CHARACTERIZATION AND HYDROCARBON SEALING CAPACITY OF THE WEATHERING MUD LAYER IN AN UNCONFORMITY

As faults and folds, unconformity is also an important structure that plays important control on hydrocarbon migration and accumulation. The unconformity-related reservoirs present high exploration potential because of its shallow burial depth and low exploration cost. An unconformity can be vertically divided into top, middle and bottom layers. However, previous studies have primarily focused on the hydrocarbon migration and accumulation of the top and bottom layers, neglecting the importance of sealing capacity of the middle layer. The middle layer of an unconformity is weathering mud layer deposited due to the long term weathering. The middle layer can present low porosity and permeability after the subsequent vertical compaction. Therefore, it can usually perform as base rock for its top reservoir or cap rock for its underlying reservoir, playing critical control on the development of unconformity-related reservoirs. Nevertheless, as the middle layer is developed above water and does not present distinct sedimentary structures, the middle layer is not easy to be recognized. Moreover, the unspecific development process and sealing capacity also baffle the exploration and resource evaluation of unconformity-related reservoirs. A series of outcrop and core observations demonstrate that the middle layer is usually purplish red or silver grey weathering mud layer. This layer presents no apparent bedding, no fossils and high content of iron and aluminum. Compared to normal mudstone, it has lower GR response, higher ICA, ICW, and IPA, lower IWP. The breakthrough pressure tests of sampled cores suggest that there is a positive correlation between the sealing capacity of wreathing mud layer and its burial depth.