Depositional and compositional controls on diagenesis of the mixed siliciclastic-volcaniclastic sandstones: A case study of the Lower Cretaceous in Erennaoer Sag, Erlian Basin, NE China

Abstract

A succession of tight-oil mixed siliciclastic-volcaniclastic sandstones (MSVS) deposited in the Lower Cretaceous in the Erlian Basin has increasingly attracted attention for its significant discovery of oil and gas. The mixed compositions have a significant influence on diagenesis and reservoir properties. Understanding the relationship between lithofacies and diagenesis is of great value to providing an understanding of hydrocarbon enrichment potential in MSVS. The current investigation utilizes a multi-method approach including core analysis, petrography, SEM, microthermometry, and stable carbon- and oxygen-isotope geochemistry to better understand the mixed sandstones' lithofacies and controls on diagenesis. Three main sandstone lithofacies are composed of non-tuffaceous sandstone deposited in fan delta front (FFD-NTss), non-tuffaceous sandstone deposited in pro-fan delta (PFD-NTss), and tuffaceous sandstone deposited in pro-fan delta and lake (PFD-Tss). The results suggest that the early diagenetic stage of the MSVS includes the formation of chlorite and subhedral micritic-crystalline dolomite-1 cement (DI). The late stage includes the precipitation of quartz and calcite and subhedral to euhedral microcrystalline dolomite-2 cements (DII). FFD-NTss suffered from strong compaction due to abundant silt-sized and clay matrix whereas PFD-NTss was influenced by abundant calcite cementation due to weak compaction and more rigid debris. PFD-Tss contains abundant volcanic materials, alteration of which not only provided abundant ions but enhanced methanogenesis during the shallow burial, therefore, leading to formation of early diagenetic DI, analcime, and chlorite. Clay minerals are predominant in FFD-NTss whereas the late diagenetic calcite and dolomite cements are common in PFD-NTss that suffered from weak compaction. With the alteration of volcanic material and clay mineral transformations, the late diagenetic carbonate and clay minerals are widely distributed in the PFD-Tss.