The Study Of Sedimentary And Sequence Stratigraphy Characteristics For Carbonera Formations Of The Llanos Basin

The study area of this article is Llanos Basin, Colombia, which is located in the north of the South American plate. The target layers are Carbonera Formations in Eocene-Miocene. As the sedimentary environment of Llanos Basin is known, this paper aims to study the rock type, depositional environment, sequence stratigraphy of the target layers deeply. Based on sequence stratigraphy, sedimentary petrology, structural geology, seismic stratigraphy and other theories, this paper will refer to well drilling and seismic data to identify the the sequence boundaries and the sequence stratigraphic framework. On this conditon, we can further study the sedimentary facies distribution and depositional environments of the target layers and the control factors of sequence development.According to the former research and current material,it is learned that the evolutionary process of Llanos Basinhas experienced five stages,which are Triassic-Jurassic synrift, Cretaceous back-arc subsidence, Paleocene-early Eocene accretion of western Cordillera and early foreland basin, middle Eocene-early Miocene pre-andean foreland basin, middle Miocene-recent Andean foreland basin.On the basis of regional background, core obeservation and seismic study, this article aims to study lithofacies assemblages, sedimentary structure,fossil assemblage and so on. It is found that Carbonera deposition system of Llanos Basin can identify delta, shelf sedimentary facies, delta can be subdivided into delta plain, delta front and pro-delta and shelf develops inner-shelf subfacies.Delta front sub-facies develops in C1, C3, C5, and C7 segments, including sandy underwater distributary channel and mud-based underwater distributary microfacies. Prodelta deposition sub-facies develops in the C2, C4, C6, and C8 segments, which can be further recognised frontal sheet sand and prodelta mud microfacies. Continental shelf develops in the C2, C4, C6, and C8 segments, which contains shelf mud and sheet sand microfacies.Under the background of the geologic structure and other fundamental geologic theories, it is concluded that in Carbonera formation there are 4 sequences(SQ1~SQ4)which have unconformable top surface. Every sequence can be delimited by mfs to 2 systems tracts(TST, HST). As the study area lacks LST because of the exposure, there are 8 plane graphs of sedimentary.Compared the sedimentary and seismic facies of single or connected wells in the Llanos Basin with the the deposition plane facies of eight Carbonera system tracts, the ariticle establishes the seismic facies of sedimentary facies in studying area. A total of three sedimentary facies unit and eight corresponding seismic facies units can be identified.What’s more, on the combination of well and seismic, the third-order sequences and systems tracts framework can be established, and eight deposition plane facies of each systems tracts are be worked out. At last, this paper also analyzesthe depositional model of the basin and the depositional model graph is also drawn. Through the analysis of the controlling factors of the sequence development, this paper illustrates that tectonic movement and sea-level fluctuation are the main causes of these systems tracts and the depositions.