鄂尔多斯盆地姬塬油区铁边城区块长8储层成岩致密化及其与油气成藏关系

Diagenetic densification of Chang 8 sandstone reservoirs and its relationship with hydrocarbon accumulation in Tiebiancheng area,Jiyuan Oilfield,Ordos Basin Kang

通过铸体薄片、扫描电镜、阴极发光、X衍射、高压压汞和流体包裹体等多种测试分析,系统研究了鄂尔多斯盆地姬塬油区东南部铁边城区块延长组长8油层组致密砂岩储层的岩石物性、成岩作用和致密化过程及其与油气充注成藏的时序关系。结果表明,研究区长8储层岩石类型以细粒岩屑长石砂岩为主,成分成熟度和结构成熟度低,储集性能较差,平均孔隙度为7.34%、平均渗透率为0.11×10^(-3)μm^(2),属于较为典型的(特)低孔-超低渗致密储层,成岩作用类型以压实、胶结和溶蚀作用为主,成岩演化阶段主体已进入中成岩A期;储层成岩-致密化过程先后经历了早成岩A-B期压实+胶结减孔、中成岩A 1期溶蚀增孔+胶结减孔、中成岩A 2期胶结减孔3个主要阶段。早白垩世末(100 Ma)最大埋深之前的压实和胶结作用是储层致密化的根本原因,造成孔隙度分别降低了21.14%和14.0%,溶蚀增加的孔隙度仅为1.76%;早白垩世中期2个主要幕次(123 Ma和105 Ma)的大规模烃类充注-成藏事件开始发生时,储层孔隙度已在晚期胶结作用影响下降至7.82%,成为典型的(特)低孔-超低渗致密砂岩储层,因而具有“先致密、后成藏”的特征。

Based on multiple testing data from the casting slice,cathodoluminescence,scanning electron microscopy,X-ray diffraction,HPMI,reservoir physical property,and fluid inclusion thermometry,systematic research was conducted to investigate the petrologic features,reservoir properties,diagenetic porosity evolution,and temporal relations between the densification and oil charging process of the Chang 8 sandstone reservoirs of the Upper Triassic Yanchang Formation in Tiebiancheng area of Jiyuan Oilfield,Ordos Basin.The results show that the Chang 8 reservoirs in the study area are mainly composed of fine-grained lithic feldspar sandstones with low maturity of composition and texture,featuring unconventional tight reservoirs with an average porosity of 7.34%and permeability of 0.112×10^(-3)μm^(2),which generally reach the middle diagenetic stage A.The diagenetic porosity evolution of the Chang 8 reservoirs mainly experienced three stages:porosity reduction by compaction and cementation in early diagenetic stage A-B,porosity increase by dissolution and reduction by cementation in middle diagenetic stage A 1,and porosity reduction to densification by cementation in middle diagenetic stage A 2 to B 1.Until the end of the Early Cretaceous,all the occurred compaction and cementation made the reservoir porosity decrements approach 21.14%and 14%,respectively,while the porosity increment by dissolution was only about 1.76%.At the beginning of two episodic large-scale oil charging events that occurred at 125 and 105 Ma during the middle to late period of the Early Cretaceous,the porosity of the Chang 8 sandstone reservoirs decreased to 7.82%under the influence of late cementation.It became a typical ultra-low porosity and ultra-low permeability tight sandstone reservoir,thus most likely indicating that the densification of the Chang 8 reservoirs occurred earlier than the hydrocarbon accumulation in this study.