Recently I read an article on vitrinite reflectance and the impact of vitrinite suppression on measured values and the relationship of rock thermal maturity to oil maturity. There is no doubt that suppression of vitrinite reflectance occurs and, when not recognized, leads to underestimation of the true level of thermal maturity. This often occurs when only good, oil prone source rocks are analyzed. When performing a maturity assessment on a well or series of wells, it is important to perform a maturity profile on the well(s) particularly utilizing any coaly intervals that are not likely oil source rocks. These coal intervals have abundant vitrinite particles as do reasonably organic rich Type III kerogens (Type III are typically kerogens having hydrogen indices (HI) less than 200; mixed Type II/III kerogens generally have HI values between 200-350 (Jones, 1984). This will allow assessment of the quality of the reflectance data in any given interval as well assessment of any changes in geothermal gradients as a result of geological or thermal events in the sedimentary column history.
Oil prone source rocks are typically lean in vitrinite particles
(that's why they're good oil source rocks!) and do not always yield
reliable reflectance values due to oil staining.
When designing a geochemical program that includes maturity
assessment, be sure to:
When possible include coaly facies from above and below the source rock
Perform a well profile whenever feasible at 250-500 ft. intervals on samples having at least 0.50% TOC
Be sure to obtain the vitrinite reflectance histograms, all recorded data points, and statistical analysis to assess the reliability of the data
Be sure to merge and interpret with the thermal alteration index, Rock-Eval Tmax and conversion data, and any extract maturity data derived from biomarker analysis
Be sure that the objective of the study is thermal maturity and not kerogen conversion or oil/gas generation; the latter is related to the former but can be quite different in different basins depending on the organic matter type and composition of the source rock(s).
Corrections for suppression of vitrinite have been suggested by Lo (1993) based on determination that "lower-than-normal" reflectance values are determined on hydrogen-rich intervals in a well profile of vitrinite reflectance through depth. In this and subsequent work Lo has provided a correction factor based on hydrogen index. Thus, to make the Lo correction for suppression the following must be available: (1) a vitrinite reflectance well profile suggesting that a hydrogen-rich zone has lower than expected vitrinite reflectance based on samples above and below the interval, and (2) the original (largely unconverted, immature organic matter) hydrogen index of the sample.
An example from the Bakken Shale in the Williston Basin illustrates this approach. A mean, indigenous measured %Ro of 0.50% was made from a population of 21 readings on a sample having a hydrogen index of 487. Samples above and below this interval suggested that the reflectance was suppressed between 0.20 to 0.30% Ro. The original hydrogen index was estimated to be 608 (based on a database of 500+ Bakken Formation samples). Using the Lo (1993) method, a correction of 0.30 was applied and a calculated vitrinite reflectance of 0.80% was obtained. We would predict that this interval was near peak oil generation (near 50% conversion) based on this maturity assessment.
However, if the true objective of the work was to relate the measured oil maturity parameters to kerogen conversion, then an incomplete analysis was performed and the original geochemical program and logic was likely flawed.
Be sure the objectives of the geochemical study are precisely defined. This allows the geochemist to design the correct analytical program to meet the study objectives.
If oil and gas accumulations have been associated with a particular level of thermal maturation, additional maturity data will complement and expand your knowledge in a particular basin but will not necessarily agree with the extent of kerogen conversion, i.e., low maturity oils will be derived from the first generated and expelled hydrocarbons from kerogen whereas high maturity oils will be derived from peak to late oil window kerogen conversion.
If trying to compare the level of thermal maturity of rock samples to oil or condensate accumulations, then kerogen kinetics are required. Kinetic data provides the level of kerogen conversion at various maturities and will vary depending on kerogen type likely due to chemical composition and structure.
Vitrinite reflectance is a direct measurement of the maximum thermal exposure a rock or sedimentary column has been exposed to (although it may be altered in the case of suppression). It is not a direct measurement of kerogen conversion that is required to assess the maturity of oil and gas accumulations
In this case the extent of kerogen conversion could be made using kerogen kinetic measurements and was determined to be only about 10-20% (Jarvie et al., 1997).
