Introduction
1. Geological setting
2. Samples and methods
2.1. Samples and procedures
2.2. Identification of laminae and bedding
2.3. Grain-size analysis
3. Types and characteristics of sedimentary facies
Table 1. Sedimentary facies of the Wufeng Formation-Long 11 shale in southern Sichuan Basin |
Type | Facies | Subfacies | Microfacies | Location | Typical well/section |
---|---|---|---|---|---|
Transitional | Delta | Delta front | Underwater distributary channel, interdistributary bay | Above topographic break 1 | Wells B1, YS201 and YS207 |
Tidal flat | Intertidal flat | Calcareous flat and clayey flat | Between average high-water line and normal wave base | Xiushan Datianba section, Well B1 | |
Marine | Shelf | Shallow shelf | Calcareous and clayey shelf | Between normal wave base and topographic break 1 | Wells W231, YQ1, YQ3 and YS128 |
Deep shelf | Deep slope | Between topographic break 1 and topographic break 2 | Wells W232, Zi207, Zu201 and DS1 | ||
Deep plain | Between topographic break 2 and topographic break 3 | Wells R202, L211, N210 and H202 | |||
Deep depression | Below topographic break 3 | Wells L208, N211 and TH1, and Changning Shuanghe section | |||
Gravity flow | Overflow | Below topographic break 1 | Wells N219, N240, W205 and H205 |
3.1. Delta facies
Fig. 2. Paleotopography and microfacies of the Wufeng Formation-Long 11 shale in the south of the Leshan-Longnüsi uplift, southern Sichuan Basin (modified from Reference [39]). |
Table 2. Microfacies of the Wufeng Formation-Long 11 shale in southern Sichuan Basin |
Item | Composition | Mineral content/% | Sedimentary structure | Kerogen type | U/Th | Topographic slope/ (m·km-1) | ||
---|---|---|---|---|---|---|---|---|
Facies | Subfacies | Microfacies | ||||||
Delta | Delta front | Underwater distributary channel | Coarse silt (>30%) | Calcite and dolomite greater than 40% | Lenticular bedding and graded bedding | Types II1 and II2 | 0.66 | 0.25 |
Interdistributary bay | Coarse silt (<30%) Fine silt (>50%) | Clay minerals greater than 40% | Massive bedding | |||||
Tidal flat | Intertidal flat | Calcareous flat | Fine silt (>95%) | Calcite and dolomite greater than 40% | Wavy bedding, small cross bedding and graded bedding | Type II 1 | 0.35 | 0.25 |
Clayey flat | Clay minerals greater than 50% | |||||||
Shelf | Shallow shelf | Calcareous shelf | Fine silt (90%-95%) | Calcite and dolomite 30%-40% | Silt-mud alternated horizontal bedding | Type I | 0.44 | 0.25 |
Clayey shelf | Clay minerals greater than 50% | |||||||
Deep shelf | Deep slope | Fine silt (70%-90%) | Calcite and dolomite 20%-30% | Silt-mud alternated horizontal bedding, Silt-mud graded horizontal bedding | Type I | 0.77 | 0.5-1 | |
Deep plain | Fine silt (50%-70%) | Calcite and dolomite 10%-20% | Shaly horizontal bedding, Silt- mud graded horizontal bedding | Type I | 1.32 | <0.1 | ||
Deep depression | Fine mud (>50%) | Silica greater than 55% | Shaly horizontal bedding | Type I | 1.44 | 0.3-0.7 | ||
Gravity flow | Overflow | Fine mud (>50%) | Clay minerals greater than 35% | Normal graded structure, deformed structure, scoured and filled structure, graded bedding, wavy bedding and horizontal bedding | Type I | 1.32 |
