Characteristics Of Hydrocarbon Source Rockes Of The Lower Cambrian Formation In Western Sichuan Basin,China

The Lower Cambrian formation in the northern Mianyang-Changning intracratonic sag?western Sichuan Basin?is deeply buried,and the current research level and exploration degree are relatively low.This paper selects the typical outcrop section in western Sichuan Basin to study the basic geological characteristics,organic geochemical characteristics and geobiological characteristics of the Maidiping and Qiongzhusi formations source rocks of the Lower Cambrian.Combined with the drilling data,the lithofacies characteristics and geochemical characteristics of the lower Cambrian source rocks in the middle section of the intracratonic sag?central Sichuan Basin?were compared with northern section?western Sichuan Basin?.Obtained the following perspectives:The existence of the Mianyang-Changning intracratonic sag controls the distribution of sedimentary facies of the Lower Cambrian formation in the western Sichuan Basin.The water gradually became deeper from the outside to inside of the sag.In the early period of Maidiping,the sedimentary facies showed ancient land?east edge outside the sag?and platform?west edge outside the sag?-shelf-slope-deep basin.In the late period of Maidiping,the sedimentary facies showed the ancient land?outside of the sag?-platform-shallow shelf-deep shelf.In the early period of Qiongzhusi,with the rising of sea level,the sedimentary facies showed shallow shelf-deep shelf.In the late period of Qiongzhusi,all the western Sichuan basin is shallow shelf.The existence of the Mianyang-Changning intracratonic sag controlled the distribution of the high-quality source rock of the Lower Cambrian.During the Early Cambrian,the extend effect reached the climax stage from the initial stage,and the transgression reached its peak.The water was generally quiet and more than 300meters black carbon-siliceous shale of deep-sea basin phase was deposited.the sag is center of source rock distribution,and has a large thickness along the direction of intracratonic sag.The western Sichuan basin is located in the central of the northern section of the sag.Source rock thickness of the Mianyang-Guangyuan area is the largest,and the source rock?TOC>0.5%?is generally thicker than 350m.The source rocks of Lower Cambrian source rocks in western Sichuan have good organic matter types,basically type I kerogen,with high organic matter abundance.The TOC distribution is between 0.16%⁴2.68%with an average of 4.51%.The maturity of organic matter is high.It is basically in a high mature-over mature stage and is high-quality source rock.The source rocks of Lower Cambrian source rocks in western Sichuan basin have good habitat types and high primary productivity,and the prosperous organisms are conducive to the generation of organic matter.In addition,the redox conditions basically belong to the oxygen-oxygen-deficient environment,which favors the preservation of organic matter.From a geobiological point of view,this set of source rocks also belongs to high quality source rock.Comparing the lithofacies and geochemical characteristics of the source rocks of the Qingping section and Dongxihe section in the northern section?Western Sichuan basin?with the Z4 well and GS17 well in the middle section?central Sichuan basin?,it was found that the source rock thickness and total organic carbon?TOC?content of the Lower Cambrian Maidiping Formation and Qiongzhusi formation are increasing from the middle section of the sag to the northern section.The western Sichuan baisn is the main hydrocarbon generation center of the Lower Cambrian.It reveals that the northern sag?western Sihchuan basin?has the similar tectonic-sedimentary features and original geologic conditions of oil and gas to the middle intracratonic sag?central Sichuan basin?,therefore,has large exploration prospect of marine oil and gas.Based on the Madiping Formation-Qizhusi Formations sedimentary sequence comparisons between the South-North,East-West of intracratonic sag,the formation and evolution of the early Cambrian intracratonic sag can be divided into six stages:?A?The last period of Dengying:In Sichuan Basin,due to the uplifting effect of the Tongwan Movement,apart from continuous deposition in the western and northern edge of the basin,Lezhi-Longchang area was the center of uplift,and the central section of Z₂dn³ and Z₂dn⁴ was eroded.Compared with the central part,the east and west sides are less eroded,and the Z₂dn⁴ section of Weiyuan and Gaoshiti-Morixi areas are not fully eroded;?B?The early period of Maidiping:Intracratonic sag begin to extend,slope is formed at the boundary of the sag,Only deposits are found in the inside of sag,and angular dolomite lithic sandstone and breccia are mainly deposited in the inside of sag;?C?The middle period of Maidiping:Tensile strength increases and the sea level rise,At this time,deposits are found in the inside of sag and west edge out the sag as well,On the west edge out the sag,phosphorus rock is mainly deposited.Inside the sag,mainly siliceous shale and carbonaceous shale are deposited,phosphorus and dolomite detrital sandstone are less deposited;?D?The late period of Maidiping:Extension is weak or inactive,the sea level is low,the erosion on the outside of the sag are filled in the sag.The main sedimentary carbonaceous mudstones are filled with phosphorus and dolomite detrital sandstone or Breccia;?E?The early period of Qiongzhusi:Quickly extened,the water quickly deepened,black shale,siliceous shale with a small amount of siltstone are deposited;?F?The late period of Qiongzhusi:Extension movement began to weaken,and at the same time it was affected by the decline of global sea levels,and the water quickly became shallow.At this time,a set of medium-coarse siltstone and dolomitic and calcareous siltstones were deposited.