| The Yanchang formation of Ordos Basin is widely known as tight reservoir, which is comprised of 10 oil groups. In these oil groups, Chang 7 oil group and Chang 4+5 oil group aren’t taken as the most important exploration strata because of the influence of obviously extensive lake transgression, relatively higher clay content, thin sand layer and the low development degree of sandstone. With continuous deepening of the exploration in the recent years, it has been found that Chang 4+5 oil group highlights the stronger potential of oil-bearing, which has carried on the more in-depth studies and achieved good exploration effects in the areas such as Jiyuan, Baibao, Shanbei. However, as the exploration degree of Chang 4+5 oil group in Zhenbei-Huanjiang area is extremely low. In 2010, the industrial oil flow has began to be obtained in Chang 4+5 oil group from the multiple wells of Zhenbei-Huanjiang area, which indicates a certain degree of urgency for the fully understanding of the distribution law of sandbody. Therefore, this article has carried on a more in-depth study for the characteristics, the origins and the distribution law of Chang 4+5 oil group in Zhenbei-Huanjiang area on the purpose of providing some exploration guidance and some academic reference value, which has been starting from the angle of new area and new formation, on the ground of the characteristic of tightness of sandbody and the relatively limited data.It is thought for the article that taking the sandbody structure as the main line of the research and referring to the previous views of predecessors on the theoretical basis of reservoir sedimentology, reservoir geology, lithofacies palaeogeography and so on. Firstly, according to the data of Outcrop, core, logging, particle size and element geochemistry, it has been analyzed the provenance and tectonic setting of the source region, the macro-structure, the characteristics and distribution of logging structure, sedimentary facies, and discussed the origins of patterns of sandbody distribution as well; Then, combined with outcrop, thin section, SEM, mercury, physical properties data, the research has been made for studying the characteristics and distribution of microscopic structure and reservoir structure, building the prediction model of logging porosity and analyzing the main controlling factors of the reservoir; Furtherly, the research was carried out the analysis of the main controlling factors of oil bearing, which conbined with the characteristics and distribution of sandbody’ oil bearing, the establishment of integrated favorable area prediction standard of Chang 4+5 oil group’s sandbody in Zhenbei-Huanjiang region by building the favorable area prediction standard of single factor. Finally, distribution of the favorable areas of sandbody was concluded. The followings are the main conclusions and innovative achievements:(1)The provenance is mainly from the Longxi ancient land in the southwest, and secondly comes from Alashan ancient land in the northwest, which contains the more felsic component and less mafic component. The provenance area is mainly located in the tectonic settings of continental island arc and active continental margin.(2) The development of sand body in the Chang 4+5 oil group is relatively low, which has the macro-structural characteristics that contain the thinner single layers, the development of barrier, sand-mud interbedded, and various bedding combinations(3)There are 4 kinds of logging structure types: box-shaped, bell-shaped, funnel-shaped and finger-shaped, being with many different kinds of superposition combinations. The former 3 types can be divided into 2 sub-types, which is based on the changes of GR curve’s amplitude forms and the values of GR; It is found in the distributed features thatⅠfunnel-shaped type has the highest proportion and the largest distribution area which is unfolded like lobes and planar shape, the second are finger-shaped type and Ⅱfunnel-shaped type, which spread around Ⅰfunnel-shaped type with the flat pattern. However, the Ⅰbox-shaped and bell-shaped take the lowest proportion and mainly develop in the west part, southwest part of the research area or the internal part of Ⅰfunnel-shaped type, which distributed with the shapes of stripe or small clumps, and Ⅱ box-shaped type distributed in the east or southeast part of the research area as the shape of isolated small clumps.(4)The sandbody of Chang 4+5 oil group is mainly located in the environment which from the delta front to the coastal-shallow lake sub-facies, in which the principal microfacies is the mouthbar that widely distributed in the horizontal and vertical orientation. The whole distribution features are showed that there is a small amount of distributary channel subfacies of the delta plain with developing large delta front lobes in the southwest, small and medium lobes in the northwest and flat microfacies of sand, mud, or mixed of the prodelta-coastal and shallow lake subfacies in the east and southeast part. Forming in the setting of shallow water delta, the sandbody of Chang 4+5 oil group is controlled by frequent changes of lake level in the short multi-phases, its distribution mechanism is mainly overlaying migration of distributary bar.(5)The reservoir lithology is mainly made up of medium-fine grained feldspar debris sandstone and debris feldspar sandstone, which have the characteristic of the low compositional and high structural maturity.The main type of the pore space is the residual primary intergranular pore, and the second is feldspar dissolved pore; The pore structure types are mainly divided into the medium-small pore microscopic fine throat and fine-microscopic pore microscopic throat, whose pore structure and space have relatively poor sorting and strong micro heterogeneity. The whole physical property is poor with characteristics of low porosity and low permeability. Major controlling factors in reservoir have closely relations with sedimentary microfacies, terrigenous clastic components and diagenesis. Subaqueous distributary channel and mouth bar have better porosity and permeability. The composition, content and characteristic of terrigenous detrital particles are the material foundation of reservoir development and late diagenesis alteration. Compaction and cementation are destructive diagenesis, which are the main factors of the reservoir tightness, while dissolution, kaolinization of feldspar and chlorite rim are constructive diagenesis, which promote the development of reservoir space.(6)The oil-bearing formation of Chang 4+5 oil group is mainly comprised of low and extremely low producing layers, with the complexity of fluid-bearing feature sandstone. The main controlling factors have closed relation with sedimentary microfacies, the physical properties and the thickness of single sandbody. The oil-producing formation is mostly distributed in the sandbody of mouth bar, a few in sand flat and sand-mud flat. The better physical property is, the higher oil-bearing possibility is, and the larger thickness of single sandbody has a positive correlation with the amounts of the medium-large production layers. Therefore, according to characteristics of logging structure being reflected to the microfacies, physical property, thickness of sand-body, the standard of comprehensive evaluation of favorable area prediction has been built on the basis of storing-capacity coefficient and logging structure. In the end, the study has arrived at the distribution of favorable sandbody of chang 4+5 formation in research area, in which, the most favorable area which owning medium-large production capacity have less development with the isolate distribution, but the favorable areas and relatively favorable areas which have medium-low production capacity are widely distributed in the research area as the lobes and planar shape. The favorable distribution range in Zhenbei area is wider than that in Huanjiang area, and the favorable area which in Chang 4+51 is larger than Chang 4+52. |
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