Structural Trapping of Buoyant Fluids
Abstract
Structural traps comprise assemblages of surfaces having high capillary threshold pressure for hydrocarbons. Individual traps may include top-, bottom- and side-seal surfaces, the latter commonly being a fault
. Faults may form sealing surfaces in 2 ways: (i) juxtaposition, so that intervals of high capillary threshold pressure (e.g. shales) are juxtaposed against the reservoir unit in the
trap
; (ii)
fault
-rock, where new material (generated during slip) forms a barrier to lateral flow even when reservoirs are juxtaposed. In order to risk prospect volumes in exploration, the first step in
fault
characterisation is to map the distribution of offset layers at the
fault
. This pattern (‘Allan diagram’) is captured by the set of separation polygons, i.e. the horizon/
fault
intersections in 3D. These same lines define the displacement pattern on the
fault
surface, and so can also be used as a QC of the interpretation, since it is well established that displacement patterns of
fault
networks show a high degree of kinematic coherence. The separation polygons are also a key part of horizon maps, since that is how faults are represented on structure maps. However, construction of
fault
polygons remains a weak step in the workflow in many E&P companies.
Fault
rocks are generated from the wall-rock layers slipping past each other and so their composition is determined by the wall-rock lithologies and amount of displacement. The detailed internal structure of the
fault
rock is impossible to predict and hence it is usual to consider an upscaled proxy such as Shale Gouge Ratio, with the assumption that high SGR represents clay smears and low SGR represents clay-poor
fault
rock such as disaggregation zone or cataclasite. Suitable estimates of hydraulic properties can then be applied to different parts of the
fault
surface. Even when the juxtaposition pattern and
fault
-rock distribution both indicate that a
fault
is likely to be sealing to cross-
fault
flow, there is a remaining risk that buoyant fluid may escape out of the
trap
up the
fault
. Such up-
fault
leakage can occur simultaneously with across-
fault
seal. It is dependent on the stress state of the
fault
, in particular how close the surface is to being critically stressed. The stress state is itself dependent on the reservoir pore-pressure, and so can be changed during
trap
filling or production. In the latter case, pore-pressure changes may be sufficient to induce slip on previously inactive faults.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014