Diamondoids and molecular biomarkers generated from modern sediments in the absence and presence of minerals during hydrous pyrolysis

Abstract

Experimental work involving artificial maturation of modern sediments was carried out to determine if diamondoids can be generated from biosynthetic precursors abundant in modern sediments. Mineral catalysts of distinctive properties, including montmorillonite K10, acidic aluminosilicate and CaCO3 were used in the hydrous pyrolysis of organic-rich modern sediments to examine which type of catalyst is active and effective in the production of diamondoids. Our results show that diamondoids are absent from modern sediments lacking petroleum contamination. Hydrous pyrolysis at 340 °C indicates that they are created coincidently with the formation of biomarkers, as the artificial maturation of organic matter proceeds. The generation of diamondoids is more significant in the presence of montmorillonite K10 and acidic aluminosilicate. This suggests that these clay minerals have a significant catalytic effect on the formation of diamondoids, and may catalyze diamondoid synthesis via carbonium ion mechanisms resulting from Lewis acid sites available on their surface. In contrast, CaCO3 has an inhibitive effect on the generation of diamondoids as well as on published diamondoid maturity parameters such as MAI and MDI. The formation of 3,4-dimethyldiamantane (DMD), 4,8-DMD, and 4,9-DMD is demonstrated to be source related. The sediment mineralogy can change the composition and distribution of both triterpanes and steranes by either catalyzing isomerization of biomarkers or cracking of side chains. Source biomarker parameters (e.g., C24Tet/C23TT, C30∗/C29Ts, C27:C28:C29 steranes) are more sensitive to the addition of minerals including montmorillonite K10, acidic aluminosilicate and CaCO3 than are maturity indicators. Our results also suggest that CaCO3 has a remarkable effect on the isomerization of hopanes at the ring system. Isomerization of steranes is probably not clay-mediated, as indicated by only minor variations in isomerization ratios, including C29 stigmastane ββ/(ββ + αα) and C29ααα-sterane 20S/(20S + 20R) observed in the presence of clay minerals.