Spatial and temporal evolution of paleo-highs/lows of the permian strata and its petroleum significance in the Sichuan basin, SW China

ABSTRACT The Sichuan basin has a great potential for hydrocarbon exploration. In recent years, Permian strata have been identified as one of the main target strata for natural gas exploration in the Sichuan basin. Not only the source rocks are abundant, but also there are good reservoirs due to the wide and complex continental oceanic interaction. However, there are still many shortcomings, especially the relationship between the tectonic evolution and the hydrocarbon accumulation model. Therefore, this research aims at understanding the characteristics of paleo-highs/lows of the Permian strata and their controlling effects on hydrocarbon migration/accumulation in the Sichuan basin. Firstly, the distribution patterns of paleo-highs/lows in the Permian strata at each critical tectonic period were constructed by integrating spatial erosion calculation. Then the temporal evolution of the Permian strata at each critical tectonic period will be revealed by restoring the upper strata sequentially. The results of this research improve the precision of paleo-highs/lows reconstruction and provide useful hints for the hydrocarbon exploration of the Sichuan basin: (i) Paleo-highs/lows directly determine the development location of the Permian key reservoirs; (ii) Tectonic evolution controls many factors and the process of hydrocarbon accumulation.


Introduction
The Sichuan basin, located to the east of the Tibetan Plateau, is an oil/gas-bearing sedimentary basin. Twentythree oil/gas-bearing layers have been discovered from the Sinian to Jurassic in the Sichuan basin (according to the internal report by the Research Institute of Petroleum Exploration & Development, PetroChina, 2019). According to recent resource evaluation, the amount of conventional natural gas geological resources is 16.45 × 10 12 m 3 from the Sinian to Jurassic (excluding shale gas) (Wei et al. 2018). More than 20 important breakthroughs on hydrocarbon exploration were discovered in middle Permian carbonates since the 1960s (e.g. Ma et al. 2019;Yi et al. 2020) in particular, as primary target layers with a series of gas fields constructed throughout the Sichuan basin, for example, the Ziliujing gas field, the Tongziyuan gas field, and the Laowenchang gas fields. The abundant oil/gas resources in the Permian carbonates have attracted many interested petroleum geologists, with dozens of studies working on the potentials of petroleum exploration in Permian, including spatial distribution of source rocks (e.g. Chen et al. 2018), the sedimentary characteristics (e.g. Yi et al. 2020), the lithofacies palaeography (e.g. Gao et al. 2021), etc.
The Sichuan basin, as a complex superposition basin, has experienced multi-staged structural evolution, including rifting stage from Sinian to late Permian, cratonic depression from late Permian to early Triassic, and foreland basin stage from late Triassic to present. Many scholars have already realized the important control of tectonic activities on the petroleum migration and accumulation of the Sichuan basin (e.g. Liang et al. 2015;Liu et al. 2015;Wu et al. 2019). However, the spatial/temporal evolution and their impacts on petroleum migration/accumulation of the Permian strata have not been well documented yet, which, to some extent, prohibits the progress of hydrocarbon exploration of the Permian strata in the Sichuan basin. Therefore, aiming at understanding the spatial/temporal evolution and their impacts on petroleum migration/accumulation of the Permian strata, multiple approaches including erosion calculation, paleo-highs/lows reconstruction and detailed reservoir analysis was employed, to reveal the basin-scale kinematic processes and the distribution features of paleo-highs/lows of the Permian strata. The relationship between basin-scale evolution, distribution of paleo-highs/lows and petroleum migration/accumulation was then systematically discussed, which can potentially provide useful hints for the following hydrocarbon exploration of the Permian strata in the Sichuan basin.

