Characteristics and Enrichment Factors of Supergiant Lower Cambrian Longwangmiao Gas Reservoir in Anyue Gas Field: The Oldest and Largest Single Monoblock Gas Reservoir in China

An ultra-deep monoblock marine carbonate gas reservoir with the highest maturity and largest individual scale in China has been discovered in Lower Cambrian Longwangmiao Formation (LF) of Anyue gas field in the last few years. Based on a thorough study on Longwangmiao gas reservoir in the Gaoshiti-Moxi(GM) area, this paper presents the characteristics of such formation in Anyue gas field as follows: ① it is a lithologic-anticlinal gas reservoir developed on a gentle paleo-uplift; ② the gas reservoir is characterized by high temperature and pressure, contains sulfur, and belongs to a dry gas reservoir; and ③ the upper part of the gas reservoir has a good connectivity along with a free of bottom water and edge water, but poor in the lower part, where small scope of enclosed CaCl₂ water exists locally. This paper further studies the main control factors for forming this supergiant gas reservoir: ① synsedimentary paleo-uplift controlled the distribution of large-area particle shoal, superimposed by multiple corrosion, which high quality reservoir was formed; ② syngenetic extended aulacogen was filled by mudstone and shale rapidly, resulted in widely distributed thick source rock; ③ synsedimentary gypsum rock and densified carbonate rock formed lateral sealing, multiple type of cap rocks superimposed vertically, therefore excellent closure conditions were formed; ④ large scale of gas reservoir trap was formed at early stage, and it was in a favorable hydrocarbon accumulation pointing area for a long time; and ⑤ the effective time-space matching of numerous hydrocarbon accumulation elements is the key to the formation of the supergiant gas reservoir. The understanding on the formation mechanism and enrichment rule of this ultra-deep, high temperature and high pressure gas reservoir will theoretically enrich the petroleum geology theory, and practically bring important reference to the exploration of large gas fields in the deep ancient craton basins with high maturity and multiple tectonic evolutions in the world.