Interpretation of Vitrinite Reflectance Measurements in Sedimentary Rocks and Determination of Burial History Using Vitrinite Reflectance and Authigenic Minerals
Vitrinite reflectance studies in non-coal-bearing rocks have been conducted over the past several years; the techniques for extraction, analysis, and interpretation of small vitrinite particles vary slightly from standard coal petrographic procedures. Two areas in California afford a comparison of thermal history utilizing authigenic minerals and vitrinite reflectance: the Great Valley sequence of late Mesozoic age near Cache Creek west of the Sacramento Valley, and Miocene to Eocene sediments in the Tejon area of the San Joaquin Valley. In the Tejon area, laumontite is a postcompaction authigenic mineral and first occurs at 9,500 ft and is abundant below 10,500 ft. Vitrinite reflectance studies of Tejon area wells demonstrate an equivalent coal rank of high-volatile C bituminous to subbituminous A at 10,500 ft. At this depth, the temperature is 218°F (104°C), and the hydrostatic pressure is 4,600 psi (317 bars). Electrical log temperatures, as corrected by comparison with accurate subsurface temperature profiles, were used to reconstruct paleothermal conditions. At Cache Creek, laumontite is found as an alteration product in sandstones formerly buried at depths greater than 32,500 ft; the calculated paleopressure at this level is 27,000 psi (1,860 bars). Vitrinite reflectance measurements within the laumontite-bearing interval indicate a thermal history equivalent to high-volatile B bituminous coal. Based on the temperature-reflectance relation at Tejon, the temperature at the top of the laumontite zone at Cache Creek is estimated to be 265°F (130°C), and the paleogeothermal gradient in the Great Valley sequence is around 0.62°F per 100 ft (1.1°C per 100 m). Because the formation of zeolites is governed by a complex interaction of physical and chemical factors, we judge that vitrinite reflectance, which is largely temperature dependent, is the more promising tool for the measurement of thermal history.