Estimation of hydrous-pyrolysis kinetic parameters for oil generation from Baltic Cambrian and Tremadocian source rocks with Type-II kerogen

Abstract

Determining kinetic parameters for oil generation from a source rock by hydrous pyrolysis requires a considerable amount of sample (kilograms) and laboratory time (several weeks). In an effort to circumvent these requirements, hydrous-pyrolysis (HP) kinetic parameters for oil generation from Upper Cambrian and Tremadocian source rocks of the Baltic region are estimated by two methods: (1) organic sulfur content in kerogen and (2) HP experiments conducted at 330 and 355°C for 72 h. Estimates for the Upper Cambrian source rocks based on organic sulfur contents gave activation energies from 47 to 56 kcal/mole and frequency factors from 1.156 ´ 1025 to 1.078 ´ 1028 m.y.-1 . Tremadocian source rocks based on organic sulfur content gave estimated activation energies from 60 to 62 kcal/mole and frequency factors from 1.790 ´ 1029 to 1.104 ´ 1030 m.y.-1 . The estimates for the Tremadocian source rocks were less affected by thermal maturation because their low kerogen S/(S + C) mole fractions (< 0.018) remained essentially constant. Conversely, the higher kerogen S/(S + C) mole fractions (>>gt; 0.018) of the Upper Cambrian source rocks decreased with thermal maturation and resulted in overestimation of the kinetic parameters. The second method was designed to estimate kinetic parameters based on two HP experiments. The assumption that the maximum yield in calculating the rate constant at 330°C (k330°C) could be determined by a second hydrous pyrolysis experiment at 355°C for 72 h proved not to be valid. Instead, a previously established relationship between Rock-Eval hydrogen index and maximum HP yield for Type-II kerogen was used to calculate k330°C from oil yields generated by the HP experiment at 330°C for 72 h assuming a first-order reaction. HP kinetic parameters were determined from relationships between k330°C and the HP kinetic parameters previously reported. These estimated HP kinetic parameters were in agreement with those obtained by the first method for immature samples, but underestimated the kinetic parameters for samples at higher thermal maturities. Applying these estimated HP kinetic parameters to geological heating rates of 1 and 10°C/m.y. indicated that the Upper Cambrian source rocks would generate oil notably earlier than the overlying Tremadocian source rocks. This was confirmed in part by available data from two neighboring boreholes in the Polish sector of the Baltic.