The quantitative characterization of stylolites in the limestone reservoirs of the Lower Triassic Feixianguan Formation, northeastern Sichuan Basin: Insights to the influence of pressure solution on the quality of carbonate reservoirs

Abstract

Pressure solution, a common and important diagenetic process in carbonates, involves physical reduction of the volume in rocks and chemical dissolution-precipitation process in pores, and thus leads to the significant heterogeneity of reservoirs. Stylolite is a typical product of pressure solution. The amplitude and residual seam thickness of stylolite can effectively characterize the intensity of pressure solution. Quantitative characterization of the density, morphology, and distribution of stylolites in the Feixianguan Formation limestones in the northeastern Sichuan Basin were performed to evaluate the intensity of pressure solution. Revealing the impact of pressure solution on the evolution of porosity and permeability is essential when looking for high-quality reservoirs by integrating quantitative statistics from cores and thin sections with geochemical data. The following four types of stylolites were observed in the Feixianguan limestones according to pure geometry, known as wave-like, rectangular, sharp-peak, and seismogram types. From matrix supported limestone to grain supported limestone, the type of stylolites gradually change from wave-like to rectangular, sharp-peak and seismogram types, accompanied by increased surface roughness and amplitudes, and significantly decreased residual seam thicknesses. The stylolite density tends to decrease from the lower to upper part of the sedimentary cycle. The dissolved calcium carbonates released via pressure solution precipitated as cement in adjacent pore spaces, forming a relatively less porous light gray band on each side of the stylolite. We implement two quantitative methods show that the intensity of pressure solution in grainstone (90.42 mm/m) is significantly smaller than that in mudstone (235.78 mm/m) suggesting that the grainstones are more resistant to chemical compaction. Moreover, pressure solution caused a decrease in the porosity of limestone by approximately 11.31%. The porosity of grainstones varies widely and decreases drastically as clay content increases, suggesting that the clean particles formed in high-energy environments are favorable for the formation of dominant reservoirs. A large number of pores, calcite cements, and bitumen within stylolites indicates that stylolites could act as fluid pathways, contributing about an order of magnitude to the reservoir permeability. Concludingly, we highlight quantitatively differences in pressure solution intensity on a millimeter scale.