Introduction
China has abundant continental petroleum resources that account for 90% of the total continental petroleum resources in the world [1]. However, the continental petroliferous basins of China have almost entered into the middle-late stage of exploration. It is more difficult to obtain new discoveries of conventional oil and gas in structural and lithologic reservoirs, so it is urgent to discover unconventional petroleum resources as successive resources to ensure national energy security [2⇓⇓⇓⇓-7]. Compared with the homogeneous shale oil geological bodies in the stable marine cratons in North America, the continental lacus-trine basins in China generally experienced multi-stage and multi-cycle tectonic evolution, forming faulted depressions and depressions in multiple structural styles. Under the influence of paleoclimate, there are diversified sedimentary water bodies such as fresh water, saline water and brackish water, resulting in complex source-reservoir combinations with supply of terrestrial, endogenous and volcanic sources [8]. In conclusion, the continental shale oil in China generally has the geological characteristics of strong reservoir heterogeneity, rapid change of thickness, low thermal maturity, high clay mineral content, unobvious abnormal high pressure and low gas-oil ratio (GOR) [9⇓-11]. Based on the successful experience of shale oil and gas exploration and development in North America, and research on basic geological theory and engineering technology, major breakthroughs have been made in discovering shale oil resources and industrial development in shale formations such as the Permian in the Junggar Basin [12], the Triassic in the Ordos Basin [13], the Cretaceous in the Songliao Basin [14], the Permian in the Santanghu Basin [15], the Paleogene in the Bohai Bay Basin [16], the Jurassic in the Sichuan Basin [17], and the Paleogene in the Subei Basin [18]. China has become the fourth country who made breakthrough in shale oil in the world.
The Ordos Basin was a typical inland depression freshwater lake basin during the sedimentation of the 7th member of the Triassic Yanchang Formation (Chang 7 Member). Under the influence of lake transgression, a source rock interval dominated by organic-rich shale was formed, which laid a material foundation for the large-scale enrichment of shale oil [19]. According to the sedimentary cycle, lithological association and formation thickness, the Chang 7 Member is divided into three sub-members, namely Chang 71, Chang 72 and Chang 73, from top to bottom. Selecting Chang 71-2, the sandstone interlayer in the semi-deep and deep lacustrine shale as a sweet zone, the PetroChina Changqing Oilfield Company discovered Qingcheng shale oil field with proven reserves of 1 billion tons, and built China's first million-ton integrated shale oil development zone. Previous studies on Chang 73 mainly focused on organic matter enrichment factors, source rock evaluation, hydrocarbon generation and expulsion simulation [20⇓-22]. The industrial breakthrough to shale-type shale oil in China confirms that the continental thick shale geological body with high TOC also has a potential of shale oil exploration and development [14]. The study on Chang 73 has also changed from simple organic geochemical research to microscopic reservoir characterization and mobility evaluation [23-24]. In 2019, the Changqing Oilfield obtained high yield of 100 t during testing wells CY1 and CY2 from Chang 73, making it possible to further find the sweet spots of shale oil in Chang 73 [25]. In 2022, continuous and stable high yield was obtained from Chang 73 (i.e., the thick shale with thin silty and fine sandstone interlayers) in Well LY1H, which started a new situation of large-scale exploration of shale oil in Chang 73. The cumulative production of this well lasted for 320 d, and the cumulative oil production exceeded 5300 t [26]. In order to explore the oil production potential of pure shale section, Changqing Oilfield carried out vertical well fracturing. Ten wells have obtained industrial oil flow in the pure shale section of Chang 73, and well test broke through the oil production threshold, showing a good prospect of shale oil exploration [27].
In order to further clarify the classification criteria and geological conditions of multiple types of shale oil in Chang 73, and find the exploration targets for shale oil in new fields in the Ordos Basin, based on logging, core and test data of Chang 73 such as in wells CY1 and H36-1, this paper analyzes the geological characteristics and enrichment laws of shale oil development in Chang 73, and discusses the exploration potential and research targets, with the intent to provide a direction for the exploration plan for Chang 73 shale oil in new fields.
1. Geologic setting
The Ordos Basin is a large superimposed basin with multiple cycles developed on the Early Proterozoic crystalline basement. It mainly experienced the development of a Middle and Late Proterozoic aulacogen basin, the development of a stable Paleozoic craton basin, the development of a Mesozoic foreland basin and the development of a Cenozoic peripheral fault basin [28]. During the sedimentary stage of the Late Triassic Yanchang Formation, a large cratonic depression lake basin was developed, and characterized by a large size, gentle slopes, shallow water, many sources and stable structures, and a set of fluvial-lacustrine clastic rock with thickness of about 1000 m was deposited. It is divided into 10 sections from Chang 1 to Chang 10 from top to bottom.
