2 ORDOVICIAN UNCONFORMITIES IN NORTH CHINA - THEIR ORIGINS AND RELATIONSHIPS TO REGIONAL CARBONATE-RESERVOIR CHARACTERISTICS

The two unconformities developed on the tops of the Lower Ordovician L iangjiashan Formation (UF1) and the Middle Ordovician Majiagou- or Fen gfeng Formation (UF2) are essential boundaries that controlled the for mation and distribution of the Lower Paleozoic karst-related reservoir s. UF1 and UF2 have been interpreted as representing short-and long-te rms of tectonic uplift, respectively, but new evidence led us to concl ude that they were created by different original mechanisms and theref ore the related reservoirs should be predicted in different ways. UF1 was commonly interpreted as the result of southern upwarping of the ba sement, but sequence-stratigraphic analysis supports its origin by eus tatic sea-level changes. Spatially, the most favorable regional reserv oirs controlled by UF1 should be located in the central area of North China, where the carbonate sediments experienced intensive shallow-sub surface dolomitization with following meteoric water leaching. UF2 was created by tectonic event which resulted in an intra-plate downward f lexure and subsequent peripheral bulge. In the depression belt of cent ral North China the younger strata (Fengfeng Fm) were protected, but a long the bulge meteoric water eroded them. As a result, the potential regional reservoirs related to UF2 are likely to be distributed along the peripheral-uplift belts, especially around the remnant of the Feng feng Formation. Based on the analysis of these two unconformities, the Early Paleozoic tectono-sedimentary evolution of North China Plate ca n be largely divided into four stages: (1) the Cambrian Period, charac terized by eustatic sea-level rise and tectonic subsidence; (2) early stage of the Early Ordovician, characterized by eustatic-sea-level fal l exceeding tectonic subsidence and development of UF1; (3) from the l ate stage of the Early Ordovician to the Middle Ordovician, featured b y eustatic-sea-level rise and slow tectonic subsidence;(4) from the la te stage of the Middle Ordovician to the Early Carboniferous, distingu ished by vigorous tectonic uplift and development of UF2.