Calculation of organic maturation levels for offshore eastern Canada—implications for general application of Lopatin's method

Abstract

Recorded maximum bottom-hole temperatures may vary significantly from true formation temperatures because of the effects of drilling fluid circulation. A theoretical temperature correction technique was applied to log-heading data to compute 191 static temperatures for 64 wells on the Scotian Shelf. A linear regression, performed on 140 computed temperatures, produced an average geothermal gradient of 2.66 °C/100 m; correlation coefficient 0.97. A geothermal gradient map constructed from the corrected data shows that areas of thicker sediment accumulation are marked by high geothermal gradients (e.g., Abenaki, Sable subbasins), whereas areas of shallow basement coincide with low gradients (e.g., LaHave Platform, Canso Ridge).It is proposed that the major control on the distribution of Scotian Shelf geothermal gradients is the thermal conductivity of the sediments. Radiogenic heat production within the sediments and subsurface fluid movement probably contribute to a lesser extent. Within the basins, higher heat flow due to thick salt accumulations at depth and the overall low conductivity of sediments above the salt lead to higher geothermal gradients. Low geothermal gradients in shallow basement areas are caused by the lack of salt and the relatively high conductivity of overlying sediments.A technique for calculating maturation levels of organic matter based on Lopatin's method and corrected bottom-hole temperatures was developed for the Scotian Shelf. A geologic model is constructed by considering the burial history of sediment for time invariant heat flow. From this, TTI (time–temperature index) values are derived to give the maturity level for specific sedimentary horizons. A comparison of 106 calculated TTI values with vitrinite reflectance measurements for 15 wells established a calibration of this technique for the Scotian Shelf. A correlation coefficient of 0.95 was obtained for the relation log TTI = 6.1841 log R0 + 2.6557.Maps showing the depth to calculated vitrinite reflectance values of 0.60 and 0.70% were constructed for the Scotian Shelf. It appears that burial rate, in addition to temperature, controls the location of various maturation levels. As one moves seaward, younger sediments increase in maturity and the oil window thickens. At equivalent depths, sediments at the basin margins are more mature than those farther seaward in the deeper parts of the basin. Sediments of the Canso Ridge area and over much of the LaHave Platform, excluding local downfaulted basins, have not attained sufficient maturity to have generated significant quantities of oil.TTI calibrations were established for the Labrador Shelf, the Grand Banks of Newfoundland, and the Canning Basin of Western Australia as above. Results indicate that tectonic history plays an important role in the calibration and that the slope of calibration lines may represent the departure from true time–temperature conditions in the modeling. Changes in heat flow with time lead to incorrect estimates of maturity when present-day geothermal gradients are used to approximate past temperature conditions. Also, uncertainties in the amount of erosion produce error in maturity estimates. The Scotian Shelf TTI calibration may be applicable to much of offshore eastern North America and parts of offshore western Europe and Africa.