Method of Determining and Displaying the Orientation of Subsurface Formations

This invention relates to the determination of geoligical orientations, and more particularly, to a method of determining and displaying these preferred orientations.

In geophysical exploration, for example, the mapping of subsurface formations is important. Plots depicting the depth, dip and azimuth (i.e., azimuth of the dip) of subsurface formations are important geophysical tools. Many different types of measurements are made to provide the raw data from which the depth, dip and azimuth of the subsurface formations are determined. Seismic exploration, well logging and gravimetric measurements, for example, have been used to provide data regarding the depth, dip and azimuth of subsurface formations. Usually, these measurements do not consistently represent the true dip and azimuth of the formations. For this reason, it is common to apply a smoothing or averaging procedure to the raw measurements in order to determine the preferred orientation of the subsurface formations. See Structural Geology, M. P. Billings, Prentice Hall, 3rd Ed. 1972, p. 377, for a discussion of the manner of determining and depicting the preferred orientation of subsurface formations.

Structural geologists, petrologists, sedimentologists and others use three-dimensional orientation analysis in their studies of the depositional environments. Examples of the gathering of measurements which supply three-dimensional data regarding the earth's subsurface include U.S. Pat. No. 4,241,429--Bloomquist et al wherein seismic measurements are made in a manner which can be used to determine the three-dimensional orientation of the subsurface layering of the earth. Another extensively used measuring technique includes the use of a dipmeter logging tool. Dipmeter logging is described in "The High Resolution Dipmeter Tool", Log Analyst, Vol. 10, No. 3 (May-June, 1969), pp. 3-11, Allaud, L. A., and Ringot, J; "Automatic Computation of Dipmeter Logs Digitally Recorded on Magnetic Tapes", JPT (July, 1962), pp. 771-782, Moran J. H., Coufleau, M. A., Miller, G. K., and Timmons, J. P.; "The Continuous Dipmeter as a Tool for Studies of Directional Sedimentation and Directional Tectonics", SPWLA 9th Annual Logging Symposium (June 23-26, 1968), pp. G1-25, Rodriquez, A. R., and Pirson, S. J.; and "A Three Dimensional Vector Method as an Aid to Continuous Dipmeter Interpretation", Geologie En Mijnbouw, Vol. 50, No. 6 (1971), pp. 725-732.

It is an object of the present invention to determine and depict the preferred orientations in depositional environments and in other orientation studies as represented by dipmeter, petrofabric and other geological and geophysical measurements.

In accordance with the present invention, the preferred geological orientation is determined by converting geological measurements, such as the dip and azimuth of the dip of the subsurface formations, into unit vectors. These unit vectors are converted into lineations which represent axes passing through the center of a unit reference sphere. Each of these axes is then converted into two oppositely directed unit vectors, one being the original vector and the other being the backward extension to the reference sphere. All of the original unit vectors are each in turn designated as a reference vector. All vectors within 90.degree. of the reference vector are selected and the selected vectors are averaged to produce a resultant vector. The resultant vector having the maximum magnitude is used to indicate the preferred orientation of the formation.

In one embodiment, the invention is applied to dipmeter logging measurements to produce a useful output plot on which lines have an inclination representing the azimuth of the resultant vector, a horizontal position representing the dip of the selected resultant vector, and a vertical position representing the depth at which the measurement was made. This produces a plot which is similar to the known dipmeter "tadpole" plot, but one on which the resultant vector is represented.

In accordance with another aspect, the invention is applied to measurements made in successive depth intervals so that the preferred orientation for each successive depth interval is determined. In accordance with another aspect of the present invention, a moving depth interval is applied to the measurements. The preferred orientation is determined with this moving depth interval at successive increments of depth.

In accordance with another aspect of the invention, the special case where more than one resultant vector has the same maximum magnitude is resolved to determine the preferred orientation.

The foregoing and other objects, features and advantages of the invention will be better understood from the following more detailed description and appended claims.