Geological setting
The Sichuan basin, located on the northwest Yangtze Plate, is bounded by the North China Plate to the north (Wei et al. 2018), and the Qiangtang Plate to the west ( Figure 1). Geographically, the Sichuan basin is constrained by the Dabashan Orogenic Belt to the northeast, the Qiyueshan Fault Zone to the southeast, the Longmenshan-Micangshan Orogenic Belt to the northwest, and the Daliangshan Orogenic Belt to the southwest (Roger et al. 2010;Li et al. 2012;Liu et al. 2013;Tian et al. 2018) .
Multiple critical tectonic activities were recorded in the evolution of the Sichuan Basin since the Carboniferous (Wei et al. 2018), including, chronologically (Figure 2), the Yunnan movement (~299 Ma), the Dongwu movement (~265 Ma), the Indochina Orogeny (~210 Ma), the Yanshan Orogeny (~90 Ma), and the Himalayan Orogeny (~25 Ma) (Yin and Harrison 2000;Jia et al. 2004Jia et al. , 2005 Being affected by these tectonic activities, the Sichuan basin experienced multi-staged tectonic evolution, including the stage of craton uplifting until late-stage of early Permian, the stage of intra-cratonic rifting from late Permian to middle Triassic, and the stage of foreland basin since late Triassic (Meng and Zhang 2000;Meng et al. 2005;Jin et al. 2010;Ding et al. 2013;Li et al. 2013a). Accordingly, several major unconformities were developed (e.g. the unconformity between the Permian formation and the pre-Permian formation), with apparent structural erosion ( Figure 2) observed throughout the Sichuan basin (Li et al. 2001). Due to the complex transitions between the tectonic stages, the sedimentary environment of the Sichuan basin also presented significant temporal variation, from marine carbonate sediments (Carboniferous to middle Triassic) to terrestrial clastic sediments (late Triassic to present) In marine carbonates of the Sichuan basin, four primary sets of source rocks were discovered in lower Cambrian, lower Silurian, lower Permian and upper Permian, which has provided abundant petroleum resources for the basin  Chen et al. 2018). For instance, as demonstrated in Figure 2, from Carboniferous to middle Triassic, there are two sets of source rocks, including the grey-black marls and mudstones of the Maokou formation in lower Permian (with thickness of 100-420 m and gas generation intensity of (10-40) ×10 8 m 3 /km 2 ), and the shales and dark grey marls of the Longtan -Wujiaping formation in upper Permian (with the thickness of 30 ~ 120 m and gas generation intensity of (10-40)×10 8 m 3 /km 2 ). Two effective assemblages of hydrocarbon accumulation elements were observed in Permian, which was the lower Permian assemblage and the upper Permian to lower Triassic assemblage (Ma et al. 2019).

Methods
In this research, multiple methods were employed to analyse the stratigraphic contact relationship and erosion distribution patterns in each critical tectonic period. The paleo-highs/lows of the Permian strata were then reconstructed, to reveal the impacts of paleo-highs/lows of the Permian strata on petroleum migration/accumulation in the Sichuan basin. As for time-depth conversion, the seismic velocity field of each individual formation was constructed based on the drilling data throughout the basin firstly. Based on the velocity fields, the time-domain erosion maps and paleo-highs/lows maps were then converted into depth domains.

Detailed seismic interpretation and delineation of stratigraphic contacts
Twelve high-resolution seismic profiles (in the time domain) were interpreted in detail (see positions of seismic traces in Figure 3), focusing on the interpretation of key horizons, and geometry of folds/faults (Yuan et al. 2008). The stratigraphic contact relationship was then delineated by analysing the key positions where truncations and onlaps were developed, Stratigraphy abbreviation: C = Carboniferous strata (among which Devonian strata do not deposit and Carboniferous strata only deposit locally); P 1 l = Liangshan formation of Early Permian; P 1 m = Maokou formation of Permian; P 2 l = Longtan formation of Permian; P 2 ch = Changxing formation of Late Permian; T 1 f = Feixianguan of Early Triassic; T 2 l = Leikoupo formation of Triassic; T 3 x = Triassic Xujiahe formation; J 1 z = Ziliujing formation of Early Jurassic.
below and above the unconformity, respectively. Three representative seismic sections with detailed delineation of stratigraphic contacts are demonstrated in this article.

