Carbonate Source-Rocks of the Ordovician in Shan-Gan-Ning Basin and Discussion on the Hydrocarbon Generation of Carbonates

Key words: 
• carbonate source rocks /
• organic abundance /
• Shan-Gan-Ning basin /
• evaluation of carbonate source rocks

Abstract: The carbonate source-rocks of the Ordovician are relatively poor in organic matter (C_org 0.24% ). According to the result on gas/source-rock correlation, it could be suggested that the carbonates with mean over 0. 2% organic carbon content at stage of over maturity should be able to form largescale oil-gas accumulation. Furthermore, this paper proposed in the carbonates with mean over 0. 15% organic carbon content the expulsion, migration and accumulation of hydrocarbons can happen based on fluorescence thin-section observation.The paper had listed the carbonate source-rocks from five kinds of sedimentary environments which have the different content of organic matter and original potential of hydrocarbon generation, and put stress on the comparison of calcareous alga-bearing carbonates with cryptalgal-bearing carbonates as well. The Cryptalgal-bearing carbonates are main sourcerocks of natural gases in the central gas-field. Finally this paper suggested following views:(1) The carbonates, rich in non-skeleton algae or other organic-shell living bodies, are good source rocks.(2) The content of organic matter in carbonates mainly depends on the oxidationreduction condition of deposition, shallow burial and the content of mudstone.(3) The oxidation-reduction condition also influences original properties of kerogen. In reduction system there is a high residual ratio to bio-remains, and original lipid properties of kerogen could be protected because oxidization not only reduces the content of organic matter,but also damages lipid properties of primary kerogen and decrease the potential of the kerogens. Although almost all kerogen in the Early Palaeozoic Period originated from precursers such as algae, plankton and palaeo-bacteria etc., they may have different potential. Based on analysis and calculation, generally, the carbonate with lower organic carbon content was merely medium potential (IH₀=250^±) of hydrocarbons generation, however, it might have high efficiency of expulsion of hydrocarbons.