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Claims  |
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I claim:
1. A well logging method of producing a map showing the structural dips of
depth zones of earth formations adjacent a borehole comprising the
following machine-implemented steps:
deriving formation dips at a succession of closely spaced depth levels in a
borehole from well log outputs of measuring devices carried on a dipmeter
tool passed through the borehole;
filtering the formation dips into a plurality of non-overlapping depth
zones each encompassing a number of formation dips most, but not
necessarily all of which are mutually consistent within the respective
zone, wherein the depth extent of a zone is not preset, but is determined
by the depth extent of the mutually consistent formation dips making up
the zone;
finding, from at least selected ones of the formation dips in the
respective depth zones, the respective structural dips of the respective
zones; and
identifying the respective structural dips of the last recited zones, and
producing respective traces on a map indicative of the last recited dips.
2. A method as in claim 1 including the machine-implemented steps of
rotating the formation dips which are within said depth zones to account
for the respective structural dips of the zones and adding, to said map,
traces showing the resulting rotated formation dips at positions which are
depth matched to the traces showing the structural dips.
3. A method as in claim 1 or 2 including the machine-implemented step of
combining at least selected ones of the dips within the respective zones,
deriving confidence limits of dip magnitude and dip azimuth for the
respective structural dips on the basis of the combined dips and showing
said confidence limits by way of traces on said map juxtaposed with said
traces showing dip magnitude and dip azimuth of the respective structural
dips.
4. A method as in claim 1 or 2 including the machine-implemented step of
locating dip patterns in the respective depth zones which correspond to
selected sedimentary patterns, including blue and red sedimentary
patterns, and producing tangible representations showing the kind and
characteristics of the located sedimentary patterns.
5. A well logging method comprising the following machine-implemented steps
of exploring subsurface formations:
deriving formation dips which are for respective closely spaced depth
levels in a borehole intersecting subsurface formations and are consistent
with logs taken therein;
filtering the formation dips to find groups thereof each of which extends
over a respective depth zone in the borehole and in each of which at least
most, but not necessarily all, of the formation dips tend to share a
respective common dip component which is likely to correspond to the
structural dip of the subsurface formations in the respective borehole
depth;
wherein the depth extent of a zone is not preset, but is determined by the
depth extent of the respective group of the formation dips sharing the
respective common dip component; and
identifying, and producing a tangible record of, said subsurface zones and
the respective structural dips therein.
6. A well logging method as in claim 5 in which the filtering comprises
building successive maps of the distribution of formation dips in dip
magnitude versus dip azimuth space, wherein a succeeding map is to a finer
resolution than a preceding map and covers a lesser range of dips than the
preceding map and is centered at the most populous partition of the
preceding map, and finding said zones on the basis of formation dips in a
selected most populous partition of the latest successive map.
7. A well logging method as in claim 6 in which said finding of zones on
the basis of formation dips in the most populous partition of the latest
successive map comprises building a depth histogram from the last recited
formation dips, finding zone on the basis of said histogram, said zone
being characterized by a succession of formation dips in which
depth-adjacent dips are within a selected distance from each other and the
dips are mutually consistent within a selected limit.
8. A well logging method as in claim 7 including finding said common dip
component for the last recited zone from a selected subset of the dips in
the last recited most populous partition.
9. A well logging method as in claim 8 in which said finding of the last
recited common dip component for a zone comprises finding trend dips which
are the dips within the most populous subpartition of the last recited
partition of the latest successive map and combining said trend dips in
three dimensional space to thereby produce said common dip component.
10. A well logging method as in claim 9 including removing the respective
structural dip components from the formation dips within the respective
depth zones to thereby produce rotated formation dips which tend to
correspond to the dips of stratigraphic earth formation features prior to
tectonic movement thereof.
11. A well logging method as in any one of claims 5-10 including finding
planar trends from the formation dips which are not included in said zones
and producing a tangible record of selected parameters of said planar
trends.
12. A well logging method as in claim 11 including the step of finding at
least one of blue and red stratigraphic patterns within said planar trends
and producing a tangible record thereof.
13. A well logging method as in claim 12 including finding the plunge of
said patterns and producing a tangible record thereof.
14. A well logging method as in claim 13 including removing said plunge
from the formation dips within said patterns to thereby produce therefrom
rotated dips corresponding to the attitudes of corresponding stratigraphic
subsurface features therein prior to tectonic movement thereof.
