Study on Diagenesis of Deeply-buried Strata in Anpeng Oilfield of Biyang Sag and Its Application to Prediction of Diagenetic Traps

Deeply buried strata refer to strata buried beneath the depth of 2 500～2 700 m .Their porosity and permeability are commonly very low. But under certain conditions, dissolution in depth can create good reservoirs and form diagenetic traps. However, prediction of diagenetic traps has long been a difficult problem, which is dealt with in this paper. Anpeng Oilfield is located in the southeast part of the Biyang Sag, Henan Province. Exploration shows that diagenetic traps exist in the sheet like sandbodies deposited in a fan delta of lower Member Ⅲ of Lower Tertiary Hetaoyuan Formation. In this paper, based on quantitative study on diagenesis and diagenetic stage, origin and distribution of diagenetic traps were discussed. The study shows that sandbodies of the studied interval experienced various diagenesis such as compaction, cementation, dissolution, etc. Different diagenesis occurs in different depth. Based on this and other data, the diagenetic stage is divided into early diagenetic stage and late diagenetic stage. Early diagenetic stage is further divided into early diagenetic stage A and early diagenetic stage B. Late diagenetic stage is further divided into late diagenetic stage A, late diagenetic stage B, and late diagenetic stage C, and late diagenetic stage A is further divided into substages A1, A2 and A3. In the study area, deeply buried strata (lower Member III of the Hetaoyuan Formation) are mainly in late diagenetic substage A3 and late diagenetic stage B. Substage A3 ranges in depth from 2 700 to 3 100 m, and characterized by strong cementation. Late diagenetic stage B ranges in depth from 3 100 to 3 900 m, and characterized by development of secondary dissolution pores, especially between 3 200～3 600 m. In the study area, sandbodies dip down from north to south. When sandbodies extend from late diagenetic substage A3 to late diagenetic stage B, the part in substage A3 becomes compact due to strong cementation and can seal oil and gas, while the part in stage B may become good reservoirs due to secondary dissolution. In this way, diagenetic traps are formed. Thus according to burial depth, characteristics of development and preservation of pores in different diagenetic stages, diagenetic traps can be predicted.