Unravelling Facies-Process Based Deepwater Sedimentary System Using Borehole Image, Seismic, Core, and NMR Logs Reduces Field Development Uncertainties, Offshore South China

Abstract

Abstract Deepwater sedimentary systems have been widely investigated for more than half a century. Geoscientists define many types of facies, elements, and/or processes separately. The continuum of processes ranging from debris flow, to turbidity flow, and to bottom-current flow indicates a continuum of facies that result from the process or process overlap. However, these essentials are always overlooked to classify deepwater processes and their definitive facies. Most deepwater reservoirs, irrespective of the process, have been uniformly called turbidites for quite a long time. The incomplete understanding on the facies-process increases drilling capital and risk because different "turbidites" have variable petrophysical properties. In the case study of the Yinggehai basin offshore south China, the main deposits are from a deepwater submarine fan system. Previous researchers inadequately defined facies as channel, lobe, levee overbank, and sheet sand without sedimentary processes. To better understand the petrophysical property variation of different facies, we integrated borehole image, seismic, core, and nuclear magnetic resonance (NMR) logs to unravel and redefine the facies-process based channels and lobes. The fit-for-purpose approach identified downslope process, alongslope process, and overlap process of downslope and alongslope; therefore, we proposed six types of facies-process based channels and four lobes. The channels include turbidity channel, sandy, muddy, and granular debris channel (downslope), internal waves-tides channel (alongslope), and internal waves-tides reworked channel (overlap). The lobes are sandy debris and turbidity lobe (downslope), internal waves-tides lobe (alongslope), and internal wave-tides reworked lobe (overlap). Among all of the facies, the internal waves-tides (affected) channel and lobe are the best reservoirs because the dispersed clay is winnowed and the sands were reworked by bottom current internal waves-tides; the turbidity channel and lobe are the second best reservoirs, followed by the sandy debris channel and lobe. The unraveling of facies-process based deepwater sedimentary systems fills the gap in understanding deepwater sedimentary systems with variable petrophysical properties and reduces the future field development uncertainties. Overview of Sediment Gravity Flow, Processes, and Facies Dott (1963) was the first to classify gravity flows based on fluid rheology. Later, Middleton and Hampton (1973) proposed to define sediment gravity flows as flows consisting of sediment moving downslope under the action of gravity. They distinguished four main types of such flows:turbidity current, in which the sediment is supported mainly by the upward component of fluid turbulence,fluidized sediment flows, in which the sediment is supported by the upward flow of fluid escaping from between the grains as the grains are settled out by gravity,grain flows, in which the sediment is supported by direct grain-to-grain interactions (collisions or close approaches), anddebris flows, in which the larger grains are supported by a "matrix"; i.e., by a mixture of interstitial fluid and fine sediment, which has a finite yield strength (Fig. 1). Sanders (1963) established the foundation of process sedimentology by interpreting fluid mechanics from sedimentary structures. Process is a mechanism of sediment erosion, transport, and deposition. Facies is a distinct sediment type, a description of lithology (color, grain size, compositional and textural maturity), and sedimentary structure and texture (laminated, graded, massive et al); therefore, processes are the conclusions from the facies.