The northwestern Sichuan Basin, one of the important areas for oil and gas exploration in the Sichuan Basin, contains several sets of source rocks in the Lower Cambrian, Permian and Upper Triassic^[1,2,3,4,5] and various reservoir rocks with marine or terrestrial phases^[6,7,8]. In the drilling cores and surface outcrops of the northwestern Sichuan thrust belt, the Paleozoic and Mesozoic have abundant hydrocarbon shows and the Sinian and Lower Paleozoic contain a large number of asphalt veins^[9,10], demonstrating good exploration prospects of oil and gas in this region. From the 1950s to the beginning of 21st century, the oil and gas exploration focused on the Mesozoic terrestrial strata inside the basin and the Sinian-Paleozoic marine strata in the surrounding thrust belts. In 1972, the drilling activities in the Zhongba anticline in the front of the northern Longmenshan Mountain discovered two sets of natural gas reservoirs, the Upper Triassic Xujiahe Formation and the Middle Triassic Leikoupo Formation^[11,12], leading to the understanding that the Indosinian paleo-tectonics had great influences on the hydrocarbon accumulation. Then several structural oil pools including Jiulongshan (1976), Hewanchang (1988) and Shejianghe (1995) were discovered in the front of the thrust belt, in which industrial gas flows were tapped in the Jurassic (Zhenzhuchong Formation), the Triassic (Xujiahe Formation, Leikoupo Formation, and Feixianguan Formation), the Permian (Wujiaping Formation, Maokou Formation, Qixia Formation, etc.) and the Lower Paleozoic (Silurian-Ordovician)^[13,14]. As the exploration went on in this area, the target strata shifted from the shallow ones to deep ones constantly, but it is more and more difficult to find large scale oil and gas reservoirs. Since 2000, advancements in petroleum geology and exploration techniques for the foreland thrust belt^[14] and marine carbonate rock^[15], have once again propelled the hydrocarbon exploration in the northwestern Sichuan Basin and surrounding fold-thrust belts. Some wells were drilled successively in the structures of Kuangshanliang (2003), Wujiaping (2005), Tianjingshan (2006), Jiulongshan (2007) and the marine platform reefs of Longgang area (2008), part of which obtained industrial natural gas flow in the Upper Triassic (Xujiahe Formation) and the Permian (Maokou Formation and Changxing Formation)^{[7, 16]}. Especially since 2011, some historic discoveries have been made in the natural gas exploration in central Sichaun Basin and northwestern Sichuan Basin, including the Upper Sinian Dengying Formation reservoir in Well Gaoshi-1, the Longwangmiao Formation reservoir of the Lower Cambrian in Well Moxi-8, and the Qixia Formation and Maokou Formation reservoirs in the Shuangyushi area (Well Shuangtan-1)^{[6, 17]}, demonstrating the deep-buried reservoirs in the Sichuan Basin have huge natural gas exploration potentials. However, due to the extremely complicated structural conditions in the northwestern Sichuan Basin and limited data available with poor quality, it was difficult to identify and understand the deep structures, seriously obstructing the deep gas exploration in this area. In recent years, the increase of seismic survey and drilling and deepening of oil and gas prospecting provided us the conditions to further study the deeply and ultra-deeply buried beds in this region. As we know, the northwestern Sichuan Basin had experienced a complicated tectonic evolution, especially the Mesozoic-Cenozoic activities of compressing and thrusting, and formed complex structures and stratigraphic distributions. Therefore, a systematic and reasonable understanding of geological characteristics on tectonic evolution, structural geometry and stratigraphic distribution is the key for the deep and ultra-deep oil and gas exploration and production. Based on integrated interpretation and analysis of seismic, drilling and geological data, this paper discusses the geometric characteristics, tectonic evolutions and favorable exploration belts, aiming to enhance the reliability of hydrocarbon evaluation and prediction and provide primary structural understandings for regional geologic study and deep gas exploration.

