Impacts of crude oil group composition on the thermal evolution characteristics of mesophase spherules

Mesophase spherules are widely distributed in solid bitumen as intermediate products formed during the thermal evolution of crude oil. As a significant bridge linking gas and oil with their source rocks， solid bitumen provides critical geological information for tracing the origin and evolution of petroleum systems， thereby playing an important role in revealing the evolution of deep and ultra-deep reservoirs. To investigate the thermal evolution characteristics of mesophase spherules and their primary controlling factors， we conduct high-temperature and high-pressure gold tube pyrolysis experiments on five oil samples with varying group component contents， that is， four crude oil samples， as well as a crude oil sample in extracted saturated hydrocarbons， aromatic hydrocarbons， saturated + aromatic hydrocarbons， and nonhydrocarbons + asphaltenes. The comparison of the optical characteristics of solid bitumen generated under four temperatures （i.e.， 450 ℃， 500 ℃， 550 ℃， and 600 ℃）， along with the areal proportions and diameters of mesophase spherules in the solid bitumen， reveals that during thermal evolution， the mesophase spherules undergo generation， growth， collision， and coalescence， ultimately evolving into solid bitumen with crystalline-domain mosaic textures. The optical characteristics and evolutionary patterns of the mesophase spherules generated during thermal pyrolysis of group component indicate that the polar compounds （i.e.， resins + asphaltenes） in crude oils act as the dominant factors controlling the formation and thermal evolution of mesophase spherules. Understanding the mesophase spherules in solid bitumen is crucial to revealing the geochemical properties and origin of paleo-oil reservoirs.