The influences of sea-level changes on the quality of bank reservoirs of the Lower Cambrian Longwangmiao Formation, in the Gaoshiti-Moxi area, Sichuan Province, China

Abstract

Reservoir properties of banks of the Longwangmiao Formation show strong lateral and vertical variations in the Gaoshiti-Moxi area, central Sichuan. In this study, we utilized outcrop, core data, thin sections, wireline logs, and seismic data to investigate the influences of sea-level changes on the development of banks and penecontemporaneous dissolution which controlled the formation of high-quality bank reservoirs. Our results revealed that the Longwangmiao Formation corresponds to a third-order sea-level change cycle, which consisted of a rapid sea-level rise and a slower sea-level fall; we further identify four fourth-order sea-level cycles. Bank reservoirs are dominated by grain dolostone, silt to fine sand-sized crystalline dolostone, and mottled silt to fine sand-sized crystalline dolostone. The main reservoir spaces were pores and vugs, with an average porosity of 4.81% and average permeability of 4.75 × 10−3 μm2. Bank deposition and dissolution during the third-order sea-level fall developed better than during the rising period, and the late fall stage was better than the early fall stage. Under the influence of fourth-order sea-level changes within the third-order sea-level change, the development of banks shows strong variations between different areas, which are evidenced by the fact that banks in the Moxi block were better developed than those in the Gaoshiti block. Bank dissolution in the Moxi block developed during the early and the late stage within the third-order sea-level fall, whereas the Gaoshiti block was developed only during the late stage. Therefore, we have concluded that sea-level changes played a significant role in controlling the quality of bank reservoirs in the Gaoshiti-Moxi area.