Sedimentary Microfacies And Dolomitization Mechanism Of Ordovician Carbonates In The North-Central North China Platform

Significant breakthroughs have been achieved in hydrocarbon exploration of deep Ordovician dolomite reservoirs in the Yangshuiwu buried-hill area of the North-Central North China Platform since 2016.The practice of exploration and development has demonstrated that the formation of this reservoir is intricately linked to the sedimentary-structural background and dolomitization.To gain insights into the formation mechanism of high-quality dolomite reservoirs,further investigation is required into the Ordovician sedimentary microfacies and the origin of dolomite in this area.This dissertation is based on previous studies and guided by theories of carbonate sedimentology,reservoir geology,and rock geochemistry,etc.Based on macroscopic field outcrops and core sampling,microscopic thin-section,scanning electron microscope（SEM）and Cathodoluminescence（CL）observations,combined with geochemical methods such as whole-rock and in-situ micro-area single mineral elements（LA-ICP-MS）,isotopes(δ¹³C,δ¹⁸O,⁸⁷Sr/⁸⁶Sr),and fluid inclusion thermometry（Th）,this dissertation aims to（1）identify the sedimentary microfacies and dolomite types of the Lower-Middle Ordovician carbonates,（2）analyze the origin of flat-pebble conglomerates,the source of dolomitization fluids,and the origin of fully crystalline dolomite and dolomitic patches,and（3）establish the sedimentary environment and occasional deposition model of the Ordovician,and the formation mechanism of dolomitization and dolomitic patches.Additionally,this study deeply investigates the characteristics of Ordovician high-quality dolomite reservoirs,reveals the main controlling factors for their formation and proposes the development model and pore evolution process of high-quality dolomite reservoirs based on the above results.The results achieved are as follows:（1）Ordovician carbonate rocks are divided into a total of twenty-two sedimentary microfacies types（MF1～MF22）and five typical sedimentary microfacies associations（MFA1～MFA5）formed in the tidal flat,including supratidal zone,intertidal zone（upper lagoon,lower shoal）,subtidal zone（upper restricted sea,lower shoal）and open sea（shelf and basin）,The main sedimentation in the study area occurred in the intertidal-subtidal zone.Five flat-pebble conglomerate types（FPC1-FPC5）comprise four FPC sedimentary associated with ribbon rocks in vertical,including layer-liked or layered FPC,"flat bottom and convex top"shaped FPC,"flat top and convex bottom"shaped FPC and irregular FPC.The flat-pebble debris is mainly derived from the fragmentation of ribbon rocks by earthquake and/or storms,followed by the drying and cracking of supratidal zones.During storms,multi-phase layer-liked or layered FPC was developed from intertidal shoal to restricted sea and/or subtidal shoal to open sea,while"flat bottom and convex top"shaped FPC was formed by the small-scale slippage of the former on the slope.The"flat top and convex bottom"shaped FPC resulted from storm throwing,whereas the irregular FPC was the in-situ diagenetic type in the burial environment.（2）Ordovician dolomites are classified into fully crystalline dolomite and dolomitic patches in"leopard spot"limestone.Fully crystalline dolomite in the Lower Ordovician Liangjiashan Formation is divided into five types:（1）planar-s to planar-e dolomicrite（LD1）,（2）planar-e to planar-s silty crystalline dolomite（LD2）,（3）planar-e to planar-s fine crystalline dolomite（LD3）,（4）planar-s to nonplanar-a coarse-medium crystalline dolomite（LD4）and（5）nonplanar-a coarse crystalline dolomite（LD5）.And it has three types in the Middle Ordovician Majiagou Formation:（1）planar-s to nonplanar-a dolomicrite（MD1）,（2）planar-s to planar-e mud-silty crystalline dolomite（MD2）,and（3）planar-s to nonplanar-a fine crystalline dolomite（MD3）.Dolomitic patches in"leopard spot"limestone,composed of silty-,fine-,medium-and coarse-grained dolomite crystals,can be categorized as follows:（1）irregular dolomitic patches associated with stylolites,（2）dolomitic patches between gravel particles,（3）pore-related dolomitic patches associated with stylolites,and（4）dolomitic patches filled in fractures.（3）Based on the crystallography（stoichiometry,order degree,unit-cell parameters）,petrology（CL,SEM,etc.）,and geochemical characteristics(δ¹³C,δ¹⁸O,⁸⁷Sr/⁸⁶Sr,REE partition pattern,Th,etc.)of different dolomite types compared to the host micrites（calcites）,it can be inferred that the dolomitized fluid is predominantly seawater or seawater-like fluid.D1 dolomite（LD1,MD1）was formed in the near-surface environment under evaporation conditions in the penecontemporaneous period,i.e.,penecontemporaneous dolomitization in the supratidal zone,and the dolomitized fluid is high Mg/Ca brine from seawater evaporation.D2 dolomite（LD2,MD2）was formed by seepage reflux dolomitization,and the dolomitized fluid is a mixture of supratidal brine and seawater seepage.The formation of D3 dolomite（LD3,MD3）occurred in shallow to medium burial dolomitization.The dolomitized fluid is the mixture of new phase seawater with the residual pore water in the strata.LD4 and LD5 dolomites resulted from recrystallization of LD3 in the burial realm.Simultaneously,the first three types of dolomitic patches coexisting with stylolites were formed in shallow-deep burial dolomitization and stylolites controlled dolomitic fluid flow in the burial realm.However,the dolomite cement in fractures was formed by hydrothermal dolomitization.Combined with the sedimentary model,the dolomitization mechanisms of the Liangjiashan Formation influenced by continuous sea level drops and the Majiagou Formation controlled by multi-stage sea level rise and fall were established.Furthermore,the origin of the derived dolomite"leopard spot"was determined.（4）Three high-quality dolomite reservoir belts（RB1～RB3）are present in the Ordovician strata from bottom to top,which develop four porosity types:intercrystalline pores,dissolution pores,caves,and fractures.The reservoirs underwent comprehensive diagenetic processes such as dolomitization,dissolution,fracture,cementation,and compaction.Dolomitization in a tidal flat sedimentary environment and variations in sea level played an essential role in the reservoirs formation,and the diagenetic evolution sequence ultimately determined the formation of porosity.Multi-stage tectonic movements facilitated the reservoirs reconstruction,while late-stage effective fractures improved the reservoirs quality.In summary,the pores mainly formed in three stages,and a high-quality dolostone reservoir development model has been established.Moreover,the favorable distribution areas of the reservoir have been predicted.