Shale oil is the crude oil occurring in organic-rich shale strata containing thin interbeds of terrigenous clastic rock, carbonate and volcanic clastic rock less than 5 m thick each and cumulatively accounting for less than 30% of the total thickness of the shale strata^[1]. Continental shale oil in China with huge resource potential will make great contribution to the increase of future oil and gas reserves and production^[1,2,3]. In recent years, some researchers have carried out studies on geological characteristics and formation mechanisms of shale oil in different regions^[4,5,6], focusing mainly on enrichment and hydrocarbon-generating evolution of organic matter, reservoir development mechanisms, oil migration and accumulation process, and “sweet spot” evaluation and prediction in shale strata. Continental shale oil is characterized by “source-reservoir integration”. Usually fine-grained, shale reservoirs have frequent changes in fabric, various lithofacies and high heterogeneity. The development degree of shale lamellation, sandy lamina and micro-lamina would directly influence the effectiveness of shale oil reservoir^{[1, 7-8]}. Therefore, to examine laminae combination characteristics of organic-rich shale and find out the rock composition and reservoir space of different types of laminae are very important for us to get a deeper understanding on the shale oil enrichment patterns. The Chang 7₃ sub-member of Triassic Yanchang Formation in the Ordos Basin has a variety of organic-rich shale laminae in complex combinations. Shale lamina is the major factor that controls the development of reservoir, oil migration and accumulation, and distribution of “sweet spot” in shale^{[6, 9-10]}. However, the oil enrichment patterns under constraint of shale laminae combination haven’t been studied thoroughly. Thus, examining the types and combination characteristics of laminae in organic-rich shale to find out the oil enrichment patterns in this type of shale can not only provide important guidance to shale oil exploration and development in the study area but also supplement the geological theory and technological development of continental shale oil.

The Ordos Basin is divided into 6 tectonic units: the Jinxi fold zone, Xiyuan thrust zone, Weibei uplift, Yimeng uplift, Tianhuan depression and Yishaan slope (Fig. 1a). During the sedimentation of the Yanchang Formation at Late Triassic, affected by the Indosinian orogeny, a large-scale inland depression lake basin with a broad water area came up. In this period, a set of terrigenous clastic rocks of mainly lacustrine-fluvial facies deposited in the basin, which can be divided into 10 members^[9]. Of them, the Chang 7 Member was deposited during the maximum transgression period, when plankton bloomed and thus large-scale lacustrine facies organic- rich shale and delta-gravity flow sands developed^{[8, 10-11]}. The Chang 7 Member can be divided into 3 sub-members, namely the Chang 7₁, Chang 7₂ and Chang 7₃. The Chang 7₃ sub-member was deposited during the prime period of lake basin when delta sands built at the margin of the lake basin (Fig. 1b) while thick-bedded organic-rich mudstone and shale with thin deep-water gravity flow sand interbeds covered a broad area of the semi-deep and deep lacustrine zone (Fig. 1b, 1c). On the plane, the Chang 7₃ sub-member is composed of widespread organic-rich mudstone and shale, flood deep-water gravity flow sandy interbeds in the southwestern part, and slump deep-water gravity flow sediments in the northeastern part (Fig. 1b). Vertically, this sub-member is buried at a depth of 1500 to 3000 m, and 28 to 42 m thick (on average 33.54 m). It is continuously distributed^[9,10] across the Ordos basin, and is deemed to be the major source rock^[5] and also the most tuff-rich section^[12] in the basin. In general, the Chang 7 Member shale, with mainly Type I and II₁ organic matters, has high organic carbon contents from 8% to 16%, 13.81% on average, and R_o of 0.9% to 1.2% that indicates the peak of oil generation. Thus, the Chang 7 Member shale is deemed to have great shale oil exploration potential and the major target of shale oil exploration in the Ordos Basin.

Shale and thin interbedded sandstone core samples used in the experiments were taken from 27 wells drilled in the Ordos Basin that basically cover the whole distribution zone of the Chang 7₃ sub-member shale. First, thin section identification, XRD analysis and XRF element measurement were done on the samples to determine the mineral composition and element content. Furthermore, organic carbon contents of these samples were measured to find out the distribution characteristics of organic-rich shale. Then, the Advanced Mineral Identification and Characterization System (AMICS), high-resolution laser Raman spectroscopy and micro-FTIR spectroscopy were used to examine the types and combinations of shale laminae, mineral content and distribution, and shale oil occurrence and maturity based on in-situ analysis of micro-zones to sort out the laminae combination characteristics of organic-rich shale and the shale oil enrichment patterns of the Yanchang Formation Chang 7₃ sub-member in the Ordos Basin.

