In the past 10 years, inspired by the commercial development of shale gas in North America, a large number of shale gas geological surveys and drilling operations on the marine shale gas in South China have been carried out^{[1,2,3,4,5,6,7,8,9,10,11,12,13,14]}, and understandings and achievements on accumulation geological conditions including lithology, physical properties, gas content, geochemical, sedimentary and reservoir characteristics of shale, etc. have been obtained^{[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]}, and breakthroughs have been achieved in shale gas exploration in the middle-upper Yangtze area^{[10,11,12,13,14,15,16,17,18,19,20,21,22]}. However, industrial development of shale gas is limited in the Sichuan-Chongqing demonstration zone in Sichuan Basin and Zhaotong, Pengshui demonstration zones near the Sichuan Basin, and the target layer is also lim-ited to the Upper Ordovician Wufeng Fm.-Lower Silurian Longmaxi Fm. The Longmaxi Fm. and other strata (Lower Sinian Doushantuo Fm., Lower Cambrian Qiongzhusi Fm., Carboniferous-Permian shale) in the periphery of Sichuan Basin have not been commercially developed yet. The shallow shale gas in this study refers to shale gas reservoir buried depth less than 2000 m, which is characterized by continuous distribution of sweet spot, high proportion of adsorbed gas, slight overpressure, low wellhead pressure and production, long production period, small horizontal stress difference, and good fracability. The shallow shale gas in this study is slightly deeper than the conventional shallow natural gas (less than 1500 m deep)^[23] but shallower than the current commercially developed shale gas (sweet spot deeper than 2000 m)^[24,25,26]. At present, the evaluation of development areas and commercial development of shallow shale gas are generally in the exploration stage, which has severely restricted the promotion of shale recovery technologies in China’s shale gas development demonstration zone and progress of commercial development of marine shale gas outside the Sichuan Basin. Based on the geological characteristics and shale gas exploration and development history inside and outside Sichuan Basin, China and the United States^[15,16,17], and in consideration of the "over maturity, strong tectonic reformation and high shear stress" of the marine shale in South China, as well as key issues of long time diffusion and leakage of shallow shale gas, and difficult preservation in the complex structural conditions^{[17,18,19,20,21,22]}, the factors controlling marine shale gas formation and sweet spot in South China are taken as the keys in selecting shallow shale gas development area. Recently, breakthroughs have been made in shallow shale gas exploration in the Taiyang anticline area in the Zhaotong National Demonstration Zone, with more than 1000 ×10⁸ m³ of proven geological reserves submitted. Based on analyzing formation conditions of shallow shale gas and controlling factors of sweet spot in the Taiyang anticline area of Zhaotong, the achievements in drilling and production, the evaluation criterion for selecting shallow shale gas sweet spot has been proposed, in the hope to provide guidance for evaluation and selection of marine shallow shale gas sweet spots in the Zhaotong demonstration zone and in the complex structural areas outside Sichuan and Chongqing and boost exploration and development of marine shale gas in South China.

Recently, breakthroughs have been made in the shallow shale gas exploration in Taiyang anticline area in the northeast of Zhaotong demonstration zone, where the proven and controlled continuous gas-bearing area is about 580 km². This area is characterized by the mountainous landforms in the Yunnan-Guizhou Plateau, and tectonically located at the transitional zone between the low-steep fold belt in the southern Sichuan basin and Dianqianbei depression (north) (Fig. 1). Marine and terrestrial shale gas strata are developed in the area^{[24, 27-28]}, among which the marine strata include Sinian, Paleozoic, and Middle-Lower Triassic, and the continental strata include Upper Triassic and Middle-Lower Jurassic, with a cumulative stratum thickness of 6000-7000 m. In addition, Triassic strata usually crop out of the surface. There are mainly three marine shale formations, including the Lower Sinian Doushantuo Fm., the Lower Cambrian Qiongzhusi Fm., and the Upper Ordovician Wufeng Fm.-Lower Silurian Longmaxi Fm, and Wufeng Fm.-Longmaxi Fm. are main shale gas exploration layers^[26,29]. In the study area, the strata overlying the main exploration target reduce significantly in thickness compared to its periphery areas. The gas-rich shale generally has shallow burial depth, the buried depth of the main structure of the anticline is 500-1500 m, and the area with the buried depth < 2000 m accounts for 65% of the study area (Figs. 1 and 2).

