Shiyou shiyan dizhi (Jan 2023)
Formation mechanism of high-quality reservoirs in deep strata of Paleogene, Bonan Subsag, Zhanhua Sag, Bohai Bay Basin
Abstract
With the continuous improvement of exploration degree, the main object for oil and gas exploration in the Bonan Subsag of Bohai Bay Basin gradually changes from medium and shallow layers to medium and deep layers. The key factor for deep exploration is the development of high-quality reservoir, which depends on the interaction between sedimentary filling and formation fluid. During the deposition of the fourth member of Paleogene Shahejie Formation, lacustrine clastic rocks and gypsum salt deposits developed in this area under a saline environment. Based on the comprehensive study of drilling data and experimental results, the sedimentary environment, formation water characteristics, reservoir petrographic characteristics and diagenetic types of the Paleogene reservoirs in the Bonan Subsag were discussed, the development and evolution mechanism of the deep and high-quality reservoirs in Paleogene was also indicated. Results show that: (1) Under the sealing of gypsum salt layer with wide distribution area and large sedimentary thickness, the clastic rocks of Es4 member in salty environment experienced alkaline diagenetic environment in the early stage. The carbonate cements formed in this period filled primary pores and effectively prevented the occurrence of compaction. (2) With the increase of the maturity of organic matters in source rocks, significant organic acid migrated to stratum driven by abnormal high pressure during the middle diagenesis stage. The early carbonate cements, feldspar and debris were generally dissolved and secondary pores were subsequently formed. Acidic and alkaline reworks are the main mechanism for the development of deep high-quality reservoirs. (3) The development of high-quality reservoirs was controlled by the distribution of gypsum salt layer, the development of source rock, fault system and the distribution of clastic sedimentary body. The front sand bodies of gentle-slope fan delta and steep-slope nearshore subaqueous fan strongly influenced by acid and alkali fluid are the high-quality reservoir development area of Es4 member in Bonan Subsag.
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