AMICS is a system that integrates the ZEISS FESEM, Burker High-Resolution EDS and AMICS analysis software which can conduct automatic quantitative mineral analysis in large area, whole mineral family and high resolution. In the automatic analysis, the thin section is made by polishing or grinding the rock sample, and then placed under SEM for mineral identification with High-Resolution EDS. AMICS analysis software is used to define the boundary of minerals. With the scanning resolution of up to 20 nm, several micro-zones can be selected to analyze and quantitatively obtain the type, content and areal distribution characteristics of minerals in the thin section. Nicole-tiN10MX infrared microspectrometer and LabRAM HR Evolution high resolution laser Raman spectroscopy were utilized to study the shale oil occurrence characteristics and maturity. In the infrared spectrum test, a microscope equipped with infrared spectroscopy was used to find the target field of vision and conduct the in-situ infrared spectrum test of organic matter and crude oil, respectively. To eliminate the effect of glass adsorbing partial infrared signals, the total wave number infrared spectrum test was conducted in two modes: the reflection mode with effective wave number of 400 to 2000 cm^-1 and the transmission mode with the effective wave number of 2000 to 4000 cm^-1. The laser Raman spectrum test was conducted with the excitation wavelength of 325 nm, grating 1800 grid line grooves, 50× object lens, at around 2 μm analysis micro-zone and exposure time of 40 s. Silicon chip was used for calibrating the wave number of the instrument and the scanning range of wave number was 150 to 4000 cm^-1.

The Yanchang Formation Chang 7₃ sub-member in the Ordos Basin is composed of thick-bedded mudstone and shale with thin interbeds of sandstone or siltstone (Fig. 2). The thick-bedded mudstone and shale exhibit two markedly different lamellation features (Fig. 2). The type I shale (encountered in Well Ning 70) has abundant grey white volcanic tuffaceous bands, high pyrite content and low clay content. The clay minerals are dominated by illite and secondarily mixed illite/smectite layer (Fig. 2a). The type II shale (encountered in Well Cai 30) contains black parallel laminae and a few discontinuously distributed grey white volcanic tuffaceous bands. It has higher clay content and lower pyrite content. Clay minerals in it are largely mixed illite/smectite, followed by illite (Fig. 2b). The two types of shale are close in total content of quartz and feldspar, but the type II shale has higher quartz content and lower feldspar content than the type I shale (Fig. 2a, 2b). Based on fluorescence spectrum element analysis, the type I shale has higher contents of some major elements (such as Fe and S) and trace elements (such as Tb, Gd, Eu, As, Co, V, P and Mo), but lower contents of some major elements (such as Si and Al) and trace element Ti (Fig. 2a, 2b) than the type II shale. Observation of thin sections under microscope shows that the type I shale has obvious sharp angular feldspar and quartz crystal pyroclasts and rod-like feldspar crystal pyroclasts (Fig. 2a); and the type II shale also has sharp angular quartz and feldspar crystal pyroclasts (Fig. 2b), besides some rounded clastic particles (Fig. 2b), indicating that the Chang 7₃ sub-member shale is rich in volcanic tuff. A comparison of microscopic features, mineral composition and element analysis results indicate that laminae in the type I shale rich in tuff are the product of direct sinking of volcanic tuff into the lake basin; but during the sedimentation of the type II shale, the volcanic tuff was affected by terrigenous transportation and terrigenous materials to some extent^{[8, 11-12]}. In addition, the ratio of Al₂O₃ to TiO₂ contents in the Chang 7₃ sub-member organic-rich shale in the Ordos Basin averages at 24.0, quite close to that of the Yanchang Formation volcanic sediments in the study area (26.5), but differs greatly from that of the typical mudstone and shale formed by terrigenous sediments (18.9)^[13,14,15]. This also suggests that the Chang 7₃ sub-member shale contains abundant volcanic tuff. Cores of thin interbedded sandstone from the type I shale strata are all saturated with oil (Fig. 2a), but cores of sandstone from the type II shale strata usually have low oil content and show oil traces (Fig. 2b).

