Integration of Well, Core, and Seismic Data for 3D Depositional Environment and Reservoir Architecture Interpretation--Example from Offshore Vietnam

摘要

Determining the distribution and uncertainty of rock properties in a reservoir is fundamental for building a robust 3D geological model for field development. Higher resolution 1D well and core information needs to be integrated with the lower resolution 3D seismic volumes with the appropriate appreciation of the scale, resolution and uncertainty of the data sets. Sub-centimeter scale resolution can be achieved from detailed core description with sedimentary structures, trace fossils, grain size distribution, bed thicknesses, bed boundary characteristics, and bed stacking patterns giving indications of sedimentary processes that can be combined to infer depositional environments in a 1D vertical arrangement. Utilizing analogs of modern day and ancient systems, models can be proposed for the 3D distribution of sedimentary body architecture and rock property distribution. These analog models serve as starting points that need to be modified to the local parameters developed from seismic data sets to build a field specific geological model. Understanding the resolution of the seismic data is necessary to determine what size and shape of the sedimentary bodies can be resolved to take the 1D depositional environments interpreted from core away from the well bore into 3D seismic space. Data from offshore Vietnam serves as an example of this integration of well, core and seismic data sets for understanding reservoir distribution. Data collection included the acquisition of 160 m of core through the reservoir intervals of a well (Fig. 1). Basic observations of sedimentary structures and trace fossils place the section in a tide influenced delta system (Fig. 2). Detailed description of the core led to the definition of five facies with related depositional environments: The delta plain is characterized by fine-medium grained sandstone holding troughcross bedding and bidirectional current ripples, mud drapes and ripped up mud clasts indicating high energy migrating bars and tides (Fig. 3a). The trace fossil assemblage and syneresis cracks indicate stressed salinities during deposition. The well appears to have encountered tidal bars and ebb/flood channels situated on a delta plain. Shoreface deposits hold fine to medium-grained sandstone that generally display an overall coarsening upwards (Fig. 3b). A diverse suite of trace fossils are indicative of marine conditions with no fluvial input. Faint crossbeds suggest moderate wave reworking. This facies is representative of transgressive shoreface reworking tidal bars or a lateral shoreface to tide dominated delta. Delta front sediments exhibit very fine-grained sandstone and mudstone interbeds (~50/50) with wave and current ripples (Fig. 3c). A brackish trace fossil suite shows marine bioturbation in mudstones but little bioturbation in sandstone beds representing tidal, slackwater, and variation in fluvial discharge. These wave influenced sands and muds of a delta front setting were penetrated in several intervals of the well. Prodelta deposition is mudstone dominated with horizontal lamination and current ripples seen in interbedded very-fine grained sands and silts (Fig. 3d). The trace fossil assemblage, including diminutive stressed burrows Schaubcylindrichnus freyi, and syneresis cracks indicate stressed salinities during deposition. Deposition was driven by suspension settlement of muds and small density driven flows. Shelfal sediments are dominated by bioturbated mudstone deposits with horizontal lamination and minor current ripples in interbedded very-fine grained sands and silts sometimes preserved (Fig. 3e). The diverse suite of trace fossils indicateds open marine conditions (e.g., Phycosphion, Chondrites, Scolicia, and Cylindricnus). The suspension settlement of muds in full marine conditions is the major depositional process occurring.