The Study on Indigenous Characteristics of Organic Matter in Marine Carbonate Hydrocarbon Source Rock with Low Abundance and High Evolution

Key words: 
• marine carbonate /
• source rock /
• indigenous characteristics of organic matter /
• effective hydrocarbon source rock /
• migration bitumen rock pyrolysis

Abstract: The indigenous characteristcs of organic matter in carbonate hydrocarbon source rock directly influences the precise evaluation of its hydrocarbon generation capability especially on marine carbonate rock with low organic matter abundance and high thermal evolution.The paper discusses that migration bitumen distribute widely in carbonate hydrocarbon source rock,meanwhile,all methods to distionguish primary or secondary organic matter in rocks are introduced in this paper.Finally,a series of measures to determine indigenous characteristics of organic matter carbonate source rock are proposed on the basis of the analyses of carbonate source rock of Lower Paleozoic in Ordos Basin. The so-called recognition of indigenous characteristics of hydrocarbon sources rock deals with two aspects：the first is the recognition of indigenous characteristics of organic matter so as to distinguish sources rock and reservoir and then determine the effective sources rock.The second is the recognition of indigenous characteristics of geochemical parameters and firsthand information so as to choose them which could really reflect the characteristics of sources rock. The carbonate rock has the dual propertes of source rock and reservoir,and the later alteration and tectonic action could make it become reservoir,especially the tectoclass or microcracks usually being the migration route or the detained space of bitumen,this phenomenon is widely distributed in the high-over mature carbonate source rock ,such as Sichuan Basin,Tarim Basin and Eerduosi Basin.The existence of migration bitumen will result in three kinds of influence on the evaluation of capability of hydrocarbons generation of organic matter in carbonate rock：First,the content of organic matter tends to be high.Second,maturation of organic matter is difficult to be determined and evaluated.In addition,it is difficult to determine the type and quality of organic matter.Therefore,the recognition of indigenous characteristics of organic matter in carbonate rock is the basis and prerequisite of all the geochemical research. On the whole,all the existed methods studying the indigenous characteristics of source rock lay particular emphasis on theory.Hence,this paper begins with rock type information as well as common used the pyrolysis data.Based on the pyrolysis principle and hydrocarbon generation theory,this paper established a series of measures that simply and practically determine the indigenous characteristics of organic matter in the carbonate source rock 1.Recognition of rock type：The carbonate source rocks usually exist in low energy environment.Due to water body being relatively quite.the rock is fine and close,mud content is high,colour is relatively dark and the rock is relatively rich in organic matter.For the water body is turbulent,the rock grain is coarse, mud content is low,the rock is relatively poor in organic matter,and the rock type is dominated by coarse-grain carbonate rock.This kind of carbonate rock could be used as reservoir but non-source rock,whose geochemical analyticd data should be removed. 2.Rock pyrolysis parameter：On the basis of pyrolysis principle and parameter,taking Ordos Basin as an example,the authors observe and measure a great deal of pyrolysis data.It is discovered that the abnormal samples whose pyrolysed hydrocarbons S2 is not proportional with the content of organic carbon resulted from filled migration bitumen and solid bitumen.They leads to high content of organic carbon and very low pyrolysed hydrocarbons S2,which is typical features of non-indigenous source rock. Thus,ws could appraise the indigenous characteristics of organic matter in source rock and geochemical parameters by the means of combining rock type with primary pyrolysis data. Though this method might be affected by organic matter type,its practical; effect is good in these areas widely coverde by marine carbonate rock.