自生“加大”含铁白云石胶结物特征及对致密砂岩储层的影响——以鄂尔多斯盆地陇东地区延长组4+5段为例

随着油气勘探的不断深入，致密砂岩储层越来越受到人们的关注，特别是关于致密化的成因及油气充注的时间。利用岩石学、岩石地球化学以及储层物性资料，综合分析鄂尔多斯盆地陇东地区延长组4+5段中含铁白云石胶结物产出特征、成因及其对储层质量的影响。研究表明：自生含铁白云石主要以白云岩岩屑的“次生加大边”形式出现，其形成与早成岩晚期碎屑白云岩的压溶和Fe质交代有关；少量含铁白云石交代长石颗粒。岩石粒度、黑云母碎屑含量、沉积微相控制含铁白云石胶结物的含量与分布，进而控制储层质量。在镇原和环县地区含铁白云石胶结物的含量高于华池和白豹地区；在水下分流河道中上部和河口坝中部，含铁白云石胶结物含量有增加的趋势。黑云母等碎屑的蚀变可能是最主要的Fe质来源。在水下分流河道底部砂体中，含铁白云石胶结物的含量一般小于3%，孔渗性整体较好；在水下分流河道顶部砂体中，这种胶结物的含量通常大于3%，储层质量一般较差。该研究成果将对相似地质条件下致密储层形成机理的分析提供一定的依据。

Cement characteristics of overgrown authigenic ferron dolomite and its impact on tight sandstone reservoirs qualities: Case study of the Chang 4 & 5 members of Triassic Yanchang Formation in Longdong area,Ordos Basin

The tight sandstone reservoirs have drawn more and more attention along with increasing hydrocarbon exploration activity.This is particularly true with regard to the mechanism of tightening and its relation with the timing of hydrocarbon charging.We analyzed the occurrence and growth mechanism of ferron dolomite cements in the Chang 4&5 members and their impact on reservoir properties in the Triassic Yanchang Formation in Longdong area,Ordos Basin,combined with a study on lithology,petrogeochemistry and physical properties of reservoirs.The results show that the authigenic ferron dolomite mainly occurs as "secondary overgrowth" of detrital dolomite,the generation of which is caused by the pressure solution of detrital dolomite and the ferron metasomasis at the late stage of early diagenesis.That is a small scale metasomasis of feldspar particles by ferron dolomite.The size of grains,the content of biotite debris as well as the sedimentary microfacies function to control the content and distribution of ferron dolomite cements,which in turn will control the reservoir property.The content of ferron dolomite cements in Zhenyuan and Huanxian areas is higher than that in Huachi and Baibao areas.The amount of ferron dolomite cements tends to increase at the middle-to-upper section of submerged distributary channels and the middle mouth bars.The ferron is mainly originated from the dissolution of biotite debris.The sand bodies of the bottom of submerged distributary channels facies usually have an overall good porosity and permeability only if the content of cements is less than 3%.However,the cement content tends to exceed 3% in the sand bodies on top of submerged distributary channels facies,resulting in lower reservoir quality.The results obtained in the study can provide indications to the analysis of tight reservoir formation mechanism in similar geological conditions.