The Research For Sedimentary Character Of Ⅱ Oil Series Of Lower Xingouzui Group In Xingou Area

Qianjiang Depression, which is located in the middle of Jianghan Basin, is also an important oil production reservoir in the basin.Under the exploration and development of dolomite reservoirs from Ⅱ oil Series in Xingou Area in South Qianjiang Depression, the result of research has showed that it has great exploration potential on tight oil resources from Ⅱ Series in Xingou Area, because we mostly focus on the II oil Series in central and northern of Xingou Area but awareness of the southern region is relatively low.This thesis focus on II oil series,which is from Lower Xingouzui group in Xingou Area of Southern Qianjiang Depression,by ways of core observation, thin section identification, comprehensively compare and systematicly divise the Ⅱ oil series from Lower Xingouzui Group, through petrology, paleontology and lithofacies characteristics, identify many kinds of subfacies and microfacies, on this basis we study on the single well sedimentary facies and well profile, make it clear that the area of influence for the main controlling factors of the distribution of sedimentary facies division and the distribution of sedimentary facies, and accomplish the establishment of the model,combined with the control effect of dolomite genesis of reservoir properties, predict the favorable facies belt and advantage the spatial position of block distribution.The types of tight oil reservoir rock in II oil series are mainly dolomite and thin interbedded mudstone,and sandstone is quite rare,which is only found in the northern part of the study area near the provenance of region. On the basis of lithofacies assemblages in the longitudinal direction, and the transverse distribution of the sedimentary section of the well connected, that this area mainly developed shallow lake sandbar, lake flat and limited lowland subfacies, because of Xingou Area away from the north of the main source area, the study area rarely develops the shallow lake sandbar; Lake flat subface is in the relatively high site of shallow lake, located between the dry season the lowest water level line and the normal wave base area, with shallow water, can be further subdivided into the mud flat microfacies and dolomite flat microfacies; limited lowland subface in the shallow lake,which is relatively low-lying and not much wide lake area of Lake flat,usually locates below the normal wave base, which is basically not influenced by wave in case of storms and other special conditions, can be divided into shale, dolomitic depression depression two microfacies.Through the research on the characteristic of rock, mineral,cathode luminescence characteristics of carbon and oxygen isotope geochemistry characteristics, combined with the summary of previous lacustrine dolomite genesis, we think only quasi contemporaneous phase penetration reflux dolomitization model can explain sedimentary characteristics of dolomite in the study area.Take part in dolomite samples observed under scanning electron microscope(SEM),it’s visable for evaporite minerals such as gypsum and salt rocks,producting coexisted with dolomite, instead of filling into the crack,intergranular pore and dissolution pore.Dolomite cathodoluminescence(CL) characteristics in this area are mainly the two types:orange and red orang.which belongs to the characteristics of the early burial.By means of the order degree of dolomite in this area of the X diffraction, they all have low values, combined with the fact that so is the characteristic values of lacustrine facies dolomite in domestic of other region,we infer that there are two main reasons on low values: ①the deposition of rift basin of China Continental dolomite formation age of late geological history in general.During the period of process is short, the transformation activity is not strong.② Lacustrine dolomite general size is fine,the mud crystal structure is very compact,the diagenetic stage or later stage of magnesium rich fluid from outside is difficult to have large-scale contacts, thus making it more difficult to dolomitization occurred many times to raise the order degree.The analysis for the result of high mole fraction of dolomite calcium carbonate and dolomite and its low order degree, indicates the formation for dolomite in the lower temperature and high speed of nucleation and crystallization of diagenetic environment. Through the study of the carbon and oxygen isotopes found in dolomite, the dolomite of δ13C values seem positive and δ18O values are negative, corresponding with the results of experiment and calculation of cathodoluminescence and the degree of order, can be inferred dolomite from Xingou Area form in the environment of blocked water with evaporation, high salinity, the characteritics of low temperature,and shallow burial refer to quasi syngenetic dolomitization characteristics. By scanning electron microscope(SEM),there is microcrystalline dolomite in this area has nice crystal structure but in small-scale with high order degree that is buried diagenetic,but it can be only taken as a geological phenomenon in the area,without explaining the genesis of the massive dolomite.Based on the analysis of core,well data, scanning electron microscope(SEM) and petrophysical data, that the carbonate reservoir in study area mainly developed three lithology types:terrigenous micrite dolomite, micritic dolomite, argillaceous dolomite.The type of terrigenous micrite dolomite formed in dolomitic flat environment,. It has the best property,including the intergranular pore, dissolved pores, dissolution fracture, good porosity and permeability; Micritic dolomite mainly developed in the dolomitic flat and dolomitic lowland environment, the pore types include intergranular pore,dissolution pore and moldic pore,without the develop of dissolution seam, its dissolution pore and moldic pore in general is filled with dolomicrite, its property is the second best after the terrigenous micrite dolomite; Argillaceous dolomite mainly occurred in dolomitic mud depression depression and sedimentary environment, the pore types are intergranular pore and dissolved pore, filled with filamentous illite dolomite crystals, is the worst in three.Synthesis of core observation, the ancient landform, the shows of other oil&gas characteristics and distribution of the results, obtained that the dolomite reservoir in dolomitic flat formed in flat sedimentary environment in the study area shows the best, as the favorable reservoir facies belts; dolomite and dolomitic reservoir sedimentary environment. In the form of difference, the oil and gas shows poor, can not be used as the favorable reservoir facies belts.