Waples 1980 AAPG Lopatin Method

The American Association of Pelroleum Geologists Bulletin V. 64, No. 6 (June 1980) P. 916-926, 13 Figs.. 4 Tables

Time and Temperature in Petroieum Formation: Application of Lopatin's iMethod to Petroieum Exploration^ DOUGLAS W. WAPLES^ AlMtract N. V. Lopatin in the Soviet Union has developed a method for taking t)oth time and temperature into account as factors in thermal maturation of kerogen. Lopatin's time-temperature index of maturity (TTI) values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons. Because such information Is potentially of great Interest for oil exploration, a calibration and evaluation have tjeen made of Lopatin's method. Within the limitations of the data presently available the following statements can be made: 1. The rate of the chemical reactions involved in thermal maturation of organic material appears to double with every I C C (18°F) rise In temperature. 2. Threshold values of Lopatin's time-temperature index of maturity (TTI) are: 15 75 160 ~500 ~ 1,000 ~ 1,500 >65,000

Onset of oil generation Peak oil generation End oil generation 40° oil preservation deadline 50° oil preservation deadline Wet gas preservation deadline Dry gas preservation deadline

3. TTI values calculated from Lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemlsts. Potential applications of Lopatin's method for oil exploration include timing of oil generation, calculation of volume of hydrocarbons generated within a basin, and determination of economic deadlines.

BSTRODUCnON It has been generally established in recent years that both time and temperature are important factors in the process of oil generation and in the subsequent cracking of oil to methane. In 1971, N. V. Lopatin in the Soviet Union published a paper which described a simple method by which the effects of both time and temperature could be considered in calculating the thermal maturity of organic material in sediments. He developed a "time-temperature index" of maturity (TTI) to quantify lus method. Lopatin's original work was greeted with some enthusiasm and much criticism. Some of the problems which surfaced could be attributed to the poor quality of the data with which Lopatin originally calibrated his model (Neruchev and Parparova, 1972; Golitsyn, 1973; Karpov et al, 1975). Despite these technical details, Lopatin's basic idea has merit. It was therefore decided to attempt to coordinate Lopatin's method with other parameters which relate to oil generation to devise a model which could predict the thermal

conditions under which hydrocarbons could be generated and preserved. CONSTRUCTION OF GEOLOGIC MODEL Implementation of Lopatin's method begins with a reconstruction of the depositional and tectonic history of the geologic section of interest. This is best accomplished by plotting depth of burial versus geologic age, as shown in the hypothetical example in Figure 1. It should be remembered that such reconstructions are not geologic cross-sections. In the example in Figure 1 a Lower Cretaceous sediment was deposited 125 m.y. B.P. at the sedimentary surface (depth = 0). Since its deposition the sediment has had the time-depth history shown by the sohd line in Figure 1, moving from left to right. Its history consisted of continual deposition at varying rates until 80 m.y.B.P., at which time a brief (2 m.y.) uplift occurred in which the sediment was raised from a depth of 7,000 ft (2,134 m) to 6,000 ft (1,829 m). UpUft was followed by renewed subsidence until a depositional hiatus was reached at 20 m.y.B.P. The hiatus persisted until 6 m.y.B.P., when subsidence commenced again. The sediment is at present (time = 0 m.y.B.P.) at a depth of 10,500 ft (3,200 m). The line in Figure 1 thus

(g)Copyriglit 1980. The American Association of Petroleum Geologists. All rights reserved. A APG grants permission for a single photocopy of this article for research purposes. Other photocopying not allowed hy the 1978 Copyright Law is prohibited. For more than one photocopy of this article, users should send request, article identification number (see below), and $3.00 per copy to Copyright Clearance Center, I n c . 21 Congress St., Salem, MA. 01970.

