Diagenesis and reservoir quality of the Upper Triassic Chang 6 member in Jingan Oilfield, Ordos Basin, China

The Chang 6 reservoir of Yanchang formation in Jingan oilfield has relatively poor quality: the porosity ranges mainly from 8 to 14%, and permeability ranges from 0.018 to 10.48 md, with a mean of 1.10 md, categorized as low porosity and low permeability reservoir type with strong heterogeneity. Petrographic and geochemical analyses of various components in these sandstones have provided clues of diagenesis. The sandstones at the early stage of diagenesis are characterized by non-ferroan calcite cementation, grain-coating, pore-lining clay minerals, and initial dissolution of detrital grains. Authigenic quartz, pore-filling and grain-replacive laumontites, albitized detrital plagioclase, authigenic K-feldspar, illite and late ferroan calcite cement dominate the late diagenesis. Reservoir quality is influenced by products of late diagenesis stage, the cement (calcite, albite, quartz, illite, and laumontite) occluded primary and secondary porosity. The organic acidic fluid derived from organic matter in the source rocks during the late diagenesis, which results in the dissolution of detrital plagioclase and laumontite cement and was the main reason for the enhancement of reservoir-quality. As a result of that, the residual primary pore and dissolution pore comprised the main accumulation space for oil and gas, and thus become important targets for hydrocarbon exploration.