PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE DATA FROM A CRATONIC BASIN ENVIRONMENT

ABSTRACT

Andrea F Brsterl and Daniel F. Merriam2

lGeoF orschungsZentrum Potsdam Potsdam, Germany

2Kansas Geological Survey University of Kansas Lawrence, Kansas USA

A thin veneer of clastic and carbonate sediments 460-1,375 m (1,500-4,500 ft) thick overlies a Precambrian crystalline basement complex in southeastern Kansas (Midcontinent, USA). Well-log temperatures were analyzed and related to modeled temperatures for different-size areas in a relatively simple structural setting of the shallow, cratonic Cherokee Basin. A statistical analysis of bottom-hole temperatures (BHTs) confirmed that (1) there is no significant change of temperature with season or through the 40 years of drilling and logging the wells in the area; and (2) the distribution of values is normal indicating they were recorded correctly on the rig within the instrumental precision of about 1 K. It was obvious from the large data set of nonequilibrium BHTs analyzed by depth and stratigraphic unit that they differed from drillstem test temperatures (DSTs) and modeled temperature­depth distributions based on heat conduction. At shallow depth (less than 500 m), BHT values as read from the logs are higher than 'true' formation temperature; in a depth range of 500-700 m, values scatter around a 'true' formation temperature; and at greater depths (up to 1,100 m), the uncorrected BHTs slightly underestimate the formation temperature. Different amounts of data scatter in the composite BHT-depth plots occur in different size areas, which also impacts the calculated geothermal gradients and empirical correction methods developed on the basis of these data. Although the BHTs in the 60x75 km area of Elk and Chautauqua counties scatter ±5-9°C around some mean value, seemingly the scatter can be reduced slightly when working with smaller areas, for example 10xl0 km. A mapping approach made to investigate the variability of the 60x75-km BHT cluster in more detail showed part of the BHT scatter to be related to regional geology. Temperature residuals of the trend are on the order of ±2.5°C with some values as large as ±7.5°C. This provides an indication of variability of BHTs resulting from other influences, which might be different perturbations as a result of drilling practices and different shut-in times of the

35

A. Förster et al. (eds.), Geothermics in Basin Analysis© Kluwer Academic/Plenum Publishers 1999

36 FORSTER AND MERRIAM

wells. In addition, some of the temperature highs are shown to be related to local, subtle anticlines developed in the sedimentary cover over faulted basement blocks and therefore contain signal. The separation of different effects on BHTs has implications on the reliability of geothermal gradient and heat-flow density estimations.

INTRODUCTION

Thermal conditions in sedimentary basins are related to heat flow from the underlying basement and the lithology and petrophysical parameters of the different sedimentary units. The accurate assessment of both the thermal structure and heat-flow distribution in basins is necessary to resolve processes, such as fluid migration, hydrocarbon maturation, and diagenesis. One of the goals in heat-flow studies is to identify and quantify the different modes of heat transfer within the sedimentary units.

Heat flow normally is determined from an average temperature gradient or a series of formation or interval temperature gradients obtained from a high-resolution temperature log made in a borehole under equilibrium in conjunction with an estimate of thermal conductivity from rock samples of the sedimentary sequence. However, the only temperature data usually available for basin analysis are bottom-hole temperatures (BHTs) or temperatures from drill stem tests (DSTs) measured in commercial boreholes drilled in the search for hydrocarbons. BHTs are measured in most hydrocarbon exploration wells, thus constituting large databases, but the data represent only one measurement from one particular depth.

Although the limitations of the data are understood, it is assumed that the lack of high-resolution continuous temperature logs can be compensated adequately by good spatial coverage of well logs with BHTs and a range of depths measured. Several studies concerned with the methodology of handling and interpreting large BHT data sets have been published in the last two decades (e.g. Majorowicz and Jessop, 1981a, 1981b; Majorowicz and others, 1984, 1985; Lam and Jones, 1984; Drury, Jessop, and Lewis, 1984; Bachu, 1985; Chapman and others, 1984; Speece and others, 1985; Deming and Chapman, 1988; Ben Dhia, 1988; Lucazeau and Ben Dhia, 1989). The general understanding is that BHTs have to be corrected to equilibrium ('true') formation temperature if they are used for heat­flow density estimates. But uncorrected BHT data have been used successfully under certain conditions for regional investigations (Bodner and Sharp, 1988).

Several methods to correct BHTs to 'true' formation temperature are available. The emphasis on these correction procedures has been either on the development of numerical methods to correct BHTs or by modeling the temperature disturbance effects in the borehole. Deming (1989) summarized the available correction methods and concluded that none are completely satisfactory because in most instances the information needed to handle the correction algorithm is not available. Hermanrud, Cao, and Lerche (1990) evaluated the error involved in correcting BHTs collected in boreholes from an oil field by using different numerical correction methods under known conditions. They concluded that advanced models gave values closest to DST ('true') temperatures, but with large standard deviations, whereas less advanced models gave temperatures too low also with large standard deviations. The error was attributed to poor quality data and physical effects not accounted for in the calculations.

Other studies were concerned with the use of inversion techniques to resolve the spatial variability of corrected BHT data (Speece and others, 1985; Willett and Chapman,

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE 37

1986, 1987; Deming and Chapman, 1988; Willet, 1990). Although the models seem to mitigate the effects of BHT data noise, the lack of available high-resolution temperature data does not allow judgement on what model best estimates actual temperature fields (McPherson and Chapman, 1996).

Some of the variability in the raw BHTs (the variability as a result of different shut­in time) obviously can be overcome by applying the proposed numerical correction procedures. The general noise in the raw data and the unknown quality of the measurements however are major problems in working with BHTs, especially when applying empirical correction methods. Empirical corrections are based on the relation between the BHT and 'true' (undisturbed) formation temperature in the measured depth interval (for example the AAPG correction method, Kehle 1972, 1973), but the original data scatter (error) will not be improved, only shifted towards a higher value. Deming (1989) concluded that the true error in raw BHT data is really unknown and that rigorous studies that compare true equilibrium temperatures to BHTs derived from the logging of oil/gas wells are warranted.

A discussion on possible influences of data quality is provided by Speece and others (1985). Some problems with the BHT quality are concerned with the way in which the measurements were taken and the conditions under which they were made. There is no evaluation as to the accuracy of the instrument nor recognition of the problems in obtaining, recording, and filing the data. The instrumental precision of industrial temperature data normally is about 1 K.

