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An Evaluation of the use of Drillers' Logs in Lithologic Studies of the Ogallala Formation of the Southern High Plains of Texas
JBty C.A. Wilson, J. T. Smith, G.L. Thompson &nd W.M.Sandeen____________
U.S. GEOLOGICAL SURVEYWATER RESOURCES DIVISION
Prepared by the U. S. Geological Survey in cooperation with the Texas Water Development Board
June 1972
7Z-
CONTENTS
Page
Abstract------------------ ------ ------ ._ __-__--.-_-------- i
Introduction ---- -- --- -- ------------------ _--_ ----- 2
Purpose and scope of the project----- ------------------- -- 2
Location and extent of the area---- -------- - 2
Methods of investigation---- ------ .--- ------------- 4
Availability of data --- -- ------ -- ------------------- 4
Computer analysis of drillers' logs and other hydrologic
data - -- ----- -- _-----_----__-_-- ------------- 7
Test-hole drilling ---- ----------- --------- ------ 12
Geophysical logging -- - - ------ -- ---------------- 15
Methods of measuring hydraulic conductivity- -- ------------ 22
Results of the investigation------- ---- - ---- ---------- 25
Utility of drillers' logs 25
Utility of computer analysis of drillers' logs---------- --- 29
Reference cited----------- ----------- -- ----__--_._-_---- 35
11
TABLES
Page
Table 1. Availability of water-well drillers' logs in
counties where data were prepared for computer
analyses _-_-______,_-___ _-_-_--___. ____»____ 5
2. Lithologic lexicon of the Ogallala Formation-- ------ n
3. Summary of geophysical logging of wells in the
Ogallala Formation- - - - - - --- -- 21
ill
AN EVALUATION OF THE USE OF DRILLERS' LOGS
IN LITHOLOGIC STUDIES OF THE
OGALLALA FORMATION OF THE
SOUTHERN HIGH PLAINS OF TEXAS
PROGRESS REPORT 1970-71
By
C. A. Wilson, J. T. Smith, G. L. Thompson,and W. M. Sandeen
U.S. Geological Survey
ABSTRACT
Logs made by water-well drillers were analyzed in conjunction with
test-hole drilling and geophysical logging to evaluate usefulness of
the driller's log in delineating areas that would be suitable for artifi
cial recharge of the Ogallala Formation. This preliminary study indicates
that lack of detailed and accurate information in many drillers' logs
prevents their use as a reliable source of lithologic information. For
many applications, such as evaluation of potential areas for artificial
recharge, the value of more complete and more accurate information will
be readily apparent as these applications become more widespread. More
effort will be required in collecting lithologic information as part of
the drilling operations.
-1-
100°
Counties in which geophysical
50 100 Miles
Counties in which geophysical logs and drillers' logs were obtained
FIGURE I.-Location of the study area and counties in which data were collected
-3-
Values given on drillers' logs such as water levels, are accurate; but
information on quantitative values such as well yields and drawdowns, which
were estimated or which are given as representative, may contain gross
errors. Many well locations, especially those given on application forms
for well permits or on well-data reports, are of questionable accuracy.
A greater degree of accuracy in determining well-site elevations is now
possible through the use of 7 1/2- and 15-minute Geological Survey topo
graphic maps.
Table 1 shows the availability of drillers' logs in counties where
the data were prepared for computer analysis. These logs were screened
to eliminate those that: (1) Did not show the base of the Ogallala;
(2) could not be located to an estimated accuracy of 1 mile on 7 1/2- or
15-minute topographic maps; or (3) did not have a continuous sequence of
lithology. Locations for most of the logs selected for computer analysis
are accurate to within 1/4 mile.
More logs were available in Lubbock County and in the northern one-
fifth of Lynn County because these areas are within the boundaries of the
High Plains Underground Water Conservation District No. 1. The District
requires that a driller's log and well-construction data be filed upon
completion of each well.
