ABSTRACT

Feldspar dissolution and precipitation of clays and quartz cements are important diagenetic reactions affecting reservoir quality evolution in sandstones with detrital feldspars. We examined two sets of sandstone reservoirs to determine whether the sandstone diagenetic systems were open or closed to the mass transfer of products from feldspar dissolution and its impact on reservoir quality. One of the reservoirs is the Eocene fan delta sandstone buried 2.5–4.0 km (1.5–2.5 mi) below sea level (BSL) in the Gaoliu (GL) area of the Nanpu sag, and the other is the Eocene subaqueous fan sandstone buried 1.5–4.5 km (1–2.8 mi) BSL in the Shengtuo (ST) area of the Dongying sag.

Both sandstones consist mainly of lithic arkoses and feldspathic litharenites, and have secondary porosity formed by dissolution of feldspars. In the GL sandstones, the absolute amounts of authigenic clays and quartz cements (generally <0.5% of the rock volume) are much lower than that of the leached feldspars. Authigenic clays in the GL sandstones are mainly kaolinite, with little illite even at a high temperature (>125°C [257°F]). The low abundance of authigenic clays and quartz cements, and low pore-water salinity indicate that much of the K+, Al3+, and SiO2(aq) released from leached K-feldspars were exported from the GL sandstone system. And the extensive feldspar dissolution enhanced much porosity and permeability.

In contrast, the ST sandstones with secondary pores formed by feldspar dissolution generally contain authigenic clays (kaolinite and illite) and quartz cements with almost identical volume of secondary pores. Kaolinite dominates in the ST sandstones at shallower depth (<3.1 km [2 mi] BSL), whereas illite dominates at greater depth (>3.1 km [2 mi] BSL) where temperature exceeds 125°C (257°F). The presence of abundant clays and quartz cements indicates that Al3+ and SiO2(aq) released from leached feldspars were retained in the ST sandstone system. The dominance of authigenic illite at greater depth indicates that sufficient K+ should have been retained within the sandstones for occurrence of illitization of kaolinite and feldspars. Secondary porosity in thin sections can be up to 3%, but little porosity (<0.25%) is enhanced. Primary macropores are lost as clays and quartz precipitate whereas the proportion of microporosity increases, occurring mainly between clay crystals. The overall result is that permeability is degraded.

The diagenetic difference between the GL and the ST sandstones can be interpreted by assessing pore-water evolution in these two areas. The current pore waters with low salinity and negative hydrogen isotopic compositions in the GL sandstone system indicate the significant impact of meteoric water, whereas the current pore waters with high salinity and the paleofluids with positive oxygen isotopic compositions in the ST sandstone system indicate little trace of meteoric water. Access of meteoric freshwater to the GL area probably occurred during the late Oligocene to Neogene through widely developed faults in the Paleogene and Neogene strata. The low-salinity water could have been responsible for flushing of solutes derived from feldspar dissolution. As such, diagenesis in the GL sandstones is considered to have occurred in an open geochemical system, whereas with limited faults and high water salinity, the ST sandstones acted as a closed geochemical system where precipitation of kaolinite, illite, and quartz cements occurred following dissolution of feldspars.

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