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.