Relations Between Depth of Burial, Vitrinite Reflectance and Geothermal Gradient

Abstract

In wells where the drilled sequence is now at its maximum temperature, relations between depth and vitrinite reflectance show three segments: an upper segment with a linear gradient from 0.2–0.25% Ro at the surface to 0.6–0.7% Ro; a middle segment in which reflectance increases rapidly to c. 1.0% R; and a lower segment in which the gradient is again linear but reflectance increases more rapidly than in the upper segment. With a linear scale for depth, the inflection represented by the short middle segment tends to be obscured by the adoption by some authors of a log scale for reflectance. The depth to the inflection systematically increases with decrease in geothermal gradient, allowing the development of a general diagram relating depth, reflectance and geothermal gradient. In wells where erosion is probable either at the present‐day surface or at an unconformity, the general diagram can be used to estimate former maximum depths of burial and paleogeothermal gradients. These estimates, together with the presence of the inflection in the depth/reflectance relation, should be part of the input into modelling of the thermal history of sedimentary basins when reflectance is used in the model. The inflection is the result of the changing chemistry of vitrinite during oil generation, and the contrast between the depth/reflectance gradients above and below the inflection comparably reflects the contrast in vitrinite chemistry.

Geochemical and Sedimentation History of Neogene Lacustrine Sediments from the Valjevo-Mionica Basin (Serbia)

“…Comparing these results to maturation ratios calculated from distribution and abundance of saturated biomarkers, we conclude that Pt(IV)-and Ru(III)-ions have much greater influence on maturation changes on the planar systems (naphthalene and phenanthrene rings), than on isomerisations in the polycyclic alkanes, steranes and terpanes (Tables 4 and 5). The above observation is in agreement with the theoretical knowledge, as it is known that transition metal ions acting as Lewis acids show an affinity for aromatic systems, and that they form stable complexes with aromatic ligands in the form of sandwich compounds (Hagen, 2006;Radke, 1987 Applying a generalized diagram that relates Ro, depth and a regional geothermal gradient (Suggate, 1998) ranging between 40 and 50 °C/km (Kostić, 2010), the minimum depth of 2300-2900 m was estimated at which the shale would become a thermally mature source rock (Fig. 10).…”

“…10). The minimum temperature necessary for catagenetic generation of hydrocarbons (temperature = depth x geothermal gradient + annual mean surface temperature; Suggate, 1998) was calculated at 103 °C (t = 2.3 x 40 + 11 = 103 °C). Using the basin-independent equation T = (lnRo+1.68)/0.0124 (Barker & Pawlewicz, 1994), and Ro value of 0.70 % is estimated to be at 107 ºC.…”

The Rank(Sr) scale: Its basis and its applicability as a maturity index for all coals

“…Corresponding relations between reflectance, depth of burial, and geothermal gradient have been demonstrated by Suggate (1998b), despite the uncertainties inherent in the comparison of reflectance determinations. The two most promising analytical parameters for determining rank increments, CV and VM, also have the largest databases.…”

“…The possible effects of time (duration of heating during the period of temperature increase to, and retention at, a maximum) were ignored by Suggate (1959Suggate ( , 1974. They were discounted by Suggate (1982Suggate ( ,1998b on the grounds that adequate time to virtually complete the reactions would normally be available throughout the metamorphic process, and that the maximum temperature reached would imprint the rank on the coal. Should time prove significant, there is still a need to identify the attained rank, using a scale such as Rank(S r ).…”

An Alternative to Conventional Rock Fragmentation Methods Using SCDA: A Review

“…However, when evaluating the behavior of SCDA under large confinements, the effect of temperature with increasing depth has to be incorporated as it may be a vital component in enhancing the expansive pressure generated by SCDA. On average the global geothermal energy gradient varies from 20 • C/km to 30 • C/km [119]. For deep mining applications, the volatility of SCDA has to be studied with respect to the rate of hydration.…”