Influence of mantle-derived CO₂ on hydrocarbon accumulation in Qinhuangdao 29 tectonic zone, Bohai Sea

Reservoirs of high CO₂ content are rarely found in Bohai Sea, offshore of the Bohai Bay Basin, and Qinhuangdao 29 tectonic zone (QHD29) is one of the few typical reservoirs of the type. To study the correlation of hydrocarbon accumulation with the migration and accumulation of high-CO₂-content gas is of great significance to oil and gas exploration and the discovery of CO₂ gas reservoirs. Based on geochemical analysis of saturated and aromatic hydrocarbons, petrologic characteristics of fluid inclusions, we systematically discuss the interrelationship between CO₂ charging and hydrocarbon accumulation in QHD29 tectonic zone and the genetic mechanism of hig-CO₂-content reservoirs in the tectonic zone. The results are shown as follows. First, the hydrocarbons sourced from the source rocks of the first and third members of Shahejie Formation (E₂s¹ and E₂s³) mainly accumulate in the E₂s¹, followed by Dongying Formation (E₃d). Second, QHD29 reservoirs underwent two stages of hydrocarbon charging. The early stage features crude oil charging dominated by north-south lateral migration from the Qinnan Sag to the uplift along the sand bodies; and the late stage features instantaneous gas accumulation caused by magmatic-hydrothermal fluid as dominated by vertical migration. The supercritical CO₂, fluid rich in hydrocarbons, moved upward along the deep-rooted faults, entered the major oil reservoirs, and then moved laterally from south to north. Third, QHD29 reservoirs obviously experienced gas washing under both early oil and late gas accumulation events, forming a hydrocarbon distribution pattern with heavy oil and bitumen enriched at the bottom, light oil and gas condensate in the middle, and CO₂ gas dominating on top of the major reservoirs.