Forming Mechanism And Prediction Of Es₃ Member Reservoir In The Northern Sub-sag Of Laizhouwan Depression

The oil and gas exploration gradually develop from shallow to middle-deep strata in the Laizhouwan Depression.The reservoir in Shahejie Formation of Paleogene has the following characteristics:deeply buried,various sedimentary types,rapid horizontal variation,complicated diagenesis,and complex reservoir space,making it difficult to predict the favorable reservoirs.Therefore,the highly efficient development of middle-deep complex reservoirs is seriously restricted.This paper is mainly focused on the third member of Shahejie Formation(Es₃ member),also the main producing oil stratum in the northern sub-sag of the Laizhouwan Depression.Based on the previous study results,the reservoir characteristics,diagenesis,and evolution are analyzed using data from core,thin section,logging,as well as geochemical examination to understand the forming mechanism and make evaluation and prediction of favorable reservoirs.The result from core observation and microscopic identification indicates that debris-feldspar and lithic arkose are the main types of Es₃ member in study area,with a middle maturity in both composition and texture.The primary pores are the main reservoir space,followed by secondary pores mainly as a result of feldspar dissolution.The reservoirs of Es₃ member in the northern sub-sag of the Laizhouwan Depression is are characterized by middle porosity and middle-low permeability with the average of21.48%and 166.31m D respectively.In the plan from east to west,the physical properties are changing in a good trend,with a better property near the faults.Two abnormal high porosity zones can be identified vertically.The paper defines there diagenetic parameters(apparent compaction rate,apparent cement rate,and apparent dissolution rate)to conduct quantitative characterization to the strengthen of diagenesis.Thus,the change of porosity from different diagenetic stages can be calculated quantitatively.As a result,compaction and cementation are the main destructive diagenesis,contributing 16.47%to the loss of primary pores totally.On the contrary,dissolution is the main constructive diagenesis and of great importance in reforming the physical properties.Based on the principle of Diagenetic Strengthen+Physical Property,the diagenetic phrases can be divided into five types.Primary sedimentary conditions,diagenesis,burial history,and abnormal high pressure are the main controlling factors of reservoir qualities of Es₃ member in the northern sub-sag of the Laizhouwan Depression.There are four main process of reservoir evolution:the pore decrease by compaction in early rapid burial stage with shallow burial depth,the pore decrease by early cementation and the pore increase by dissolution in early-middle slow burial stage with shallow burial depth,as well as the pore decrease by late cementation in middle rapid burial stage with deep burial depth.Using data from physical experiments,mercury injection,and oil test,the lower limit values of effective porosity and permeability are 14.5%and 8.25m D respectively.The result of Q cluster and discriminant analysis indicates that the reservoirs of Es₃member in the northern sub-sag of the Laizhouwan Depression can be divided into five types.Based on the research coupling the history of sedimentary evolution,tectonic evolution,strata burial,and diagenetic evolution,the paper sets up the evaluation standard of clastic reservoirs of Es₃ member and predicts the favorable reservoir developed area in the northern sub-sag of the Laizhouwan Depression.The results show that the underwater distributary channels are favorable phases for oil accumulation and the reservoir qualities become poorer from the west to east.