--> Lithofacies and Deposition Environment of the Fine-grained Depositional Rocks in Saline Lake: A Case Study From the Permian Lucaogou Formation in Jimsar Sag, Junggar, China

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Lithofacies and Deposition Environment of the Fine-grained Depositional Rocks in Saline Lake: A Case Study From the Permian Lucaogou Formation in Jimsar Sag, Junggar, China

Abstract

The Lucaogou Formation of the Jimsar Sag is a classic tight-oil play in China. Despite its economic importance, the sequence stratigraphy and depositional processes of the lacustrine fine-grained rocks is still unclear. The Lucaogou Formation comprises a variety of facies but is dominated by fine-grained particles. Ten general lithofacies are identified on the basis of mineralogy, sedimentary structures, textures and composition: (1) brecciated dolomite mudstone; (2) intraclastic conglomerate; (3) mixed siliclastic and intraclast grainstone;(4) dolomitic siltstone; (5) dolomitic mudstone; (6) delta deposits; (7) carbonate grainstones; (8) massive siliceous mudstone;(9) organic–rich laminated siliceous mudstone; (10) turbidites. The sequence stratigraphic framework reveals that two sweet spot intervals of Lucaogou Formation were deposited during the lake contraction phase, and they exhibits high-frequency cycles comprising three successive facies associations: littoral-sublittoral, profundal and palustrine, which recording distinct lacustrine expansions and contractions. The development of abundant mudcracks, paleosol and vertical dissolution fracture indicated these facies record deposition in ephemeral to perennial lake of aridity. Fluvio-deltaic and eolian transport is the main mode of transport for siliciclastic component that ultimately accumulated in subaqueous and subaerial environments. The dolomitic mudstone exhibits a distinct low 87Sr/86Sr isotopic ratio (0.7048-0.7056) that is consistent with volcanic rock, which indicated the fall out volcanic glass offer the calcium and magnesium ions after devitrification.