15. A well logging method as in claim 5 including removing the respective
structural dip components from the formation dips within the respective
depth zones to thereby produce rotated formation dips which tend to
correspond to the dips of stratigraphic earth formation features prior to
tectonic movement thereof.
16. A well logging method as in claim 5 including finding planar trends
from the formation dips which are not included in said zones and producing
a tangible record of selected parameters of said planar trends.
17. A well logging method as in claim 16 including finding the respective
plunge of the respective planar trends and producing a tangible record
thereof.
18. A well logging method as in claim 16 including finding blue and red
stratigraphic patterns within said planar trends and producing a tangible
record thereof.
19. A well logging method as in claim 18 including finding the respective
plunge of said red and blue stratigraphic patterns, removing the
respective plunge from the formation dips within the respective patterns
to thereby produce rotated dips therefrom, and producing a tangible record
of the last recited rotated dips.
20. A well logging method as in claim 16 in which said finding of a planar
trend comprises building a Schmidt map of the distribution, in dip
magnitude versus dip azimuth space, of the formation dips which are not
within said zones, selecting the most populous angular sector of a
selected size within the last recited map, finding in said sector a string
of formation dips characterized by (i) formation dips which are depth
contiguous within selected criteria and (ii) formation dips which are
mutually consistent within selected criteria, and producing a tangible
representation of a planar trend consistent with the last recited string
of formation dips.
21. A well logging method as in claim 5 in which said tangible record of
zones and structural dips comprises a visible trace on a record medium
having borehole depth versus dip magnitude coordinates, wherein the trace
for a zone comprises a straight line extending from the top to the bottom
of the zone at a position on the dip magnitude coordinate corresponding to
the structural dip of the zone, together with a visible indication of the
azimuth of the structural dip of the zone.
22. A well logging method as in claim 21 in which said visible record of
the structural dip for a zone includes a visible graphic indication of the
confidence limit on the structural dip magnitude and a visible indication
of the structural dip azimuth and the confidence limit thereon.
23. A well logging method comprising the following machine-implemented
steps:
deriving formation dips which are for respective depth levels in a borehole
depth interval and are consistent with logs taken therein;
filtering the formation dips to find respective common dip components for
respective groups of formation dips which are likely to correspond to
respective planar trends in respective non-overlapping depth zones of the
subsurface formation within said depth interval; and
producing a tangible record of said planar trends.
24. A well logging method as in claim 23 including finding blue and red
stratigraphic patterns within said planar trends and producing a tangible
record thereof.
25. A well logging method as in claim 24 including finding the respective
plunge of the respective planar trends and producing a tangible record
thereof.
26. A well logging method as in claim 25 including removing the respective
plunge from the formation dips in the respective planar trends to thereby
produce therefrom rotated formation dips corresponding to the attitudes of
the respective subsurface features prior to tectonic movement thereof.
27. A well logging method as in claim 23 including finding the respective
plunge for each respective planar trend and producing a tangible record
thereof.
28. A well logging method as in claim 27 including removing the respective
plunge from the formation dips within the respective planar trends to
thereby produce therefrom rotated formation dips which are likely to
correspond to the attitudes of the respective subsurface features prior to
tectonic movement thereof.
29. A well logging method as in claim 23 in which said tangible record
comprises a visible arrow plot on a record medium in which the arrows for
formation dips retain the dip magnitude of the original formation dips but
have their azimuth indication corrected to remove therefrom the plunge of
the respective planar trend and including an arrow of different
characteristics for the dip magnitude and azimuth of the plunge for the
respective planar trend.
30. A well logging method as in claim 23 including filtering the formation
dips to find zones other than planar trends in which the formation dips
have respective common dip components which are likely to correspond to
the structural dips of the respective zones, and producing a tangible
record of said zones and structural dips thereof.
31. A well logging method comprising the following machine-implemented
steps:
logging a borehole depth interval with a dipmeter tool and deriving
therefrom formation dips which are for respective depth levels in the
borehole and are consistent with the dipmeter logs;
filtering the formation dips to find non-overlapping depth zones within
said borehole depth interval, wherein the depth extent of each respective
zone is determined by finding a sequence of formation dips having a common
dip component which is likely to correspond to the structural dip of the
respective zone; and
producing a tangible record of said zones and likely structural dips.
