ABSTRACT: Sequence Stratigraphy and Gas Reservoirs of Ordovician Carbonate in Ordos Basin, Northcentral China

XU, HUAIDA, China University of Geosciences, Beijing, People's Republic of China

Ordos, a cratonic basin in northcentral China is covered by stratigraphy from Sinian to Middle Ordovician and from Middle Carboniferous to Tertiary with a low dip angle less than 1 degree westward, and consists of four tectonic elements: North Uplift, Central Uplift, East Shallow Intracratonic Basin, and West Foreland Basin. Some large and giant gas fields with high production were discovered from the carbonate just beneath the unconformity at the top of Ordovician which is covered by Carboniferous coal bearing series. By means of sequence stratigraphic study, differential intraformation velocity analysis, pseudosonic logs and high precision gravitational survey, many buildups within Lower Ordovician carbonate were discovered. All of them are related with gas production as they are ab ndant in original porosity and secondary porosity formed by wheathering and dolomitization. Seven cycles of sea level changes are recognized in Lower Ordovician and all of buildups are developed in highstand system tract in the cycles. Four types of buildups are identified: Shelf margin buildups, platforms margin buildups, pinnacles and patch reefs. They are attributed to reefs, algae reefs and limemud mounds. More than 9 layers of salt are distributed in the East Intracratonic Basin.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)