-5-
METHODS OF INVESTIGATION
Availability of Data
Drillers' logs of water wells are sources of extensive data on the litho-
logy and structure of the Ogallala Formation of the Southern High Plains; in
1969, there were about 59,000 irrigation wells in the area. Most of the logs
are in the files of the Texas Water Development Board, the High Plains Under
ground Water Conservation District No. 1, and the U.S. Geological Survey.
Other, but less extensive data include well schedules, water-level measure
ments, drawdown-yield measurements, pumpage inventories, well-location maps,
results of aquifer tests, and water-quality analyses. Very little of the
data in the Geological Survey files have been, gathered since 1958. Data in
the files of the Texas Water Development Board date mostly from 1964 to 1971;
the most recent material is the most abundant. Some geophysical logs may
be available from oil-company files.
A limited amount of geologic data and water-quality analyses are available
from the Geology Departments of Texas Tech and Texas A§M Universities. Some
field data and ground-water depletion surveys are available from the files of
consulting geologists and engineers who have worked in the area. Well-
construction data are available from the records of water-well drilling
contractors.
-4-
Tabl
e 1.
--Av
aila
bili
ty of w
ater-well
dril
lers
' lo
gs in
counties wh
ere
data
were prepared for
computer an
alys
es
Appr
oxim
ate
number of
Coun
ty
irrigation we
lls
I/
Arms
tron
g
Briscoe
Cros
by
Garz
a
Hale
Lubbock
Lynn
Swisher
212
650
2,082 60
4,400
6,200
2,466
4,60
0
Approximate
number of
Number of lo
gs
Number of lo
gs selected
dril
lers
' lo
gs available
avai
labl
e to
in
sa
mple
se
ts an
d to
th
e st
udy
samp
le population
coded
on formats
149
213
267 39
1,33
4
2,66
4 2J
2,470
3/
2,19
2
811
97 117
139 39
672
2,39
4 2J
+2,1
00 3/
1,992
452
66 93 128 27 326
786
2/
831
3/
489
275
I/
Texa
s Water
Deve
lopm
ent
Boar
d, 1969.
2j
Nort
hern
one-half o
f Lubbock
Coun
ty (north of
la
titu
de 33
°37'
30")
.
3/
Sout
hern
one-half o
f Lu
bboc
k County.
Computer Analysis of Drillers' Logs and Other Hydrologic Data
The formats developed for coding drillers' logs and other geologic and
hydrologic data are shown on figures 2a and 2b. The lithologic lexicon for
the Ogallala Formation is given in table 2.
The procedures used in manual processing of drillers' logs for coding
are illustrated by the systems flowchart shown on figure 3. The coded
formats were mailed to the New Mexico District for conversion into punched
cards for computer input.
A sample of the available logs were used for an initial evaluation of
the usefulness of drillers' logs. Sample set No. 1 consists of one log
per 2 1/2-^minute quadrangle, and set No. 2 consists of two logs per 2 1/2-
minute quadrangle, or 25 percent of all logs in the quadrangle, whichever
is greater. Not all quadrangles, however, will have enough logs (minimum
of three) for a complete sample selection.
The purpose of the second set was to compare the results based on the
greater sampling with that obtained from the first set. If the results
were not comparable, successive samplings would be tried to see if a suit
able match was obtained.
-7-
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Explanation of ayiabola \ Plot location 7\4 field no. in /\2 1/2 ain. grid/\ontopo. nap. /\ Sanple /\ population /
\Detarmina lat.,\long., * elev. itfroa topo.napi/\for lag* in /laanple seta/
Figure 3. Flowchart for manu&l procenini; of drillers' logs
-10-
Tabl
e 2.