32. A well logging method as in claim 31 including filtering the formation
dips to find planar trends in the subsurface formation logged by said
dipmeter tool which are not included in said zones and producing a
tangible record of selected parameters of said planar trends.
33. A well logging method comprising the following machine-implemented
steps:
logging a borehole with a dipmeter tool and deriving therefrom formation
dips which are for respective closely spaced depth levels in the borehole
and are consistent with said dipmeter logs;
filtering the formation dips to find respective common dip components for
respective groups of formation dips which are likely to correspond to
respective planar trends of respective non-overlapping depth zones in the
subsurface formation; and
producing a tangible record of said planar trends.
34. A well logging method as in any one of claim 31-33 in which said
tangible record comprises a visible log trace on a record medium.
35. A well logging system comprising:
first means for deriving formation dips which are for respective depth
levels in a borehole depth interval and are consistent with logs taken
therein; and
second means for filtering the formation dips to find respective
non-overlapping depth zones within said borehole depth interval, wherein
the depth extent of each respective zone is determined by the depth extent
of a sequence of formation dips having a common dip component which is
likely to correspond to the structural dip of the respective zone and for
producing a tangible record of said zones and structural dips.
36. A well logging system as in claim 35 in which the second means includes
means for further filtering of the formation dips to find, from formation
dips which are not included in said zones, planar trends of subsurface
formations and for producing a tangible record of selected parameters of
said planar trends.
37. A well logging system comprising:
means for derving formation dips which are for respective depth levels in a
borehole and are consistent with logs taken therein; and
means for filtering the formation dips to find respective common dip
components for respective groups of formation dips which are likely to
correspond to respective planar trends respective non-overlapping depth
zones in the subsurface formations and for producing a tangible record of
said planar trends.
38. A well logging system as in any one of claims 35-37 in which said
tangible record comprises one or more visible logs on a record medium.
39. A process for improving dipmeter logs confused by the unseparated
influence of structural dip, due to geological processes such as tectonic
movements, and stratigraphic dip, due to geological processes such as
depositional and/or erosional events, said logs being derived from the
outputs of a dipmeter tool passed through a borehole interval intersecting
subsurface formations likely to have been subjected to both kinds of
geological processes, comprising the following machine-implemented steps:
finding the respective structural dip component of the dips of subsurface
formations in respective non-overlapping zones in said borehole interval
which are likely to have undergone geological processes such as tectonic
movements; and
filtering the respective structural dip component from the respective dips
to thereby produce an improved dipmeter log in which the confusing
influence of structural dip is reduced and the stratigraphic dip influence
is emphasized.
40. A process as in claim 39 in which the finding of the structural dip in
a zone comprises building successive maps of the distribution of formation
dips in dip magnitude versus dip azimuth space, wherein a succeeding map
is to a finer resolution than a preceding map and covers a lesser range of
dips than the preceding map and is centered at the most populous partition
of the preceding map, and finding said zone on the basis of formation dips
in a selected most populous partition of the latest successive map.
41. A well logging method as in claim 39 or 40 including the
machine-implemented step of finding planar trends in subsurface formations
which are not included in said zones.
42. A process as in claim 39 or 40 including the machine-implemented step
of producing a record medium trace of said improved dipmeter logs. |
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Claims |
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Description |
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DESCRIPTION
1. Background and Summary of the Invention
The invention is in the field of well logging and relates particularly to
producing maps of underground earth formations in a way which enhances the
showing of formation features which are believed most important in the
search for hydrocarbons or other valuable underground resources.
One of the valuable aids in locating and exploiting underground resources
such as oil and gas are maps of the attitudes of underground formations in
the vicinity of a borehole. Variants of such maps are derived from the
outputs of a dipmeter tool which is passed through a borehole in the earth
formation and carries three or more well logging instruments which trace
respective paths spaced from each other along the cirumference of the
borehole. The outputs of these well logging instruments are combined in
various ways (see, e.g., U.S. patent application Ser. No. 537,998, hereby
incorporated by reference, filed on Dec. 30, 1974 in the name of C.
