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LØSETH, HELGE*, MARITA GADING and GUNN MARI GRIMSMO TEIGE, Statoil R & D Centre,Trondheim, Norway

Abstract: How to Interpret Leakage Zones on Seismic Data

Introduction

Seismic interpreters often observe anomalies which cannot be explained by primary depositional processes or processing errors. Some of these anomalies reflect interesting events in the underground that may give valuable information if interpreted correctly. One group of such anomalies is related to the process of leaking hydrocarbons. They have been studied and this work intends to give a quick overview of common seismic expressions of leakage anomalies, their terminology and how they can be interpreted.

How to interpret leakage anomalies

The first step for a seismic interpreter looking for leakage observations is to scan through the seismic data in the study area and observe and describe anomalies that are difficult to explain as depositional features or processing artifacts and that may be related to leakage. In this context it is valuable to use purely descriptive terms without considering the processes forming the anomalies.

The next step is to interpret the individual anomalies. Leakage processes can result not only in changes to the rock properties but also in physical deformation of the sediments. The seismic anomalies that are caused by changes in the rock properties are mainly due to hydrocarbons replacing brine, e.g., gas replacing brine forming brights, (Figure 1). Leaking hydrocarbons can also generate processes that distorts the primary layering, e.g., erosion as gas is released at the seabed forming pockmarks. Where hydrocarbons seep at the seabed sedimentary features like bioherms may develop. The physical changes result in permanent rock deformation while the acoustic changes formed by hydrocarbons replacing brine may be reversible. Other types of leakage anomalies like diagenesis related to leaking hydrocarbons (O'Brian et al. 1998) or gas hydrates and bottom simulating reflectors (Hovland and Judd 1988) can be observed on seismic data.

After observing, describing and interpreting the various seismic anomalies related to leakage, they must be organized in a way that makes it possible to interpret their implications. In this work three new terms have been defined: leakage zone, top of leakage zone and root of leakage zone. A leakage zone (Figure 2) is defined as the total rock volume influenced by vertically migrating hydrocarbons and is bounded by a root and a top.

The base of the leakage zone is termed the root (Figure 2). The root of a leakage zone is located where the hydrocarbons start to migrate or leak vertically from the reservoir into the cap rock.The leakage from the reservoir can either occur over a large area, i.e., a root zone or it can occur in a small area, i.e., a root point. On seismic data the root of the leakage zone is defined at the deepest located seismic anomalies in the leakage zone. Mapping of roots can help to locate where a pressure compartment leaks, it can subdivide a structure into pressure compartments and may also locate wedge-out points of stratigraphic traps.

The upper termination of the leakage zone, i.e., the top of the leakage zone, (Figure 2) is set at the uppermost level of vertical migration or leaking hydrocarbons through cap rocks. On seismic data the top of the leakage zone is defined where the uppermost leakage anomaly is.observed.The top of a leakage zone can be used to:
1) Define when the leakage was active,
2) Define levels of laterally distributed reservoir rocks or "gathering systems",
3) Indicate the hydrocarbon-sourcing points, e.g., the point or area where hydrocarbons from Jurassic reservoirs migrate vertically through the Cretaceous succession into Paleocene sands, and
4) Suggest levels of effective migration seals, e.g., salt.

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