For companies that only wish they had deep enough pockets for the prolific basins offshore West Africa, the speaker at the recent Geophysical Society of Houston’s technical luncheon outlined the wild ride the lucky ones have had. Kurt Rudolph, chief geoscientist for ExxonMobil, said that this play has been “one of the most efficient exploration programs undertaken by the industry.” Several reasons exist. Firstly, the regulatory climate forced companies to make aggressive development plans or risk forfeiting their acreage. Secondly, the technology was mature when it needed to be – good seismic data was available to image the very complex subsurface. Companies were able to image the reservoirs and the fluids. Overall, while volume discoveries on the continental shelf show a steady increase over time, deepwater discoveries show a much more accelerated accumulation over a much shorter period. Rudolph discussed four major aspects of the play – the petroleum system, the reservoir and seal systems, direct hydrocarbon indicator (DHI) analysis, and production. The two major areas of his focus are the Niger Delta and the Congo Basin, and each have very different petroleum systems. The Niger Delta is characterized by a large clastic wedge over the oceanic crust with a series of normal faults collapsing downdip. It contains an abundance of traps. The Congo Basin has salt, which always tends to complicate things. Here the synrift has opportunities for both carbonate and clastic reservoirs. Over a large part of the slope the prime source is mature, but there is increased reservoir risk as one goes toward the outboard part of the basin. So far there is no documentation of presalt hydrocarbons. In terms of reservoir and seal systems, the main producing reservoirs tend to be in confined channel complexes. These tend to have good reservoir and resource quality, but, Rudolph said, ‘it’s a complicated plumbing system.” It’s not a simple matter of drilling anticlines; good seismic definition is a must. He next compared the company’s success rate using DHI analysis (primarily spectral decomposition) vs. not using it. In most cases the success rate using DHI is much higher – a 60% success rate with DHI support vs. 35% without. In one case, the company had a deep objective under shallower known reservoirs. The seismic was not providing particularly good resolution at the deeper level, but using frequency-based analysis at 11 Hz, interpreters made another discovery beneath a producing oil field, “always a good thing,” Rudolph noted. ExxonMobil has also had good success using time-lapse seismic over its deepwater West Africa fields, he said. In the Zafiro field, for example, impedance contrasts between the datasets helped geoscientists see oil being displaced by water. In Kizomba A, modeled sweep using a producer/injector pair at the north end of the field showed fairly uniform sweep; the actual 4-D data indicated that the sweep became less efficient farther away from the wells. “Four-D is a very powerful surveillance tool,” he said. “We’re often surprised at the results.” Overall, he said, deepwater West Africa offers a good prolific system with good-quality reservoirs but also heterogeneities that force a new level of understanding. Future opportunities, he added, will be more subtle, requiring further improvements in technology.
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