Formation Mechanism And Controlling Factors Of Intermediate-Deep Effective Reservoirs In The Shahejie Formation In The Raoyang Sag Of Jizhong Subbasin

The Raoyang Sag of Jizhong Subbasin has become the focus area for oil and gas exploration in China,which is a main oil producing area.But petroleum exploration theories on Shahejie Formation,with deep burial depth of reservoirs and complicated diagenetic evolution,were not perfect.Therefore,the studies of formation mechanism,especially of the high property reservoirs under tight condition are of important and pratical values,which will show new insights into the exploration of deep reservoirs in Jizhong area.Based on the observations of bore cores and thin sections combined with well logging and drilling data,this study documents the characteristics of diagenesis and diagenetic facies of the sandstones in the Shahejie Formation of the Raoyang Sag,and decipher the distribution mechanism with the key factors controlling the clastic reservoirs.It was conducted within the sequence stratigraphic framework from two aspects: control of sequence boundary on diagenesis and diagenetic evolution of sandstones in different systems tracts.The results indicate that the Sha-3 Member of the Shahejie Formation comprises one second-order sequence and three third-older sequences.Each of them can be subdivided into three system tracts: low stand(LST),transgressive(TST)and high stand(HST)systems tracts.The sequence boundary controlled carbonate cementation and dissolution in the underlying sandstones.However,it did not act as the preferential migration pathway of fluids which facilitated the dissolution in the sandstones overlying the sequence boundary.Sands in the LST tend to be mixed with clays and were less resistant to compaction so that primary porosity was most likely to be lost due to compaction and cementation of early carbonates from the over-saturated alkaline solutions.Although sandstones in TST are generally fine grained,they are relatively well sorted.As a result,considerable primary porosity could be preserved at the early diagenetic stage.Furthermore,secondary porosity could be developed since sandstones in TST were in direct contact with source rocks and could be easily get accessed by organic acids.Sands in HST consist of well-sorted larger grains and were more resistant to compaction.Their relatively shallow burial made it possible for primary porosity to be preserved more easily.In addition,the leaching by the atmospheric waters in the early diagenetic stage and the subsequent dissolution of organic acids led to the significant development of secondary porosity,which improved the reservoir quality of the sands considerably.Thus,sandstones in HST possess the best reservoir quality.This study proposed the diagenetic evolution models for sands in different systems tracts at syngenetic,eogenetic,and telogenetic stages.The study results made it possible to delineate the high quality reservoirs.Also,based on the characteristics of diagenesis and diagenetic facies of the sandstones in the Shahejie Formation of the Raoyang Sag,the sandstones have underwent compaction,cementation,dissolution and fracturing,and they are at the phase A of middle diagenetic stage.Five types of diagenetic facies are recognized on the basis of the type and intensity of diagenesis and diagenetic minerals.They are tightly compacted,clay mineral filling,carbonate cemented,unstable components dissolved and moderately compacted-weakly cemented diagenetic facies.The sonic,resistivity,and gamma ray logs,which are sensitive to diagenetic facies,were selected to establish the logging criteria by calibrating cores with logging curves.There are two types of constructive diagenetic facies: moderately compacted-weakly cemented diagenetic facies and unstable components dissolved facies.With the criteria,subdivision and recognition of diagenetic faceis were carried out for individual wells.The identified diagenetic facies match well with the results of oil testing and petro-physical analysis.A sophisticated approach of diagenetic facies is an important method for predicting sweat spots of tight sand reservoirs.The moderately compacted-weakly cemented diagenetic facies is characterized by low GR and medium to high acoustic response.It has a wide range of electrical resistivities.The development of intermediate-deep good quality reservoirs is attributed to three mechanisms: high compositon and tecture maturity,early oil charging,fluid overpressuring and dissolution.Therefore,it is recommended that deep petroleum exploration should focus on the delineation of “Sweet Spots” in deep layers by taking these mechanisms into consideration.