The Snignificance of Bitumen Reflectance as a Mature Parameter of Source Rocks

Whether bitumen reflectance can be used as a reliable mature parameter of source rocks is an unsolved problem for a long time. In this paper, the detailed geological characteristics and optical properties of bitumen occurring in source rocks are investigated with the organic petrological methods. Combining the results of thermal simulating tests of bitumen, the authors obtain the following acknowdges: 1. Bitumen belongs to a maceral group in the classification system of dispersed organic matter in sedimentary rocks. Bitumen occurring in source rocks may be divided into three macerals accouding to its origin: autobitumen, allobitumen and reworked bitumen. Forthermore, autobitumen is subdivided into three submacerals: pre-oil-bitjimen, post-oil-bitumen and oxidized bitumen on the basis of their forming pattern, and allobitumen into two submaceralss: early allobitumen and late allobitumen accouding to the relative maturation of the source rocks into which the bitumes pour. 2. Autobitumen is the most abundant bitumen in all types of source rocks, and it is characterized by a secounday form, and fills into the microfissures or microholes in source rocks. The distribution of allobitumen depends on the regional geological background to great extent. It appears often in the basins in which there are more than one oil source rocks seams with different geological time. The fundamental difference between autobitumen and allobitumen is in that the reflectance of allobitumen is unfit with the maturation level of the source rock in which it occurs. Reworked bitumen usually occurs in clay source rocks, and is characterized by original deposited occurrence and paralleling with stratification of the source rock. 3. That the significance of bitumen reflectance are used as a parameter depends upon, on one hand, its origin types and regional geological background, on the other hand, the maturation level of the source rocks and its optical structure. In most cases, the reflectance of autobitumes can be only used as a mature parameter. However, in some cases, the reflectance of the allobitumen in continous deposition basin and the reworked bitumen in very high mature stage source rocks can be also used to determine its source rocks maturation level. The reflectance of autobitumen has a good relation with the vitrinite reflectance or mature stage within the region of high mature stage to dry gas stage(about vitrinite reflectance from 0.01 to 2.0%), and is an excellent mature parameter, but, the reflectance of bitumen can be only used as a reference mature parameter when the mature level of source rocks is too high or two low,