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