Kerogen kinetic measurements are basically the rate at which organic matter decomposes into hydrocarbons under varying levels of thermal stress (increasing temperature). Kerogen conversion is related to measured maturity parameters such as vitrinite reflectance and peak pyrolysis temperatures (Tmax or cTemp) but will vary depending on organic matter composition and structure. Thus, at a given vitrinite reflectance value one kerogen may be more or less converted into hydrocarbons than second, chemically different kerogen. For example, the Point Arguello field, offshore California, based strictly on maturity parameters would have a low reflectance value (<0.60% Ro) despite the fact that an estimated 1 billion barrels of oil are in-place derived from rocks at these maturity levels. How can this be? Because, as the kerogen kinetics of the Monterey Formation indicate, these rocks are up to 50% converted at these low maturity levels (Jarvie and Lundell, 1996) or even higher if the kinetics of Hunt et al. (1991) are used. Thus, in a given sedimentary column the oil window will vary depending on organic matter composition. Figure 1 below illustrates the levels of kerogen conversion with associated vitrinite reflectance values.
Figure 1. Comparison of the extent of kerogen conversion at a given level of thermal maturity (0.70% Ro) using kerogen kinetics and an arbitrary constant heating rate of 10° C/my. A high heating rate was used to compare kerogen conversions to the high heating rates found in the Santa Maria Basin, California. Lower heating rates would lower the extent of conversion but the same levels of conversion would be reached at lower temperatures due to longer residence times
It is easy to determine kinetic parameters using nonisothermal, open-system pyrolysis techniques. However, assessing the derived kinetic data to determine which data are correct is not a trivial task and few companies offer the necessary services needed to accurately determine these data. This assessment requires proper selection of analytical conditions, heating rates and temperature conditions, independent determination of A (Arrhenius) factors, comparison of results to other experimental data, and proper selection of the correct mathematical model.
Furthermore, bulk kinetic parameters do not describe the earliest generated products very accurately nor to they give any indication as to the product distribution. A new analytical technique to measure the composition of hydrocarbons as they evolve at different temperatures has been developed by Humble. In the past it has not been possible to trap and resolve the light gases formed during nonisothermal pyrolysis by gas chromatography. Humble developed a technique that permits the complete separation and resolution of these compounds as well as the reproducible release of higher molecular weight hydrocarbons to C40+ (Figure 2).
This technique allows us to describe the decomposition of kerogen into any hydrocarbon fractions. We use a distribution of activation energies, for example, to describe the rate of generation of…
Figure 2. Pyrolysis/gas chromatographic fingerprint showing the complete resolution of C1, C2, C3, and hydrocarbons extending to C30+ using the Humble Gas Trapper technology.
The yield of these or other components, e.g., aromatic hydrocarbons, can be described at various levels of maturity or temperatures. Illustrated in Figure 3 is the distribution of products formed from the complete conversion of a mixed Type II/III (mixed oil/gas prone) kerogen.
Figure 3. Distribution of products formed by the complete decomposition of a mixed Type II/III kerogen. Yields of products at different levels of thermal maturation or geological temperatures can be computed from the kinetic data
It is likewise possible to predict the gas-to-oil ratios (GOR) from these data given the inherent limitations of laboratory analytical techniques.
Dan Jarvie
President
Humble Geochemical Services has an extensive collection of source rocks from around the world available for purchase in our World Source Rock Geochemical Library (companies must first join the Library by contributing samples). In addition we have numerous source rocks from which we have completed compositional kinetic and yields analysis in several regional studies:
References
Lo, H. B. (1993) Correction criteria for the suppression of vitrinite reflectance in hydrogen-rich kerogens: preliminary guidelines, Org. Geochem., Vol. 20, No. 6, pp. 653-657.
and , 1996, Evaluation of the Composition and Rates of Kerogen Decomposition of the Miocene Monterey Formation, USGS Cooperative Monterey Organic Geochemistry Study Proceedings, Columbia University Press, submitted.
, , and R. F. Inden, 1996, A comparison of the rates of hydrocarbon generation, from Lodgepole, False Bakken, and Bakken Formation Petroleum Source Rocks, Williston Basin, U.S.A., 1996 Rocky Mountain Section Meeting Amer. Assoc. Pet. Geologists, Billings, MT, oral presentation.
Jones, R. W., 1984, Comparison of Carbonate and Shale Source Rocks, in Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, AAPG Studies in Geology 18, J. Palacas, ed., pp. 163-180.
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