• Manuscript received, July 13, 1979; accepted, November 5, 1979. 2Chevron Oil Field Research Co., La Habra, California 90631. Present address: Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401. 1 thank Chevron Oil Field Research Co. for support of this work and for permission to publish the findings. Among the many geologists who helped in the development of the methods I particularly thank Boone Warner, Jack Nelson, and Ed DeFeu of Chevron U.S.A., Denver, and Don Kushnir, Dean Bamum, Denny Jizba, and Bob Jones of COFRC for stimulating discussions and many new ideas. Tom Edison and Dave Baskin performed the TAI analyses. Article Identification Nundber 0149-1423/80/B006-0005$03.00/0

916

Time and Temperature in Petroleum Formation

917

FIG. 1—Depositional and tectonic history of a Lower Cretaceous sediment.

FIG. 2—Depositional and tectonic history of several sedimentary horizons.

FIG. 3—Complex subsurface temperature grid.

FIG. 4—Illustration of section thinning by erosion.

traces the depth-time relation for the sediment. Any shallower strata, such as those shown in Figure 2, will have depth-time lines subparallel with the first line, commencing with their deposition. A set of these lines, as in Figure 2, forms Lopatin's geologic reconstruction. Except in certain situations (to be dealt with later in the section entitled "Special Cases") the depth-time line segments for the various horizons will always be parallel. The geologic reconstruction is based on the best information available. Some reconstructions will be easy to make with a high level of confidence, particularly where deposition has been continuous. For sediments which have had complex histories, however, the reconstruction may represent only a best guess. The second aspect of the geologic model is the temperature grid. The subsurface temperature

must be specified for every depth throughout the geologic past. The simplest way to do this is to compute the present-day geothermal gradient and assume that both the gradient and the surface temperature have been constant throughout the time interval covered by the reconstruction; therefore, the temperature grid is simply a series of equally spaced lines of constant depth. A 10°C spacing is convenient. Figure 3 shows a more comphcated situation, in which there is a break in the present-day geothermal gradient. The upper part of the section, which is mainly sand, has a low gradient, but the lower shaly part has a high gradient. If it is assumed that the geothermal gradient is related to lithology, the geothermal gradient prior to 88 m.y. B.P. must have been high for the entire section, for only shales were present. The low gradient came into existence after 88 m.y.B.P., when depo-

918

Douglas W. Waples

sition of sand began. The isotherms (dashed lines) in Figure 3 thus represent the subsurface temperatures as a function of geologic time. There is no theoretical limit to the complexity which can be introduced into the temperature history of a section. However, most data necessary for a highly sophisticated temperature reconstruction wiU simply not be available. Lopatin's method can be applied to any geologic model, regardless of the model's crudeness or complexity. A well thought out, detailed reconstruction will obviously yield more rehable results than one which is based largely on guesswork. These hmitations should be borne in mind in any subsequent interpretation of Lopatin data. However, even a very crude or approximate model may be able to answer important questions about hydrocarbon generation or preservation. SPECIAL CASES Although many geologic models can be constructed in a strai^tforward manner, there are some situations in which caution is advisable, or where special techniques are necessary. When uplift and erosion occur, some section is lost. TTius although the horizon Unes remain parallel after such an event, the distance between them will be reduced, as illustrated in Figure 4. Another problem can arise when the section under examination is cut by a fault. Such sections above and below the fault may have had different thermal histories. It is thus necessary to make two different geologic reconstructions for the two different sides of the fault and combine them to obtain the complete reconstruction for the section. THEORY OF LOPATIN'S METHOD Lopatin and many others believe that two factors, time and temperature, are important in oil generation and destruction. These two factors are interchangeable: a high temperature acting for a short time can have the same maturation effect as a low temperature acting over a long time. Lopatin assumed that the dependence of maturity on time is linear—doubling the cooking time at a constant temperature doubles the maturity. Chemical reaction rate theory predicts that the temperature dependence of maturity will be exponential. To take into account this relation between reaction rate and temperature, Lopatin divided the temperature profile into ICC intervals and drew the isotherms, as in Figure 3. He then chose the 100 to 110°C interval as the base interval and assigned to it an index value of n = 0. The other intervals were assigned index values as shown in Table 1. Lopatin then defined a y factor, which reflects the exponential depen-

Table 1. Temperature Factors for Different Temperature Intervals Temperature Interval (°C)

Index Value n

30- 40 40- 50 50- 60 60- 70 70- 80 80- 90 90-100 100-110 110-120 120-130 130-140 140-150 150-160

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 m

.. ..'