A single BHT value, even corrected to some formation temperature, is suspect, but the problem of the inaccuracy can be reduced by using large BHT data sets. From oil and gas fields tight clusters of corrected, as well as uncorrected, BHTs are known, which show that the scatter of the BHTs is about 15 to 20°C. This suggests that the random noise in the corrected BHT data may be on the order of ± 1 OOC (Deming and others, 1990). A study in the deep Northeast German Basin utilizing multiple BHTs (three or more BHTs measured at the same depth, but at different shut-in times) that were corrected using the Bullard (1947) equation and its simplification (the Homer plot) have shown for boreholes with multiple­depth BHTs that the data after correction scatter around some mean value by about ± 1 0-15°C (A. Forster and coworkers, unpublished data). In addition it can be shown that the BHT correction surprisingly underestimates the equilibrium temperature known from continuous temperature logs by about 5-15°C.

Considering the high variability in the data even after the correction, and that only one corrected BHT value may be available for a well, it is necessary to know how many BHTs are needed for (1) composite temperature-depth plots to obtain a meaningful temperature-depth relation and geothermal gradient for an area or vice versa, and (2) how large a geologic area can be to obtain a sufficient number of BHTs for composite plots with no increase in the basic data scatter. In addition, investigations of possible sources of variation other than depth in BHT measurements are desirable.

This study is a reconnaissance of commercial temperature data from the shallow, cratonic Cherokee Basin in southeastern Kansas (Midcontinent, USA: Fig. 1) where the Precambrian crystalline basement is overlain by a thin sedimentary veneer. In the U.S. Midcontinent, archives of commercial wireline logs are available. Petrophysical logs vastly outnumber high-precision temperature logs and data on subsurface thermal conductivity. However, by integrating thermal measurements with commercial logs, a high-quality database can be extended to viable synthetic thermal conductivity profiles for large regions which then can be used in combination with commercial temperature data to determine heat flow and evaluate the thermal structure.

38

DASSM ENT Sl"I\UCTURE 0' th~

MID. CONTINENT

FORSTER AND MERRIAM

c:=:J 'A1.EOZCJC 'ElSIC Vo..CAHIC ROCKS (St. (IIi. I I .

CJ PAlEOZOIC OJ4C .. ", fACC:S , ~ht~ , . n) A.,.. . t~ C"tUUYI 1.w~ I )

1:1 =, :::'!f:t =, :::f:=:::':f:=~'YO ..... n

Figure I. Generalized regional map of Midcontinent USA showing major basement structures and outcrop of Precambrian rocks. Cherokee Basin in southeastern Kansas where studies were made is highlighted. Also shown are Nemaha Anticline and Sedgwick and Forest City Basins (not identified) west and north of Cherokee Basin. Contours on top of basement (subsea); CI = 1,000 ft (from Adler and others, 1971).

Here, we focus on the problems and limitations of working with single temperature measurements in such a shallow sedimentary environment and to what extent available data might add information on subsurface geothermal conditions using simple plotting and mapping techniques. The area we have selected is one of several subareas in southeastern Kansas of well-known geology where the structural situation is simple, which is a prerequisite to learn about the general variability of commercial temperature data.

Figure 2 is an index map outlining the subareas in southeastern Kansas within this framework that were investigated previously by us. Forster and Merriam (1993), utilizing a map comparison/integration technique, and Forster, Merriam, and Brower (1993), utilizing multivariate statistics, analyzed the usefulness of (1) average gradients from continuous thermal logs at shallow depth, and (2) average gradients from uncorrected bottom-hole temperatures (BHTs) obtained from depths deeper than the continuous temperature logs in their relation to known geologic conditions. It was shown that the patterns of both types of geothermal gradient maps, as originally published by Stavnes (1982), have only vague similarity and that the interpretation of average geothermal gradients in terms of subsurface geology can be misleading. To work on the methodology of interpreting BHT data in southeastern Kansas, Forster and Merriam (1995) and Forster, Merriam, and Davis (1997) analyzed in more detail a subset of available BHTs and DSTs as a basis for making interpretations on a basinwide scale.

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE

a I o

I

SO MILES I

50 KILOMETERS

39

Figure 2. Map of eastern Kansas showing in detail subareas of fonner investigations: hatched = area of map comparison/integration and cross sections for thennal modeling (Forster and Merriam, 1993); stippled = area analyzed by multivariate statistics (Forster, Merriam, and Brower, 1993); cross-hatched = area ofBHTIDST analysis (Forster and Merriam 1995; Forster, Merriam, and Davis, 1996; this paper); double outline = area of thennal subsurface model (this paper).

GEOLOGICAL CONDITIONS

Southeastern Kansas is part of the stable US Midcontinent where Precambrian crystalline rock is covered by a thin veneer of Phanerozoic sediments. This veneer consists of Paleozoic strata ranging in age from Late Cambrian to Permian, capped by some Pleistocene and Recent sediments (Merriam, 1963). Figure 3 shows the main stratigraphic units, their general lithology, and approximate thickness.

The total thickness of the sedimentary veneer ranges from about 460-1,375 m (1,500 to 4,500 ft) (Fig. 4). Generally, the sediments dip gently westward 30 to 40 feet per mile except where a reversal of dip occurs on local structures. The sediment veneer thins toward the western slope of the Ozark Uplift located in western Missouri. Whereas the sedimentary section in the east comprises mostly carbonates (Mississippian and Cambro-Ordovician) because of the truncation, the section increases in clastic content to the west because of the presence of the Permo-Pennsylvanian on top of the Lower Paleozoic carbonates. The northeast-trending, southerly plunging Nemaha Anticline forms the western flank of the basin and is recognizable by a thinning of sediments over the crest. A south-plunging syncline subparalleling the eastern side of the Nemaha Anticline is the axis of the Cherokee Basin, which is a northern shelf extension of the Anadarko/Arkoma basin complex located farther south in Oklahoma (see also Fig. 1).

The regionally continuous Lower Paleozoic carbonates comprising the Mississippian and the Cambro-Ordovician Arbuckle Group are parts of the Ozark Plateau and the Western Interior Plains Aquifer Systems of the Midcontinent. As outlined in papers by Jorgensen

40 FORSTER AND MERRIAM

General Stratigraphic age thickness I ft. rock type

Column

............ AHemating thick shale,

- lenticular sandstone, and

Permo-Penn 0- 2000 thin limestone in cyclic

7':--:.7.-::7-:: :- pattern.

~'~':.::'':'

7:'::+::';';:.~ .;-;-.. -;-; .. -:::;,.

Mainly limestone with some dolomite and shale,

Miss-Dev 200 -450 cherty in places. Upper (0 -100) surface is karsted. Black

shale at base (Dev.l ///

Mainly dolomite wnh some limestone. Sandstone at

Cambro-Ord 700 -950 base (Cambrian). Upper surface karsted; lower surface uneven.

