Thermal stability of 2-thiadiamondoids determined by pyrolysis experiments in a closed system and its geochemical implications

Abstract

Abstract The abundance of 2-thiadiamondoids is currently utilized to assess the extent of thermochemical sulfate reduction (TSR). However, these compounds are ultimately degraded to small molecules with increasing temperature in deep hot reservoirs, resulting in uncertainty in their use as a proxy to evaluate the extent of TSR. In this study, the thermal stability of 2-thiadiamondoids was determined from variations in their abundance upon pyrolysis at different temperatures using an oil enriched in thiadiamondoids. The results demonstrated that these compounds were generally stable at Easy%Ro\u202f≤\u202f1.0, followed by partial destruction at Easy%Ro\u202f=\u202f1.3–2.0 and intensive decomposition at Easy%Ro\u202f≥\u202f2.0–2.3, suggesting the potential underestimation of the TSR extent at the highly mature to over mature stages. The thermal stability of 2-thiadiamondoids and diamondoids strongly depended on their cage numbers, which presented an increasing trend with higher cage numbers. However, diamondoids were far more stable than 2-thiadiamondoids and were still thermally stable even up to Easy%Ro\u202f=\u202f1.8–2.3. The thermal stability of dibenzothiophenes (DBTs) showed a decreasing trend with increases in alkyl substituted carbon numbers. They were thermally stable at Easy%Ro\u202f≤\u202f1.0–1.8 and can be seriously cracked at Easy%Ro\u202f≥\u202f1.8–3.0. The C0-DBT and C1-DBTs were more stable than thiadiamondoids, but C3-DBTs showed the opposite trend. The thermal stability of gasoline range hydrocarbons showed a trend of CH4\u202f>\u202fC2H6\u202f>\u202fC3H8\u202f>\u202fC4–5. CH4 and C2H6 were much more stable than diamondoids, while C3–5 hydrocarbons were only thermally stable at Easy%Ro\u202f≤\u202f1.8–2.3 and severely degraded at Easy%Ro\u202f≥\u202f2.6–3.0. The C3H8 and 2-cage diamantanes, C4–5 compounds and 1-cage adamantanes had a similar thermal stability. Therefore, to accurately assess the TSR extent should be made using thiadiamondoids, diamondoids and C0–1-DBTs, even gasoline range hydrocarbons, rather than thiadiamondoids alone, if gases are well preserved within petroleum reservoirs in the subsurface.