Diagenesis of sandstone in the Majiashan-Xiaojianzi oilfield, Ordos basin and its effect on reservoirs

Abstract：Integrated methods, including thin section identification, cast thin section examination, mercury penetration, scanning electron microscopy (SEM), X-ray diffraction (XRD) and fluid inclusion analysis, were used to reveal the controls of sandstone diagenesis in the Majiashan-Xiaojianzi oilfield on reservoirs. Study indicates that the diagenesis of sandstone of the Chang 61 and Chang 4+5 members in the Yanchang Formation has reached stage A of the middle diagenetic phase. The distribution of diagenetic facies exhibits a distinctive regularity: dissolution pore facies, chlorite cement facies, carbonate cement facies and compact matrix facies occur in succession from the sand bodies of subaqueous distributary channels and center of river mouth sandbars to margins and down to the subaqueous distributive bay. The physical properties of sandstone become poor successively. The favorable reservoirs are dissolution pore facies and chlorite cement facies in the sand bodies of subaqueous distributary channels and center of river mouth sandbars.