Compositional Features of Molecular Compounds of the Ultra-Deep Condensate Oil, Bohai Bay Basin, China

Zhu, Guang Y.; Zhu, Guangyou; Zhang, Shuichang; Wang, Huitong; Weng, Na

The Bohai Bay Basin in China is a Cenozoic continental rifted basin, in which the target strata of exploration are mostly more than 4000 m deep. Recently, a high-yield industrial flow of oil and gas is achieved in buried-hill carbonate reservoirs which are 5641-6027m in depth and 190-201 centigradein temperature. The condensate oil density is 0.7792g/cm3, with the sulphur content less than 0.04%. The composition of the condensate oil is analyzed using comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOFMS). A total of 4955 compounds are detected, of them, the contents of saturated hydrocarbons, aromatic hydrocarbons and non-hydrocarbon are 94.8%, 5.1% and 0.02%, respectively, and the content of normal-alkanes is 53.6%. The normal-alkanes and long branch-chain substituted compounds remain intact. The detected results contain 25 adamantanes, 16 diamantanes and 3 triamantanes. The natural gas is wet gas, which has 81.77-86.47% methane, 6.73% ethane, and more than 10% heavy hydrocarbons. The fact that the hydrocarbon reservoir is up to 200 centigrade and liquid oil exists in great quantity suggests that the thermal stability of crude oil is much higher than traditionally thought, the exploration for oil and gas in deep strata has a great potential in the Bohai Bay Basin.

The ND1 well is located in the Baxian sag of the Jizhong depression, Bohai Bay Basin. The Baxian sag is a Cenozoic, dustpan-shaped fault subsidence, which is 2400 km2 in area and has a maximum depth of basement of about 10000m. The main exploration beds of interest in the sag are the Neogene, Paleogene, and Ordovician to Pre-Cambrian. The condensate oil in the ND1 well is an oil sample that is from the highest-temperature reservoir ever found in China. The well is also the deepest well of commercial oil and gas in the Bohai Bay Basin. The discovery of the oil provides important reference for re-understanding the oil and gas resources in deep strata of the Bohai Bay Basin. Using comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOFMS) and other methods, we found that normal-alkanes are rich and long branch-chain substituted compounds are also kept intact. In combination with the distribution of adamantine compounds, it can be determined that the oil has cracked but the cracking degree is not high, suggesting that liquid petroleum can exist in the geological conditions of reservoirs at 200 centigrade.

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