Research On Sedimentary Facies Of Paleogene Source Rocks In Nima Basin,Tibet

The Cenozoic Nima Basin is in center Tibet and is closely similar to the nearby Lunpola Basin from which commercial oils have been produced.Oil saturated sandstones and disseminated asphaltic material have recently been found in the Nima Basin resent years.The Paleogene Niubao Formation and Dingqinghu Formation are most important hydrocarbon source rock.Most previous studies have focused on their organic geochemical studies,but rare sedimentology studies have yet been published.This paper is based on the theory of sedimentology and petroleum geology,using methods of Lithofacies analysis,organic geochemistry,organic petrology,in order to study sedimentology and source rock evaluation of The Paleogene,further more,We combine those two aspects to figure out the Factors controlling the development of lacustrine source rocks.Prensently,the following acquisition have been stated:17 main types of lithofacies can be recognized from the Niubao formation and the Dingqinghu formation,and they can be aligned to 7 lithofacies associationtins.Based on the features of lithofacies associationtins,the sedimentary facies of Xiabie Lake section can be recognized as fan delta facise.The sedimentary facies of Langnongba section can be recognized as delta facies and lacustrine facies.The sedimentary facies of Chaangba section can be recognized as lacustrine facies The sedimentary facies of Ni-1 well can be recognized as delta facies and playa facies.For Niubao formation,The TOC contents are high,the TOC of most sample are >0.4%,generally speaking,the shales has higher TOC than marls?The organic matters are classified as oil-prone type ?1,and it is in the thermal evolutio stage of low mature to maturelow mature.the Dingqinghu formation has much lower TOC except Xiabie lake section,The organic matters are classified as gas-prone type ?2 or type ?,and it is in the thermal evolution stage of immature to low mature.Above all,hydrocarbon generating conditions is more excellent in Niubao formation.Chaangba area has thick source rock of Niubao formation with good hydrocarbon generating conditions,thus,becoming an important area for oil and gas exploration.Nima Ancient lake basin accompanied by low lake productivity,which is sensative to salinty,and the lake level is relatively low,Terrestrial higher plant is important input of Parent material for source rock.Lake level in the east and middle area is relatively high,when it becomes semi-deep lake up-deep lake,Low aquatic organisms becomes active because of the middle salienty,those conditions favor the formation of organic-rich source rock.The east “dry salt lake”area has lower lake productivity due to the high salinty,so the soure rocks are poor.The north “fan delt” area are dominate by Terrestrial higher plant input,and is good for type ?2 or type ? kerogen.