Formation of Secondary Porosity With Implications for Reservoir Quality in Deeply Buried Sandstones: A Case Study From Sublacustrine Fan Deposits in the Eocene Es4 Interval, Dongying Depression, Bohai Bay Basin, Northeastern China

Abstract

This case study on sublacustrine-fan deposits of the Eocene Es4 rift succession in the Dongying Depression, Bohai Bay Basin, China evaluates whether or not secondary porosity enhanced the total porosity of deeply buried sandstone reservoirs. Based on petrographic observations, secondary porosity developed as result of dissolution of feldspar and calcite, dolomite and anhydrite cements during mesogenesis. Dissolution of feldspar is spatially associated with by-products such as authigenic quartz and kaolinite. Quartz cements do not show an increasing trend with depth but have highest abundances at depths of 2750-3750 m, which correspond with zones of extensive feldspar dissolution. Based on mass-balance calculations, quartz cement was sourced internally within sandstones and not derived from clay transformations in adjacent mudstones. Carbonate dissolution is kinetically rapid at high temperatures and carbonate cements have retrograde solubility with temperature. Consequently, dissolved early-formed carbonate cements re-precipitated as ferroan carbonate cements during mesogenesis. δ13CPDB for ferroan carbonate cements in Es4s are depleted (+1.04 to +3.29‰) relative to earlier calcite and dolomite (-0.65 to +5.59‰) and, thus, probably indicate a mixture of carbon derived from dissolution of early formed calcite and dolomite cements in the sandstones, as well as minor contributions of organic carbon from adjacent mudstones. δ13CPDB values of ankerite cements in Es4x range from -7.12 to -3.7‰; this is a similar range in δ13CPDB values to early-formed dolomite (-7.45 to -2.57‰) in Es4x. Depleted δ13CPDB in ankerite are consistent with derivation from the early-formed dolomite cements and/or from organic carbon from adjacent mudstones. Dissolution of anhydrite related to thermochemical sulfate reduction (TSR) generated ankerite and nodular pyrite as by-products that precipitated in adjacent pores. Nodular pyrite has comparable δ34SCDT values (+17.1 to +37‰) to precursor anhydrite (+21.2 to +37.8‰). Therefore, lack of sulfur isotope fractionation occurred between parent sulfate and reduced products and indicates a relatively closed system during TSR. Minerals that were dissolved to form secondary porosity were re-precipitated as other authigenic mineral phases within sandstones during mesogenesis. Therefore, the formation of secondary porosity occurred in a relatively closed system and did not greatly enhance reservoir quality.

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