IN SITU STRESS AND FRACTURES IN THE COLVILLE BASIN, ALASKA
KLEK, Alfred, Geophysical Institute, University of Alaska, Fairbanks, Department of Geology and Geophysics, Natural Sciences Building, 900 Yukon Drive, Fairbanks, AK 99775, [email protected] and HANKS, Catherine, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775
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 (Shmin and SHmax 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 (Sv)
to which a rock is subjected effects its porosity through compaction and
diagenesis. The magnitude of Sv
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 Sv.
Breakouts occur in the plane occupied by s1
and s3 therefore measurements of Sv
may show that Sv must exceed Shmin
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, Sv
and open fractures in the Colville Basin. The scope of this work includes
(1) the regional mapping of Shmin
and SHmax 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 Sv from density
logs, and (4) quantification of Shmin
from leak off tests.