Features And Forming Mechanism Of Magmatic-hydrothermal Exhalative Sedimentary Rocks In Permian Lucaogou Formation,Santanghu Basin,Xinjiang

The Permian Lucaogou Formation in northern Xinjiang Provence is an important tight oil reservoir with huge thickness and broad distribution.For decades,the Formation has been regarded as a series of lacustrine organic matter-rich mudrocks.Many reservoir characteristics and organic geochemical analyses were done.But the microscopic texture and forming mechanism were ignored.This paper focus on black organic-rich mudrocks and composite quartz grains-bearing shale,laminated micritic dolostones and a special type of intervals in Lucaogou Formation in Santanghu Basin using macro seismic interpretation,and micro petrology,mineralogy and geochemistry.These intervals dominantly contain magmatic and hydrothermal minerals with low grade of compositional and texture maturity.Features and forming mechanism of ancient magmatic-hydrothermal exhalative sedimentary rocks(MHESR)as well as the complex sedimentary environments affected by interaction of magma,hydrothermal fluid,lake water,and organisms are discussed.Finally,the standards of identification and classification of different types of exhalative sedimentary rocks are proposed.Main results and knowledge are as follows:1.Features of stratigraphic morphology from 3-D seismic interpretation and lithofacies distribution from drilling cores observation indicate abundant stratigraphic buildups by volcanic and hydrothermal activities.That suggest volcanic and hydrothermal activities occur during the Lucaogou depositional period.Hence,the rocks researched in this paper are related to the activities.2.Black mudstones are fine laminated and interlaminated with or intercalated with in dolostones,and locally silicified.Grains in the mudstones are clay-fine silt sized,angular,poorly sorted,and contain abundant crystal fragments but lack of clay minerals.These observations suggested that the mudstones have a subaqueous eruptive origin and were transported by lake water,and deposited from volcanic eruption-fed turbidity currents.Then the mudstones mixed with lacustrine carbonate rocks and normal distal detrital clasts.Acidic fluids related to volcanic activities generate early devitrification of fine sized fragments erupted by volcanic activities,which result decreasing the content unstable vitirc fragments.Furthermore,early and later alterations by weathering are blocked since the hydrothermal environments are relatively closed,which lead to the rare clay minerals.3.Laminated dolostones show features of primary dolostone,and are mainly composed of<5 ?m,subhedral to euhedral dolomite or ankerite.Small amount of angular and shard-like quartz,alkaline feldspar,and analcime grains are also common in dolostones.Abundant of filamentary mineral,possibly related to microbial activities,occur in the intercrystalline pore of dolomite.Features of main,trace elements and Sr-Nd isotope suggest that Mg cation have a mixed origin,Fe cations form by the processes of devitrification.Features of C-O isotope indicate that microbial and hydrothermal activities influence the formation of dolostone.Above all,the laminated dolostone is interpreted as a primary dolostone directly precipitated from hydrothermal fluid affected by magmatic and microbial activities.4.The four types of special intervals are interpreted as hydrothermal dominantly controlled magmatic-hydrothermal exhalative sedimentary rocks(MHESR).These four lithofacies are characterized by locally distribution,simple kinds of minerals component,and extremely low content.Individual lamina or thin bed of dolomite and diopside MHESR are relatively thick,mass.Grains in these two lithofacies are mass and contain framework grains and matrix.The framework grains are composed of dolomite-smectite or diopside-wollastonite and are similar to phenocryst in volcanic rocks observed on hand samples,but formed by aggregation of different minerals under microscope.The matrices are smectite and alkaline feldspars respectively.These features indicate a high density granular flow deposit.And some dolomite crystals are altered into serpentine.Analcime and felsic MHESR are laminated,and show sharp,irregular base,suggesting an erosional origin.Framework grains in individual lamina show crude orientation parallel to bedding plane and cemented by ankerite or silica minerals,suggesting relatively low density turbidity deposit.The most unique characteristics of the framework grains,including dolomite,diopside,alkaline feldspar,and wollastonite are the common shard-like forms with embayed and curved edges and sharp corners,and intergrowth,indicating igneous origin.The features of geochemistry of dolomite MHESR suggesting deep material derived origin mixed with shallow material.The smectite and serpentine indicate the alterations from volcanic materials by hydrothermal fluids.Euhedral analcime,and cements of ankerite and quartz crystals indicate hydrothermal chemical precipitation.5.Some black shale contains abundant composite quartz(Qc)grains.The grains are Qc grains occur as framework grains in volcanic originated matrix and are uniform size,elongate shape,and bedding-parallel.Qc grains are composed of a core of anhedral microcrystalline quartz and an outer part of subhedral mega quartz grains,whose edges are composed of small euhedral quartz crystals,indicating multiple episodes of recrystallization.Abundance of Al and Ti in quartz crystals,and estimated temperature from fluid inclusions in Qc grains indicate recrystallization and overgrowth of quartz crystals by hydrothermal fluids.The uniform shape and size of Qc grains and preferential occurrence in some laminae and the radial internal pattern of individual quartz crystals suggest a biogenic origin.The Qc grains are interpreted as original microorganism tests,which were altered by hydrothermal fluids to recrystalize and overgrow during penecontemporaneous shallow burial.This interpretation suggests that volcanic and hydrothermal activities would inject episodically extra nutrients into the lake water,which would cause active production of microorganisms,and the strong diagenetic alteration carried on by hydrothermal fluid related to volcanic activities.6.MHESRs are a series of subaqueous volcanic-hydrothermal eruptive-fed sedimentary rocks,including effusive,explosive,exhalative,and mixed rocks.Exhalative and mixed rocks may show broad distribution and totally large thickness;whereas effusive and explosive rocks only show local distribution,very low content and serve as intervals other sedimentary rocks.Ancient MHESRs are easily to be indentified from the host rocks,e.g.normal profundal sedimentary rocks with mature mineral compositions and textures,on the basis of MHESRs'simple mineral compositions,low grade of compositional and texture maturity.Based on the mineral characteristics,transportation,and deposition,the rocks studied in this paper are classified into dolomite efusive rock,diopside efusive rock,analcime explosive-exhalative rocks,and felsic explosive rock.The black mudstones are tuffaceous sedimentary shale mainly formed by volcanic ash and can be termed as mixed rocks,due to their mixture with other types of MHESRs and normal detrital and chemical lacustrine rocks.As an important tight oil reservoir contained formation in northern Xinjiang,carefully researched on the Lucaogou Formation using mineralogy,petrology,and geochemistry is very significant to understand the forming mechanism of hydrocarbon.And this work may lead scientists to explore and develop oil-gas with new thinking.However,this paper haven't studied on the relationship between magmatic-hydrothermal activities and forming processes of hydrocarbon.That research is the future orientation.