ordos Basin. |
4.1.2. Vertical distribution of natural gas is controlled by charging mode and charging channel type
(1) Natural gas directly enters pores and microfractures to control the distribution within and near the source. In the close source-reservoir assemblage, namely inner- source and near-source assemblages, after generated and expelled from the source rock, natural gas directly enters the tight reservoirs surrounding or adjacent the source rock (Fig. 10 a, 10b). This kind of gas reservoir has a large area of source rock supply gas, and generally contains gas with the characteristics of large-scale continuous distribution. The above mentioned Sulige gas field, Shanxi Formation of Yulin Gas Field and He 8 Member are of this model. In addition to pores, microfractures also act as transport channels. A considerable amount of natural gas is not expelled, but retains in the source rocks to form shale gas accumulation, like the model of the Permian Shan 2 Member in the eastern margin of the Ordos Basin.
Fig. 10. Microscopic gas charging geological models of tight reservoir. (a) Inner-source gas accumulation; (b) Near-source gas accumulation; (c) Far-source gas accumulation through fault transport. |
(2) Natural gas transports along faults to form far- source assemblage. This type of transport model is a special type of tight gas migration and accumulation, and its transport channels are faults and micro-fractures. Tight gas is mainly distributed in the areas with faults at the margin of the Ordos Basin, showing the characteristics of far-source hydrocarbon accumulation. On the plane, the gas-bearing area is developed next to the faults, and vertically the far-source gas-bearing strata are controlled by the distribution of faults. In addition, in the fault development area, there is a certain order in the inner-source, near-source and far-source hydrocarbon accumulation, which is mainly affected by the time of fault development, such as the Linxing and Shenfu areas [40]. Before fault developed, gas mainly charged in the source and near the source. After fault development, far-source reservoirs began to be charged. From the perspective of charging time, the order is inner-source, near-source and far- source. From the perspective of gas-bearing area, inner- source and near-source assemblages are mainly controlled by the hydrocarbon supply capacity of source rocks and the physical properties of reservoirs, and far-source assemblages are controlled by fault development and the physical properties of reservoirs (Fig. 10 c).
4.1.3. Reservoir physical properties control natural gas accumulation and the development of sweet spots
Generally, given the certain gas supply capacity of the source rock, the physical properties of the reservoir determine whether natural gas can enter the tight reservoir, and then control the gas-bearing range. In fact, the physical properties of the reservoirs have an obvious control effect on the accumulation and distribution of natural gas. On the plane, the area with relatively good reservoir physical properties has generally good gas bearing properties, so industrial gas flow wells may be drilled. On the other hand, the area with poor reservoir physical properties has poor gas-bearing properties (Fig. 9 ). However, it should also be noted that the gas-bearing range is controlled by the physical properties of the reservoir after the expulsion intensity of the source rock reaches a certain value. If the expulsion intensity of the source rock is relatively low, even if the physical properties of the reservoir is good, the gas-bearing properties may be very poor or even no gas, such as the area near the northern margin of the basin.
4.2. Geological models of gas accumulation in the total petroleum system
4.2.1. Gas retaining in the source and accumulating inside and near the source in the central part (without fault)
The coal seams in the Benxi, Taiyuan and Shanxi formations in the Ordos Basin are primary source rocks, in which dark mudstone also has a certain hydrocarbon generation capacity, and generally has good hydrocarbon supply conditions. The tight reservoirs in the source rocks are relatively developed, and have the conditions allowing gas to directly charge, resulting in a large area of tight gas enrichment in the source rock. At the same time, in the source rock system, mud shale and coal measures could retain natural gas, and thus form inner-source gas accumulation, such as coalbed methane and shale gas, which are potential targets for future exploration. In addition, the central structure of the Ordos Basin is stable, and faults are not developed. Most of the gas reservoirs are inside and near the source rocks. For example in Sulige gas field, near-source and continuous tight sandstone gas reservoirs are dominant. The enrichment of natural gas is mainly controlled by the hydrocarbon generation ability of source rocks and the physical properties of tight reservoirs. In the central area with a strong hydrocarbon generation capacity, gas enrichment scale is relatively large, and the sweet spots zone can be defined from the perspective of reservoir physical properties (Fig. 11 ).
