Sequence Stratigraphy And Sedimentary System Of U-T Section In Block T, Oriente Basin, Ecuador

The clastic rocks of the U-T member of the Upper Cretaceous Napo Formation in the Oriente Basin,Ecuador are the most important oil-bearing strata in the Tarapaa block.At present,the provenance of the U-T member of the study area,the type of sedimentary system,and the distribution pattern of sedimentary facies are still unclear,which limits the next oil and gas exploration and development.In this paper,cores,thin slices,wireline logging,mud logging,and geochemistry analysis data are used to study the sequence stratigraphy and sedimentary system of U-T Member of Upper Cretaceous Napo Formation in the Oriente Basin.Studies show:(1)There are five 3^rd-order sequences in the Napo Formation,which are KSQ2,KSQ3,KSQ4,KSQ5,and KSQ6.T member corresponds to the shelf marginsystem tract and transgressive system tract of KSQ2;B limestone between U and T corresponds to the highstand system tract of KSQ2;U member corresponds to the shelf margin system tract and transgressive system tract of KSQ3;The limestone A overlying the U member corresponds to the highstand system tract of KSQ3.The T member is further divided into 4 parasequences,and the U sandstone is subdivided into6 parasequences.(2)Based on the core description,a large number of bidirectional cross bedding and mud drapes indicating tidal action,and the vertical succession of the U and T members are all progressively deepening upwards.The geochemical characteristics of mud drapes indicate thetransition environment.It is believed that the sedimentary systems in the U and T members are mainly tidal-dominated estuary and tidal-dominated shelf sedimentary systems.(3)Ten types of lithofacies are recognized,which are further grouped into 6 types of microfacies,including tidal channel-tidal bar microfacies,sanddy flat microfacies,muddy tidal flat microfacies,glauconitic sheet sandstone microfacies,shell beach microfacies,and muddy shelf microfacies;The study area mainly develops the middle and outer estuary subfacies.The middle estuary subfacies mainly develop tidal channel microfacies.The outer estuary subfacies develop tidal bar and sandy tidal flat microfacies.Tide-dominated shelf developed glauconitic sheet sandstone,shell beach,and muddy shelf facies.(4)Both U and T members experienced rapid regressive-slow transgressive sedimentary cycles,and the sedimentary facies belt moved from the southwest to the northeast of the study area.The river flows from the northeast to the southwest of the study area,and gradually transitions to the tidal-dominated estuary and tidal-dominated shelf environment.(5)The tidal bar microfacies extending from southwest to northeast are developed on the outer estuary axis,which are reservoir facies?.Tidal channel developed in middle estuary are considered as reservoir facies?.Reservoir facies?is sandy tidal flat facies while glauconitic sheet sand facies represent Resevoir facies ?.