FIG. 1 depicts a cross-section of the earth upon which a dipmeter logging operation is performed;

FIG. 2 is a flow chart depicting the invention;

FIG. 2A is a flow chart of subroutine VECMAX1 which determines the vector having the maximum magnitude;

FIG. 2B is a flow chart of subroutine REFVEC which is called up if there is more than one resultant vector having the same maximum magnitude;

FIG. 3 depicts an example of 15 measurements of azimuth and dip;

FIGS. 4 and 5 show the coordinate system used in describing the example;

FIG. 6 shows the extension of the vectors of FIG. 3 into lineations;

FIG. 7 depicts the axes representation of FIG. 6;

FIG. 8 shows the initial reference vector and related half axes associated with FIG. 3;

FIG. 9 shows the vectors associated with reference vector no. 1;

FIG. 10 shows the original unit vectors of another example;

FIG. 11 shows the axes representation of FIG. 10;

FIG. 12 shows the resultant vectors with identical magnitudes equal to the maximum value;

FIG. 13 shows the axes representation of FIG. 12;

FIG. 14 shows the original vectors, primed vectors, and final reference vector associated with FIG. 10;

FIG. 15 shows the vectors associated with the final reference vector of the second example with conventional dipmeter measurements on the left and plots of the preferred orientation analysis of the present invention on the right;

FIG. 17 is similar to FIG. 16 but additionally shows the results of a conventional vector averaging procedure;

FIG. 18 is similar and additionally shows the results of a conventional pole averaging procedure;

FIG. 19 shows an output plot produced with a 60-foot moving window to which the process of the present invention has been applied;

FIG. 20 shows output plots produced by the present invention applied to 20-foot intervals and to a 60-foot moving window;

FIG. 21 depicts conventional vector averaging on a simple example;

FIG. 22 depicts conventional pole averaging;

FIG. 23 depicts the preferred azimuth orientation on another example with conventional vector averaging; and

FIG. 24 shows the preferred azimuth orientation on the same problem with the axes representation of FIG. 23.

Referring to FIG. 1, which represents an east-west vertical cross-section, a dipmeter logging tool is shown traversing a borehole through the formations. In this instance, the formations include a layer 12 having a dip of approximately 45.degree. and having an azimuth in the westerly direction. Formation 14 has essentially no apparent dip in the east-west vertical plane, but actually dips 45.degree. in approximately the northerly direction. Formation 16 has a dip of approximately 45.degree. with an azimuth in the easterly direction and formation 18 has a dip of about 45.degree. with an orientation, or azimuth, generally in the westerly direction. From the dipmeter measurements, the preferred orientation of each of the formations is to be determined.

Commonly, dipmeters have a plurality of electrical resistance measuring elements spaced around the circumference of the logging tool. The measurements of the earth's resistivity at these measuring element locations are converted into values of the dip and azimuth of the formation surrounding the borehole. A plurality of these measurements are made over the depth interval that is logged in the borehole. The left-hand side of FIG. 17 depicts the display resulting from a plurality of dipmeter measurements. Each of the solid circles on the left-hand side of FIG. 17 is placed at a horizontal position relative to the dip scale at the top which indicates the dip of the formation, in this case from 0.degree. to 70.degree.. Each of the lines directed outward from the solid circles has an inclination which indicates the azimuth of the formation; i.e., straight up is north, straight down is south, left is west and right is east. Each of the solid circles is positioned relative to a vertical depth scale to indicate the depth of the formation. From this confusing plethora of measurement results, the preferred orientation of the formations must be determined. This is a long-standing problem in geophysics, and it will subsequently be shown that prior art techniques of determining preferred orientation are ambiguous and misleading. The present invention produces, in the dipmeter case, a display shown at the right-hand side of FIG. 17 wherein the lines having arrowheads indicate the preferred orientation of the formations obtained from the measurement results depicted on the left-hand side. It will be shown that the present invention produces more reliable and accurate results than prior art techniques of determining preferred orientation.

FIGS. 2, 2A and 2B depict the operation of the present invention as performed on a digital computer. Before describing this process in detail it is useful to consider FIGS. 3-15 which explain the invention with a series of graphical illustrations. Although the invention is most useful for three-dimensional orientation studies, it is advantageous for simplicity and ease of comprehension to restrict the following discussion to two dimensional azimuth orientation problems. The procedure followed is the same in either case.

First, consider the azimuth orientation example illustrated in FIG. 3. Each of the vectors represents a measurement result. Since this is an azimuth problem (i.e., all dips are zero), imagine the unit vectors shown as lying on a flat horizontal surface. Refer to these vectors as the "original vectors". The .theta..sub.i designate the azimuths and the .phi..sub.i represent the dips plus 90.degree.. FIGS. 4 and 5 depict the coordinate system and the .phi. and .phi. angles.