Fig. 3. Microscopic characteristics of conventional thin sections of the Wufeng Formation-Long 11 shale, southern Sichuan Basin. (a) Underwater distributary channel, hybrid accumulation, Well YS201, 2764.02 m, under cross-polarized light; (b) Tidal flat, grain-supported structure, Xiushan Datianba section, under cross-polarized light; (c) Shallow shelf, grain-supported structure, normal graded bedding, Well Zi203, 3006.76 m, under cross-polarized light; (d) Deep slope, grain-supported structure, Well 206, 4275.10 m, under cross-polarized light; (e) Deep plain, matrix-supported structure, Well L208, 3840.77 m, under cross-polarized light; (f) Deep depression, fine mud in dominance, Huaying Sanbaiti section, under cross-polarized light. |
Fig. 4. SEM characteristics of the Wufeng Formation-Long 11 shale in southern Sichuan Basin. (a) Delta front, scattered sapropelinite, Xiushan Datianba section; (b) Tidal flat, layer #8, Xiushan Datianba section; (c) Shallow shelf, grain-supported structure, Well W210, 3228.70 m; (d) Deep slope, Well W206, 3753.86 m; (e) Deep plain, dispersed sapropelinite, vitrinite as small flakes, Well N211, 2350.25 m; (f) Deep depression, Well H206, 4355.50 m; (g) Gravity flow, organic matter in scattered distribution, Well W210, 3248.91 m; (h) Gravity flow, organic matter in scattered distribution, Well Y101H3-8, 3747.18 m; (i) Gravity flow, organic matter in scattered distribution, Well N211, 2321.05 m. |
3.2. Tidal flat facies
Fig. 5. Bedding characteristics of the Wufeng Formation-Long 11 shale in southern Sichuan Basin. (a) Tidal flat, small cross bedding, Xiushan Datianba section; (b) Tidal flat, small wavy bedding, Xiushan Datianba section; (c) Shallow shelf, silt-mud alternated horizontal bedding, 2570.09 m, Well W202; (d) Shallow shelf, small normal graded bedding, 2573.80 m, Well W202; (e) Deep slope, silt-mud alternated horizontal bedding, Longmaxi Formation, Changning Shuanghe section; (f) Deep slope, silt-mud graded horizontal bedding, Longmaxi Formation, Changning Shuanghe section; (g) Deep plain, shaly horizontal bedding, 3668.80 m, Well Zi201; (h) Deep depression, shaly horizontal bedding, 2342.05 m, Well N211; (i) Deep depression, shaly horizontal bedding, 3783.40 m, Well Y101H3-8. |
3.3. Shelf facies
3.3.1. Shallow shelf subfacies
3.3.2. Deep shelf subfacies
3.3.3. Gravity flow subfacies
Fig. 6. SEM photos of large thin sections of the Wufeng Formation-Long 11 shale showing characteristics of gravity flow deposits. (a) Matrix-supported structure, normal graded bedding, Well H203, 3756.64 m; (b) Deformed structure, Well Zi203, 2983.40 m; (c) Scoured-filled structure, Well W201, 1497.35 m; (d) Graded bedding, Well N212, 2065.01 m; (e) Wavy bedding, Well Y101H3-8, 3757.66 m; (f) Wavy bedding, Well Y101H3-8, 3759.27 m. |
4. Distribution of sedimentary facies
4.1. Single-well facies
Fig. 7. Sedimentary microfacies of the Wufeng Formation-Long 11 in (a) Well W206 and (b) Well L202. |
4.1.1. Well W202
4.1.2. Well L206
4.2. Well-tie facies
Fig. 8. Well-tie facies correlation of the Wufeng Formation-Long 11 crossing Well WJ1-Well JYT1 in southern Sichuan Basin. GR—natural gamma; Δt—interval transit time. |
4.3. Plane distribution of sedimentary microfacies
Fig. 9. Plane distribution of sedimentary microfacies of the Wufeng Formation-Long 11 in southern Sichuan Basin. |
4.4. Distribution models of sedimentary microfacies in the epicontinental shale
Fig. 10. Distribution models of sedimentary facies in the Wufeng Formation-Long 11 epicontinental shale in southern Sichuan Basin. |