Quantitative calculation of erosion amounts
Based on seismic interpretation and stratigraphic contact delineation, the minimum erosion amounts were calculated for each critical formation at its corresponding tectonic period (Jing et al. 2021). Both traditional method (Figure 4a,b) and improved method (Figure 4c, d) for erosion calculation were illustrated in Figure 4. As demonstrated in Figure 4a,c, the layers A and B are strata above and below an unconformity, respectively. In the traditional method, the erosion amount of layer B (marked with red vertical arrows) can be restored by calculating the difference between the maximum thickness and residual thickness (Figure 4b), with an assumption of a constant thickness of layer B when deposited. However, a layer may present thickness with spatial variations when deposited, which fails this traditional method for erosion calculation. Therefore, in this research, an improved method considering the internal stratigraphic textures within layer B was employed for the calculation of erosion amounts (Figure 4c,d). As demonstrated in Figure 4c, the lower portion of layer B, which has changing thickness laterally, is not eroded by the unconformity (below the blue line), whereas only the upper portion of layer B should be considered when calculating the erosion amount. The erosion amount of layer B (marked with red vertical arrows) is then restored by calculating the difference between the maximum thickness and residual thickness of the upper portion of layer B (Figure 4d). Apparently, if the internal stratigraphic textures of layer B are considered, the results of erosion amount using the improved method are more accurate compared to the results using the conventional method (Figure 4c,d).
In this research, the erosion amounts of Jurassic to Quaternary (J 1 z to present), late-Triassic (T 3 x), early-Triassic (T 1 f-T 2 l), late-Permian (P 2 ch), mid-Permian (P 1 m), and early-Permian (P 1 l) in sections were calculated, which were then utilized to construct planview maps of erosion distribution by using the Surface Construction Module in 3D Move of Midland Valley. The erosion maps allowed us to observe the distribution patterns in each critical period and analyse their controlling factors.

Reconstruction of paleo-highs/lows
The 3D spatial models and distribution maps of each key horizons were constructed using Midland Valley by considering erosion calculation. The detailed workflow of this method was described in Figure 5 and related texts in Miao et al. (2022). For instance, targeting on the paleohighs/lows of the Maokou formation (P 1 m) in the late Triassic, i.e. before the deposition of the Xujiahe formation (T 3 x 1 ) (237 Ma): firstly, the spatial model of the Xujiahe formation (T 3 x 1 ) at present was constructed; secondly, the erosion amount of the Xujiahe formation (T 3 x 1 ) was then overlaid on its present spatial model; finally, the paleohighs/lows of the Maokou formation (P 1 m) at 237 Ma was reconstructed by unfolding the spatial model of the Xujiahe formation (T 3 x 1 ) with erosion restored. (e.g. Jiu et al. 2012;Jing et al. 2021)

