Distribution of diagenetic alterations within depositional facies and sequence stratigraphic framework of fan delta and subaqueous fan sandstones: evidence from the Lower Cretaceous Bayingebi Formation, Chagan sag, China–Mongolia frontier area

Abstract

Linking siliciclastic diagenesis to sequence stratigraphy allows a better understanding of the parameters controlling the spatial and temporal distribution of diagenetic alterations. A study of the subaqueous fan and fan delta sandstones of the Bayingebi Formation, Lower Cretaceous, Chagan sag (China), reveals that the distribution of diagenetic alterations can be constrained within a sequence stratigraphic framework. Eogenetic alterations, which were encountered in the sandstones of subaqueous fan and fan delta, include: (1) replacement of detrital silicates and mud matrix by kaolinites in sandstones of the late highstand systems tract (HST) and the lowstand systems tract (LST). The formation of kaolinites is attributed to the influx of meteoric water when the base level fell and the channels migrated laterally on preexisting floodplains or channel deposits. (2) Infiltrated clays, which rim around detrital grains, mainly occur in channel sandstones of the LST due to the percolation of mud-rich surface waters. (3) Calcite I (δ18OVPDB = −20.19‰ to −3.64‰ and δ13CVPDB = −0.67‰ to −3.1‰), Calcite II (δ18OVPDB = −14‰ to −18.28‰ and δ13CVPDB = −2.2‰ to −3.60‰), dolomite (δ18OVPDB = −17.91‰ to −21.04‰ and δ13CVPDB = −0.46‰ to −1.66‰) and ankerite (δ18OVPDB = −15.02 to −20.42‰ and δ13CVPDB = −0.074 to −1.43‰), which are associated with palaeosol horizons mainly developed on top of overbank and channel sandstones of transgressive systems tract (TST) and early stage of the HST. Such extensive eogenetic calcite cements may act as potential layers for the formation of reservoir compartments for underlying sandstones. Mesogenetic alterations include: (1) abundant quartz overgrowths in the overbank sandstones of subaqueous fan in the LST and pro-fan-delta sandstones in the HST, in which early carbonate cements are lacking and grain-coats are thin. (2) Illite, chlorite and albitized feldspars, which occur in small amounts, are most common in the LST and late stage of the HST sandstones. Such cementation by calcite, dolomite, quartz overgrowths and formation of illite led to porosity and permeability deterioration during mesodiagenesis. This study revealed the importance of integration of diagenesis with sequence stratigraphy of subaqueous fan and fan delta sandstones in improving our ability to predict the spatial and temporal distribution of eogenetic alterations and their subsequent impact on mesogenetic alterations, and thus on reservoir quality modifications.