Clavier, A. Dumestre and V. Hepp and assigned to the assignee of this
invention now U.S. Pat. No. 4,348,748, granted on Sept. 7, 1982) so as to
find formation dips at a succession of depths in the borehole, for example
at each one-foot increment of the borehole. One common way of defining the
dip of a plane intersecting the borehole is by way of two characteristics
of a unit vector normal to that plane: the dip magnitude, which is the
angle between the vertical and that unit vector, and dip azimuth, which is
the angle in the horizontal plane measured clockwise (looking down in the
borehole) between true north and the projection of that unit vector on the
horizontal plane. A common way of showing such dips is on an arrow plot in
which the vertical dimension is depth and the horizontal dimension is dip
magnitude. The dips are shown on this plot as "tadpoles"--small circles
with "tails" or lines emanating from them. The position of one of these
small circles on the arrow plot shows the depth at which the dip occurs in
the borehole and the dip magnitude while the direction of the tail
emanating from the circle shows the dip azimuth.
One difficulty with the common arrow plot is that each tadpole shows the
scrambled contribution of at least two different kinds of dips: structural
dip and sedimentary dip. Structural dip can be thought of as dip resulting
from tectonic movements, e.g., a common movement of many sedimentary
layers, and sedimentary dip is associated with dip resulting from, for
example, depositional or sedimentation processes as opposed to other
planar events such as fractures. Particularly in the case of high
definition dipmeter tools, where 50 or more dips may show per 100 feet of
depth in an arrow plot, it is difficult to find the clues which are
thought to be most useful in searching for and exploiting underground
resources such as oil and gas. One example of a needed clue is sedimentary
patterns, such as patterns due to deposition of layers from erosion events
at different ages. Another example of a useful clue is the structural dip
common to a number of sedimentary layers. Those clues, however, are
difficult to extract from the common arrow plot because it does not
identify the structural dip shared by a reasonably consistent succession
of sedimentary layers, or of the dips of such a group of sedimentary
layers after rotation to account for their common structural dip, or for
that matter which group of formation dips corresponds to a group of
sedimentary layers sharing a common structural dip.
Before this invention it had been common to seek such clues from arrow
plots by way of subjective interpretation of their contents. Only a few
experts have been considered competent in such a time-consuming process.
Understandably, such process is believed to be prone to error and is not
believed to be a reliable and efficient way to find a useful and accurate
map of the characteristics of underground formations which are believed of
greatest importance in the search for resources such as oil and gas.
In contrast, this invention makes it possible not only to map the
underground formation dips more quickly, more accurately and more reliably
than with the known prior art but also to produce a map of a kind that is
not known to have been produced before--a map which expressly shows by way
of a new kind and new juxtaposition of traces, characteristics of the
underground formations which are believed to be the ones most needed in
searching for and exploiting underground resources.
In accordance with one example of the invention, the output of a three or
more trace dipmeter tool is used in a filtering process such as that
described in the Clavier et al patent identified above, to find the depth,
dip magnitude and dip azimuth of formation dips at a succession of depth
levels in a borehole, and to associate each of these formation dips with a
cell in a hemispherical equal area map. One way to visualize this
collection of cells is to think of a hemisphere divided into some number
of equal area cells. The radius of the hemisphere is a unit and the
hemisphere is resting on a horizontal plane. Each unit vector representing
a formation dip originates at the center of the hemisphere and ends at a
point on the hemisphere. A unit vector representing a horizontal formation
will end at a point on the top of the hemisphere and a unit vector
representing a vertical formation will end at a point on the circumference
of the hemisphere. The unit vectors representing formations having the
same dip will end at the same point on the hemisphere. Of course a point
on the hemisphere does not indicate the depth in the borehole at which a
particular dip occurs; it only indicates the magnitude and azimuth of the
dip. A dip can thus be identified by four characteristics: the depth at
which it occurs, its dip magnitude angle, its dip azimuth angle, and the
cell of the hemispherical collection of cells to which it belongs. The
number of equal area cells on the hemisphere is arbitrary; in one example
of this invention a 45 by 45 collection of cells is used.
The formation dips derived and characterized in this manner are then
filtered into depth zones made up of dips which are mutually consistent
within a zone. A depth zone can be thought of as portion of the borehole
which is between a top depth and a bottom depth and contains formations
having dips which are mutually consistent within a selected criteria. For
example, a depth zone may contain formations which are associated with a
string of dips which is nearly continuous and consists of dips which are
the same within a selected tolerance. In this context, "nearly continuous"
can mean, for example, that no two members of the string can be separated
in depth by more than a preselected small number of dips which do not
belong to the string and that the string should consist of at least a
selected number of dips which belong to it. In physical terms it means a
sequence of earth formations that tend to share a common structural dip,
e.g., a pattern of sedimentary dips which have been rotated together in
some tectonic movement.
