Diagenesis of volcanic-rich tight sandstones and conglomerates: a case study from Cretaceous Yingcheng Formation, Changling Sag, Songliao Basin, China

Thick conglomerate and sandstone lithofacies of Yingcheng Formation in Changling Sag are tight gas reservoirs. The formation contains a fair amount of volcanic clasts and accumulated in a deltaic system that spans from delta plain to prodelta depositional setting. High-quality reservoirs are only a small fraction of these thick siliciclastic rocks. Petrographic analysis has been used to assess diagenetic processes, paragenetic sequence, and their effects on the reservoir qualities. The major loss of primary porosity is due to compaction along with grain-coating, pore-filling clay, and quartz cements. Alteration of volcanic rock fragments supplied alkali cations which favored formation of chlorite, smectite, and zeolite cements. The thickness of the chlorite coating decreases from the delta plain to the distal part of the delta front. On the contrary, the content of the zeolite cement increases toward the distal part of the delta front. Quartz and feldspar cements were ubiquitous due to high SiO2 concentration and alkali cations. Early dissolution of volcanic rock fragments and feldspars by freshwater increased porosity but did not contribute much to the permeability because pore-throats are predominantly occluded with clay and pseudomatrix. Late-stage dissolution of the zeolite cement was caused by organic acid expelled from maturation of organic matter. Although the reservoirs with chlorite coatings have moderate porosity, the permeability in situ is very low and pore-throat sizes are small. The high-quality reservoirs are medium- to fine-grained zeolite-dissolved sandstones developed in the distal part of the delta front and shallow lacustrine.