An investigation of sealing and episodic pulsing of fluids at a minibasin-bounding growth fault from seismic reflection images

That faults are of fundamental importance in petroleum systems cannot be stressed enough. The role the faults play, however, is complicated by their dual nature as fluid seals and conduits. Understanding a fault’s behavior demands extensive knowledge of its past geologic history, core samples, pressure measurements, and well logs. Further complicating matters, the sealing capacity of a fault usually varies directionally and the along-fault permeability episodically increases in response to slip. Seismic reflection images offer an attractive means to measure the internal properties of fault zones remotely; for instance, direct evidence of faults (e.g., fault-plane reflections) occasionally shows up on migrated seismic data. We describe three basic models that give rise to fault-plane reflections: juxtaposition (e.g., sand/shale) contacts, effective linear-slip behavior at the fault, and large pore pressure jumps across the fault. From the point of view of fault properties, the first of these three models, juxtaposition contacts, is the least interesting. Reflectivity from a fault originating from one of the other two models directly relates to the sealing and conducting behavior of the fault. To enhance the information on the sealing and conducting properties of a fault, we propose a seismic data processing technique to attenuate the reflectivity due to juxtaposition contacts and layer boundaries. Utilizing a 2D spectral-element implementation of the elastic wave equation, we model examples and combinations of these three basic models and their imprint on reflected waves. A 3D seismic survey from Blocks 314, 315, 330, and 331 of the South Eugene Island field, offshore Louisiana, contains reflections from a major growth fault system. We find that differences in pore pressure across a fault known as the A-fault give rise to reflections from the fault-plane. Thus, the presence of the reflections point to the fault providing a significant seal. The ability of the reflected waves to sense the sharp onsets of overpressure, or pore pressure in excess of hydrostatic, has implications for the prediction of potential drilling hazards. Along another fault known as the B-fault, we study the possibility of geologically fast, pressure-driven fluid flow along the fault. We observe the first evidence from seismic reflection images of a fluid pulse, or “fault burp”, propagating up the B-fault. In reflection images from 1985 and 1992, areas of high reflectivity systematically move up the fault 1 km, for an average speed of 140m/yr. The average pulse speed can be explained with a model of a permeable fault zone connecting the shallow, normally pressured sediments to a deep, overpressured compartment. The presence of geologically fast fluid movement along growth faults sheds light on hydrocarbon migration mechanisms and the rate of reservoir recharge at South Eugene Island.