Temperature Factor 7 _-7 _-6 _-5 _-4 ^-3 -2 J.-1

_2

J.3 _4

J.5

,m

' Data not available. dence of maturity oil temperature. He assumed that the rate of maturation increased by a factor r for every 10°C rise in reaction temperature. Thus for any temperature interval the temperature factor y = r°, where n is the appropriate index value given in Table 1. For his time factor Lopatin used the length Of time (in m.y.) that the sediment spent in each temperature interval. The maturity added in any temperature interval i is given by AMaturityi = (ATi)(r"i), where ATi is the length of time spent by the sediment in the temperature interval i. Because maturation effects on the organic material are additive, the total maturity (or TTI) of a given sediment is given by the sum of the maturities acquired in each interval. Thiis nmax nmin where nmax and nmin are the n-values of the highest and lowest temperature intervals encountered. If Lopatin's idea is correct, the TTI value should correlate with data obtained using other methods for evaluating the thermal maturity of organic material. The present work attempted first to choose a value for r, and second to estabhsh a correlation between TTI and vitrinite reflectance and thermal alteration index (TAI) measurements.

Time and Temperature in Petroleum Formation Table 2. Interconveision of Thermal Alteration Index (TAl) and Vitrinite Reflectance Values (Rg) Ro

TAI

Ro

TAl

0.30 0.34 0.38 0.40 0.42 0.44 0.46 0.48 0.50 0.55 0.60 0.65 0.70 0.77 0.85 0.93 1.00 1.07 1.15 1.19 1.22

2.0 2.1 2.2 2.25 2.3 2.35 2.4 2.45 2.5 2.55 2.6 2.65 2.7 2.75 2.8 2.85 2.9 2.95 3.0 3.05 3.1

1.26 1.30 1.33 1.36 1.39 1.42 1.46 1.50 1.62 1.75 1.87 2.0 2.25 2.5 2.75 3.0 3.25 3.5 4.0 4.5 5.0

3.15 3.2 3.25 3.3 3.35 3.4 3.45 3.5 3.55 3.6 3.65 3.7 3.75 3.8 3.85 3.9 3.95 4.0 4.0 4.0 4.0

919

between measured and calculated maturities are f>oor at the extreme values of r, but are generally good for values of r between 1.6 and 2.5. The plot of TTI versus Ro for r = 2 is shown in Figure 5. There is significant scatter in the data, probably due to two main factors: error in TAI or Ro measurements, and error in the geologic models used. Because of the large number of samples, however, the average TTI-Ro line (as shown in Fig. 5) is probably satisfactory. As there was no strong evidence indicating a better choice for r, a value of 2 (representing doubling of the reaction rate with every 10°C temperature rise) was selected. All further discussions in this paper assume that r = 2.

00,000:

19

•\ •

\ L •

10,000:

\ *, T« • \ •*

i •» \ • M* •

m\

CHOOSING VALUE FOR r The Arrhenius equation states that the rates of chemical reactions approximately double for every 10°C rise in temperature. Lopatin himself accepted this rule, and thus selected 2 for r. Other workers, however, have disputed this choice (Neruchev and Parparova, 1972; Golitsyn, 1973). Because of the complexity of the chemical reactions actually involved and the broad temperature ranges over which these reactions occur, it is not possible to make a sound theoretical prediction about the best value for r. It was therefore decided to try to evaluate r empirically by looking at a large quantity of TAI and vitrinite reflectance (Ro) data and choosing the r value which gave the best correlation between calculated and measured maturities. Thermal maturity data for 402 samples from 31 worldwide wells were tabulated. The sediments sampled ranged in age from early Paleozoic to Quaternary, and thus represent a broad time interval. Maturities were measured by either TAI or Ro. To compare TTI values with a single maturity parameter, TAI values were converted to their Ro equivalent according to the scale in Table 2. The range of reflectance values of the samples is about 0.4 to 6. To test empirically for the most appropriate value of r, TTI was plotted versus Ro for various values of r, ranging from 1.0 to 10.0. Correlations