The main target layer, Long1₁ sub-member is stable in the east-west direction but thins from north to south (Fig. 3). The Wufeng Fm. is composed of black graptolite shale and argillaceous shell limestone and has the small thickness of 2.0-6.0 m. The Long1₁ sub-member is made up of mainly black siliceous shale and carbonaceous shale, rich in pyrite and graptolite, and 20.0-40.0 m thick (Fig. 3). Based on lithology, electrical property, and graptolite fossil characteristics, Longmaxi Fm. is vertically divided into the 1^st Member (hereinafter referred to as Long-1 Member) and the 2^nd Member (Long-2 Member). Long-1 Member is further subdivided into 1^st sub-member (Long-1₁) and 2^nd sub-member (Long-1₂). The Long1₁ sub-member is divided into Long-1₁¹, Long-1₁², Long-1₁³, and Long-1₁⁴ sublayers (Fig. 4). Long-1₁¹ shale is rich in silicon and carbon^{[30,31,32,33]} and 2.0-3.0 m thick. Long-1₁² shale has high silicon and carbon contents^{[30,31,32,33]} and is 4.0-7.0 m thick. Long-1₁³ shale has high silicon content and medium carbon content^{[30,31,32,33]} and is 9.0-11.0 m thick. Long-1₁⁴ shale has medium silicon and carbon contents^{[30,31,32,33]} and is 12.0-16.0 m thick. All the sublayers basically thin from northwest to southeast (Fig. 3).

Regional stratigraphic correlation shows that the Wufeng Fm. in the demonstration zone is 5 m (YS108 well) to 10.8 m (YS128 well) thick in the west, 1.8 m (Y107 well) to 4.59 m (YS109 well) thick in the east, and 10.0 m (YS203 well) to 26.33 m (YQ8 well) thick in the south, and 1.8 m (Y107 well) to 4.59 m (YS109 well) thick in the north, indicating it is thinnest in the Taiyang anticline area. The main target layer, the Long-1¹ sub-member shale of the Longmaxi Fm. is relatively stable in thickness with little change, but it is thicker in the study area (30-40 m, Fig. 3). The overlying Long-1² sub-member is 95.46 m (YS112 well) to 143.33 m (YS111 well) thick in the west, 68.13 m (Y102 well) to 78.4 m (Y105 well) thick in the east, and 68.13 m (Y102 well) to 78.4 m (Y105 well) thick in the north, and 41.99 m (Shuangqiao) to 55.55 m (YS203 well) thick in the south, and it is thinnest in the Taiyang anticline area. The Long-2 Member is similar in characteristics with Long-1² sub-member (Fig. 2). The Lower Permian Qixia Fm. and Maokou Fm., the Lower Triassic Feixianguan Fm. and Jialingjiang Fm. all show a thinning trend towards the Taiyang anticline area. The Tongjiezi Fm. below the Lower Triassic Jialingjiang Fm. is dominated by argillaceous rocks, and varies to argillaceous limestone and merges with the Jialingjiang Fm. in the east Taiyang anticline area. The Upper Permian Emeishan Fm. basalt wedges out from west to east, and the Lower Silurian Hanjiadian Fm. thins most significantly (Fig. 2). It can be seen that the Long-1 ¹ sub-member shale is shallow in burial depth in the Taiyang anticline area (less than 2000 m in most part).

According to the data of 21 shale gas evaluation wells (Fig. 4), the sea level in the study area rose two times and fell two times during the sedimentary period of Longmaxi Fm.^[31,32,33]. The first transgression occurred in the sedimentary period of Long-1₁¹ and reached the maximum marine flooding surface at the end of the period. The regression happened in Long-1₁², Long-1₁³ and Long-1₁⁴ sedimentary periods, and the high level occurred at the end of Long-1₁⁴. The second transgression occurred in the sedimentary period of Long-1₂ sub-member, and reached the maximum marine flooding surface at the end of this period. The regression occurred in the sedimentary period from the Long-2 Member to the bottom of the Shiniulan Fm., and high stand systems tract occurred at the end of this period. The two times of sea level fluctuation show a rapid rise in the transgressive period and a slow decline in the receding period, reflecting the history of early transgression and late regression of the Longmaxi Fm.

