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 devel-oped a method for taking t)oth time and temperature into account as factors in thermal maturation of kero-gen. Lopatin's time-temperature index of maturity (TTI) values correlate with the thermal regimes correspond-ing to generation and preservation of hydrocarbons. Because such information Is potentially of great Inter-est 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 dou-ble with every I C C (18F) rise In temperature.

2. Threshold values of Lopatin's time-temperature index of maturity (TTI) are:

15 Onset of oil generation 75 Peak oil generation

160 End oil generation ~500 40 oil preservation deadline

~ 1,000 50 oil preservation deadline ~ 1,500 Wet gas preservation deadline

>65,000 Dry gas preservation deadline

3. TTI values calculated from Lopatin reconstruc-tions consistently agree with other maturation parame-ters commonly used by petroleum geochemlsts.

Potential applications of Lopatin's method for oil ex-ploration 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 tec-tonic history of the geologic section of interest. This is best accomplished by plotting depth of burial versus geologic age, as shown in the hypo-thetical example in Figure 1. It should be remem-bered that such reconstructions are not geologic cross-sections. In the example in Figure 1 a Low-er 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 Fig-ure 1, moving from left to right. Its history con-sisted of continual deposition at varying rates un-til 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 subsi-dence 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 sedi-ment 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, Inc . 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. 1Depositional and tectonic history of a Lower FIG. 2Depositional and tectonic history of several Cretaceous sediment. sedimentary horizons.

FIG. 3Complex subsurface temperature grid. FIG. 4Illustration of section thinning by erosion.

traces the depth-time relation for the sediment. Any shallower strata, such as those shown in Fig-ure 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 cer-tain situations (to be dealt with later in the sec-tion 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 confi-dence, particularly where deposition has been continuous. For sediments which have had com-plex 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 10C spacing is convenient.

Figure 3 shows a more comphcated situation, in which there is a break in the present-day geo-thermal 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 as-sumed 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 temper-atures as a function of geologic time.

There is no theoretical limit to the complexity which can be introduced into the temperature his-tory of a section. However, most data necessary for a highly sophisticated temperature reconstruc-tion wiU simply not be available.

Lopatin's method can be applied to any geo-logic model, regardless of the model's crudeness or complexity. A well thought out, detailed recon-struction 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. How-ever, 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 con-structed 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 dif-ferent sides of the fault and combine them to ob-tain the complete reconstruction for the section. THEORY OF LOPATIN'S METHOD

Lopatin and many others believe that two fac-tors, 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. Lopa-tin assumed that the dependence of maturity on time is lineardoubling the cooking time at a constant temperature doubles the maturity.

Chemical reaction rate theory predicts that the temperature dependence of maturity will be expo-nential. To take into account this relation be-tween reaction rate and temperature, Lopatin di-vided the temperature profile into ICC intervals and drew the isotherms, as in Figure 3.

He then chose the 100 to 110C 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)

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 .. ..'

Index Value

n

-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 10C rise in reaction temperature. Thus for any temperature interval the temperature fac-tor 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. Be-cause 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 encoun-tered. 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 ther-mal alteration index (TAI) measurements.

Time and Temperature in Petroleum Formation 919

Table 2. Interconveision of Thermal Alteration Index (TAl) and Vitrinite Reflectance Values (Rg)

Ro

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

TAI

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

Ro

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

TAl

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

CHOOSING VALUE FOR r The Arrhenius equation states that the rates of

chemical reactions approximately double for ev-ery 10C rise in temperature. Lopatin himself ac-cepted this rule, and thus selected 2 for r. Other workers, however, have disputed this choice (Ner-uchev and Parparova, 1972; Golitsyn, 1973). Be-cause 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 decid-ed 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 in-terval. 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

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, prob-ably 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 dou-bling of the reaction rate with every 10C temper-ature rise) was selected. All further discussions in this paper assume that r = 2.

TTI

00,000:

10,000:

1,000:

100:

"

.

