准噶尔盆地AH5井区八道湾组碳酸盐胶结物成因及对储层影响分析

Genesis of Carbonate Cement and Influence on Reservoir Quality of the Badaowan Formation in AH5 Well Block of Junggar Basin

深入分析准噶尔盆地AH5井区侏罗系八道湾组储层碳酸盐胶结物形成原因,探讨碳酸盐胶结物对储层物性、含油性的影响,以指导后期储层评价及预测,本文综合运用岩石、铸体、荧光薄片以及碳酸盐胶结物的碳氧同位素等多种方法,对研究区碳酸盐胶结物的类型、形成期次、成因及其对储集层的影响进行研究。结果表明,研究区八道湾组至少存在两期碳酸盐胶结物,成岩早期碳酸盐胶结物以泥晶状菱铁矿和连晶状方解石为主,分布较为局限,其形成受控于凝灰质等易溶物质的溶蚀。晚期发育团粒状菱铁矿、铁方解石和铁白云石充填次生溶孔,铁白云石体积分数最大;晚期碳酸盐胶结物的δ^(13)C_(PDB)分布于-13.6~-6.5‰,δ^(18)O_(PDB)分布于-21~-18.1‰。通过古盐度和古温度的计算,发现碳酸盐胶结物的成岩流体为淡水环境,形成温度分布于112~136.1℃,推测形成于中成岩A期。结合碳图解和天然气同位素分析,碳酸盐胶结物中的碳来源主要为有机成因。通过物性资料、荧光薄片和胶结物含量分析,发现碳酸盐胶结的储层物性往往较差,以铁白云石为主的储层物性相对最好;同时碳酸盐胶结物体积分数越高,反映前期次生溶孔越多,含油性越好。

To deeply investigate the genesis of carbonate cement in reservoirs of Jurassic Badaowan Formation in AH5 well block of the Junggar Basin,and to discuss the influence on the reservoir physical property and oiliness for evaluating/predicting reservoirs in later period,we studied the types,forming period,genesis and the influence on reservoir quality through combining thin-section petrography and carbon-oxygen isotope analyses in carbonate cement.The results show at least two periods of carbonate cement in the Badaowan Formation:micritic siderite and crystalline calcite in the early diagenetic stage(limited distribution),and granular siderite,ferrocalcite and ankerite developed in the late diagenetic stage(secondary dissolution pore infill),and ankerite has the highest volume.δ^(13) C_(PDB) of carbonate cement=-13.6‰to-6.5‰,andδ^(18 )O_(PDB)=-21‰to-18.1‰.The paleosalinity and paleotemperature estimated from the carbon-oxygen isotope compositions show that the carbonate cement was formed in freshwater environment at 112 to 136.1℃,based on that the forming time was the middle diagenetic period.The carbon source in carbonate cement was of organic origin through carbon diagram and analysis on natural gas isotopes.Reservoirs with carbonate cement have always bad physical property,and reservoirs with ankerite cement have better physical property through combining physical property,fluorescence thin-section and carbonate cement content analysis.The higher volume of ankerite would have more early secondary dissolved pores and better reservoir oiliness.