Late-stage Hydrocarbon Accumulation and Enrichment Pattern in Liaozhong Sag, Northern Offshore Bohai Bay Basin

The Liaozhong Sag is one of the major oil-bearing areas in offshore Bohai Bay Basin and is obviously characterized by late-stage hydrocarbon accumulation. In this paper our study was focused on strong activities of tectonic movement in the Liaozhong sag. In terms of source rock thermal history analysis, generation, migration and accumulation of hydrocarbon, late-stage hydrocarbon accumulation and enrichment pattern were summarized. Apatite fission track analysis and evolutionary history of the Easy% Ro model simulation of source rock showed that major source rocks became matured rapidly at the time 5.1Ma from the present. Fault analysis, showed that hydrocarbon migration in the study area was mainly the result of fault transportation. Fluid inclusions indicated that the accumulation period of oil and gas reservoirs was 5.1Ma from the present. Our study showed that late-stage rapid hydrocarbon generation, late-stage vertical fault transportation, hydrocarbon accumulation near source rocks and late-stage rapid in-filling are the main features of hydrocarbon accumulation in the Liaozhong Sag. The abnormal high pressure and fluid inclusion homogenization temperature anomaly are indications of late-stage episodic reservoiring and in-filling. Regional reservoir-cap rock combination of Eocene Shahejie Formation and Oligocene Dongying Formation, predominant transport channels and relevant traps formed by the complex tectonic movement of the Tan-Lu Fault are major factors controlling hydrocarbon accumulation. Episodic reservoiring of formation overpressure, near-source accumulation and fault control constitute the oil and gas enrichment pattern of the Liaozhong Sag.