Geological Genesis Of Low Permeability Reservoir And Disstribution Law For High-quality Reservoir Of The1^st Member Of Xishanyao Formation In Baijiahai High, Junggar Basin

Baijiahai high is located in the centre of Junggar Basin. It is a nose-like high trendingsouthwest and one of the important exploratory and developmental blocks of Cainan oilfieldof PetroChina Xinjiang Oilfield Company. The large-scale structural and lithologic trapsystem was developed in1stmember of Xishanyao Formation in structural high part of theuplift (Cai9-Cai10well area). It is generally characterized by low-middle porosity and lowpermeability (the average porosity is14.6%and the average permeability is2.81×10-3μm2).The block has been in high water cut stage during the middle and later Period of OilfieldDevelopment. However, the knowledge of geological genesis of low permeability reservoirand distribution of high quality reservoir are not clear which makes it difficult to resolve theproblem of the oil-water distribution law in development. Furthermore, high water cut stageoccurs during fracturing development period presenting serious injection-productionproblem which restricts the efficient development of the oilfield.Under the guidance of sequence stratigraphy and reservoir sedimentology and throughthe analysis of core description, logging, seismic and production performance data, Isystematically study stratigraphic framework, structural feature, paleogeo graphy, faciesdistribution and reservoir diagenesis. By integrating with typical well production history,main geological genesis of low-permeability reservoir and distribution law of high-qualityreservoirs are deeply analysed which provide reliable basis for subsequent developmentadjustment, progressive evaluation of exploration and development of the oil region. Themain innovation and conclusion of the thesis are as follows:(1) Through the study of the palaeogeomorphology by analyzing well data and seismicdata, the results indicate the research area was a wide-gentle NW-trend homocline duringthe period of Xishanyao Formation. Two large-scale depositional systems were developed inthe East(Cai10-well area) and West(Cai9-well area) under the influence of sedimentsupply from the north and northeast source area.Two lobate–type deltas with thick layer ofsand bodies which stacked and distributed contiguously were formed in the west area.Owing to multiphase stacked braided subaqueous distributary channels, single well dataindicates the average cumulative thickness of sand was23.1m and is characterized bybox-shaped in common. In contrast, four birdfoot-type deltas were formed in the east. Thecharacteristics of sand bodies are moderate-thin layer of sand and isolated distribution.Single well data indicated average cumulative thickness was10.3m, and well logging curveshape is bell.(2) Three major aspects about geological origin of low permeability reservoirs in thestudy were obtained. Firstly, because of the fine particle size (mean3Φ) and high plastic volcanic debris content (average content of17%), the compaction rate of pore throats issignificantly greater than that of pore. Secondly, throats were plugged by kaolinite cementslargely. Thirdly, the acidic water solution generated from overlying coal seam acceleratesthe dissolution of silicate minerals. Due to the combined effects of these three aspects, thethroat of sandstone reservoirs rapidly becomes thinner (throat radius range from0to5μm),reducing the permeability of the reservoir greatly, and the reservoir is characterized bymoderate porosity and low permeability. Furthermore, the high reservoir irreducible watersaturation results in water flooded seriously during the early fracturing development stage,whereas oil production remained relatively stable in the later stage.(3) High quality reservoir is mainly controlled by the sedimentary hydrodynamiccondition; The strong hydrodynamic channels at gentle slope (main channel, subaqueousdistributary channel) are characterized by less plastic particles and good physical property.These channels belong to reservoir typeⅠ. Whereas the mouth bar are always formed atsites where gradients of slope are changing. The sedimentation rate, particle size and thecontent of plastic particles are larger than the channel sandstone which are classified asreservoir type II. The characteristics of reservoir of this type are poor physical property.(4) The high-quality reservoir are influenced and controlled by the scale of sand body,distribution of sedimentary facies and effective thickness of sandstone. According to theanalysis of these three controlling factors, the distribution of reservoir was predicted. Thedirection of reservoir distribution in the west is consistent with the direction of the mainsubaqueous distributary channel. The shape of the reservoir on flat surface in the west islike string whereas the shape of reservoir in the east is isolated.