| As the main sediment transporting mechanism,the deep-water gravity flow can lead to thick and large-scale coarse-grained sandbodies and accompanied fine-grained sediments in seafloor and deep lake.Sandbodies often occurred with a big accumulation of oil and gas,making them the focus of current hydrocarbon explorations.Taking the Yanchang Formation in Longdong area,Ordos Basin for example,this paper systematically documented the lithofacies types and associations,sedimentary facies and models,as well as the triggers of the deep-water gravity flows in lacustrine basin.Basing on the analysis of tectonic settings,light and heavy mineral,rock fragments components and geophysics,the provenances are from three different directions: southwest,northeast and south.Due to the various distances from the source area and different terrain slopes,in the shallow lake of the study area,the braided river delta developed in the west and south.The braided river delta was in the southwest and west while the meandering river delta was in the northeast in the shallow lake.The large area of semi-deep and deep lake provided enough accommodation space for the deposition of gravity flows.Based on cores and well loggings,gravity-flow deposits in the study area are divided into seven lithofacies types(Lf1-Lf7),they are,respectively,slides(Lf1),slumps(Lf2),thick-bedded massive sandstone(Lf3),thin-to medium-bedded massive sandstone(Lf4),massive pebbly mudstone and silty mudstone(Lf5),thin-to medium-bedded sandstone and siltstone with graded bedding,parallel bedding,ripple cross bedding,wavy bedding,horizontal-laminated bedding(Lf6),and mudstone and shale(Lf7).They are categorized into four groups:(1)mass transport-dominated lithofacies(Lf1,Lf2),(2)debris flow-dominated lithofacies(Lf3,Lf4,Lf5),the major part of gravity flows in the study area,(3)turbidity current-dominated lithofacies(Lf6),and(4)deep water mudstone-dominated lithofacies(Lf7).The seven lithofacies form two sedimentary entities—the sublacustrine fan and the slump olistolith.The former is composed of three subfacies—inner fan,middle fan and outer fan—whereas the latter is composed of proximal lobe and distal lobe.Inner fan is divided into main channel and main channel lateral margin with fining-/thinning-upward sequences.The main channel is characterized by sandy debris flow-dominated,thick-bedded massive sandstones,and the main channel lateral margin is sandy debris flow-dominated,thin-to medium-bedded massive sandstones and turbidites.The middle fan is distributary channelized lobes,which can be further subdivided into distributary channel,channel lateral margin and inter-channel.The distributary channel is characterized by sandy debris flow-dominated,medium-bedded sandstones,and upward,the distributary channel lateral margin comprises thin-bedded sandstones,siltstones and turbidites with thin mudstones interbedded which belong to inter-channel.The outer fan is non-channelized lobes locating at the periphery and front of middle fan,including sheeted turbidites and deep lake mudstones,which are characterized by interbedded Bouma Tcde sequences and mudstones.The proximal lobe of slump olistolith consists of slides and slumps,mainly containing sandstone,siltstone and muddy-siltstone with glide plane and deformation structures.The distal lobe comprises debris flow lobe and turbidites lobe.It is characterized by sandy debris flow-dominated sandstone,siltstone,muddy debris flow-dominated silty mudstone and mudstone,and Bouma Tabcde sequences.Because of the basement subsidence in the Chang 7 period,the lake basin expanded rapidly and reached its peak,with large areas of gravity-flow sediments deposited.The sublacustrine fans controlled by west and south provenances were larger in size than the northeast.The slump olistolith mainly deposited in the northwest of deep lake and had a small scale.The basin shrank continuously in the Chang 6 period,construction of delta strengthened,and deep lake area reduced.The sublacustrine fan controlled by northeast provenance was larger than the south.And the slump olistolith deposited only in the west with a limited scale.The evolution of the lake basin during the Chang 7 to Chang 6 period was a complete cycle of transgression and regression controlled by tectonic settings.Several factors caused the formation of gravity flows in the Ordos Basin during the period of the Yanchang Formation,including source supply,terrain slope and trigger mechanisms,such as volcanic events,earthquakes,and seasonal floods.During the Late Triassic,the slope of the lacustrine basin was gentle in the east and steep in the west and southwest.The analysis of the zircon fission track of sandstones shows that tectonic-thermal events occurred during the deposition period of Chang 7 and Chang 6,this coincides with the Qinling orogenic movement during Indosinian movement.Several phenomena observed from cores in the study area indicate paleo-earthquake events,consisting of synsedimentary micro-faults,load cast,ball and pillow structure,twisting deformation and sand injections.The tuffs interbedded with shale and mudstones also indicate the volcano events.The microelement analysis and spore-pollen analysis suggest a warm and humid climate during Chang 7 and Chang 6,which can be the foundation of seasonal floods.Different source supply,slope and trigger mechanism can result in different sedimentary types of gravity flow deposits.The source supply of the sublacustrine fan was most likely from seasonal floods with a high net: gross and incised channels.It is widely distributed in the plane.Most likely triggered by volcanic events and earthquakes,the slump olistolith is deposited by the slumping and secondary transport of unconsolidated sandy and muddy sediments in the delta front or the prodelta with a low net:gross.The olistolith has no incised channels and has a confined distribution.Based on the physical property data and oil test results,the relation between sedimentary facies and reservoir quality,oil enrichment of gravity flow reservoirs was studied.The main channel of sublacustrine fan had the best physical property of all the microfacies,followed by distributary channel and debris flow lobe.The oil enrichment was measured by oil production and producing pay results in the oil test data.They are the main channel,distributary channel and debris flow lobe in the order of from high to low.The distributary channel developed far more frequent than the main channel,while the main channel developed more frequent than debris flow lobe.On the account of the reservoir quality,oil enrichment and frequency of gravity flow microfacies,it was concluded that the distributary channel of sublacustrine fan was the most favorable exploration target with superior porosity and permeability,high oil enrichment and wide distribution.The main channel had the best physical property,highest oil enrichment but well limited distribution,the debris flow lobe of slump olistolith had the superior physical property and relatively limited distribution,the two microfacies can also serve as the favorable exploration targets. |
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