The study area is located in the northwestern area of the Sichuan at the junction of the eastern margin of the Qinghai-Tibet Plateau and the southern front of the Qinling orogenic belt. Subjected to multi-stage compressional tectonics since the late Indosinian, the area has complex surface geological structures characterized by thrusts and folds (Fig. 1a). The collision between the South China Plate and the North China Plate during the Indosinian period gave rise to the Longmenshan thrust belt and the western Sichuan foreland basin, which latterly suffered the intracontinental transformation during the Yanshanian and Himalayan periods^{[18,19,20,21]}. In particular, the thrusting and uplifting of the eastern margin of the Tibet Plateau in late Cenozoic caused the complex fold-thrust structures and basin-mountain landforms in the northwestern Sichuan region^[22,23,24]. Adjacent to the northern Longmenshan fold-trust belt in the west and the southern Micangshan fold-trust belt in the north, the northwestern Sichuan Basin has rows of fold belts. By observing surface structures, the folds and faults have 3 strikes (Fig. 1b). The first is the NE strike represented by Indosinian (T3) thrusting structures such as the Kuangshanliang, Tianjingshan and Zhongba anticlines in the front of the northern Longmenshan; the second is the nearly EW strike represented by Yanshanian (J₃-K₁) fold-thrust structures such as the Dalianghui and Wujiaba anticlines; the third is the NEE strike represented by rows of fold belts in Guangyuan-Bazhong area which has an angle of 10° to 15° clockwisely from the former Indosinian thrusts, including the anticlines of Hewanchang, Shejianghe, Tongziguang, Jiulongshan, Fuyangchang, and the synclines of Cangxi and Zitong. Generally, the northern part of the northwestern Sichan Basin experienced a strong later compressing and uplifting and formed fold rows with a consistent strike which involved the Jurassic and Lower Cretaceous, while the southern part relatively showed no obvious folds and thrusts inside the basin, indicating the later compressive strength varying along the NW-SE strike.

The northern Longmenshan and the Micangshan fold-thrust belt are showing the strong thrusting and folding deformation outcropped with the basement, Sinian, Paleozoic and Triassic. The northwestern Sichuan basin interior mainly exposes the Jurassic and Lower Cretaceous in surface, where the structures are clearly controlled by the northern Longmenshan (Fig. 1b). From the hinterland (west) to foreland (east) of the northern Longmenshan fold-thrust belt, the thickness and composition of the strata are significantly different between the over thrust nappes and the basin margin (Fig. 1b). Allochthonous nappe structural unit, on the northwest side of the Majiaoba fault, consists of the thrust nappes of Tangwangzhai and Jiaoziding with the main sedimentary rocks from the Sinian to the Carboniferous. In this allochthonous unit, the Ordovician, Silurian and Devonian sediments had been successively developed and experienced low-grade metamorphism, among them the Silurian and Devonian had the maximum cumulative thickness up to 10 000 m ^[21]. In the thrust belt on the southeast side of the Majiaoba fault, Tianjingshan anticline and its southwestern area have no Ordovician and Silurian remnants and only the Middle-Lower Devonian with a thickness of 100-200 m exposes. In the Kuangshanliang and Nianziba structures in the northern segment, only 150 m thick Middle Silurian, 15-30 m thick Middle Ordovician Baota Formation and 175 m thick Middle-Lower Devonian crop out. Well Heshen-1 drilled on the Hewanchang anticline at the northern edge of the basin also only encountered 594.5 m thick Middle Silurian, 28 m thick Middle Ordovician Baota Formation and 82.5 m thick Middle-Lower Devonian. The distribution of these strata reflects that the northwestern Sichuan Basin experienced complex tectonic evolution, uplifting and denudation, and sedimentation in the Paleozoic. The Permian and overlying strata, especially the Mesozoic, are relatively continuous and mainly suffered from multi-stage later-period compressing and folding, giving rise to near-surface thrusts and folds. In summary, the northwestern Sichuan Basin has experienced the tectonic evolution of the craton Basin from the Neoproterozoic to Early Mesozoic and the multi-period intra-continental tectonic activities since the Late Triassic, with a superimposed sequence of marine and terrestrial strata developed (Fig. 2). The Sinian to Middle Triassic are mainly composed of marine deposits on the craton carbonate platform and passive continental margin, and the Upper Triassic to Lower Cretaceous are of deposits in the non-marine foreland basin and intra-continental depression basin. The marine mud shale in the Cambrian and Permian and the coal beds in the Upper Triassic Xujiahe Formation constitute the effective source rocks in this region. The carbonate in the Sinian and Paleozoic and sandstone in the Devonian and Mesozoic serve as good reservoirs. At present, oil and gas explorations are mainly focusing on the autochthonous thrust belts at the eastern side of the Majiaoba fault and the deeply buried Sinian-Paleozoic marine strata inside the basin. Because of the complex multi-stage tectonic evolution, the spatial distribution and structural deformations of the remnant strata in the basin deep remain unclear, which will be the focus of following discussion.