The present invention takes the original vectors and projects them backwards through the unit sphere associated with these vectors to obtain "primed vectors". This step is illustrated in FIG. 6 where, for the two dimensional case under consideration, the unit sphere is represented as a unit circle. The solid lines designate the original vectors and the dashed lines the primed vectors. The primed vectors are defined by .theta..sub.i '. In three dimensions, these are designated by .theta..sub.i ' and .phi..sub.i '.

In the next step, each original vector and its primed counterpart is transformed into an axis through the unit sphere (or circle). For this example, this amounts to removing the vector arrowheads and constructing unit circle diameters, which are referred to as lineations, or axes, as shown in FIG. 7. These lineations represent the conversion of each original vector to a diameter through a unit sphere. From inspection of FIG. 7, we can see that the solution to the problem should indicate a preferred north-south orientation.

The present invention designates the original vector defined by .theta..sub.1 and .phi..sub.1, as "reference vector" number 1. The half-axes of all lineations within less than 90.degree. of this reference vector are selected as illustrated in FIG. 8.

The half-axes associated with reference vector number 1 are then converted into vectors as shown in FIG. 9. Conventional vector averaging procedures are applied and the magnitude of the resultant vector associated with reference vector number 1 is determined.

The process then chooses the original vector defined by .theta..sub.2 and .phi..sub.2 as reference vector number 2 and repeates the same operations as performed with reference vector number 1. The magnitude of the resultant vector associated with reference vector number 2 is then determined.

This procedure is continued until all original vectors have been used as reference vectors. Table 1 below lists the components and magnitude of the resultant vector associated with each reference vector for the case under consideration.

The direction of the resultant vector exhibiting the greatest magnitude is then chosen as the preferred orientation solution to the problem. Note that Table I lists five resultant vectors with the same maximum magnitude of 12.075. Those associated with reference vectors 1, 2, and 3 describe a preferred orientation in the positive Y-direction or a dip of 0.degree. and an azimuth of 0.degree. (due north). But those corresponding to reference vectors 14 and 15 produce a preferred orientation result in the negative Y-direction or a dip of 0.degree. and an azimuth of 180.degree. (due south). However, both of these solutions are equivalent. The bi-modal nature of the azimuth is due to the two dimensional properties of the example problem. In the general three dimensional case, if the resultant dip (other than 0.degree.) is taken as positive downward from the horizontal and the direction of the azimuth is associated with positive dip, then the bi-modal influence disappears and the azimuth solution is unique.

In accordance with an important aspect of the invention, special cases are resolved in a manner which determines preferred orientation. Under certain circumstances it is possible to obtain more than one resultant vector with the same maximum magnitude, and with none of these resultants, a solution to the preferred orientation problem. This can be demonstrated by adding an original vector oriented due east (azimuth=90.degree.) to the previous example and obtaining FIG. 10. The axes representation for the problem as presented in FIG. 11 shows that the preferred orientation is again north-south. If the same procedure as above is followed, Table I is replaced by Table II below.

In this Table, the maximum magnitude of the resultant vectors is 12.116 and is shared by the resultants associated with reference vectors 2, 3, 15, and 16. Those corresponding to reference vectors 2 and 3 describe a preferred orientation with a dip of 0.degree. and an azimuth of 4.73.degree. (or 184.73.degree.). The resultants listed with reference vectors 15 and 16 produce a dip of 0.degree. and an azimuth of 175.27.degree. (or 355.27.degree.). We already have seen from inspection of FIG. 11 that the preferred orientation should be in a north-south direction. Therefore, none of the resultants obtained provide the preferred orientation solution for this example.

In such a situation, the present invention performs extended steps to determine the correct preferred orientation. At the point where more than one maximum magnitude resultant vector is obtained, the program temporarily takes these resultants and treats them as original unit vectors. In the case under consideration, the problem then becomes that illustrated in FIG. 12. The axes representation is then obtained as shown in FIG. 13 and the resultant vector is determined to be defined by .phi.=90.degree. and .theta.=0.degree. (or 180.degree.).

The invention then uses this resultant vector as a final single reference unit vector along with all original vectors and primed vectors as illustrated in FIG. 14. The vector representation of the half-axes associated with the final reference vector (FIG. 15) is obtained and conventional vector averaging techniques produce the correct preferred orientation solution to this problem with dip=0.degree. and azimuth=0.degree..