Erosion distribution in the critical tectonic period
Considering the present topography, structural features, and oil/gas exploration progress of the basin, the Sichuan basin can be sub-divided into five first-order structural units (e.g. Wei et al. 2018), including the Central Uplift Zone (I), the Western Depression (II), the Northern Depression (III), the Eastern Fold Belt (IV), and the Southern Fold Belt (V) (Figure 3).
Twelve seismic sections were interpreted within 3D Move of Midland Valley, which allowed our understanding of the structural styles (including unconformity, folds and faults) and strain distribution throughout the Sichuan basin ( Figure 3). Three  (c,d). Layers A and B are strata above and below an unconformity, respectively. In the conventional method, the erosion amount of layer B is restored (mark with red vertical arrow lines) by calculating the difference between the maximum thickness and residual thickness (a,b). In the optimized method, the internal stratigraphic textures within layer B are also considered (c,d).
representative seismic sections are demonstrated in this paper, which are sections A-A', B-B' and C-C' (Figure 3, 6 -7, 8). The two sub-parallel NW-striking sections are distributed in the north and centre of the Sichuan basin, respectively. The section A-A' goes across the Longmenshan Mountain, the Northern Depression (III) and the Eastern Fold Belt (IV) from west to east; the section B-B' goes across the Western Depression (II), the Central Uplift Zone (I) and the Eastern Fold Belt (IV) from west to east; while the NEstriking section C-C', distributed in the west Sichuan basin, goes across the Daliangshan Mountain, the Western Depression (II) and the Northern Depression (III) from south to north.
Based on the seismic interpretation of sections A-A' (Figure 6a), it is observed that the east portion of the Sichuan basin is dominated by a series of sub-parallel fold belts with symmetric geometry in section view, which is highly affected by the existence of multiple decollement layers (i.e. the Eastern Fold Belt). In section A-A' (Figure 6a), the west end is characterized by multiple SE-directing thrust faults due to the Longmenshan orogeny, whereas the central portion of this section experienced limited structural deformation (i.e. the Northern Depression). In addition, the key unconformities were also picked up in section A-A' (Figure 6b), corresponding to the critical tectonic activities documented in the stratigraphic and tectonic columns in Figure 2. The truncations/onlaps below/above the unconformities provide important evidence for the processes of uplift and subsidence. For example, the upper portion of the Xujiahe formation (T 3 x 1 ) was truncated by the unconformity and then onlapped by the Ziliujing formation (J 1 z) (Figure 6c). Similar truncation/onlap relationships were also depicted on the Leiloupo (T 2 l)/ Xujiahe (T 3 x 1 ) unconformity ( Figure 6d) and the Silurian to Carboniferous (S-C)/Liangshan (P 1 l) unconformity ( Figure 6e).
Similar to section A-A', the southeast portion of section B-B' is also characterized by a series of sub-parallel fold belts with symmetric geometry in section view ( Figure 7a). However, being different to section A-A', the central portion and northwest portion of section B-B' are featured by uplift and depression, respectively. The strata of the Xujiahe formation (T 3 x) and Leikoupo formation (T 2 l) are thinning from the northwest depression (II) to the central uplift (I) of the Sichuan basin. In addition, there are also multiple unconformities picked up in section B-B'. For example, the Leikoupo formation (T 2 l) was onlapped by the lower portion of the Xujiahe formation (T 3 x 1 ) above the unconformity ( Figure 7c) and the Silurian to Carboniferous (S-C) strata were truncated by the unconformity ( Figures. 6d and 7e).
In the NE-striking section C-C', being controlled by the complex fault and fold systems in the Daliangshan and Micangshan, both the north and south ends are characterized by a series of basinward directing thrust faults and back-thrusts ( Figure 8a). The central portion of section C-C' is a large-scale depression adjacent to the Longmenshan located to the west of Sichuan basin. However, being influenced by the central uplift of the Sichuan basin, a small-scale uplift is also observed in the depression. Multiple unconformities are also picked up in section C-C'. For example, the upper Leikoupo formation (T 2 l) was onlapped by the lower Xujiahe formation (T 3 x 1 ) above the unconformity (Figure 8c), the upper Xujiahe formation (T 3 x) was truncated by the unconformity and then onlapped    by the Ziliujing formation (J 1 z) (Figure 8d), and the Silurian to Carboniferous (S-C) strata were truncated by the unconformity (Figure 8e). Based on the features depicted in the 12 seismic sections (see traces in Figure 3), the erosion amounts for each formation were calculated using the method described in Figure 4. The spatial models of erosion accounted by the key tectonic events were built by using the surface modelling function of 3D MOVE, Midland Valley. Plan view maps of erosion were also constructed to observe the distribution patterns in each critical tectonic period (Figure 9). To make an easy comparison of erosion amounts in different geological periods, the maps in Figure 9 were set with identical colour scales. Apparently, the features of the distribution of erosion amount present significant variations in each critical

