Compressively matured solid bitumen and its geochemical significance

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 differ-ent geochemical characteristics from thermally matured bitumen: very high bitumen reflectance (ROB) 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 composi-tion, molecular evolution parameters (associated with their chemical components) and hydrocarbon-pro-ducing 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 ROB values and molecular structures. The above contrasting geochemical characteristics can be used to distinguish the compressively matured bitumens from thermally matured bitumens. However, the maturity estimated by the ROB val-ues of primary anisotropic bitumens in the lower Triassic (T1) carbonate rocks of the Shiwan Dashan basin, south China, is much higher than that estimated by RO, according to the published relationships between ROB and RO. Gao and Chen (1998) suggested that the difference in maturation pressure could have caused these different types of bitumens. This study was conducted to under-stand the origin of these bitumens based on the structural and optical characters and results of high pressure and high temperature experiments