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Late Stage Hydrocarbon Accumulations and Enrichment Pattern in Liaozhong Sag, Northern Offshore Bohai Bay Basin, Eastern China

Kunsheng Qiang; Xiuxiang Lü; Xinhuai Zhou; Changgui Xu; Xiaodong Lan

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. Under the guidance of the late-stage accumulation theory a series of medium-to-large oil and gas fields were found in the salients and sags in offshore Bohai Bay Basin since the end of 1990. Bohai Oilfield production surpassed 30 million tons in 2011(CNOOC LIMITED Official website, 2011)

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. The inclusion samples were retrieved from J21-1and J1-1 structural units of the Liaozhong Sag,Total of 16 samples were tested, and fluid inclusions were found in 15samples. 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. The Easy% Ro model analysis shows that hydrocarbon generation period in the Liaozhong sag was mainly at the time since 12.0My from the present, and the Neotectonic movement period (5.1Ma before the present) was the peak period of hydrocarbon generation and expulsion of the major source rock E3s3. 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. Abnormal high formation pressure, non-uniform temperature in the interior of reservoirs, and hydrocarbon inclusion homogenization temperature anomaly are indications of late-stage rapid in-filling in the Liaozhong sag. The regional reservoir-cap rock combination and predominant migration pathway controlled the horizons for oil and gas accumulation. Hydrocarbon was accumulated in large traps transformed by complex tectonic activities. These factors constitute the enrichment pattern in the Liaozhong Sag.

 

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013