Based on the seismic profiles, three regional cross-sections of the northwestern Sichuan Basin were constructed to illustrate the structural features in front of the Micangshan and the northern Longmenshan Mountain (Figs. 3-5). Fig. 3 shows the structures in the NS direction from the Dalianghui anticline in the southern front of the Micangshan to the Longgang area inside the basin. Figs. 4 and 5 show structural features in the EW direction from the front of the northern Longmenshan to the eastern area of the Jiulongshan anticline.

The seismic profile of Fig. 3a is splicing together with three seismic lines, which cuts through the south front of Micangshan, the Jiulongshan anticline and the eastward flank of the Zitong syncline from north to south. The structural interpretation results show that the southern front of the Micangshan develops an upper monocline structure and a deep buried base-involved imbricates, which are separated by a regional distributed salt detachment in the Lower Triassic Jialingjiang Formation (Fig. 3b).

The southern Micangshan fold-thrust belt has deeply buried active thrusting and passive roof detaching. The passive roof detaching mainly refers to the back thrust and uplift of the Mesozoic strata above the salt layer of the Jialingjiang Formation. Its structural deformation is mainly concentrated in the monoclinic zone of the south flank of the Dalianghui anticline. And there is no obvious displacement and deformation propagating forward into the basin via the detachment (Fig. 3). The upper structural layer above the salt detachment has no visible thrust faults and shows simple complete configuration of the syncline and anticline consistent with the occurrence of the detachment. The deformation position and strength are mainly controlled by the deep structures under the detachment. And the folding deformation gradually weakens and disappears from the piedmont to the basin inside.

In the southern front of the Micangshan belt, the strata under the detachment have undergone strong shortening and active thrusting. The deep thrusting is transmitted from the Micangshan uplift to the basin, and the overall performance is characterized by the base involved imbricates. From south to north, the deep-buried Sinian steps up and finally exposes out of the surface, with structures including the platform margin of the 4^th Member of Upper Sinian Dengying Formation, Jiulongshan anticline, Huangyangchang thrust belt, Dalianghui and Yanzixia anticlines etc developed. The deep deformation of the southern Micangshan belt is represented by the basement involved folds and thrusts. There are several major thrust faults cut steeply into the basement with high dip angles and large vertical and small horizontal displacements. All of them cut upward and disappear in the Lower Triassic salt detachment layer. Therefore, several imbricate thrusting fault blocks came about under the upper monocline in the southern Micangshan belt. And each fault block contains secondary forward or backward thrust faults. Thin-skinning thrust happened in local part with the Cambrian shale acting as another detachment layer, making the faultblocks more complex and fragmented. In Fig. 3, the deep structure of Jiulongshan is a gentle anticline higher in the north wing and lower in the south wing, and the underlying fault controlling its formation is located in the deeper basement. Based on the geometry of horizons and the angular unconformity, it is considered that the current structural pattern of the Jiulongshan anticline is mainly controlled by the late compression since the Late Cretaceous. The Zitong syncline in the south side has generally stable formation structure and no visible thrusting and folding deformation, and shows deep-buried structures of paleo-highs and paleo-depressions only experienced the later transformation processes of burial, uplifting and erosion.

