Insights to Thermal Maturation Alterations of Kerogen, Oil, and Gas Using Compound Specific Sulfur Isotope Analysis
Sulfur has a unique role in petroleum systems as it is involved in the preservation of source rocks, the generation of oil and gas along the thermal maturation path, and in post-generation processes such as thermal sulfate reduction. Organic sulfur compounds (OSC) in oil and gas can be divided into families that differ in their thermal stability including: thiols (mercaptans), thiolanes, thiophenes, benzothiophenes and dibenzothiophenes. Recent analytical developments enabled the measurement of compound specific 34S analysis of these compounds both in the oil and the gas phase with high sensitivity (picomoles) and accuracy (Amrani et al., 2009; Said-Ahmad et al., 2017). The 34S of organic sulfur compounds depends on source rock and fluid preservation, maturation, and post-generation processes, and therefore this new analytical frontier will provide better understanding of petroleum systems. Here we focus on insights to thermal maturation alterations of kerogen, oil, and gas using compound specific sulfur isotope analysis. To this end, semi-open pyrolysis experiments were conducted on a thermally immature, organic and sulfur-rich source rock (Ghareb formation, Israel). Oil and gas samples were collected sequentially in several points along the maturation path and were analyzed (Rosenberg et al., 2017). In addition, four natural crude oils from Israel were analyzed and the results were compared to the pyrolytic oils. The 34S of the different OSC showed relatively large variability (～10‰) in the immature bitumen and first pyrolytic oil and gas. Upon thermal maturation the reactivity of the different families of OSC inversely follows their thermal stability; that is, thiols > thiophenes > benzothiophenes > dibenzothiophenes. This order of reactivity is manifested both in the concentration profiles with maturation, and in a decrease in 34S variability. As a results of the decrease in 34S variability, the maturation process yields OSC with δ34S values that closely reflect the kerogen and can be used as a fingerprint for oil-gas-source rock correlation in wide ranges of thermal maturities. This can be especially useful in gas and condensates plays, where biomarkers are absent. Moreover, volatile OSC begin to form before oil generation, thus providing a unique insight into kerogen transformation at low levels of thermal maturation (EasyRo of ~0.3). Because thiols are so reactive, they attained a fast isotopic equilibrium with the coexisting H2S (the main S phase generated from the kerogen), and their formation was systematically shown to be correlated with the [H2]/[H2S] ratio. On the other hand, thiophenes approached isotopic equilibrium only at higher pyrolysis temperature (i.e., higher degrees of maturation) reflecting their higher thermal stability. This order isotopic equilibrium can be utilized as a proxy for the time and temperature exposure of OSC with a reduced source of S (e.g., H2S), either generated by thermal maturation or other processes such as thermochemical and microbial sulfate reduction as well as H2S migration from deeper strata.
AAPG Datapages/Search and Discovery Article #90341 ©2019 AAPG Geoscience Technology Workshop, Exploration and Development of Siliciclastic and Carbonate Reservoirs in the Eastern Mediterranean, Tel Aviv, Israel, February 26-27, 2019