|
Chukchi pressures suggest hydrocarbons Overpressures at depth in Chukchi Sea wells indicate hydrocarbon generation below the Brookian sequence, BOEM scientists think Alan Bailey Petroleum News
Abnormally high downhole pressures in four of the five exploration wells drilled in the Chukchi Sea between 1989 and 1991 indicate hydrocarbon generation in the subsurface, Kristin Elowe from the Bureau of Ocean Energy Management told the American Association of Petroleum Geologists Pacific Section’s annual meeting on May 22. The data do not encompass results from the one well that Shell completed in 2015 in its more recent Chukchi Sea exploration program. Evaluating the pressures BOEM scientists have been able to use log data from the five earlier wells to evaluate the pore pressures in the rocks penetrated by the wells, and to determine the pressure trend that would result from normal compaction of the rocks. Under normal circumstances the subsurface pressure should reflect the hydrostatic pressure associated with water trapped in the rock sequence. The other form of pressure consists of the lithostatic pressure, the pressure resulting from the weight of the rocks that are stacked above the measurement point in the subsurface. The actual subsurface pressure may be below hydrostatic level if under pressured and above that level if over pressured. If the pressure does deviate from the hydrostatic pressure, there must be some form of fluid seal that isolates the subsurface region where the pressure deviation exists, Elowe explained.
There are three potential explanations for pressure deviations: disequilibrium in the manner in which the sediments that form the rocks have become compressed; pressure changes resulting from the shifting of rocks from one pressure regime to another, as different parts of the crust move or deform; or hydrocarbon generation causing an increase in the amount of fluid held in the rocks.
Overpressure in four wells In four of the Chukchi Sea wells the log analysis indicated significant overpressures at depths below 3,600 to 7,600 feet. It is possible to infer that in the case of these particular wells the overpressure results from hydrocarbon generation: The overpressure is associated with carbon-rich rocks that have been exposed to temperatures conducive to oil formation, in particular in association with the Shublik formation, a major Arctic Alaska oil source rock, Elowe said.
The pressure anomalies tend to start below the Brookian unconformity, a major discontinuity in the rock sequence above the Shublik. The Brookian sediments above that unconformity are believed to have pushed the older rocks down to greater depths, into the depth window where oil would have formed. And the BOEM scientists determined that the overpressures determined from the Chukchi wells tend to increase to the south, towards the inferred main center of hydrocarbon generation under the basin in which Brookian sediments were deposited. The pressure gradients in the subsurface also increase towards the south, Elowe said.
The only well with no overpressure, the Popcorn well, penetrated rocks that do not appear to be in the oil window, at a location where the Shublik is absent and the lower part of the Brookian sequence is particularly thin.
Hydrocarbon trap Under onshore Arctic Alaska a major rock discontinuity, the lower Cretaceous unconformity, plays a major role in trapping hydrocarbons in the subsurface. Under the Chukchi Sea the Jurassic unconformity, an older regional discontinuity, is thought to perform a similar role. At the Crackerjack well, there are free hydrocarbons sitting right under the Jurassic unconformity, with corresponding overpressures.
Elowe also commented that, although the wells provided evidence for hydrocarbons in the subsurface, none of the wells encountered economic hydrocarbon pools.
|