The deformation characteristics of the northwestern Sichuan belt are showed from west to east via two blank seismic images and their structural interpretations (Figs. 4 and 5). Generally, the northern Longmenshan frontal belt can be separated by a blind major thrust fault (Fig. 5, F₁) into two structural units including the hanging wall of para-autochthonous thrusts (such as the Tianjingshan anticline^[25]) and the footwall of in-situ structures. There are two regional detachments situated respectively in the Lower Triassic Jialingjiang Formation (Figs. 4 and 5, upper detachment) and the Lower Cambrian (Figs. 4 and 5, lower detachment)^[26,27]. The in-situ structural unit of the Longmenshan front belt shows an obvious multi-level deformation with the upper folds, the intermediate thin-skin thrusts and the deeper basement involved folds (Figs. 4 and 5).

The upper folds refer to the folds in the Mesozoic strata above the Lower Triassic salt detachment, including the piedmont monoclinic belt, the Shuangyushi anticline, Hewanchang anticline, Tongziguan anticline, Jiulongshan anticline, Guangyuan syncline, and Tongzi syncline etc (Figs. 4 and 5). The upper structural layer has no obvious displacement and transfer and thrusting forward into the basin via the upper detachment, and has similar geometry on the whole with the folded upper detachment and only some small-scale salt detachment folds in the east of the Jiulongshan anticline (Fig. 5).

The middle structural layer includes a series of forward and backward thin-skin thrust structures located between the upper and lower detachments, the most typical ones are the Shuangyushi and the Hewanchang anticlines, with pop-up and pop-down structures developed (Figs. 4 and 5). The long-dis- tance basinward propagation of thrust and the non-imbricate structure pattern indicate that there is a detachment with lower friction coefficient below. The strata (part of Paleozoic and Lower Triassic) in this structural layer are more fragmented with small-size fault blocks. Moreover, the deformation of the middle layer is mainly concentrated between the hidden fault F₁ and the Tongziguan anticline, and there is no obvious detached structure in the middle-deep structural deformation layers inside the basin (Figs. 4 and 5). The range of this thin-skin thrust structure is consistent with the distribution of the thick Cambrian, indicating that the detachment in the Lower Cambrian affects primarily the development of the middle thin-skin structural layer in the west.

The structural layer under the Lower Cambrian detachment has several thrust folds in the basement from the north part of Longmenshan to the basin. In the north of the northern Longmenshan front belt, a number of faults cut into the basement and form several anticlines. The Sinian Dengying Formation is obviously deeper and deeper in burial depth toward the basin inside, which means the tectonic uplift is getting lower and lower (Fig. 4). In contrast, in the front of the Tianjingshan, there are no strongly deformed basement involved structures in the deeper structural layer, and the Jiulongshan anticline in the front also has weak deformation (Fig. 5). These phenomena reflect that the current rows of anticline and syncline structures in the northwestern Sichuan region were mainly controlled by the lately strong compression of the western Longmenshan Mountain. From the southeast to the northwest, the deformed basement gradually widens and becomes shallower in burial depth, which reflects the enhancement of the late compressional strength from south to north, which caused the multi-row folds inside the basin and the complex structure with superimposed deformations in the northwestern Longmenshan frontal fold-trust belt.

The Micangshan thrust belt appear as a large-scale fold structure with basement involved, where the vertical uplifting and southward thrusting gave rise to the EW striking Dalianghui anticline and the EW thrust belt in its southern front (Fig. 3). The west wing of the Micangshan belt has the overall appearance of a stable monoclinic structure plunging into the Longmenshan Mountain (Figs. 1 and 6). The seismic interpretation reveals that the northern Longmenshan fold-thrust belt is on the stable west wing of the Micangshan uplift and has clear features of multi-layer structural deformation (Fig. 6). The upper structural layer above the Lower Triassic salt detachment is the Guangyuan syncline which involves the Middle-Upper Triassic and Jurassic and presents as continuous folding deformation affected by the deeper structure. The middle structural layer is controlled by the deep-seated Lower Cambrian detachment and contains several small-scale thrust faults, and the width of the deformation zone is related to the distribution of Cambrian below. The deep structural layer has folds and thrusts involving the basement. All these phenomena reflect that the multi-stage compression tectonism led to the thrusting structure and deformation in the front of the northern Longmenshan fold-thrust belt.