--Li
thol
ogic
le
xico
n of the
Ogal
lala
Formation
Rock
type
Bentonite
Blue
Boul
der
Boulders
Break
Cali
che
Capr
ock
Cavi
tyCh
alk
Clay
Cong
lome
rate
Dirt
Fullers
eart
hGr
avel
Gumb
oHardpan
Hone
ycom
bLakebed
Laye
rsLe
dge
Ledges
Lime
Lira
eroc
kLimestone
Mixture
Muck
Mud
Packsand
Pebble
Red
Redbed
Redbeds
Riverbed
Rock
Rocks
Quar
tzSand
Sandrock
Sand
rock
sSandstone
Shal
eSh
ell
Shel
lsSilt
Slag
Soapstone
Stone
Soil
Streak(s)
Subs
oil
Surf
ace
Tops
oil
Water
Whit
e rock
Wate
rsan
d
Code
BENT
BLU
BLDR
BLDRS
BRK
CLCH
CPRK
CAVT
CHK
CLY
CGL
DIRT
FLET
HGV
LGMB0
HDP
HNYCB
LKEBD
LYRS
LDG
LOGS
LME
LMRK
LS MXTRE
MUCK
MD PCKS
DPBL
REDD
RDBD
RDBDS
RVRB
DRK RK
SQT
ZSD SD
RKSDRKS
SS SH SHEL
SHELS
SLT
SLAG
SPST
NST S0IL
STR(S)
SS0I
LSU
RFTS
0IL
WTR
WHRK
WTSD
Rock
-type
modifier
Argillaceous
Bed
Bentonite
Calcareous
Cali
che
Cap
Capr
ock
Chal
kCh
alky
Clea
nCr
ysta
llin
eDense
Flin
tFo
ssil
ifer
ous
Glassy
Gran
ite
Gum
Gumbo
Gumm
yGy
psum
Hone
ycom
bLake
Lime
Lime
yLoam
Mica
Mired
Mixture
Muddy
Multicolor
Pea
Pebble
Quar
tzQu
ick
Rive
rRocky
Sand
Sand
ySh
aly
Soli
dSu
gar
Water
Code
ARC
BD BENT
LIMY
CLCH
CP CPRK
CHK
CHKY
CLN
XLN
DNS
FLNT
F0SL
GL GRNT
GUM
GMB0
GUMM
YGYP
HNYC
BLAKE
LME
LMY
LOAM
MICA
MIRD
MXTRE
MDY
MLC0L
PEA
PEBB
LEQT
ZQC
KRVR
RKY
SD SDY
SHY
S0LID
SGR
WTR
Color
Blac
kBlue
Bluish gray
Brow
nBuff
Crea
mDark
Gray
Grayish
whit
eGreen
Gree
nish
Light
Multicolor
Orange
Pale blue
Pink
Red
Redd
ish
Redd
ish
brow
nTan
Whit
eYe
llow
Yellowish
Code BLK
BL BLGY
BRN
BUFF
CRM
DK GY GYWH
GN GNIS
HLT MLC0
L0RNG
PLBL
PK RED
RDSH
RBR
TAN
WH YEL
YWSH
Cementation
Ceme
nted
Cement
Crystallized
\
Grain
size
Coar
se
Code
CMTD
CMT
CRYLD
Code
CCoarse-grained CG
RDFine
Fine-grained
' Me
dium
Medium-fine
Medi
um-c
oars
eSmall
Angularity
Sharper
Angular
Hardness
Hard
Soft
F FGRD
M MF MC SML
Code
SHRPR
ANG
Code
HD SFT
Miscellaneous
Bear
ing
Bloc
kyBreaks
Brok
enCa
ves
Caving
Chip
sCrumbly
Doug
hDry
Firm
Fres
hHeavy
Heav
ier
Joint
Jointed
Ledge
Ledg
esLe
nse
Lenses
Loose
Mostly
Medium- loose
Pack
Packed
Poro
usPo
wder
ySeams
Shee
tsShell
Shel
lySp
ots
Sticky
Streak
Streaks
Stringer
sStrip
Stri
psSu
rfac
eTi
ght
Tough
Code
BRNG
BLKY
BRKS
BRKN
CVS
CVG
CHIP
SCR
MBY
DGH
DRY
FIRM
FRES
HHVY
HVR
J0NT
JTD
LDG
LOGS
LENS
ELNS
LSE
MSTLY
MLSE
PCK
PCKD
P0RS
PWDY
SEAMS
SHTS
SHEL
SHEL
YSPS
STCK
YSTR
STRS
STRGS
STRP
STRP
SSURF
TT TGH
Neutral
Amount
And
At Ball
sBase
Big
Bits
Clea
tyDe
epEq
ual
Fair
Few
Free
Good
In Layer
Laye
rsLi
ttle
Mixed
Not
Not
repo
rted
Of Or Real
Scat
tere
dSlightly
Small
So Some
Thin
To Very
Weak
W|th
Code
AMT
+ , AND
AT BALL
SBA
SEBIG
BTS
CLTY
DEEP
=, EQ
UAL
FR FEW
FREE
G IN LYR
LYRS
LTL
MXD
N0T
NREP
T0F 0R RE
ALSCAT
SL SML
S0 S0ME
THN
T0 V WEAK
WITH
The general processing systems used by the New Mexico District are shown
on figures 4, 5, and 6. After a series of error checks, updating, sorting,
and merging, all coded data for each county are stored on magnetic tape. The
computer output is obtained by using a program that lists the desired terms