Spatial and temporal evolution of paleo-highs/lows of the permian strata
The 3D paleo-highs/lows models of the Permian strata were built using Midland Valley by integrating erosion calculation (detailed methodology was described in section 3 and Figure 4). The workflow of paleo-highs/lows map construction was shown in Figure 5. Plan view maps of the paleo-highs/lows of the Maokou formation (P 1 m) and the Changxing formation (P 2 ch) were then constructed for each critical tectonic period (Figures 10,11), including the Early Permian (~273 Ma), the Middle Permian (~263 Ma), the Late Permian (~252 Ma), the Middle Triassic (~237 Ma), the Late Jurassic (~201 Ma) and the Early Jurassic ( ~189 Ma).
As demonstrated by Figure 10, the paleo-highs/lows of the Maokou formation (P1m) presented significant spatial and temporal variations throughout the Sichuan basin. In the Early Permian (~273 Ma, Figure 10a), although the whole Sichuan basin was in a high position, the Maokou formation (P1m) did not present much fluctuation throughout the basin, with a couple of abnormal highs/lows in west rim of the basin due to the limited coverage of seismic data. In the Middle Permian (~263 Ma, Figure 10b), the Maokou formation (P1m) did not present much fluctuation as well, although several paleo-lows were observed in northeast Sichuan basin. In the Late Permian (~252 Ma, Figure 10c), the structural contours of the Maokou formation (P1m) started to fluctuate significantly compared to the Early-Middle Permian (Figure 10a, b), with multiple paleo-highs  appearing to the south of Chengdu-Suining-Chongqing and a northwest-striking lowland observed in the northeast Sichuan basin. In the Middle Triassic (~ 237 Ma, Figure 10d) and the Late Triassic (~201 Ma, Figure 10e), the Maokou formation (P1m) presented similar distribution of paleohighs compared to the Late Permian (Figure 10c), however, with a northeast-striking lowland developed in the west rim of the Sichuan basin since the Middle Triassic (Figure 10c,d).
In the Early Jurassic (~189 Ma, Figure 10f), on the one hand, the paleo-highs in the south Sichuan basin were continuously strengthened; on the other hand, a paleo-low was developed in the north Sichuan basin.
Plan view maps of paleo-highs/lows of the Changxing formation (P 2 ch) were also constructed, from the Late Permian (~252 Ma) to the Early Jurassic (~189 Ma, Figure 11a,d). The interactive observation of Figures 10,11 suggested that the Changxing formation (Figure 11a,d) presented spatial and temporal characteristics with high similarity to the Maokou formation (Figure 10c,f). Apparently, the areas of paleo-highs in the south Sichuan basin were continuously strengthened from the Late Permian to the Early Jurassic. However, the areas of paleolows presented significant variations. In the Late Permian (Figure 11a), the lowland (in blue) was distributed in the north rim of the basin. Since the Middle Triassic (Figure 11b), the primary lowland was migrated to the west rim of the basin, which was also inherited in the Late Triassic ( Figure 11c) and the Early Jurassic (Figure 11d). In addition, a highland was also developed in the north rim of the Sichuan basin in the Early Jurassic (Figure 11d).

Impacts of tectonic evolution on distribution of plaeo-highs/lows of permian strata
The Sichuan basin, located on the northwestern rim of the Yangtze plate, was highly affected by the convergence between the Yangtze plate and its neighbouring plates. From late Palaeozoic to Mesozoic, the closure of the Paleotethys ocean and plate collisions dominated tectonic activities occurring in the Sichuan Basin.
During the Permian, to south of the Yangtze plate, the Paleotethys ocean had been closing due to northeast subduction towards the Yangtze plate (Sengör and Hsü 1984;Zhang et al. 1999;). Meanwhile, the Figure 13. The distribution of paleo-highs/lows and the discovered marine gas fields in the Permian strata: (a) paleo-geomorphology map of the P 1 m in late Triassic; (b) distribution of the Kaijiang-Liangping rift and discovered gas fields; (c) paleo-geomorphology map of the P 2 ch in early Triassic; and, (d) distribution of the Luzhou-Kaijiang paleo-uplift and discovered gas fields.
Emeishan mantle plume was intruded in the southern portion of the Yangtze plate in Middle Permian (Figure 12a). In this period, to north of the Emeishan mantle plume, the back-arc portion was still dominated by tensional stress field. This resulted in the nonuniform distribution of paleo-highs and paleo-lows in ancient Sichuan Basin in Late Permian, corresponding to the paleo-tectonic maps in Figures 10c,11a.
In Early to Middle Triassic, to north of the Yangtze plate, the South Qinling ocean was closed to form the Mianlue suture zone, resulting in collision between the Yangtze plate and the North China Plate. A northeast trending foreland basin, together with postcollisional ganite, was developed in southeast of the Mianlue suture zone (Figure 12b). Meanwhile, to south of the Yangtze plate, the Qiangtang plate moved northeastward, forming the Jinshajiang subduction zone between the Qiangtang plate and the Yangtze plate.
In late Triassic, the tectonic setting of the Yangtze plate was changed significantly due to the successive convergence of the North China plate to the north, the Qiangtang plate to the south, and the ancient Pacific plate to the east (Figure 12c). To south of the Yangtze plate, being controlled by the collision between the Qiangtang plate and the Yangtze plate, the Jinshajiang suture zone was developed. In the junction area among the North China plate, the Qiangtang plate and the Yangtze plate, the Longmenshan orogen was uplifted, giving rise to the development of a foreland basin in its east (Yong et al. 2003;Jia et al. 2006;Tang et al. 2011;Hang et al. 2015). In the east of the ancient Sichuan basin, the Luzhou paleo-uplift was also developed as the forebulge of the foreland basin. This was validated by the distribution of lows in the northwest and highs in the southeast of the Sichuan Basin (e.g. Figures 10e and 11c).
In Early and Middle Jurassic, to northeast of the Sichuan Basin, the Dabashan orogen was uplifted due to the successive collision between the North China plate and the Yangtze plate (Sheng et al. 2007;Dong et al. 2013). Consequently, a foreland basin was developed in the south of the Dabashan orogen. In this period, the paleo-highs were primarily distributed in south of the Sichuan Basin, whereas the west and north portions were dominated by paleo-lows (e.g. Figures 10f and 11d).