10: ; ^

"

1:

~

-

-

. 1 :

-

.01-

1 9 \

\ L \ *, T

\ * i m\

\ M * .\o i**. a *

isJ wR #^ iV 'vB* Sua ^ *%/

% t i

1

1

t 1

>

Mil 1 1 11 i i i i i r f 11 1 10

FIG. 5^Time-temperature index of maturity (TTI) ver-sus 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)

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

Horizon C 20- 30 30- 40

r"

2' r' T" T' r" r^ r= r' 1

r r' T' T' T"

T" f

A Time* (m.y.)

15 5 5

10 3.5

(3.5+6.5) (4.5+37.5)

10.5 24

3.5 (3.5+2.5)

(5 + 38) 12.5 24.5

10.5 29.5

Interval TTI

0.06 0.04 0.08 0.31 0.22 1.25

10.5 5.3

24.0

0.01 0.05 0.67 0.39 1.53

0.17 0.22

Total TTI

0.06 0.10 0.18 0.49 0.71 1.96

12.5 17.8 41.8

0.01 0.06 0.73 1.12 2.65

0.17 0.39

* 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 calculated from our geologic models. Results of The principles involved in calculating TTI val- *se correlations, which are based on the previ-

ues have been explained in the foregoing; here we "sly mentioned statistical analysis of 402 sam-shall go t h r o u ^ a specific example. Figure 6 ples from 31 worldwide reconstructions, are given shows a geologic model having three sediment ' Table 4. ^ , ^ ,^^^ horizons (A, B, and C) and a moderately complex Table 5 shows R

Time and Temperature in Petroleum Formation 921

its are based upon unpubUshed Chevron data. These correlations effectively define the TTI range in which oil generation occurs (15 to 160); the highest TTI values at which oils of 40 and 50 API will be preserved (approximately 500 and 1,000, respectively); and the highest TTI values at which wet gas can be preserved (1,500). Dry gas is produced in the Union of California 1-33 Bruner, Beckham County, Oklahoma, from a horizon having a TTI of about 65,000, but it has not yet been established that this is the maximum possi-ble TTI at which methane is still stable.

CORRELATION OF TU WITH OTHER GEOCHEMICAL DATA

Calculated TTI values were compared with measured data from many worldwide samples

Table 4. Correlation of Time-Temperature Index of Maturity (TTI) with Vitrinite Reflectance (RQ)

representing a variety of ages and Uthologies. The correlations of TTI with TAI, vitrinite reflec-tance, bitumen/organic carbon ratio (Bit/Corg), carbon preference index (CPI), kerogen hydro-gen/carbon (H/C) ratio, percent expandable clays, and gravity (API) are presented graphically in Figures 7 through 11. Results from each of these relations lead to the same conclusion: TTI is a valid measure of thermal maturity of organic material.

Bltmneii/Orgaiiic Carbon Ratios It is generally accepted among organic geo-

chemists that Bit/Corg ratios should show an in-crease within the oil-generation window. In actual practice, however, Siis increase is not always clearly visible, because such factors as migration and kerogen type may strongly affect the bitumen content. When a large number of samples is ana-lyzed statistically, however, a maximum in the av-erage Bit/Corg ratio should be visible within the generative window.

Figure 7 shows a plot of TTI versus Bit/Corg where each point represents the average of up to 46 individual pieces of data. A visible maximum where the ratio is about two times that of the baseUne is apparent between TTI = 25 and TTI = 200. This region corresponds rather well to the oil generative window (TTI = 15 to 160) as de-termined from TAI and Ro data.

