Hydrocarbon Expulsion Characteristics And Effectiveness Evaluation Of Low Abundance Source Rock Of Ordovician,Platform Of Tarim Basin

Amounts of hydrocarbons have been discovered in the platform by 2013,Tarim Basin.However,there are still many problems about how to determine the origins of the marine oil and gas.The marine hydrocarbon is mainly from a mix of Cambrian–Lower Ordovician and Middle-Upper Ordovician source rocks.Researchers disagree over which set of source rocks primarily contributes to oil accumulation.The results of resource evaluation based on the contribution of source rocks with high abundance(TOC > 0.5%)are different from the results of the latest exploration practices,and the former are always smaller.The essential problems is that the contribution of source rocks with low abundance(TOC < 0.5%)was not taken into account.Source rocks with low abundance(TOC < 0.5%)can be divided into two types,the first one has high organic carbon content in ancient times but the hydrocarbon generation and expulsion lead to the low organic carbon content,which are beneficial to hydrocarbon accumulation.The second one has low organic carbon content in ancient times and could hardly generate and expel hydrocarbon,which contribute little to hydrocarbon accumulation.It is significant to solve the problem how to identify the two types of hydrocarbon source rocks with low abundance(TOC < 0.5%),and evaluate the effective contribution to oil and gas accumulation.The Ordovician source rocks with low abundance,in Tarim Basin,were taken as an example during the study.According to the theory of hydrocarbon expulsion threshold,based on the relationship between actual residual hydrocarbon amount of Middle-Upper Ordovician rocks with low abundance and the residual hydrocarbon amount of expulsion threshold of source rocks,the two types of Ordovician source rocks with low abundance were distinguished,Tarim Basin;according to the comparison of the geochemical,geological and well logging characteristics of the two types of source rocks,through aspects including the types and distribution of hydrocarbon generation materials and enrichment and preservation conditions of organic matters,the genesis mechanism of effective source rocks with low abundance was revealed as that,in stable ecological system,the rich hydrogen planktonic algae or suspected source can be used as hydrocarbon generation material,and appropriate reduction environment can preserve the organic matter.On this basis,through analysis of the main factors controlling carbonate source rocks with low abundance that have expelled hydrocarbon,by multiple linear stepwise regression method,the model was built using Index of geochemical parameters(Ig)to quantitatively predict and evaluate effective low abundance source rocks,which is related to five main controlling factors,i.e.hydrocarbon generation potential,TOC,maturity,S2 /S1 and S2.According to the well logging reflections of carbonate source rocks with low abundance that have expelled hydrocarbon,by means of multiple non-linear curve fitting method,the model was built using Index of well logging(Iw)to quantitatively predict and evaluate effective low abundance source rocks,which is related to four well logging curves,i.e.GR,RD and U/TH.The above criteria and quantitative forecasting method were applied in the Tazhong area,Tarim Basin,and the hydrocarbon expulsion amount and relative contribution ratio from effective source rocks with low abundance were calculated and evaluated in the area.Also,the most favorable formation and areas were predicted of effective source rocks with low abundance in the platform,Tarim Basin.The results showed that,the hydrocarbon expulsion amount of effective the Middle-Upper Ordovician source rocks with low abundance in Tazhong Area is 4.09 billion tons,with 18% of relative contribution to hydrocarbon accumulation;the most favorable formation is the Yijianfang Formation of Middle Ordovician in the platform,Tarim Basin,which the volume of effective source rocks with low abundance accounted for 23.46% to the formation volume,followed by the Lianglitage Formation and Tumuxiuke Formation of Upper Ordovician,and the most favorable areas are the Tabei uplift and the south slope,where the hydrocarbon expulsion amount is 16.36 billion tons.