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

Since 2011, CNPC Southwest Oil and Gas field Company had deployed two exploration wells-Gs1 and Mx8, acquired more than 35.3 mmcf of gas. Since that, the exploration of Lower Cambrian has been strengthened, resulting in the... [ view full abstract ]

Since 2011, CNPC Southwest Oil and Gas field Company had deployed two exploration wells-Gs1 and Mx8, acquired more than 35.3 mmcf of gas. Since that, the exploration of Lower Cambrian has been strengthened, resulting in the discovery of the oldest and largest marine carbonate gas reservoir in the Longwangmiao Formation. High-quality reservoirs of the giant gas fields of Longwangmiao Formation in the Gaoshiti-Moxi area, central Sichuan was clearly controlled by bank facies. Reservoir properties of banks showed strong lateral and vertical variations. In this study, we utilized core data, thin sections and outcrop to investigate influences of sea-level changes on the development of banks and penecontemporaneous dissolution which controled the formation of high-quality bank reservoirs. Our results revealed that the Longwangmiao Formation corresponded to a third-order eustatic cycle which consisted of a rapid sea-level rose and a slow sea-level fell. The regressive stage could be subdivided into 3.5 forth-order eustatic cycles. The transgressive stage was unbeneficial to the deposition and dissolution of banks. This could be evidenced by the banks-deposition rate, percentage of banks thickness to the strata thickness, which was 12.7%, and dissolution porosity abundance which was 4.79/m2. In regressive stage, the banks-deposition rate and the dissolution porosity abundance were higher than those in transgressive stage, especially in the late regressive stage. The banks-deposition rate was 58.9% while the dissolution porosity abundance was 31.82/m2. Forth-order sea-level fluctuations played a significant role in the formation of the reservoirs in the Moxi area, which had a slightly higher topography than the Gaoshiti area. The Moxi area had a banks-deposition rate of 54.4% and a dissolution porosity abundance of 42.44/m2, while the Gaoshiti area had a banks-deposition rate of 47.7% and a dissolution porosity abundance of 11.15/m2. Therefore, it was concluded that sea-level changes played a significant role in controlling the quality of bank reservoirs in the Gaoshiti-Moxi area.