The Late Ordovician to Early Silurian was a transitional period of sedimentary-structural evolution of the Yangtze Block. The study area was in the Sichuan, Chongqing-Hunan and western Hubei Caledonian foreland basin area held between the Jiangnan-Xuefeng orogenic belt formed in Caledonian orogeny in the south-east of the Yangtze Block and the middle Sichuan paleo-high in the northwest part. As the global sea level rose, the carbonate platform before the Late Ordovician was submerged, and the study area turned into epeiric sea type, water-retention deep-water shelf-reduction sedimentary environment, which provided good conditions for the reproduction of rich organic matter dominated by graptolite for Upper Ordovician Wufeng Fm. black shale and the Silurian Longmaxi Fm. black shale. In the early sedimentary period, the Longmaxi Fm. was dominated by terrigenous clastic rock, and the provenance was mainly from the Yunnan ancient paleo-land in the west part of the demonstration zone and the Qianzhong paleo- land in the south (Figs. 1 and 3). The Long-1₁ and Long-1₂ sub- members are dominated by a set of dark graptolite-bearing shale stably distributed, representing products under reduction conditions. In the late period, with the uplift of the southern paleo-land, regression occurred in the sedimentary period of the Long-2 Member, and gray shale and limestone interbeds of shelf facies developed. After the deposition of Longmaxi Fm., the sedimentary center of the study area gradually moved northward, and lithological differentiation occurred. In southeastern Sichuan area, Xiaoheba Fm. fine sandstone deposited^[34], which turned westward and southward to the Lower Silurian Luoreping Fm. siltstone and limestone, or the Shiniulan Fm. biolithic limestone and the marl intercalated with the shale facies. In the sedimentary period of the Middle Silurian Hanjiadian Fm., gray-green-gray sandy shale and sandstone deposited, with commonly purple-red shale at the bottom, which generally reflects the gradual regression of sea water. By the end of the Middle Silurian, the study area completely exposed out of water. Regional geological surveys show that the Silurian is in unconformable contact with Permian in the Taiyang anticline, and the Upper Silurian to Carboniferous strata are missing, indicating that the area suffered wide denudation^[35]. After the Hercynian period, the Taiyang anticline area received Upper Paleozoic and Mesozoic sediments gradually.

The Zhaotong demonstration zone is in the Wumeng Mountain area at the juncture of Yunnan, Guizhou and Sichuan. The main part is in the Dianqianbei depression (Fig. 1) in the periphery of the southern Sichuan Basin. The shallow shale gas exploration here started back in 2009, when Well YQ1, the first shale gas geological survey well was drilled at the beginning of the construction of the demonstration zone^[24]. As on one hand, the exploration area has complex structural geology and geographical conditions, making drilling and production of the medium-deep shale gas and factory-like operation more difficult and engineering investment and costs rise constantly^[25,26], on the other hand, shale gas occurs in the micro-nano pores in the poorly-connected shale reservoirs, it was considered that although the shallow buried marine shale gas in the study area had some vertical dissipation, under the sealing of the direct tight caprock, the shallow shale reservoir might still had some gas retained, in April 2017, Y1 well and Y102 well in the Taiyang anticline area with favorable shale gas enrichment and preservation conditions were rechecked for shallow shale gas layers at buried depth of less than 1 000 m (Fig. 1). The Long-1₁¹ and Long-1₁³ sublayers (at the burial depth between 768.5-778.8 m and 976.2-986.0 m respectively) in the Long-1₁ sub-member were volume fractured and tested for gas, obtaining an industrial gas flow of over 1 × 10⁴ m³/d, making a breakthrough in shallow shale gas exploration in the Zhaotong demonstration zone, and verifying the exploration potential of shallow shale gas in the mountainous area with complex structure. Based on the success of the above two old wells and re-evaluation of previous results of exploration in shallow shale gas in South China, two new evaluation wells of Y103 and Y105 (vertical wells) were deployed in the Taiyang anticline at the end of 2017, and they also obtained industrial gas flow after fracturing (Table 1). Productivity evaluation continued in early 2018, horizontal well of Y102H1-1 targeting Long-1₁ sub-member was drilled for productivity evaluation, which was tested a high production gas flow of 5.6 × 10⁴ m³/d.

