Study Of Sedimentary Microfacies Of Ordovician Yijianfang Formation In Gucheng Area,tarim Basin

The thick marine carbonate deposits are widely developed in the Lower Paleozoic of Tarim Basin.The Middle-Lower Ordovician contains rich oil and gas resources and constitutes an important hydrocarbon reservoir in the basin.The Gucheng area is located on the eastern side of the Tazhong uplift and the southern margin of the Mangal hydrocarbon-bearing depression.It has a good paleo-tectonic development background and high-quality reservoir-cover combination development conditions,and its petroleum geological conditions are superior.The high-frequency sequence and sedimentary microfacies of Yijianfang Formation in the Gucheng area were studied,the distribution and evolution of sedimentary microfacies in the high-frequency sequence framework were identified,which is of important significance for further study of reservoir characteristics and optimization of favorable sedimentary facies.Based on the multi-disciplinary theory of marine carbonate sedimentology,sequence stratigraphy and logging geology,this paper is based on the summary of previous research results,combines the regional geological background and comprehensively utilizes the core,conventional logging and 3D seismic data,to study the characteristics of high-frequency sequence and sedimentary microfacies of the Ordovician Yijianfang Formation in the Gucheng area of the Tarim Basin.The main points were as follows:1.Using core,conventional logging and 3D seismic data,identifying the sequence interface,combined with the quantitative research results of GR curve and U curve,the high-frequency sequence stratigraphy of Yijianfang Formation was studied.A four-level sequence interface is identified within Yijianfang Formation,and the Yijianfang Formation is divided into two four-level sequences SQ1 and SQ2.The maximum flooding surface can be identified within each four-level sequence,and the transgressive systems tract and highstand systems tract are classified accordingly.The high-frequency sequence stratigraphic framework of Yijianfang Formation in the Gucheng area was established by using the well-shaped cross-section profile.2.On the basis of core observation and thin section identification,the carbonate lithofacies of Yijianfang Formation in Gucheng area are divided into calcsparite-grained limestone facies,micrite-calcsparite grained limestone facies,micrite grained limestone facies and micrite limestone facies,then subdivided into 10 rock types.The calcsparite-grained limestone facies include calcsparite oolitic limestone,calcsparite gravel-bearing calcarenite,calcsparite oolitic calcarenite,calcsparite medium-fine calcarenite and calcsparite calcisiltite.The micrite-calcsparite grained limestone facies include micrite-calcsparite fine-powder arene limestone and micrite-calcsparite calcarenite,and the micrite grained limestone facies only includes micrite calcisiltite,micrite limestone facies includes dust-bearing micrite limestone and micrite limestone.3.Based on core observation,thin section identification and logging-lithology interpretation,the sedimentary background of Yijianfang Formation in Gucheng area is generally interpreted as carbonate ramp,which mainly develops inner ramp and middle ramp.The inner ramp includes two subfacies of the grain beach and interbank sea,and three microfacies of the high energy beach,the middle energy beach and the interbank sea.The middle ramp includes three subfacies of the grain beach,the storm deposit,the static water mud,and three microfacies of the low energy beach,the storm beach and the static water mud,then summarized the rock-electrical comprehensive response characteristics of each microfacies.4.The SQ1 period of Yijianfang Formation is dominated by inner ramp and middle ramp.The inner ramp are mainly distributed in the central and western parts of the study area,and the middle ramp are distributed in the northeast and southeast regions.Among the inner ramp,the high-energy beaches are mainly distributed in the southwestern part of the study area,and the distribution of the middle-energy beaches is relatively scattered.The interbank sea between the grain beaches are the most widely distributed.The middle ramp is dominated by static water mud,low-energy beaches scattered among them,and the storm beach deposits showed up in a few wells.During the SQ2 period,the inner ramp and middle ramp deposition was still dominant,and the sedimentary pattern of the SQ1 period was inherited.Compared with the SQ1 period,the inner ramp retrogradation toward the west,the middle ramp progradation toward the west,and the high-energy beach migrates to the north.The lower part of the single fourth-order sequence of Yijianfang Formation developed interbank sea-static water mud deposition.The upper part of the sequence is mainly inner ramp sedimentary,and a little middle ramp deposit is developed in the east.There are several medium-thick grain beaches developed within the four-level sequence,which are obviously progradation toward the east.At the top of the sequence,the middle-energy beaches and high-energy beaches are stacked on each other,often forming a super thick beach.Compared with the SQ1 period,the grain beaches in the SQ2 is still progradation toward the east,but the development range is reduced to the west.In general,during the deposition period of Yijianfang Formation,the Gucheng area was in a process of transgression to the west.