Reservoir Characteristics And Control Factors Of The Lower Cambrian Longwangmiao Formation In Northern Sichuan Basin

This paper selects the reservoir of the Lower Cambrian Longwangmiao Formation in the northern part of the Sichuan Basin as the research object.Using petrology,sedimentology and geochemistry as the theoretical guidance,based on the field measured profiles,and through the use of cores,logs,and other data,the characteristics of the sedimentary facies of the Longwangmiao Formation in the northern Sichuan basin are recognized;and by means of microscopic identification of thin slices,cathodoluminescence,scanning electron microscopy combined with pore permeability parameters,the reservoir type was analyzed;Besides,through a series of geochemical test parameters such as major elements,trace elements,rare earth elements,carbon and oxygen isotopes,lanthanum isotopes,and ordering degrees,elements of enrichment and migration were found,the properties of diagenetic fluids were determined,and dolomization patterns were established.Finally,concluding the characteristics of reservoirs to establish the diagenesis model,analyze the causes of dolomite formation,and summarize various factors controlling the development of reservoirs in the Longwangmiao Formation.Through the study,the following achievements and understanding are obtained.?1?Using the field survey profile,logging data and other methods,combined with the characteristics of the beach grain deposition in the study area,on the basis of the analysis of phase signs,the long Longwangmiao formation in the study area is divided into limited platform facies,mixed tidal flat facies,slope facies and open platform facies.Then,further analysis of subfacies and microfacies to clarify the distribution characteristics of the sedimentary facies.Overall,from east to west,the sedimentary facies showed a transition from the platform edge to the constrained mesa facies,representing a sedimentary phase transition from the offshore to the continental side.?2?The reservoir rocks of the Longwangmiao Formation in the northern Sichuan region are dominated by fine-grained dolomite,and the reservoir space is dominated by intergranular and intergranular dissolution pores,intragranular pores,and intragranular dissolved pores.The pore size is approximately 1mm.Cracks developed,and the cracks showed obvious characteristics of two-stage filling under cathodoluminescence.On the whole,the reservoir in the study area is characterized by low porosity,low permeability,and poor reservoir properties.?3?Geochemical tracers in the study area show many sense:the major elements indicate that dolomite formed by sedimentation;the low ratio of Sr/Ba may be affected by low salinity fluids;the content of Fe and Mn increases with increasing crystal size Trends;The positive anomaly of Ce indicates that the reservoir rock is formed in the weak reduction environment.However,the no Eu anomalies indicating that the diagenetic process is not affected by deep hydrothermal fluids,and dolomite are products of shallow burial periods.The distribution curve which is relatively flat imply that the nature of the fluid did not change significantly;⁸⁷Sr/⁸⁶Sr ratios reveal that sediment was significantly affected by terrestrial origin,and the Sr isotope of veins were related to that of the surrounding rocks;carbon and oxygen isotopes show the diagenetic fluid had higher salinity,and with the degree of ordering,the dolomitization pattern in northern Sichuan basin is considered as evaporative enrichment and osmotic dolomitization by reflow,and the burial effect is superimposed in the later stage.?4?The reservoir in the study area is controlled by sedimentary facies and diagenesis.First of all,sedimentary facies controlled the material basis for the formation of reservoirs.The main reservoir rock,fine-grained dolomite,is controlled by the granular beach microfacies,and the porosity is significantly better than that of other non-reservoir rocks controlled by other microfacies;Secondly,the destructive diagenesis such as pressure dissolution,cementation and filling effect,and excessive dolomitization reducing the reservoir porosity and permeability,make the reservoir densified,while constructive diagenesis including dissolution and appropriate dolomitization,can optimize reservoir space and improve properties reservoir.