Fluid geochemical response recorded in the alteration of marine carbonate reservoirs: the Silurian Shiniulan Formation, southeast Sichuan Basin, China

Abstract

Natural gas resources in marine carbonate rocks comprise the most significant remaining oil and gas resources in China. Preservation conditions assessment and destruction mechanisms of high-quality carbonate reservoirs play an important role in petroleum resource evaluation in carbonate reservoirs. In the Silurian Shiniulan carbonate gas reservoirs of southeast Sichuan Basin, mineral sequences in fracture-cavity veins, calcite-precipitating fluid sources, and paleo-pressure evolution histories were investigated by Raman quantitative analysis, element characteristics and isotope compositions. The process of reservoir decompression indicated by inclusion analysis and the enrichment of trace elements are all due to the response of reservoir reconstruction. Petrographic observations show two stages of calcite vein occur commonly in fracture-cavity. Stage-1 calcite veins mineralized in a reduction environment as results of hydrothermal activity. Gas accumulation was accompanied by the crystallization and precipitation of the stage-1 calcite veins. Stage-2 calcite vein precipitated during Himalayan tectonics and derived from intensive invaded penetrating fluid, which might be affected by atmospheric freshwater. The density, pressure, and temperature for CH4-bearing fluid inclusions in the stage-1 calcite veins filling was determined by quantitative Raman spectroscopy. Thermodynamic simulations were conducted to constrain pore pressure evolution histories with the establishment of trapping condition of the geo-fluids. Our results suggest that gas generation and charging comprise the main mechanisms for paleo-overpressure development. Identification of fluid sources and understanding pore pressure evolution are keys to study the oil and gas migration and accumulation.