~~ v v· . ..,'" Granite I rhyolne crystallines

If' v v v PC

wnh some metasediments. ~,,'~' ... v v"'.., + Faulted in block pattem. ,' .. ' ... ' ... ' v v v

~;~;~; ;~~ 1.1 -1.4 Ga

',... ,","',',

Figure 3. Generalized stratigraphic column of units present in southeastern Kansas and their relative thickness and lithologic type (3.28 ft = 1 m).

(1989), Macfarlane and Hathaway (1987), and Musgrove and Banner (1993), these two aquifer systems have three regional patterns of groundwater flow. Groundwater in southern Missouri migrates radially outward from the Ozark Uplift through the Ozark Plateau Aquifer System; the resulting flow is westward into southwestern Missouri and southeastern Kansas. To the south in Oklahoma, groundwater flows out of the Anadarko/Arkoma basin complex northward into southern Kansas. In western and central Kansas, groundwater migrates to the east and southeast through the Western Interior Plains Aquifer System. These three regional flow components of groundwater meet and mingle in and near the study area.

GEOTHERMAL CONDITIONS

Only two heat-flow measurements are published for southeastern Kansas (Blackwell and Steele, 1989). The heat-flow density value determined in Miami County is 60 mWm-2,

the other value from Labette County yields 62 mWm2 • From high-precision temperature logs in these boreholes and from additional investigations in boreholes in central Kansas, it is known that formation temperature gradients differ vertically through a large range

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE 41

because of the contrasts in thermal conductivity of the sediments. Blackwell and Steele determined that the mean gradients in the carbonate sections of different boreholes were nearly identical, averaging 17-21°Ckm·1 for those which are predominantly limestone (Mississippian) and 13_17oCkm·1 for those which include dolomite (Arbuckle Group). Gradients in the predominantly shale sections (Pennsylvanian) range from 35 to 53°Ckm·1

depending on the shale content. It also was recognized that the heat-flow density in the boreholes is constant because thermal conductivity offsets the variation in formation temperature gradients. Thermal-conductivity contrasts in the subsurface result in a ratio of 2.5 : 1 between limestone and shale and up to 4 : 1 between dolomite and shale. As a consequence of the different lithology and thermal conductivity in the sedimentary sequence, significantly different temperatures occur at the same depth in those parts of the area that differ structurally and stratigraphically.

T o W N S H I p

31

J1

JJ

J4

JSS 10 II 12 13 14 15 16 17 18 19 10 21 22 2J 24 lS I'!

RANGE

Figure 4. Total thickness of sedimentary section in southeastern Kansas (from top Precambrian to surface). CI = 500 ft. Prominent Nemaha Anticline in western part of area; note thinning of stratigraphic section to east. Area is subdivided into counties and townships (N-S) and ranges (E-W); township is 36 sq mi.

Because of the lack of measurements of radioactivity in the basement, the influence geographically of a changing heat-production rate within the basement on the sedimentary temperature changes can not be determined. However, no extreme changes in the heat-

42 FORSTER AND MERRIAM

production rate in the underlying basement rocks that would cause lateral temperature changes in the area are suspected from the range of heat-flow density measured in eastern and central Kansas. Also lateral temperature variations in the sediments resulting from fluid flow and causing large-scale lateral transfer of heat are not evident in any of the aquifer systems (for example, see Blackwell and Steele, 1989).

Geothermal information on southeastern Kansas and other parts of the Midcontinent also is available from isotherm and gradient maps that have been published resulting from an analysis ofBHT data (AAPG - USGS, 1976a, 1976b). These average gradients computed on the basis of a two-point-straightline computation between BHT data and a mean-surface temperature value are manyfold in their interpretation as already shown by Forster and Merriam (1993) and Forster, Merriam, and Brower (1993) on two types of gradient maps originated by Stavnes (1982) and therefore did not add information on the area not already known from the few thermal logs measured.

MODEL OF THE THERMAL STRUCTURE

In areas where no, or only a few, subsurface temperature data are available and steady-state heat conduction is assumed, temperatures as a function of depth can be predicted using Fourier's law, q = k(dT/dz), where q is heat-flow density, k is thermal conductivity, t is temperature, and z is depth. In our situation of known heat-flow density for the area and sedimentary units with contrasting thermal conductivity, the subsurface temperature should be modeled considering the thickness of the various subsurface units and their characteristic thermal conductivity as:

n T = To + q I (Z; / k)

;=1 (1)

where Tis temperature eC), To is ambient surface temperature eC), z; (m) and k; (Wm·1K·1)

are thickness and thermal conductivity of the i th unit from the surface, and q (m Wm·2) is heat-flow density.

The equation was used in computing temperature-depth profiles at borehole locations along two E - W cross sections (Fig. 2) (Forster and Merriam, 1993), and for the purpose of this study, temperature-depth data at additional locations were generated to give a relatively homogeneous data distribution for mapping. In such a manner, a thermal structure within the sediments of southeastern Kansas, comprising all or parts of 32 counties from T 13 S to T 35 S, and R 1 E to R 25 E, was developed that is dependent on the thickness and lithology changes within the sedimentary section.

Table 1 lists the units (stratigraphic groups) in which the stratigraphic sequence of Figure 3 is organized. To involve the units in the subsurface temperature modeling, a lithotype was assigned to each group summarizing the percentages of lithology making up the interval. Considering the uniformity of the stratigraphic section regionally, the percentage of lithologies for each stratigraphic unit does not differ significantly in the area modeled. Thermal-conductivity values as measured by Blackwell and Steele (1989) in four boreholes in eastern and central Kansas were assigned to each lithotype by weighting the values according to the thickness of lithology. In such a way, the model reflects the conditions in the area in which lateral variation of measured thermal-conductivity values is small in comparison to the vertical variation because of alternating lithological units.

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE 43

Table 1. Thennal-conductivity data used in modeling.

