Effects of oil cracking on fluorescence color, homogenization temperature and trapping pressure reconstruction of oil inclusions from deeply buried reservoirs, Northern Donging Depression, China

The effects of oil cracking on fluorescence color, homogenization temperature (T_ho) and trapping pressure (P_t) of oil inclusions from deeply buried reservoirs (DBRs) (3672-4359 m) in the northern Dongying Depression were determined based on fluorescence spectroscopy and homogenization temperatures of oil inclusions, kinetic modeling of crude oil cracking, and petroleum inclusion thermodynamics modeling. The modeling results demonstrate that fluorescence color, T_ho and predicted P_t have strong relationships with the transformation rate via cracking of oil to gas (T_r), and the formation temperature (T_f) that the inclusions experienced. The fluorescence color is hardly influenced at all during the initial stages of oil cracking (Tr < 13%, T_f < 160 ºC), but fluorescence color begins to shift toward shorter wavelengths (blue shift) during progressive oil cracking (T_r < 24%, T_f < 190 ºC). With further oil cracking, the fluorescence color may either experience no change or continue its blue shift. Eventually the fluorescence color will disappear as the aromatic compounds are completely cracked. The Tho increases at first (T_r < 24%, T_f < 190 ºC), but then decreases or even becomes negative during major oil cracking. The reconstructed Pt values show a corresponding reverse trend. Oil inclusions from DBRs and other shallow reservoirs in the Dongying Depression show an obvious blue shift in fluorescence color at a depth of approximate 4000 m (T_f = 160 ºC) and generally contain solid bitumen below 4000 m, supporting the effect of oil cracking on fluorescence variation, consistent with the modeling result. The T_ho from DBRs in the Minfeng area increases with increasing burial depth (T_f < 190 ºC), which is also consistent with the modeling results. However, the Tho of oil inclusions with blue-white fluorescence from DBRs in the Shengtuo area did not show such a trend. Recent trapping, high trapping pressure and higher-maturity oil may have led to a low-degree of oil cracking, and thus less modification of Tho in the Shengtuo area. Oil cracking results in consistent volume ratios of pyrobitumen to oil inclusions (F_vpy) in the same fluid inclusion assemblage, and the F_vpy value increases with oil cracking level, which can be used to recognize if oil cracking has occurred in oil inclusions and what level of oil cracking they have experienced. As the oil cracking model used in this study did not account for the role of pressure, it is more applicable for oil inclusions that were trapped under normally pressured conditions. Oil inclusions trapped under overpressured conditions will be less influenced by oil cracking.