Stratigraphic and sedimentary characteristics of Middle Permian Qixia Formation in Beichuan Tongkou section, middle Longmen Mountain

Abstract

The Qixia Formation in the foothills of Longmen Mountain in the western Sichuan Basin is gaining attention for oil and gas exploration. However, exploration of the Qixia Formation in the middle Longmen Mountain is insufficient. Consequently, it is essential to understand its stratigraphic sedimentary characteristics for guiding natural gas exploration. This study focuses on the Beichuan Tongkou section, combines field measurements, thin-section observation and trace element analyses to investigate the sequence stratigraphy and sedimentary features of the Qixia Formation, thereby defining its sedimentary environment. Within the Beichuan Tongkou section, which spans from the Liangshan Formation to the Qixia Formation, researchers have identified two third-order and one fourth-order sequence boundaries, delineating one third-order sequence and two fourth-order sequences. The sedimentary facies found in this interval include shore-swamp, open platform, and platform edge environments, which are further classified into four subfacies including open sea, intertidal sea, intra-platform beach, and platform edge beach. Additionally, three microfacies are identified including intra-platform bioclastic beach, platform edge bioclastic beach, and platform edge sand debris and bioclastic beach. The Qi1 member and lower part of the Qi2 member is located on an open platform, while the upper-middle part of the Qi2 member is found at the platform edge. This setup defines a sedimentary model for a carbonate-rimmed platform in the Qixia Formation of the middle Longmen Mountain. During the sedimentary period of the Qixia Formation, a warm, humid, and oxygen-rich marine environment supported abundant ancient organisms. In the highstand system tract of the upper-middle part of the Qi2 member, the Sr/Ba ratio is lowest, while the MgO/CaO ratio is highest, indicating elevated ancient water temperatures and specific chemical compositions. The m value, reflective of paleosalinity, is notably high, suggesting a shallow, high-energy platform edge environment away from the mainland. Thick, high-energy platform edge sand debris and bioclastic beaches in this area promote the development of high-quality porous dolomite reservoirs of beach facies. The sedimentary facies and reservoir conditions of the Qi2 member in the middle Longmen Mountain are comparable to those in other sections, making it a promising target for oil and gas exploration within the Qixia Formation and a potential strategic replacement area for reservoir expansion.

Keywords

• major and trace element
• sequence stratigraphy
• sedimentary facies
• sedimentary model
• qixia formation
• middle permian
• longmen mountain