Feldspar Dissolution, Authigenic Clays and Quartz Cements in Buried Sandstones With Open or Closed Geochemical Systems

Abstract

Feldspar dissolution and precipitation of clays and quartz cements are important diagenetic reactions affecting reservoir quality evolution in buried sandstones. We examined two sets of sandstone reservoir to determine whether the geochemical 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 below sea level (BSL) in the Nanpu sag, and the other is the Eocene subaqueous fan sandstone buried 1.5–4.5 km BSL in the Dongying sag. Both the sandstones consist mainly of lithic arkoses and feldspathic litharenites, and these have secondary porosity formed by feldspar dissolution. In the Nanpu sandstones, the absolute amounts of authigenic clays and quartz cements are much lower than that of the leached feldspars. Authigenic clays in the GL sandstones are mainly kaolinite, with little illite even at high temperature (>125°C). 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. The extensive feldspar dissolution enhanced much porosity and permeability. In contrast, the Dongying sandstones with secondary pores formed by feldspar dissolution generally contain authigenic kaolinite, illite and quartz cements with almost identical volume of secondary pores. Kaolinite dominates in the ST sandstones at shallower depth (<3.1 km BSL), whereas illite dominates at greater depth (>3.1 km BSL) where temperature exceeds 125°C. The presence of abundant clays and quartz cements indicates that Al3+ and SiO₂(aq) released from leached feldspars were retained in the sandstone system. The dominance of authigenic illite at greater depth indicates that sufficient K+ should have been retained within the sandstones for 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. Thus, prediction of porosity and permeability of sandstones with feldspar pores should be based on careful consideration of the sandstone system status as openness versus closeness with respect to products released from feldspar dissolution.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016