Study On Cretaceous Hollin Sedimentary Facies And Sequence Stratigraphy In The Middle And North Of Oriente Basin Of Ecuador

Dozens of oil and gas fields in Oriente basin have been found since 1921.Primary reservoirs are the Cretaceous Napo and Hollin sandstones(Higley,2000;Lee et al.,2004).However,the sedimentary and sequence stratigraphy study of Hollin formation is less than its above strata(Napo U,T,M1 and M2).With the exploration level increasing,the target is moving towards Hollin formation.A lithology cycle varying from medium-coarse grained sandstone,glauconite sandstone and sandstone to shale and micritic packstone develops in study strata.Three types of sedimentary environments have separately developed in Main Hollin Formation,Upper Hollin Formation and Lower Napo Shale Formation(C Limestone Formation).Mainly,fluvial sedimentary system developed in Main Hollin Formation: a.oil immersed coarse-medium grained sandstone with inclined bedding,parallel bedding and erosion surface;b.SP normal cycle combination;c.imbricated seismic reflection.The subfacies are channel fill and channel margin.The microfacies are channel,natural levee and crevasse splay.Barrier coast sedimentary system developed in Upper Hollin Formation: a.shale-oil bearing fine grained sandstone with flaser bedding,lenticular bedding,deformation bedding,bidirectional cross bedding,bioturbation,ripple mark and mud pebble;b.bell,linear and finger-shaped well loggings;c.hummocky,lenticular,concave-down,filling shaped seismic reflection.The subfacies are barrier island,tidal flat,tidal channel-flood tidal delta and lagoon.The microfacies are barrier flat,sand flat,mixed flat,mud flat and storm sandstone,tidal channel,lagoon mud and lagoon shoal.Lower Napo Shale and C Limestone develop shelf face-inner shelf subface-shelf mud,shelf packstone,glauconite sandstone and quartz arenite.Study strata(Main Hollin?Upper Hollin?Lower Napo Shale and C Limestone)is identified as one 3rd order sequence(Sequence 1).Sequence 1 bottom boundary is an unconformity truncation in seismic reflection,so the sequence boundary 1 is defined as type I.As a result,sequence 1 is divided into LST,TST and HST.LST,TST,HST are respectively corresponded to MAIN_HOLLIN_SS,UPR_HOLLIN_SS(HOLLIN_FM)and LWR_NAPO_SH(C_LS).