The sedimentary period of the Chang 7 Member is the heyday of the development of the inland depression lake basin in the Ordos Basin, when a semi-deep to deep lake area appeared with an area of 6.5×104 km2, and a set of mud shale interval dominated by organic-rich shale and dark mudstone with thin silty-fine sandstone layers was deposited, with thickness of 110 m (Fig. 1 ). During the sedimentation of the Chang 7 Member, the paleoclimate in the Ordos Basin was warm and humid. It was a typical freshwater lake basin where the water was poor in oxygen, and the reducing environment was conducive to the preservation of organic matter in semi-deep to deep lake subfacies. The sedimentary period of Chang 73 was not only the maximum flooding period, but also the peak period of lake thermal fluid activities. The prosperity of algae and plankton not only laid a material foundation for the deposition of organic-rich shale, but also created favorable material conditions for the enrichment of large-scale shale oil. During the sedimentation of Chang 73, three secondary paleogeomorphological units were developed in the lake basin: ancient gentle slope, ancient slope and ancient sag. The ancient gentle slope mainly developed delta sand bodies. The sand bodies formed by sandy debris flow were pushed from the ancient slope in the semi-deep lake to the ancient sag in the deep lake, and the proportion of sand bodies of turbidity current began to increase [29]. With the increase of transport distance, the vertical layers of the sand bodies become more but the thickness becomes thinner, and the farthest end evolves into micron-millimeter terrestrial felsic laminae in shale. The sand bodies of different scales and genesis are superimposed with organic-rich shale in different degrees in the semi-deep to deep lake area, forming various types of lithofacies assemblages. This lays a geological foundation for the development of multiple types of shale oil in Chang 73.
Fig. 1. Sedimentary facies map and stratigraphic column of Chang 73 in the Ordos Basin. |
2. Classification of shale oil
According to the maturity of organic matter, continental shale oil can be divided into medium-high maturity shale oil and low-medium maturity shale oil [30]. According to the source-reservoir relationship, it can be divided into three types: separate source-reservoir type, coexistent source-reservoir type and integrated source-reservoir type [31]. According to the geological conditions and sedimentary characteristics, it can be divided into three types: interlayer type, mixed type and shale type [32]. According to the accumulation mode of hydrocarbons, it can be divided into two types: migrating and retained shale oil, and retained shale oil [33]. By referring to the classification method of Jiao et al. [32], the shale oil in the Chang 7 Member in the Ordos Basin is divided into two types: interlayered shale oil and shale oil. The interlayered shale oil is mainly developed in Chang 71-2 in the central part of the lake basin, and large-scale exploration and economic development have been realized. The shale oil is mainly developed in Chang 73 in the central part of the lake basin, and many risk exploration wells have high oil production, showing the potential of large-scale exploration. Although Chang 73 is almost shale, thin interlayers of silty-fine sandstone caused by gravity flow are also developed to a certain extent. Compared with the sandstone interlayer in Chang 71-2, the sandstone interlayer in Chang 73 is thinner, and has worse vertical and horizontal continuity, finer grains and stronger reservoir heterogeneity. In order to distinguish the types of shale oil in Chang 73, based on the classification from Fu et al. [19], this paper refers to the maturity of organic matter, hydrocarbon accumulation mode and the development degree of sandstone interlayer /lamina and microlamina to divide the shale oil in Chang 73 into two types and three sub-types: migratory-retained shale oil and retained shale oil (low-medium maturity, and medium-high maturity) (Table 1 , Fig. 2 ). The migratory-retained shale oil in Chang 73 is mainly distributed around the gentle slope belt at the edge of the deep sub-sag at the lake bottom. The retained shale oil is mainly distributed in the deep sub-sag at the lake bottom. In the area strongly affected by terrestrial source input, migratory-retained shale oil is locally developed (Fig. 3 ).