4.1.1. Tuff-rich lamina

The tuff-rich lamina is light brown under plane-polarized light, composed of primarily volcanic tuffaceous materials, and nearly horizontal layered. These laminae were 100 to 500 μm thick and contain discontinuously distributed organic matter bands, dispersed organic matter fragments, and angular volcanic vitric and crystal pyroclasts (Fig. 3a). AMICS mineral analysis shows that the tuff-rich laminae have 80% of illite, and authigenic pyrite commonly seen at the places where volcanic tuffaceous materials contact with the banded or dispersed organic matter (Fig. 3b). They contain a small amount of other minerals such as albite, ankerite, quartz and mica, and have an organic content (the area proportion of organic matter on the plane, similarly hereinafter) of 6% (Fig. 3b, 3c).

4.1.2. Organic-rich lamina

This kind of lamina is black brown under plane-polarized light, high in organic matter content, and appears as continuous horizontal layer. They contain discontinuously distributed volcanic tuff lenticular masses, and are 300 to 1000 μm in thickness (Fig. 3d). AMICS mineral analysis shows that the organic-rich laminae have illite contents of over 50%, large amounts of authigenic quartz occurring in places where the lenticular tuff masses contact with organic matter, and granular authigenic pyrite commonly (Fig. 3e). They also contain a small amount of other minerals, such as albite, K-feldspar, apatite, dolomite and muscovite, and have an organic matter content of around 12% (Fig. 3e, 3f).

4.1.3. Silt-grade feldspar-quartz lamina

This kind of lamina is light colored under plane-polarized light, in silt-grade clastic texture, and contains a small amount of dispersed organic matter fragments. Grains of this kind of lamina are generally moderate-good in sorting, angular to sub-angular in roundness, and related to volcanic material sources. Suffering strong compaction, these laminae have grains in close contact and thicknesses from 500 to 1000 μm each (Fig. 3g). AMICS mineral analysis shows that these laminae have K-feldspar contents mainly of 70%, followed by quartz and albite (Fig. 3h, 3i). Therefore, this kind of lamina is named silt-grade feldspar-quartz lamina. Moreover, these laminae contain a small amount of mica fragments and authigenic minerals, such as pyrite, dolomite and apatite (Fig. 3h, 3i).

4.1.4. Clay lamina

This kind of lamina is dark brown under plane-polarized light and of argillaceous clastic texture. They are composed of primarily clay minerals and minor extreme-fine silts. Quartz and feldspar in them are sub-rounded to rounded (Fig. 3j). AMICS mineral analysis shows that these laminae have illite in dominance (80%), followed by quartz and K-feldspar. They have a very small amount of pyrite and white mica, and very low organic matter content (Fig. 3k, 3l).

4.2.1. Massive mudstone

The massive mudstone is made up of clay minerals and a small amount of fine to extreme-fine silts. It is of argillaceous clastic texture, generally poor in sorting, and strongly compacted (Fig. 4a). It contains a small amount of scattered organic matters (Fig. 4a), low organic carbon content, and poor oil-generating capacity. With little reservoir spaces, it is poor in storage capacity.

4.2.2. “Organic-rich + silt-grade feldspar-quartz” lamina combination shale

This type of shale, usually dark colored and high in organic carbon content (on average 23.12%), is made up of alternate organic-rich laminae and silt-grade feldspar-quartz laminae. The silt-grade feldspar-quartz laminae often have weak scouring marks at the bottom, and the organic-rich laminae often contain lenticular tuffaceous masses (Fig. 4b). Under a relatively strong transportation, silt-grade volcanic materials depositing on the land at early stage could be transported by flowing water into the deep-water zone to form silt-grade feldspar-quartz laminae^[16]. Under relatively weak transportation, argillaceous sediments were mainly transported. Tuffaceous materials formed by volcanism subsided into the lake basin, and the water-rich tuffaceous sediments after a short period of sedimentation could be eroded by flowing water and then transported into the deep-water zone to form discontinuously distributed lenticular masses mixed with fine-grained sediments^[17]. Appropriate volcanic tuffaceous materials could carry adequate nutrient substance into water body, leading to algae bloom and improving the productivity of lake. Accordingly, abundant organic matters would be accumulated to form organic-rich laminae^[18,19,20].

