Depositional facies, diagenesis and their impact on the reservoir quality of tight oil sandstones: Evidence from Chang-7 sandstones in Ordos basin, China

The seventh member of the Upper Triassic Yanchang Formation (Chang-7) tight oil sandstone reservoirs is the main exploration target and a significant amount of hydrocarbons have been discovered in such reservoirs in Ordos... [ view full abstract ]

The seventh member of the Upper Triassic Yanchang Formation (Chang-7) tight oil sandstone reservoirs is the main exploration target and a significant amount of hydrocarbons have been discovered in such reservoirs in Ordos basin, China. The Chang-7 sandstones are characterized as tight with low porosity, low permeability and strong heterogeneity. This study investigates the sedimentary facies, and diagenesis and their impact on reservoir quality of the Chang-7 tight oil sandstones. In spite of their large lateral extent, exploration of these sandstones is complicated by variation of sediment facies and intense diagenesis, which strongly affected reservoir quality. These sandstones were deposited in a lacustrine-deltaic setting and been divied into front delta, semi-deep to deep lacustrine and slump turbidite fan facies. Depositional environment exerted an essential control on the reservoir quality. Higher energy, well-sorted and relatively coarser grained sandstones have better quality than low-energy fine-grained, clay-rich sandstones. Diagenesis was mainly composed of mechanical compaction and dissolution and cementation by carbonates and various clay minerals. The reservoir properties of the Chang-7 sandstones are generally poor with a porosity ranging from 1.4% to 20.7% ( average porosity of 10.1%) and permeability ranging from 0.001 to116.7 mD (permeability of 0.18 mD), which are attributed to the significant compaction and cementation. Porosity systems of the Chang-7 sandstones are of primary and secondary origins. The primary intergranular porosity mainly reserved by chlorite coatings ranges from trace levels to 6.20%, with an average of 1.30%. The secondary porosity ranging from trace levels to 6.70%, averaging 2.80%) was derived from partial to complete dissolution of framework grains (mainly feldspar) and is of great importance for the Chang-7 sandstones. By linking diagenesis to depositional facies, it can be concluded that the better quality reservoirs in Chang-7 tight oil sandstones are associated with fine to medium-grained sandstones of distributary channel facies and turbidite facies, development of secondary porosity (e.g. feldspar dissolution), and low content of cements such as carbonate and clay minerals (e.g. illite). The best reservoir quality is potentially encountered in the sandstones at deeper depths, which experienced a higher degree of dissolution due to closer proximity to the underlying Chang-73 source rocks.