There were at least three stages of strong compression tectonic activities including the Late Triassic, Late Jurassic-Early Cretaceous and Late-Cenozoic in the northwestern Sichuan region^{[28,29,30,31]}. The Late Triassic compression is mainly concentrated in the northern Longmenshan belt characterized by strong thrusting and folding in outcrops, the Kuangshanluang-Tianjingshan-Zhongba structural belt is the front of the thrust. The EW striking folds in the south front of the Micangshan belt may be formed during the Late Jurassic-Early Cretaceous, for the foredeep in front of Micangshan Mountain is filled with thick Upper Jurassic and Lower Cretaceous monastic deposits^[19,20]. The regional uplifting and extrusion of the Late Cenozoic Tibet Plateau caused the current topography and geological structures in the northwestern Sichuan Basin and adjacent areas, strongly reshaping the earlier northern Longmenshan and Micangshan thrust belts. This late tectonic process led to the formation of rows of anticlines and synclines in NEE strike controlled by the faults rooted in basement and caused strong deformation involving all sedimentary caprocks and former structures.

In summary, the frontal thrust belt of the northern Longmenshan is an independent deformation system developed on the west wing of the Micangshan uplift and is basically not affected by the Micangshan thrust belt. Whereas the Micangshan thrust belt is transformed by the late basement folding in the west from the northern Longmenshan belt, giving rise to the NEE striking large-scale thrusts and folds such as the Jiulongshan anticline and the Cangxi syncline (Fig. 4).

The northwestern Sichuan Basin has experienced a sophisticated tectonic evolution history from the marine Craton basin during the Sinian-Middle Triassic to the intra-continental basin during the Meso-Cenozoic, resulting in the complicated distribution of the old strata and geological structures (Figs. 7 and 2). Taphrogenic trough^[27], stair-stepping paleo-platform and platform margin developed during sedimentation of the Dengying Formation of the Upper Sinian, which is overlaid by the Lower Cambrian thin in the east and thick in the west (Fig. 7a and 7b), indicating that there are superior conditions for oil and gas accumulation in northwestern Sichuan Basin similar to Sinian-Cambrian Anyue giant gas field in the central Sichuan Basin^{[16, 27]}. At the end of Silurian, a tectonic uplifting caused the regional denudation and formed the Caledonian paleo-high in the west, where the Lower Cambrian is directly covered by the Upper Paleozoic (Fig. 7c, 7d). During the Late Paleozoic to Early Mesozoic, this area mainly had passive marginal basin and marine craton basin deposits (Fig. 7d). At the present location of the northern Longmenshan frontal belt, strata of the Devonian and Carboniferous deposited in local parts. And subsequent regional transgression formed the large scale Middle Permian and Middle Triassic carbonate platform. During the Late Paleozoic, the Dongwu rifting activity^[32] at the end of the Middle Permian caused the uplift and erosion of the Middle Permian and its later resting induced the overlying Kaijiang-Liangping marine trough during the Late Permian and the Early Triassic^[7]. As a result, this region developed several sets of dolomite reservoir and limestone karst reservoir in the Upper Paleozoic and high-quality source rocks in the Middle-Upper Permian. Afterward, the collision between the South China and North China plates and subsequent intercontinental tectonic activities formed the Indosinian northern Longmenshan fold-trust belt and frontal western Sichuan foreland basin (Fig. 7e), as well as the intercontinental depression basin of the Jurassic-Cretaceous (Fig. 7f). Lastly, affected by the Tibet Plateau Uplifting, the northwestern Sichuan region experienced the rejuvenated foreland folding and thrusting, finalizing into the complex geologic structure today (Fig. 7g). On seismic profile, the Mesozoic-Cenozoic above the upper detachment has simple folding deformation and clear stratigraphic sequence (Figs. 3-6). But the deep- buried strata under the upper detachment, which are the major targets for hydrocarbon exploration, including the Sinian Dengying Formation, the Lower Cambrian and the Upper Paleozoic, have mazy stratigraphic structure affected by above-mentioned multi-stage tectonic processes and activities.

