Diagenesis and Reservoir-Quality Evolution of Paralic, Shallow Marine and Fluvio-lacustrine Deposits Links to Depositional Facies and Sequence Stratigraphy

Abstract: Linking diagenesis to depositional facies and sequence stratigraphy enables better prediction of spatial and temporal distribution of diagenetic alterations, and thus of evolution of reservoir quality in sandstones. This thesis demonstrates that employing this approach is possible because depositional facies and sequence stratigraphy can provide useful information on parameters controlling the near-surface diagenesis, such as changes in: (i) pore-water chemistry, (ii) residence time of sediments under certain geochemical conditions, (iii) detrital composition and proportion of extra- and intra-basinal grains, and (iv) types and amounts of organic matter.Evidence from four case studies enabled the development of conceptual models for distribution of diagenetic alterations and of their impact on evolution of reservoir quality in sandstones deposited in paralic, shallow marine and fluvio-lacustrine environments. Diagenetic alterations that have been constrained within the context of depositional facies and sequence stratigraphy include: (i) carbonate cement (microcrystalline and equant calcite spars dolomite over poikilotopic calcite), pyrite and glaucony are most abundant in progradational braid-delta fan sequences, particularly along the topsets (i.e. maxiumum flooding surface, MFS) and along parasequences boundaries in the deltaic facies of the early highstand systems tract HST, (ii) cementation by coarse spar calcite, dolomite, and the formation of moldic porosity by the dissolution of framework carbonate grains are most abundant in the aggradational fan deltas sequences, (iii) eogenetic kaolinitization of framework silicates is largely restricted to the fluvial and paralic HST sandstones, whereas telogenetic kaolinite may occur in the transgressive systems tract TST sandstones too, (iv) formation of goethite ooids in the TST sediments, (v) formation of glaucony, siderite spherules, and extensive grain-coatings, grain-replacing and ooidal berthierine, more in the TST than in the HST sediments, particularly below the transgressive surface TS and MFS, (vi) cementation by calcite with (?18OV-PDB = -11.5‰ to -5.4‰) and Fe-dolomite/ankerite (?18OV-PDB = -10.8‰ to -9.6‰) occurs in both TST and HST sandstones, (vii) syntaxial quartz overgrowths are most extensive in the HST sandstones owing to the presence of incomplete grain-coating berthierine/chlorite, (viii) greater amounts of micro-porosity in the TST sandstones than in the HST sandstones are related to the greater amounts of berthierine/chlorite in the former sandstones, and (ix) chlorite rims around quartz grains retarded the precipitation of quartz overgrowths, and hence prevented a greater loss of primary intergranular porosity in fluvio-lacustrine sandstones. Therefore, constraining the distribution of diagenetic alterations in the contexts of depositional facies and sequence stratigraphic context is a powerful approach to be used in hydrocarbon exploration.