Reservoir Performance And Dissolution Filling Of Pagoda Formation In Sichuan Basin

The Sichuan Basin is a large hydrocarbon-bearing superimposed basin in China,and the exploration of Ordovician strata in the region has not made a major breakthrough.The Pagoda formation of the Upper Ordovician is a set of limestone with the unique “network structure”,and also a potential reservoir.For the recent years,the research focus has been mainly on the discussion of sedimentary facies,diagenesis process and mechanism,and little work has been done on the dissolution closely related to reservoir.Guided by the multidisciplinary theories of carbonate sedimentary petrology,carbonate reservoir geology,sequence stratigraphy and sedimentology,and combining with the results of fieldwork and regional geological data,this paper makes a comprehensive analysis using thin sections observation,cathodoluminescence and geochemical element test.While expounding the basic lithological characteristics of the Pagoda formation,this paper focuses on its internal corrosion.A number of studies on the characteristics,period and formation mechanism of dissolution filling have been carried out.The following conclusions have been obtained:1.Bitumen marks commonly exist in Pagoda formation limestone.Trace of organic matter contamination can be seen in the field and various micro-fractures in the thin slices,indicating that paleo reservoir migration exists in the layers.The related work of this paper shows that the reservoir physical property of Pagoda formation is poor.The dissolution filling and bitumen marks prove that Pagoda formation has the conditions for reservoir formation in geological history.Diagenesis have destroyed most of the original reservoir space,and the possibility of forming large-scale reservoirs is small.But it is still has the possibility of forming industrial capacity in some area.2.Lithology of Pagoda formation is compact.Diagenesis of the Pagoda formation has compaction,pressure-solution,fracturing,dissolution and so on,among which fracturing and dissolution are important constructive diagenesis.Reservoir space of Pagoda formation can be divided into two types: fracture and pore.The main type is fracture,and the pore is relatively undeveloped.Fractures can be divided into two stages:in the first stage fractures are mainly filled with calcite;the later stage is mainly filled with organic matter,and a very small number of intergranular pores.3.The dissolution of Pagoda formation is mainly divided into two stages,one is supergene corrosion caused by structural uplift and exposure,the other is buried corrosion related to organic acids in the burial diagenetic environment,and two phases of calcite fillings were formed.The chemical composition of the fillings is obviously different from the surrounding rock,showing the geochemical characteristics affected by fresh water.4.The Caledonian tectonic exposed the Pagoda formation to the surface,forming local corrosion holes and filling cracks.Organic acids produced by hydrocarbon evolution dissolve and reconstruct the original pores and fractures.The change of fluid environment resulted in the crystallization of early dissolved carbonate and filled in the dissolution pores and cracks.Finally,the residual asphalt occupied the space of the remaining pores.As a result of regional tectonic activity,the extent of dissolution in the northern part of the basin is slightly stronger than that in other parts of the basin.