In the study area, the top of Dengying Formation can be clearly identified on the seismic profile and shows obvious topographic relief (Figs. 5 and 7a). The Lower Cambrian is in angular unconformable contact with the Dengying Formation (Fig. 8a-c). In the east, around the Jiulongshan anticline, the Dengying Formation is the thickest, with the entire sequence from the 1^st to the 4^th members developed and is at the structural high (Figs. 7a and 8a-b). To the west Shuangyushi structure, the Dengying Formation thins gradually and the 3^rd member, the 2^nd member and the 1^st member crops out in sequence, showing apparent negative terrain (Fig. 8 a-c). The origin of the structure remains unclear, but there are several distinct structural and sedimentary characteristics: (1) Both the 2^nd and the 4^th member of the Dengying Formation have west-dipping scarps (the platform margin), in between the scarps, a gentle platform slope about 40 km wide crops out, west of the 2^nd member scarp only the 1^st member crops out and the 2^nd-4^th members of the Dengying Formation are almost missing completely. (2) There are no obvious underlying extensional structures such as pre-existing normal faults controlling the formation of these scarps, and the Dengying Formation in the Northwestern Sichuan mainly shows overall folding deformation. (3) The Lower Cambrian overlaps on different members of the Dengying Formation, thick in the west and thin in the east, and has a maximum thickness of more than 2 000 m.

The Dengying Formation is distributed along certain direction on the plane. The scarps (platform margins) indicated by the stratigraphic lines extend from south to north and appear as the westward pinch-out of different members of the Dengying Formation (Fig. 9a). The Dengying Formation in the east area, along the direction of Wangcang-Langzhong in front of the Micangshan is complete, for example, in Zitongguan anticline and Jiulongshan anticline, and shows paleo-uplift frame (Fig. 7a).

From the Late Silurian to the Early Devonian, the west part of the northwestern Sichuan uplifted, resulting in intense denudation of the Paleozoic and the formation of the paleo-high and the regional unconformity (Figs. 7b and 8a). The Ordovician and Silurian gradually uplift to the west and pinch out near Shuangyushi structure, forming wedge-shape residual strata (thin in the west and thick in the east) (Fig. 8b). In some parts, the Permian directly covers on the Cambrian even the Lower Cambrian, with the absence of most Ordovician-Carboniferous (Fig. 8b-c); the residual Devonian-Carboniferous exist locally in the western margin (Fig. 8c-d), indicating a Paleo-uplift occurred in the Northwestern Sichuan region at the end of the Early Paleozoic. That there are no obvious syn-tectonic folds and thrusts under the unconformity at the bottom of the Devonian or Permian suggests the regional vertical uplift and denudation. The Paleo-uplift strikes north- south on the whole, with Cambrian, Ordovician and Silurian exposing in sequence from west to east (Fig. 9b). The residual Ordovician and Silurian mainly are distributed in front of the Micangshan and in the Guangyuan area in front of the northern Longmenshan, with a maximum thickness of 1 000 m. The north Longmenshan section from Jiangyou to Majiaoba was the structural high of the paleo-uplift, where the Middle-Upper Cambrian, Ordovician and Silurian are all eroded (Figs. 8c and 9b).

During the Paleozoic, regional extension of the Paleo-Tethys Mianlue Ocean controlled the evolution of the passive continental margin basin in the northwestern Sichuan area^[33]. During this period, the northwestern Sichuan area experienced several tectonic stages including the Late Paleozoic intercontinental extension, the Middle-Late Devonian to Early Carboniferous initial ocean, the Carboniferous-Early Permian limited ocean, the Middle Permian-Middle Triassic subduction of the ocean crust, and the Middle-Late Triassic intercontinental collision and orogeny^[33]. Part of the Devonian and the Carboniferous deposited on the earlier Caledonian paleo-high, thinning out from northwest to southeast (Fig. 8c-d). At the core of the Caledonian paleo-high, the Lower Cambrian is directly covered by the Devonian, and the two are in obvious unconformable contact (Figs. 8c and 9b). The remnant Devonian-Carboniferous are distributed along the Jiangyou-Guangyuan in the north Longmenshan thrust belt and basin margin, but absent inside the northwestern Sichuan Basin and around Micangshan (Figs. 1 and 9b). Since the Middle Permian, the northwestern Sichuan area entered a stable carbonate platform deposition stage, when regional transgression occurred and formed hugely thick carbonate rock (Fig. 7d). And the Middle Permian platform margin belt developed along the northern Longmenshan (Fig. 10). During this period, this basin underwent the Dongwu movement of the Late Permian^[32], resulting in the subsequent differential sedimentation and erosion, the karst reservoirs of the Maokou Formation of the Middle Permian and the Kaijiang-Liangping Trough from the Late Permian to the Early Triassic.

