Fault
and Top Seal Strength at Nansen Field,
East Breaks, Gulf of Mexico
A trap
analysis of a highly faulted Pliocene-Pleistocene turbidite sequence is performed in order to understand the controls on column
heights, which range from ~150-2400 ft in 21 reservoir compartments. A kinematically viable structural and stratigraphic model is generated from an
interpretation of 3D seismic data, stratigraphic picks from >25 wells, and
systematic petrophysical analysis of well logs to develop a field-wide model of
shale volumes. This model forms the framework for analyzing reservoir
juxtapositions and estimating
fault
properties in five main reservoirs
intervals that are distributed throughout four main
fault
blocks. The field
lies above an allochthonous salt-high with a series of steeply-dipping,
anti-listric, ESE dipping normal faults that accommodate bending during the
formation of a deep mini-basin to the SE. The structurally shallowest blocks
are significantly overpressured (>1000 psi) relative to the deeper
fault
blocks, and appear limited by top seal strength as indicated by reservoir
pressure data that approach the minimum principal stress and the presence of
gas chimneys above the
trap
crests. Hydrocarbon columns in the deeper
fault
blocks are all
fault
dependent despite being presently active and conductive, as
deduced from the associated topographic scarps and pock marks visible on the
seafloor. While a few hydrocarbon columns are controlled by self-juxtaposition
of reservoir units across faults, most fluid contact depths lie above
self-juxtaposition points, indicating
fault
-rock seal capacity has been
reached. Secondary migration pathways defined by these weak/leak points appear
consistent with hydrocarbon maturity differences inferred from sterane
distributions between
fault
blocks. Weak points from these traps, defined as
the highest ratios of buoyancy pressure to shale gouge ratio identified along
the
fault
, form a maximum
fault
seal capacity envelope that is significantly
higher than previously published relations, but appear consistent with
laboratory derived data collected from core in this and analogous reservoirs.
These results suggest that maximum
fault
seal strength could be erroneously
calculated when columns are controlled by self-reservoir juxtaposition leak
points or top seal capacity, and highlight the importance of building a viable
structural model and performing an integrated
trap
analysis at early stages of
exploration and appraisal.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California