Role of K-feldspar dissolution in chloritization of smectite: Insights from hydrothermal experiments

Chlorite, a critical diagenetic clay mineral, significantly influences porosity and permeability in deeply buried sandstone reservoirs. Authigenic chlorite develops from various clay and non-clay mineral precursors. Although smectite-to-chlorite transformation has been widely documented, experimental constraints on how dissolution of common framework minerals in natural sandstones supplies solutes that trigger smectite nucleation and subsequent chlorite authigenesis remain underexplored. This study presents a systematic hydrothermal-reactor experiments demonstrating a dissolution-driven pathway linking K-feldspar and detrital kaolinite alteration to authigenic development of smectite and its subsequent chloritization, using arkosic sandstone (Subbat Member, Jauf Formation, NW Saudi Arabia). Closed-system autoclave experiments were conducted at 80–250 °C and ∼ 10–16 MPa for 2–4 weeks, with a fluid-to-rock ratio of 10:1, using natural Red Sea water and a synthetic multi-salt brine. Integrated petrographic, mineralogical and geochemical analyses reveal a multistage reaction pathway for chlorite authigenesis: (1) early dissolution/etching of detrital feldspar, kaolinite, and hematite cements from 80 °C releases Al, Si, K, and Fe into the reacting fluid; (2) nucleation of smectite occurs at ∼150 °C, forming honeycomb and grain-coating textures; (3) smectite transforms into well-crystallized chlorite between 200 and 250 °C via a dissolution–precipitation mechanism, with fluid chemistry dictating Fe vs. Mg enrichment; and (4) silica released from feldspar and kaolinite dissolution precipitates as quartz overgrowths on detrital quartz. Continuous chlorite grain coatings inhibit quartz cementation, preserving intergranular porosity, whereas pore-filling chlorite reduces porosity from 24.2% to 19.5%, underscoring the importance of chlorite texture and distribution for reservoir quality. These findings demonstrate that K-feldspar dissolution can drive a smectite-mediated chloritization pathway in arkosic sandstone, with significant implications for predicting reservoir quality in K-feldspar–rich sandstone formations worldwide.