Oil and source correlation and sub-source hydrocarbon accumulation mechanisms of the third member of Eocene Liushagang Formation in Weixinan Depression, Beibu Gulf Basin: a case study of Weizhou 11 area

The third member of the Eocene Liushagang Formation (LF) in the Weixinan Depression, Beibu Gulf Basin, has relatively low exploration levels and is an important area for increasing crude oil reserves and production. In recent years, the first exploration breakthrough was achieved in the sub-source reservoirs in the third member of LF in the Weizhou 11 area, but limited understanding of its oil source and accumulation mechanism has restricted exploration progress in this area. By comparing the geochemical characteristics of source rocks and crude oil, combined with analysis of hydrocarbon accumulation history, driving forces, and accumulation and migration pathways, the sub-source accumulation characteristics were clarified, and the accumulation model for the third member of LF in the Weizhou 11 area of the Weixinan Depression was established. The crude oil in the third member of LF could be classified into two types. Type Ⅰ crude oil generally showed high abundance of C₃₀ 4-methylsteranes, V-shaped distribution of regular steranes C₂₇, C₂₈, C₂₉, and relatively heavy carbon isotopes in both whole oil and saturated aromatic hydrocarbons, originating from oil shales of the lower submember of the second member of LF during the peak oil generation stage. Type Ⅱ crude oil exhibited low abundance of C₃₀4-methylsteranes, L-shaped distribution of regular steranes C₂₇, C₂₈, and C₂₉, and lighter carbon isotopes of whole oil and saturated aromatic hydrocarbons, indicating mixed contributions from the lower submember of the second member and upper submember of the third member of LF source rocks during the peak and high maturity stages. Intra-granular fractures and healed fractures were widely developed in quartz grains within LF3 reservoirs. The intra-granular fractures of quartz grains contained yellow-green and blue-green fluorescent inclusions, corresponding to crude oil with maturity less than 1%. The healed fractures of quartz grains contained blue fluorescent inclusions, corresponding to crude oil with maturity greater than 1%. This revealed key hydrocarbon accumulation stages at 25-16 Ma, 16-7.5 Ma, and 7.5 Ma to present. The residual pressure and buoyancy of source rocks were the main driving forces for sub-source hydrocarbon accumulation. In the early stage, LF3 reservoirs connected with oil shales in the central sag through fault displacement, with buoyancy-driven accumulation. In the middle to late stages, local oil shales entered the hydrocarbon generation peak, with sub-source injection occurring under residual pressure. High-quality source rocks and regional residual pressure jointly controlled the enrichment of crude oil in the Weizhou 11 area.