Two 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 Liangjiashan Formation (UF1) and the Middle Ordovician Majiagou- or Fengfeng Formation (UF2) are essential boundaries that controlled the formation and distribution of the Lower Paleozoic karst-related reservoirs. UF1 and UF2 have been interpreted as representing short-and long-terms of tectonic uplift, respectively, but new evidence led us to conclude that they were created by different original mechanisms and therefore the related reservoirs should be predicted in different ways. UF1 was commonly interpreted as the result of southern upwarping of the basement, but sequence-stratigraphic analysis supports its origin by eustatic sea-level changes. Spatially, the most favorable regional reservoirs controlled by UF1 should be located in the central area of North China, where the carbonate sediments experienced intensive shallow-subsurface dolomitization with following meteoric water leaching. UF2 was created by tectonic event which resulted in an intra-plate downward flexure and subsequent peripheral bulge. In the depression belt of central North China the younger strata (Fengfeng Fm) wer proted, but along 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 Fengfeng Formation. Based on the analysis of these two unconformities, the Early Paleozoic tectono-sedimentary evolution of North China Plate can be largely divided into four stages: (1)the Cambrian Period, characterized by eustatic-sea-level rise and tectonic subsidence; (2)early stage of the Early Ordovician, characterized by eustatic-sea-level fall exceeding tectonic subsidence and development of UF1;(3)from the late stage of the Early Ordovician to the Middle Ordovician, featured by eustatic-sea-level rise and slow tectonic subsidence; (4)from the late stage of the Middle Ordovician to the Early Carboniferous, distinguished by vigorous tectonic uplift and development of UF2.