Origin and accumulation of natural gas in the Carboniferous carbonate in the buried hill A area, Weixinan Depression, Beibu Gulf Basin

Buried hills constitute a significant oil and gas exploration area in the Weixinan Depression. The A area is a typical buried hill, and carbonate hills are considered major reservoir reserves with rich resources. Geochemical, core observation, microsection, and drilling data have been extracted to study the tectonic control factor of the carbonate buried hill reservoir from the A area in the Weixinan Depression. The results indicate that the carbonate buried hill in the study area mainly consists of karst reservoirs, which are divided from top to bottom into the epikarst, vertical percolate-corrosion, runoff corrosion, and phreatic corrosion zones. The epikarst zone and runoff corrosion zone exhibit the best reservoir physical properties among all zones. The natural gas in the reservoir producing oil-related gas, staying in mature stage without experiencing secondary modification effects. Through a comparison of carbon isotope and parameters of light hydrocarbon, the natural gases can be deemed to probably be sourced from the oil shale of the middle of the Liushagang Formation in an adjacent depression. The fault was connected the source rocks and reservoirs, which were active during the early Holocene to early Miocene. Meanwhile, the oil shale in the middle Liushagang Formation was in the mature stage, resulting in the expulsion of natural gases. Multistage tectonic uplift has subjected the buried hill to long-term weathering and leaching, leading to the formation of storage space. Additionally, secondary development and optimal fault opening positions provide favorable migration conditions for hydrocarbon to accumulate in buried-hill reservoirs.