Investigation and prediction of diagenetic facies using well logs in tight gas reservoirs: Evidences from the Xu-2 member in the Xinchang structural belt of the western Sichuan Basin, western China

Abstract

The second member of the Xujiahe Formation (Xu-2 Member) is one of the most significant gas reservoir units in the Sichuan Basin. The Xu-2 sandstones in the Xinchang structural belt are characterized by tight reservoirs with both low porosity and low permeability owing to diagenetic effects. The tight sandstones are commonly deeply buried and test samples from cores for research are limited. Therefore the investigation of diagenesis and diagenetic facies using well logs is reasonable and necessary. The Xu-2 sandstones are mainly litharenites, with less lithic quartz arenite and feldspathic litharenite. Secondary pores make up the majority of tight reservoir porosity, while remaining primary pores are still observable. Diagenetic minerals are mainly composed of carbonates, quartz cement and clay minerals. Carbonate and quartz cementation are two key reservoir quality destructive mechanisms. Illite generally has an important influence on diminishing the reservoir quality, whereas chlorite is associated with primary pore preservation. Five diagenetic facies are recognized, namely, tightly compacted facies, dissolution facies, carbonate cemented facies, quartz cemented facies and clay mineral cemented facies. By correlating diagenetic facies with characteristics of well log responses, various diagenetic facies can be recognized, even in the intervals that lack core control. Tightly compacted and dissolution facies are the two key diagenetic facies in the target stratum of the study area, with favorable reservoirs generally associated with the dissolution facies.