AAPG Geoscience Technology Workshop

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A Method of Reflect Oil Cracking and Oxidization

Abstract

Experimental and case studies have shown that stability of oils in deep or super-deep reservoirs is not controlled only by burial depths or temperatures the reservoirs experienced. Other factors such as burial and heating history, formation pressure, formation water distribution, and thermochemical sulfate reduction (TSR) may have exerted important effects on oil stability. Here we present a revised method to reflect oil cracking and oxidization extents based on the experiments from Fang et al. (2013), which shows that adamantanes (As) and diamantanes (Ds) can be generated during oil cracking when EasyRo is >1.1%. Thus, the oil cracking extent expressed as (1 - C0/Cc)100 (Dahl et al., 1999), which is based on assumption that no diamondoids are generated or destroyed during oil cracking, should be changed to [1 - C0/(Cc - Cnewly generated)100], where C0 and Cc are initial and final concentration of 3- + 4- methyl diamantine, respectively; and Cnewly generated is the concentration of newly generated 3M- + 4M-D and is the function of EasyRo as shown by the linear increase in As and Ds concentrations with increasing EasyRo during the cracking of saturated, aromatic, resin and asphaletene fractions (Fig. 1). About 100 Tarim oils from Tazhong and Tabei areas have been newly analyzed and collated, and then calculated for aromatic and diamondoid maturity indices to determine their maturity, which is related to EasyRo, thus, the cracking extents for these oils can be determined using our new method. On the other hand, we find that an oil is being oxidized and cracked during TSR. We temporally use both thiaadamantanes and diamondoids concentrations to determine the ratio of amount of residual oil after oxidization and cracking to that of initial oil, and find more than 80% oil in well ZS1C has been oxidized and cracked to CO2 and gaseous C1 to C4 alkanes.