Application of seismic sequence stratigraphy and seismic facies analyses for identifying stratigraphic trap system within Rawat central sub-basin, White Nile State, Sudan

Abstract

Identifying stratigraphic traps in highly faulted rift basins such as the Rawat Basin in southeastern Sudan is challenging as a result of the intensive tectonic events. Sedimentological studies in the basin indicate the possibility of enormous petroleum potential locked up within several stratigraphic traps. This study examined the petroleum prospects in the basin beyond the conventional structural trapped systems that have yielded limited hydrocarbon prospectivity. This article utilizes core feature analysis to identify the primary sedimentary environments, employs seismic stratigraphy to determine sedimentary cycles, and conducts seismic facies analysis. Sequence stratigraphic and seismic facies analyses, core data, and log signatures from eight exploratory wells were analysed and employed to define the dynamic depositional processes of the fluvial and lacustrine sandstones and mudstones of the lower to upper Cretaceous first rift cycle in the Rawat central sub-basin. The core analysis indicates fluvial to deltaic depositional environment, while the log signatures, which showed bell, funnel, and serrated shapes, correspond to braided delta and lacustrine environments in the Galhak sand, Galhak, and Mulet Formations, respectively. The motifs and lithology were interpreted as indicating variations in lithology. The sediments were composed of low-energy particle sizes and were highly well sorted with a laminated bed of silt clay, and the variation in clay color indicates the deposition in a weak reduction–oxidation environment with shallow water; all these indicate the fluvial lacustrine setting. The concept of seismic stratigraphy applied identified a first-order cycle that includes low-stand system tract, transgressive systems tract, and high-stand systems tract. The first-order cycle is subdivided into two major second-order cycles, which are further divided into five para-sequence sets. The sets from the bottom to the top include SQ1, SQ2, SQ3, SQ4, and SQ5. Seismic facies analysis identified wedge chaotic progradational configuration, parallel seismic facies, sub-parallel, hummocky, and discontinuous reflectors within the first-order cycle, which confirmed the setting to be fluvial lacustrine. The stratigraphic traps recognised in the central sub-basin are lateral facies change as one of the depositional traps and the truncation in the northeast of the central sub-basin. The combined strengths of seismic stratigraphy and seismic facies analysis with the litho-facies association have greatly enhanced the identification of stratigraphic traps, some of which could have significant hydrocarbon prospects.