The deep-buried favorable exploration reservoirs in the northwestern Sichuan Basin actually present zoning characteristics (Table 1). The Sinian-Lower Paleozoic is distributed in the central-eastern region, the Upper Paleozoic in the southwestern region, and the 2^nd member of the Dengying Formation and the Paleozoic mainly in the northwest region. The favorable belts for hydrocarbon exploration in the northwestern Sichuan include the Sinian paleo-platform and marginal belts in the east, the Upper Paleozoic in-situ blind thrusts and folds in the west, as well as the Lower Paleozoic paleo-slope in the central region.

Nowadays, the Upper Sinian Dengying formation is one of the key target beds for hydrocarbon exploration in the Sichuan Basin. The 2^nd and 4^th members of this formation contain the dolomite reservoirs of high quality^{[6, 26]}. In northwestern Sichuan, the Dengying Formation is mainly distributed in the inside and north part of the northwestern Sichuan Basin. The most favorable structure for exploration is the Jiulongshan anticline structural belt, where the Dengying Formation has been on the structural high and covered by the thick Lower Cambrian caprock, and the anticline formed late has been well preserved (Fig. 7). Especially in the west and south of the Jiulongshan area, there is a prominent platform margin belt of the 4^th members, and Lower Cambrian source rocks in the western depression, constituting lateral and vertical oil and gas migration and accumulation conditions (Fig. 8a). The second best area is the Yanting slope belt, where the entire 1^st-4^th Members and platform margin structure developed, and the slope belt has been stable tectonically, the only disadvantage is that the belt uplifts toward north, which is not favorable for in-situ oil and gas accumulation. The third favorable area is the relatively shallow-buried frontal thrust belt along Guangyuan to Wangcang in the north part of the basin, including the structures of the Kuangshanliang, Shilongliang, Tongziguan, and so on. In this belt, the northern segment of the northern Longmenshan mainly has the 2^nd member of Dengying Formation, while the southern frontal belt of the Micangshan has the whole Dengying Formation. However, the Dengying Formation close to piedmont appears in complex fault blocks or nose structures, with severe deformation and poor preservation conditions. Therefore, it is necessary to confirm the target traps in this belts.

The favorable reservoirs in the Lower Paleozoic in Sichuan Basin include the Canglangpu Formation and the Longwangmiao Formation of the Lower Cambrian, the Middle-Upper Cambrian and the Baota Formation of the Ordovician. Among them, the Longwangmiao Formation revealed by the Anyue gas field in Central Sichuan is the main target layer^[17]. As shown in Fig. 8, the depression on the top of Dengying Formation is filled up by the Maidiping and Qiongzhusi formations of the Lower Cambrian. The Maidiping and Qiongzhusi formations are wedge shape and compose the main source rocks. After that, the overlying strata of the Cambrian-Silurian have sediments in roughly uniform thickness, and show differential distribution in the EW direction after the denudation at the end of earlier Paleozoic (Fig. 8b-8c). The Longwangmiao Formation thins out westward, reflecting the micro-terrain high in the west and low in the east during the deposition of Longwangmiao Formation. According to the basin evolution process in Fig. 7, we think that Caledonian West Paleo-high controlled the distribution of the Lower Paleozoic and the structural high was the east slope of the paleo-uplift before Middle Triassic; during the Middle-Late Mesozoic, the structural high was at the Shuangyushi structure and Zitong synclinal belt (Fig. 7f); finally, after the Late Cenozoic compression, Jiulongshan area in the east became the structural high of the Lower Paleozoic (Fig. 7g). For hydrocarbon-bearing systems with Lower Cambrian source rock, the structural high of ancient structures are favorable sites for hydrocarbon accumulation only. Therefore, the favorable exploration areas of Paleozoic in northwestern Sichuan are the Shuangyushi structural belt and deep formation in Zitong syncline, which were located at structural high in late Paleozoic and Mesozoic, on the direction of oil and gas migration and accumulation.

