282 PETROLEUM GEOLOGY/The Petroleum System generation levels ($ 1.0%Ro) If the vitrinite is anisotropic and maximum and minimum reflectances are determined, then the mean of the maximum values may be reported (%Ro, mean-max) Theoretically the maximum %Ro, max is the most significant indicator of maturity The interpretation of the mean vitrinite reflectance values is normally undertaken by plotting %Ro on a log-scale against sample depth (Figure 20C), where anomalies may be recognized as plotting away from the linear trend The interpretation then uses the industry-standard relationship with both oil and gas generation windows and coal rank (Figure 20D) Discontinuities in the maturity trend may be seen at unconformities (where the amount of uplift may be estimated from the offset), and excursions may relate to intrusions or hydrothermal flow As well as defining the maturity of the sampled section, the reflectance trend can be projected to predict—in the absence of major unconformities—maturity levels ahead of the drilling bit In addition to vitrinite reflectance two other maturity parameters are commonly reported, Rock-Eval Tmax (Figure 15), and kerogen or spore colour estimated on TAI (1–5) and SCI (1–10) scales, respectively Acronyms are defined and equivalences detailed for oil and gas generation in Table and Table 2, respectively Although the Tmax values are often available in large numbers, in a single well they often show substantial scatter with respect to depth In contrast, fewer visual estimates of kerogen colour are determined, but they often produce a better maturity trend with depth Spore colour estimates are particularly reliable since reworked or caved spores and spores introduced with drilling mud or other contamination can be readily identified as being out of stratigraphic sequence Finally the maturity involved in a petroleum system will normally be interpreted in the context of basin modelling and a burial history plot (Figure 21) The burial history plot (time and hence stratigraphy versus depth) traces the burial depth (and temperature) for each modelled strata (black lines) from deposition to the present day, including periods of uplift reflecting erosion at the palaeosurface Compaction is modelled by various methods as the sediments are progressively buried, and the temperature grid (red lines) calculated from geothermal gradients or mantle heat flow The heat flow calculation requires the knowledge of the thermal conductivity of each sediment unit—a function of mineralogy and porosity (compaction): Figure 21 Burial history model showing present day and palaeotemperature and early , mid , and late mature oil windows defined by vitrinite reflectance