Mixed Fine-Grained Rock Reservoir Internal Architecture And High-Quality Reservoir Distribution

The composition of the Permian Luchaogou Formation in the Jimsar Sag,Junggar Basin is a typical brackish lake facies mixed fine-grained sedimentary shale oil reservoir,which has now achieved the large-scale production stage.Due to its complex and heterogeneity lithology,it is extremely difficult to develop the benefits.The role of reservoir heterogeneity and reservoir architecture in controlling the distribution of high-quality reservoirs has been not clear.Faced with the critical period when the shale oil development of the Luchaogou Formation is moving from the testing stage to the economic benefit stage,there is an urgent need for more prefound research on reservoir heterogeneity.In this paper,the research objective is to study the reservoir architecture of mixed fine-grained rocks in the brackish lake facies.Based on 91wells of logging data,cores and outcrop,the research workflow of fine observation and description of cores,hierarchical structure and facies-controlled constraint are used to carry out.The type identification of lacustrine facies,characteristics of architecture elements of mixed fine-grained rock reservoir,fine characterization and distribution of mixed fine-grained rock reservoir architecture,quality difference of facies-controlled mixed fine-grained rock reservoir and distribution characteristics of high-quality reservoir are studied.The study shows that:（1）The type of brackish lake facies facies was identified.Based on core observation and outcrop analysis,sedimentary facies markers,combined with logging data,the target layer was identified as the brackish lake facies,subdivided into three sub-facies:debris shallow lake,mixed shallow lake and semi-deep lake-deep lake,and six micro-facies types:shallow lake sand dam,debris shallow lake mud,mixed shallow lake mud and semi-deep lake mud.（2）The characteristics of the reservoir architecture elements were determined.The fifth level order elements correspond to the subfacies level,the fourthlevel order elements correspond to the microfacies,and the third level order elements are mainly the internal accretion of the shallow lake sand bars,dolomitic flat and mixed beach reservoirs,and their lithological,physical and logging response characteristics were summarized.（3）A study of reservoir architecture boundary identification was carried out.Three methods were used to identify reservoir architecture boundaries and scales:lithological variability,nuclear magnetic resonance（NMR）parameter variability and horizontal well trajectory boundaries.Three types of mixed fine-grained reservoir architecture styles are proposed:isolated,stacked and migrated stacked.（4）The distribution characteristics of the reservoir architecture are presented.Sand layers plane contours of lithologies such as feldspathic clastic sandstone,siltstone,sand clastic dolomite and dolomitic siltstone are used.The lithological contours are superimposed,and the reservoir architecture planar distribution is modified and constrained by combining with the reservoir architecture boundary identification method,which can visually and accurately show the distribution range of various reservoir architectures and the extension scale of sand bars.（5）The characteristics of quality differences of facies-controlled reservoirs are illustrated.The NMR porosity,oil-bearing saturation,brittleness and TOC of different conformational units are significantly different.The shallow lake sand bars are represented with poor inter-well connectivity and strong heterogeneity,with significant differences in production capacity in different areas.The mixed beach is widespread,thin in overall thickness and stable in distribution.The dolomitic flat is developed in plan as a large contiguous area with a few isolated areas.（6）The role of reservoir architecture in controlling the distribution of high-quality reservoirs is established.Based on the NMR porosity,saturation,TOC and brittleness parameters,the high-quality reservoirs are divided into three categories.Firstly,the mainstay P₂l₂^2-2develops a thick layer of shallow lake sand bar,with the main favourable areas in the southeast where is high pore,high saturate and relatively high TOC.Secondly,the mainstay P₂l₂^2-2develops a thin layer of mixed beach,with the main favourable areas in the northeast where the medium pore is moderately saturated and relatively high TOC,and in the southwest where the high pore is moderately saturated and relatively high TOC.Thirdly,the mainstay P₂l₂^2-2develops a thin layer of dolomitic flat and the southeastern medium-hole medium-saturated relatively high brittleness and high TOC;targeted deployment and development recommendations are made to address the variability in the development of high-quality reservoirs.The dolomitic flat has high overall NMR saturation values and weak heterogeneity,and is the most effective section for shale oil development,and should be developed by large-scale horizontal well deployment and platform operations.The mixed beach NMR saturation is not high and the heterogeneity is strong,which makes it an average section for development and a suitable area for horizontal well deployment.Although the NMR saturation value of the shallow lake sand bars is high,the whole section is scattered and has poor continuity,reflecting strong heterogeneity.