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Quantitative Prediction Methods of the Thin Interbedded Beach-Bar Sandstone in Lacustrine Basin

Abstract

The beach-bar reservoir has become a great prospect area for exploration in Liaohe Basin. The sedimentary characteristics, distribution patterns and formation mechanisms of beach-bar sand bodies in the second member of the Dongying Formation in Liaohe Tidal Zone were studied in detail based on the comprehensive analysis of the data of well logging, drilling core and seismic. The sandy beach-bars are characterized by siltstones, fine-grained silty sandstones interbedded with thin mudstone units. The presence of well-developed sedimentary structures, such as swash bedding, parting lineation, parallel bedding, ellipsoidal mud clasts, ripples, terrestrial plant debris and vertical burrows, suggests that beach-bars were deposited in a relatively shallow water environment under the influence of strong hydrodynamics. The formation and distribution of beach-bar sand bodies was influenced by palaeogeomorphology, waves, palaeowater depth, palaeoshoreline and distribution of other associated sedimentary systems. This research suggests that the formation and distribution are primarily controlled by wave, specifically, the types of the beach-bar are various in different wave zone. Moreover, the longshorebar was deposited in the surfing backflow zone; the nearshore bar was deposited in the breaker zone; and the distal bar was deposited in the swash backwash zone. In this study, the formation mechanisms of large beach-bar were investigated through the reconstruction of palaeogeomorphology, hydrodynamics, lake levels and sediment source systems. Consequently, a beach-bar distribution model was established in Liaohe Tidal Zone. Through analysis of controlling parameters especially the hydrodynamic parameter, the scale of the beach bar in the gentle east slope in Liaohe Tidal Zone is quantitatively forecasted. And therefore it is revised by inputing the palaeogeomorphology parameter and the wind parameter. Finally, we draw the conclusion that the beach bar in Liaohe Tidal Zone is formed under an average velocity of wind moving at 24.4m/s, and the scale of the beach-bar in the gentle east slope is 3 times of that in the west steep slope.