Differences And Genetic Analysis Of Intra-platform Reservoirs In The Upper Ordovician Lianglitage Formation In The Central Tazhong Area

The Tarim basin is rich in oil and gas resources.The Lianglitage Formation of Upper Ordovician platform margin in Tazhong area,many in-depth researches have been done;To date,researches on the open platform are at a low degree and there lacks systematic study on regularity in reservoir development and differences of the platform margin reservoirs,which lead to a lower success rate of oil and gas exploration within the platform.Guided by principles of carbonate sedimentary petrology,carbonate diagenesis,and reservoir geology theoretical methods,this paper analyzed the differences in sedimentary characteristics and reservoir development between the marginal and open platform of the Orioletic Lianglitage Formation by using core data,thin section,logging,and seismic data and techniques.Also,main controlling factors of the platform margin and open platform reef-shoal complex reservoirs have been analyzed,and the open platform and platform margins reef-shoal complex reservoirs have been established,superimposed evolution pattern of the reservoirs was established and the prediction and exploration ideas were respectively pointed out.The differences of petrological characteristics between the platform margin and open platform reef-shoal complex reservoirs of Lianglitage Formation in the Tazhong area is as follows:the thickness of platform marginal reef-shoal complex reservoirs is large,and sedimentary facies are mainly granular beach,reefs and mud mounds,with abundant content of bioclasts;while the thickness of open platform reef-shoal complex reservoirs is relatively thin,the types of sedimentary facies are mainly shoal,mud mounds and open platform depression.In terms of types of reservoir space,fabric-selective intragranular dissolution pores,intergranular dissolution pores and mold pores as well as and non-fabric selective karst dissolved fractures and tectonic fractures are developed in platform marginal reef-shoal complex.Fabric-selective pores in platform margin reef-shoal complex reservoirs are more developed than those in open platform reef and grain bank reservoirs,while dissolved fractures,tectonic fractures are more developed in platform margin.In terms of physical properties,the properties of platform margin are better than those of open platform reservoirs,but both show the characteristics of low porosity and low permeability.From the type of reservoirs,the platform margin reservoirs are mainly characterized by pore-type reservoirs while the open platform by fracture-cavity reservoirs.In terms of reservoir distribution,the platform margin reef-shoal complex reservoirs develop from Liang 5 to Liang 1 member,and 2-6 cycles were shown vertically.The thickness of single layer and cumulative thickness are large and the lateral distribution is quasi-layered and continuity.The open platform reef-shoal complex reservoirs are mainly distributed in the Liang 2-1 member.The thickness of the mono-layer is small and the vertical cycles are not obvious.In Liang 5-Liang 3 member,a small number of reservoirs have an isolated distribution and the continuity in the transverse direction is poor.The platform marginal reef-shoal complex reservoirs are controlled by sedimentary landforms and karst landforms.The strong effects of near-surface karstification of quasi contemporaneous atmospheric waters and early diagenesis cause the form of multiple sets of dissolved pores.The late burial dissolution and three-stage tectonism inherited and reconstructed the early cavities.Open platform reef-shoal complex reservoirs are controlled by sedimentary landforms and grain shoal distribution,high-quality reservoirs are controlled by the three-stage fractures and late burial dissolution,and the atmospheric freshwater karstification during the early diagenesis period is relatively weak in open platform than that in platform margin.For the platform margin reef and grain bank reservoirs,priority should be attached to finding high-energy reef-shoal complex facies as well as superimposed development zones including burial karst and tectonic fracture.In addition,attention should be paid to the search for favorable pore development areas under the control of quasi-contemporaneous freshwater and early near-surface karst dissolved effects.As to open platform reef and grain bank reservoirs,attetion should be focused on the areas under combined control of fractures and burial dissolution,especially the intersection areas of strike-slip faults and early faults,and it is advised to weaken the atmospheric corrosion of early diagenesis and near-surface karstification which are associated with reef and grain bank geomorphology.