Dynamic Fault Behaviour Characterization Using 4-D Seismic Data

Castilla, Raymi ¹; Gatefoin, Rudy ¹; Brechet, Emmanuelle ²; Prat, Philippe ²; Marlot, Vincent ³; Zuckmeyer, Eric ³; Ruelland, Philippe ¹
(1)Geoscience Research Centre, TOTAL E&P UK, Aberdeen, United Kingdom. (2) CSTJF, TOTAL SA, Pau, France. (3) TOTAL E&P ANGOLA, Luanda, Angola.

4D attributes such as changes of acoustic P-velocities (dVp/Vp) can show the spatial distribution of fluid pressure drop and water and gas saturation change around wells. The 4D information has been used here as a proxy for fault sealing or baffling capacity in dynamic conditions by looking at the distribution of those changes and its relationship to interpreted faults. The faults systems on which the methodology described is built are located in the deep-offshore Angola. The area sits on the transition between the extensional and compressional structural domains of this part of the East-African margin. Two fault families, one oriented N45° and another one oriented N135°, have been identified.

Water injection and depletion associated to oil production show clear indications of some faults acting as barriers/baffles to fluid flow. Negative 4D effects on the DVp/Vp attribute start to be observable when the bubble point pressure is reached and gas comes out of solution. If initial pressure conditions are far from the bubble point, some degree of depletion can take place without being detected by the 4D signal. This implies that the fact that a 4D effect is limited by a fault does not necessarily means that the fault acts as a strict barrier. In these cases the fault can be acting as a baffle with depletion in both sides, one side being below the 4D detection threshold.

The 4D response close to faults has first been analysed in terms of reservoirs juxtaposition. Both geometrical and lithological juxtapositions have been scrutinised. Cases where fault sealing cannot be explained in terms of juxtaposition have been compared to the “standard” and purely static estimation of fault sealing capacities (i.e. SGR, CSP). In addition, the estimated in-situ stresses have been used to assess the mechanical state of faults and its impact on fault sealing behaviour.

Dynamic fault baffling is defined as the absolute difference in DVp/Vp from the most depleted side of the fault to the least depleted side. Dynamic baffling is observed on intervals where sand-rich and shale-rich units are juxtaposed across a fault and where estimated values of SGR predict clay content in the fault zone ranging between 10-30%.

Refining this approach and comparing the 4D signal both with fault properties with fault slip tendencies could enable to determine some of the key factors that control the dynamic behaviour of faults in the depositional context of the deep offshore Angola.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.