Click to view this article in PDF format.
3-D Seismic in Identifying Spatially Variant Fracture Orientation in the Manderson Field, Wyoming*
By
David Gray1
Search and Discovery Article #40049 (2002)
*Adapted
for online presentation from an article by the same author in AAPG Explorer
(September, 1999), entitled “Using
3-D Seismic to Identity Spatially Variant Fracture Orientation in the Manderson
Field." Appreciation is expressed to the
author and to M. Ray Thomasson, former Chairman of the AAPG Geophysical
Integration Committee, and Larry Nation, AAPG Communications Director, for their
support of this online version.
1Research programmer ([email protected]), Veritas DGC Inc., Calgary (www.veritasdgc.com). Co-authors for this article are Kim Head, Veritas Exploration Services, Calgary, Kent Chamberlain, Gordon Olson, and John Sinclair, KCS Mountain Resources, Worland, Wyoming, and Chris Besler, Interactive Earth Sciences, Denver.
Abstract
Oil
is produced from the Manderson Field, in Wyoming's Big Horn Basin, from a
fracture system with possible significant lateral connectivity; therefore,
characterizing the fractures in this field is of great importance. Amplitude
Versus Offset (AVO) analysis
of azimuthally restricted seismic data is used to
determine the orientation and density of fractures at selected locations
corresponding to well sites in the Manderson Field. The result of this
analysis
is compared to fracture strike
analysis
from oriented cores for three wells.
|
The Manderson Field in Big Horn Basin, discovered in 1951, is located on a sharp, asymmetric, northwest-plunging anticline. It produces oil and gas from Pennsylvanian, Permian and Cretaceous horizons, although the Permian Phosphoria is the most productive zone. It is a complex interval consisting of a thick unit of medium-to-thick bedded, fractured carbonate. Purchased
by KCS Mountain Resources in 1995, the field showed several wells with
cumulative production in excess of original oil-in-place estimates. Low
matrix porosity, production history and strong pressure support suggest
that oil is produced from a fracture system with significant lateral
connectivity. Four oriented cores and one Formation Micro-Imager (FMI)
log were taken by KCS between 1996 and 1998. A 3-D seismic survey was undertaken in 1996 to improve structural definition of the reservoir. The survey was reprocessed in 1998 to further improve structural characteristics--and to detect those, fractures that strongly influence production. The
seismic data from the Manderson 3-D was acquired over a full 360 degree
azimuthal range--a suitable candidate for the test of measurements of
azimuthal anisotropy from pre-stack seismic data The
Manderson 3-D covers the locations of three wells for which oriented
cores have been analyzed for fracture azimuth. This experiment is to
determine whether fracture strike and fracture density can be determined
from 3-D seismic data in the Manderson Field. The requirements for this
The
five locations chosen for these tests are centered on the wells 4333P,
34-28P, 34-18P, 42-24P and 1218P. Well 43-33P is the most productive
well in the field, and wells 34-28P, 34-18P, and 42-24P have fracture
strike A
modification of the AVO method of Lynn et al. (1996) is used to estimate
the fracture strike and density at these locations. The theory behind
this method is that the acoustic The
primary direction of the anticlinal fold axis is 140 degrees (Figure
2).
The most dominant fracture direction may be expected to be parallel and
perpendicular to the fold axis at 140 degrees or 50 degrees,
respectively. The AVO azimuthal Azimuthal differences in AVO response are shown in the change in amplitudes with shot-receiver offset as shown in the boxes in Figure 3. These amplitudes are larger at long offsets in the gather on the left than the one on the right. The fracture azimuth predicted by the seismic data falls within the range of values estimated from the oriented core (Table 1). This azimuth appears to be the average value for the open fractures. For example, the strike orientation rose diagram for the open fractures of the 34-28P well (Figure 4) shows three significant strike directions, at 0, 60 and 105 degrees. If all these fracture strikes are averaged then 84 degrees is the expected response. The average fracture strike (Figure 5) is a useful value, because the largest volume of open fractures will be encountered by drilling horizontally perpendicular to this average fracture direction. Well 43-33P is the most prolific well in the Manderson Field, and it has the largest value of crack density. Wells 34-28P, 34-18P and 42-24P indicate that there may be correlation between crack density and open fracture aperture. Combine this with the high value observed for well 43-33P, and crack density appears to have some correlation with open fracture aperture. There
appear to be two predominant fracture strikes indicated by the azimuthal
AVO Fracture
strike and crack density are estimated from the seismic AVO response.
The seismic AVO results show consistent fracture strikes and crack
densities at the test locations, implying that these values are robust.
The most predominant estimated fracture strikes at these locations
coincide with the major geologic features in the area, a correlation
that makes sense from a geologic standpoint. The fracture strikes
derived It is important to note that this AVO method probably finds the average fracture direction. If the fractures have different aperture or spacing in different directions, then the seismic may find a weighted average. Far example, if fractures at 0 degrees are 0.1mm thick and fractures at 60 degrees are 0.2mm thick, then the weighted average is 40 degrees.
Lynn, H.B., Simon, K.M., and Bates, C.R., 1996, Correlation between P-wave AVOA and S-wave traveltime anisotropy in a naturally fractured gas reservoir. |