Seismic Facies Analysis and Hydrocarbon potential Evaluation of parts of Orange basin,Offshore south Africa
The study identified, mapped and named all the depositional sequences defined on the seismic data, described each sequence in chronostratigraphic order, produced a regional/local scale characteristics of each sequence, described the seismic facies properties of each identified sequences and analyzed maps for closures that poses efficient trapping style for hydrocarbon accumulation. This was with a view to assessing the hydrocarbon viability of each of the depositional sequences. The study materials which included a set of processed 2-D seismic, core, biostratigraphic as well as a suite of composite well log data were analyzed using Petrel version 2009 and IHS Kingdom Advanced software. Structural and seismic sequence stratigraphic framework were developed and used as input for seismic facies interpretation. Reflection attributes were used to discriminate between different seismic facies. A structure map was generated for surface of interest and analyzed for closures capable for holding hydrocarbon. The hydrocarbon potential of each depositional sequence were then assessed and integrated into the regional dip line so as to make predictions beyond the study area. Ten (10) seismic sequences (sequences B through K) representing either individual or composite depositional sequences were delineated within a spectrum of environments and their regional/local scale characteristics described in chronostratigraphic order. Seismic interpretation revealed NE-SW and NW-SE trending faults which terminated against major sequences boundaries notably the 22At1 and 15A2t1 surfaces. All the mapped sequence boundaries were relatively continuous except for where they were truncated by erosion and they also exhibited strong reflections which qualified them as major stratigraphic markers. No well penetrated sequence K and little was deduced about its hydrocarbon potential. Sequences J, I and H represented a composite transitional to drift successions. They contained facies that were potentially important reservoir rocks (sequence J) and condensed sections (Aptian shale of sequence I) that included significant source rocks known to be responsible for most of the hydrocarbon generation within the basin. The structure map for the top of sequence J revealed a fault dependent closure with an areal extent of about 50 km2 and the inter-bedded shale within this sequence serves as prospective seal. Sequences G and F contained essentially no mature source rocks and reservoir quality sands but constituted a regional seal within the study area. Sequences E, D, C and B represented a drift succession characterized by argillaceous sediments in which there were no interval of organic enrichment and reservoir potential combinations. Careful integration of present work into the regional dip line enabled the correlation of seismically interpreted sections into the bathyal region of the basin. This region may represent frontier area for future exploration works. The study concluded that the deeper portion of the Orange Basin correlated to the identified distinct depositional sequences could be more viable for hydrocarbon prospectivity within the basin.