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, quartz cementation, and clay cementation are significant diagenetic processes in deep-water fan feldspar-rich sandstones in the Shengtuo area, Dongying sag (East China). The timing and origin of these water-rock interactions, as well as the paleo-fluids in which relevant chemical reactions occurred were deduced using data from microscopy, fluid inclusion microthermometry, oxygen isotope microanalysis and trace element microanalysis.
Three distinct, separate episodes of quartz overgrowths (referred as Q1, Q2 and Q3) were distinguished by cathodoluminescence (CL) microscopy. Q1 quartz, identified in all porous sandstones from 2500 m (8200 ft) to 3600 m (11800 ft), was formed at approximately 100-115 °C (212-239 °F) before oil filled the reservoirs. Q2 quartz was mainly precipitated at 115-130 °C (239-275 °F), accompanying or slightly postdating the main phase of oil filling, and was identified in samples from 2800 m (9200 ft) to 3600 m (11800 ft). Q3 quartz was only identified in sandstones buried deeper than 3500 m (11400 ft), and was likely precipitated recently when temperature exceeded 130-135 °C (266-275 °F). Secondary ion mass spectrometry (SIMS) oxygen isotope microanalyses yielded δ18OV-SMOW values ranging from 21.42‰ to 24.35‰ for Q1 quartz, from 22.03‰ to 24.99‰ for Q2 quartz, and from 21.72‰ to 22.91‰ for Q3 quartz.
A mass balance calculation and quantitative petrography data of the amount of leached feldspars and associated secondary minerals suggest that the internal feldspar dissolution is likely the primary source for the authigenic clays and quartz cements in these sandstones. Positive δ18O(water) values (+0.5‰ to +4.5‰ V-SMOW) of quartz-forming waters indicates that quartz cementation and feldspar dissolution occurred in a geochemical system with a limited volume of diagenetically modified connate water. The variations of δ18O(cements) and trace element compositions from Q1 quartz to Q2 quartz in individual overgrowth suggest that hydrocarbon filling changed the chemistry of the pore fluid significantly; further, δ18O(water) values of the pore water increased by about 2‰ V-SMOW after oil filling.
Meteoric freshwater did not contribute to quartz cementation and simultaneous feldspar dissolution. The detected CO2 and hydrocarbons in fluid inclusions in the quartz cements, the existence of CO2 in hydrocarbon-rich natural gas and the bitumen in the feldspar-hosted pores suggest that acids derived from organic matter (kerogen in source rocks and hydrocarbons in reservoirs) probably have promoted the extensive subsurface leaching reactions of feldspars in these sandstones. The ongoing development of Q2 and Q3 quartz suggested that quartz precipitation did not cease after oil filling. Further, diagenetic reactions likely have proceeded from water-rock interactions to hydrocarbon-water-rock interactions.
Key Words: Quartz cementation; Subsurface feldspar dissolution; Fluid inclusion; Oxygen isotope; Secondary ion mass spectrometry (SIMS); Trace element.
ABSTRACT: Feldspar dissolution, quartz cementation, and clay cementation are significant diagenetic processes in deep-water fan feldspar-rich sandstones in the Shengtuo area, Dongying sag (East China). The timing and origin of these water-rock interactions, as well as the paleo-fluids in which relevant chemical reactions occurred were deduced using data from microscopy, fluid inclusion microthermometry, oxygen isotope microanalysis and trace element microanalysis.
Three distinct, separate episodes of quartz overgrowths (referred as Q1, Q2 and Q3) were distinguished by cathodoluminescence (CL) microscopy. Q1 quartz, identified in all porous sandstones from 2500 m (8200 ft) to 3600 m (11800 ft), was formed at approximately 100-115 °C (212-239 °F) before oil filled the reservoirs. Q2 quartz was mainly precipitated at 115-130 °C (239-275 °F), accompanying or slightly postdating the main phase of oil filling, and was identified in samples from 2800 m (9200 ft) to 3600 m (11800 ft). Q3 quartz was only identified in sandstones buried deeper than 3500 m (11400 ft), and was likely precipitated recently when temperature exceeded 130-135 °C (266-275 °F). Secondary ion mass spectrometry (SIMS) oxygen isotope microanalyses yielded δ18OV-SMOW values ranging from 21.42‰ to 24.35‰ for Q1 quartz, from 22.03‰ to 24.99‰ for Q2 quartz, and from 21.72‰ to 22.91‰ for Q3 quartz.
A mass balance calculation and quantitative petrography data of the amount of leached feldspars and associated secondary minerals suggest that the internal feldspar dissolution is likely the primary source for the authigenic clays and quartz cements in these sandstones. Positive δ18O(water) values (+0.5‰ to +4.5‰ V-SMOW) of quartz-forming waters indicates that quartz cementation and feldspar dissolution occurred in a geochemical system with a limited volume of diagenetically modified connate water. The variations of δ18O(cements) and trace element compositions from Q1 quartz to Q2 quartz in individual overgrowth suggest that hydrocarbon filling changed the chemistry of the pore fluid significantly; further, δ18O(water) values of the pore water increased by about 2‰ V-SMOW after oil filling.
Meteoric freshwater did not contribute to quartz cementation and simultaneous feldspar dissolution. The detected CO2 and hydrocarbons in fluid inclusions in the quartz cements, the existence of CO2 in hydrocarbon-rich natural gas and the bitumen in the feldspar-hosted pores suggest that acids derived from organic matter (kerogen in source rocks and hydrocarbons in reservoirs) probably have promoted the extensive subsurface leaching reactions of feldspars in these sandstones. The ongoing development of Q2 and Q3 quartz suggested that quartz precipitation did not cease after oil filling. Further, diagenetic reactions likely have proceeded from water-rock interactions to hydrocarbon-water-rock interactions.
Key Words: Quartz cementation; Subsurface feldspar dissolution; Fluid inclusion; Oxygen isotope; Secondary ion mass spectrometry (SIMS); Trace element.
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