The Research On Comprehensive Evaluation And Reservoir Characteristics Of The Silurian And Devonian In Bachu-maigaiti Area, Tarim Basin

Bachu-Maigaiti area is a rich oil and gas bearing area and located in the Midwestof Tarim Basin, across two subtectonic units of the Bachu broken ridge and Markitslope. The structure conditions are complicated, especially the western part of Bachuuplift. Since2000, oil and gas shows and low-yielding reservoirs have been seen manydrilling wells in the Devonian reservoir such as Qun6, BT4and BK8, which indicatesthat the Silurian-Devonian clastic reservoirs that the research area have goodexploration prospects. However, the exploration degree is relatively low and there arelots of factors affecting the good reservoirs, which hindered the further exploration anddevelopment of oil and gas.Using core and outcrop observation and description, well logging and seismic dataanalysis, and microscopic identification and test analysis of rock samples, combinedwith the research results of r sedimentary facies and diagenesis, we complete thereservoir sand body genesis and distribution, the reservoir physical propertydistribution, reservoir heterogeneity and its influence factors. Then we choose thequalitative and quantitative parameters which can reflect reservoir characteristics to dothe mathematical statistical analysis and use the fuzzy mathematics assessmentmethods to complete the comprehensive evaluation of the reservoirs in this area andthen predict favorable exploration areas on this basis.The reservoir rock characteristics analysis showed that the rock types of Silurianreservoirs are mainly debris quartz sandstones and debris sandstones; the rock types ofDonghetang Formation in Silurian reservoirs are mainly quartz sandstones andfollowed by debris quartz sandstones; the rock types of Keziertage Formation aremainly debris quartz sandstones and quartz sandstones. Bottom up, debris content is in adecreasing trend in all the formations while the feldspars do not develop well and areonly rich in some layers of some areas. The quartz sandstones and debris sandstonescan be been, too.Profiles and drilling core observations in Bachu-Maigaiti area show that reservoirrocks of Silurian-Devonian were formed in the coast of shallow water, and there alsodeveloped wave based near-shore deposits and tidal-based tidal flat deposits. The camemainly from the southwest and southeast directions, and a certain size of deltadepositional system developed along the materials source directions. The distributioncharacteristics of the sand bodies in the research area are mainly affected by thestructural control of the source supply and the syngenetic faults in the northeast. The thickness of downthrown side of syngenetic faults increased significantly, and the sandbodies distributed along the north-east obviously.With the analysis of the physical property data of the core samples, we establish thelinear relationship between porosity, permeability and acoustic time in Silurian-Devonian reservoirs of Bachu-Maigaiti area. The results show that KeziertageFormation has the best reservoirs with the an average porosity of8.77%and an averagepermeability of15.46×10-3um2, which are special-low porosity and permeabilityreservoirs; the Donghetang Formation has the second best reservoirs with the anaverage porosity of4.46%and an average permeability of2.12×10-3um2, which isspecial-low porosity and permeability reservoirs and low porosity and permeabilityreservoirs; followed by the Kepingtage Formation and the Tataaiertage Formation, theaverage porosities are3.59%and4.63%respectively while the the averagepermeability are2.18×10-3um2and0.19×10-3um2respectively, which are special-lowporosity and permeability reservoirs and ultra-low porosity and permeability reservoirs;the Yimugantawu Formation has the worst reservoirs with the an average porosity of2.47%and an average permeability of0.24×10-3um2, which are ultra-low porosity andpermeability reservoirs.On constraint conditions of individual wells’ physical data, seismic velocity data isinversed in order to grasp the reservoir development characteristics and the reservoirdistribution in the drilling blind area. The Devonian reservoirs are mainly distributed inthe north-west and central parts of the study area while the Silurian reservoirs aremainly located in the northwest of the study area and He4wells area in the east with theoverall performance of physical properties decreasing from north to south and fromwest to east. In addition, the reservoir properties changes with the sedimentary facies.For example, in the delta depositional system, the distributary channel sand bodies havethe best physical properties, followed by sheet sand bodies and then the mouth bar sandbodies; in the near shore depositional system, the beach sand bodies have the bestphysical properties while the shoreface sand bodies have the worst. Diagenesis andearly hydrocarbon injection are the key factors of reservoir properties changes.Selecting12parameters of4main categories such as reservoir depositional setting,lithology, physical properties, spatial pore structure, we evaluate the Silurian-Devonian reservoirs comprehensively. The results show that there developed the betterclass of reservoirs (II) in Donghetang Formation and Keziertage Formation, mediumclass of reservoirs (III) in Kepingtage Formation and the Tataaiertage Formation andbad class of reservoirs (IV) in Yimugantawu Formation. According to the classificationstandards of reservoirs’ profitable areas, the Devonian stratums mainly develop bestand better profitable reservoirs, and some prospect reservoirs, while the Silurianstratums mainly develop better profitable reservoirs and prospect reservoirs.