Depositional Model And Reservoir Development Mechanism Of The Dengying Formation In Central To Northern Sichuan Basin

The discovery of the Anyue gas field in central Sichuan Basin revealed that there developed high-quality microbial carbonate reservoirs in the Dengying Formation.Recently,several high-yield gas wells drilled in the north slope area of central Sichuan paleo-uplift further confirmed the existence of good reservoirs in central to northern Sichuan Basin.However,previous exploration mainly focused on the central Sichuan Basin and the studies on the northern Sichuan Basin were relatively few.And the development degree and quality of microbial carbonate reservoirs in the second and fourth members of the Dengying Formation,as well as in different regions,are quite different.Therefore,platform development and evolution,reservoir development mechanism and its controlling factors are of great significance for further natural gas exploration.Based on many core observations and field outcrop observations,combined with drilling and logging data and seismic profiles,the classification and distribution of microbial carbonates and other lithofacies were clarified.Then,the development and evolution of the platform in central to northern Sichuan Basin was reconstructed,and the favorable paleoenvironmental conditions for the development of microbial carbonates were restored and clarified.Besides,the control of cyclic sedimentation on microbial carbonate reservoirs under different paleogeographic backgrounds was analyzed.Finally,the development mechanism of reservoirs was illuminated,and the reservoir development model was established.The following conclusions were obtained:(1)Two transgressive-regressive depositional cycles were recognized in the Dengying Formation.Microbial carbonates were mainly developed in the warm and semi-arid climate during the regression.There was a ramp-like epeiric carbonate platform in the first regression,and it evolved into a siliciclastic shelf during the second transgression.Finally,a rimmed carbonate platform developed in the second regression.The evolution of the platform was mainly controlled by intense differential subsidence and sedimentary rates between the rift trough and platform under the effect of pre-existing basement rifts and coeval growth fault activities.(2)The suboxic,high-salinity seawater of the Dengying Formation is conducive to the development of large-scale microbial carbonate rocks.From the second member to the fourth member of the Dengying Formation,the oxygen concentration of seawater increases and the salinity decreases,which could promote the emergence of metazoans.The decline of microbial carbonate content in the fourth member of the Dengying Formation may also be concerned with this.(3)The reservoirs of the Dengying Formation are mainly microbial framestone consisting of thrombolite and stromatolite and grainstone.In the second member,they are mainly developed in the shallow subtidal cycles at the edge of the platform while there are less reservoirs developed in the peritidal cycles inside the platform.In the fourth member,reservoirs mainly develop in the shallow subtidal cycles and the peritidal cycles at the platform margin,and they are rarely developed in the subtidal cycles in platform interior.(4)With high primary porosity,microbial framestone and grainstone lay firm foundation for the formation of reservoirs of the Dengying Formation.Increasing reservoir spaces significantly,penecontemporaneous dissolution is key to reservoir development.The seafloor cementation and compaction-pressure dissolution with the resulting cementation are the main porosity reduction effects.The sedimentary cycles at platform edge have higher proportions of microbial framestone and grainstone than those inside the platform.They also suffer stronger penecontemporaneous dissolution and thus produce more porosity,which is more conducive to the development of reservoirs.There are more extensive microbial framestone on the ramp-like epeiric platform in the second member than on the rimmed platform in the fourth member,heralding considerable potential for the development of large-scale high-quality reservoirs in the second member of the Dengying Formation.