Sedimentary characterization of a braided delta using well logs: The Upper Triassic Xujiahe Formation in Central Sichuan Basin, China

Abstract

The image logs played a key role in interpreting the lithology and sedimentary structures of uncored units. An integrated analysis of the borehole images and open-hole logs, which are calibrated with core observations, is conducted to reveal the detailed sedimentary characteristics of the Upper Triassic Xujiahe Formation in the Central Sichuan Basin. To establish borehole image interpretation guidelines, the interpretations of sedimentary textures and lithology from cores and outcrops are calibrated with borehole images and conventional logs. Lithologies in the Xujiahe Formation include conglomerates, fine to medium-grained sandstones, siltstones, mudstones and coals. Sedimentary structures in terms of bedding, scour surface and stacking style of beds are interpreted from the image and open-hole logs. Depositional microfacies are defined on the basis of grain size and sediment texture, sedimentary structures, thickness, basal and upper contacts derived from core observations, openhole logs and image logs. The Xujiahe Formation sandstones in Central Sichuan Basin are deposited in a braided delta front, of which the depositional microfacies consist of underwater distributary channel, river mouth bar, distal bar or sand sheet and underwater interdistributary bay. The depositional microfacies is predicted in a single well using a combination of openhole conventional logs, borehole images, and cores. The distributary channels, which show box-shaped or bell-shaped GR log character, are generally infilled with fine to medium-grained sandstone with massive, tabular, wedge-shaped or trough-cross-laminated bedding. Repeated fining-upwards cycles, as well as the stacked nature of individual bedsets and the presence of scour surfaces represent multistory, amalgamated, channel-fill deposits. The underwater distributary channel deposits can also be overlain by interdistributary bay mudstones, known as channel abandonment. Mouth-bar deposits, which are characterized by coarsening- up successions, infill the channels as they migrate laterally, and sometimes could be overlain by a channel deposit. The methodology has potential application to other braided delta front reservoirs with appropriate calibration and scaling.