After comprehensive shallow shale gas development area selection and target evaluation and upgrade of engineering technologies, 6 wells in 3 well-pad in the Taiyang anticline area were put into production test to evaluate their productivity in 2018. After further optimization of evaluation design and drilling engineering technology integration, 13 wells have been tested after fracturing, and all of them have obtained high industrial gas flow (Table 1).Analysis of field test and well test data (Tables 1-2 and Figs. 1-3) shows that the shale gas producing layers in the study area are vertically continuous and laterally stable and have no interlayers. Vertically, the shale gas producing layers are concentrated in Wufeng Fm. to the bottom of Long-1 Member, and the Long-1₁ sub-member has higher gas production than the Wufeng Fm. Laterally, the area with high-silicon content and carbon-rich shale and the anticline high point area have the highest production.Compared with the medium-deep shale gas in Huangjinba-Zijinba area which have put into development in the demonstration zone, the shallow shale gas in the Taiyang anticline area has five characteristics. a) Shallower burial depth and higher adsorbed gas content: the shale gas layers are generally 500-2000 m deep, and have a total gas content of 3.81-5.34 m³/t, 4.3 m³/t on average, which is similar with the medium-deep shale gas (4.4-5.9 m³/t). The gas is dominated by methane, with a methane content of 91.19%-98.98%, 95.94% on average. The gas has a dry coefficient C₁ /C_1-5 of 0.9866- 0.9986, 0.9939 on average, indicating the gas is typical dry gas in the over-maturity stage. The gas layers have higher adsorbed gas content of over 50% (Table 2), indicating the free gas in the shallow shale has significant dissipation. b) Slight overpressure: The gas layers have pressure coefficients of 1.2-1.6 generally, indicating slight overpressure. The wells with shale gas reservoir at the buried depth of 700-2000 m have wellhead pressures of 6-12 MPa, and wells with shale gas reservoir at the buried depth of over 2000 m have wellhead pressures of more than 20 MPa. c) Big thickness, stable distribution and good fracability: The shale layers with TOC over 1% are 50.3-57.0 m thickness, free of interlayer, continuous vertically and stable in lateral distribution; the shale layers have a brittleness index of 50%-65%, and porosity of 4%-9%, 5.67% on average, matrix permeability of 0.000 017- 0.044 353×10^-3 μm², permeability of (0.015 246-6.178 450)× 10^-3 μm² in fracture-rich area, representing the physical properties of medium-low porosity and ultra-low permeability; the shale layers have horizontal stress difference of 4-15 MPa, which is obviously lower than the medium-deep shale layers (15-25 MPa), so the shale has better fracability. d) Lower initial production of wells: Test data shows vertical wells and horizontal wells in these reservoirs have a production of (0.5-2.0)× 10⁴ m³/d and (2.63-20.70)×10⁴ m³/d, respectively, and the wells produce stably at (2-5)×10⁴ m³/ d in the initial stage. e) Stable production and low decline rate: Wells in the shallow shale gas reservoirs have a decline rate between 30% and 40% in the first year and long production life of over 30 years from prediction.

3.1.1. Developed caprock in shale gas area

The Silurian system exposes out of surface in the core area of the Taiyang anticline structure. The residual stratum overlying the main target reservoir of Long-1₁ sub-member is only 500 m thick. The residual strata increase from Long-1₂ sub-member, the Silurian Shiniulan Fm. and Hanjiadian Fm. at the core of the anticline to the Permian and Triassic at the flank of the study area, and increase in thickness from 800 m to more than 1500 m. Among them, the Shiniulan Fm. + Hanjiadian Fm. is a set of gray-dark and gray marlstone, gray-green and green-gray mudstone, and gray mudstone intercalated with argillaceous siltstone and brown-gray limestone, with wide distribution area and large cumulative thickness (600-900 m), and it acts as the regional cap and keeps the temperature and pressure field stable for the gas-bearing shale of the Wufeng Fm.-Long-1₁ sub-member in the study area^{[31, 36-40]}.

3.1.2. Developed shale reservoir with overall sealed system

The roof and floor of the gas reservoir of the Wufeng Fm.-Long-1₁ sub-member depositing continuously with the shale gas layers (Figs. 2 and 4) are characterized by tight lithology, large thickness, stable distribution, high breakthrough pressure, and good sealing property. The roof is the Long-2 Member gray-dark and gray medium-thick limy mudstone, argillaceous marlstone or siltstone, and argillaceous siltstone interbedded with thin layer silty mudstone, with an average porosity of 3.8% and an average permeability of 7.69 × 10^-8 μm². The floor is gray nodular marl, cryptocrystalline and micrite limestone of the Ordovician Linxiang Fm. and Baota Fm. with an average porosity of 1.58% and an average permeability of 0.0017×10^-8 μm². According to the physical properties, both the roof and floor of the shale reservoir are low-porosity, ultra-low permeability tight formations, which have provided good sealing and preservation for shale gas during formation of the shale gas reservoir and in the later tectonic reformation^{[31, 37]}.

3.1.3. Weaker tectonic reformation in late stage

The study area shows an anticline structural pattern of "sag around uplift" (Fig. 1), which is characterized by "inherited fold deformation and multi-stage weak fault reformation". Specifically, anticlines and synclines are adjacent to each other, and the structures are the Xuyong syncline, the Taiyang anticline, the Yunshanba syncline, the Baiyangping syncline, and Haiba anticline from north to south in the study area. The faults of multiple directions and orders superimpose, and the near EW reverse faults and near NS strike- slip faults take the majority^[28,41].Taiyang anticline started to develop in the Caledonian period. After the Indosinian period, affected by the NS compression stress, it became a near EW trending anticline, with near EW trending thrust faults formed at the top and flanks. With continuous compression after the Yanshanian period and compressional strike-slip in Himalayan period, the anticlinoria deformed further progressively, and the early near EW anticlinoria was superimposed and reformed by the near NS compression-shear strike-slip fault. The complete EW anticlinoria was divided into two blocks, meanwhile, north-south trending small faults were formed in the south and north flanks of the anticline, but the anticline structure remained intact (Fig. 5). Although the Wufeng-Longmaxi shale experienced uplift, the strike-slip-thrust faults cutting the top have remained in compressed state, the tight rock layers on both sides of the fault juxtapose well, keeping the fault in good sealing, and the anticline of the target layer has maintained complete. The Silurian Shiniulan Fm. has conventional gas and the shale gas layer in the Longmaxi Fm. has overpressure, indicating that the anticline structural area has good preservation conditions and is a 3D overall closed system.

