A method for predicting the fault induced lag degree of seal formation time of mudstone cap rocks and its application

It is very important to accurately predict the fault induced lag degree of seal formation time of mudstone cap rocks for correctly understanding the oil and gas distribution laws near faults, but there are few relevant researches and lack of theoretical basis. In order to correctly understand the oil and gas distribution laws near faults, this paper studies the fault induced lag degree of seal formation time of mudstone cap rocks and its influential factors. Based on this, the seal formation time of mudstone cap rocks and fault rocks was determined. In addition, a set of methods for evaluating the fault induced lag degree of seal formation time of mudstone cap rocks was put forward and verified based on case study. And the following research results are obtained. First, the fault induced lag degree of seal formation time of mudstone cap rocks is mainly affected by the relative relationship between the seal formation time of mudstone cap rocks and that of fault rocks. The later the seal formation time of fault rocks is than that of mudstone cap rocks, the greater the fault induced lag degree of seal formation time of mudstone cap rocks is, and vice versa. Second, the new method overcomes the shortcomings of the previous methods, which are difficult to accurately calculate the compaction diagenetic depth of fault rocks and take 1 MPa as the lower limit of the seal formation time of cap rocks and fault rocks. Third, the prediction results of the new method show that the seal formation time of the fault rocks of F4 fault of Nanpu No. 2 structure in the Bohai Bay Basin within the mudstone cap rocks of the second Member of Paleogene Dongying Formation (Dong 2 Member) is 1.1 Ma, and that of the Dong 2 Member mudstone cap rocks is 25.8 Ma. The lag degree of the seal formation time of Dong 2 Member mudstone cap rocks caused by F4 fault is 0.96. F4 fault is unfavorable to the accumulation and preservation of the oil and gas sourced from the first and third Member of Paleogene Shahejie Formation in the Dong 2 Member sandstone reservoir. In conclusion, the prediction results of F4 fault by the new method are consistent with the current oil and gas exploration practice, so this method can be used to predict the fault induced lag degree of seal formation time of mudstone cap rocks. © 2021, Natural Gas Industry Journal Agency. All right reserved.