Carbon Preference Index It is generally accepted that CPI values de-

crease with increasing thermal maturity. The data in Figure 8 bear out the truth of that statement. CPI values for immature samples range from low to very high, but among the more mature samples very high values are conspicuously absent. The maximum CPI value possible for a given TTI value decreases as TTI increases, as shown by the

Table 5. Correlation of TTI with Important Stages of Oil Generation and Preservation

Ro

0.30 0.40 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 1.26 1.30

TTI

< 1 < 1

3 7

10 15 20 30 40 56 75 92

110 120 130 140 160

Ro

1.36 1.39 1.46 1.50 1.62 1.75 1.87 2.00 2.25 2.50 2.75 3.00 3.25 3.50 4.00 4.50 5.00

TTI

180 200 260 300 370 500 650 900

1,600 2,700 4,000 6,000 9,000

12,000 23,000 42,000 85,000

Stage TTI TAI

Onset of oil generation Peak oil generation End of oil generation Upper TTI limit for occurrence of oil

with API gravity 5.0 >4.0

922 Douglas W. Waples

I.OOO.OOOn

100,000-

10,000-

1,000

TTI

100

I-

(7)

(61

(32)

.(38)

(42) '(36) (40)

,(46)

OIL GENERATION WINDOW

(15) NO. OF SAMPLES

(25)

L(35)

BIT/C, org

FIG. 7Time-temperature index of maturity (TTI) ver-sus ratio of bitumen/organic carbon (Bit/Corg)-

envelope Une in Figure 8. According to these data, bitumens associated with kerogens within the oil generative window can have CPI values of up to 1.75 (at TTI = 15) or 1.2 (at TTI = 160). TTius although a decrease in maximum CPI corre-lates with oil generation, the actual CPI's of new-ly generated bitumens do not necessarily reach tike low values normally found in crude oils (CPI < I . l ) , indicating that thermal maturity is only one of the possible factors which can strongly in-fluence n-paraffin distributions (Tissot et al, 1977).

Kerogen H/C Ratios Figure 9 shows a plot of TTI versus average

kerogen H/C ratios, with each point representing up to 49 individual samples. Again TTI directly reflects thermal maturity, for the average H/C ratios decrease with increasing TTI values.

Percent Expandable Clays Although the transformation of montmorillon-

ite (expandable clay) to illite (nonexpandable clay) has often been thought to be associated with a definite subsurface temperature, the M ^ I transformation may actually be a kinetically con-trolled process. It therefore should be possible to apply Lopatin's method to clay transformations in the same manner in which it is applied to cata-genesis of organic material.

100,000:

10,000

1,000:

100:

TTI

CPI

FIG. 8Time-temperature index of maturity (TTI) ver-sus carbon preference index (CPI).

Time and Temperature in Petroleum Formation 923

10,000:

OIL GENERATION WINDOW

(30) NO. OF SAMPLES

(30)

I 1 'I 1 " - 1 1 1

0 .2 .4 .6 JB 1.0 1.2 1.4

FIG. 9Time-temperature index of maturity (TTI) ver-sus kerogen hydrogen/carbon ratios (H/C).

To this end a plot was made of TTI versus per-cent expandable clay layers (Fig. 10). There is a strong correlation between the maximum percent expandable layers and the TTI, as shown by the solid line. Samples lying significantly left of the line in the thermally immature region probably represent material which contained less than l()b% expandable layers when it was originally deposited.

About 50% of the interlayer water has already been lost from between the layers before the on-set of oil generation, and another 25% prior to peak oil generation. The quantity of expelled in-terlayer water available for transport of newly generated oil is therefore much smaller (except perhaps in supemormally pressured regimes) than some workers have estimated. This observation should be important in future studies of the mechanism of primary migration.

API Gravity of Oils API gravity data were available for 57 world-

wide oils from reconstructed sections. TTI is plot-

ted versus oil gravity in Figure 11. Most oUs, in-cluding all those of low gravity 3 0 API), show TTI values of 40) were recovered from horizons having TTI values >160; that is, from reservoirs which have been subjected to additional thermal maturation

1,000,000

100,000;

10,000:

TTI

1,000:

100:

-I r 40 60

% EXPANDABLE LAYERS

FIG. 10Time-temperature index of maturity (TTI) versus % expandable layers in mixed-layer clays.

