Sedimentary Environment And Paleoclimate Analysis Of LTA Formation In The North Of South Sumatra Basin

Predecessors' understanding towards LTA formation in the north of South Sumatra Basin concentrate on tectonic evolution,the law of hydrocarbon accumulation and so on,while the study of types of sedimentary micro-facies and their characteristics under the control of a high-resolution sequence stratigraphy framework is relatively rare.There has been a dispute over whether transgression began during LTA formation,that is,whether it was marine or continental facies.Taking the theory of high-resolution sequence stratigraphy as a guide,this paper finely depicts evolution of depositional systems at S-G-R fields in the north of South Sumatra Basin based on an isochronous stratigraphic framework during LTA formation according to data of 3D seismic,conventional logging curves,FMI imaging logging as well as core photographs.This paper holds that G deep subsag and R deep subsag can be divided into 5 fourth-order sequences,however,S uplift-subsag can only be divided into 3 fourth-order sequences.Located in marine-continental transitional zone,G field and R field contain multiple kinds of sedimentary micro-facies.Among them,alluvial fan,braided river and meandering river are found at G field in the beginning,and later tidal-influenced delta until estuary and tidal flat.On the other hand,R field deposited lagoon in LST,with its water limited and low-energy,which belongs to barrier coast sedimentary system.It gradually converted into tidal flat in TST,and ultimately meandering river in HST.As for S field,since it was positioned close to paleouplift,erosion occurred in LST.Then in TST,expanding-lake and lake-bay came into being.Finally,there came the development of normal shore-shallow lacustrine in HST.In addition,thin coal seam always developed within study area during LTA formation,which is nowadays one of the most significant exploration strata of lithologic-stratigraphic oil and gas reservoirs.This paper shows that coal seam that has a mean thickness of 0.3m,usually between 0.1m and 0.8m,generally appeared amid tidal plat deposition of fourth-order sequence 4 and flood plain swamp deposition of fourth-order sequence 5,accompanied by pyrite,making LTA formation a favorable reservoir facies belt.Furthermore,this paper analyzes the control function of paleoclimate and eustatic sea level variation over evolution of sedimentary systems within study area for the first time.Through paleoclimate change revealed by spore-pollen data,the author draws a conclusion that it was the frequently periodically fluctuating paleoclimate during LTA formation that dominated the growth of expanding-lake and lake-bay at S field.Moreover,eustatic sea level variation during LTA formation is bound to be the direct cause of evolution of sedimentary systems at G field and R field that possess the characteristics of both marine facies and continental facies.