Variable rock-electrical saturation model for dual-medium tight sandstones: A case from the second member of the Upper Triassic Xujiahe Formation, Western Sichuan Depression

doi:10.13745/j.esf.sf.2025.8.55

Abstract

Abstract:

The second member of the Upper Triassic Xujiahe Formation in the Western Sichuan Depression holds substantial potential for tight sandstone gas exploration and development. However, the deeply buried reservoirs exhibit complex structures, low porosity and permeability, and a multi-phase superimposed fracture system, resulting in diverse reservoir types formed through fracture-matrix coupling. Conventional Archie-based water saturation models perform poorly in dual-medium systems, causing low interpretation accuracy and hindering efficient development. This study integrates core observations, thin-section analyses, rock physics experiments, and well-logging data to classify reservoirs into three types: fractured, fracture-pore composite, and porous. Significant variations in pore structure and flow capacity among these types cause pronounced differences in petrophysical parameters. To address these differences, we developed a reservoir-classification-based saturation model with variable rock-electrical parameters. The model demonstrates strong agreement with measured water saturation from sealed core samples and gas testing results, yielding an average absolute error of 4.35%—a substantial improvement over the 50.25% error of the traditional Archie model. This research offers an effective approach for fluid saturation evaluation in dual-medium tight sandstone reservoirs and provides a transferable methodology for analogous unconventional gas systems. These findings ultimately contribute to enhanced hydrocarbon recovery and more efficient field development.

Key words: dual-medium tight sandstone, reservoir classification, variable rock-electrical parameters, saturation model, the second member of the Upper Triassic Xujiahe Formation

CLC Number:

P618.13

QU Linbo, YUE Dali, WANG Wurong, JIN Wujun, LAI Hechuan, WU Qingzhao, LIAO Changzhen, FU Jialin, ZHANG Jiarui, LI Wei. Variable rock-electrical saturation model for dual-medium tight sandstones: A case from the second member of the Upper Triassic Xujiahe Formation, Western Sichuan Depression[J]. Earth Science Frontiers, 2025, 32(5): 377-388.

Figures/Tables 12

Fig.1 Regional geological setting of the study area. (a) and (b) modified from [27].

Fig.2 Fractured reservoir type and characteristics (well XS204H, 4686-4706 m)

Fig.3 Fracture-pore composite reservoir type and characteristics (well X3, 4946-4966 m)

Fig.4 Porous reservoir type and characteristics (well XS204H, 4590-4615 m)

Fig.5 Differences in pore structure among reservoir types

Fig.6 Rock-electrical response characteristics of tight sandstones in the study area

Fig.7 Correlation matrix between cementation exponent m and related parameters

Fig.8 Scatter plots of measured vs. calculated cementation exponent m from different models

Fig.9 Relationship between core-measured water saturation and resistivity index

Table 1 Variable rock-electrical parameter saturation models based on reservoir classification

储层类型	变岩电参数含水饱和度计算公式	胶结指数m计算公式	饱和度指数n	岩性指数b
裂缝型储层	S_w=${\left(\frac{{b}_{1}{R}_{w}}{{Ф}^{{m}_{1}}{R}_{t}}\right)}^{\frac{1}{{n}_{1}}}$	m₁=0.69Ф-0.12R_D-0.33RQI-0.05AC-0.17GR	n₁=3.626	b₁=1.126 3
裂缝-孔隙复合型储层	S_w=${\left(\frac{{b}_{2}{R}_{w}}{{Ф}^{{m}_{2}}{R}_{t}}\right)}^{\frac{1}{{n}_{2}}}$	m₂=1.23Ф-0.34R_D-0.33RQI-0.14AC-0.41GR	n₂=3.023	b₂=1.108 9
孔隙型储层	S_w=${\left(\frac{{b}_{3}{R}_{w}}{{Ф}^{{m}_{3}}{R}_{t}}\right)}^{\frac{1}{{n}_{3}}}$	m₃=0.69Ф-0.16R_D+0.05RQI+0.51AC	n₃=2.068	b₃=1.010 9

Fig.10 Integrated logging-based reservoir evaluation results for well XS204H

Fig.11 Error comparison of water saturation results calculated by different methods

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