Generation temperatures for oils sourced from sulfur-rich kerogens using aromatic and light hydrocarbon isomers

Abstract

Isomer ratios of certain hydrocarbons change with temperature, and this relationship can be used as a parameter for assessing thermal maturity and guiding petroleum exploration. The C7 dimethylpentanes (DMPs) are frequently used in the literature to calculate a temperature of generation for produced oils. These light hydrocarbons are extremely useful, when present, but can also be affected by source mixing, thermochemical sulfate reduction, water washing, and evaporative fractionation. The accurate calculation of temperatures is especially important for oils sourced from sulfur-rich kerogens (i.e., Type II-S) as the onset of oil generation commences at lower temperatures due to early cracking of carbon bonds. Due to the sulfur enrichment of kerogen, standard maturity methods may overestimate the thermal maturity of Type II-S oils; therefore, alternative methods specific to sulfur-rich sources, such as the aromatic methyldibenzothiophene (MDBT) isomers, are needed to accurately calculate generation temperatures and maturity. Studies linking light hydrocarbons to larger-chained aromatic hydrocarbons are limited. This study integrates the light hydrocarbon DMPs with the aromatic, sulfur-bearing MDBTs to calculate maturity and generation temperatures for Type II-S oils directly from the 4-MDBT/1-MDBT ratio. Results from Type II-S Middle Eastern oils indicate the calculated MDBT temperatures have an average standard deviation of 0.7% compared to calculated C7 temperatures. Furthermore, they also have an average standard deviation of 1% compared to present-day bottom-hole temperatures (BHT). The MDBT isomers’ thermal stability between the immature to the wet gas window enables the use of this technique to assess and map fluids over a wide temperature spectrum in a basin. This stability makes these aromatic compounds especially useful in the absence of light hydrocarbons and in cases where oils have been altered by secondary processes.