鄂尔多斯盆地姬塬地区侏罗系延安组古河道砂岩储集层成岩演化及主要胶结物来源

Diagenetic evolution and sources of main cements within palaeochannel sandstone reservoirs of the Jurassic Yan'an Formation in Jiyuan area,Ordos Basin

成岩作用直接影响储集层物性演化进而控制储集层品质,厘清储集层成岩作用的相对先后顺序对油气勘探具有重要意义。通过岩心观察、普通薄片、铸体(染色)薄片、扫描电镜(SEM)、阴极发光(CL)、X衍射、流体包裹体以及碳氧同位素等多种测试手段,对鄂尔多斯盆地姬塬地区侏罗系延安组古河道砂岩成岩演化过程及其主要胶结物来源进行研究,并定量分析了胶结作用和压实作用对粒间孔隙变化的影响。结果表明:延安组古河道砂岩储集层主要发育长石砂岩和岩屑长石砂岩,平均孔隙度15.1%,平均渗透率19.5×10^(-3)μm^(2),属中孔低渗储集层;先后经历压实、早期胶结、中期溶蚀、晚期胶结和晚期交代等成岩作用;其中,铁方解石胶结物含量最高,高岭石分布最广泛。胶结作用是原始粒间孔隙体积减少的最主要贡献者,平均贡献量为37.5%;压实作用次之,平均贡献量为29.4%。方解石胶结物直接来源于与大气降水相关的成岩流体,白云石是早期方解石与薄煤层、泥岩热演化产物联合供源,铁方解石则直接来源于薄煤层和泥岩夹层的热演化产物。本次研究建立了姬塬地区侏罗系延安组古河道砂岩储集层的成岩演化序列,可为后续深入勘探、开发提供储集层评价基础。

Diagenesis directly affects the evolution of reservoir physical properties,and further controls reservoir quality.It is of great significance for oil and gas exploration to clarify the relative sequence of diagenesis.The diagenetic evolution of Jurassic Yan'an Formation palaeochannel sandstone in Jiyuan area of Ordos Basin and its main sources of cements were studied based on core observation,common thin sections,cast(stained)thin sections,scanning electron microscopy(SEM),cathodoluminescence(CL),X-ray diffraction,fluid inclusion and carbon&oxygen isotopes.Also the impact of cements and compaction on the loss of intergranular pores was quantitatively analyzed.The results show that paleochannel sandstone reservoir of Yan'an Formation is mainly composed of arkoses and lithic arkoses,with average porosity of 15.1%and average permeability of 19.5×10^(-3)μm^(2),which belongs to medium-porosity and low-permeability reservoir.The diagenetic process has experienced compaction,early cementation,medium dissolution,late cementation and late replacement.As far as the cements are concerned,the content of ferrocalcite is the highest,and kaolinite is the most widely distributed.Cements are the main contributor to the reduction of original intergranular pore volume,with an average contribution of 37.5%,and compaction was the minor contributor,with an average contribution of 29.4%.In addition,calcites directly derived from diagenetic fluid related to atmospheric water;dolomite was a joint source of early calcite,coal and mudstone thermal evolution products,but ferrocalcite primarily originated from thermal evolution of coal and mudstone interlayer.This study establishes a diagenetic sequence of Jurassic Yan'an Formation palaeochannel sandstone reservoirs in Jiyuan area,which is able to provide a foundation for reservoir assessment for further exploration and development.