Thomas Finkbeiner¹, Mark D. Zoback², Beth B. Stump³, Peter B. Flemings⁴
(1) GeoMechanics International, Palo Alto, CA
(2) Stanford University, Stanford, CA
(3) Texaco Worldwide Exploration & Production, New Orleans, LA
(4) The Pennsylvania State University, University Park, PA

Abstract: Stress, Pore Pressure, and Dynamically Constrained Hydrocarbon Columns in the South Eugene Island 330 Field, Gulf of Mexico

There are two limiting values on hydrocarbon phase pore pressures at the top of a reservoir that are related to the current in situ state of stress: 1) hydraulic fracturing of the overlying caprock and 2) frictional slip along a reservoir bounding fault. As the hydrocarbon phase pressure at the top of the reservoir results from both the buoyancy of the hydrocarbon column and the initial water phase pore pressure, it is possible to quantitatively determine whether either of these two critical pressure values are reached and the observed column heights are at their dynamic capacity. We utilized in situ stress and pore pressure data from nearly 50 wells in the South Eugene Island 330 field to examine the dynamic capacity of the various reservoirs in this field. These data indicate that in moderately overpressured reservoirs at intermediate depths, hydrocarbon phase pressures at the top of the sands in two fault blocks are observed to be within the range of theoretically required pressures to induce slip on the reservoir bounding growth faults. Hydrocarbon column heights in these sands are dynamically controlled by the capacity of the bounding growth faults to resist frictional sliding. At greater depth in two severely overpressured reservoirs, hydrocarbon phase pressures at the top of the structure are extremely close to both the minimum principle stress of the bounding caprock and the vertical stress. We suggest that the system is dynamically constrained but cannot discriminate between the fault slip and caprock hydraulic fracturing mechanisms. The dynamic capacity concept appears to be useful for understanding i) marked variations of hydrocarbon columns at moderate overpressure, ii) the absence of significant columns at severe overpressure, iii) the mechanisms by which hydrocarbons migrated into the extremely young sand reservoirs of this field.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana