Controls of paleoclimate and sea−level changes on the high−frequency sequence of shallow−water carbonates: A case study of the Longwangmiao Formation in the central Sichuan Basin

This paper is the result of oil and gas exploration engineering.

Objective Clarifying the control mechanisms of ancient climate and sea level changes on high−frequency sequences and favorable reservoirs of shallow water carbonate rocks is of great significance for clarifying the genesis and distribution patterns of reservoirs.

Methods Microfacies and sequence stratigraphy analysis and geochemical analysis including carbon and oxygen isotopes, major and trace elements with core samples were used.

Results The Four types of microfacies including dolomitic lagoon, inter−shoal, grain shoal and tidal flat were identified in the Longwangmiao Formation. The Formation was subdivided into two fourth-order sequences which was regionally correlated. The δ¹³C of the Longwangmiao Formation vertically shows two periodical changes, which indicates two episodes of sea-level changes. This trend of sea-level change is consistent with the change of water depth reflected by the variations of Al₂O₃, TiO₂, B and V values. Using carbon and oxygen isotopes coupled with the well−correlated Mg and Ca values, the estimated Z values and seawater temperaturesduring the deposition of the Formation collectively reflect arid and hot climates and a subtropical marine environment. Systematic variations in MgO/CaO, MnO₂, Fe₂O₃/MnO₂, and Al₂O₃/MgO values indicate a brief transition to warm and humid conditions during the early stages of the two successions, and the Sr/Ba values reflect the marked increase in late−stage seawater salinity during deposition.

Conclusions A warm and humid climatecoupled with a rapid rise in sea level, led to the formation of transgressive systems tracts. During the hot and arid climates, rapid deposition of grain shoals and dolomitic tidal flat constitutes the highstand systems tracts. The grain shoal and dolomitic tidal−flat deposits serves as the material basis for high−quality reservoirs. Early dissolution and dolomitization processes occurring during sea−level fall and a hot and arid climate are crucial for reservoir development.