Reservoir characteristics of lacustrine mixed siliciclastic-carbonate sedimentary rocks: A case study of the Paleogene Funing 2lt;supgt;ndlt;/supgt; Member in Subei Basin, China

Abstract

Unique lacustrine mixed siliciclastic-carbonate sedimentary rocks accumulated during deposition of the second member of the Paleogene Funing Formation in the Jinhu Sag and Gaoyou Sag, Subei Basin, which has significant scientific and petroleum exploration values. 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 deltas, and were also found in gravity flow deposits of a deep lake. The results show that lithology and dissolution of early calcite cements 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 reservoirs in the study area mainly includes compaction, cementation and dissolution. Reservoirs with massive or graded bedding were impacted by compaction, with current porosity less than 5%. Reservoirs with wavy cross-lamination could form high-quality reservoirs with porosity up to 22%, in which 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.