| The dolomitic fine-grained mixed sedimentary rock,as the representative rock type of the Permian Lucaogou Formation of Junggar Basin with rich organic matter,is a research focus in the exploration of tight oil and gas reservoir.The exploration of the petrological characteristics and sedimentary processes of organic-rich mixed sedimentary rocks are considered a weak spot,and the sedimentary types and controlling factors are not clear.By taking the Permian Lucaogou Formation of Jimusaer Sag in Junggar Basin as the research target,this paper summarizes the sedimentary types and provides the basis for the exploration of the organic-rich mixed sedimentary rocks in the saline lacustrine basin.The research result was obtained by comprehensively using technical methods,which included core observation,rock thin section/fluorescent thin section identification,X-ray diffraction mineral analysis,scanning electron microscopy analysis,cathodoluminescence image analysis,carbon and oxygen stable isotope composition features,the characteristics of major and trace elements,the identification of kerogen components,the bulk-rock analysis technology of organic petrology and other technical methods.Based on the data analysis of the seimic,logging,core and the organic geochemistry,the stratigraphy of the Lucaogou Formation in the Jimusaer Sag was studied under the third-level lithologic sequences division scheme.The Lucaogou Formation developed two third-order sequences and five system tracts.The sequence?developed the LST,the TST,and the HST.Sequence?compromised the TST and the HST.The lacustrine sedimentation was the main sedimentary type of the Lucaogou Formation during its sedimentary period.The depth of water varies frequently.The sedimentary environment included the shallow water environment,deep water environment,developed delta front,carbonate shoal,sandy flat,dolomitic flat,mixed flat and mud falt,semi-deep lake and deep lake sedimentary subfacies.The dominant weather during the sedimentary period was hot and dry.Influenced by the precipitation and the evaporation,the lake water level changes periodically.The lower sweet interval were characterized by open lake with short periodic fresh water injection during early stage and by highly saline lakes with long hydraulic residence time and high confine during late stage,the upper sweet interval were characterized by highly saline lakes with long hydraulic residence time and high confine.The salinity of lake water during the sedimentation was high and presented the gradually increasing trend from the early sedimentary stage to its late stage.The Lucaogou Formation in the Jimusaer Sag contains various minerals compositions and multiple rock components.The content of quartz,feldspar and dolomite are high and equivalent to each other.The content of clay minerals is comparatively low.The rock components include terrigenous detrital,carbonate,pyroclast debris and organic matter.Based on the research on rock component types,characteristics and contents,this paper took the 5%and 50%as the boundary point for dividing four rock components into three end-members.Besides,the petrographic division also took three-stage nomenclature as the reference.The contents of terrigenous detrital,carbonate and pyroclastic debris are taken as the end-member.And rocks in this area were divided into four categories:terrigenous detrital rocks,carbonate rocks,pyroclastic rocks and normal mixed sedimentary rocks.By taking the organic carbon content of 1.5%as the boundary,the rock types were divided into rich-organic mixed sedimentary rock and poor-organic matter mixed sedimentary rock.Combined with the sedimentary structural characteristics,the petrographic types of mixed sedimentary rock were further divided.There were nine organic-rich matter petrographic types,which were mainly composed of carbonate mixed sedimentary rocks and pyroclastic mixed sedimentary rocks.The type,content,occurrence and distribution of organic matter were clarified by using the organic petrology,geochemical parameter identification and other methods.The organic matter was composed of a majority of type?and type?1,a small amount of type?.The organic macerals were composed of the sapropelinite,followed by vitrinite and inertinite,and the exinite was few.The secondary maceral were comparatively high.The sapropelinite was mainly composed of unstructural algae and amorphous organic matter,deriving from algae and aquatic organisms with algae layer and orange-orange red fluorescence.The inertinite was mainly from terrestrial higher plants with partial rich lithologic stratum.The secondary maceral comprised the hydrogen-depleting secondary macerals and the hydrogen-rich secondary macerals.According to the content,shape and fluorescence of organic maceral,the organic matter occurrence were divided into five types,namely,the stratified type,stratiform-like type,enrichment type,part-enrichment type and dispersion type.The stratified type,the stratiform-like type and the enrichment type were mainly developed in the low-energy and anoxic deep-semi-deep lake sedimentary environment.The part-enrichment type and dispersion type were mainly developed in the high-energy and oxygen-enriched shore-shallow lake.Five sedimentary organic facies were divided according to their features of the organic petrology,sedimentary environmenti and geochemistry parameters.The organic facies A,B and C of sedimentary rocks were favorable facies.Based on the analysis of single well,well-tie and distribution of sedimentary organic facies.The favorable facies belt of the lower sweet interval extensively developed in the middle and west area of the Sag.The upper sweet interval mainly develops in the east area of the sag in the south-north direction,and a small scale in west area of the Sag.Based on the research result of rock mineral components and organic matters,combined the analysis of paleoclimate and palaeowater environment,this paper summarized the causes of rich organic matter in this area:rich nutrient provided by volcanic ash,water with high productivity and the anoxic environment.The majority of Lucaogou Formation organic components were hydrobiontic algae,which flourished in the environment with appropriate temperature and abundant nutrient.During the sedimentary period,volcanic activity was frequent in the basin and produced a large amount of volcanic ash.Those ashes falling into the lake provided plenty of nutrients for the algae.Under high-temperature conditions,the algae in the lake were multiplied and maintained the high sustained biological productivity.Influenced by the strong evaporation,the saline water became the main lake basin water type,which created a strong reductive environment.Silt-mud-type sediments are favorable for the deposition of organic matter components.Most rich-organic rocks were deposited in the still and hypoxic deep lake-semi-deep lake or other low-lying areas.The hydrodynamic power of the lake basin margin was strong and created an oxidation sedimentary environment,which was disadvantageous for preserve the algae.Periodically injected freshwater may carry terrestrial higher plants into the lake.Due to large particles size,most terrestrial higher plants were deposited in the shallow lake.The main controlling factors of the mixed sedimentary rock were deduced from the research result of the mixed sedimentary characteristics and types.The mixed sedimentary rocks can be divided into intraformational mixed sedimentary and strata mixed sedimentary.The mixed sedimentary type mainly included the blended mixed deposits and the salutatory mixed deposits.The palaeoclimate,tectonic movement,lake level change,and provennances influenced the types,characteristics and distribution of mixed sedimentary rocks of the Lugoucao Formation.Tectonic activity controlled the basin shape and influenced the distribution of sedimentary area.The tectonic activity led to the volcanic eruption and provided a special component type.The volcanic activity increased the environment temperature.Meanwhile,plenty of volcanic materials fell into the water and provided sufficient nutrient for the aquatic organism to bloom.In this environment,Ca and Mg,released from the volcanic materials,conduced to the formation of carbonate rock.The sedimentary model which the terrestrial brackish lake with mixed sedimentary components and high organic matter enrichment were established. |
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