from the Ogallala lithologic lexicon (table 2) or terms used in the New
Mexico District data file (OMNIANA). The requested information is retrieved
from storage by the computer and can be converted into a variety of data out
put such as coordinate points, value plots, tables, graphs, or visual displays
This study is primarily concerned with an output series of contour maps
based on lithologic or structural data. Coordinate points, based on lati
tude and longtitude, are plotted by an off-line plotting device. The desired
values are printed adjacent to the coordinate point, and these values may
then be contoured by hand. The values could be contoured by machine if the
computer were programmed for this operation»
Test-Hole Drilling
During fiscal year 1971, 58 test holes (total footage of 5,617 feet)
were completed in 80 days of auger drilling in Briscoe, Hale, and Swisher
Counties. The drilling unit, which was provided by the U.S. Geological
Survey from Denver, Colorado, is equipped with 5-foot sections of a 5 3/4-
inch diameter hollow-stem auger. The unit is also equipped with a split-
spoon sampling device for taking undisturbed formation samples for laboratory
analysis.
-12-
Keypunch data on cards
& verify
Old 0g lexicon listings Haw data deck
Good datareturned to
Ogallala project
Error check program
Oldmaster
storage ifavailable
Correct & return
Bad cards returned to
Ogallala project
Error-printout to Ogallala project
Explanation of symbols
r Manual operation
Input/ output
Punched cards
Document
ieti< tape
Figure k. Systems flowchart for processing coded formats
-13-
\Punch desired\ lithologic\ terms forSubroutines
Subroutines:Call- up tables (class ification, manip ulation & accu mulation of data)
Resultsproofed byOgallalaproj ectpersonnel
Program ordata
errors corrected
Explanation of symbols
letic tape
Punched cards
Figure 5. Systems flowchart for table-generating programs
-14-
De»ired \data )
points I
Figure 6. Systems flowchart for plotting data positions and values
-IS-
Two test holes, one deep (100 to 150 feet) and one shallow (40 to 50
feet) were drilled at most sites. First the deeper hole was drilled, logged,
and sampled at 5-foot intervals or at changes in lithology. After the hole
was completed, a gamma-ray log was made through the hollow-stem auger, and
then the auger was removed. Another gamma-ray log was made in the uncased
hole. In addition, electrical and caliper logs were made at some sites,
and some undisturbed samples were taken at the bottom of the holes. All of
the data acquired by these operations were used to construct a lithologic
log of the test hole.
The second, shallower hole was usually drilled about 10 feet or less
from the first hole and was bottomed in a relatively impermeable layer of
clay or silt. The desired depth was determined from analysis of the logs
of the deeper hole, and an undisturbed sample was taken with the split-spoon
sampler to determine the hydraulic conductivity.