Petroleum implications of paleo-highs/lows on of the permian reservoirs
The Sichuan basin is one of the largest petroliferous intracontinental basins in western China. In the past decades, dozens of gas fields have been discovered in the Permian strata, including the Maokou formation (P 1 m) and the Changxing formation (P 2 ch). Previous studies (e.g. Guo et al. 2014;Wu et al. 2019;Huang et al. 2019a;Ma et al. 2020) proposed that the hydrocarbon accumulation patterns of the Permian strata present the following features, including, (i) multiple hydrocarbon supplies, (ii) near-source accumulation, (iii) lithology reservoirs, (iv) oil-gas conversion and (v) later adjustment. By integrating the outcomes of petroleum exploration and the paleo-highs/lows reconstruction, it is believed that the distribution of discovered marine gas fields is highly coupled with the paleo-highs/lows in critical tectonic periods, in particular, the gas fields in the Maokou formation that developed in the Luzhou-Kaijiang paleo-uplift (Figure 13a, b) and the gas fields in the Changxing formation that developed surrounding the Kaijiang-Liangping rift (Figure 13c, d).
(1) The Maokou formation and the Luzhou-Kaijiang paleo-uplift In Middle Permian, being controlled by the Dongwu movement, the southern Sichuan basin was uplifted. The Maokou formation in the southern basin was sitting in a shallow water environment (Xiao et al. 2015) and that resulted in a high level of karstification in the upper limestones of the Maokou formation (Xu et al. 2017). Huang et al. (2019b) suggested that the karstification happened in the Maokou formation was eogenetic karst, forming a large number of dissolved fissures and caves. In late Permian, being controlled by the Emei taphrogenic activity (He et al. 2006), the southern Sichuan basin was significantly uplifted on an overall falling background. In this period, a series of vertical joints or fractures were developed, providing effective conduits for karst water transportation, promoting the expansion of karst fractures and caves in the Maokou formation. From late Triassic to early Jurassic, the Luzhou-Kaijiang area was significantly uplifted due to the Indosinian orogenic activity. As a consequence, tectonic faults were highly developed within the Luzhou-Kaijiang paleo-uplift, which could probably provide effective conduits for vertical fluid migration . In Jurassic, geometrically, the Luzhou paleo-uplift could be considered as a large anticline trap. In this period, the argillaceous limestone of the Liangshan formation (P 1 l) and the mudstone of the Silurian Longmaxi formation in the Luzhou area were at the peak of oil generation (Supp .  Table 1), providing abundant hydrocarbon through the faults to the overlying Maokou formation within the Luzhou paleo-uplift (Figure 14a).
(2) The Changxing formation and the Kaijiang-Liangping rift Although the paleo-geomorphological maps of the Maokou formation (P 1 m) and the Changxing formation (P 2 ch) presented a high level of similarity ( Figures. 10  and 11), the Changxing formation presented a different pattern of petroleum migration and accumulation compared to the Maokou formation.
From late Permian to early Triassic, being influenced by the Emei taphrogenic activity (Wu et al. 2019;Ma et al. 2020), the northern Sichuan basin had experienced a NE-SW extension, forming a large-scale NW striking Kaijiang-Liangping rift. The eastern side of Kaijiang-Liangping rift was control by large contemporaneous faults, but the western side of the rift mainly showed stratigraphic overlap on the slope with fewer contemporaneous faults. However, large contemporaneous faults and the stratigraphic overlap both were well migration pathway (Mao et al. 2011). As a consequence, thick organic reefs of the Changxing formation were developed in both the north and south edges of the Kaijiang-Liangping rift, with vertical accretion growth, multi-phase superposition, and limited distribution along the steep slopes. The mudstones of the Dalong formation (P 2 d), high-quality source rocks, were then deposited in the centre of the Kaijiang-Liangping rift . In addition, the gypsum layers of the Jianglingjiang formation (T 1 j) above the organic reefs can perform as high-quality cap rocks . The oil generation of the Dalong formation provided abundant petroleum sources for the Changxing formation (Supp. Table 1). The oil was then laterally migrated northward and southward, to form the reef-bank gas fields along the two edges of the Kaijiang-Liangping rift (Figure 14b).

