The Effects Of Clay Minerals On Production Capacity Of The Chang-8 Formation Sandstone Reservoirs In Jiyuan-Haqing Area

Ordos Basin, Jiyuan-Huaqing region is the main output regions of the oil. Chang-8 formation is the main producing oil reservoir spaces, but the layer reservoirs with low porosity and permeability characteristics. By study on the clay minerals of Chang-8 formation, the result play the great role on the exploration and development of oil field.The paper discuss the petrological characteristics,reservoir properties,pore structure and distribution, combined type and existing state of clay minerals of Chang-8 formation sandstones on the basis of lots of data of X-ray diffraction (XRD), scanning electron microscopy (ESM), polarizing microscope, cathodoluminescence, mercury injection, conventional petrophysical properties analysis etc. It is indicated that rock type are lithic feldspathic sandstone and feldspathic lithic sandstone, compositional maturity of rock is lower, Q1/(F1+R1) index is below the 0.8, maturity structure of the rock is comparatively high with the fine or medium of the grain size, the middle or good of the sorting and the second angular of the roundness in the area, which are the material resources of the clay minerals diagenesis and is also a basic condition of properties and production of sandstone reservoir. Three main clay minerals can be identified in Chang-8 formation reservoir sandstones in Jiyuan-Huaqing area, they are illite, chlorite and kaolinite. Among them, the content of illite is at the utmost and chlorite has the secondary content. The existing state of illite which is fibrous, hair-like, flaky and particle coating presence in the region; The existing state of chlorite is pore lining which mainly distribute in Huaqing region; Pore-filling kaolinite which primarily concentrated in Jiyuan area. In the layer, the content of illite, chlorite and kaolinite have shown more in the upper section of Chang-8 formation than the lower section. In addition, the porosity characteristics in sandstone reservoir of the study area show that the core plane porosity of Jiyuan region is better than Huaqing region's, and the core plane porosity of the lower Chang-8 formation is better than the upper face. The pore types are mainly the primary intergranular pores and secondary solution pores which are better in Jiyuan area than Huaqing where has the better micro-cracks. Upper layer of Chang-8 formation relatively developed secondary porosity, while the lower of Chang-8 formation with the intergranular pores.The distribution and existing station of clay minerals are mainly controlled by the deposition and diagenesis. Lots of illite is the matrix in the reservoir sandstone in the study area, and some authigenic illite is the alteration of feldspar in the conditions which formation fluid is alkaline. The relationships between porosity/permeability and illite are negative relationship, so a lot of illite in reservoir sandstone have great destructive effect on porosity and permeability of sandstone reservoir. The analysis of single-well reveals that, as long as relatively high content of illite layers, the result of formation testing and well log interpretation is almost water layer or dense layer, while the result of sensitivity test data analysis show that illite has some speed-sensitive and water sensitive to the external fluid in sandstone reservoirs. So we can infer that illite has the great negative impact on reservoir capacity in the reservoir sandstone. In the early diagenetic stage of iron, magnesium and other conditions (such as the delta environment, the rich volcanic rocks chips) generated a large number of authigenic chlorite and continue to growth in the late diagenetic stage which protect those of secondary porosity and the original intergranular pores by resistance to the compaction of overlying rocks and control the increase of secondary quartz, making the reservoirs have favorable physical properties. Authigenic chlorite with a propoetional interrelation with permeability or porosity, whlic play a positive role to the protection of the reservoir properties. But because of chlorite which grow along the porewall and narrow the pore radius so that pore structures worse. The result of formation testing and logging interpretation conclusions is oil or water reservoir in the layers with more content chlorite, well productivity is also better, such as Huang-55 well what oil production was 16.15t. But in the development process needs note that the sensitivity of chlorite to the external sour fluids damage reservoir. Authigenic kaolinite are mainly relate to the solution of aluminum silicate in acidic conditions, for it was the main secondary minerals of feldspar dissolution. There has relatively developed secondary pores as long as the development of kaolinite in sandstone reservoirs. However, kaolinite filled the pores so that it has a negative impact on pore structure. Second, the result of formation testing and well log interpretation conclusions is dry layer in the layers with more kaolinite content, in particularly association with illite, reservoir properties are relatively poor. However, there are plenty of kaolinite and chlorite in the well section with lower illite what the reservoir properties is better, the productivityof single well is better. In addition, because of its multi-sensitivity to external fluid makes production capacity of the reservoir decreased.