Geophysical Logging
Geophysical well logs, including gamma-ray, electrical, caliper, and
temperature, were used mainly to identify and to determine the boundaries
of various lithologic units penetrated by the well. Some additional data
on water quality, water temperature, and sediment consolidation can be
obtained from the logs.
-16-
A total of 309 logs was made in 222 wells and test holes during fiscal
years 1970 and 1971. Gamma-ray logs were made in all test holes and wells.
Electrical logs were made in some uncased wells and other types of logs were
made in a few selected wells and test holes. Table 3 gives the number and
types of logs made in the various counties of the study area.
A comparison of the lithologic log and the gamma-ray log for test hole
AK-11-12-8A is shown on figure 7. On completion of drilling a test hole,
it is best to make a gamma-ray log through the hollow-stem auger or drill
stem because caving of the hole may occur when the drilling tools are removed,
A second gamma-ray log is made after the auger or drill stem is removed.
Casing, auger, and drill stem all tend to subdue the response received by
the logging instruments. The difference in the character of the gamma-ray
log made through the hollow-stem auger and in the same hole after the auger
was removed is shown on figure 8.
The effect of logging through the casing of a large-diameter well is
shown by the gamma-ray log of well XT-11-20-5A (fig. 8), located about 800
feet east of well XT-11-20-5B. Logging below the water table increases the
loss of response in addition to the loss caused by the casing. Gamma-ray
logs made in gravel-packed or cemented wells are not accurate because the
measured radioactivity is caused by both the sediments and the gravel pack
or cement. Figure 9 shows a comparison of gamma-ray logs in well SP-24-32-8A
made first in the open test hole and then after the well was reamed out,
cased, gravel-packed, and cemented.
-17-
AK IH2-8A
Armstrong County 34°46'26"N I0r32'5l" Elev. 3412 feet
Gamma- ray Uncased 5 3/4-inch
dia. test hole
Radiation increases
-i 0Lithologic log
Soil
501
I001
ISO1
200'
oL
10 20i
30i
40i
50
COUNTS PER SECOND
Clay
Silty clay
Sandycaliche(caproc
Finesandandinter-
bedded caliche layers
Sand
Sandand
gravel
0
(red bed)
FIGURE 7-Gamma-ray log and lithologic log of test-hole AK IH2-8A in Armstrong County
-18-
XT
ll-
aO
-5B
XT
II-2
0-5A
Sw
ishe
r C
ount
y34
°41'
22"N
IO
I°32
'46"
Elev
. 34
03 f
eet
Gam
ma-
ray
tog
Cas
ing
5 3/
4-in
ch
$tee
l au
ger
Rad
iatio
n in
crea
ses
Uth
o lo
gic
log
CO i
50'
100'
150'
1020
_i
CO
UN
TS
PE
R
SEC
ON
D
Son
Silt
y ca
liche
Cal
iche
No
retu
rns
Sand
(p
oor
retu
rns)
Gam
ma-
ray
log
Unc
ased
5 3
/4 i
nche
s
Rad
iatio
n in
crea
ses
-i
0 50'
100'
-J
150'
102
0
i30
40
_i
CO
UN
TS
PE
R
SEC
ON
D
Sw
ishe
r C
ount
y34
° 41
* 12
" N
101°
32'
46"
Ele
v. 3
4OI
feet
Gam
ma-
ray
log
Cos
ing
15 i
nche
s
Rad
iatio
n in
crea
ses
50'
100'
150'
200'
1020 i
30
_IC
OU
NTS
P
ER
SE
CO
ND
FIG
UR
E 8.
-Com
paris
on o
f ga
mm
a-ra
y lo
gs m
ade
thro
ugh
a ho
llow
-ste
m a
uger
, in
the
ope
n ho
le,
nnrf
in
a
nan
rhw
r.n
<u*r
i ir
rin
nti
nn
SP
24-3
2-8
AS
P 2
4-3
2-8
A
Lubb
ock
Cou
nty
33°
30'2
8"
N 10
2° 0
2'5
4'
Ele
v. 3
30
0 f
eet
Lubb
ock
Cou
nty
33°
30'2
8"
N
!02
°02
'54
"E
lev.
