Origin and significance of carbonate shoal depositional cycles: A case study of the Cambrian Longwangmiao Formation, Sichuan Basin, SW China

Abstract

• Different cycles of the Longwangmiao Formation have different petrological and petrophysical properties. • Spatial variation of the depositional cycles was mainly controlled by paleobathymetry. • The grain-riched cycles were more than other cycles to develop in the upper parts of the sequences. • High dolomite and oolitic content, strong dissolution of grainstone facilitate the formation of good reservoirs. The carbonate shoals are generally considered as the critical reservoirs in the carbonate sediments. However, whether the quality of reservoir directly related to the development of carbonate shoal is continuously debated. To further explore the control of carbonate shoal on the quality of reservoirs, the Longwangmiao Formation is selected as an example to examine this hypothesis. Based on the multiple techniques, four types of deposition environments were recognized, including the shoal, tidal flat, mixed tidal flat and intershoal. Moreover, three types of depositional stacking pattern (e.g., type A, B and C) were identified. Type A is characterized by continuously carbonate shoal deposits. Type B has two different sub-types which consist of basal shoal deposits that grade upward into tidal flat or mixed tidal flat deposits. For type C, one of the sub-types is dominated by the mudstone and upward evolve to the grainstone in the top. In contrast, the other sub-type consists of basal intershoal deposits that grade upward first into shoal deposits, and then into tidal flat deposits. The spatial variation of depositional cycles was mainly related to paleobathymetry and relative sea-level changes. In terms of physical properties, type A is favorable for the formation of the reservoir as the result of the high primary porosity and experienced meteoric dissolution and dolomitization. Inversely, the content of grain-riched sediments is less in type C and leads to low porosity. This study confirms that type A cycles are the key factor in the formation of carbonate reservoirs.