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Evidences of CO 2 induced diagenesis in the Quantou Formation, Southern Songliao Basin, have been investigated using petrographic analyses, X-ray diffraction, stable isotopes, fluid inclusions and kinetic-thermodynamic modeling. The studied sandstones have two distinct zones based on their mineralogy, one “pristine” being composed of quartz, plagioclase, chlorite, calcite and dolomite, and one “altered” mainly consisting of quartz, illite and illite/smectite, ankerite, and dawsonite. The ankerite/dawsonite and secondary clay-bearing zones are related to the major deep rooted faults in this area. The δ 13 C value of carbonate cements in the altered zone (mainly centered on −0.9‰ to −8.0‰) is higher than that in the “pristine” sediments. We therefore proposed that these zones have experienced CO 2 charged hydrothermal (∼120–140 °C) alteration and with a similar precursor mineral assemblage as in the plagioclase/chlorite rich zones. Kinetic modeling furthermore suggests that the observed alteration must have been under high CO 2 partial pressures, with the closest match between simulations and XRD data found at the highest attempted pressure of 100 bar. The modeling furthermore suggests that using kinetic data from the literature together with reactive surface areas normalized to the surface area of the pores provides good estimates for the alteration of tight sandstones such as the Quantou Fm (∼8% porosity). [ABSTRACT FROM AUTHOR]

The Eocene sandstones in the northern Dongying Sag, Bohai Bay Basin, China, are reservoirs for large accumulations of hydrocarbons. The sandstones are mainly lithic arkoses and feldspathic litharenites, texturally and compositionally immature. These sandstones have a wide range of porosity (0.4–37%) and permeability (0.004–6969 mD) and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level. The reduction in reservoir quality is a product of several digenetic processes; these include compaction, precipitation of dolomite and calcite in eodiagenetic stage; compaction, feldspar dissolution, precipitation of quartz cements and clays (kaolin and illite) and precipitation of ferrocalcite and ankerite in mesodiagenetic stage. Mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones. Carbonate cementation, together with compaction reduced the sandstones’ porosity and permeability significantly. In a sandstone bed, marginal sandstones with distance to sandstone/mudstone interface less than one meter always have lower porosity than central sandstones. As burial depth exceeds 4000 m, marginal sandstones have very low porosities (<5%), indicating that thin sandstone beds (<2 m) were totally destroyed by cementation and compaction, and only thick sandstone beds (>2 m) can be potential effective reservoirs. Feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity. Mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones, and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0.25%. However, although there is little or no net import of matter to the sandstones, the pore architecture changes dramatically. Primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases, occurring mainly between clay crystals. The overall result is that permeability is significantly degraded. [ABSTRACT FROM AUTHOR]