Table 1. Types of Chang 73 shale oil in the Ordos Basin |
| Type | Maturity | Hydrocarbon accumulation | Thickness of sandstone layer/lamina | Reservoir | Sweet spot zone | Development method |
|---|---|---|---|---|---|---|
| Migrating- retained shale of | Medium-high maturity Ro>0.8% | Micro-migration in sandstone laminae and in situ retention in shale laminae | Single layer: 0.01-1.00 m; Large gross thickness | The silty laminae of sandy debris flow and low density turbidite flow are interbedded with deep lake shale laminae. The porosity and permeability of the silty laminae are mainly 2%-6% and (0.01-0.03)×10-3 μm2. | The zone with rich sandstone and micro-fractures, and high maturity and GOR. | Horizontal well + fracturing stimulation |
| Retained shale oil | Micro-migration in felsic and tuffaceous laminae and bedding fractures, and in-situ retention in clay and organic laminae | Micro- to nano-level | Pores in tuffaceous and felsic laminae are developed with rich free hydrocarbon, which is conducive to fracturing stimulation. Pores in organic and clay laminae are less developed with absorbed hydrocarbon, which is difficult to fracture. | Felsic black shale with high TOC, developed bedding fractures, high retained hydrocarbon content and brittle minerals. | ||
| Low-medium maturity, Ro<0.8% | In situ retention in micro-clay and organic laminae | The organic matters are mainly type I and type II1, not converted. The proportion of organic matter and retained hydrocarbon, the content of organic matter, the hydrogen index and the in situ conversion rate are high. | High TOC (>15%) Medium Ro (0.5%-0.8%) Large thickness (>15 m) Moderate depth (<2000 m) | In situ heating in horizontal wells, and hydraulic fracturing in horizontal wells for local zones with relatively high pressure and good oil-bearing property |
Fig. 2. Characteristics of shale oil in Chang 73 in the Ordos Basin. OL—organic lamina; FL—felsic lamina; CL—clay laminae. |
Fig. 3. Distribution of shale oil in Chang 7 in the Ordos Basin. |
2.1. Migratory-retained shale oil
Migratory-retained shale oil is the result of frequent combination and interaction between hydrocarbon micro-migration in sandstone laminae and hydrocarbon in-situ retention in shale laminae. The sweet spot zone is the combination of silty laminae caused by sandy debris flow and low-density turbidity current and frequent interbedded laminae of semi-deep to deep lake organic-rich shale. The laminae of this type of shale oil are macroscopic laminae that can be observed by the naked eye. The thickness of a lamina is 0.01-1.00 m, and the combined section is sandy. The proportion of laminae is more than 50%, and the cumulative thickness is more than 5 m. Due to the strong hydrocarbon generation and expulsion of organic-rich shale laminae, the sandy laminae with good physical properties that are frequently interbedded with organic-rich shale laminae generally have the characteristics of high oil saturation, and the major contributor to well productivity. Organic-rich shale laminae also contribute to well productivity. The density of crude oil is 0.83 g/cm3, and the viscosity is 5.8 mPa·s.
2.2. Retained shale oil
Retained shale oil refers to the solid and liquid hydrocarbons that are generated during the thermal evolution of organic matter and retained in situ. According to the maturity of organic matter, retained shale oil can be divided into the shale oil with medium-high maturity and that with low-medium maturity.
2.2.1. Medium-high maturity retained shale oil
Medium-high maturity retained shale oil refers to the hydrocarbon retained in the organic-rich shale with Ro higher than 0.8%. The sweet spot zone is the black felsic shale with bedding and laminae and high content of brittle minerals. The laminae refer to the microstructure of frequent interaction and combination of micron-scale and millimeter-scale structures that can be observed under a microscope. The proportion of movable hydrocarbon is high in the composite unit composed of organic matter + terrigenous felsic laminae' and bedding fractures, and organic matter + tuffaceous laminae' and bedding fractures, which is the main factor affecting well productivity. The density of crude oil is 0.84 g/cm3, and the viscosity is 6.4 mPa·s.
2.2.2. Low-medium maturity retained shale oil
Low-medium maturity retained shale oil refers to the hydrocarbon retained in the organic-rich shale with Ro less than 0.8%. It is mainly based on the solid hydrocarbon retained in the original microscopic clay lamina and organic lamina. It is difficult to achieve commercial development of the shale oil through conventional fracturing stimulation, but can be effectively developed by in-situ heating transformation.
Although the Chang 73 was deposited in the maximum flooding period, frequent gravity flow events led to the development of sandstone in the central part of the lake basin, which is characterized by the interaction of multiple types of shale oil in the vertical direction (Fig. 4 ).