3300 f
eet
Gam
ma-
ray
log
Unc
ased
9
5/8
-in
ch
dia
test
ho
le
Rad
iatio
n in
crea
ses
-i
0
Drilli
ng
flu
id
leve
l
501
I00
1
I501
Dril
lers
lo
g
Gam
ma-
ray
log
10-in
ch d
ia.
stee
l ca
sing
, gr
avel
pa
cked
in
20-
inch
dia
. ho
le.
Cem
ente
d 0
lo
±7
0
feet
gr
avel
pac
ked
±70
to 1
78 f
eet
Rad
iatio
n in
crea
ses
Cal
iper
lo
g U
ncas
ed 9
5/8
-in
ch
dia.
hol
e
J
200'
1020
30
_I
CO
UN
TS
PE
R
SEC
ON
D
Cal
iche
Sand
y cl
oy
and
sand
st
one
Gra
vel
and
cong
lorr
erat
e
San
d
and
grav
el
Cla
ySa
nd
and
grav
el
-Soi
l an
d cl
ay
Har
d sa
ndro
ckE
lect
rical
lo
g
Spo
ntan
eous
P
oten
tial
Res
istiv
ity
-Cla
yW
ater
lev
el
lOO
mv
50 o
hms
0 501
I00
1
150'
1014
1822
2630
2001
DIA
ME
TER
OF
WE
LL B
OR
E,
IN I
NC
HE
S
FIG
UR
E 9
.-Com
paris
on o
f drille
rs'
log
and
vario
us g
eoph
ysic
al l
ogs
mad
e in
wel
l S
P 2
4-3
2-8
A i
n Lu
bboc
k C
ount
y
Tabl
e 3.--Summary
of geophysical
logg
ing
of wel
ls in th
e Og
alla
la Fo
rmat
ion
Coun
ty
Armstrong
Bail
ey
Bris
coe
Castro
Cros
by
Floy
d
Garz
a
Hale
Lamb
Lubbock
Lynn
Martin
Farmer
Randall
Swisher
Tota
l number of
well
s lo
gged
21
1 21 1 10 8 2 74 3 24 13 1 2 2 39
Gamm
a -ray
24
1
20
1 13 9 3 79 3 28 12
1 2 2 45
Type
of
Resi
stiv
ity
and
spontaneous
pote
ntia
l
2 1 8 - - 1 - 13 - 7 1 - - - 5
geophysical
log
Cali
per
Temperature
- 1 2 - - - - 3 - 3 9
1 - - - 2
Conductivity
- - 1 - - - - - - 4 - - - - -
Tota
l22
2 243
3812
Methods of Measuring Hydraulic Conductivity
The methods used in determining the hydraulic conductivity of the
sediments were by laboratory analyses of core samples.
Core samples taken during the test-hole drilling program were analyzed
by the U.S. Geological Survey in Denver, Colorado, or by the Texas Water
Development Board. The core samples were analyzed for vertical hydraulic
conductivity, porosity, specific yield, and grain-size distribution. Some
samples from the deeper intervals were analyzed for horizontal hydraulic
conductivity. In addition, 10 bag samples were repacked and analyzed for
hydraulic conductivity, porosity, specific yield, and grain-size distribution,
The results of laboratory analyses of most of the samples are shown by
the graphs on figures 10 and 11. The minimum vertical hydraulic conductiv-r Q
ity of the samples was 5.8x10 ft/day (feet per day) or 2.05x10 cm/sec
(centimeters per second) in test-hole 11-45-2B (Briscoe County) at a depth
of 15 to 16 feet. Grain-size analysis for the sample in hole 11-45-2B shows
that 45 percent of the sediment is of clay size or finer (less than 4 micro
meters) and 75 percent is silt size or less (less than 0.0625 millimeter).