The invention can be carried out with a general purpose digital computer which performs the steps shown in the flow sheet of FIGS. 2, 2A and 2B. An exemplary program listing is given in the Appendix. FIG. 2 depicts the main program. The step 20 reads in a plurality of measurement results for a particular depth, such as the fifteen values of dip and azimuth depicted as vectors in FIG. 3. These original vectors are extended backwards, as indicated at 22, to produce the vector shown in FIG. 6 and the lineations shown in FIG. 7.

As indicated at 24 in FIG. 2, the subroutine VECMAX1 is called. This subroutine returns the rectangular components and magnitude of the resultant vector which has the maximum magnitude along with a value of KEY2. If more than one resultant has the same identical maximum magnitude, subroutine REFVEC and subroutine VECMAX2 are used along with subroutine VECMAX1.

The step 26 determines if there is a preferred dip orientation. If KEY2 is not equal to 0, there is no preferred dip and azimuth and the program terminates with an appropriate message. If there is a preferred dip and azimuth, the components of the preferred orientation are determined as indicated at 28.

If the azimuth for the preferred orientation vector is indeterminate, no final solution is possible and the program terminates with an appropriate message as indicated at 30.

Step 32 is a call for subroutine DISPER which will be subsequently discussed.

FIG. 2A depicts subroutine VECMAX1 which selects each vector as a reference and determines the magnitude of the resultant. Step 34 chooses the reference vector as previously depicted with regard to FIG. 8. The steps between 34 and 36 in FIG. 2A determine the magnitude of the resultant vector associated with the reference vector of FIGS. 8 and 9. As indicated at 38 these steps are repeated using each original vector as a reference vector. The resultant vector which has the maximum magnitude is determined as indicated at 40.

The special case where more than one resultant vector has the same maximum magnitude is dealt with by the subroutines called in step 42. As indicated at 44, the subroutine returns to the main program with the rectangular components and magnitude of the resultant vector which has the maximum magnitude along with a value of KEY2.

FIG. 2B is a flow chart of subroutine REFVEC. Step 46 determines how many resultant vectors have the same maximum magnitude and it determines their components.

Step 48 determines the orientations of the maximum magnitude resultant vectors in terms of angles. Step 50 obtains these angles for the associated primed vectors.

As indicated at 52, subroutine VECMAX2 is called. This uses the maximum magnitude resultant vectors and their primed counterparts in the same manner as the original vectors and primed vectors in subroutine VECMAX1 to determine a final single maximum magnitude resultant vector and a value KEY2. This is used as the final reference vector.

The step 54 obtains the angles for the final reference vector. There is a return to subroutine VECMAX1 with the angles for the final reference vector along with the value of KEY2. Subroutine VECMAX2 is essentially the same as subroutine VECMAX1 without the call for subroutine REFVEC. It uses the maximum magnitude resultant vectors and their primed counterparts in the same manner as original vectors and primed vectors in subroutine VECMAX1 to determine a final single reference vector and a value for KEY2. Return is made to subroutine REFVEC.

Subroutine DISPER performs a dispersion analysis on the group of lineations to determine how good an estimate has been obtained. The computer program listed in the Appendix contains a dispersion analysis based on the work of Fisher, R. A., "Dispersion on a Sphere", Proc. Roy. Soc., London, Ser. A, Vol. 217, (1953), pp. 295-306. The dispersion analysis results are included in the printout from the program and the nomenclature is provided in the Appendices.

FIGS. 16-20 depict results obtained with the present invention practiced on dipmeter measurements of an actual well. FIGS. 16-19 depict the measurement results performed only on an interval of the well between 5,900 feet and 6,040 feet. As previously indicated, the left hand side of these figures shows the display of these measurements by prior art technique without using any averaging or determination of preferred orientation. The measurements are plotted to represent dip magnitude as a function of depth. The dip magnitude scale at the top ranges from 0.degree. to 70.degree. dip. The straight line originating at the center of each solid circle represents the azimuth of the true dip and is analogous to the north-seeking end of a compass needle. In these plots, north is taken in the upward direction.

FIG. 16 illustrates the results obtained when the three-dimensional axes analysis of the present invention is applied to each of the seven 20' intervals which are shown. These are plotted to the right of the conventional plots. This interval by interval analysis using the invention eliminates much of the overall scatter of the original tadpole plot and suggests that certain combinations of intervals represent continuity while others indicate change.

