Lithologic Reservoirs ›› 2019, Vol. 31 ›› Issue (1): 12-19.doi: 10.12108/yxyqc.20190102

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Mechanism of sealing oil and gas with cap-rocks

LI Chuanliang1, ZHU Suyang1, LIU Donghua2   

  1. 1. School of Petroleum Engineering, Southwest Petroleum University, Chengdu 610599, China;
    2. Zhanjiang Branch of CNOOC Ltd., Zhanjiang 524057, Guangdong, China
  • Received:2018-06-16 Revised:2018-07-28 Online:2019-01-18 Published:2019-01-18

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Abstract: Underground accumulation of oil and gas is the result of cap-rock's sealing. Without the sealing of caprocks, oil and gas will migrate to the earth's surface and be lost away. Therefore,cap-rocks play an important role in the accumulation of oil and gas. In order to clarify the sealing mechanism of cap-rocks,a deep study was carried out based on the related knowledge of petroleum geology,petrophysics and fluid mechanics,which reached the following conclusions. Cap-rocks seal oil and gas by capillary pressure. The stronger the heterogeneity of formations is,the larger the capillary pressure differential between the cap-rock and the reservoir rock will be, and the more the oil and gas can be sealed by the cap-rocks. Homogeneous formations cannot accumulate oil and gas. The heterogeneity of formations has made the petroleum industry. Oil and gas can migrate from the smaller pores of hydrocarbon source rocks into the larger pores of reservoir rocks,but cannot migrate from the larger pores of reservoir rocks into the smaller pores of cap-rocks,which is the marvelous point of oil and gas migration. The wettability of rocks is also very important. The oil-wet rocks cannot seal oil and gas,while the water-wet rocks can do. The water-wettability of rocks has made the petroleum industry. The hydrocarbon concentration of water is quite low in cap-rocks,being impossibly higher than that in reservoirs,so the hydrocarbon concentration sealing of cap-rocks cannot take place. Cap-rocks can seal oil and gas rather than formation water,and cap-rocks can seal the phase of oil and gas rather than the molecules of oil and gas, which is the marvelous point of sealing mechanism of cap-rocks. Overpressure of cap-rocks is calculated by the equivalent depth method,which is a misleading appearance. Cap-rocks are not really overpressured,so they cannot apply overpressure sealing to oil and gas.

Key words: cap-rock sealing, capillary pressure, hydrocarbon migration, hydrocarbon concentration sealing, overpressure sealing, undercompaction

CLC Number: 

  • TE342
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