Reservoir characteristics of lacustrine mixed siliciclastic-carbonate sedimentary rocks: A case study of the Paleogene Funing second member in the Subei Basin, China

Unique lacustrine mixed siliciclastic-carbonate sedimentary rocks accumulated during the deposition of the second member of the Paleogene Funing Formation in the Jinhu and Gaoyou Sags, Subei Basin, which has significant scientific and petroleum exploration value. The reservoir characteristics of the deposits were summarized by means of core observation, petrographic analyses, scanning electron microscope analysis, porosity and permeability tests, carbonate content measurement, and X-ray diffraction clay mineral analyses. These special deposits consisting of siliciclastics and allochems were mainly found in a shallow lake at the front and side of the deltas and also were found in the gravity flow deposits of a deep lake. The results indicate that the lithology and dissolution of early calcite blocks of cement together influence the reservoir performance. From a sedimentological perspective, two types of lacustrine deposits are present in the study area. The first one preserves massive or graded bedding composed of allochems (ooids and intraclasts) and siliciclastics (mainly quartz) of various grain sizes as well as marl; this type of reservoir is poorly sorted. The second one preserves wavy cross-lamination composed of ooids and quartz of similar size; this type of reservoir is well sorted. The diagenesis of the reservoirs in the study area mainly includes compaction, cementation, and dissolution. Reservoirs with massive or graded bedding were impacted by compaction, with current porosity of less than 5%. Reservoirs with wavy cross-lamination could form high-quality reservoirs with porosity of up to 22%, in which the secondary dissolution pores represent the main type of porosity. The early calcite cementation may have inhibited compaction prior to the formation of solution vugs and the formation of high-quality reservoirs.