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.
Acknowledgments
The work was Supported by China National Petroleum Company (2010D-5006-0103) and Funded by State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (PLN201616) and Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education (TPR-2013-16). We are grateful to the Core analysis Unit at the Chengdu Institute of Sedimentology and Mineralogy. We thank the Stable Isotope Unit at the Central Laboratory of Changqing Oilfield. We thank Zhao Jingzhe, the director of exploration and development, Liu Lili and Wei Liping, for assistance with collecting materials and valuable discussions, and Zheng Guanghui for assistance with our field work. The manuscript also benefited from suggestions by Wang Qi, a research scientist at Lanzhou Institute of Geology.
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