1,000:

.•\»o

•§• •i**. •a*

100: TTI

" . 10:

; ^ " 1:

~ .1:

« .01-

M i l 1 1 1—1

isJ wR

#^iV«

'vB*

Sua •

•^ *%/• •• • • % ti • »• •• 1 •t 1• •1 ••«•> • i i i i i r f 1—1

1

10

FIG. 5—^Time-temperature index of maturity (TTI) versus vitrinite reflectance (Ro) for r = 2.

920

Douglas W. Waples Table 3. Calculation of Present TTI Values for Geologic Model (Fig. 6)

Temp. Interval

rc)

r"

A Time* (m.y.)

Interval TTI

Total TTI

15 5 5 10 3.5 (3.5+6.5) (4.5+37.5) 10.5 24

0.06 0.04 0.08 0.31 0.22 1.25 10.5 5.3 24.0

0.06 0.10 0.18 0.49 0.71 1.96 12.5 17.8 41.8

3.5 (3.5+2.5) (5 + 38) 12.5 24.5

0.01 0.05 0.67 0.39 1.53

0.01 0.06 0.73 1.12 2.65

10.5 29.5

0.17 0.22

0.17 0.39

Horizon A 20- 30 30- 40 40- 50 50- 60 60- 70 70- 80 80- 90 90-100 100-110 Horizon B 20- 30 30- 40 40- 50 50- 60 60- 70

2'

r' T" T'

r" r^ r= r' 1 r« r' T' T' T"

Horizon C 20- 30 30- 40

T"

f

* AT for a particular interval is merely the age at which the sediment enters that interval, minus the age at which it enters the next interval. CALCULATION OF TTI The principles involved in calculating TTI values have been explained in the foregoing; here we shall go t h r o u ^ a specific example. Figure 6 shows a geologic model having three sediment horizons (A, B, and C) and a moderately complex temperature grid. The calculation for each horizon is given in Table 3. Computer calculation of TTI values is feasible. It is also possible to calculate the TTI value at any time in the past in the same way. Suppose we are interested in the TTI value of horizon A 60 m.y. ago (represented by point P in Fig. 6). The calculations are carried out in a manner analogous to that done previously but stop 60 m.y.B.P. instead of at the present. The calculated TTI value for point P in Figure 6 is 5.9. INTERPRETATION OF TTI VALUES

calculated from our geologic models. Results of *«se correlations, which are based on the previ«"sly mentioned statistical analysis of 402 samples from 31 worldwide reconstructions, are given ' ° Table 4. ^ , ^ ,^^^ Table 5 shows R 5 % C2+) can be preserved, a TTI value of about 1,500 might be a reasonable estimate for the wet-gas deadline. The dry-gas deadline (below which methane will not be found) could not be exactly determined, but appears to he at TTI >65,0(X). Dry

-Z''^.

BASIN OUTLINE PRESENT-DAY TTI OF OIL SOURCE

ROCK

CONTOUR OF ONSET OF OIL GENERATION

FIG, 12—Present-day TTI values of organically rich shale in hypothetical basin.