Fig. 11. The total petroleum system and geological model of natural gas distribution of the Upper Paleozoic in the Ordos Basin. |
4.2.2. Inner-source, near-source, and far-source gas accumulation in the basin margin area (fault development area)
Relatively developed faults at the margin of the Ordos Basin make the region not only develop inner-source and near-source gas accumulation, but also develop far-source assemblages relative to the central area of the basin. For example, the natural gas accumulation system of the Upper Shihezi Formation and Shiqianfeng Formation of Linxing and Shenfu areas in the eastern margin is far-source gas accumulation transported by faults. In addition, the margin of the basin is relatively shallow. Under the premise of considering economic benefits, the shale gas in the margin area deserves more attention, and has achieved preliminary exploration benefits.
To sum up, to explore the Upper Paleozoic natural gas in the Ordos Basin, the gas accumulation model of total petroleum system should be the basis. We need to change the exploration idea, and transfer the exploration model from tight sandstone reservoirs to hydrocarbon source rocks and coal seams, that is, from "following the vine (tight reservoir rocks) to get the melon (gas accumulation)" to "tracing the root (source rock and coal seam)", which requires to consider all elements. In the central area of the Ordos Basin, we should focus on inner-source and near-source gas accumulation, and consider the coupling relationship between the hydrocarbon retention capacity, hydrocarbon supply capacity of source rocks and reservoir physical properties, and first select it as a potential replacement area. In the basin margin area, we should focus on the combination between fault transport and reservoir to provide theoretical guidance and basis for searching for far-source gas accumulation. At the same time, fully consider the geological characteristics of shallow burial in the basin margin to explore the residual resources inside the source rock, i.e. shale gas (Fig. 11 ).
5. Conclusions
A total petroleum system is an effective model to study hydrocarbon accumulation in a petroliferous basin. Its core lies in the systematic evaluation of source rocks. The hydrocarbon supply capacity of source rocks and the hydrocarbon accumulation capacity of different types of reservoirs determine the accumulation and distribution range of shale, tight and conventional oil/gas, which are comprehensively controlled by fault development and reservoir physical properties. The Upper Paleozoic reservoirs in the Ordos Basin are widely developed and tight. According to the conceptual model of total petroleum system, the gas accumulation models of the Upper Paleozoic include a central model and a margin model of the basin. The natural gas accumulation model in the center of the basin has four types: inner-source residual shale gas, coalbed methane, inner-source tight gas, and near-source tight gas. The gas accumulation model at the margin of the basin has five types: inner-source residual shale gas, coalbed methane, inner-source tight gas, near-source tight gas and far-source tight gas. The difference mainly lies in the development of faults at the margin of the basin, so the latter has far-source gas accumulation. In general, future exploration of the Upper Paleozoic natural gas in the Ordos Basin should take the theory of total petroleum system as a basis, and focus on the gas supply capacity of source rocks and the gas accumulation capacity of different types of reservoirs, to predict favorable exploration areas more effectively.
Nomenclature
g—gravitational acceleration, 9.8 m/s2 in this paper;
H—height of water column, m;
Hs—source rock thickness, m;
M—molar mass of natural gas, 16 kg/mol in this paper;
qe—hydrocarbon expulsion intensity, m3/km2;
R—gas constant, 8.314 33 J/(mol·K) in this paper;
r—critical pore throat radius, μm;
T—thermodynamic formation temperature, K;
Z—gas compression constant, dimensionless;
ρgl—subsurface gas density, kg/m3;
ρw—water density, kg/m3;
σ—gas-water interfacial tension, N;
θ—wetted contact angle, (°);
ϕ—source rock porosity, %.