Characteristics and controlling factors of Silurian reservoir in northern slope of central uplift in Tarim Basin

Tazhong northern slope is located in the central uplifting belt in Tarim Basin. Silurian is mainly tidal-flat facies sandstone and mudstone overlapped on the surface of Ordovician palaeotopography and palaeogeomorphology in the area. On the basis of detailed petrologic study, this paper expounds the (characteristics) of the Silurian (mainly lower section sandstone) reservoir. The compositional maturity of Silurian sandstone is totally lower, that is, the lithology mainly be lithic sandstone. But there exists several interlayers of higher mature quartz sandstone in the lower subsection of the lower section sandstone. Their textural maturity is higher. The compactional diagenesis of the reservoir is stronger (leading to the linear inter-granular contaction generally), so is the cementation (mainly that of carbonate, quartz and kaolinite). Dissolution in the reservoir is commonly weaker. Yet there locally exists layers in which (dissolution) (mainly that of carbonate) is stronger and they may have significant effect on the characteristics of reservoir. The main pore types in the reservoir are as follows: primary intergranular pore, innergranular (feldspar and rock debris) dissolution pore, carbonate cement dissolution pore and micropore. The pore types and combinations vary horizontall and vertically. Intergranular pore and micropore are dominant in most layers, whereas carbonate cement dissolution pore has a relatively big ratio in some layers. Further the paper sums up the dominant factors affecting the reservoir characteristics. Among them, compaction and cementation are most important. The lithologic features, such as clastic component and grain-size, are also important factors, and the relatively good quality reservoirs are basically middle-fine grained quartz sandstone or close quartz sandstone in compositionally.