Abstract:

The Hierarchy of stratigraphy unconformities and sequence interfaces in different tectonic parts has played an important role in distribution of reservoirs of carbonate rocks. An integrated study of Caledonian unconformities (seismic reflection of stratigraphic surfaces of T27 and T47), structure patterns of margin and platform in the Late Ordovician, depositional sequence and statistic data of reservoirs developed in 32 wells, as well as carbon and oxygen and strontium isotope for calcites filled in fracturesvugs and the Ordovician carbonate rocks in 11 wells, in the Western Tazhong is reported. The results show that the first episode of the Middle and Late Caledonian is of the characteristic of transformation from tectonic extensional to compressional loading, and the stratigraphy unconformities of T47 is composed of toptruncation and erosion developed from the margin into intraplatform as well as a strong karstification, and later attributed to deepened submerge sequence (drowned) with fillingup; while the stratigraphy unconformities of T27 would be defined as overlying deepened submerge sequence and above possibly forming of condense section, and erosion and subaerialkarstification seemed to occur in highposition area. The aggradation would be gradually replaced by progradation in the Upper Ordovician margin toward the interior platform, associated with the highstand systems tract (HST) predominantly and the transgressive systems tract (TST) secondary; while distribution of larger slope angle of the Upper and Middle Ordovician would have significantly contributed to a greater remanent strata of Lialitaga and Yijianfang Group. The stack patterns of 3th or above parasequence or cycles in the Ordovician are much similar to each other in the Western Tazhong, the similar average value of δ18O and δ13C, 87Sr/86Sr for either Ordovician carbonate rocks and calcites or dolomites in vugs and fractures suggested the latter intimately inherited from or associated with that of the pervious matrix rocks; the significantly depleted δ13C PDB and δ18O PDB in calcareous dolostone and dolostone and calcites or dolomites in vugs and fractures in Yingshan group of the Middle and Lower Ordovician may respond to the degree of erosion of strata or karstification developed in different tectonic belts; unlike lower 87Sr/86Sr in rocks or calcites in vugs and fractures of Lialitaga and Yijianfang Group, the rather higher contents of 87Sr/86Sr in Yingshan Group is composed of terrestrial strontium and the dissolved strontium with a rather higher terrestrialderived strontium, indicating the different sedimentary environment and widespread development of karstification for Yingshan group. The reservoirs of Yingshan formation of the Middle and Lower Ordovician, in great extent, depended on the development of karstification that was closely associated with faultrelated folding uplift, denudation and subaerialunconformity development(T47). In the areas of inherited developed structures, topographic rising and the hanging wall of thrustingfaults, the forming of reservoirs of carbonate rocks would be defined as “residues of phreatic zone”, which was formed by a series of continued geological stages, i.e., uplift linked to tectonic compressional loading, development and partly erosion of freshwater phreatic zone, and overlapping of multiepisode karstification；whereas “undifferentiated freshwater phreatic and leaching zone” likely occurred either in the weak transformation parts or in the gentle topographic slope or in the footwall.

Key words:  Caledonian unconformities, sequence boundary, isotope, residues of phreatic zone, carbonate reservoirs, Ordovician, the Western Tazhong area