Abstract
The maximum paleotemperature or paleo-geothermal gradient is one of the important parameters for evaluating hydrocarbon potential in the sedimentary basins.
Authigenic zeolites, found in Japanese Tertiary and Cretaceous oil/coal fields, show systematic distribution of a vertical zonal arrangement: the fresh glass zone, the clinoptilolite zone, the analcime zone, and the albite zone in descending order. The zeolites in sedimentary rocks are formed mainly through the function of temperature during the burial diagenesis. The temperatures of the first appearences of the minerals are particularly affected by heating time, geothermal gradient and the concentration of Na+ in pore water.
The present paper studied the relationship between the present geothermal gradient and the appearance of zeolite observed in twenty wells in the Akita and Niigata oil fields of which the burial depth of hole rocks are geologically considered to be maximum at the present time. The temperatures (T, °C) and the depths of the first appearances (Z, m) can be expressed by functions of the geothermal gradient (G, °C/100m):
Clinoptilo1ite: Tc=70.4-2.42G, Zc=5271/G-141,
Analcime: Ta=145.6-10.6G, Za=15434/G-1501,
Albite: Tab=191.1-11.3G, Zab=19103/G-1434.
Thethicknesses of the clinoptilolite zone (ΔZc)and the analcime zone (ΔZa) are respectively given by
ΔZc=10050/G-1317 and ΔZa=3669/G+67.
The paleo-geothermal gradient and the maximum paleotemperature can be estimated from the above relations whenever the appearances of zeolites will be clear.
