Sedimentary Characteristics and Evolution of He-3 Formation in Deep Area of Biyang Sag,Nanxiang Basin

Based on logging and particle size analyses in combination with regional geological data,results of previous studies,core observation and seismic reflection data,the authors consider that braided river delta, near-shore subaqueous fan,slump turbidite fans and lacustrine sedimentary facies are developed in the He-3 Formation.On such a basis,five subfacies and six microfacies can be identified.The near-shore subaqueous fan is developed in southwest and southeast margin-controlling faults of the study area,braided river delta is developed in the northeast,and slump turbidite fan is developed at the semi-deep lake-deep lake area in front of braided river delta.The lithology of near-shore subaqueous fan is mainly the medium to fine gray conglomerate characterized by multiple composition,poor sorting and poor roundness.Braided river delta consists of medium-thick gray bedded conglomerate,pebbly sandstone,sandstone with parallel bedding,cross bedding and scour-and-fill structure. Vertically,braided river delta shows disconnected positive rhythm.Spontaneous potential and resistivity curves are bell-shaped or box-shaped.Where the sand of braided river delta front is instable,slump would occur and,at last, the slump turbidite fans would form in deep lake.The lithology of slump turbidite fans includes fine-grained sandstone,siltstone and argillaceous siltstone with positive rhythm,massive bedding,liquefied deformation bedding,wave bedding,mudstone rip-up,flute mold,furrow membrane and other structures.Electric curves show bell,box and funnel shapes.Lacustrine sedimentary facies is mainly developed in the furrow area in front of near-shore subaqueous fan and delta front.The water depth of the study area has experienced the evolution from deep to shallow and then a little deeper in H38-H31 area.