Characteristics Of Hydrocarbon Source Rocks And The Resource Potential Of Lower Cambrian Niutitang Formation In Southern Sichuan And Northern Guizhou Area

The Lower Paleozoic and Sinian natural gas exploration in the Sichuan Basin has been for more than 50 years.It has been found that the Moxi gas field,Weiyuan gas field,Ziyang gas reservoir,and gas-bearing structures such as Anpingdian and Longnvsi.However,The discovery of the Cambrian Mianyang-Changning intracratonic sag provides new thoughts and new ideas for the exploration of the Lower Paleozoic natural gas.It runs through the northwest and south of the Sichuan Basin form north to south,covering an area of 54,000 square kilometers.The current exploration is mainly concentrated in central Sichuan Paleo-uplift of the middle of the intracratonic sag,while the lower-combination?Cambrian-Sinian?of the southern Sichuan and northern Guizhou area has a lower degree of natural gas exploration.So there should be a large prospect of exploration.Basing on the collection of 12 typical well geochemical data,8 field profiles and surveys and the relevant organic geochemical experiments of the samples,and the development characteristics of the Mianyang-Changning intracratonic sag,referencing to previous knowledge,this study researched about the sedimentary environment of source rocks in Niutitang Formation of Lower Cambrian in the research area,Geochemical characteristics of source rocks,Plane spread of various parameters of source rock,Hydrocarbon generation intensity and resource potential,and Predicting favorable zones of source rocks.Early Cambrian period is the strong period of development of Mianyang-Changning intracratonic sag,it controls the sedimentary and developmental characteristics of the Lower Cambrian Niutitang Formation in southern Sichuan.Comparing the stratigraphic thickness of the Niutitang Formation in the north-south and west-east of the study area,It can be found that the stretched trough extends southward from the Sichuan Basin to the northern Guizhou.Therefore,it proves that the Niutitang Formation in the northren area of the study area is also controlled by the Mianyang-Changning intracratonic sag.In the early stage of sedimentation of the Niutitang Formation?Qianzhusi Formation?,the water level became deeper from the outside of the intracratonic sag gradually.There Mainly be black-gray carbonaceous mud/shale,black-grey mud/shale,black-gray silty mud/shale.So the research prove that the deep-water argillaceous shelf are deposited in and around the intracratonic sag.The water levels on both sides of the intracratonic sag is shallow,which deposited in a shallow-water shelf.The Main features of source rocks are black-grey silty mudstone,black-grey argillaceous siltstone,gray or gray-green siltstone.The Jinsha-Zunyi-Songlin area is intensified due to tectonic thermal subsidence,accompanied by rising sea levels and the influx of nutrient-rich ocean currents.The Main features of source rocks are Black mud shale,siliceous mudstone and phosphorus-bearing rock formations,which deposited in deep-hot-water shelf.The deep-water argillaceous shelf controlled by intracratonic sag,in which the water lever is deep and the hydrodynamic force is weak.It is a favorable area for the development of source rocks of the Lower Cambrian Niutitang Formation in the research area.The deepest Formation thickness of the Effective source rock?TOC>0.5%?is about 400m.,and most are 300m⁴00m.The area south of the basin to the north of the area is also controlled by the intracratonic sag.The source rock thickness is 250m¹00m,and belong to Well-developed source rock area.The shallow shelf facies mainly develops on both sides of the Mianyang-Changning intracratonic sag.The thickness of source rocks to the west of intracratonic sag gradually decreases.The thickness of source rocks around Leshan is just 50m.The effective source rock thickness from east of intracratonic sag area to Yongchuan-Chongqing area almost be0m.The deepest thickness of high quality source rocks?TOC>2.0%?in the research area could be more than 100m,and most of them are 60m¹00m.It mainly develops in the area of intracratonic sag.The thickness of high-quality source rocks in the northern part of the research area has been reduced to about 20 m.Because of the poor organic in Yongchuan-Chongqing-Changshou area,the most thickness of high quality source rocks is about 0m;Due to the sedimentary control of deep-hot-water shelf in northern Guizhou,the thickness of effective source rocks is relatively large,and the predicted distribution is about 80m.Basing on the analysis and statistics of geochemical data of source rocks,the organic matter type of source rocks of Niutitang Formation of Lower Cambrian in the research area is sapropelic?type I?.Primitive organic matter of research area mainly low-grade aquatic organisms in sedimentary environment of basin facies.Water body is anoxic,sediment grains are fine,and the hydrocarbon generation ability is strong.Black mudstones are well developed in the underpart of deep-water argillaceous shelf facies and deep-hot-water shelf facies.The organic carbon content of it is high,and the average value is between 1.0%to 4.0%.It is a good source rock grade.Outside the stretching trough,the main source rocks are deposited in shallow water environment.TOC is below 1.0%,which is a general grade source rock.TOC in Hechuan-Chongqing-Xishui area and to the East is less than 0.5%.There are no source rocks or poor source rocks.According to the result of conversion of asphaltene reflectance test to equivalent vitrinite reflectance,it shows that the maturity of organic matter in most areas of Niutitang Formation in the study area has reached more than2.0%,and it has entered into the stage of over-maturity,and producted dry gas,S1+S2<0.5mg/g.This indicates that the source rocks of this series have been expelled a lot of hydrocarbons,so that they have no hydrocarbon generating capacity any more.According to the original organic matter by whole rock thermal simulation parameter method in genetic method,and using DFDraw4.0,it shows that there are two Hydrocarbon generating centerin the research area:Around Well Gaoshi 17 and the northern of it,Yibin-Changning-Luzhou area.The highest hydrocarbon-generating strength is 146×10⁸m³/km².The intensity of hydrocarbon generation of source rocks developed in the trough ranges from 60×10⁸m³/km² to 146×10⁸m³/km²;The intensity of hydrocarbon generation of Source rocks of Niutitang Formation in the western of Mabian-Leshan area are below 40×10⁸m³/km².The hydrocarbon generation capacity of Niutitang Formation source rocks in the whole area is 1421.6×10¹²m³,and the Natural gas resources are about 4.02×10¹²m³.The hydrocarbon generation capacity of Niutitang Formation source rocks in the whole research area is 1048.3×10¹²m³,and the Natural gas resources are about 3.04×10¹²m³.There are two favorable zones in the research area are predicted:?1?The part of the northern part of the southern Sichuan Basin and the northern part of Guizhou,as same as Yibin-Changning-Junlian-Weixin Area;?2?The developed part of intracratonic sag,as same as Neijiang-Zizhong area.These results could be useful for further research and exploration.