4.2.3. “Organic-rich + tuff-rich” lamina combination shale

This type of shale is relatively light colored, and 8.76% in organic carbon content on average. In it, organic-rich laminae are frequently and regularly interbedded with tuff-rich laminae, and the lamina interfaces are relatively flat and straight. The organic-rich laminae contain discontinuously distributed lenticular tuffaceous masses, and the tuff-rich laminae contain organic matters in discontinuously bands or dispersed patches (Fig. 4c). During the sedimentation of Chang 7₃ sub-member of the Triassic Yanchang Formation, intensive volcanic activities produced large amounts of tuffaceous materials^[21]. This would facilitate the formation of organic-rich lamina, on the other hand, large amounts of tuffaceous materials could accumulate to form layers in the lake during the extensive volcanic eruption period, bringing about an extremely oxygen-deficient environment, thereby leading to reduction of the lake productivity unfavorable for enrichment of organic matters^[18]. Under this condition, tuff-rich laminae with relatively low organic carbon content would be formed.

5.1.1. Characteristics of reservoir space

Major reservoir space in this kind of shale oil reservoir is K-feldspar dissolved pores in silt-grade feldspar-quartz laminae (Fig. 5a). With high K-feldspar content, the silt-grade feldspar- quartz laminae have intragranular dissolved pores in common with authigenic kaolinite fillings (Fig. 5b, 5c). The K-feldspar dissolved pores are mostly in micrometer scale, over 100 μm in maximum diameter, and almost all filled by crude oil (Fig. 5a, 5b). Some K-feldspar dissolution pores are filled with authigenic kaolinite to form nanoscale intercrystalline pores (Fig. 5c). The organic-rich laminae contain mostly clay mineral intercrystalline pores (Fig. 5d) and pyrite intercrystalline pores (Fig. 5e). This type of pore is usually small in size, predominately nanoscale (Fig. 5d, 5e) and poorly connected. Only some of them have minor oil films. In addition, the organic- rich shale commonly contains microfractures, predominately overpressured microfractures. The microfractures are largely micrometer in scale and 10 to 100 μm wide, may cut through the laminae, and are all filled with crude oil (Fig. 5f). There are bedding fractures between the shale laminae distributed along lamellation. They usually extend over relatively long distance but are small in width (mainly 0.5 to 2.0 μm) (Fig. 5g).

5.1.2. Reservoir space in thin interbedded sandstone

In this type of shale, thin interbedded sandstone are strongly compacted, so the grains in the sandstone are in close contact, leaving only a small amount of primary pores preserved (Fig. 5m, 5n). Some feldspar grains were dissolved, forming feldspar intragranular dissolution pores and intergranular dissolution-enlarged pores (Fig. 5h, 5i). Residual primary pores left after compaction in the sandstone are charged by oil (Fig. 5i), but feldspar dissolution pores showing poor connectivity and low charging ability, as evidenced by low oil contents of cores (Fig. 5).

5.2.1. Characteristics of reservoir space

In this kind of shale, the organic-rich laminae have little reservoir space. The organic-rich laminae and tuff-rich laminae both have no good reservoir space, so this kind of shale is poor in storage capacity. The two kinds of laminae have similar types of reservoir space, largely clay mineral intercrystalline pore (Fig. 6a) and strawberry-like authigenic pyrite intercrystalline pore (Fig. 6b). These pores are nanoscale in size and poorly connected (Fig. 6a, 6b). The majority of intercrystalline pores are not charged by oil, except a small amount of larger-sized pores that have oil film.

5.2.2. Characteristics of reservoir space in the thin interbedded sandstone

In the “organic-rich + tuff-rich” lamina combination shale of the Yangchang Formation Chang 7₃ sub-member in the Ordos Basin, thin interbedded sandstones are weakly compacted, so in the sandstone, grains are mostly in point contact and rarely in line contact; intergranular primary pores are well-preserved, and some feldspar grains have been dissolved at the edge, giving rise to intergranular dissolution-enlarged pores (Fig. 6c). The primary pores have large radius and good connectivity, and are commonly charged by oil (Fig. 6c).

