In order to reveal the reservoir characteristics and their influences on poroperm characteristics of the Lower Cretaceous in Chagan Depression

core

thin-section

and reservoir analysis data were utilized to research the petrological characteristics

sedimentation and diagenesis of the reservoirs.The results show that the sandbodies of fan-delta facies in the Lower Cretaceous consist of lithicarkose and feldspathic litharenite with low compositional maturity

low maturity structure and high cement content

which are originated from proximal and rapid accumulation.The reservoir is relatively tight as a whole and is of ultra-low porosity and permeability reservoir type(with average porosity of 6.3% and permeability of 7.78×10

-3

μm

2

).Thin-tiny throats are well developed and the reservoir spaces are dominated by residual primary and dissolved intergranular pores

and their development are controlled by particle size

compaction

cementation

content of carbonate and the origin of organic acid.The favorable reservoir is mainly distributed in the sandstones(conglomerates)of the subaqueous distributary channel of fan delta front

followed by fine sandstones in sand sheet and mouth bar.Vertically

the secondary porosity zone is mainly developed in the depth interval of 2 100-2 800 m

which corresponds to the maturity stage of organic matters.The mature organic matter released large amount of organic acid which strongly dissolved the feldspar particles and carbonate cement

enlarged intergranular and intragranular pores and effectively enhanced reservoir quality.Obviously

favorable sedimentary microfacies

strong dissolution zone and secondary porosity zone are the main factors controlling favorable reservoir distribution.