Defining a Trap: Implications of (Not) Understanding Fault Dimensions From Seismic Data in Exploration and Production

Rotevatn, Atle; Fossen, Haakon

It is easy to fall for the temptation to believe that the seismically mapped tipline of a fault represents the line where displacement is zero and the fault actually ends. Yet, all faults in the subsurface have portions that fall below the resolution of any seismic dataset. In this contribution we explore the effects of this sub-seismic fault domain and discuss the implications of appreciating or ignoring it in exploration and production settings. This is done by the use of two examples:

First, we look at an outcrop example, where we study the length and character of the tip zone. We use reservoir modeling and fluid flow simulation to quantify the effect fault tip zone on fluid flow and reservoir compartmentalization. The studied fault exemplifies that the fault tip zone may extend several hundred meters beyond the seismically mapped tip, depending on vertical seismic resolution and fault displacement gradients. The flow simulation results demonstrate that the low-permeable tip zone may generate steep pressure gradients in a reservoir and may affect the tortuosity of reservoir fluid flow. As such, the sub-seismic continuity of seismically mapped faults should not be ignored in production.

Second, we discuss the implications of understanding (or not understanding) fault dimensions in an exploration setting. We do so by applying the lessons of the outcrop and flow simulation example to a real subsurface example where we discuss how this may affect the delineation of fault-controlled prospects. We demonstrate how small adjustments in fault interpretations in the sub-seismic domain may significantly affect trap definition, prospect volumes, project economics and the selection of exploration well locations.

Overall, the results and examples of our contribution highlight the importance of accounting for the fact that faults do indeed continue past the seismically mappable fault tip. Therefore, and finally, we discuss a very simple method for the estimation of sub-seismic fault continuity past the seismically mapped tip.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013