Distribution of diagenetic alterations within depositional facies and sequence stratigraphic framework of fluvial sandstones: Evidence from the Petrohan Terrigenous Group, Lower Triassic, NW Bulgaria

Sequence stratigraphy of fluvial deposits is a controversial topic because changes in relative sea level will eventually have indirect impact on the spatial and temporal distribution of depositional facies. Changes in the relative sea level may influence the accommodation space in fluvial plains, and hence have impact on types of fluvial system, frequency of avulsion, and style of vertical and lateral accretion. This study aims to investigate whether depositional facies and changes in the fluvial system of the Lower Triassic Petrohan Terrigenous Group sandstones (NW Bulgaria) in response to changes in the relative sea level have an impact on the spatial and temporal distribution of diagenetic alterations. Eogenetic alterations, which were encountered in the fluvial sandstones, include: (i) mechanically infiltrated clays, particularly in channel and crevasse splay sandstones towards the top of the lowstand systems tract (LST) and the base of the highstand systems tract (HST). (ii) Pseudomatrix, which resulted from mechanical compaction of mud intraclasts, occurs mainly in channel sandstones at the base of the LST and towards the top of the HST and thus led to porosity and permeability deterioration. (iii) Calcite (δ¹⁸O_VPDB = -8.1‰ to -7.5‰ and δ¹³C_VPDB = -7.8‰ to -6.3‰) and dolomite (δ¹⁸O_VPDB = -8.3‰ to -5.2‰ and δ¹³C_VPDB = -8.3‰ to -7.1‰), which are associated with palaeosol horizons developed on top of crevasse splay and channel sandstones of transgressive systems tract (TST) and LST. Such extensive eogenetic calcite cements may act as potential layers for the formation of reservoir compartments for underlying sandstones. Mesogenetic alterations include: (i) calcite (δ¹⁸O_VPDB = -18.4‰ to -12.8‰ and δ¹³C_VPDB = -8.6‰ to -6.8‰) and dolomite (δ¹⁸O_VPDB = -14.7‰ to -12.4‰ and δ¹³C_VPDB = -8.0‰ to -7.0‰), which were formed in all depositional facies and systems tract sandstones, (ii) illite, which is the dominant diagenetic clay mineral in all depositional facies and systems tracts, was associated with albitization of detrital K-feldspars, and (iii) quartz overgrowths, which are most abundant in TST rather than LST and HST sandstones, because of the presence of suitable infiltrated clays and pseudomatrix in the latter sandstones. Such cementation by calcite, dolomite, and quartz overgrowths and formation of illite led to porosity and permeability deterioration during mesodiagenesis. The results of this study revealed the importance of integration of diagenesis with depositional facies and sequence stratigraphy of fluvial 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.