3.1.4. Relatively complete anticline structure

According to statistics of wells of Y1, Y102, Y103, Y104, Y105, Y102H1-1, YS109, YS117, and YS118 which have made breakthroughs in the study area (Table 1 and Table 2), all gas-producing wells are in the Taiyang anticline trap, and wells in the anticline core have higher yield and wells in the anticline edges have slightly lower yield. Comprehensive analysis of logging and seismic data shows that the stratigraphic dips in the study area are complex and vary widely. The stratigraphic dips are 0°-10° in the anticline core, 10°-15° in the south flank and 15°-40° in the north flank in general, indicating the anticline is asymmetric in shape with a steep slope in the north and a gentle slope in the south (Figs. 1 and 5). Based on analysis of fine interpretation map and coherence map of 3D seismic data (Fig. 6), the faults in the study area are divided into II, III, and IV 3 orders, in which there are 2 faults of II order (with fault throw of over 1 km and longer extension), 4 of III order (with fault throw mainly between 80-300 m and longer extension), and 17 of IV order (with fault throw mainly between 20-80 m and short extension). The faults are largely III and IV compressive reverse faults with good sealing performance. Only 1 IV normal fault is open (F4-15 fault in the east, Fig. 6). The faults have been in a state of compressive-shear stress, and the rocks juxtaposing on both sides of the faults are tight marlstone and limestone, so the faults have good lateral sealing. The geological logging and test of cores from wells drilled on both sides of faults show high gas content; fracturing test of the shale gas reservoir shows high production and formation pressure coefficient of more than 1.2; conventional gas overflow is commonly observed during drilling of the overlying Shiniulan Fm., all indicating that the anticline has few faults, gentle strata in the main part, and good sealing performance on the whole. The sweet spot has the enrichment mode of typical compression-shear anticline gas reservoir (Fig. 7).

According to statistics of gas content of the cores from the Wufeng Fm.-the Long-1₁ sub-member of exploration wells in the study area (Fig. 8), the main target layers in the area have shale gas contents of 1.31-6.18 m³/t in general, 3.03 m³/t on average. Long-1₁¹ sublayer has the highest gas content of 6.00 m³/t on average, Long-1₁² sublayer comes second with 4.90 m³/t, Long-1₁³ sublayer is the third with 3.95 m³/t, and Long-1₁⁴ layer has the lowest gas content of 3.32 m³/t on average. Wufeng Fm. has an average gas content of 4.49 m³/t. Calibrated with the gas content data from gas desorption in drilling site, the gas content of shale gas target reservoir in the study area predicted by seismic attribute inversion under well log-seismic constraint is 3.30-5.51 m³/t, 4.27 m³/t on average (Fig. 9). The study results show that the shale gas reservoirs are continuously distributed in the study area, showing the characteristics of large-scale monolithic gas reservoir with large-scale gas accumulation.

According to the law of effective stress, the effective stress is uniform under the same porosity. The acoustic logging log can be used to quantitatively calculate the abnormal formation pressure in the study area. The formula of the equilibrium depth method is:

$p_{z}=\rho_{r}gz+\frac{(\rho_{r}-\rho_{w})g}{c}ln\frac{\Delta t}{\Delta t_{0}}$

Single well formation pressure is calculated by first calculating the original acoustic time difference Δt0 and the compaction coefficient c from the normal compaction curve of mudstone, and then the formation pressure is calcualted by using the acoustic difference of specific well. This method was used to calculate pressure of gas layers in 9 wells (Table 3). In combination with the prediction results of the 3D seismic attribute inversion in the study area, it can be seen that the shale gas layers in the main target formation of the Taiyang anticline zone are all in a critical overpressure state (Fig. 10).

