oa Characterization of Fault Opening for Hydrocarbon Migration

In order to characterize the sealing capacity of faults during hydrocarbon migration most studies use sealing indexes based on one or two parameters. However, these indexes, may successfully used in some areas but not in others, since fault sealing is the consequence of many geological processes that cannot be simply described by so few factors. We present an empirical method (termed fault-connectivity probability method) for assessing the long-term sealing capacity of a fault for hydrocarbon migration. This method is based on the observational evidence of the opening or closing behavior of the fault during the entire process of hydrocarbon migration. In practice, petroleum leakage through an element of fault is identified by the existence (or not) of hydrocarbon-bearing layers on both sides of this element. The data from the Wangjiagang Oil Field in the South Slope of the Dongying Depression in the Jiyang Sub-basin in Bohai Bay Basin, NE China, are used to develop this method. Fluid pressure in mud-rocks, normal stress perpendicular to fault plane, and clay smear are identified as the key factors representing fault seal capacity. They are combined to compose a non-dimensional Fault Opening Index, FOI. The values of FOI are calculated from the key factors measured on elements, and the relationship between FOI and fault-opening probability on any an element is established through a statistical analysis: when FOI is <1.0, the fault-opening probability is 0; when FOI is in the range (1-3.5), its relationship with fault-opening probability is quadratic polynomial; when FOI is > 3.5, the fault-opening probability is 1. Then, the values of fault-opening probability can be contoured on a fault plane, to characterize the variations of seal capacity on the fault plane.