Sedimentary Sequence,Provenance Study And Hydrocarbon Potential Of The Eocene-Oligocene Siliciclastic Rocks Exposed In The Ngahlaingdwin Area,the Salin Sub-Basin,Myanmar

Aim of the current study was to evaluate the actual potential for hydrocarbon prospects in association with interpretation of depositional environments and processes as well as the modal compositions of the sandstones,provenance characteristics,tectonic setting,degree of chemical weathering and paleo-oxidation conditions for the Eocene-Oligocene deposits of the Ngahlaingdwin Area,the western Salin Sub-Basin which is a well-known Cenozoic hydrocarbon-rich basin in central Myanmar,and contains a succession of siliciclastic sedimentary rocks.Nine stratigraphic units are well exposed within the study area.Among them,five units: the Pondaung Formation(late Middle Eocene),the Yaw Formation(Late Eocene),the Shwezetaw Formation(Early Oligocene),the Padaung Formation(Middle Oligocene),the Okhmintaung Formation(Late Oligocene),are chosen for the sequence stratigraphic approach conjunction with the lithofacies analysis.Regarding the lateral and vertical stratigraphic relationship,genetically related lithofacies associations on the basis of the combinations of physical,chemical and biological processes that have been identified from analysis of the facies were grouped into three principal depositional systems(i)Fluvio-deltaic system,(ii)Estuarine system and(iii)Transgressive shelf marine system.Additionally,sequence stratigraphic approaches combination with lithofacies analysis which was performed with a view to deduce the nature of depositional environments can recognize four depositional sequences bounded four sequence boundaries in the study area.The Tabyin sequence(SQ-1)comprises transgressive-and highstand systems tracts that are deposited in fluvio-deltaic environments.The Yaw sequence(SQ-2)encompasses falling-stage-,lowstand-,transgressive-,and highstand systems tracts,that occur during the facies transitions ranging from transgressive marine environments through immature estuary environments to deltaic environments.The lower Padaung and upper Padaung sequences(SQ-3 and SQ-4,respectively)contain falling-stage-,lowstand-,transgressive-and highstand systems tracts that are deposited in the transition conditions from transgressive marine environments relative sea level rises through deltaic environments to estuary environments.The falling-stage-,lowstand-,highstand-,and transgressive system tracts were interpreted to be controlled by three episodes of base level changes: base-level fall,rapid base-level rise and subsequent slow down of base-level rise,respectively.Furthermore,the analytical results of petrographic and whole-rock geochemical data indicate that the sandstone sequences in the Salin Sub-Basin were derived mainly from at least three different provenances: the Western Myanmar Arc,the Myanmar terrane basement,and the westerly Indo-Myanmar(Burman)Ranges(IMR),where sediments were subjected to initial-intermediate weathering conditions and rapid erosions,and were finally transported to the study area.As a result of major and trace-element tectonic setting discrimination diagrams and geochemical parameters,sedimentation in the study area was likely to occur at the Continental Island Arc(CIA)setting under a well-oxygenated condition.It was most probably formed under the site of a fore-arc setting associated with the India-Asia convergence and collisional processes.Investigations for hydrocarbon potentials of the study area have also been made using conventional logs coupled with seismic data.The potential for stratigraphic discontinuities related to erosional sequence boundaries(regional truncation and incised valleys)led to the most important stratigraphic features with traps potential(Upper Pondaung and Lower Padaung incised valley fill).Incised valley fills are especially significant because potential reservoir sands in either valley fills or inter-valley,paleohills,may be draped by relatively organic-rich shales in the overlying transgressive system tract.Thrust faults associated with potential traps and reservoir sands are important because they may act as the most important migration pathways.