Porosity and permeability cutoffs for calculating effective thickness of different types of low-permeability reservoirs and causes of their differences: a case study of the Mesozoic in S region of central Shaanbei slope
Zhang F.,Xi'an Shiyou University |
Wu F.,Xi'an Shiyou University |
Meng X.,Petrochina |
Gao X.,Yanchang Petroleum Coporation |
And 3 more authors.
Oil and Gas Geology | Year: 2015
Analytic means including thin section examination, SEM, cathode luminescence, mercury penetration and so on, were applied to determine the cutoffs of porosity and permeability for calculating the effective thickness of the major Mesozoic oil-bearing formations in S region of Ordos Basin. The reasons for the varying cutoffs were also discussed based on quantitative characterization of physical property changes of the formations. The results show that the cutoffs (porosity and permeability) decline with the increase of burial depth. Samples from Yan 9, Chang 2, Chang 4+5 and Chang 6 formations, have porosity cutoffs of 15%, 14%, 8% respectively, and permeability cutoffs of 4×10-3, 1×10-3, 0.15×10-3 μm2 respectively. The differences in content and grain size of rigid particles in mineral composition of each reservoir caused mainly by sedimentation processes are regarded as the basic factor for the difference of cutoffs, while diagenesis is the major factor. The processes, including compaction, cementation and corrosion, dented the formations in different ways. All the formations had the similar porosity at first. But later on it turned out that Yan 9 had a higher porosity than that of Chang 2 because of their different response to corrosion and Chang 4+5 had lower porosity than that of Chang 2 because of their different behavior under cementation. ©, 2015, Oil and Gas Geology. All right reserved.