Abstract:

The Tazhong-Shuntuoguole area has been shown to have rich hydrocarbon resources as major breakthroughs in hydrocarbon exploration continue to be made in this area. To understand the regional sedimentary environment and for reservoir prediction purposes it is important, both theoretically and practically, to clarify the sedimentary characteristics of shoals. Through extensive thin section analysis we find the carbonate shoals are mainly intraclastic shoals with underdeveloped biotic reefs, and composed largely of sparry calcite cement that are partly dolomitic. The intraclastic shoals contain some ooid and algae grains, with a little bioclastics often found in micrite. Ooid gains are usually small, with radial texture, and formed in a low-energy environment; while algae grains are usually mixed with intraclast and some bioclastics indicating they are formed in a mid-high-energy environment. The Yingshan Formation can be subdivided into two third-order sequences according to well log analysis. The stacking pattern of carbonate shoals typically shows thin-layered retrogradation within the ascending semi-cycle and thick-layered aggradation within the descending semi-cycle, whilst small-scale shoals develop during the maximum flooding period. Beach-interacting sea facies often develop near the maximum flooding surface. In the Yingshan Formation, open-platform and restricted-platform intraclastic shoals are recognized. The open-platform shoals are thick and distributed as clumps along the platform margin in a thick-layered aggradational stacking pattern, whilst the restricted-platform margin shoals are thin and distributed in small clumps or strips in a similar stacking pattern.

Key words: Tarim Basin, Yingshan Formation, intraclastic shoals, sedimentary characteristics, sedimentary model