Genetic Analysis Of Intra-platform Reservoirs In The Upper Ordovician Lianglitage Formation In The Western Part Of The Tarim Basin

Based on contrastive analysis with a large number of cores,thin sections,logging and seismic data,and combined with the disciplinary theories of Carbonate Sedimentology,Reservoir Geology,Carbonate Diagenesis and geophysical and geochemical methods as guidance,to precisely and meticulously identify characteristics of stratigraphic distribution,types and distribution of sedimentary facies,basic characteristics and distribution of reservoirs of Lianglitage Formation in western Tazhong Area.The main controlling factors and development model of reservoirs are analyzed and studied,and the favorable development zones of reservoirs are evaluated and predicted.This paper have mainly acquired the following results and findings:It is considered that the Lianglitag Formation in Western Tazhong Area can be divided into three lithologic sections and 12 sub-layers from top to bottom.From NW to SE,the thickness of the formation as a whole shows a trend of slight thinning firstly and then gradually thickening.Based on the analysis of sedimentary facies marks.It is considered that open platform facies are mainly developed in the study area,and their types of rocks are mainly micrite,grainstone and less biolimestone.Sedimentary facies can be further subdivided into intra-platform banks,mud mound and interbank sea.The interbank sea are the most developed subfacies in open platform and is widely distributed.The types of intra-platform include sandy clastic banks,bioclastic banks and oncolite banks,which presenting a banding distribution along the NW-SE direction.The mud mounds developed in early time and have a small scale,and presenting a punctiform distribution in higher spots in local palaeogeomorphology.The mud mounds often overlap with the intra-platform banks to form the intra-platform mound-bank complex.The scale and scope of the mound-bank complex are gradually increased from the lower Lianglitage Formation to the middle Lianglitage Formation.But the development scale of the mound-bank complex became the smallest in the upper Lianglitage Formation.However,many sets of mound-bank complex were developed in every period,reflecting the characteristics of water fluctuation decreasing and rising again.The main rock types of reservoirs in the study area are sandy clastic limestone,bioclastic limestone and micrite.The main types of reservoirs are vuggy reservoirs,fracture-vuggy reservoirs and fracture reservoirs are taken second and third place,while the cave reservoirs are not developed.The overall reservoir level is dominated by type ? reservoirs in the study area.The reservoirs have an average porosity of 2.93%and permeability of 0.216 mD,which also reservoir heterogeneity is strong.the properties and permeabilities of reservoirs in the middle and lower parts are generally better.And the properties and permeabilities of reservoirs also show a trend of increased from west to east.The Western Tazhong Area is located in a relatively low-energy environment as a whole,with unfavorable sedimentary facies zones and lithology and ptrofacies are weak at control over reservoirs.The atmospheric freshwater dissolution is weak dvuring the quasi-contemporaneous period.And the scale of early diagenetic near-surface karst is small.As a result,there is no large-scale cave-fracture system has been formed in early stage.Therefore,the reservoirs development are mainly affected by buried karst and fractures.This is reflected in the reservoirs of Lianglitag Formation mainly distributed in the fracture development area and the developmental area of mound-bank complex in later period and so on.There are three main burial and dissolution stages in the research area,are the shallow burial stage in the late Caledonian-early Hercynian period,the middle-deep burial stage in the late Hercynian-Indosinian period and the Himalayan period,respectively.With each burial period,the fracture,acidic formation water or hydrothermal fluid generated by tectonic activity expand the dissolution along the pre-fractures,seam and dissolved pore,forming a considerable scale of non-fabric selective dissolution fracture-cave system.The subsequent oil and gas or hydrocarbon have prevented and reduced the burial cementation in the pore-cave system.Thus the porosity of reservoirs have been protected effectively.At the same time,Three times stages burial dissolution and structural fracture also improve the permeability and connectivity of the reservoir,and increase the effective reservoir capacity of the reservoir.The reservoirs of Lianglitag Formation in the Western Tazhong Area are mainly controlled by fracture and burial dissolution.Therefore,the reservoirs are banded distributed in the vicinity of the fault zone,such as ZL1,Z1,Z15 wells and other southeastern wells.In addition,the mound-bank complex development areas in later period,such as Zhongl,Zhong20 and Zhongl22 wells,are also the most favorable development areas of reservoirs,with punctiform distribution and poor regularity.Generally speaking,reservoirs in the study area are mainly distributes along the periphery of the most favorable area,which is mainly affected by the degree of fracture development and the scale of the mound-bank complex.However,the area where have no fractures and the favorable petrofacies(such as mound-bank complex)are go against for the form of reservoirs.