Limitations
In this research, the erosion amounts were calculated by employing an optimized seismic method considering the internal stratigraphic textures (Figure 4), which provided minimum values of erosion in each period. In another word, the real erosion amounts might be, to some extent, higher than what we calculated. However, as the limited wells and geochemical data could not guarantee the erosion map construction of the whole Sichuan basin, it was reasonable to employ the optimized seismic method for erosion calculation in this research.
In addition, given the limited seismic data used in erosion calculation and paleo-geomorphology reconstruction, the accuracy of both the 3D models and plan view maps was still potential to be improved in the future. On one hand, both the quantity and quality of the seismic sections resulted in a moderate level of area coverage in the Sichuan basin, which limited the resolution of 3D models and plan-view maps. On the other hand, as the majority of the seismic sections were not long enough to cover the basin boundaries, the sims of the basin were not properly constrained, with some abnormal values observed in the rims of the maps ( Figures. 9-11).

Conclusions
1. During the Permian, the erosion amount of the Sichuan basin was low due to the marine sedimentary environment. But there were few portions with higher erosion due to tectonic activity. However, since Triassic, the erosion distribution of this basin presented significant highs and lows, because of the change from marine to continental sedimentary. In addition, there was a huge gap between the erosion values of highs and lows.
2. From early Permian to middle Triassic, being affected by the Emei trphrogenesis, the southwest portion of the Sichuan basin was featured with paleo-highs, while the northeast portion was dominated by paleo-lows (e.g. the Kaijiang-Liangping rift). Since late Triassic, being affected by the orogenic activities in the Indosinian movement (e.g. the Longmenshan orogen), the paleo-geomorphology of the Sichuan basin varied horizontally, with the southeast and northwest portions dominated by paleo-highs (e.g. the Luzhou-Kaijiang paleo-uplift) and paleo-lows (e.g. the west Sichuan foreland basin), respectively.
3. The control of paleo-highs/lows on hydrocarbon accumulation can be divided into the following two aspects: (i)The location of paleo-highs/lows controlled the distribution, type and quality of reservoirs; (ii) The paleo-highs/lows controlled the hydrocarbon accumulation patterns.

Highlights:
• The basin-scale erosion distribution was quantitatively calculated based on unconformable contact • The model of paleo-uplifts/lows of the Permian strata were reconstructed at key geological periods • The distribution of paleo-uplifts/lows may present important hints for hydrocarbon exploration