Introduction
1. Geological setting
Fig. 1. Geologic structures and cross-well section of T3x2 in Xinchang structural belt in the West Sichuan Depression. |
2. Features of fault-fold-fracture body
2.1. Concept of FFFB
Fig. 2. Schematic diagram of FFFB. |
2.2. Types of FFFB
2.2.1. Types of promising matrix reservoirs
Fig. 3. Classification and petrophysical properties of promising matrix reservoirs in T3x2 in Xinchang structural belt. |
Table 1. Petrophysical properties and pore structures of promising matrix reservoirs in T3x2 in Xinchang structural belt |
Type of promising lithofacies | Lithofacies assemblage | Development of bedding fractures | Petrophysical property | Pore structure | |||||
---|---|---|---|---|---|---|---|---|---|
Porosity/ % | Permeability/ 10-3 μm2 | Coefficient of mean value | Sorting coefficient | Variation coefficient | Flexure coefficient | Median radius/μm | |||
Layer- cake | Medium to coarse sandstone with trough/parallel bedding | Developed horizontal bedding fractures | 4.50-5.60 (4.85) | 0.01-33.20 (19.95) | 11.49 | 2.84 | 0.23 | 1.00 | 0.028-0.056 (0.042) |
Blocky | Medium sandstone with massive bedding and diagonal bedding | Undeveloped horizontal bedding fractures | 4.00-4.90 (4.35) | 0.002-4.500 (0.300) | 11.53 | 2.96 | 0.33 | 0.31 | 0.005-0.018 (0.013) |
Note: The value in the parentheses is an average. Data were derived from 115 samples acquired from 7 wells including XC12, XC8, and L150. |
2.2.2. Types of effective fracture bodies
Fig. 4. Core photos showing various fractures in T3x2 in Xinchang structural belt. |
Fig. 5. Classification of fracture bodies in T3x2 in Xinchang structural belt. |
2.3. Formation feature of FFFB
2.3.1. Fault-associated fractures and major controls
Fig. 6. Field outcrops and geologic models of fault-fracture and fold-fracture bodies in Xujiahe Formation, West Sichuan Depression. |
Fig. 7. Crossplots of fracture density vs. key parameters of faults and folds in T3x2 in Xinchang structural belt. |
2.3.2. Fold-associated fractures and major controls
3. Geologic models and promising areas of FFFBs
3.1. Geologic models of FFFBs
Table 2. Comprehensive evaluation of sweet spots in T3x2 in Xinchang structural belt |
Type of sweet spots | Reservoir space | Type of promising matrix reservoir | Petrophysical properties | Pore structure | Gas saturation/% | Pore type | |||
---|---|---|---|---|---|---|---|---|---|
Permeability/ 10-3 μm2 | Porosity/ % | Median radius/μm | Description | ||||||
Fault-fold fracture | Network fracture | Structural fractures, bedding fractures, matrix pores | Layer-cake | ≥100 | ≥3 | ≥0.06 | Very good | ≥60 | Porous- fractured |
Single structural fracture | Structural fractures, matrix pores | Blocky | 10-100 | ≥3 | ≥0.06 | ≥60 | |||
Matrix storage & permea- bility | Bedding fracture | Bedding fractures, matrix pores | Layer-cake | 0.30-10.0 | ≥3 | 0.02-0.06 | Good | ≥55 | Fractured- porous |
Low-per- meability pore | Matrix pores | Blocky | 0.03-0.30 | ≥3 | <0.02 | Moderate | ≥50 | Porous | |
Extremely low permeability pore | Matrix pores | Blocky | <0.03 | ≥3 | <0.02 | Poor | ≥50 | Tight |
Fig. 8. Classification of sweet spots in T3x2 in Xinchang structural belt. |
Fig. 9. Geologic model of FFFBs in T3x2 gas reservoirs in Xinchang structural belt. |
3.2. Promising prospects
Fig. 10. Promising prospects in T3x22 in Xinchang structural belt. |