Study On The Characteristics Of Lithology,Sedimentary Facies,Hydrocarbon Source Rock And Reservoirs Of The Mesozoic-Cenozoic Of Linhe Depression In Hetao Basin

In 2018,commercial oil flow was obtained from the Early Cretaceous Guyang Formation and Oligocene Linhe Formation in Well Jihua 2 and Well Song 5 in the Hetao Basin,which has become a hot spot for oil and gas exploration.However,because the Early Cretaceous and Oligocene in the main depression area of the Hetao Basin are relatively deep?6000?11000 meter?,exploration drilling in the early stage mainly focuses on the areas with a depth of less than 6000 meter in the southeastern margin of the basin.Although Oligocene and Early Cretaceous source rocks were found in drilling,their monolayer and accumulative thickness were thin and their hydrocarbon supply capacity was limited.Whether there are better hydrocarbon source rocks in the deep part of the basin still lacks a stronger basis.It is also unknown whether there have reservoirs at a further depth.Therefore,the distribution of hydrocarbon source rocks and the development characteristics of deep reservoirs in the deep sag area have become important problems restricting the further exploration of Linhe Depression in Hetao Basin.To solve these problems,based on the drilling and core analysis data of Well Long 1 and Well Song 5,combined with seismic and well logging data,this paper carried out further research on characteristics of the lithology,sedimentary facies,hydrocarbon source rocks and reservoir of the Mesozoic-Cenozoic of Linhe Depression in Hetao Basin.The main understandings obtained are as follows:1. There is a semi-deep lake sedimentary facies belt in the Linhe Formation in the main depression of the Hetao Basin in the northwest of Well Long1,based on the following:?1?There are dark micritic carbonate rocks in the core of the Linhe Formation in Well Long1,most of which are thin layers of less than 1 meter,with a cumulative thickness of 8.2meters,indicating that there is a lacustrine environment in Linhe Formation of Hetao Basin with little terrestrial influence and deep water.Boron content is high,Sr/Ba value is medium and high,indicating brackish water or salt water sedimentary environment.?2?The mud sand ratio of the Linhe Formation gradually increases from Well Lintian 1 to Well Long 1 to Well Linshen 1,reflecting that the sedimentary facies belt became thinner from southeast to northwest.?3?According to seismic facies analysis,the Oligocene and Early Cretaceous in the deep depression of Well Long 1 in the northwest of the basin have continuous parallel reflection facies,with strong amplitude and low frequency.The distribution range from Xilinhuduge to Wuyuan,which reflects the lithologically stable and fine deep-water sedimentary environment in this area.?4?There are wedge-shaped disordered reflectors surrounded by continuous parallel reflectors in the area where Fault Hanghou 2,which is active in the sedimentary period of Linhe formation,disappears at the downdip end in the northwest direction,while the updip end of the fault in the southeast direction is proved to be shore shallow lake facies belt by drilling.This reflects the development of subaqueous fan deposits with semi-deep lake symbiosis in the downdip part of Fault Hanghou 2 in the northwest during the sedimentary period of Linhe Formation.2. The TOC of Linhe Formation hydrocarbon source rocks in Well Long 1 is 0.76%on average,up to 4.09%.The organic matter types are mainly type?₁ and type?,and the thermal evolution degree of organic matter R_o is 0.76?1.06%,which is medium-better hydrocarbon source rock.The semi-deep lacustrine hydrocarbon source rocks in the main depression area of the basin in the northwest of the well are thicker and have a higher degree of evolution,making them better hydrocarbon source rocks.3. There are four main types of reservoirs in the Mesozoic-Cenozoic of Linhe Depression in Hetao Basin.The first is the porosity sandstone reservoir,the second is the sand and mud combination reservoir,the third is the fractured argillaceous rock reservoir,and the fourth is the bedrock reservoir.The porosity sandstone reservoir has been proved to contain oil in the Linhe Formation of Well Song 5 and Well Jihua 2.According to the reservoir variation trend with the depth of Well Long 1 and Well Lintan 1,the medium porosity-medium to high permeability reservoir is developed at the buried depth of 3000?4000 meters,and the reservoir with certain pores is still developed at the buried depth of 5200?5500 meters.