6.1.1. Crude oil enrichment process in “organic-rich + silt-grade feldspar-quartz” lamina combination shale strata

In the “organic-rich + silt-grade feldspar-quartz” lamina combination shale stratum, oil is trapped primarily in K- feldspar dissolution pores within the silt-grade feldspar-quartz laminae (Fig. 5a). Crude oil trapped in the K-feldspar dissolved pores within the silt-grade feldspar-quartz laminae has basically the same laser Raman spectrum features with the residual oil adsorbed in the organic-rich lamina. The displacement of the D peak coincides completely with that of the G peak (Fig. 7a, 7b), indicating that the crude oil generated in the organic- rich lamina migrated a very short distance into the K-feldspar dissolved pores within the silt-grade feldspar- quartz lamina. In this type of shale, the longer distance to the interface between organic-rich lamina and silt-grade feld-spar-quartz lamina, the weaker intensity of dissolution of K-feldspar in silt-grade feldspar-quartz lamina will be (Fig. 7c). In the burial process of shale, with increasing burial depth, organic matters would become mature gradually and expel organic acid. Then the organic acid discharged out of the organic-rich lamina would dissolve the K-feldspar particles in silt-grade feldspar-quartz laminae nearby to form large amounts of dissolved pores. Meanwhile, the organic acid would be consumed^[22], which was unfavorable for dissolution of feldspar in thin interbedded sandstone. During the oil-generation stage, K-feldspar dissolution pores in silt-grade feldspar-quartz laminae in close proximity to the organic-rich laminae would be charged by crude oil generated by the organic-rich lamina first (Figs. 5a and 7c), forming the in-situ “oil generation- migration-accumulation” process between laminae within the shale,. Meanwhile, crude oil generated by organic-rich lamina could further migrate through numerous overpressured microfractures and bedding fractures present in shale laminae to the K-feldspar dissolution pores of the silt-grade feldspar-quartz laminae, as a result, the silt-grade feldspar-quartz laminae are saturated with oil. Crude oils retained in organic-rich laminae, trapped in overpressured fractures and K-feldspar dissolved pores have quite similar infrared spectrum characteristics (Fig. 7d to 7f), indicating that they are close “relatives”^[23,24] and originated from the organic-rich laminae. As crude oil generated by the organic- rich laminae accumulated first in the silt-grade feldspar-quartz laminae, only a small amount of the oil was expelled out of the shale and migrated into the thin sandstone interbeds, moreover, the thin sandstone interbeds in this kind of shale have limited reservoir space, so these sandstone interbeds usually have low oil saturations (Fig. 2a).

In the “organic-rich + silt-grade feldspar-quartz” lamina combination shale stratum, oil is trapped primarily in K- feldspar dissolution pores within the silt-grade feldspar-quartz laminae (Fig. 5a). Crude oil trapped in the K-feldspar dissolved pores within the silt-grade feldspar-quartz laminae has basically the same laser Raman spectrum features with the residual oil adsorbed in the organic-rich lamina. The displacement of the D peak coincides completely with that of the G peak (Fig. 7a, 7b), indicating that the crude oil generated in the organic- rich lamina migrated a very short distance into the K-feldspar dissolved pores within the silt-grade feldspar- quartz lamina. In this type of shale, the longer distance to the interface between organic-rich lamina and silt-grade feld-spar-quartz lamina, the weaker intensity of dissolution of K-feldspar in silt-grade feldspar-quartz lamina will be (Fig. 7c). In the burial process of shale, with increasing burial depth, organic matters would become mature gradually and expel organic acid. Then the organic acid discharged out of the organic-rich lamina would dissolve the K-feldspar particles in silt-grade feldspar-quartz laminae nearby to form large amounts of dissolved pores. Meanwhile, the organic acid would be consumed^[22], which was unfavorable for dissolution of feldspar in thin interbedded sandstone. During the oil-generation stage, K-feldspar dissolution pores in silt-grade feldspar-quartz laminae in close proximity to the organic-rich laminae would be charged by crude oil generated by the organic-rich lamina first (Figs. 5a and 7c), forming the in-situ “oil generation- migration-accumulation” process between laminae within the shale,. Meanwhile, crude oil generated by organic-rich lamina could further migrate through numerous overpressured microfractures and bedding fractures present in shale laminae to the K-feldspar dissolution pores of the silt-grade feldspar-quartz laminae, as a result, the silt-grade feldspar-quartz laminae are saturated with oil. Crude oils retained in organic-rich laminae, trapped in overpressured fractures and K-feldspar dissolved pores have quite similar infrared spectrum characteristics (Fig. 7d to 7f), indicating that they are close “relatives”^[23,24] and originated from the organic-rich laminae. As crude oil generated by the organic- rich laminae accumulated first in the silt-grade feldspar-quartz laminae, only a small amount of the oil was expelled out of the shale and migrated into the thin sandstone interbeds, moreover, the thin sandstone interbeds in this kind of shale have limited reservoir space, so these sandstone interbeds usually have low oil saturations (Fig. 2a).

