Study on Correlation between Carbonate Reservoirs and Transgression or Regression of Sea water, Uplifting or Sinking of Shoal Flat, and Dolomitized Karst——A case study of the Longwangmiao Formation of the Yangtze Platform

Key words: 
• carbonate platform /
• shoal flat assemblage /
• reservoir /
• control factors /
• shoal /
• dolomitization shoal /
• dolomitization and erosional shoal

Abstract: There are four types of reservoirs in the carbonate rocks confirmed by the practices of oil and gas exploration and production, namely reef and shoal reservoirs, dolomite reservoirs, erosion(karst) reservoirs, and fracture reservoirs. Therefore, the control factors of the reservoirs play a very important role in the practices. Six main outcrop profiles, 2 main well sections, and 81 various information points are analyzed using the methodology of "rapid sedimentary facies analysis of the shoal and flat assemblage", as well as rock slices and scanning electron microscopy (SEM) in the laboratory.On this basis, a large number of single factor maps, comprehensive sedimentary model maps, reservoir distribution prediction maps, and other maps have been completed, and finally summed up by the "Shoal,Dolomitization Shoal, Dolomitization and erosional shoal" (with fractures);the reservoir physical properties gradually improve.Therefore, it is considered that:1) the Longwangmiao Formation carbonates formed under arid climate conditions and were frequently influenced by storm activities. The carbonate reservoirs are controlled by various factors, including transgression or regression, shoal and tidal flat over or below the water, dolomitization, and dissolution. 2) Because of the benefitof primary porosity systems and their evolution, the erosions and dolomitizations during the contemporaneous period, penecontemporaneous period, and the early period following the contemporaneous are extremely important. The surface karsts not only improve the reservoir properties, but also increase the reservoir spaces. 3) Influenced by sea level and palaeogeomorphology, a combination of grain banks and dolomite flats or a combination of grain banks and limestone flats are widely distributed. The methodology of rapid sedimentary facies analysis of the shoal and flat assemblage in a carbonate platform can be applied to the unusual periods and conditions in the Yangtze Platform:the smooth bottom,steep slope, and the deep basin in the Longwangmiao Formation of the Early Cambrian. The method takes the shoal as the base point, giving there lative paleowater deptha value of 10 m (optimal interval value 5-15 m); giving the shoal and belt(limestones or dolostones) assemblage 20 m; and the shoal and lamina(laminatedcontemporaneous dolomites) assemblage 0 m. Thus, the curve of sea level changes can be drawn quickly, and the sequences and system tracts also can be recognized. Finally, a new sedimentary model of "shoal-to-shoal on the platform" and "Shoal flat assemblage" is identified in the study area, which provides a reliable geological foundation for the study of reservoir genesis and distribution.