Factors influencing stylolite formation in the Cambrian Longwangmiao Formation, Sichuan Basin, SW China

Abstract

In the deep and ultra-deep carbonate reservoirs, the porosity will be reduced due to the compaction, pressure solution, and cementation which are considered as the crucial factors for the mechanism of high-quality carbonate reservoirs. However, it is still a challenge to quantify the influence of stylolites on the Longwangmiao Formation carbonate reservoirs. To better understand the controls on the formation and distribution of stylolites, the Longwangmiao Formation is selected as an example to evaluate the effect of stylolites on the carbonate reservoirs. The mineralogy of the Longwangmiao Formation reservoirs is mainly dominated by dolomite (79.6%) with minor calcite (11.4%), and terrigenous minerals (clay and quartz) (5.6%). The rocks with dolomite content of more than 80%, calcite content of less than 30%, and terrigenous minerals of less than 1% have lower stylolite density and amplitude. The content of dolomite can affect the occurrence of stylolite indicating that the higher abundance of dolomite has a lower density and amplitude of stylolite. Based on the multiple techniques, four types of carbonate lithofacies were recognized, including the grainstone (dolo-grainstone, grainstone), packstone (dolo-packstone, packstone), wackestone (dolo-wackestone, wackestone), and mudstone (dolo-mudstone, mudstone). The distribution of stylolite also shows the lithofacies-specific behavior in the Longwangmiao Formation carbonate reservoirs. The lithofacies of grainstone shows the lowest density and amplitude of stylolite, while the occurrence of stylolite in the mudstone shows the highest abundance. Vertically, the stylolite density and amplitude in the lower part of the fourth-order sequences is higher than this in the upper part. This study suggested that the dolomitized lithofacies with relatively high grain (50–70%) and cement (∼25%) contents can resist the pressure solution and can preserve the porosity of carbonate reservoirs.