Stratigraphic Unit Lithotype Thennal Conductivity (Wm·1K·1)

Sumner Group sh4/salt1 1.40 Chase Group sh11ls1 1.62 Council Grove Group Is3/sh1 2.05 Admire Group 1sl/sh1 1.70 Wabaunsee Group 1sllsh3 1.45 Shawnee Group 1 s lIshb2 1.52 Douglas Group Isllsh3 1.70 LansinglKC Groups Isllshllsstl 1.90 Pleasanton Group sstllsh2 1.51 Mannaton Group Isllshllsstl 1.90 Cherokee Group sstllsh2 1.25 Mississippian units Islldol 3.33 Chattanooga Shale sh 1.10 Hunton Group Is 3.15 Maquoketa Shale sh 1.25 Viola dolomite do 2.92 Simpson Group shllsslllslldol 2.20 Arbuckle Group do 4.00 Reagan Sandstone ss 2.47

sh = shale, ss = sandstone, Is = limestone, sst = siltstone, do = dolomite, salt - salt, shb - black shale

A surface temperature of l3°C (ambient present-day surface temperature in Kansas) and a heat-flow density of 60 m Wm'2 were used as input parameters. Figure 5 shows exemplarily for one borehole that the modeled temperature-depth values based on these input parameters coincide reasonably well with the actual measured temperature log (see Blackwell and Steele, 1989, for the temperature logs). Differences between modeled and measured temperatures that can be observed for example in the 500-m (1,650-ft) depth map (Fig. 6) are caused partially by differences in the measured surface temperature (differs between 13 and 15°C) compared to the input surface temperature for modeling (constant l30 C). The temperature conditions at the 500-m depth level are 35°C (95°F) in the western part of the area, but are 25°C (77°F) or lower in the eastern part. Differences in the thickness and lithology (thennal conductivity) of the sediments making up the sequence account for the different temperature conditions.

To illustrate the influence of sediment thickness/structural conditions on the subsurface temperature in detail, the calculated temperatures were mapped separately for four different subsurface (stratigraphic) units (Fig. 7); these are (from youngest to oldest): (A) Pennsylvanian (LansinglKansas City); (B) top of Mississippian; (C) top of Cambro­Ordovician Arbuckle; and (D) top of Precambrian. The Nemaha Anticline in the west (compare with Fig. 4) is reflected by local temperature anomalies over the crest. The anomalies are more pronounced on maps of the older units (Fig. 7B-D) and are an effect of shallower depths of these stratigraphic units and a thinner Penno-Pennsylvanian section. The thennal model in general shows that all stratigraphic units are characterized by a slight temperature increase to the west and southwest which is the result of the westward regional

44 FORSTER AND MERRIAM

Temperature ( DC )

o 10 20 30 40 50 60 O~~~~~~~~~~~-r~~~~~

200

400

E ..c 600 a Q)

Cl

800

1000

1200

14ooL-----------~----~-----L----~----~

Figure 5. Comparison of measured (solid line) and modeled temperature-depth profile (+) from Butler County. Measured temperature profile from Blackwell and Steele (1989).

dip of the units. Lowest temperature values on top of all units occur in the southeastern part of the area. The temperature differences between the southeast and the southwest of all maps are on the order of 40°C, except for the Pennsylvanian map (32°C) where the mean ambient surface temperature for the outcrop area is l3°C.

In contrast, at a depth of 500 m (Fig. 6), the temperature difference over the area is smaller and on the order of about 10°e. Again, higher temperatures occur in the southwestern part of the area because of the relatively thick, low heat-conducting Pennsylvanian clastic (shaly) sequence. Small anomalies of temperature highs and lows are associated with the Nemaha Anticline as indicated on the stratigraphic temperature maps (Fig. 7B-D). Temperature highs shown on the 500-m map are caused by thinning of the high heat-conducting Arbuckle and Mississippian units.

From this regional temperature model for southeastern Kansas, we selected the area of Elk and Chautauqua counties for the analysis of commercial well-log temperatures (see Fig. 2). This area is not affected by pronounced structural features, and therefore the modeled temperature for each stratigraphic unit increases steadily over the entire area with increasing depth of the layers. There is no complication in the structure (see Fig. 4) nor in the modeled regional temperature field.

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE 45

o 100 miles

~I ----------------~I----------~I o 100 kilometers

Figure 6. Modeled temperatures at 500m depth. Temperatures at locations A-D are from temperature logs; at locations A-C thermal-conductivity measurements were made (Blackwell and Steele, 1989); in parentheses are the measured surface temperatures. CI = 1 ·C.

ANALYSIS OF WELL-LOG TEMPERATURES

The Data

Bottom-Hole Temperatures. The data for Elk and Chautauqua counties, which is an area of about 60x75 km (36x45 mi), were compiled from well-log headers from the files of the Kansas Geological Survey. BHTs were recorded on resistivity and spontaneous potential logs, laterologs, density, and sonic logs. The data set comprises 637 BHT values, each value taken at total depth at a single borehole site. Despite the high density of boreholes, in most instances, available infonnation is limited. There is practically no infonnation on time since the circulation of drill mud ceased and the time the BHT value was obtained. Also, there are no multiple BHT recordings at the same depth, which would allow the application of a numerical correction procedure based on the temperature recovery in the well. This is the typical situation when working with BHTs in a mature hydrocarbon province such as the US Midcontinent.

BHTs as reported in this study, can be likened to working with well data such as fonnation tops reported on scout tickets, which are widely and successfully used in the

46 FORSTER AND MERRIAM

CJ <20'C

CJ 20' 28

[]]I] 28·36

IillIllTIiiil 36' 44

44·52

> 52aC

Figure 7. Contour maps of modeled temperature on top of different stratigraphic horizons in southeastern Kansas. CI = 4°C. (+) stands for well sites that were modeled. A, Temperature on top of Pennsylvanian; B, temperature on top of Mississippian; C, temperature on top of Arbuckle Group; D, temperature on top of Precambrian.

petroleum industry. Scout tickets may be all that is available when lacking well-log infonnation. Scout tickets may have errors in the reported depth of tops, misidentification of stratigraphic units, incorrect well elevations, etc., but in most instances these errors are negligible or obvious. IfBHTs are used in a regional study, then individual measurements are important only in contributing to the general pattern. Adjacent, similar values fonn part of the pattern and erroneous individual values usually are obvious and appear as anomalous values. Values deemed incorrect can be given less, or no, weight when interpreting the pattern. This is the same procedure geologists use when using scout tickets or other well data of unknown quality for structural mapping. We acknowledge that BHTs also may be

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE 47

incorrect in value or depth, but can be screened for useful information prior and after applying any empirical correction factor.

