IN SITU STRESS AND FRACTURES IN THE COLVILLE BASIN, ALASKA

In a petroleum system, fluid flow is central to that system's evolution and productivity.  In areas that are undergoing tectonic deformation such as the Colville Basin, fluid flow depends greatly on in situ stresses.  The minimum and maximum horizontal stresses (S_hmin and S_Hmax respectively), along with the inherent strength of the rock in which they are measured, determine the existence and orientation of fractures in that rock.  These fractures, if present, often have permeablities that are orders of magnitude greater than that of the unfractured rock in which they form.  An understanding of fracture location, orientation, and timing is therefore an invaluable tool for understanding (1) fluid migration pathways from the source rock to the reservoir rock, (2) the timing of that fluid migration, and (3) how best to utilize fractures during well production. 

Along with the roll of fluids (hydrocarbons and water) in the retention of pore space, the maximum vertical stress (S_v) to which a rock is subjected effects its porosity through compaction and diagenesis.  The magnitude of S_v is primarily controlled by pressure exerted at depth by overburden.  It therefore stands to reason that the apparent correlation between depth and occurrence of borehole breakouts may also depend on S_v.  Breakouts occur in the plane occupied by s₁ and s₃ therefore measurements of S_v may show that S_v must exceed S_hmin for breakout formation to occur.  This minimum depth of burial, used in conjunction with the depositional history of a basin, may be useful in determining the timing and location of fracture formation.

A regional study is underway that explores the relationship between horizontal in situ stress, S_v and open fractures in the Colville Basin.  The scope of this work includes (1) the regional mapping of S_hmin and S_Hmax orientations attained from observations of borehole breakouts in wells in the Colville Basin, (2) comparison of this orientation with those of regional map scale structures, (3) quantification of S_v from density logs, and (4) quantification of S_hmin from leak off tests.