Fig. 4. Composite column of Chang 73 in Well CY1 in the Ordos Basin. |
3. Geological conditions of shale oil development
The organic-poor interval with a good seepage capacity and the organic-rich interval with a weak seepage capacity in Chang 73 depositional stage are frequently overlapped on different scales. This constitutes good in-source hydrocarbon generation and expulsion, micro-migration and accumulation and sealing conditions, and lays a good geological foundation for the large-scale enrichment and preservation of multiple types of shale oil.
3.1. Conditions of source rock
Chang 73 was deposited during the largest flooding of the whole Yanchang Formation. Frequent volcanic activities led to the bloom of algae and microorganisms in the lake, forming an ultra-eutrophic water body. The large and gentle lake basin bottom, high productivity and strong reduction conditions jointly controlled the large-scale deposition of shale with abnormally high organic matter in Chang 73.
3.1.1. Source rock
The black shale of Chang 73 covers 4.3×104 km2, the maximum cumulative thickness is more than 50 m, and the average cumulative thickness is 13.9 m (Fig. 5 ). The organic matter is dominated by aquatic algae in the reducing environment, and mainly types I and II1. The average TOC is 13.8%. The type of the organic matter is good, the abundance is high, and the oil generation potential is large. The distribution of hydrocarbon generation activation energy is relatively concentrated, and the average activation energy is low. It has the characteristics of a short hydrocarbon generation period, a fast hydrocarbon generation rate and a high total oil production rate [34]. The dark mudstone of Chang 73 covers 6.2×104 km2, the maximum cumulative thickness is 40 m, and the average cumulative thickness is 11.3 m. The organic matter is mainly composed of aquatic algae and terrestrial higher plants, and mainly Type II1, Type II2, and a small amount of Type III. The average TOC is 3.8%. Although the organic matter abundance is lower than that of black shale, it is still a high-quality source rock compared with the shale oil source rocks in other continental lake basins in China. The activation energy of hydrocarbon generation is relatively dispersed, and the average activation energy is higher. It has the characteristics of a long hydrocarbon generation duration, a slow hydrocarbon generation rate and a relatively low total oil production rate [34]. The widely distributed black shale and dark mudstone source rocks have superior conditions. The results of hydrocarbon generation simulation experiments show that the hydrocarbon generation potential of organic matter in the Chang 7 Member is strong, and the hydrocarbon generation potential is about 400 kg/t. The average hydrocarbon generation intensity of high-quality source rocks is 495×104 t/km2 [13], which lays a good material foundation for the large-scale development of shale oil in the Chang 7 Member.
Fig. 5. TOC contour map of Chang 73 black shale (modified according to Reference [33]). |
3.1.2. Maturity limit
The thermal simulation carried out by Qi et al. [22] shows that the Ro of the main hydrocarbon generation period of the black shale in the Chang 7 Member in the Ordos Basin is 0.70% to 0.87%, and the Ro of the main hydrocarbon generation period of the dark mudstone is 1.06% to 1.72% (Fig. 6a ). The thermal simulation conducted by Liu et al. [35] shows that when the Ro is 0.75% to 0.95%, the shale has the highest yield of n-hexane and products extracted with mixed solvent (Fig. 6b ). The mathematical model of adsorption capacity prediction by Dang et al. [36] shows that the Ro of 0.75% is the maturity limit of shale oil occurrence state and mobility transform, while the Ro of 0.85% to 0.90% is the lower limit of the best maturity window for shale’s 'high oil content and good mobility' (Fig. 6c ). Although there are deviations in the corresponding Ro parameters given by different scholars for the main hydrocarbon generation period and movable limit of Chang 73 shale oil, they all show that due to the relatively low activation energy of hydrocarbon generation, Chang 73 shale can achieve relatively high hydrocarbon generation capacity at lower maturity, which enhances the mobility of shale oil. Li et al. [37] studied that types I and II1 organic matter in organic-rich shale had a significant degree of inhibition on Ro, and the measured maturity was generally lower than the real maturity by more than 0.2%.
Fig. 6. Thermal evolution model of Chang 7 source rock in the Ordos Basin. |
Combined with previous research results, Li et al. [27] took the Ro of 0.8% as a standard for the selection of medium and high maturity shale oil in Chang 73, and the corresponding true maturity is above 1.0%, which is relatively reasonable. In addition to the southern part of the Ordos Basin, the Ro of the Chang 7 Member is less than 0.8%, and the Ro in most areas of the central part of the basin is 0.8% to 1.2% (Fig. 7 ) [38]. According to the standard of 0.8%, it has reached the mature stage of oil generation and is in the peak period of oil generation. This provides a large number of high-potential hydrocarbon-rich high-quality fluids for shale oil accumulation in Chang 73.
Fig. 7. Contour map of organic matter maturity in Chang 7 Member. |
3.2. Reservoir conditions
3.2.1. Retained shale oil reservoir
Under the influence of paleoclimate, terrigenous clastic supply and volcanic activity, a large number of micron- scale and millimeter-scale microstructures with frequent interaction of different lithologies are developed in the pure shale of Chang 73, including high-TOC organic-rich laminae, medium-TOC clay laminae and low-TOC felsic laminae and tuffaceous laminae (Fig. 8a -8c). A large number of intergranular pores and dissolution pores are developed in the felsic lamina (Fig. 8d ). The dissolution pores of feldspar crystal debris in the tuffaceous lamina are generally developed (Fig. 8e ). A large number of intergranular pores of clay minerals are developed in the clay lamina (Fig. 8f ). A large amount of strawberry-like pyrite is generally associated in the organic lamina, and pyrite intergranular pores and organic matter shrinkage fractures are generally developed (Fig. 8g ). Wu et al. [39] found that the dominant pore of the felsic laminae in Chang 73 shale was more than 500 nm, that of the clay mineral laminae was 30 nm to 300 nm, and that of the organic laminae was smaller. The felsic lamina and the tuffaceous lamina are high-porosity and high-permeability laminae in pure shale, which effectively improve the storage capacity of the shale and the mobility of crude oil. Fluorescent thin sections show that the fluorescence of the felsic lamina and the tuffaceous lamina is significantly stronger than that of the organic lamina and the clay lamina, which are enrichment units of high-quality hydrocarbon (Fig. 8h ). The retained shale oil in Chang 73 shows the characteristics of micro-migration and accumulation from organic lamina and clay lamina to felsic lamina and tuffaceous lamina, so the development of felsic lamina and tuffaceous lamina determines the content of movable hydrocarbons in Chang 73 shale.
Fig. 8. Reservoir characteristics of retained shale oil in Chang 73 in the Ordos Basin. (a) Well L57, 2318.45 m, frequently developed laminae in black shale, thin-section; (b) Well CY1, 2018.71 m, scanning imaging of laminated minerals in black shale; (c) Well CY1, 2018.71 m, the laminae in shale are frequently interbedded, argon ion polishing electron microscope photo; (d) Well CY1, 2010.21 m, Intergranular pores in felsic laminae, argon ion polishing electron microscope photo; (e) Well Z9, 1329.80 m, tuffaceous lamina dissolution pores, casting thin section; (f) Well CY1, 2011.21 m, intergranular pores of clay minerals in shale, argon ion polishing electron microscope photo; (g) Well Z22, 1551.01 m, pyrite enriched in organic laminae in shale, SEM; (h) Well L57, 2334.00 m, fluorescence characteristics of different laminae in shale, fluorescent flakes. |
3.2.2. Migratory-retained shale oil reservoir
The good development of sandstone interlayers/laminae in Chang 73 provides space for hydrocarbon migration and accumulation, and it is the decisive factor for the mobility of the migratory-retained shale oil in Chang 73. Zhao et al. [40] analyzed that the pore structure and physical properties of the sandstone interlayer / laminae are significantly better than those in pure mudstone, the volume of medium and large pores is larger, and the reservoir seepage and storage of fluid is better. In the sandstone interlayer/laminae, there are residual intergranular pores supported by rigid minerals (Fig. 9a , 9b), feldspar dissolution pores (Fig. 9c , 9d), and intergranular pores of clay mineral. Fluorescent thin sections show that intergranular pores and feldspar dissolution pores are rich in relatively light hydrocarbons, and the fluorescence is generally bright yellow. The clay mineral film at the edge of the pore adsorbs relatively heavy hydrocarbons, and the fluorescence is generally grayish black (Fig. 9e , 9f), indicating that the widely developed intergranular pores and feldspar dissolution pores with large pore diameters and support of rigid mineral are the main reservoir space for free hydrocarbons. The intergranular pores of clay minerals are mainly enriched by adsorbed hydrocarbons. The reservoir characteristics of shale lamina are consistent with those of retained shale oil.
