Fluid Flow and Related Diagenetic Processes in a Rift Basin: Evidence From the Eocene Es4 Interval, Dongying Depression, Bohai Bay Basin, China

Abstract

Fluid flow in a geochemically open versus closed system has a critical impact on the distribution of diagenetic minerals. Based on petrographic observations and geochemical analysis, distribution of authigenic minerals was recognized which reflect the sources and patterns of fluid flow in deeply buried sandstones in the Eocene Es4 interval. Extensive precipitation of calcite and dolomite at or near mudstone-sandstone contacts during eogenesis was a result of large-scale, mass transfer between sandstones and adjacent mudstones. This process was driven by compactional or advective fluid flow in a relatively open geochemical system. Support for this model is provided by large sulfur isotope fractionation between framboidal pyrite (δ34SCDT from -3.9 to +5.7‰) and precursor anhydrite (δ34SCDT from +21.2 to +37.8‰). Dissolution of feldspar and non-ferroan carbonate cements occurred during early mesogenesis and is related to organic acids derived from adjacent mudstones and transported by compactional fluids. Feldspar dissolution is closely associated with authigenic quartz and kaolinite. Early-formed carbonate were dissolved and re-precipitated as ferroan carbonate cements as evidenced by early-formed carbonate engulfed or replaced by ferroan carbonate cements. δ13CPDB for ferroan carbonate are depleted (+1.04 to +3.29‰) relative to earlier carbonate (-0.65 to +5.59‰). Thus, ferroan carbonate probably resulted from a mixture of carbon derived from dissolution of early-formed carbonate in the sandstones as well as minor organic carbon from adjacent mudstones. Dissolution of anhydrite cements related to thermochemical sulfate reduction (TSR) during late mesogenesis generated ankerite and nodular pyrite that precipitated in adjacent pores. Velocities of pore-fluid flows are low during mesogenesis and quantitative calculations indicate large-scale thermal convection and advection probably did not occur. Rather, diffusion is inferred as the dominant transport mechanism for dissolved solids over short distances that resulted in precipitation of dissolution by-products either in-situ or in adjacent pores. A lack of sulfur isotope fractionation between parent anhydrite (δ34SCDT from +21.2 to +37.8‰) and nodular pyrite (δ34SCDT from +17.1 to +37.0‰) supports a relatively closed system. Therefore, water-rock interaction during eogenesis and early mesogenesis occurred in a geochemically open system whereas a geochemically closed system dominated during late mesogenesis.

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