6.1.2. Crude oil enrichment process in “organic-rich + tuff-rich” lamina combination shale strata

In this kind of shale stratum, the organic-rich laminae and tuff-rich laminae have little reservoir space and poor storage capacity, so the crude oil occurs primarily in the interbedded sandstones (Fig. 6c). Crude oil trapped in the thin interbedded sandstone has similar laser Raman spectrum features and approximately the same D peak with retained oil absorbed in the organic-rich lamina. But the G peak of spectrum line for retained oil in organic rich lamina is slightly left-shifted compared with the oil in thin interbedded sandstone (Fig. 8a-8c), possibly because the organic-rich lamina in the shale went through thermal evolution during burial process after charged by oil. Pressure increase induced by hydrocarbon generation in organic-rich laminae could restrain the compaction on thin interbedded sandstones and thus protect intergranular pores, and also create the formation of large amounts of overpressured fractures with maximum width of 200 μm (Fig. 8d). Crude oil could be expelled out of the organic-rich shale through these fractures and migrate into the thin interbedded sandstones to form high oil saturation intervals (Fig. 2b). Crude oil and organic matters in fractures within the tuff-rich laminae have markedly signs of compaction and deformation (Fig. 8d), indicating that the crude oil generated by the organic-rich laminae might migrate into thin interbedded sandstones driven by overpressure, forming a “oil generation-migration-accumulation” process between the organic-rich shale and thin interbedded sandstone; that is, crude oil expelled from shale migrated to and accumulated in the interbedded sandstone.

High-resolution laser Raman tests were conducted on thin sections of interbedded sandstones from the “organic-rich+ silt-grade feldspar-quartz” lamina combination (encountered by Well Cai 30) and “organic-rich + tuff-rich” lamina combination (encountered by Well Ning 70) shale stratum. The results show that the crude oil in the former has great Raman peak intensity ratio and area ratio than the crude oil in the latter (Fig. 9a, 9b), suggesting that crude oil in thin interbedded sandstone of “organic-rich + silt-grade feldspar-quartz” lamina combination shale stratum has higher maturity than that in the thin interbedded sandstone of “organic-rich + tuff-rich” lamina combination shale stratum ^[25,26]. Similarly, the laser Raman parameters of crude oil filling in K-feldspar dissolution pores in the silt-grade feldspar-quartz lamina of “organic-rich + silt-grade feldspar-quartz” lamina combination shale stratum and crude oil in thin interbedded sandstone of “organic-rich + tuff-rich” lamina combination shale stratum were compared, the results show the crude oil in the feldspar dissolution pores has great Raman peak intensity ratio and area ratio than crude oil in the thin sandstone interbed (Fig. 9c, 9d), indicating that the crude oil filling in K-feldspar dissolved pores in silt-grade feldspar-quartz laminae has higher maturity than that in the thin interbedded sandstone of “organic-rich + tuff-rich” lamina combination shale stratum ^[25,26]. In the “organic-rich + tuff-rich” lamina combination shale strata, the thin interbedded sandstones have an average bitumen reflectance of 0.59%, and the organic-rich laminae have an average bitumen reflectance of 0.62%. In the “organic-rich + silt-grade feldspar-quartz” lamina combination shale strata, the thin interbedded sandstones, silt-grade feldspar-quartz laminae and organic-rich laminae have average bitumen reflectance values of 0.94%, 0.93%, and 0.96% respectively. Clearly, the crude oil in the “organic-rich + tuff-rich” lamina combination shale strata were generated and charged earlier than that in the “organic-rich + silt-grade feldspar-quartz” lamina combination shale strata.