-22-
12 z u g
60
d o z < s £ t
160
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1 1
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RA
ULI
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ND
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EN
TIM
ET
ER
S
PE
R S
ECO
ND
FIG
UR
E 10
.-V
ertic
al h
ydra
ulic
con
duct
ivity
of
care
sam
ples
0
40
u H! Ul u <r 3 Ul
CD
80
120
160
200
240
1
- - - - -
1
1 »
X
*
1
!>?f?
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x
X v
^
x x
FT.
%
x *
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* i
*
1
1
1
Texa
s of
co
X
US
. G
e co
re-
I
EX
PL
AN
AT
I
Wat
er D
evel
oj
ire- s
ampl
e an
ol
ogic
al
Sur
ve
sam
ple
anal
ys
1
ON
>men
t B
oard
a
a lys
is
y an
alys
is o
f is
na ly
sis
- - - - -
1020
30
40
50
PE
RC
EN
T
PO
RO
SIT
Y
6070
80
90
100
FIG
UR
E 11
.-P
oros
ity
of c
ore
sam
ples
_2 The maximum vertical hydraulic conductivity was 34.5 ft/day (1.22x10
cm/sec) in test-hole 11-60-2A (Hale County) at a depth of 47 to 47.5 feet.
In the sample from test-hole 11-60-2A, about 20 percent of the material is
of silt size or less, and 75 percent is smaller than fine, sand (less than
0.25 millimeter). Grain-size percentages varied over a wide range, but the
average was about 64 percent clay and silt. The porosities of the samples
ranged from 25.1 to 45.5 percent.
RESULTS OF THE INVESTIGATION
Utility of Drillers 1 Logs
Many of the lithologic terms used on drillers' logs are vague or may
have several different meanings. For example, some of the descriptions of
rock types include such terms as "blue," "red," "break," "dirt," "lakebed,"
"ledge," "mixture," and "mud and rock" (table 2). Very often, an interval
of the formation may be described as "clay, sand, and caliche" or "sand and
rock layers."
The term "sand" means little on a driller's log because "sand" by
definition, is a particle size, and much of the Ogallala Formation is composed
of fine-grained sediments of low hydraulic conductivity. Few drillers use
standard particle-size ranges (American Geological Institute, 1960 p. 253),
so sediments described as "sand" may be very fine and practically imperme
able or may be very coarse and highly permeable. The term "silt" is seldom
used by the drillers, but a high percentage of the Ogallala Formation is
composed of silt-size particles.
-25-
A series of commercial drillers' logs, the corresponding geophysical
logs, and the interpretations of the geophysical logs are shown on
figure 12.
Good correlation for both lithology and boundaries of the units is
shown for logs AK-11-12-9A and KY-11-49-6A. Log XT-11-17-2A correlates
well witn the Geological Survey log of the samples collected at the well,
but mucn less so with the commercial driller's log. Drillers' logs for
wells XT-11-36-5A and JW-23-04-9A also cannot be correlated well with the
geophys i cal logs.
The preparation of good lithologic logs of the Ogallala Formation is
difficult because of the complex and rather variable lithology of the
deposits. In general, the thicknesses and depths of the different rock
types shown on drillers' logs are not closely comparable with those shown
by geophysical logs. Differences, many of them amounting to several tens
of feet, are attributed to difficulty in interpretation of depth of samples
during drilling. Much of the difficulty stems from the fact that the drill
commonly has reached a greater depth by the time material from a given
depth reacnes the surface. In consequence, the driller's logs commonly
indicate a greater thickness of water-bearing sand or gravel than is shown
on the geophysical log. Great care is needed in order to estimate more
closely, from drilling characteristics (for example, difficulty of drilling),
the depths at which significant changes in material occur.