In one example of the invention this filtering of the formation dips into
depth zones involves first finding the zone which tends to be of best
quality, i.e., tends to contain the most mutually consistent dips and
tends to be the longest zone containing such dips. One way to do this is
to pan through the hemispherical map in a way discussed in greater detail
below so as to find the most populous 5 by 5 window of equal area cells,
i.e., the window which contains the greatest number of points representing
unit vectors of formation dips. The formation dips within this 5 by 5 cell
window are further filtered into depth sequences, e.g., they are arranged
by the depth at which the dips within the window occur in the borehole.
These depth sequences are further filtered to find among them the longest
sequence (string) which is an almost continuous one. For example, the
arrangement by depth may be into cells of a depth histogram where each
depth cell covers a selected number of feet of depth, say enough so it is
likely that it covers about 5 to 10 consecutive dip levels. Then, two
consecutive empty depth cells in this depth histogram arrangement may
signify the end of one string and the beginning of another. The longest
string found in this manner defines a depth zone.
In order to use the best quality formation dips within this zone so as to
find the likely structural dip of the zone, the formation dips within the
zone are further filtered to find among them the "trend dips", i.e., the
dips within the zone which are believed to be most representative of the
likely structural dip of that zone. One example of doing this is to pan
the 5.times.5 window of the hemispherical map which produced the zone with
a smaller, 2.times.2 window, so as to find the most populous position of
that smaller window. The dips within this smaller window are believed to
tend to be the most reliable indication of what the structural dip of the
zone may be, and are considered to be "trend dips". These trend dips are
combined with each other as described in more detail below to find a
combined "trend dip" which tends to be indicative of the structural dip of
the zone. The trend dip so found is then treated as the structural dip of
the zone. Of course this structural dip, since it results from combining a
number of trend dips, is not necessarily equal to any one of the trend
dips, although the trend dips do tend to be within a close range of the
structural dip by virtue of the technique described above. The probable
error in the dip magnitude of the found structural dip is related to the
dispersion figure of the trend dips on which it is based. If the
structural dip is found by accumulating the three vector components (in
Cartesian coordinates) of the trend dips, then the dispersion figure can
be thought of as the arc cosine of the ratio of the length of the
resultant to the number of vectors included in the accumulation. The
probable error in the structural dip azimuth is a function of both the
dispersion figure and the structural dip magnitude.
Trend dips should be represented throughout the zone to which they belong.
Each fraction of the zone should contain a few representatives of the
trend dip, i.e., a few participants in the 2.times.2 window used to define
the trend dip area. If all participants were found to be grouped in one
small fraction of the zone, leaving the larger fraction free of occurrence
of trend dips, those participants could not be considered representative
of the zone. In order to check for this the system finds a randomness
measure by considering the average distance between pairs of trend dips
chosen on either side of the median depth of the zone to which they belong
and comparing it to half the length of the zone itself. If the ratio is
unity, the degree of randomness is regarded as perfect. If the ratio is
much smaller, participants are considered to be non-representative and the
zone is rejected. The ratio could reach a maximum of two for only two
participants happening to fall at the zone boundaries; this zone would
also be rejected as containing too few participants to be representative.
The system may apply further criteria to the zone. As an example the length
of a zone may have to exceed one depth histogram cell length; the number
of trend dips in the zone must exceed an arbitrary minimum, such as five;
and the randomness measure must exceed a minimum value such as 0.1. If the
zone fails any of these criteria it may be discarded by the system and all
the formation dips used in defining the zone released for consideration in
finding other zones.
Once a zone has been defined and characterized as discussed above, the
system resumes considering all dips in the equal area map except those
belonging to formerly defined zones whether accepted or discarded. There
is thus a progressive exhaustion of the population of the equal area map
until no more than an arbitrary few are left. The identified zones may be
arranged in some convenient order, such as in order of increasing depths.
At this time a preliminary map may be produced showing, by way of map
traces, the dip azimuths of the structural dips of the respective zones.
