Fault sealing effect on hydrocarbon accumulation in the Yuquan area of the Xihu Sag, East China Sea Basin

Abstract

This study examines the effect of fault sealing on hydrocarbon accumulation in the Xihu Sag where NE- and NNE-striking faults within the sag prevent the escape of trapped oil and gas. Analyses like the shale gouge ratio (SGR), shale smear factor (SSF), and fault lateral sealing coefficient (F_h) are used for a quantitative evaluation of the lateral sealing capacity of faults in the Yuquan area of the Xihu Sag. Paleo-structural sections reconstructed using cross-section balancing techniques are used to analyze the evolutionary characteristics of fault sealing. The results indicate that the lateral sealing capacity of faults is mainly controlled by fault throw, clay content, and sand–shale distribution characteristics; in the Yuquan area, fault segments exhibit good lateral sealing when the SGR exceeds 0.5, the SSF is less than 2.5, and the F_h is greater than 10.5. The methodology for evaluating fault sealing evolution, established using structural reconstructions during key geological periods, shows that fault sealing capacity increased gradually over time; in the Xihu Sag, effective sealing of the main hydrocarbon-bearing intervals began during the Liulang period. Evaluating fault sealing capacity is also crucial for estimating the rate of hydrocarbon accumulation, and typically shows a positive correlation with gas (oil) column height. In the Yuquan area, the volume of hydrocarbons sealed by the faults increased progressively over time, with a relatively high overall fault sealing capacity during the second hydrocarbon accumulation period. This study provides valuable insights into the evaluation of fault sealing capacity as well as an investigation of the relationship between faults and hydrocarbon migration–accumulation in the Xihu Sag and analogous geological settings.