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The AAPG/Datapages Combined Publications Database
Journal of Petroleum Geology
Abstract
Journal of Petroleum Geology, vol.
DETERMINATION OF PALAEOSTRESSES
FROM MESOSCALE SHEAR FRACTURES IN CORE SAMPLES USING THE MULTI-INVERSE
METHOD
Y. Yamada* and A. Yamaji**
*JAPEX Research Centre, 1-2-1, Hamada, Mihama, Chiba 261-0025, Japan. corresponding author: [email protected]
**Division of Earth and Planetary Sciences, Graduate School of Science, University of Kyoto, Kyoto 606-8502, Japan.
Palaeostress reconstruction is the key to quantitative characterization of fractured
hydrocarbon reservoirs. In order to calculate the palaeostresses responsible for fault
activity, faults in oriented core samples can be analysed and the optimal stresses can then
be determined from fault-slip data in a least
-
squares
sense. Many fractured reservoir
rocks, however, include faults generated during a number of different tectonic phases. In
these cases, the optimal stresses can be determined for subgroups of faults which are
considered to be of approximately the same age. The difficulty with this is that criteria for
accurate age-dating are often absent, especially from core samples.
Recently, the so-called "multi-inverse
" method has been proposed to address this
problem. This is a numerical technique for separating palaeostresses within a
heterogeneous fault assemblage in the absence of a priori information on the faults' relative
ages. In this paper, we apply the method to eighteen faults in two short (9m) cores from an
exploration well in Hokkaido, northern Japan. The cores came from a fractured
hydrocarbon reservoir and were oriented by correlation with borehole images. Core
lithologies included massive Cretaceous basalts in which there was no layering which
would show fault offsets; for these samples, we determined the sense of shear by studying
asymmetric microstructures on the fault surfaces. The results show that the rocks have
experienced six different stress regimes. In general, these are compatible with the tectonic
history of the study area as inferred from the regional stratigraphy and from macroscale
geological structures.
These results show that the multi-inverse
method can be used to extract palaeostress
data from cores, thereby providing information which will assist with the exploration of
(and production from) fractured hydrocarbon reservoirs.
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