Diagenesis And Pore Evolution Of The 4+5 Low Permeability Reservoir Of The Yanchang Formation In The Eastern Hebei Province

Based on the previous studies, taking advantages of data from cast slice, size analysis, X-ray diffraction and SEM, this paper conducted a research on petrological characteristics, physical property and pore structure as well as diagenetic characteristics of sandstone in the study area. This paper discussed how diagenesis controlled the pore evolution, and summarized the characteristics of relatively high-quality reservoirs in the study area. The results show as follow:Characteristics of petrology of Chang4+5 reservoir group in Longdong area exhibited a northeast-southwest difference:in western and southwestern source areas, quarts and lithic tended to have a high content, the major debris were dolomite, quartzite and phyllite, and the predominant rock type was feldspathic lithic sandstone; while in northeastern source area, feldspar tended to be higher, and there were large amounts of mica flakes, and the rock type was mainly feldspar sandstone. As to the physical property and porosity structure, the physical property of Chang4+5 reservoir group was worse in general, among which the physical property of southwestern and northeastern source areas tended to keep the best, the western source area tended to be relatively better, while the southern part was the worst. The types of pore of Chang4+5 reservoir group were mainly of residual intergranular pores, then followed by intragranular holes. Statistics from mercury injection showed that the porosity structure of Chang4+5 reservoir group in the southwestern source areas was relatively better, while the southern part was the worst. The compaction of Chang4+5 reservoir group in the study area was strong, and some of quartz, feldspar and other rigid particles were breaked, while mica, phyllite and other plastic components exhibited bending deformation; the cementation was also strong, and the cement type of reservoir was dominated by iron calcite, iron dolomite, silicon and chlorite. The solution was developed, and the dissolution of feldspar and lithic was the most common type. In terms of diagenetic intensity, the apparent compaction, cementation and dissolution rates of Chang4+5 reservoir group were 47.25%, 81%,41.5% respectively, among which material sources reservoirs in southwest and west were stronger than that in northeast whether in destructive or constructive diagenesis(compaction and cementation). Chang 4+5 reservoir group in the study area was in the A period of middle diagenetic evolution stage, and its diagenetic sequence was as follow:early compaction?plastic debris extrusion and deformation?rock particles compression and break?formation of thin chlorite film?grade I quartz overgrowth?mica alteration?feldspa, lithic dissolution?clay minerals (kaolinite, illite) filling and cementation ?authigenic quartz, grade ? quartz overgrowth?iron carbonate filling and replacement. The dense reservoirs of Chang4+5 reservoir group in Longdong area were mainly affected by sedimentation and diagenesis, the relatively high-quality reservoirs were distributed in BaiBao-WuBao-ZiFangpan in northeastern source area, TongChuan,TaiBailiang in southwestern source area and MuBo in western source area.The microscopic characteristics were that they were dominated by rigid particles, including quartz, feldspar, dolomite and quartzite lithic debris, and the grain surface tended to appear chlorite film cementation. The type of combination of pore and throat was mainly macropore with meso-leptothroat and the solution was developed in feldspars.