Time and Temperature in Petroleum Formation

925

AGE (MY) 1^0

FIG. 13—Iso-TTI lines on geologic model. whether or not the thermal maturity necessary for hydrocarbon generation has occurred in a region. For example, an organically rich shale has been found in a basin, and we want to know whether this shale has reached thermal maturity. By making time-depth reconstructions for several points in the basin, we can calculate present TTI values for the shale at these points, as shown in the hypothetical example in Figure 12. By contouring the TTI values we can get an idea of the areal extent of rich shale which has entered the generative window. In the example in Figure 12 the generative area (within the TTI = 15 contours) represents only a small part of the total basin; hence only a small fraction of the rich shale could have begun to generate oil. Thus the exploration risk in prospects adjacent to this basin would be considerably higher than if the whole basin had akeady reached tiiermal maturity. A third appUcation of TTI data in exploration is in answering questions about timing of generation. Figure 13 shows a geologic model in which TTI values of 15 and 160 have been located on each of several horizons. If we contour iso-TTI values on this model we have two lines which delimit the oil-generative window for the entire sec-

tion throughout the geologic past. The shaded region in Figure 13 indicates the generative window. Let us suppose that one particular formation, indicated as "Oil Source Rock" in Figure 13, is the only plausible oil source rock (OSR) for this region. We can determine when in the geologic past the OSR generated oil by inspection of Figure 13. The OSR entered the generative window 181 m.y.B.P. and ceased generating 120 m.y. B.P. The region in which the time-depth con(fitions are appropriate for oil generation in the OSR is shown in Figure 13 in black. As we now know the time span during which oil generation occurred (from 181 to 120 m.y.B.P.), we can begin to answer important questions about the timing of oil generation and trap formation. Suppose that the only structural traps in the region were created during the uplift lasting from 100 to 90 m.y.B.P. Because trap formation occurred at least 20 m.y. subsequent to the end of oil generation, the probability is low that this oil could have been captured by these local traps. It is more likely that by the time these traps were formed the oil had already migrated out of the region because there was no barrier to its movement. This Ust of potential applications of Lopatin's

926

Douglas W. Waples

method is doubtless incomplete, for the method is very versatile. Creative geologists will certainly discover new ways to use TTI data to answer specific questions important in their own particular exploration areas. CONCLUSIONS This study has verified that the maturation of organic material in sediments depends upon both time and temperature. There is good correlation between calculated TTI values and measured geochemical-maturity parameters. A scale correlating TTI values with TAI and Ro data has been constructed. TTI values corresponding to the oil generative region have been determined. Using these TTI values it is possible to predict wheSier a given sediment has reached thermal maturity and, if so, at what time in the geologic past. TTI values corresponding to deadlines for preservation of various kinds of hydrocarbon deposits have also been determined. These TTI values effectively delimit the depth limits in each area at which oil, wet gas, and dry gas can be expected. TTI values calculated from Lopatin reconstructions agree consistently with other parameters commonly employed by petroleum geochemists in estimating thermal maturity of organic material. Potential application of Lopatin's method for petroleum exploration is considerable. Among

the more obvious possibilities are quantitative basin analysis, comparison of timing of oil generation with trap formation, and determination of economic basement. Further applications will undoubtedly be discovered by exploration geologists as Lopatin's method begins to be employed routinely. REFERENCES CITED Dow, W. G., 1977, Kerogen studies and geological interpretations: Jour. Geochem. Exploration, v. 7, p. 79-99. Golitsyn, M. V., 1973, The duration of the process of coal metamorphism (in Russian): Akad. Nauk SSSR Izv. Ser. Geol., no. 8, p. 90-97. Karpov, P. A., et al, 1975, Quantitative evaluation of temperature and geologic time as factors in the coalification of dispersed coaly remains and the possibility of its application to petroleum geology (in Russian): Akad. Nauk SSSR Izv. Ser. Geol., no. 3, p. 103-113. Lopatin, N. V., 1971, Temperature and geologic time as factors in coalification (in Russian): Akad. Nauk SSSR Izv. Ser. Geol., no. 3, p. 95-106. Neruchev, S. G., and G. M. Parparova, 1972, The role of geologic time in the process of the metamorphism of coal and dispersed organic material in rocks (in Russian): Akad. Nauk SSSR Sibirsk. Otdeleniye Geologia i Geofizika, no. 10, p. 3-10. Tissot, B., et al, 1977, Alakanes as geochemical fossils— indicators of geological environments (in French), in Advances in Organic Geochemistry 1975: Madrid, ENADISMAp. 117-154.