In our approach, we collected the BHTs and coded them according to the three major stratigraphic units of relatively homogeneous geology: (1) the Pennsylvanian; (2) the Mississippian; and (3) the Cambro-Ordovician Arbuckle. The homogeneous nature of the lithology is obvious for the Mississippian and Arbuckle carbonate units. Because of the thin-bedded nature of the Pennsylvanian section comprised of alternating clastic and carbonate rocks (see Fig. 3), the entire sequence can be considered homogeneous representing a typical mean geothermal gradient derived from logging on the order of 38±6°Ckm·1•

To test the possible range of subsurface temperatures for each of the three stratigraphic units in the area we applied two empirical correction factors: (1) the ForsterlMerriamlDavis (1996) correction factor; and (2) the average AAPG correction factor (Kehle, 1972, 1973). Maps made using original raw data and the two corrected BHT data sets are shown in Figure 8A, 8B, and 8C. The patterns are almost identical indicating the differences are merely shifts in average values. All three maps were contoured using the same algorithm and are directly comparable. The FIMID correction changes values from 0 to 3SC with larger changes in the west and small changes in the east (Fig. 8D) in the shallower part of the basin. The AAPG correction ranges from 1.5 to 2SC with a gradual increase to the west (Fig. 8E). This correction factor was designed for deeper measurements and has little effect on the values in the shallow Cherokee Basin, whereas the FIMID correction was constructed specifically for the Cherokee Basin and provides the 'best' empirical correction (Forster, Merriam, and Davis, 1996).

Drillstem-Test Temperatures. In processing available well-log data, it was obvious, as expected, that the temperatures recorded in drillstem tests usually were slightly higher than those recorded as BHTs (maximum mud temperatures). Vik and Hermanrud (1993) reported that temperatures from drillstem tests with high flow rates and low pressure drawdown can be expected to yield values close to 'true' formation temperatures. They applied a t-test statistic to their two subsets of data and determined that the BHT and DST data sets were independent and could not be integrated without a correction factor. This, also not surprisingly, is the situation for the Cherokee Basin BHT and DST data sets.

On the whole, 88 temperatures from drillstem tests (DSTs) were collected from the same three stratigraphic units as the BHTs. Only those values from the tests which showed oil or water inflow into the borehole were used. Excluded were those temperatures measured mostly in mud which do not represent 'true' formation temperature (Forster and Merriam, 1995).

IDENTIFICATION OF SIGNAL AND NOISE

A statistical test of month differences in BHTs showed there is no significant change of temperature with season. BHTs were measured for a 40-year period and show a slight increase of about 1°C through this time. The increase may result from (1) drilling to test deeper targets; (2) a change from drilling with cable tools to rotary rigs; (3) a technical improvement in the BHT measurement tool; or (4) unknown factors. Although drilling deeper will result in higher BHT measurements, the regression uses depth as an independent variable so this factor already is considered; there is an increase with time that cannot be attributed to deeper wells. The most likely cause for the increase in BHTs may

48 FORSTER AND MERRIAM

D E

Figure 8. Maps showing pattern of: A, uncorrected Mississippian BHT data; B, FIMID correction applied to BHT data; C, AAPG average correction applied to BHT data; A, B, and C contoured with same algorithm; CI = 2°C. D, Difference in uncorrected and FIMID corrected values; E, difference in uncorrected and AAPG average corrected values; CI = O.5°C. Configuration shows influence of westward regional dip and local structure. See text for explanation.

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE 49

be changes in drilling practices through time. The frequency with which specific temperature values were recorded was tested to see if some temperature, such as 100°F, occurred with unusual frequency. However, the distribution of values are normal suggesting that the values were recorded correctly on the rig.

Composite TemperaturelDepth Plots

In general, it should be possible to calculate an average temperature gradient on the basis of a BHT /depth plot if temperatures are well distributed along the depth profile. To get a statistically meaningful result, it is practice to generate a composite temperature/depth plot and work with confidence bounds for the BHT data scatter. A disadvantage of using composite plots can arise when the basic scatter in data (noise) is enlarged by signal from changes in structural conditions in the area.

To illustrate the problems, we plotted the BHTs from Elk and Chautauqua counties versus depth separately for each stratigraphic/lithologic unit (Fig. 9A-C). Our area of 60x75 km from which the BHTs are compiled for example is larger than the area of 30x30 km used by Jessop (1990) for comparison of uncorrected BHTs and equilibrium temperature logs in the Western Canada Sedimentary Basin. Each of our BHT data sets shows a large scatter in values with maximum departure from a mean value on the order of about 10°C. For comparison, Jessops' BHT values, even from a smaller area also scatter by the same amount determined by data regression. These scatter plots alone would confirm previous results on the variation of BHT data as reported by Deming and others (1990). However, the modeled temperature-depth profiles related to the plots (Fig. 9A-C; see also the section on modeling) reflect that in our situation some of the variability in the BHT values is a result of the gentle dipping of the layers in the area and a thickening of the Pennsylvanian units towards the west.

The modeled temperature-depth profiles El and Cl (Fig. 9A-C) reflect the thermal conditions in western Elk and Chautauqua counties in a comparable structural situation. The profile E2 represents eastern Elk and C2 eastern Chautauqua County. The changes in the stratigraphic section from east to west are indicated by the break in the temperature-depth curves caused by high heat-conducting Mississippian and Arbuckle carbonates, which in the eastern part of the area is located at shallower depths. On all plots it can be observed that some BHTs are higher than the 'true' subsurface temperature from modeling. This might be an effect of the used surface temperature of l3°C. If a value of 15°C would be used, as it was observed by continuous temperature logs at some locations, the entire temperature­depth curve would shift towards a higher value by 2°C and better approach the upper bound of the BHT scatter.

Pennsylvanian BHTs (Fig. 9A) show a scatter of 12-18°C. At about 500 m, the BHTs are near 'true' formation temperature; at depths shallower than 500 m, the BHTs are higher; and at depths greater than 500 m, they are slightly below the modeled formation temperature. In this depth range almost all BHTs are from the western part of the area and from slightly deeper depth and therefore have to be related to profiles El and Cl. The abnormally high values at shallow depths give rise to a couple of questions. Normally, by rotary drilling a temperature obtained at the bottom of a well shortly after cessation of drillmud circulation will result in a value lower than the equilibrium formation temperature. So the BHTs in the shallow subsurface might have been affected by other influences, which are not defined yet.

50 FORSTER AND MERRIAM

Temperature rC) Temperature ('"C) Temperature rC)

,0 ,S 20 25 30 35 40 45 50 ,0 ,S 20 25 30 3S 40 45 50 ,0 'S 20 25 30 3S 40 45 50 0 0 0

200 __ .l __ _C~ __ L __ ~ _________

200 200 , 0 !: OC

fl' 6 : c .

