| The Longmaxi Formation black shale is a very important which has become the key stratum of breakthroughs in developing Chinese shale gas.With the in-depth study of resource evaluation,the research of controlling factors of organic matter enrichment has become the focus of this industry.The Yanyuan basin is located on the west side of the Kangdian ancient land,which is the blank area of the shale gas geological survey.Therefore,in this paper,through the detailed geological investigation and systematic sample analysis,the black shale's mineralogy,elemental geochemistry,organic geochemistry characteristics are studied.Systematically study the geochemical characteristics and analyze the depositional environment,and discussed the controlling factors of the organic matter source rocks so as to prvide a basis view to the later shale gas exploration Work.To sum up,the main conclusion include the following aspects:Based on the observation of the lithologic assemblage and sedimentary characteristics of the Longmaxi Formation black shale,It is found that there are muddy deep-water shelf,muddy shallow shelf,silty mudstone shallow shelf and limestone argillaceous shallow shelf in the Yanyuan basin.And the muddy deep-water shelf,muddy shallow shelf are normal.Generally know the distribution characteristics of Longmaxi formation.Through the analysis of lithology,mineral,elemental geochemistry and geochemical data of Baizitian section and Lizihe section in the study area,we found that the main minerals are the quartz and clay minerals,The illite is the main clay mineral composition.Brittle mineral content is high,accounting for about 60%-90%of the total,is conducive to generation of cracks and fracturing transformation.The formation is rich in organic matter and the rich organic shale section is more than 30 meters(TOC>2).The organic matter maturity Ro is higher and the major type of organic matter is I type,which indicates that the source rock has high abundance organic matter and good type,and it has a good resource potential abundant in organic matters and already into the oil generation threshold,and favorable thickness r,so it has the good resource potential.Longmaxi formation black shale has high siliceous content,and it is not pure siliceous rocka which has more terrigenous detrital silicon.The siliceous rocks are mainly controlled by biological and affected by low and medium hot water,meanwhile we found the influence of water in Baizitian area is bigger than the lizihe area.The biological siliceous rocks is often rich in organic matter,and the upwelling oceanic curuent can bring nutrient elements from the bottom to the surface in the form of hot seawater to provide nutrients for the enrichment of surface organic matter,both of them can contribute to biological preservation and organic matter rock formation.Combined with the tectonic background,use of chemical elements ratio and related illustrations,it is found that the provenance area has been suffered low and moderate chemical weathering,and the neutral and acid rock mixing origin of the source-rock.The study area is passive continental margin deposition and the relatively warm and humid climate in the early time of Longmaxi fomation,and it is a little arid in the late stage.lt is a low energy,reduced,vulcanized deep water shelf environment of the sedimentary water,and it is affected by rising ocean currents and deposited with a large number of deep-sea sediments.and we think the baizitian section water depth is larger than Lizihe section and such point of view is confirmed by the enrichment of trace elements.The paleosalinity is low-moderate and the salinity span is larger.It is inferred that the water of the source area is stratified,and the upper layer water is slightly diluted due to the continuous injection of the terrigenous material,while the lower seawater is slightly salty due to the hydrostatic reduction.The paleoproductivity is higher,therefore it can provide a lot of organic matter,which is an important factor to rich organic matter source rock's formation.There are well correlations among the various parameters of the sedimentary environment,but in a particular historical period,it can be seen that the parameters are not isolated but interdependent,so that the chemical elements are used to identify the sedimentary environment should be more cautiou.Sedimentary environment plays an vital role in the formation of organic matter abundance source rocks,in which the paleo-productivity,reducing environment,water salinity,sedimentation rate,and sea level rise are good associated with TOC,especially reducing environment and the productivity have a high correlations with the TOC content.The reducing environment will affect the preservation of organic matter,while the productivity will influence the boom of organic biological.Therefore,it is considered that the paleoproductivity,the redox condition,the paleosalinity and the sedimentation rate are all important sedimentary environment controlling factors for formation rich organic shale. |
PDF Full Text Request