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1 Department of Petroleum Geology, Xi'an Petroleum Institute, Xi'an, 710065, People's Republic of China; bqwang{at}xapi.edu.cn
2 Department of Earth Sciences, School of Physical Sciences, University of Windsor, Windsor, Ontario, Canada, N9B 3P4; alaasm{at}uwindsor.ca
Baoqing Wang received B.Sc. and M.Sc. degrees from Chengdu College of Geology, China. He is now a professor in the Department of Petroleum Geology at Xi'an Petroleum Institute. His research interests include sedimentology and petroleum geology. His present research focuses on carbonate and sandstone reservoirs and diagenesis.Ihsan Al-Aasm earned his B.Sc. (1974) and M.Sc. (1977) degrees in geology from the University of Baghdad, Iraq, and his Ph.D. (1985) in geology from the University of Ottawa, Canada. From 1985 to 1986, he was a postdoctoral fellow at the University of Ottawa, then assistant professor from 1986 to 1988. In 1988, he joined the Department of Earth Sciences at the University of Windsor as assistant professor, and he is now a full professor at that university. He is also an adjunct professor at the University of Western Ontario. His principal area of research is the petrologic and chemical attributes of carbonate and siliciclastic diagenesis, dolomitization, and environmental geochemistry. He is a past associate editor for the Journal of Sedimentary Research. His professional activities include memberships in AAPG, SEPM, the International Association of Sedimentologists, and the Geological Association of Canada.
The Ordovician Majiagou Formation contains the main reservoir of the Ordos Central gas field in the Ordos basin. The producing zone at the top of member 5 consists of carbonate rocks modified during a long period of subaerial exposure and karstification from the Late Ordovician to the middle Carboniferous. On the eastern margin of the Ordos basin, the porosity of the exposed carbonate rocks of this unit ranges from 0.5 to 15.1%, and permeability ranges from <0.01 to 1224 md. In the subsurface of the basin, the porosity of the reservoir rocks ranges from 0.5 to >8%, and permeability ranges from <0.1 to 5 md. The main reservoir porosity is a dissolution-enhanced vuggy porosity, associated with dolomite. The carbonate rocks show great heterogeneity, reflecting the varying effects of karstification in creating and modifying porosity.
Petrographic and geochemical analyses of various components in these carbonates provided evidence for depositional and diagenetic processes. The reservoir carbonates were deposited in shallow and restricted hypersaline environments and were later modified by karstification and burial diagenesis. Dolomitization appears to have resulted from mixing of marine and meteoric waters and probably occurred in both shallow and deep burial settings. Cementation by calcite also occurred in both shallow and deep environments, under different hydrodynamic conditions. Both depositional settings and diagenetic processes, such as leaching by meteoric water, paleokarstification, dolomitization, and cementation, controlled reservoir development. The outcrop and subsurface samples show similar petrographic features, porosity types, and geochemical characteristics, but the exposed section of the formation shows evidence of more alteration by meteoric water.
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  Tectonic and stratigraphic controls of hydrocarbon systems in the Ordos basin: A multicycle cratonic basin in central China AAPG Bulletin, February 1, 2005; 89(2): 255 - 269.
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