Deep-lacustrine transformation of sandy debrites into turbidites, Upper Triassic, Central China

Abstract

Deep-lacustrine transformation of sandy debrites into turbidites in the downslope direction is evident in the Upper Triassic Yanchang Formation of the Ordos Basin, central China. This facies trend is used as a template for predicting the distribution of reservoir facies of the Huaqing oilfield, which contains 100-million-tonnes of oil reserves. Based on examination of conventional cores from 30 wells, four major types of lithofacies have been recognized: (1) fine-grained massive sandstone with floating mudstone clasts and planar clast fabric (sandy debrite); (2) fine-grained sandstone and siltstone showing contorted bedding, sand injection, and ptygmatic folding (sandy slump), (3) fine-grained sandstone with thin layers of normal grading and flute casts (turbidite), and (4) mudstone with faint laminae (suspension fallout). Thick sandy debrite units occur beneath the delta-front facies, implying progradation. The lake margin is dominated by sandy debrites, whereas the lake center is dominated by turbidites. In our study area, sandy debrites constitute the producing petroleum reservoirs, but turbidites are non reservoirs. The proposed model is characterized by (1) delta-fed multiple source (line source), (2) absence of channels, and (3) downslope transformation of proximal sandy debrites into distal turbidites. This downslope transformation of sandy debrites into turbidites may be applicable to other deep-lacustrine basins worldwide for predicting reservoir distribution.