ABSTRACT

Solid bitumens found in carbonate rocks of the Shiwan Dashan basin, south China, are suggested to have been matured under high pressure and moderate temperature conditions. These bitumens have different geochemical characteristics from thermally matured bitumen: very high bitumen reflectance (R_OB) values and compact molecular structures (bigger size of crystal nucleus, more layer number of aromatic cycle, lower lamellar distance and so on) in spite of the chemical components reflecting lower thermal maturity. Therefore these are named as compressively matured bitumen. The geochemical characteristics of the compressively matured bitumens result from increase in its crystalline degree (grain size) under high pressures, not at high temperatures. The compressively matured bitumens retain chemical composition, molecular evolution parameters (associated with their chemical components) and hydrocarbon-producing potential similar to those of thermally less mature bitumens. High temperature and high pressure experiments proved that pressure and temperature have similar effect on the maturation indices, such as R_OB values and molecular structures. The above contrasting geochemical characteristics can be used to distinguish the compressively matured bitumens from thermally matured bitumens.