THE GEOCHEMICAL FEATURES OF OIL GENERATION IN DEVONIAN-CARBONIFEROUS OF XIANG-ZHONG DEPRESSION, HUNAN, AND THE DIRECTION IN OIL AND GAS EXPLORATION

Xiang-Zhong was an area of relatively stable and secular depression in Late paleozoic. It is thousands of metre thick with marine facies carbonate rocks as the bulk of the sedimentation which was possessed of certain oil forming geological conditions. In this paper, the abundance, type and evolution of organic matter are studied through the analysis of organic carbon,gaseous hydrocarbons, GR/G chloroform extracts, group compositon of the extracts, GC of saturated hydrocarbons, infrared and ultraviolet spectra of aromatic hydrocarbons; and the vitrinite reflectance, elemental composition, paramagnetic spectra,scanning electron microscopic observation thermal-thermogravimetric analysis of kerogen. Meanwhile, as well as computer simulation using the relationship of time versus temperature,the history of the formation of oil-gas is inferred.The basic suggestions are: The Middle Devonian Qiziqiao Formation and Lower Carboniferous of Xiang-Zhong depression; and the Upper Devonian Shetianqiao Formation of Xin Hua depression are the major horizons of oil generation. The evolution of oil-gas has reached a stage of overmature and the prospecting has to be focussed on natural gas. The Middle-Late Carboniferous and Permain were the main stage of oil formation. Around the episoda of the Indosinian movement there came the prosperous stage of the formation of natural gas. Therefore, the structure of early Indosinian and pre-Indosinian, and the Devonian-Carboniferous reef-bank facies provided favourable condition for forming pools. So, it should be a prior target for oil prospecting.