Main factors influencing the formation of thermogenic solid bitumen

Highly mature solid bitumen, a residue of oil cracking, is widespread in the lower Paleozoic paleo-oil reservoirs of southern China. Solid bitumen is not a simple, pure component, but rather a compositionally and structurally variable mixture of materials. This study investigated the formation of thermogenic solid bitumen and the effects of oil composition and reservoir environment. A series of seven gold-tube pyrolysis experiments were conducted: three used the main fraction groups (i.e., saturated, aromatic, and resin + asphaltene fractions) of crude oil to evaluate the effect of oil composition on the formation of solid bitumen during cracking; the other four tested the effects of water and pressure in reservoirs by simulating the cracking of crude oil under different reservoir conditions. Quantitative analyses of pyrolytic products (including methane, C2-C5 gaseous hydrocarbons, C6-C13 light hydrocarbons, C13+ heavy hydrocarbons, and solid bitumen) indicated that thermogenic solid bitumen formed at different stages of oil cracking, and its formation was clearly affected by oil composition. In contrast, water and pressure in the reservoir had little effect on the formation of solid bitumen.