400 --~--~---~c -" ~- - - - ----- 400 400 , ;'lI 000

I : ~ I ~ 600 ' I I 0 I

600 ~ 600 --7---'---'---0 0

c!l , ~ 5 - c c ,

eo' 600

o Penn. ~ ___ ~ __ ~ __ - - 800 800 -E, -C,

,000 -E2 -- - --- '000 -E2 '000 -C2 -C2 -'CO, -'CC,

'200 '200 ,200

A B C

Figure 9. Plots ofBHTs versus depth for Elk and Chautauqua counties in comparison with modeled (C1, E1, C2, E2) temperatures and temperature profile (lCQ1) in shallow depth logged by Stavnes (1982). A, Pennsylvanian BHTs; B, Mississippian BHTs; C, Arbuckle BHTs. Data are compiled from area about 60x75 km (36x45 mi).

Mississippian BHTs (Fig. 9B) are mostly from the central and western part of the area with a few from the eastern part (shallow depth). These values scatter around the 'true' formation temperature. BHTs from the central and western part plot in the depth interval where the two sets of curves spread apart. Again, it is observed that the BHTs are scattered by about 18°C. The scatter in the data masks any relationship with the generally assumed trend of temperature disturbance by drilling.

Arbuckle BHTs (Fig. 9C) also are mostly from the central and western part of the area and therefore have to be related mostly to curves El and Cl. Again as observed with the Mississippian data, the BHT values generally fit in the envelope given by the two sets of curves, whereby the values from the west (from deeper depth) logically are more reduced and thus lower as the formation temperature indicated by the corresponding El and Cl curves. BHTs from the eastern and central area (depth interval 550-800m) are partly in ex­cess of the modeled temperatures (E2 and C2) and partly below.

The formation geothermal gradients based on regression analysis are about 16°Ckm·1

for all three stratigraphic units (Table 2), but a poor fit is indicated by the correlation coefficient. Whereas, the regressed BHT gradients in the Mississippian and Arbuckle car­bonates fall in the range of those gradients observed by thermal logging under equilibrium conditions, which are 21±3°Ckm-1 and 15±I°Ckm-1, respectively (see Forster and others, 1997), the gradient in the Pennsylvanian is far too low compared with the equilibrium gradient of 38±6°Ckm-l . The Pennsylvanian is the uppermost part of the sequence in southeastern Kansas and the regression line calculated for this unit should intercept the X axis of the plot at an ambient surface temperature of about 13°C (Fig. 9A). The actual value however is about 24°C indicating that the BHTs in the upper part of the sequence (at a depth of 200-500 m) are higher than 'true' formation temperature resulting in a temperature gradient that is too low. To analyze how robust those formation gradients are in terms of the number of data points and the depth range from which they are compiled, we computed formation gradients of BHTs from an interval of 500-800 m (Table 2); this is the depth from which most BHTs are available. The gradients obtained for this depth range differ slightly

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE 51

from the previous gradients comprising data from the entire depth range: the highest gra­dient is for the Arbuckle Group (21.6°Ckm-1) followed by the Mississippian (18_7°Ckm-1)

and Pennsylvanian (13.0°Ckm-1). In both situations, the order of formation gradients is a mismatch with the order of gradients known from continuous temperature logging. Also it is shown that the gradients do not reflect the expected disturbance effect on the raw BHT data because of drilling. As a result from plotting the data from different depth intervals it can be summarized that the general data scatter provides equivocal formation gradients.

Table 2. Statistical data from BHTIDST-depth plots.

BHT BHT DST

Unit Depth: 0-1100m Depth: 500-800m Depth: 0-1100m

no.point sd roc gradient no. point sd roc gradient no. sd roo gradient

Pennsylvanian . 141 4.0 0.60 16.0 65 3.4 0.28 13.0 40 4.5 0.95 32.8

Mississippian 313 3.4 0.45 15.7 251 3.3 0.37 18.7 38 3.3 0.78 24.2

Arbuckle 183 3.3 0.52 15.9 119 3.3 0.45 21.6 10 4.0 0.97 41.3

sd = standard deviation, r c:c = correlation coefficient

In a next step to investigate the sensitivity of the data scatter and its impact on the formation gradients, composite temperature-depth profiles from several different-size areas were analyzed. We determined that in our particular geological situation a reference area of one township (10xlO km; 6x6 mi) is too small to obtain a meaningful relation of temperature with depth (Fig. 10). In this situation the data scatter may be large compared to depth difference. Moving into larger areas of 3x3 townships (30x30 km; 18x18 mi), it was observed that the composite BHT-depth relations plotted for two different 30x30-km clusters give different results in terms of formation gradients, but also in terms of mean gradients averaged over the entire borehole depth. The difference in average gradient obtained from the two clusters was on the order of 8°Ckm-1, which amounts to 21 % gradient change compared to mean gradient known from logging. The number and distribution of data points available in the different areas also affect the correlation coefficient.

Temperatures measured during drillstem tests in Elk and Chautauqua counties are plotted in Figure llA. Where the data points are from small, producing oil fields (about 1I2x1l2 mi), only 1 or 2 values from the remainder of data for each field were used for the analysis. The DST data, which are mostly from the western part of the area better match the modeled temperature profiles (E1 and C1) than the BHTs (compare with Fig. 9), but in general are slightly higher. This might be an indication that the modeled temperature profiles are in error by about 2°C as a result of the surface temperature used in modeling as it was reported previously in this study. However, the overall match between DSTs and modeled temperature is most striking for the Pennsylvanian where the DST scatter is less than 8°C at any depth. The DSTs in the Mississippian and Arbuckle carbonates show a scatter of about 12°C which is comparable to the scatter of BHT data. Results of the regression analysis of DSTs for all three stratigraphic/lithologic units are shown in Figure lIB. The formation gradient indicated by the regression line is highest in Arbuckle dolomites and lowest in Mississippian limestones (Table 2). Although the DST gradients in the Pennsylvanian and Mississippian are about in the range of temperature gradients

52

Temperature (OC)

15 20 25 30 35 40 45 50 O~~~~~~~~~

200

400

800

1000

I I I I j I

_ L __ 1 __ 1. __ 1 __ .1 __ I I I I I

1 1 1

--1--t--1 1 I I 0 I 1 1 I I I I

_J __ !...~_I_Q _ 1 1 1 I

. . -~--~--:--~-:-

I I I I Q I

D Penn. • Miss.

o Arb.

-Cl

10 1

f» 0:

A

FORSTER AND MERRIAM

Temperature ("C)

15 20 25 30 35 40 45 50 O~~~~~~~~~

200

400

600

800

1000

1 I I

--l--+-­I I

1

1

1 I 1

_1.. __ 1 __ ...i __ 1 I 1

1- - + - -1- - -f --1 1 1

I

• Miss. - -;- - ~ - -:- - ~ --

o Arb.