XRF fluorescence spectrum element analysis results show that the “organic-rich + tuff-rich” lamina combination shale (encountered in Well Ning 70) has higher contents of transitional elements such as Fe, Co and V than “organic- rich + silt-grade feldspar-quartz” lamina combination shale (encountered in Well Cai 30) (Fig. 2a, 2b). These transitional elements have better catalytic property, which can catalyze and hydrogenize the hydrocarbon generation of organic matter, and make the source rock generate more oil and gas under lower temperature and pressure^[27,28]. Thus, the organic matters were greatly consumed. But for the mudstone with relatively weak catalytic effect, the heat-trapping effect of source rock caused further thermal evolution of organic matters. As a result, source rocks generated large amount of crude oil, accumulating in silt-grade feldspar-quartz laminae, when reaching the oil-generation peak. Thus, “organic-rich + silt-grade feldspar-quartz” lamina combination shale strata generated oil later have higher bitumen reflectance than “organic-rich + tuff-rich” lamina combination shale strata that generated oil earlier.

Therefore, in the presence of rich volcanic tuffaceous materials, organic-rich laminae in “organic-rich + tuff- rich” lamina combination shale went through a short period of organic acid generation, and generated large amount of oil and gas during the early stage of burial under shallow depth and low temperature (at R_o of 0.6%). At that time, the thin interbedded sandstones were weakly compacted, and crude oil would be expelled from source rock through overpressure fractures and accumulated in intergranular pores in the thin interbedded sandstones (Fig. 10). On one hand, pressure increase induced by hydrocarbon generation in the organic-rich shale can restrain the compaction on thin interbedded sandstones, on the other hand, substantial crude oil charging would enhance the resistance to compression of sandstone reservoir, so large amounts of intergranular pores in the thin sandstone interbeds of this type of shale can be preserved (Fig. 6c).

The “organic-rich + silt-grade feldspar-quartz” lamina combination shale was less influenced by volcanic tuff during burial evolution. With the increase of burial depth and temperature, the organic matter in it would generate organic acid first^[22], the organic acid would intensively dissolve the K-feldspar particles in the silt-grade feldspar-quartz laminae to form large amounts of intergranular pores (Fig. 5a). As the organic acids were largely consumed in the shale, only a small amount of organic acid would get into the thin interbedded sandstone, and feldspar dissolution in the interbedded sandstone became weaker (Fig. 5h, 5i). As the burial depth and temperature further increased, organic matters became thermally mature and began to generate hydrocarbons. Crude oil entered into K-feldspar dissolution pores in the silt-grade feldspar-quartz laminae first (Fig. 7c, 7d and 10). Although pressure increase induced by hydrocarbon generation of organic matters would restrain the compaction on thin interbedded sandstones the sandstone reservoirs in this kind of shale had been greatly compacted and lost a large proportion of intergranular pores since the overpressure occurred late (Fig. 5h, 5i). In addition, substantial crude oil had accumulated in silt-grade feldspar-quartz laminae in the shale, the interbedded thin sandstone in this kind of shale strata have lower oil saturation (Fig. 10).

The Chang 7₃ sub-member of Yanchang Formation in the Ordos Basin has 4 types of laminae: i.e., tuff-rich lamina, organic-rich lamina, silt-grade feldspar-quartz lamina and clay lamina, and these laminae constitute “organic-rich + silt-grade feldspar-quartz” and “organic-rich + tuff-rich” lamina combination shale oil strata.

In the “organic-rich + silt-grade feldspar- quartz” lamina combination shale stratum of Chang 7₃ sub-member in the Ordos Basin, crude oil is trapped primarily in K-feldspar dissolved pores of silt-grade feldspar-quartz laminae within the organic-rich shale rather than in primary pores and feldspar dissolution pores of thin interbedded sandstones. In contrast, in the “organic-rich + tuff-rich” lamina combination shale stratum, crude oil is trapped in primarily intergranular pores remained in the thin interbedded sandstones as the organic-rich shale has little reservoir space.

Crude oil in the “organic-rich + silt-grade feldspar-quartz” lamina combination shale strata of Chang 7₃ sub-member in the Ordos Basin has higher maturity, and experienced the “generation-migration-accumulation” process between lamina, resulting in oil enrichment in laminae inside the shale. In contrast, crude oil in the “organic-rich + tuff-rich” lamina combination shale strata has lower maturity, but as the transitional elements present in the tuff-rich lamina served as catalysts, the organic-rich laminae could generate large amount of crude oil in a short period of time under relatively low temperature. The oil would migrate into thin interbedded sandstones through overpressure microfractures and go through a “generation- migration-accumulation” process from the organic-rich shale to thin interbedded sandstones.