924 Douglas W. Waples

FIG. 11Time-temperature index of maturity (TTI) versus oil gravity (API).

after oil generation. Because oil gravity is general-ly conceded to be at least in part related to ther-mal maturity, it is significant that low-gravity (immature) oils are not found associated with liigh TTI values.

From these limited data it is possible to esti-mate tentative "deadlines" for the preservation of oils of a certain API gravity. The solid line in Figure 11 approximately indicates the upper TTI value at which a given gravity of oil can be pre-served. Thus the maximum TTI value for finding a 40 oil would be about 500, and for a 50 oil, about 1,0(X). These numbers should, however, be viewed cautiously as merely the best extrapola-tions possible at present on the basis of our limit-ed data.

Natural Gas Data from 36 natural gas localities were avail-

able. Although the data are too sparse to allow an accurate calculation of the maximum TTI value at which wet gas ( > 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 de-termined, but appears to he at TTI >65,0(X). Dry

gas is produced from the Union of California 1-33 Bruner well in Beckham County, Oklahoma, at a TTI of about 65,000. The Lone Star 1 Baden well in Washita County, Oklahoma, struck liquid sulfur at a TTI of 972,000, indicating that the dry gas deadUne probably lies at a TTI value between 65,000 and 972,000. It is interesting that the Socal 1 James dry gas discovery in Wlieeler County, Texas, the world's deepest producer at 22,918 to 23,938 ft (6,985 to 7,296 m), is at a TTI of only 17,500.

APPLICATION OF TTI DATA TO EXPLORATION TTI values obtained by application of

Lopatin's method can be useful in several ways for oil exploration. If we are concerned with how deep we can expect to find preserved accumula-tions of oil, wet gas, or dry gas, we need only calculate the present-day TTI values of the sus-pected reservoirs and find the TTI regime into which they fall. For example, suppose it is expect-ed that a certain reservoir rock will be encoun-tered at 12,000 ft (3,758 m) in a proposed well. Can oil or gas be expected, and if oil, of what gravity?

Suppose that we calculate a TTI of 1,2(X) for the reservoir formation. This means that the res-ervoir has a higher TTI value than that at which a 50 oil can be preserved (1,000 from Table 5). We would predict from the TTI calibrations that the reservoir lies beond the oil deadUne, and could therefore contain only wet or dry gas. As stated previously, the confidence level of this interpreta-tion would depend upon the quaUty of the geo-logic model.

A second way in which TTI values can assist in oil exploration is in answering the question of

-Z' '^. BASIN OUTLINE PRESENT-DAY TTI OF OIL SOURCE ROCK

CONTOUR OF ONSET OF OIL GENERATION

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

Time and Temperature in Petroleum Formation 925

AGE (MY) 1^0

FIG. 13Iso-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 mak-ing 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 hy-pothetical 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 genera-tive window. In the example in Figure 12 the gen-erative area (within the TTI = 15 contours) rep-resents 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 consid-erably 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 genera-tion. 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 del-imit the oil-generative window for the entire sec-

tion throughout the geologic past. The shaded re-gion in Figure 13 indicates the generative window. Let us suppose that one particular for-mation, 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 geo-logic past the OSR generated oil by inspection of Figure 13. The OSR entered the generative win-dow 181 m.y.B.P. and ceased generating 120 m.y. B.P. The region in which the time-depth con(fi-tions 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 spe-cific 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 corre-lating 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 pres-ervation of various kinds of hydrocarbon deposits have also been determined. These TTI values ef-fectively delimit the depth limits in each area at which oil, wet gas, and dry gas can be expected.

TTI values calculated from Lopatin recon-structions agree consistently with other parame-ters commonly employed by petroleum geochem-ists 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 ba-sin analysis, comparison of timing of oil genera-tion with trap formation, and determination of economic basement. Further applications will un-doubtedly be discovered by exploration geologists as Lopatin's method begins to be employed rou-tinely.

REFERENCES CITED Dow, W. G., 1977, Kerogen studies and geological in-

terpretations: 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 coali-fication 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.