A Research Of Semdimentary And Reservoir Characteristics Of Fuyu Reservoir In Dan Bei Area

Daan area is loacated in the Daan county of Jilin province, and lies in Daan structure in Honggang terrance of the Central Depression in the south of the Songliao Basin, west to westen slope, east to Gulong Sag. Daan structure is a big nose structure as a whole, controlled by Daan reverse fault.The reservoir is controlled by structure in the middle-east area respectively, by both structure and lithology in the west-north area. The target strata of research area is the Fuyu Reservior in the forth member of Quantou Formation of lower Cretaceous system, with the area of180km2and180wells.The already founded research show that Daan area has superior hydrocarbon accumulation conditions, however, Fuyu Reservoir has low porosity and ultra-low permeability, and the distributions of oil and water are controlled by multiple factors of structure, faults, lithology and sedimentation, therefore, present a very complicated situation. At the same time, as exploration and appraisal wells drilling in fuyu reservoir arerelatively are relatively less and unevenly distribute, and the existing studis of the sedimentation and reservoir characteristics, treating sand group as sedimentary units longitudinally, have not carried out in-depth enoughly to meet the need of fine reservoir description. Therefore, it is worth to establish isochronous stratigraphy framework, and deepen the understanding of the semdimentation and reservoir characteristic, search the main factors causing reservoir densification, and finally make a comprehensive evaluation for Fuyu Reservoir, lay a significant geological foundation for the further promate oilfield exploitation benefit.Based on the core observation and integrated logging data interpretation, using rock stratigraphy and high-resolution sequence stratigraphy as guide, applying point, line and surface trinity method, the isochronous stratigraphy framework of Fuyu reservoir of180wells is etabloshed.Analysing the core data, log data, particle size and regional geological background comprehensively, source area,macrofacies types and sedimentary evolution rule are discussed.The studis show that the main deposit sediment of Fuyu Reservoir in Daan area came from the northwest Yingtai-Honggang system, and there is also a minor source from southwest. Fuyu Reservoir deposited delta plain subfacies and delta front subfacies at the background of delta sediment.Twelve sedimentary microfacies, including distributary channel, natural levee, overbank sand, crevasse fan, crevasse channel, abandoned channel, interdistributary bay, subsea distributary channel, subsea overbank sand, mouth bar, sheet sand and subsea interdistributary bay, have been recognized, and the distributary and subsea distributary sands are the most important reservoir.By comprehensive application of the data of the microscope observation, electron microscope scanning and thin section identification, in-depth and detailed analysis has been conducted about the lithology characteristics and diagensis. The research shows that the composition maturity of Fuyu Reservoir is on a low side, and the sandstones are main feldspathic lithic sandstones, there is also a small amount of lithic arcose sandstone. The main diagenesis of Fuyu reservoir includes compaction, cementation, metasomatism and dissolution, etc. which belongs to stage A of middle diagenesis phase. Compaction and cementation change the physical property for worse, although the dissolution of unstable mineral, such as feldspar, is commonly seen, for the dissolved degree is weak, dissolution contribute a little to better physical property. As a result, the reservoir become tight.The main type of pore of reservoir rock is interparticle pore, followed by interparticle dissolved pore.The main throat type is micro throat and lamellar throat, then converging throats.The connectivity between different pores and throat coordination number are both very low. Fuyu Reservoir as a whole has low porosity and ultra-low permeability.Based on the studies of sedimentary characteristic,reservoir characteristic and diagensis, factors effecting reservoir physical property have been analysed and mainly controlling factors causing reservoir densification have been discussed. The studies show that deposit facies, compaction and cementation are the main factors controlling reservoir physical property development.High content of unstable component of clastic grains,such as feldspar and debris, and matrixs, sharply porous diminution caused by compaction and cementation, and lack of secondary pores jointly lead to reservoir densification.In order to characterize the tight sandstone more factually in research area, the study select characteristic factors including porosity,permeability, shale content, throat diameter, sandstone thickness and diagenetic facies type as well to conduct a comprehensive evaluation of the reservoir. There are4reservoir categories in the study area, that I is favorable reservoir, II is less favorable reservoir, III is moderate reservoir and Ⅳ is poor reservoir. Based on the comprehensive evaluation of the reservoir, under the guidance of the hydrocarbon accumulation pattern, the well blocks of priority hydrocarbon accumulation are selected, through the overlap between Planar graph of different reservoir types of every thin layer and Micro-structure map.