Fluid evolution and related fluid–rock interactions of the Oligocene Zhuhai Sandstones in the Baiyun Sag, North Margin of the South China Sea

Abstract

Abstract The pore fluids control the diagenetic processes and storage spaces of the deep clastic rock reservoirs and have become a major area of interest within the fields of sedimentology and petroleum geology. This paper aims to relate diagenetic processes of the Oligocene Zhuhai sandstones in the Baiyun Sag, to pore fluids varying with burial depth. The types and distribution patterns of authigenic minerals are investigated by analyzing petrographic, mineralogical, and geochemical features, to illustrate the originand flow patterns of pore fluids and their influences on reservoir diagenesis. Strong cementation of eogenetic carbonate cement near the sandstone–mudstone interface was a consequence of material migration from adjacent mudstones on a large scale. The pore fluids were mainly affected by microbial methanogenesis and carbonate mineral dissolution in adjacent mudstones during eogenesis. It was diffusively transported in a relatively open geochemical system within a local range. Support for this model is provided by heavier stable isotopic values in eogenetic calcite and dolmite. Feldspar dissolution during early mesogenesis is spatially accompanied by the precipitation of authigenic quartz and ferroan carbonate cement. Pore fluids in this period were rich in organic acid and CO2, and its migration mechanism was diffusive transport. The obviously lighter carbon and oxygen isotopic compositions of ferroan calcite supports this deduction. During late mesogenesis, the input of deep hydrothermal fluid might be partly responsible for the precipitation of ankerite, barite and authigenic albite. Oil charge may inhibit the carbonate cementation and compaction, accordingly preserving porosity, and together with the authigenic kaolinite, might promote the reservoir from water wet to oil wet, to the benefit of oil entrapment. The findings reported here shed new light on the evaluation and prediction of sandstone reservoirs that have experienced multiple periods of fluid flow.