Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 240,
part 2
Comparison of Maturation Data and Fluid-inclusion Homogenization Temperatures to Simple Thermal Models--part 3 of 10
TTI MODELING FOR LOCALITIES IN CENTRAL KANSAS
Procedures for TTI Calculation
A time-temperature index model (Waples, 1980; 1981), based on work by Lopatin (1971), takes into account both time and temperature in calculating a cumulative thermal maturity for sedimentary organic matter. This technique assumes that the rates of chemical reactions involved in the conversion of kerogen to petroleum can be mathematically approximated by a first-order chemical reaction in which the rate of maturation doubles for every 10°C rise in temperature. The Lopatin technique generates a dimensionless number, called the "time-temperature index" (TTI) that Waples (1981) empirically correlated to vitrinite reflectivity and stages of oil and gas generation (table 1). The TTI models were run with a geothermal gradient that did not vary over geologic time, but with a surface temperature that changed according to paleoclimate and global position of Kansas over geologic time. The tectonic history was depicted by plotting subsidence and uplift of various strata against time. Subsidence was determined by thicknesses and ages of strata; uplift was determined by estimating erosion that occurred at major unconformities. The graphical history of subsidence and uplift and of subsurface temperatures was also utilized to infer the maximum temperature and depth a given stratigraphic interval experienced and the time at which various levels of maturation were achieved. Refinements can be added to the TTI calculation, such as decompacting the sedimentary column and taking into account heat flow and thermal conductivity of individual rock layers to determine the geothermal gradient. In this study, however, only limited organic and thermal maturation data were available with which to compare the results of such elaborate modeling, so the models presented were kept simple.
Table 1. Correlation of time-temperature index (TTI), Rock-Eval transformation ratio (TR), and Rock-Eval Tmax to vitrinite reflectance (Ro). TTI to Ro is from Waples (1981); TR and Tmax to Ro is from Peters (1986).
| Stage |
TTI |
TR |
Tmax (°C) |
Ro(%) |
| Onset of oil generation (Peters) |
|
~0.1 |
~435-445 |
~0.6 |
| Onset of oil generation (Waples) |
15 |
|
|
0.6 |
| Peak oil generation (Waples) |
75 |
|
|
1.1 |
| End oil generation (Waples) |
160 |
|
|
1.3 |
| End oil generation (Peters) |
|
~0.4 |
~470 |
~1.4 |
| Upper TTI limit for wet gas (Waples) |
1,500 |
|
|
~3.8 |
TTI calculations are specific only to a single locality--usually one well. In this study, two localities were selected: a location at the crest of the Central Kansas arch in western McPherson County (T. 19 S., R. 4 W.), and a location in southern Harper County (T. 33 S., R. 7 W.) where the Viola Limestone is deepest in central Kansas (fig. 1). The localities were selected to provide answers to the following questions:
- What is the maximum temperature and depth to which the Upper Ordovician Viola Limestone in central Kansas has been subjected?
- When did the Viola Limestone experience these conditions?
- How well do direct maturation indicators compare to the thermal maturity indicated by the TTI modeling?
- Are additional thermal events necessary to better correlate the TTI modeling to the direct maturation indicators?
Fig. 1. A geothermal gradient map of Kansas based on 43,348 bottom-hole temperatures (BHT's) from oil and gas tests (Stavnes and Steeples, 1982). Counties targeted for TTI modeling are shaded; wells in which saddle dolomites were analyzed for fluid-inclusion homogenization temperatures are superimposed on the geothermal gradient map. Temperature-logged wells (Stavnes, 1982) in the vicinity of the modeling localities are also shown, along with their determined geothermal gradients. Isopleths are at 5°C/km. Generally higher geothermal gradients in eastern Kansas are attributable to data derived from many relatively shallow wells that were drilled only into Pennsylvanian strata. Because strata are characterized by relatively low thermal conductivity, temperatures rise relatively rapidly with depth; consequently, a BHT measurement from a shallow well in this locality generally will yield an anomalously higher geothermal gradient (Förster and Merriam, 1993).
Kansas Geological Survey
Web version March 16, 1998
http://www.kgs.ku.edu/Current/1997/newell/newell3.html
email:lbrosius@kgs.ku.edu