Differential Analysis Of Sedimentation And Diagenesis Of Interlayer Sand Bodies In Cretaceous Baxigai Formation In Western Tabei Uplift

The Tarim Basin is a typical large inland superimposed basin in China.As one of the important oil-controlling structural units in the basin,the Tabei Uplift is a key oil and gas resource block in China and contains abundant oil and gas resources.As an important oil-bearing interval,the Cretaceous has received extensive attention from experts and scholars in recent years.The sedimentary period of the Baxigai Formation is stable.Although the buried depth is more than 5000 m,it still maintains a high porosity.However,the internal heterogeneity of the reservoir is strong,and the highquality reservoir is mixed with the poor reservoir,which brings trouble to the efficient development of oil and gas.On the basis of previous research results,this study systematically collected and sorted out the data of ordinary thin sections,conventional core physical property analysis and cast thin sections in the study area,and characterized the reservoir characteristics of interlayer sand bodies from three aspects of material composition,reservoir physical property and pore type,so as to find out the differences.At the same time,logging and seismic data are combined to make full use of grain size analysis,heavy mineral analysis and other data.In addition,core observation is carried out in the core database of Tarim Oilfield.The influence of source supply direction and sedimentation on the physical properties of interlayer sand bodies is clarified.Through the observation of casting thin sections and X-ray diffraction analysis of whole rock clay,the constructive and destructive effects of calcite cements on reservoir physical properties are revealed,supplemented by means of cathodoluminescence,fluid inclusion temperature measurement,main and trace element measurement,etc.The output characteristics,plane and vertical distribution of calcite cements and the main controlling factors of their interlayer distribution differences are clarified.In order to guide the exploration and prediction of dominant reservoirs and provide theoretical support for oilfield production and development,the characteristics and formation mechanism of physical property differences between deep clastic reservoirs in Tarim Oilfield are analyzed from two aspects of sedimentation and diagenesis by means of macro and micro combination.The results show :The provenance of the Cretaceous Baxigai Formation in the western Tabei uplift comes from the southeast.The lithology of the whole reservoir section is dominated by lithic feldspar sandstone and feldspar lithic sandstone,and the compositional maturity of the reservoir is low.The study area is dominated by the sandstone of the second member of the Brazilian Gai Formation,which is characterized by the upper thin sand body and the lower massive sand body.The massive sand body can be subdivided into massive sand 1 layer and massive sand 2 layer.The reservoir physical properties of the target layer in the study area are generally good,and the porosity is mostly distributed in the range of 15 % ～ 20 % and 20 % ～ 30 %.The vertical reservoir heterogeneity is strong,and the physical properties of massive sand bodies are better than those of thin sand bodies.The physical properties of massive sand layer 1 are better than those of massive sand layer 2.On the plane,the physical properties of Yingmai well area are better than those of Yangta and Yudong well areas.The primary intergranular pores are the main reservoir space of the target interval in the study area,followed by intragranular and intergranular dissolution pores.Sedimentation has a certain restriction on the reservoir properties of the Cretaceous Baxigai Formation in the western Tabei uplift.The thin sand body of the Baxigai Formation is a seasonal flood overflow delta deposit.The channel is narrow,the sedimentary slope is slow,the sand body is small,the continuity is poor,the source supply is relatively small,and the reservoir space is small.The bottom massive sand bodies are seasonal braided river delta deposits,with wide channels,large sedimentary slope,large scale,good continuity and patchy distribution of sand bodies.The channels are superimposed with each other,the source supply is sufficient,and the reservoir space is relatively large.Therefore,the overall reservoir physical properties of the bottom massive sand bodies are better than those of the thin layer sand bodies.Yangta well area and Yudong well area are farther from the source than Yingmai well area,so the physical properties are poor.Diagenesis is the main controlling factor for the obvious difference in reservoir physical properties of the Cretaceous Baxigai Formation interlayer sand bodies in the western Tabei uplift,among which calcite cement has the most significant influence on the reservoir.The Baxigai Formation in the western part of the Tabei uplift is in a stable sedimentary environment,and its overall thickness is thin.The thick-thin sand bodies are affected by medium compaction,and the interlayer differences are not obvious.The cementation is mainly the destructive diagenesis of plugging pores and a small amount of anti-compaction constructive effect.The cement content and porosity reduction rate of thin layer sand body are higher than those of thick layer bottom massive sand body.The cement content and porosity reduction rate of 2 layers of massive sand body are higher than those of 1 layer of massive sand body.Compared with other types of interstitial materials,calcite cements occupy a large area of reservoir space,which seriously reduces the reservoir physical properties.There are mainly two phases of calcite cements in Brazilian reconstruction,among which the early continuous crystal calcite has obvious effect on reservoir physical properties.The calcite cements above the plane are enriched in the braided river delta front subfacies.Vertically,two enrichment zones can be identified : one of the calcareous cement layers enriched at the top of the massive sand body is obviously affected by the transformation of clay minerals in the overlying mudstone of the sand body,and the other is enriched at the junction of the first and second layers of the massive sand body.The calcite cement is significantly affected by the flooding surface.The high-quality reservoirs of the Cretaceous Baxigai Formation in the western part of the Tabei uplift are developed in the middle-upper to bottom of the massive sand bodies of the braided river delta plain subfacies and delta front subfacies.The oil layers of the target interval in the study area are distributed in the underdevelopment zone of calcite cement in the 1st and 2nd layers of massive sand,and the strength of calcite cementation affects the distribution of oil layers.