The map may show the zone tops and bottoms related to borehole depth and
the dip magnitude and dip azimuth of the structural dips within the zones.
In addition, the same map or a separate printout may show various
characteristics associated with the found and shown structural dips, such
as the probable errors, the randomness measure, the number of depth levels
included in each zone, the number of trend dips and other possible
characteristics.
There may be, and typically are, gaps between the zones defined as
described above. These gaps may include intervals of depth which were
initial candidates to zones but were rejected for one of the three
criteria already mentioned. They may be intervals where no clusters of
formation dips could be found. This includes intervals where the
structural dip would vary smoothly as well as intervals where it would
vary chaotically and no discernible trend may be found. Such gaps may also
be due to intervals where no formation dips exist. This includes intervals
of zero thickness, i.e., of direct transition from one zone to another.
Some possible treatments of such gaps are discussed in more detail below.
An important step used in the invention after finding the zones is to
convert the original formation dips to rotated or relative dips. In this
context, a rotated or relative dip is the dip within a zone as it would
have been before the physical movement which resulted in the structural
dip associated with the zone. For example, if a sedimentary layer was at
one time horizontal but is not tilted due to a tectonic movement, the
rotated or relative dip of that layer would be that of a horizontal plane.
The rotated dips that relate to sedimentary layers thus tend to represent
the attitudes of those layers before they were subjected to movements tha
resulted in structural dip. In this manner, in accordance with the
invention the effects of sedimentary dip and structural dip can be
separated from each other.
Once rotated dips are available, the system can seek dip patterns in the
respective depth zones which correspond to selected sedimentary patterns.
For example a sedimentary pattern called a "red pattern" is characterized
by sedimentary layers which have about the same dip azimuth but have dip
magnitudes which increase the depth. Conversely, a sedimentary pattern
called a "blue pattern" is characterized by sedimentary layers which have
about the same dip azimuth but have dip magnitudes which tend to decrease
with increasing; depth in the borehole. The system seeks such patterns
using a procedure similar to that used to pan through the equal area map
of formation dips discussed in connection with finding zones. As one
example, the number of occurrences of azimuth values are plotted on an
azimuth frequency histogram and the most populous angular increment of the
map is found. The depths of the dips in that increment are examined to
find any sequence of two or more which are consecutive. If they are found
to be so, their relative dip magnitudes are tested. If they are found to
increase with depth, this is a "red pattern". If they are found to
decrease with depth, they are a "blue pattern". A correlation measure may
be found to cover intermediate cases. Once the dips belonging to the
current most populous lobe or angular increment of the azimuth frequency
plot have been so combined, the system resumes the same process for the
participants in the next-most populous lobe of the plot, and so on until a
lobe is found containing less than an arbitrary small number of dips, such
as two dips.
At this time the system may produce the final map showing the results of
separating structural dip from sedimentary dip and the results of finding
sedimentary patterns. One example of such a map may be produced on arrow
plot paper to show each zone as a vertical line drawn at the location of
the found structural dip as measured on the dip magnitude grid of the
arrow plot paper and extending from top to bottom of the zone to which it
belongs. At the midpoint of this line a small circle may appear. Centered
about this circle a horizontal bar shows the dispersion figure on dip
magnitude of the structural dip and a fan of opening equal to the
dispersion figure on dip azimuth is symetrically drawn with respect to the
azimuth direction of the structural dip. In addition, the locations and
characteristics of the found patterns of sedimentary dips may be shown in
a convenient manner. As an alternative, in addition to the map traces
discussed above, the map may show the individual dips after rotation, as
conventional tadpole symbols on arrow plot paper but after accounting for
the structural dip, and may show those that are associated with trend
dips. Other traces may include azimuth frequency plots and perhaps the
original formation dips on an adjacent, depth registered strip of arrow
plot paper. The map may thus conveniently show at the same time both the
structural and the sedimentary dip as well as the addition clues to the
underground formation discussed above.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an overall illustration of a well logging system making use of
the invention.
FIG. 2 illustrates a part of a dipmeter tool shown in FIG. 1.
FIG. 3 illustrates the dip of a formation feature intersecting a borehole.
FIG. 4 illustrates a hemispherical map and a unit dip vector.
FIG. 5 illustrates an equal area Schmidt plot of formation dips.
FIG. 6 is a flow chart illustrating some of the major steps in the
operation of a system embodying the invention.
