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Petrography, Fluid Inclusion, Isotope and Trace Element Constraints on the Origin of Quartz Cementation and Feldspar Dissolution and the Associated Fluid Evolution in Arkosic Sandstones

Abstract

Feldspar dissolution and quartz cementation are significant diagenetic reactions in arkosic sandstones in the Dongying Sag, East China. The timing and origin of these water-rock interactions, as well as paleofluids in which relevant mineral cementation and dissolution occurred was deduced using data from the analysis of oxygen isotope microanalysis, element microanalysis, fluid inclusion microthermometry, and microscopy. Three distinct, separate episodes of quartz overgrowths (Q1, Q2 and Q3) were distinguished by cathodoluminescence microscopy. These three episodes were also recognized in the distribution of fluid inclusions, oxygen isotope and trace element microanalyses. Q1 quartz formed at approximately 100-115°C before oil filled the reservoirs and was identified in all porous sandstones from 2500m to 3600m. Q2 quartz mainly precipitated at 115-130°C, accompanying or slightly postdating the main phase of oil filling, and was identified in samples from 2800-3600m. Q3 quartz was only identified in sandstones deeper than 3500m, and was likely precipitated recently when temperature exceeded 130-135°C. Secondary ion mass spectrometry (SIMS) oxygen isotope microanalyses yielded δ18O values ranging from 21.42‰ to 24.35‰ for Q1 quartz, from 22.03‰ to 24.99‰ for Q2 quartz, and from 21.72‰-22.91‰ for Q3 quartz, respectively. Internal feldspar dissolution was likely the primary silica source for the quartz cements. Calculated positive δ18O(water) values (+0.5‰ to +4.5‰) of quartz-forming waters indicates that quartz cementation and feldspar dissolution occurred in geochemical system with limited volume of diagenetically modified connate water. The variations of δ18O(cements) and trace element compositions from Q1 quartz to Q2 quartz in individual overgrowths suggest that hydrocarbon filling changed the chemistry of pore fluid significantly; further, δ18O(water) values of pore water were increased by about 2‰ after oil filling. The continuous development of Q2 and Q3 quartz suggests that chemical diagenesis did not cease after oil filling. Further, diagenetic reactions likely have proceeded from water-rock interactions to hydrocarbon-water-rock interactions.