The structure of Jiyuan region in the Ordos Basin was relatively stable during the deposition of Chang 8

2

.In this period

a shallow water delta depositional system was developed

the lake shore line slightly migrated in the southeast

and distributary channel and underwater distributary channel constituted the deposition framework.This paper studied the diagenesis of Chang 8

2

reservoir based on large amount of thin sections

core observation

SEM samples

X-ray analysis

and Cathodoluminescence analysis.The study shows that the diagenesis has double effects on the reservoir.On one hand

early development of chlorite film has a positive effect on anti-compaction.The early calcite is easy to be dissolved by organic acid

resulting in secondary pores and improving poroperm characteristics.Dissolution is the major mechanism of secondary pore formation and can greatly improve reservoir poroperm characteristics.On the other hand

compaction destroys the primary porosity formed in early digenetic process

and late ferrocalcite fills pores

resulting in local strong calcareous cementation zone.In general

the sedimentary facies and diagenetic facies control the distribution of oil and gas.The former determines the reservoir shape and dimensions.While the later modifies reservoir pore space.Late diagenesis-modified chlorite-cemented sedimentary facies with residual inter-granular pores and that with feldspar-dissolution plus residual inter-granular pores are the major reservoir spaces for hydrocarbon accumulation.