-26-
The completeness and accuracy of the additional information given on
drillers' logs (such as location, well-construction data, static water level,
yield, and drawdown) differs considerably from one log to another. The
casing size, screened or slotted intervals and size, gravel-pack or straight-
wall designations, pump description, and engine or motor size are incompletely
described in many logs. Measurements or estimates of static water levels
and pumping levels are commonly included, but in many logs, the time or date
of measurement is not given. Yields estimated or based on the pump discharge-
pipe diameter (a convenient method commonly used) are at best approximate,
and may be greatly in error. The specific-capacity values calculated from
the given discharge values are therefore approximations also.
In general, the commercial driller's log is more accurate for the inter
vals below the water table than for those above it. Greater accuracy below
the water table probably reflects closer attention given to the drilling
and logging procedures. The base of the Ogallala Formation is usually picked
where the lithology changes from sand or sand and gravel to red clay or shale,
blue clay or shale, or limestone.
As the usefulness of this information becomes more widely recognized,
greater emphasis will be placed on the observation and recording of these data.
It should be noted that the wells have usually been drilled to extract water
for the least possible cost, and in practice there has been little demand for
the accuracy and detail that can be obtained from careful construction of a
lithologic log during drilling and from a complete summary of the test data.
-28-
Utility of Computer Analysis of Drillers' Logs
To determine the utility of computer analysis, data from drillers' logs
of wells in Briscoe, Hale, and Swisher Counties were used as input. A sample
of the initial computer printout showing a tabulation of the data is shown
on figure 13.
After tabulation, the data can be manipulated by using different table-
generating programs (figure 5). For example, the "total selected sands"
routine is a program designed to examine all lithologic data on each log
and to accumulate the thickness of permeable lithologic units such as sand
and gravel.
Lithologic rock terms and modifiers used by drillers to indicate good
permeability are: Medium-grained sand; medium sand; coarse-grained sand;
coarse sand; medium-coarse sand; sand and gravel; gravel; medium-coarse
gravel; medium gravel; fine gravel; coarse-grained gravel; river gravel;
layers of gravel; small gravel; some gravel; big sand; big gravel; scattered
boulders; loose boulders; and some boulders.
In general, the program is written so that the "total selected sands"
include all intervals containing gravel, boulders, and sands coarser than
medium. A page of computer output, showing the lithology of a log and the
accumulated "total selected sands" for the entire log and for the total above
the base of the Ogallala, is shown on figure 14.
-29-
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FOR UNIQUc NUHBER GQQOOOQQQ^
TOTAL S£LcC(£D SANO THICKNESS FOR THIS LOG
33.0
TOTAL SELEGTEO SAND THICKNESS TO
QGALLALA BASE =
33.0
The data selected and accumulated by the table-generating programs are
best analyzed by plotting the values on a map. Figure 15 is a Calcom plot
of the locations of drillers' logs in southeastern Hale County for sample
sets 1 and 2. The locations, which are represented by squares, are plotted
by latitude and longitude. The series of numbers to the left of the point
designate the county and the well number. The well number used for this
plot, whicn is a "unique number" ("figs. 2a and 2b) assigned by the programmer
is not related to the State well number. The numbers to the right show the
land-surface elevation. The values given as "0.0" indicate the placement of
other data that may be desired for that point. Latitude and longitude are
shown next to tne intersection symbols (+); however, not all values are shown
on tnis sample of the Calcom plot.
This project was terminated before preparation of a computer program to
combine the plotting program with the manipulating and accumulating programs,
such as "total selected sands." If this had been done, values of "total
selected sands" would be printed in place of one of the "0.0" shown on
figure 15.
Other data that could be generated with the plotting routine include
tne altitude of tne base of the Ogallala Formation and the total thickness
of clay, silt, and other similar rock types that occur in the upper 40 and
100 feet of the Ogallala Formation (fig. 16).
-32-
REFERENCE CITED
American Geological Institute, 1960, Glossary of geology and related
sciences, with supplement: American Geol. Institute, Washington,
D.C., 397 p.
-35-