Comparison of diagenetic characteristics and pore evolution in outcrops and cores of tight sandstone reservoirs in the Triassic Yanchang Formation, the Ordos Basin, China

Abstract

The sedimentary and diagenetic control on tight sandstone reservoir quality is a key focus of hydrocarbon exploration. This paper compares reservoir characteristics, such as diagenetic processes, pore types, and porosity evolution, of cores and outcrops of the Yanchang Formation, the Ordos Basin, northwestern China, to identify diagenetic trends and processes responsible for improving reservoir quality and to develop a model for identifying reservoir sweet spots in tight sandstone formations. Samples of the Yanchang Formation sandstone are taken from wells and adjacent outcrops in the Ordos Basin. No significant differences were found in porosity-destructive diagenesis such as compaction and cementation between the core and outcrop samples. However, the effects of dissolution are far more pronounced in cores than in the outcrop, resulting in a higher core sample porosity. High-density sampling of the outcrop revealed that the reservoir sweet-spot model is controlled by sedimentation, whereas three distinct vertical diagenetic zones were identified in adjacent wells. A comprehensive analysis of cores and outcrop samples shows that both areas experienced similar sedimentation and early diagenetic patterns. However, the diagenetic processes experienced by core and outcrop sandstones diverged during the later part of burial and uplift, with dissolution being responsible for the contrasting reservoir porosities between core and outcrop.