Affected by the Xuefeng-Qianzhong uplift event at the southern margin of Yangtze block in the Caledonian period, the study area entered the deep-water continental shelf deposition stage of the foreland occlusion bay basin in the early Silurian^{[28-29, 35]} (Fig. 3) and had relatively quiet water in lag anoxic-dysoxic environment, where the planktonite fauna developed, which was favorable for enrichment of organic matter and formation of source rock. The dark argillaceous source rock layers (TOC≥1.0%) in the study area are widely distributed with large thicknesses typically between 60-180 m. The organic-rich shale (TOC≥2%) is concentrated at the bottom of Wufeng Fm.-Longmaxi Fm. and generally 30-40 m thick (Figs. 2-4). Analysis of 17 samples from the Wufeng Fm.-Long-1₁ sub-member in wells of YS109 and YS117 shows the samples have a saprolite content of 62%-86%, 77.95% on average in the kerogen maceral, Type II₁ organic matter, and high vitrinite reflectance (R_o) generally between 1.99% and 3.08% with the average of 2.41%, indicating they are in the over maturity-dry gas stage. Although the source rock has passed the main hydrocarbon generation period (Fig. 11), and the source rock has experienced pyrolysis and cracking gas generation peak in the shale burial and reformation period, and most of generated hydrocarbon gas has retained in the shale source rock. In addition, the Taiyang anticline had a prototype of underwater paleo-uplift in the Sinian Period and the anticline has developed successively and remained in good sealing from Caledonian-Indochina period to now, so it has become a self-generation and self-storage sweet spot enrichment area^{[36,37,38,39]}.

The organic carbon content of Wufeng Fm.-Long-1₁ sub-member in the study area calculated with log data after calibration with core analysis and test results is 0.42%-9.05%, averages at 2.92%. Vertically, Long-1₁¹ sublayer has the highest TOC between 2.40%-9.05%, 5.24% on average, Long-1₁² sublayer and Wufeng Fm. follow with 2.49%-5.58%, 3.53% on average, and 0.42%-5.82%, 3.68% on average respectively, and Long-1₁³ and Long-1₁⁴ sublayer have the lowest TOC between 1.83%-3.97%, 2.71% on average, and 1.15%-3.28%, 2.33% on average, respectively.

Comprehensive prediction of organic carbon content by combining the core analysis results, well logging calculation, and 3D seismic attribute inversion shows the Wufeng Fm.-the Long-1₁ sub-member in the study area has stable TOC between 2.54%-3.14%, 2.92% on average (Fig. 12), which means the shale layers reaches the TOC criterion of Types I+II high-quality reservoirs, and the material basis for hydrocarbon generation is good^{[29,31,36-37]}.

The shale reservoirs in the Wufeng Fm.-Long-1₁ sub- member in the study area have fairly good physical properties, with a measured porosity of 2.92%-8.01%, 4.98% on average (Fig. 13), and logging interpretation porosity of 3.52%-5.96%, 4.29% on average (Fig. 14). Long-1 ₁¹ sublayer has the highest porosity, with an average measured value of 6.40% and average log interpretation value of 5.28% (Fig. 14), which are in good consistent with the porosity of 3.98%-5.41%, 4.29% on average from seismic inversion (Fig. 15). The shallow gas reservoirs are characterized by low porosity and low permeability, which is similar with the middle-deep shale gas reservoirs. Storage space in the reservoirs includes organic pores, inorganic pores, and micro-fractures.

Organic pores are the main matrix storage space, and inorganic pores are dominated by dissolved pores and intergranular pores^[31,40] (Fig. 16). The shale reservoirs have total pore specific surface areas of 15.5-31.7 m²/g, total pore volumes of 0.024-0.037 ml/g, and average pore sizes of 7.2-10.4 nm. Pore specific surface areas of the sublayers in decrease order are Long-1₁¹, Wufeng Fm., Long-1₁², Long-1₁³, and Long-1₁⁴. The shale pores size are mainly 2-50 nm, and meso-pores account for 87.32% of all pores and is the dominant pore type for shale gas adsorption and storage in the target reservoir. Fractures in the shale reservoirs are dominated by micro-fractures. The shale reservoirs also have some horizontal fractures and high-angle fractures, which are generally less than 30 cm long and some filled by pyrite and calcite (Fig. 17). The gas-rich shale sections have horizontal fractures, such as interlaminar seams and layer sliding joints (Fig. 17), are concentrated at the bottom of the Wufeng Fm.-Long-1₁ sub-member.

According to analysis of imaging log data, the shale reservoirs of the Wufeng Fm.-Long-1₁ sub-member in the study area have NNE-SSW fractures, and most of the fractures are medium and high angle ones. These are in consistent with the characteristics of fractures and faults predicted by seismic data (Fig. 18). Considering that the main part of the study area is the Taiyang anticline structure, dense micro-fracture systems are generally developed in the high parts of the anticline structure under continuous compression (Figs. 17 and 18), and some are subtle micro-fractures and fissures invisible for the naked eye. These micro-fractures and fissures reduce the horizontal stress difference of the shale and connects micro-nano pores during the reservoir fracturing, forming complex man-made fracture networks to achieve volume fracturing. This is possibly an important reason for the stable gas production of shale gas wells in the shallow part of the anticline structure.

