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通过对盆地北部近百件下奥陶统马家沟组碳酸盐岩的有机岩石学观察发现，碳酸盐岩中的沥青赋存形式多样，大致可分为基质型和充填型。前者呈分散状、絮状和顺层分布状等；后者则主要为孔洞型（ 晶间孔、晶间溶孔和膏膜孔） 和裂缝型（ 缝和线、构造缝等）。根据上述各种沥青赋存形式（ 产状） 和烃类色层效应将这些沥青划属原生—同层沥青。并认为普遍存在的基质型沥青代表了碳酸盐岩中原始有机质的输入和向油气的转化（ 形成原生沥青）；而充填型沥青则指示了新生成油气的排驱和初次运移（ 相应形成初次运移沥青） 。正是这种原生沥青和初次运移沥青的共同存在，对碳酸盐岩烃源岩的确认提供了最直观可信的岩石学依据。此外，从原生—同层沥青含量和残余有机碳、热解生烃潜量等大致呈正相关变化中，进一步说明了原生—同层沥青含量可作为碳酸盐岩（ 尤其是高演化碳酸盐岩） 烃源岩评价的重要指标。据此，对盆地北部马家沟组碳酸盐岩进行了生烃潜力评价。

AN ANALYSIS OF THE ORIGINAL COEXISTING BITUMEN IN LOWER ORDOVICIAN IN EERDU OSI BASIN

Through an organo petrological observation on the carbonate rocks in Majiagou Formation of Lower Ordovician in the north part of the basin for near one hundred times,it is found that the existing form of the bitumen in the carbonate rocks is various,being roughly divided into matrix type and filling type.The former is dispersed,floccular and bedded and the latter is mainly the pore cavity type (including the intercrystalline pores,intercrystalline solution pores and anhydrite moldic pores)and fracture type(including the stylolites and structural fractures).According to various existing forms (occurrences) of the bitumen mentioned above and the chromatographic effects of the hydrocarbon,such kind of bitumen is belonged in the original coexisting bitumen.It is considered that the generally existing matrix type bitumen represents the entrance of the original organic matters and their transformation into oil and gas (forming original bitumen) but the filling type bitumen indicates the displacement and primary migration of the neogenic oil and gas (correspondently forming primary migration bitumen). Exactly because of the coexistance of the original bitumen and primary migration bitumen,the most audio visual and creditable petrological basis can be provided for determining the carbonate source rocks.In addition,according to the roughly positive correlation between the original coexisting bitumen content and the residual organic carbon or pyrolytic hydrocarbon generating potential,etc.,it is further indicated that the original coexisting bitumen content may be taken as an important index of evaluating the carbonate source rocks(especially the high evolution carbonate source rocks).In view of the above,the evaluation of hydrocarbon generating potential was carried out for the carbonate rocks in Majiagou Formation in the north part of the basin.