Abstract:

The Lower Cambrian Longwangmiao Formation gas reservoir in the Moxi Block of the Anyue gas field is the largest complete carbonate gas reservoir discovered in China so far. The gas reservoir, developed in the eastern end of the paleo-uplift in the central Sichuan Basin, has a proven gas-bearing area of 805 km² and proven gas geological reserves of 4403.8×10⁸ m³. Its high efficiency development is of strategic significance for ensuring natural gas supply in the Sichuan-Chongqing area. Reservoir development in the Longwangmiao Formation shows strong heterogeneity, and the gas well productivity varies greatly across the reservoir. For gas reservoir development and deployment, it is very important to study both macroscopic and microscopic characteristics of the reservoir and evaluate the heterogeneity of the reservoir storage space. Based on core, imaging log and cast slice data as well as CT scan, nuclear magnetic resonance, mercury injection, etc., we conducted a systematic study on the stratigraphic sequence division, correlation, karst pattern, reservoir characteristics and main controlling factors of the Lower Cambrian Longwangmiao Formation in the Anyue gas field of the central Sichuan Basin. Results show that there are three types of reservoir storage spaces in the Longwangmiao Formation: millimeter-sized dissolved vugs, solution pores, and intergrain/intercrystal pores. Reservoir of the Longwangmiao Formation is mainly developed in sand-clastic dolomite, featuring small to medium-sized dissolved vugs or solution pores, with medium-low porosity and medium-high permeability. Porosity of the full diameter samples from the Longwangmiao Formation was between 2%-18.48%, averaging at 4.81%, and the full diameter permeability ranged between 0.01-78.5 mD, averaging at 3.91 mD. Due to fracture and cavity, reservoir permeability acquired at different measurement scales varied greatly. The log interpreted permeability ranged between 0.1-10 mD, which was slightly higher than the core tested permeability; whereas well tested permeability ranged between 3.24-925 mD, about 1-2 orders of magnitude higher than the core tested permeability. The Moxi area developed four stages of shaols in the vertical and two main shoals with one trench in the horizontal directions, while the second and third stage shaols distribute most extensively. The Longwangmiao Formation experienced three stages of reservoir dissolution, namely the pene-sedimentary, supergene reformation and burial stages. In the pene-sedimentary stage, frequent short time exposures and leaching and dissolution of meteoric fresh water resulted in the formation of early micropores as well as solution pinholes in shoal carbonate deposits. Caledonian period karstification was the key factor for the formation of the vuggy reservoir of the Longwangmiao Formation. Along the vast karst slope, bed-parallel dissolved vugs and pores were well developed, with the karst vuggy reservoirs superimposing vertically and compacting laterally. The favorable spatial allocation of the bed-parallel dissolved vugs and the high-steep structure fissures resulted in the formation of the apparently homogeneous fractured vuggy reservoir. High-quality reservoir is mainly distributed in the MX11- MX8 and MX10-MX12-MX9 well fields, with a reservoir thickness of 40-50 m.

Key words: Anyue gas field, Longwangmiao Formation, shoal, reservoir space, karst pattern, reservoir characteristics, main controlling factors