According to the criterion for shale gas reservoir in the Sichuan-Chongqing demonstration zone of PetroChina^[42] (Table 4), the Wufeng Fm. and Long-1₁¹, Long-1₁², and Long-1₁³ sublayers of the Longmaxi Fm. in the study area are evaluated as Type I reservoirs, and Long-1₁⁴ sublayer is evaluated as Type II reservoir. The reservoir characteristics of the sublayers in Wufeng Fm.-the Long-1₁ sub-member were evaluated comprehensively by combining the seismic attribute inversion and logging interpretation results. The sublayers with the area of Type I and Type II reservoirs in descending order in the study area are Long-1₁¹, Wufeng Fm., Long-1₁², Long-1₁³, and Long-1₁⁴ (Fig. 19), and Type I reservoirs of Wufeng Fm., Long-1₁¹, and Long-1₁² cover the main part of the Taiyang anticline, Long-1₁³ and Long-1₁⁴ cover most of the anticline area, and Type II reservoir cover other areas, indicating that the shale reservoirs of the main target formation in Taiyang anticline area have good quality and provide enough space for shale gas accumulation and are ideal places for shale gas accumulation and storage.

According to statistics on mineral composition of the Wufeng Fm.-Long-1₁ sub-member in the study area, the main mineral components are quartz, feldspar, carbonate, pyrite, and clay minerals, etc., and the measured content of brittle minerals (including quartz, feldspar, calcite, and dolomite) is 70.2% on average (Fig. 20), and 71.6% on average from logging. Generally, Long-1₁¹, Wufeng Fm., and Long-1₁² have higher brittle mineral contents, while Long-1₁³, and Long-1₁⁴ sublayers have lower brittle mineral content. Long-1₁¹ has a brittle mineral content of 74.0% from measurement and 75.16% from logging interpretation.

By combining seismic attribute inversion and logging interpretation, the brittle mineral contents of Wufeng Fm.- Long-1₁ sub-member in the study area are between 51% and 75%, 71.6% on average (Fig. 21). The study area has a maxi-mum horizontal principal stress of 24.2-71.6 MPa, a minimum horizontal principal stress of 16.0-51.6 MPa, and a horizontal stress difference of 4.1-17.6 MPa. Laterally, the anticline uplift has lower horizontal stress difference, and the syncline trough has higher horizontal stress difference (Fig. 22). Overall, the horizontal stress difference increases with the increase of burial depth. Putting all these together, we concluded that the shale gas reservoirs in Taiyang anticline area are favorable for SRV fracturing, and the shale layers can be easily "crushed" during hydraulic fracturing, forming man-made fractured gas reservoirs.

In summary, the Taiyang anticline area in the Zhaotong demonstration zone has superior conditions for shallow shale gas enrichment and accumulation. The sweet spot area has advantages in five aspects: a) Favorable sedimentary environment for high-quality shale. Silicon-rich and carbon-rich, high silicon and carbon content, and high silicon and medium carbon content shale developed in the deep-water continental shelf anaerobic environment has a continuous thickness of more than 30 m, indicating favorable shale source and reservoir conditions. b) High-quality shale reservoirs. The shale layers have a lot of organic pores and micro-fractures, good physical properties, and are largely Type I reservoirs favorable for shale gas storage and enrichment. c) The roof and floor of the gas-rich shale reservoirs depositing continuously with the shale reservoirs have tight lithology, large thickness, stable distribution, high breakthrough pressure, and good sealing performance, providing favorable shale gas sealing and preservation conditions. The structure has weak deformation and reformation. The faults under continuous compression have good lateral sealing performance. The anticline has favorable 3D sealing performance and contains typical anticline shale gas reservoir under compression-shear. d) The gas-rich shale sections have higher gas content, and are all in an overpressure state and high in exploration potential, providing the resource base for high production. e) Gas-rich shale layers have high brittle mineral content, and the anticline area has smaller the horizontal stress difference, which are favorable engineering conditions for fracturing to make high yield man-made gas reservoir, and complex fracture network is likely to be formed to realize SRV.

Based on the available criterion of selecting marine shale gas areas in South China^[25,26], and according to the recent data of drilling and production pilot in the shallow shale gas reservoirs of the Taiyang anticline area, the criterion of shallow shale gas area selection in Zhaotong demonstration zone has been summarized, as shown in Table 5.

According to statistics of the available data, the buried depth of Wufeng Fm.-Longmaxi Fm. in Zhaotong demonstration zone is shallower than that in the Sichuan Basin, and is mostly between 500-2500 m. The potential shallow shale gas area less than 2 000 m in buried depth is nearly 2 000 km², and has preliminary estimated resources of over 10 000×10⁸ m³, accounting for nearly half of the total area and resources of the demonstration zone. Hence, PetroChina Zhejiang Oilfield Company deployed shallow geological wells of YQ6, YQ7, and YQ8 in the anticline shallow buried area between Haoba, Luowang, and Hualang synclines in the middle of Zhaotong demonstration zone (Fig. 23) in August 2018. It has been confirmed by drilling that the shallow shale gas reservoirs are well developed, and have good gas display and rich resources, and are the key domain for the next step of shale gas exploration and development in the demonstration zone, and they have become hot spots for exploration and evaluation of shallow shale gas sweet spots in the near future. In June 2019, two shallow shale gas evaluation vertical wells YS137 and YS203 drilled on the north and south flanks of the Haiba anticline in the south of the Taiyang anticline area made major breakthroughs. The shale gas reservoirs in them at the depth of 1032 m and 1643 m respectively were tested a gas production of over 4.4×10⁴ m³/d and 4.2×10⁴ m³/d respectively after fracturing, demonstrating bright prospect of shallow shale gas exploration in the demonstration zone.

