Shallow Gas Accumulations in the Neogene Strata of the Jiyang Superdepression, Bohai Bay Basin

Guangyou Zhu¹, Shuichang Zhang¹, Jinxing Dai¹, Qiang Jin², Linye Zhang³, and Jian Li^1
1 RIPED, PetroChina Company Limited, Beijing, China
² The University of Petroleum, Dongying, Shandong, China
³ SINOPEC Shengli Oilfield Company, Dongying, Shandong, China

Natural gas occurs extensively in eastern China, with a number of large and medium-sized gas fields having been discovered in recent years. Gas reservoirs occur in both Cenozoic strata and underlying basement. Almost 90% of the total proven natural gas reserves in the Jiyang Superdepression, Bohai Bay Basin are found in the shallow Neogene gas pools, in traps formed on top of the paleotopographic highs and along the margin of individual grabens or half grabens. The gases are closely associated with heavy oils, mostly as gas caps, associated gases within the heavy oil pools, or in separate gas pools above or updip. The gases contain over 95% methane and small quantities of C2+ alkanes, nitrogen and carbon dioxide. The stable carbon isotopes of methane in these gases are up to 10‰ more positive than those of the thermogenic gases in the deep Paleogene reservoirs, with the propane being more depleted in 12C than butane. This study demonstrated that the majority of the petroleum source rocks in the Jiyang Superdepression tend to be oil-prone, and are currently within or above conventional oil window (with 0.45%-1.0% Ro). The chemical and carbon isotopic compositions of the gases, together with the moderate-severe biodegradation of the associated heavy oils in the shallow Neogene strata, suggest that the shallow Neogene gas accumulatins in the Jiyang Superdepression result from in-reservoir anaerobic degradation of accumulated oils.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005