Study of Origin and Accumulation Period of Mass CO2 in Paleogene of Bohai Area: A Case Study of QHD29-2 Oil Field in Qinnan Depression

Abstract

Natural gas discoveries with CO₂ volume proportion ranging from 43.93% to 90.61% and the mean of 57.82% were drilled in QHD29-2 oil field, a condensate gas reservoir with oil ring in Bohai area, whose reserves is mainly distributed in Donging formation of Paleogene. The gas components were examined, which includes CH4 with a volume proportion ranging from 7.40% to 46.75% and the mean of 34.77%; N2 with a volume proportion ranging from 0.26% to 0.58% and the mean of 0.46%; There is also a certain portion of rare gas, He, blended in the main contents. The stable carbon isotope of CO₂ in gas samples ranges from 6.40‰ to 5.20‰ and the ratio of two other stable isotopes (3He and 4He) is 5.49Ra to 6.21Ra (Ra is the ratio of 3He and 4He isotope abundances in atmosphere). Based on these statistic data, it can be determined that the CO₂ in QHD29-2 condensate gas field is mainly volcanic mantle origin. In addition, under the observation of fluorescence microscope the gas reservoir contains two stage inclusions of hydrocarbon, which can be revealed by two peak value regions of homogenization temperature. Most components of these inclusions can be recognized as CO₂ and CH4 by Laser Raman composition testing, and their volum ratio is about from 1.5/1 to 2.5/1. The CO₂ originated from mantle should be filled at the same period as the filling period of hydrocarbon, and there are at least two filling periods. Most CO₂ discoveries were distributed in the traps connected with faults that were active after Pliocene (fault activity rate >10m/Ma). The geological evolution history preclude the possibility of the CO₂ filling time that earlier than 5.1Ma, which means that it has the characteristic of “multi-stages charging, late-stage accumulation”. Thus, the fine studies of structure, especially the studies of major faults' activity and volcanic activity after the Neogene Pliocene should be strengthened in the coming exploration. This will contribute to the recognition of the distribution and accumulation law of CO₂ in Bohai area.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015