In summary, the sweet spot selection method and exploration technology in shallow shale gas reservoir through pilot of shallow shale gas drilling, fracturing, and productivity evaluation in Zhaotong demonstration zone has not only promoted the technological progress of shale gas exploration and development in the demonstration zone, but also boosted the commercial development of marine shale gas in Yunnan, Guizhou and Guangxi, and played a leading role in technical popularization and demonstration in the Yangtze River Economic Belt. According to the preliminary shale gas evaluation, the marine shale gas prospective area in the complex structure residual basin of the Yangtze River Economic Belt has an acreage of around 15×10⁴ km² and estimated resources of (16.70-18.15)×10¹² m³. In Junlian of south Sichuan, Xishui of Guizhou, Qianjiang of Chongqing, and Yichang of Hubei, active shale gas shows have been found at the buried depths of 156-1453 m, and some geological survey wells have tested thousands of cubic meters of production after fracturing, indicating promising prospect of shallow shale gas exploration.

Sedimentary environment is the main factor controlling shallow shale gas “source and reservoir”, which are the geological basis for hydrocarbon generation. In the Caledonian period, the Taiyang anticline area in Zhaotong demonstration zone in the southeastern edge of the Upper Yangtze region entered the sedimentary environment of anaerobic foreland basin occlusion bay in the deep-water shelf, and received thick siliceous shale and carbonaceous shale sediments. The shale of Wufeng Fm.-Long-1₁ sub-member has TOC of more than 2%, and an average measured porosity of 4.98%, and is in the stage of over-maturity dry gas (with an average vitrinite reflectance of 2.41%). All these show the study area has good shale source and reservoir conditions and is a favorable exploration area for shallow shale gas.

The main target reservoir has regional cap, the roof and floor of the reservoir are tight and have good sealing performance, and the structure has suffered weak reformation after deposition, these are prerequisites for preservation of shallow shale gas in the demonstration zone. Exploration practice proves that the stable area with relatively weak structural reformation (trough-like structural belt) in the central and northern part of Zhaotong demonstration zone is the favorable area for shallow shale gas exploration.

The shallow shale gas in the high-yield and gas-rich region of the Taiyang anticline has enriched in the typical compression-shear reformed trap model. This area has Type I reservoirs, high gas content, formation overpressure, high brittle mineral content, slightly lower horizontal stress difference, and good reservoir quality, which are favorable for gas enrichment and accumulation, and creating man-made fracture-type gas reservoirs by volume fracturing.

The distribution and enrichment of shallow shale gas sweet spot and high-yield areas in the demonstration zone are controlled by lithofacies in the sedimentary period, deformation strength in the reformation period, and sealing in the reservoir-forming period. The silicon-rich and carbon-rich shale depositing in the anoxic deep-water continental shelf is the main factor controlling development of shallow shale gas source and reservoir. The anticline structure with weak structural deformation and closed faults and the wide and gentle syncline provide favorable conditions for shallow shale gas enrichment and accumulation. The 3D sealing system of regional cap+ roof and floor is a key factor for shallow shale gas accumulation.

In South China complex structure area outside the Sichuan Basin, the paleozoic marine shale gas shows considerable exploration potential and is a key area of realizing breakthroughs in marine shale gas exploration in South China. The breakthroughs in shallow shale gas exploration and the innovative practice of productivity construction and core technologies in the Zhaotong demonstration zone will play a demonstration and leading role in shale gas development.

c—compaction coefficient of normal compacted mudstone, m^-1;

g—gravitational acceleration, 9.8 m/s²;

GR—natural gamma, API;

p_z—pore pressure or formation pressure of undercompacted mudstone, Pa;

R_d—resistivity, Ω·m;

R_o—vitrinite reflectance, %;

TOC—total organic carbon content, %;

z—buried depth of under-compacted mudstone, m;

ρ_r— average density of sedimentary rock, kg/m³;

ρ_w—the density of pore water in the formation, kg/m³;

Δt—interval transit time of under-compacted mudstone, μs/m;

Δt₀—interval transit time of original surface acoustic wave, μs/m.