Sedimentary evolution and petroleum geological significance of Permian Maokou Formation in eastern and northern Sichuan Basin

Abstract

In recent years, the Permian Maokou Formation of the Sichuan Basin shifted focus. Previously, targeting high points and exploring along the long axis were common. Now, the strategy emphasizes finding shoal grainstone reservoirs with pores, leading to significant breakthroughs and making the Maokou Formation a hotspot for exploration. This change requires strong support from sedimentary evolution studies. The eastern and northern Sichuan regions are notable for distinct tectonic and sedimentary features within the Maokou Formation, suggesting promising exploration potential. Analysis of the logging data from 386 wells was conducted and sedimentary facies analysis was carried out using field profiles and drilling cores. Combining the logging curves with sedimentary facies, the sedimentary evolution of the Maokou Formation in the study area was analyzed. Findings reveal a gentle carbonate slope from the first member to the lower submember of the second member. The stratigraphic thickness and sedimentary facies are generally influenced by the ancient Central Sichuan Palaeouplift, with low-energy shoals developing locally. A transition to a rimmed carbonate platform occurs from the upper submember of the second member to the third member, with stratigraphic thickness and sedimentary facies showing a northwest to southeast orientation. The platform develops sequentially from platform facies to platform margin, slope, and shelf facies from west to east, with high-energy shoals developing in the platform margin. Sedimentation history indicates two distinct rise to fall processes of the relative sea level during the Maokou Formation sedimentary period, corresponding to the first member to the lower submember of the second member, and the upper submember of the second member to the third member. The formation of shoal grainstone reservoirs with karst and dolomitization in the gentle slope during the first period was controlled by regional sea level changes, tectonic activities, and the type of carbonate platform. Potential hydrocarbon source rocks developed in submarine troughs during the second period. The region holds promise for unconventional oil and gas exploration, particularly on the west side of the trough with high-energy shoals on platform margins.

Keywords

• sedimentary evolution
• marine trough
• oil and gas exploration
• maokou formation
• eastern and northern sichuan basin