The target reservoirs of the Upper Paleozoic include the sandstone of the Devonian Jinbaoshi Formation and dolomite of the Devonian-Carboniferous and the Middle Permian. According to the distribution of the residual strata and the favorable sedimentary facies, the most favorable reservoirs must be along the northern Longmenshan frontal belt (Figs. 9b and 10), especially the area west and south of the Shuangyushi structure. In this area, the Lower Cambrian is directly covered by the Upper Paleozoic and faults and fractures can connect deep source rock. The deep-buried reservoirs and structures kept stable during the earlier geological history. And under the late Himalayan tectonic compression, the thin-skinned thrust belt was formed above Lower Cambrian detachment in the front of Longmenshan fold-thrust belt, which is the key hydrocarbon exploration target of the Upper Paleozoic.

The northwestern Sichuan Basin has experienced the tectonic evolution of marine Craton basin from the Sinian to the Middle Triassic and developed distinctive strata distributions and sedimentary structures at several key stages. The Sinian Dengying Formation is thin in the west and thick in the east on the whole and has two platform-platform margin in the 2^nd and 4^th members, controlling the east uplift and west depression. Above the Dengying Formation deposited the Lower Cambrian thick in the west and thin in the east, indicating there is high-quality source rocks in the deep part of Longmenshan thrust belt and adjacent area. At the end of the Silurian, the west part of the area was an uplift, experiencing extensive uplifting and erosion, the Lower Cambrian exposed, and later was covered by the Devonian-Carboniferous or the Permian directly. From the Middle Permian to Middle Triassic, regional transgression gave rise to large-scale carbonate platform with the margin belt along the northern Longmenshan fold-thrust belt. Since the Late Triassic, the multi-stage tectonic compressions in different directions caused the foreland basin filled with terrestrial sequences and surrounding fold-thrust belts with vertically multi-level deformed structures at last.

The northern Longmenshan frontal belt includes the autochthonous nappe structural unit in the hanging wall and the autochthonous units in the footwall. The autochthonous thrust structures at the basin margin and inside basin show obvious multi-layer deformation separated by two detachment layers seated in the Lower Triassic and the Lower Cambrian, consisting of the folds in the upper layer, the thin-skinned thrusts in the middle layer and basement involved folds in the deep layer. Whereas the southern front of the Micangshan shows a structural pattern with two deformation layers controlled by the Lower Triassic detachment layer, including the upper back thrusting monocline and the deeper forward thrusting imbricate. Inside the basin, there are several rows of large-scale basement-involved fold belts, including Zitong syncline, Jiulongshan anticline, Cangxi syncline, Fuyangchang anticline, and so on.

There are several sets of source-reservoir-caprock assemblages in the northwestern Sichuan Basin in differential distribution on the plane, and the deep-buried exploration reservoirs have obvious zoning characteristic. The Upper Sinian Dengying Formation reservoir-cap assemblage is largely distributed in the east and north part of the northwestern Sichuan, where the Jiulongshan anticline and surrounding areas are the most favorable exploration belts. The Lower Paleozoic is mainly controlled by the Caledonian west paleo-high and transformed by late folding. The Zitong syncline and the Shuangyushi structure in the west part of the basin are the most favorable sites for the exploration of this assemblage. The Upper Paleozoic was controlled by the deposition at the continental margin, in the southwest part, the residual Devonian-Carboniferous and the Middle Permian platform margin may exist, so the front thrust belt of the north Longmenshan and the deep in-situ structural belt are favorable targets of the Upper Paleozoic assemblage. From the structural points of view, the favorable exploration belts for the deep formations of the northwestern Sichuan Basin mainly include the folds seated on the paleo-high, the hidden in-situ thrusts and the Lower Paleozoic paleo-slope belt controlled by the Caledonian paleo-high at the west margin.