Indicators for identification of lacustrine sandstones of sandy debris‐flow origin: A case study of Es1 Member, Palaeogene Shahejie Formation, Nanpu Sag, Bohai Bay Basin, China

Abstract

The first member of Shahejie Formation of Eocene–Early Oligocene (Es1, Nanpu Sag) consists of fluvio-deltaic to deep lacustrine shale-sandstone accumulations, and the deep lacustrine deposits include sandy debris flow (SDF), turbiditic and mudrock facies. In the process of hydrocarbon exploration, it was found that that SDF sandstones in the deep lacustrine deposits can constitute excellent hydrocarbon reservoirs. Therefore, the identification of these lithofacies, especially SDF sandstones, is crucial for hydrocarbon exploration. We document here parameters that discriminate lacustrine SDF sandstones from turbiditic and mudrock deposits. Data from cores, wireline logs, and seismic sections are integrated and interpreted. Core-based identification indicators of the SDF sandstones include massive structures with upper and lower sharp contacts with interbedded mudrocks, lateral pinch-outs, locally imbricated to floating pebble-size grains, and elongated (teared) mudrock clasts within the middle and upper parts of normally- or inversely-grading sandstone and poor to high matrix content. These properties are attributed to deposition of non-Newtonian flows with sediment-support mechanism of dispersive pressure, matrix strength and buoyancy, sediment-transportation mechanism, and sediment-settling properties (hindered settling and freezing of the SDF). Log-facies identification indicators of the SDF sandstones are characterized by higher RLLD/RLLS values (40–80 Ω m), higher SP values (35–70 mV), lower Gamma ray (GR) values (70–120 API), and strongly serrated curve motifs. Seismic facies identification indicators of the SDF sandstones have mound-like, lenticular, or wedge-shaped external geometries, and interior worm-like and fusiform chaotic reflections with medium to strong amplitude, medium to high frequency, and medium to low continuity. The results and interpretation of the integrated core-log-seismic data as identification indicators are useful tools for identifying SDF sandstones and effectively distinguishing them from other deep lacustrine deposits.