Fig. 9. Reservoir characteristics of migratory-retained shale oil in Chang 73 in the Ordos Basin. (a) Organic matter in intergranular pores in sandstone, 1640.21 m, Well Z148, common thin section; (b) intergranular pores in sandstone, 2281.86 m, Well Y297, SEM; (c) feldspar dissolution pores in sandstone, 1886.90 m, Well C37, casting thin sections; (d) feldspar erosion, 1786.20 m, Well Z204, SEM; (e) intergranular pores filled with oil in sandstone, 2021.77 m, Well CY1, common thin section; (f) sandstone in the same field of view with (e), 2021.77 m, Well CY1, fluorescent photo. |
3.3. Accumulation conditions
Micro-migration within source rock is the primary mechanism for oil-bearing and sweet spot enrichment in multiple types of shale oil reservoirs. Xi et al. [41] found that the characteristics of laser Raman spectroscopy of the crude oil in the organic-poor felsic laminae and thin sandstone interlayers in Chang 73 are similar to those of the residual crude oil adsorbed in the organic-rich laminae, indicating that the crude oil generated in the organic-rich laminae in the pure shale migrated in the source rock at different scales.
3.3.1. Source-reservoir pressure difference is the power for in-source hydrocarbon migration
The high-quality source rocks in Chang 73 have high hydrocarbon generation intensity and high oil production efficiency, and the expansion resulting from hydrocarbon generation is very obvious. The overpressure caused by strong hydrocarbon generation and expulsion provides a sufficient power for in-source hydrocarbon migration. The resistance of in-source micro-migration in Chang 7 is capillary pressure. Its average is 1.17 MPa [42] in Chang 7, while the hydrocarbon generation pressure of the source rock is about 10 MPa. The overpressure caused by hydrocarbon generation can completely overcome the capillary pressure, provide a power for in-source hydrocarbon expulsion in the organic-rich lamina, and promote the formation of sweet spot zones with high shale oil saturation in the thin interlayer of the organic-poor sandstone with good seepage conditions, and felsic and tuffaceous laminae in Chang 73 shale.
3.3.2. Hydrocarbon generation induced fractures, bedding fractures and microfractures are high-rate channels for in-source hydrocarbon migration.
Affected by the strong hydrocarbon generation of organic matter, the organic-rich laminae in Chang 73 pure shale generally have hydrocarbon-generation-induced fractures (Fig. 10a ), which provide channels for the vertical migration of hydrocarbon at the micron-millimeter scale. The frequently developed bedding fractures provide channels for the lateral migration of hydrocarbon (Fig. 10b , 10c). These two types of fractures are efficient channels for the high crude oil saturation in the felsic and tuffaceous laminae.
Fig. 10. Hydrocarbon generation induced fractures, bedding fractures and microfractures in Chang 73 in the Ordos Basin. (a) Hydrocarbon generation induced fracture network in organic matter laminae rich in shale, 2023.02 m, Well CY1, SEM; (b) bedding fractures in black shale, 2380.25 m, Well H36-1, core photos; (c) shale bedding surface with crude oil immersion, 2084.65 m, Well L231, core photo;(d) spillover of crude oil in high-angle fractures in siltstone, 2371.51 m, Well H36-1, core photos; (e) obvious fracture fluorescence in the same field of view with (d), core fluorescence photo; (f) siltstone fracture surface with oil immersion, 1767.40 m, Well N228, core photo. |
The results of field profile, core observation and imaging logging analysis show that the natural fractures in Chang 73 are relatively developed (Figs. 10d-10f and 11), which are almost unfilled NE-SW high-angle fractures. The micro-fractures in Chang 7 are mainly formed in the Yanshan Movement period, which corresponds to the primary hydrocarbon accumulation period. The core observation shows that the oil-bearing level on the fracture surface is high, and obvious under fluorescent light. The development of micro-fractures reduces the difficulty for oil filling in sandstone interlayers/laminae, and improves the micro-migration of shale oil in Chang 73.
Fig. 11. Identification results of fracture imaging logging data in Chang 7 in Well H36-1, the Ordos Basin. |
3.3.3. The widely developed micro- and nano-pore-throat system provides space for hydrocarbon accumulation
The results of micro- and nano-CT experiments show that Chang 73 not only has a large number of micro- and nano-pore-throat systems densely distributed in the sandstone interlayers/laminae (Fig. 12a , 12b), but also in the shale (Fig. 12c ). They effectively improve the reservoir capacity. The pore-fracture network composed of micro- and nano-pores and various fractures provides space for large-scale accumulation of multiple types of shale oil in Chang 73.
