Impact of diagenesis on the reservoir quality of tight oil sandstones: The case of Upper Triassic Yanchang Formation Chang 7 oil layers in Ordos Basin, China

The Chang 7 tight oil reservoirs are important oil exploration targets in the Ordos basin. The reservoirs are generally characterized by low porosity, low permeability and strong microscopic heterogeneity. Mineralogical, petrographic, and geochemical analyses have been used to investigate the type and degree of diagenesis and diagenetic history of Chang 7 tight oil reservoirs. The influences of composition, texture and diagenesis on reservoir quality were also discussed in this article. Diagenesis of the Chang 7 tight oil reservoirs was mainly composed of mechanical compaction, grain dissolution and cementation by quartz, carbonates and various clay minerals. Reduction of porosity by mechanical compaction was more significant than by cementation. Eodiagenesis mainly includes (1) mechanical compaction and mechanically infiltrated clays; (2) cementation by calcite, pyrite, and clay minerals; and (3) leaching of feldspars. Mesodiagenesis mainly includes (1) further mechanical compaction; (2) cementation by late stage carbonates; (2) formation of illite and mixed-layered illite-smectite; (3) quartz cements; (4) dissolution of feldspars. The porosity was decreased by compaction and cementation and then increased by dissolution of the framework grains. Primary porosity is higher in sandstones with abundant detrital quartz. Secondary dissolution pores are mainly associated with those feldspar-rich samples. Sandstones which have undergone the most feldspar dissolution are the cleaner (abundant in both detrital quartz and feldspar), better sorted, and coarser-grained samples. Reservoir quality of Chang 7 tight oil reservoirs is also largely controlled by pore occluding cements. The composition and texture have played an important role on intergranular volume and subsequent diagenetic modifications of the Chang 7 tight oil reservoirs. Good quality reservoir intervals are characterized by fine-grained, moderately-well sorted with high percentages of detrital quartz and feldspar but low content of detrital clay and cements. (C) 2016 Elsevier B.V. All rights reserved.