Bedload transport and deposition of mud-grade sediments in deep-lacustrine settings: A case study in the Triassic Yanchang Formation
ABSTRACT
With ongoing shale oil and gas exploration and development across the world,the sedimentology of deep-water fine-grained sedimentary rocks remains animportant focus for research. In deep-lacustrine settings, in particular,mud-grade sediments are distributed widely. However, the very fine particlesize of mud-grade sediments limits the clear identification ofmicro-sedimentary structure; the mud-grade sediments are in most faciesmodels believed to be deposited via suspension settling in deep-lacustrine settings.In this study, thin-section observations, advanced mineral identificationand characterization system analyses, scanning electron microscopy observationsand three-dimensional reconstruction analysis by a focused ionbeam-electron beam dual-beam system were conducted on deep-lacustrinemud-grade sediments of the Chang 73 sub-member of Triassic Yanchang Formationin the Ordos Basin. Through this multi-proxy approach, evidence of bedloadtransport of the mud-grade sediments in the deep-lacustrine settings isprovided. During the deposition of deep-lacustrine sediments, a significantamount of volcanic material within the mud-grade sediments provided largequantities of high-valence metal cations into the lake waters, which promotedthe formation of flocs through electrostatic forces. Subsequently, these flocs aretransported as bedload and deposited on the substrate, forming numerous flocculeripples in the deep-lacustrine settings. When floccule ripples form, theflocs continuously pass over the ripple crests under the action of underflowand accumulate downward along the sloping surface of the floccule ripples,developing multiple foresets. Lateral migration and frontal progradation of foresetsresults in multiple stacked and laterally-connected foresets, ultimately controllingthe accumulation and preservation sites of these deep-lacustrinemud-grade sediments. The discovery of floccule ripples and foresets confirms adynamic deep-lacustrine setting, in which the mud-grade particles can be transportedas bedload under the action of underflow and then deposited. A newunderstanding of the sedimentary process of deep-lacustrine mud-grade particlesprovides a theoretical basis for clarifying the distribution of mineral compositionand organic matter content in mud-grade sediments. It furthersupports the prediction of favourable shale oil/gas reservoirs, promoting theefficient exploration and development of shale oil and gas resources, globally.
Keywords
Bedload transport, deep-lacustrine setting, flocculation, mudgradesediments, Ordos Basin, sedimentation.
ABSTRACT
With ongoing shale oil and gas exploration and development across the world,the sedimentology of deep-water fine-grained sedimentary rocks remains animportant focus for research. In deep-lacustrine settings, in particular,mud-grade sediments are distributed widely. However, the very fine particlesize of mud-grade sediments limits the clear identification ofmicro-sedimentary structure; the mud-grade sediments are in most faciesmodels believed to be deposited via suspension settling in deep-lacustrine settings.In this study, thin-section observations, advanced mineral identificationand characterization system analyses, scanning electron microscopy observationsand three-dimensional reconstruction analysis by a focused ionbeam-electron beam dual-beam system were conducted on deep-lacustrinemud-grade sediments of the Chang 73 sub-member of Triassic Yanchang Formationin the Ordos Basin. Through this multi-proxy approach, evidence of bedloadtransport of the mud-grade sediments in the deep-lacustrine settings isprovided. During the deposition of deep-lacustrine sediments, a significantamount of volcanic material within the mud-grade sediments provided largequantities of high-valence metal cations into the lake waters, which promotedthe formation of flocs through electrostatic forces. Subsequently, these flocs aretransported as bedload and deposited on the substrate, forming numerous flocculeripples in the deep-lacustrine settings. When floccule ripples form, theflocs continuously pass over the ripple crests under the action of underflowand accumulate downward along the sloping surface of the floccule ripples,developing multiple foresets. Lateral migration and frontal progradation of foresetsresults in multiple stacked and laterally-connected foresets, ultimately controllingthe accumulation and preservation sites of these deep-lacustrinemud-grade sediments. The discovery of floccule ripples and foresets confirms adynamic deep-lacustrine setting, in which the mud-grade particles can be transportedas bedload under the action of underflow and then deposited. A newunderstanding of the sedimentary process of deep-lacustrine mud-grade particlesprovides a theoretical basis for clarifying the distribution of mineral compositionand organic matter content in mud-grade sediments. It furthersupports the prediction of favourable shale oil/gas reservoirs, promoting theefficient exploration and development of shale oil and gas resources, globally.
Keywords
Bedload transport, deep-lacustrine setting, flocculation, mudgradesediments, Ordos Basin, sedimentation.
|
|
|