Abstract

Hydraulic behaviors of faults in sedimentary basins have been paid close attention in studies of hydrocarbon migration and accumulation because of their important functions in basin hydraulic circulations. In previous studies, however, the function of faults in hydrocarbon migration is characterized by the sealing capacity of faults. In fact, sealing is only an impressive and time-dependent aspect of the hydraulic behavior of faults, which may act as seals during some periods and as pathways some time later. Therefore, in hydrocarbon migration studies, sealing indices may successfully be used in some cases but not in others. In this article, we introduce an empirical method (termed the fault-connectivity probability method) for assessing the hydraulic connecting capacity of a fault for hydrocarbon migration over geological time scales. The method is based on the recognition that observable hydrocarbon in reservoirs should result from the opening and closing behavior of the fault during the entire process of hydrocarbon migration. In practice, the cumulative petroleum migration through a segment of the fault zone is identified by the presence (or not) of hydrocarbon-bearing layers on both sides of the segment. Data from the Chengbei step-fault zone (CSFZ) in the Qikou depression, Bohai Bay Basin, northeast China, were used to develop this method. Fluid pressure in mudstones, normal stress perpendicular to fault plane, and shale gouge ratio are identified as the key factors representing fault-seal capacity. They are combined to define a nondimensional fault opening index (FOI). The values of FOI are calculated from the measured values of the key factors, and the relationship between FOI and fault-connectivity probability on any fault segment is established through statistical analysis. Based on the data from the CSFZ, when the FOI is less than 0.75, the fault-connectivity probability is 0; when FOI ranges from 0.75 to 3.25, the corresponding fault-connectivity probability increases from 0 to 1 following a quadratic polynomial relationship; when FOI is greater than 3.25, the fault-connectivity probability is 1. The values of fault-connectivity probability can be contoured on a fault plane to characterize the variations of hydraulic connective capacity on the fault plane. The applicability of this concept for other oil fields (in particular, the quantitative relationship between FOI and fault-connectivity probability) has still to be ascertained.

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