FIG. 17 includes the results of the conventional vector averaging procedure (CVAP) (without arrowheads) along with those from the invention (with arrowheads). Many of the azimuths determined by the CVAP tend to parallel those obtained from the invention, but the dips vary widely and are greater in magnitude in almost every case. Dips greater than 70.degree. are plotted as 70.degree..

FIG. 18 compares the conventional pole averaging procedure (CPAP) with the results of the present invention. Here again, the tadpoles with arrowheads represent the dips and azimuths as determined by the invention. In these plots, it appears that the two techniques produce azimuths that are more or less comparable, but the dips tend to be lower for the CPAP than the results of the invention for most intervals.

Another approach to analyzing dipmeter data is to use three-dimensional axes analysis along with a moving window. For this, let us use a 60' interval and a 20' step increment. This means that we apply the process of the invention over a 60' interval. If this is done, the plot of FIG. 19 is produced.

FIG. 20 is a condensed version of the total length of the well which was logged. In this figure, the continuity of azimuth and dip is impressive, particularly where this continuity carries straight through sections that appear to be poorly oriented on the original plot. It illustrates the differences in results obtained when the invention is used for interval by interval analysis and when the moving window technique is used. Both representations have their own advantages.

The moving window results suggest two major events. One of these occurs in the vicinity of 6,000' and the other apparently at 6,240'. The latter event demonstrates an abrupt and permanent change in dip on the order of 10.degree.. The first event is comparable in magnitude but apparently not permanent since the appearance of the plot is quite similar above and below this event.

In order to further demonstrate the usefulness of the invention, the procedure of the present invention is compared with commonly used vector averaging techniques. FIG. 21 illustrates the conventional vector averaging procedure, referred to as CVAP. Here, the vectors lie on the dipping planes, point in the direction of the dip, and are plotted on the projection of the lower hemisphere. If vector averaging is applied to these vectors, the resultant vector obtained is:

Resultant:

Azimuth=270.0.degree.

Dip=45.1.degree.

FIG. 22 illustrates the use of poles for the same problem. Here, the poles or normals to the dipping planes are plotted on the projection of the upper hemisphere.

It should be pointed out that a pole is also the representation of a dip vector that has been rotated 90.degree. upwards in the vertical plane containing the dip vector. Except for a dip of exactly 90.degree., all such rotations will result in vectors confined to the upper hemisphere where the dip angle is then given by the angle between the pole vector and the vertical axis of the coordinate system.

If vector averaging is applied to the poles shown in FIG. 22, the resultant pole vector obtained is:

Resultant:

Azimuth=270.0.degree.

Dip=0.4.degree.

This procedure where the poles to the dipping planes are used is referred to herein as the conventional pole averaging procedure or CPAP.

Upon inspection of the results of the two commonly used averaging procedures as applied above to a simple example, it is obvious that something is wrong. One resultant dip is 45.1.degree. while the other is only 0.4.degree., a difference of two orders of magnitude. The azimuths are identical in this example, but it can be shown that in many cases, they will not be so. Actually, neither of these results are valid. Application of the procedure of the present invention demonstrates that the correct resultant vector should be:

Resultant:

Azimuth=0.0.degree. (or 180.0.degree.)

Dip=0.0.degree.

Consider the azimuth orientation example illustrated in FIG. 23. Since this is an azimuth problem (i.e., all dips are zero), imagine the vectors shown as lying on a flat horizontal surface. Visual inspection suggests that, due to symmetry and location of the vectors, the preferred resultant azimuth is due east. This is actually found to be the result if the conventional vector averaging procedure (CVAP) is applied to this example. The conventional pole averaging procedure (CPAP) is useless in this case since the resultant azimuth is indeterminate.

FIG. 24 is the axes representation for this same problem. This illustrates a preferred north-south orientation of axes and an east-west symmetry plane. Axes analysis demonstrates that this is true.

Therefore, for this example, the correct preferred azimuth as obtained by axes analysis is either 0.degree. or 180.degree. and not 90.degree. as calculated using the conventional vector averaging procedure. In other words, the CVAP resultant is in error by 90.degree..

While a particular embodiment of the invention has been shown and described, various modifications are within the true spirit and scope of the invention. The appended claims, therefore, are intended to cover such modifications. ##SPC1##