-Cl

B

Figure 10. Temperature/depth plots for two townships in Chautauqua County. A, depth/plot for T 32 S, R8 E has data points from area with no data from oil fields; B, township T 32 S, R 9 E contains Hylton oil field with anomalously high temperatures that give large data spread for both Mississippian and Arbuckle. This example demonstrates differences that can be obtained by different data sets in similar geologic settings.

§: ~ Q)

0

Temperature rC)

10 15 20 25 30 35 40 45 50 55 60 65 O~~~~~~~~~~~~~~

, , , , , , 200 --f-- ---: .. ,

--' .. _-, --I -1--, , , , , , , , , , , , , , '. , 400 --t--~---~ . -r -" --:- --~- -{--, , , , , , , , .: , ,

600 DST

-E1

600 -C1 , , -E2 ' , , , -C2 , ,

1000 o Penn. --~---~--~--, , . Miss . ' , , , , o Arb.

, , 1200

,

A

§: ~ Gl 0

Temperature rC)

10 15 20 25 30 35 40 45 50 55 60 65 O~~~~~~~~~~~~~~~

200

400

600

800

1000

1200

, , , , I I I I I I I -T---,---r"--l---,---r "-I I I I I I

" , " , " , " , 1 I, I I I I

_1 ___ L __ J. __ J ___ L __ .1 __ _ I I I I I I

I ::

, , , I I I I I I

- -+- - -/- - -1--- -+-- -1- --l-I I L I I I I t I I I ,0

~ ~ : : : . i DST : : : : I

I I I I I

-e-Penn. -i--~---:---i--~-

-Miss. :: :: , " ..... Arb. I :: , ,

,

, , , , , , ---1---1----4---, , , , , ,

I : : , , , , , , t---i---

, , , ,

B Figure II. A, Plots of DSTs versus depth for Elk and Chautauqua counties for Pennsylvanian, Mississippian, and Arbuckle units compared with modeled (EI, CI, E2, C2) temperatures; B, data analyzed by linear regression. Statistical data given in Table 2.

obtained from high-precision temperature logging, the obtained temperature gradient for the Arbuckle carbonates of 41 °Ckm-I is too high compared to the logging results (13-17°Ckm- I).

The misfit of the DST Arbuckle gradient and temperature gradient from logging is

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE 53

surprising considering the high correlation coefficient of the DST data, but could be the result of too few data points.

The observed results in the BHTIDST-depth plots can be summarized as follows: (1) Uncorrected BHTs from shallow depths (less than 500 m) reflect temperature

conditions that are higher than undisturbed formation temperatures. In a depth range of 500-700 m, the BHT values scatter around a 'true' formation temperature. At greater depths a cooling effect on BHTs can be observed. The disturbance effect can be seen best on a BHTIDST depth plot (Forster and Merriam 1995).

(2) The composite BHT -depth plots for the three stratigraphic units show a total scatter of about 18°C. Some of this scatter is related to background, some is the result of different adjustment to thermal equilibrium, and some seemingly is the result of regional structure. Different BHT subsets show different statistical characteristics in the temperature­depth plots depending on area size and number of data points. Consequently, each data set reflects different temperature gradients for die stratigraphic units as well as the entire depth interval.

(3) As was expected, the DSTs better reflect 'true' formation temperatures and apparently are less variable than the BHTs. The 'best' correlation with depth is observed in the relatively thick Pennsylvanian and consequently, the DST gradient is closest to the formation gradient obtained from logging.

(4) We have observed in our study area that both, BHTs and DSTs are well suited for determining mean temperature gradients when averaged over some depth interval of at least several hundred meters. Even with a reduced DST scatter compared to the scatter of uncorrected BHTs, the variability of the values does not allow to calculate formation gradients unequivocally.

Temperature Maps

To analyze the temperature distribution in the different stratigraphic/lithologic units in more detail and delineate local and regional trends that might provide hints for signal in the data, the BHTs were mapped separately for the three different stratigraphic units (Fig. 12A-C). The maps help to interpret the overall BHT data scatter observed in the temperature-depth plots and identify the basic error in the data.

Although all three maps show a similar overall pattern, the distribution and amount of data differed for each map. A general trend of increasing temperatures towards the west can be observed on all three maps. This trend is controlled by the increased depth of the westward-dipping stratigraphic units. The pattern of the BHT maps however is not as regular as observed on the modeled temperature maps (Fig. 7 A-C).

Data for the Pennsylvanian map (Fig. 12A), comprising a depth interval from about 200 to 800 m, are scarce compared to the other two maps. Because of BHTs are scarce in the northern part of the area this part of the map cannot be interpreted. The data over the remainder of the map range from 21 to 43°C with most values in the range of25 to 35°C. A few small hot spots with temperatures from 35 to 43°C occur in the western part of the area. The pattern on the Pennsylvanian BHT map is difficult to interpret because of the considerable thickness of the unit and different depths from which the BHTs were taken. This is because most of the Pennsylvanian is an exploration target for both structural and stratigraphic traps.

The MiSSiSSippian map (Fig. 12B) comprising a depth interval of about 450 to 900 m is the most detailed because the most data are available for that unit. The Mississippian is a

54 FORSTER AND MERRIAM

major oil and gas producer in this part of Kansas, and therefore the large number of boreholes drilled to this unit. For the Mississippian as well as the Arbuckle units, the BHT values all are compiled from near-top of the unit. The exploration target for these units are hydrocarbon traps linked with local structural features. Although the data are from near top of the units, the contours of BHTs on the Mississippian are highly irregular showing an alternation of highs and lows especially in the central and western part where most of the boreholes are located. The lateral change in temperature on the map ranges between 25 and 45°C. The regional trend however shows an increase in values from 32°C in the east to 38°C (~T=6°C) in the west. For comparison, the temperature model for the Mississippian surface (Fig. 7) shows for the same area a more pronounced increase of temperature from about 28° to 44°C (~T=16°C), indicating that the BHTs in the west (in the deeper part of the basin) are somewhat cooler than the near-equilibrium conditions, but in the eastern (shallower) part may not deviate from them. Temperature anomalies as isolated by trend analysis are in the range of ±2SC with a few values of ±7.5°C. The anomalously high temperatures in excess of 38°C are scattered mostly over the central and western part of the map. The remainder of the map in the southeastern and northern part is devoid of data so that the smooth pattern of low values there is not reliable.