Fig. 12. Micro-CT scanning characteristics of Chang 73 reservoir in the Ordos Basin. |
3.3.4. Hydrocarbon differentiation improves the mobility of shale oil
The contents of saturated hydrocarbons and aromatic hydrocarbons in the extract components on Chang 73 sandstone interlayers/lamina are 66.43% and 8.86%, respectively, and those in the pure shale are 37.72% and 19.74%, respectively. The light components in the sandstone interlayers/laminae are significantly higher than those in the pure shale. The density and viscosity of the surface crude oil are also lower than those of the oil in the shale, and the fluidity is relatively good. The results of fluorescent thin sections of black shale also show that the quality of hydrocarbons in felsic and tuffaceous laminae is better than that in clay mineral laminae and organic matter laminae. This is the result of adsorption of polar components with weak migration ability such as non-hydrocarbons and asphaltenes in organic matter laminae and clay mineral laminae. The differentiation of hydrocarbon in the Chang 73 source rock makes the non-polar hydrocarbon components with the strongest migration ability, such as saturated hydrocarbon and aromatic hydrocarbon, accumulate in the sandstone interlayers/laminae and the felsic and tuffaceous laminae, which improves the mobility of shale oil.
3.4. Brittleness
The average contents of quartz, feldspar, carbonate and total brittle minerals of the sandy laminae in the Chang 73 migratory-retained shale oil reservoir are 37.18%, 45.43%, 6.38%, and 88.99%, respectively. The average contents of quartz, feldspar, carbonate, pyrite and total brittle minerals of the retained shale oil reservoir are 31.6%, 12.20%, 6.14%, 15.53% and 65.49%, respectively. They are conducive to fracturing stimulation. However, there is a certain proportion of authigenic silicon in the Chang 73 shale. The authigenic siliceous particles in the shale with a high clay content are small and mostly dispersed in the clay minerals in a floating form. The self-healable clay mineral is unhelpful for fracturing stimulation.
The content of brittle minerals in the retained shale oil reservoir is obviously affected by the development and type of microscopic laminae. The development of the laminae in Fig. 13a is high, so the total content of brittle minerals such as quartz, feldspar, carbonate and pyrite reaches 74.29%. The laminae are less developed in Fig. 13b , so the content of brittle minerals is only 23.46%. The organic-rich laminae are generally accompanied by a large amount of authigenic pyrite. The more developed the organic laminae and microscopic laminae with better brittleness, the higher the brittleness index. The brittleness index of the retained shale oil reservoir calculated according to mineral content cannot fully reflect the fracturing effect of the shale oil reservoir, and the genesis and occurrence of authigenic silicon need to be considered [43]. The brittleness index of the migratory-retained shale oil reservoir is also significantly affected by the development and cumulative thickness of the sandy laminae.
Fig. 13. Comparison of mineral components between shale with developed laminae and shale without developed laminae. (a) Black shale with frequent laminae and brittle minerals of 74.29%, Chang 73, Well N150; (b) black shale with undeveloped laminae and brittle minerals of 23.46%, Chang 73, Well N150. |
4. Exploration direction and challenges
Affected by the thickness of black shale, the distribution of industrial oil flow wells in Chang 73 is almost in the area with thick black shale (Fig. 14a ). According to the sedimentary thickness of black shale and the degree of thermal evolution of organic matter, the Chang 73 in the Ordos Basin is divided into three deep sags: Maling-Huachi (Class I), Huanxian-Jiyuan (Class II) and Zhengning-Xunyi (Class III). In the Class I sag, the Ro is higher than 0.9%, the TOC is higher than 16%, and the average thickness of black shale is 23 m. In the Class II sag, the Ro is higher than 0.8%, the TOC is higher than 8%, the average thickness of black shale is 25 m, the fracture development is relatively complex, and the preservation conditions are poor. The Ro is less than 0.8%, the TOC is higher than 10%, and the average thickness of black shale is 22 m in the Class III sag. The deep sags and the zones around the sags are favorable for exploration and deployment of shale oil (Fig. 14b ).