FIGS. 7, 8 and 9 illustrate various equal area maps related to an
explanation of steps 202, 204 and 206 of FIG. 6.
FIG. 10 illustrates a histogram related to step 208 in FIG. 6.
FIG. 11 illustrates a minimatrix of dip values in connection with the
operation of step 212.
FIG. 12 illustrates the combination of unit dip vectors in connection with
step 214 of FIG. 6.
FIG. 13 illustrates simplified earth formation features intersecting a
borehole.
FIG. 14 illustrates the depth locations of dips forming a zone.
FIG. 15 is another flow chart showing other major steps in the operation of
a system embodying the invention.
FIG. 16A is an illustration of a hemispherical map plot of a planar trend.
FIG. 16 is an illustration of a digest map produced in accordance with the
invention.
FIG. 17 is an example of a dual presentation map produced in accordance
with the invention.
FIG. 18 is another flow chart showing some other major steps carried out by
a system embodying the invention.
FIG. 19 is a map of a planar trend showing plunge, right ascensions and
declinations relating to step 248 of FIG. 18.
DETAILED DESCRIPTION
Referring to FIG. 1 for an overall illustration of a well logging system
making use of the invention, a multipad investigating tool 10, commonly
referred to as a dipmeter, is lowered on an armored multiconductor cable
12 into a borehole 14 to investigate a subsurface earth formation 16. The
tool 10 is adapted for movement up and down the borehole 14 and may
include four pads 18, 20, 22 and 24 (the front pad 18 obscures the view of
the back pad 22 which is not shown). The pads 18, 20, 22 and 24 are
uniformly angularly spaced from each other along the circumference of the
borehole 14. Each pad carries one or two (or more) measuring devices each
adapted to derive well logging measurements, comprising sets of samples,
or logs, at the wall of the borehole 14. The pads 18, 20, 22 and 24 may
each carry, for example, a survey electrode designated A.sub.o, and one of
the pads, for example, in this instance pad 18, may carry an additional
survey electrode A'.sub.o, useful in determining the speed of the tool 10.
Each survey electrode A.sub.o is surrounded by an insulating material 26.
The insulating material 26 and the survey electrode A.sub.o are further
surrounded by a main metal portion 28 of the pad. The metal portion 28 of
each pad, along with certain other parts of the tool, may comprise a
focusing system for confining the survey current emitted from each of the
different survey electrodes into the desired focus pattern. Survey signals
representative of changes in the earth formation characteristics along a
path in the borehole opposite the path inscribed by the movement of the
respective electrode are produced from circuits comprising the A.sub.o
electrodes, focusing elements, and current return electrode B. In
addition, the tool may contain devices (such as magnetic compass and a
device for detecting the tool inclination from the vertical, which devices
are not shown) to provide signals from which the attitude of the tool
itself can be found each time the devices on its pads take log samples. A
detailed description of the multipad (and therefore multipath)
investigating tool is disclosed in U.S. Pat. No. 3,521,154 issued to J. J.
Maricelli on July 21, 1970 and entitled, "Methods and Apparatus for
Enhancing Well Logging Signals by the Use of Multiple Measurements of the
Same Formation Characteristic".
The upper end of the multipad investigating tool 10, as shown in FIG. 1, is
connected by means of the armored multiconductor cable 12 to suitable
apparatus at the surface of the borehole 14 for raising and lowering the
tool 10 therethrough. Mechanical and electrical control of the tool 10 may
be accomplished with the cable 12 which passes from the tool 10, up
through the borehole 14 to a sheave wheel 32 at the surface and then to a
suitable drum and winch mechanism 34.
Electrical connections between various conductors of the cable 12, which
are connected to the previously described electrodes, and various
electrical circuits at the surface of the earth are accomplished by means
of a suitable multi-element slip ring and brush contact assembly 36. In
this manner the signals which originate from the tool 10 are supplied to
signal processing circuits 38, which in turn supply the signals to a
signal conditioner 40 and a recorder 42. A suitable signal generator 44
supplies current to the tool 10 via a transformer 46 and, as may be
needed, to the various signal processing circuits at the surface. Further
details of such circuits are described in the aforementioned Maricelli
patent.
The log signals from the investigating tool 10 may be recorded graphically
by a film recorder 42. One su | | |