Values for the Arbuckle Group overlying the Precambrian basement (Fig. 12C) are from a depth interval of about 550 to 1050 m and range from 26°C (eastern part) to 43°C (western part). This temperature range is the same as recognized for the overlying Mississippian. The map has the best spatial data distribution of all three maps. Again, as outlined for the Mississippian, there is an overall trend of increasing values from 32°C in the east to 40°C (~T=8°C) in the west. For comparison, modeled temperatures range from 32° to 46°C (~T=14°C). Several hot spots of temperatures between 37° and 43°C are scat­tered over the central and western part of the map. The values of hot spots on the Mississippian BHT map are in about the same range as the maximum values in the Ar­buckle.

The regional trend of increasing BHT values towards the west can be related visually to the westward dip of the stratigraphic units as shown on a configuration map on top of the Mississippian (Fig. 12D). Although the top of the Mississippian is a karst surface, at this scale and contour interval the pattern reflects the regional structure. In addition to the regional trend, a series of small high and low features is visible. These local features are elongated northeast-southwest and northwest-southeast, reflecting the Precambrian basement fracture/fault system. These local features, known as 'plains-type folds,' were developed by differential compaction over the buried fault blocks (Merriam and Forster 1994, 1996).

From a visual comparison of the Mississippian BHT map (Fig. 12B) with the Mississippian structural map (Fig. 12D), it can be seen that the alignment of small highs and lows on the temperature map semiparallels the grain of the structural pattern. It also is obvious that some of the anomalies of higher temperatures locally occur coincident with structural highs, for example in the southwestern and south-central part of the area. The higher temperature at shallower depth is in contradiction to the general prediction in the area that temperature increases with increasing depth of a unit.

A detailed study of several subtle anticlines in the area substantiates the relationship of higher temperature and local positive structures in the Lower Paleozoic carbonates (Fig. 13). Temperature anomalies on the order of about 7°C (15°F) occur in both the BHT and the DST data sets. These anomalies are not a reflection of variation in stratigraphic/lithologic properties within these subtle structures, and if not a result of noise,

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE 55

D<350 C A B

25 Miles i' ,

25 Kilometers

Figure 12. Contour maps of bottom-hole temperatures (BHTs) recorded in Elk and Chautauqua counties; CI = SoC. A, BHTs recorded in Pennsylvanian. B, BHTs on top Mississippian. C, BHTs on top Arbuckle Group; and D, structural contour map on top of Mississippian rocks in Elk and Chautauqua counties; CI=lOO ft. Datum: sealevel.

could be a reflection of a change in the reservoir conditions in the carbonates. The contained hydrocarbons in the anticlines in conjunction with fluid movements along the developed fracture system on top of those structures may cause the temperature anomalies.

56 FORSTER AND MERRIAM

SUMMARY OF RESULTS

The BHTIDST analysis of our area demonstrates the problems of working with temperature data from well logs to obtain information on the subsurface temperature distribution and temperature gradients. The major problem with single temperature data is their reliability. In our situation, a lack of data on circulation time and on the more important shut-in time does not allow temperature corrections fur drilling disturbance, but it is questionable whether such a correction actually would improve the data resolution.

o 1 mile ~I ----------~i------~i o 1 kilometer

1

32

S 11

14 R 9 E

Figure 13. Hylton oil field in Chautauqua County showing structure on top of Mississippian (CI = 10 ft, 3.3 m) and DSTs (CI = 5°F, 2.8°C). Anticline has closure of'" 40 ft (12 m). Well control shown by solid circles; wells with DSTs shown by solid diamonds. Temperature difference on structure'" 15°F (7°C).

Although we have checked the BHT data carefully for several influences that might affect their value during measurement, none of them actually explain or reduce the variability (basic noise) inherent in the data. Therefore, clusters of BHT data are needed to average values and extract a meaningful temperature/depth relation, which then can be corrected to formation temperature using empirical methods. However, different data scatter obviously occur in different size areas, which also impacts the geothermal gradients that are obtained. A data scatter ofBHTs around some mean value of ±5-9°C was observed in the 60x75 km area comprising Elk and Chautauqua counties. Seemingly, the data scatter can be reduced to ± 7°C when working with composite BHT -depth plots for smaller areas of lOxlO km. But this requires good data coverage, which in most situations is not available. The disadvantage of working with small areas also can be that the data do not have vertical resolution because of clustering at about the same depth. There is no way to determine from the scatter plots how much scatter actually is caused by different adjustment of the BHTs to steady-state equilibrium temperature. The DSTs, which in general are closer to true

PROBLEMS AND POTENTIAL OF INDUSTRIAL TEMPERATURE 57

formation temperatures, seemingly are less scattered and may contain less error than raw BHT data (see Forster and others, 1997).

We used a mapping technique to investigate the variability of BHTs as shown in the scatter plots in terms of signal and noise. We attribute signal in the BHTs to be related to regional and local geological conditions in the area. By separating the BHTs according to their stratigraphic unit and plotting their spatial distribution, it is obvious that, in addition to a regional trend of increasing temperature with increasing depth, local anomalies complicate the temperature pattern. Thus, for example, the regional BHT increase from east to west amounts 6°C on the Mississippian surface. The internal 'noise' not related to regional geology (here the depth change of the subsurface) is on the order of ±2.5°C with a few values of ± 7 .5°C as indicated by the residuals of the trend. This amount of noise is less than would be obtained from BHT scatter plots, where the regional geological variation is not as obvious.

Part of the anomalous values (some of the temperature highs) are shown to be correlated with local, subtle anticlines developed in the sedimentary cover over faulted basement blocks and contain signal. Moreover, from the BHT trend being coincident with the trend of regional structure it also can be concluded that for deeper units below 500 m depth the amount of cooling in the BHTs seemingly is correlated with depth and that BHTs measured at the same depth have undergone about the same disturbance during drilling. This implies that anomalies on the map not related to local structure could be a result of different shut-in times compared with the remainder ofBHT data, but this cannot be proved.

The mapping approach we used in our study is a valid tool to search for the variability in the industrial temperature data to delineate and emphasize local influences. The search for spatial properties in the data set helps identify and select proper values to be used in temperature-depth plots and thus to improve the data resolution prior to the application of empirical correction procedures, which do not affect the statistical properties of the data set. The separation of different effects on BHTs is a necessity to improve the reliability of geothermal gradient and heat-flow density estimations using these data.

ACKNOWLEDGMENTS

We would like to thank Lynn Watney and David Newell of the Kansas Geological Survey and Phil Armstrong of the University of Utah for reading a preliminary copy of the manuscript and making helpful suggestions. John Davis of the Survey did the statistical analysis and helped with the interpretation.

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