Tracing the source of oil in an oilfield can be a bit like finding a crack in a leaking roof. Your first guess is that the leak is close to the water stain in the ceiling. But the water could equally easily have run along the inside of the roof for some distance before reaching the ceiling inside.
Take, for example, the giant Prudhoe Bay field on Alaska’s North Slope. The field reservoir is in the Triassic Ivishak formation, with the oil in a huge structural and stratigraphic trap, where the Ivishak formation is beveled off against the lower Cretaceous unconformity at the crest of a structural high called the Barrow Arch (an unconformity occurs where ancient surface erosion has cut across relatively old rocks and then younger rocks have formed on top of the eroded surface).
As with the leak in the roof, conventional wisdom would suggest that the prime source of oil in the Ivishak reservoir should be the Triassic Shublik formation, a prolific oil source rock that’s adjacent to the Ivishak in the stratigraphy of the North Slope.
But there are several other major and younger source rocks under the North Slope, including the Jurassic Kingak shale, the Cretaceous Hue shale/Gamma Ray Zone and the Cretaceous Pebble shale. To what extent have these sources contributed to the Prudhoe Bay oil? And how did the oil migrate into North Slope fields such as
Prudhoe Bay?Kenneth Peters a senior research geologist from the U.S. Geological Survey talked about some current research into Alaska oil sources to an audience at the joint meeting of the Cordilleran Section of the Geological Society of America, the Pacific Section of the American Society of Petroleum Geologists and the Western Region of the Society of Petroleum Engineers on May 9 in Anchorage, Alaska.
Circum-Arctic crude oilsPeters explained how a geological team has analyzed more than 1,000 crude oil samples from the circum-Arctic region. From those analyses the team selected 20 parameters that would enable different types of source oil to be distinguished. The parameters consisted of biomarkers and chemical isotopes (biomarkers are organic compounds that are characteristic of the organisms from which the oil formed).
Peters said that the chosen parameters are associated with oil sources and chemically stable.
“They’re resistant to secondary processes like biodegradation and thermal maturation,” he said.
The team plugged the parameter data into a statistical technique called principal components analysis. This technique finds components that express the variability of the data — by plotting components against each other it is possible to discover statistical associations between samples. In the case of the oil samples, the associations point to groupings of oil samples with similar characteristics.
The principal components analysis indicated that, on the basis of the 20 chosen parameters, the circum-Arctic oils form five major groupings.
The team then carried out a principal components analysis on each of the five groupings, to further refine the distinctions between the oils. That resulted in subgroupings that could then be analyzed further into even smaller subgroups. The result was a kind of “decision tree,” in which the oil samples were slotted into smaller and smaller groupings.
“The resolving power of this new approach that we’re using here increases as you get further down into this decision tree,” Peters said.
By carrying this statistical procedure to the finest useful resolution, the geologists found 31 oil groups for the circum-Arctic. Nine of those groups occur in the Alaska North Slope.
The groups correspond to oil from specific sources or specific sets of sources. For example, one group exhibits the chemical characteristics of the Shublik formation, a marine carbonate rock, while another group is associated with the Hue shale/Gamma Ray Zone, rocks consisting predominantly of shales.
Barrow Arch trendAnd a plot of the principal components of oil samples along the Barrow Arch showed how the various oil sources may have contributed to oil accumulations along the arch. The plot showed samples from the northwest end of the coastal plain forming a group associated with a Shublik source. Another group associated with a Hue shale source came from east of Prudhoe Bay. Samples from the Kuparuk River and Prudhoe Bay fields fell in an intermediate group, between the Shublik and Hue groups.
“No matter how you rotate this (plot) … the Prudhoe Bay oils remain intermediate,” Peters said.
The geologists then looked at the source oil profiles along the Barrow Arch using a statistical technique called alternating least squares, to match oil samples to combinations of different source oils, again using the 20 analytical parameters. First of all they found that the oils along the Barrow Arch could be accounted for, essentially, as combinations of source oil from the Shublik formation, the Hue shale and the Kingak shale, with the Kingak oil being a fairly minor component.
And they found that oil found near Barrow, at the northwest end of the coastal plain, appears to contain 98 percent Shublik oil, with a transition in oil source to about 95 percent Hue shale to the east of the Endicott field area, east of Prudhoe Bay.
The Kuparuk River field, to the northwest of Prudhoe Bay, contains dominantly Shublik oil but with a significant amount of Hue oil. But what about the oil in the Prudhoe Bay field itself?
“The penultimate question here is that we want to test the concept that the crude oils from Prudhoe Bay are mainly from the Shublik source rock,” Peters said.
That question brought a surprising answer — on average, 53 percent of the Prudhoe Bay oil appears to have originated from the relatively young Hue shale/Gamma Ray Zone.
“Within the Prudhoe Bay field the split indicates that the Hue GRZ is the dominant source rock,” Peters said. “… It contributes more oil to Prudhoe Bay than does the Shublik. That’s new and I think that’s quite exciting.”
And an analysis of oils across just the Prudhoe Bay field showed the same general trend as that for the Barrow Arch as a whole — the oil from the west side of the field appears to contain a somewhat higher Shublik component than the oil from the east side of the field.
“We’re seeing the same trend within the field that we see along the whole Barrow Arch from all of the geochemical data,” Peters said.
The geologists found the analysis results to be consistent across different samples from the same rock intervals in the field. And they also found that they could quite accurately predict the sulfur contents for the oils, using the estimated source oil components, thus supporting the results of the statistical analysis of the oil samples.
Petroleum systemSo how have the various mixes of source oil formed, especially since geologists think that the Hue oil generated and migrated at some time after the Shublik oil?
Peters said that the relative absence of the Shublik oils east of Prudhoe Bay probably results from the lower Cretaceous unconformity cutting out the Shublik formation — the Shublik formation simply does not exist in the eastern part of the area where the Shublik oil is largely absent. However, it appears that, where the Shublik formation was preserved, the Shublik oil migrated south to north into the Ivishak and into the sands of the Kuparuk River field reservoir. That migration happened perhaps about 50 million years ago.
Subsequent deformation of the rock strata along the Barrow Arch dropped the Hue shale/Gamma Ray Zone to the east of Prudhoe Bay below the level of the Ivishak formation. Oil generated in the Hue shale/Gamma Ray Zone could then have flowed up along the lower Cretaceous unconformity to fill the oil reservoirs east of Prudhoe Bay (unlike the water from a leaking roof, underground oil tends to migrate upwards). Around the area of Prudhoe Bay and Kuparuk this younger oil would have flowed into the older reservoirs, to mix with the Shublik oil that was already in place.
Hence the pattern of source oil content found from the statistical information.
“This information is, I think, very useful in rationalizing the North Slope migration history,” Peters said.
Meantime the team of geologists is experimenting with mixes of actual source oils to calibrate and check the analysis results.