Fig. 14. Thickness contours of black shale and distribution of paleotopographic units in Chang 73 in the Ordos Basin. |
4.1. Risk exploration of retained shale oil with medium-high maturity in deep mature sags
At present, the wells with industrial oil flow tested in pure shale are mainly located in the deep mature sags. Risk exploration of retained shale oil with medium-high maturity is mainly carried out in Maling-Huachi (Class I sag) with relatively high organic matter maturity and good preservation conditions. Previous studies have shown that felsic black shale with TOC of 4%-14%, Ro higher than 0.8%, well-developed bedding and laminae, and high content of brittle mineral is a favorable exploration target for retained shale oil with medium-high maturity in Chang 73. High-yield industrial oil flow has been obtained in production test (Fig. 15a ). It is roughly estimated that the movable retained shale oil is 60×108 t, indicating rich resources and broad prospects. The oil-bearing property of medium-high maturity shale oil reservoirs is obviously controlled by multiple types of micro-laminae. It is necessary to further optimize the logging sequence, clarify the vertical and plane distribution of the dominant laminated shale, and evaluate the oil content, reservoir seepage and fracturability. The fractures in Chang 7 are relatively developed, but the preservation conditions of shale oil are relatively poor. In particular, the development of fractures in Huanxian- Jiyuan (Class II sag) is relatively complex. It is necessary to further clarify the influence of fractures of different scales on the medium-high maturity retained shale oil, and actively explore the zones with good preservation conditions in the mature deep sags.
Fig. 15. Production test results of Chang 73 reservoir in the Ordos Basin. |
4.2. In-situ conversion of low-medium maturity retained shale oil in low-maturity deep sags
More than 50% of the organic matter in the Chang 73 shale in the Ordos Basin has not been converted into hydrocarbons. The technically recoverable resources from in-situ conversion are about (400-450)×108 t, indicating a huge resource potential [38,43⇓ -45]. The deep sag area, i.e., Zhengning-Xunyi (Class III) in the southeast of the basin is shallow (less than 1500 m), with low maturity of organic matter (Ro<0.8%), low water cut (less than 0.3%), and a large favorable area of in-situ conversion (greater than 1.5×104 km2). A stable shale section of 13.9 m was drilled in Well Z75GC1, with average TOC of 14.1%, average hydrocarbon generation potential of 68.75 mg/g, and average hydrogen index of 467.85 mg/g. Future study will investigate the retained hydrocarbon in the low-medium maturity shale, clarify the hydrocarbon generation characteristics and kinetic process, establish a dynamic and basic reservoir simulation model for in-situ conversion, and develop fine characterization technology for in-situ conversion heated structures. Finally, considering the energy consumption ratio, a reasonable heating-production well network will be built, which can provide theoretical guidance for the promotion of field application.
4.3. Exploration of migratory-retained shale oil in around-sag sandstone-rich zones
Migratory-retained shale oil is a realistic target for large-scale exploration in Chang 73. The successful well LY1H further enhanced the confidence of large-scale exploration in the sandstone-rich zones in mature deep sags (Class I and Class II). In 2022, the new predicted geological reserves of migratory-retained shale oil exceeded 2×108 t. The sandstone interlayers/laminae have relatively better porosity and permeability, so they are the destination where hydrocarbon is expulsed in the deep sag area, and easy to develop high shale oil saturation. Affected by organic-rich shale, thin silty-fine sandstone layers in Chang 73 are very subtle, and include multiple cycles of silty-fine sandstone, so they are difficult to identify by logging data. In addition to normal sandstone, sandstone with high GR values is frequent in Chang 73, even higher than 300 API locally (Fig. 15b ), which puts forward higher requirements for logging evaluation technology.
5. Conclusions
Based on organic matter maturity, hydrocarbon accumulation model and the development degree of sandy and microscopic laminae, the shale oil in Chang 73 can be divided into two types: migratory-retained shale oil and retained shale oil.
High-quality source rocks provide rich fluid with high potential and rich hydrocarbon. The pressure difference between source rock and reservoir provides a power for the accumulation of crude oil in thin organic-poor sandstone interlayers, felsic and tuffaceous laminae and bedding fractures with good seepage conditions. Hydrocarbon generation induced fractures, bedding fractures and microfractures provide high-rate channels for the micro-migration of crude oil. The widely developed micro- and nano-pore-throat system provides space for large-scale hydrocarbon accumulation. The differentiation of hydrocarbons in source rocks makes the non-polar hydrocarbon components with the strongest migrating ability, such as saturated hydrocarbon and aromatic hydrocarbon, accumulate in sandstone interlayers/laminae and felsic and tuffaceous laminae in shale, which improves the mobility of shale oil.
Migratory-retained shale oil is the primary target for large-scale exploration in the around-sag sand-rich zone, which depends on the fine identification of high-GR sandstone interlayers/laminae. The target of risk exploration is the medium-high maturity retained shale oil in deep mature sags, which depends on the identification of dominant laminae and preservation conditions. The potential of low-medium maturity retained shale oil resources is huge. Future study should focus on theoretical research and field application of in-situ conversion for developing this type of shale oil in deep sags.