Evidences on Effective Carbonate Source Rock of Low Organic Matter Abundance and its Lower Limit of TOC

The carbonate rocks are widely distributed,whose thermal evolution is generally high and organic matter abundance are lower in China. Whether or not they can be effective source rocks has great significance for the evaluation of hydrocarbon generation potential and exploration prospect of carbonate rocks. Many studies show that as the depth or thermal evolution degree increase,TOC of source rocks decreases,and hydrocarbon generation potential and residual amount increase and then decrease,besides the results of thermal simulation experiment,which indicate the source rocks of low organic matter abundance could can generate and expulse a great number of hydrocarbon. Hydrocarbon sources of some typical oil and gas fields are the carbonate rocks of low organic matter abundance in the world. A few factors could be considered to determine the lower limit of TOC of carbonate source rocks. The analysis suggests the lower limit of TOC of carbonate source rocks is less than 0. 5% of mud rocks. The lower limit of TOC of the effective source rocks with different maturity was gained by the method of hydrocarbon generation potential and simulative calculation based on material balance principle according to the hydrocarbon expulsion threshold. Referring to the study results of predecessors and exploration practice,The lower limit of TOC of the effective carbonate source rocks with low maturity,maturity and high—over maturity stages respectively are TOC≥1. 4%,TOC = 0. 1% ~ 1. 4% and TOC 0. 1%.