Characteristics and genesis of deeply buried under-compacted sandstone and its comparison with normal Sandstone-A case in the lenghu tectonic belt, northern Qaidam Basin

Based on recovered core observations and thin section identifications combined with scanning electron microscopy images and statistics for physical properties, XRD analysis, and data for trace elements and carbon and oxygen isotopes, the genesis of under-compacted sandstone in the Lower Ganchaigou Formation of the Paleogene in the Lenghu VII structure in the northern Qaidam Basin has been confirmed through a detailed comparison with normal intervals for petrology, sedimentary environments and diagenetic characteristics. The results showed that under-compacted sandstone, developing in underwater distributary channels, was characterized by abnormal high porosity and large amount of intergranular pores. The genesis of intergranular pores is the intrusion of organic acid fluids, dissolution of calcite cements and restoration of primary pores. Conclusions could be drawn from the comprehensive comparison with normal interval. First, honeycomb-like chlorite grain coatings were much abundant (33%–58% in total clays) in the under-compacted sandstone comparing to it in normal interval and sandstone in adjacent area (<30%), indicating an intrusion of Mg-rich fluid leading to transformation from I/S to chlorite. Second, the carbonate cements content of the under-compacted sandstone (5.7%–17.3%) and the normal range (4.1%–20.3%) were both dominated by micrite calcite, and large amounts of early calcite residues resulting from dissolution were only found in the under-compacted sandstone. The δ¹⁸O value (17.36‰ ∼ −10.54‰) and the δ¹³C values (−5.12 to −3.51‰) indicated inorganic carbon dominated in carbonate cements; however, the small negative bias for δ13C in the under-compacted sandstone indicated that more organic carbon was contained than is typical. Third, the minor changes in the V/Zr, Ni/Zr, Co/Zr, Sr/Zr and Mo/Zr values suggested similar and stable paleoenvironments in both intervals. The weaker correlations among U/Zr, Ni/Zr and V/Zr indicated that U/Zr was affected by both the environment and fluid intrusion in the under-compacted sandstone. However, U/Zr was much larger (3.6–23.9) in the under-compacted sandstone than it is in normal sandstones (0.83–3.69), which indicated the invasion of acidic fluids originating from source rock.