NEW YORK Feb 26 (Reuters) - A study of a fresh sample of crude oil from the Bakken shale in North Dakota published this week showed sharply lower levels of volatile vapors compared to previous tests, potentially raising new questions about the danger of shipping it by rail.
The latest data from Marathon Petroleum's Capline Pipeline unit, which publishes so-called "assays" on the quality of over 100 types of crude on its website, showed a sharp fall in the oil's vapor pressure, a common measure of a fuel's ability to evaporate and give off combustible gases.
While the data offers only a single snapshot of the properties for a batch of so-called "North Dakota Sweet", a term for Bakken crude, it may raise more questions about the combustibility of the oil, which has been cited in several fiery derailments in recent months.
The data emerges just as U.S. regulators impose new rules requiring more testing of Bakken crude for fear it is prone to explosion during accidents. Industry officials are appearing before a House committee in Washington on Wednesday to discuss the issue.
Capline's latest Bakken assay, dated 14 January, was posted on its website this week, shortly after a Wall Street Journal story on Monday used older Capline data to show Bakken crude carries more combustible gases than other varieties. While Bakken's ultra-light properties are generally well known, hard data is rarely made public.
Regulators are seeking more information on Bakken crude from oil shippers and are also conducting their own data collection and sampling. It is not clear when or if those results will be released.
Capline's newest test showed a vapor pressure reading of 5.94 pounds per square inch (psi). The reading compares with a psi of 8.75 for a test done in February 2013 and is nearly 4 pounds per square inch lower than the highest reading, 9.7 psi, recorded by Capline in December 2010. A higher psi reading generally indicates a liquid fuel is more prone to give off combustible gases.
It was not clear why the rate had declined so sharply, or whether that decline is broadly reflective of the region as a whole. One expert who reviewed the data said the wide fluctuation appeared unusual, but not conclusive of any trend.
"I would expect it to go up and down, it's going to vary, but that's a big drop," Connie M. Hendrickson, chemist with Arkon Consultants in Dallas, Texas, told Reuters. "Without extra sampling and extra testing, we just don't know."
A spokesman for Marathon declined to comment on the decrease, other than to say the firm has "a process in place to test crude oil for quality oversight purposes". The samples are generally taken at the same spot on the pipeline, the spokesman said.
It is not clear how much Bakken runs through the Capline, a 1.3 million barrels-per-day (bpd) pipeline running north from St. James, Louisiana, to Illinois. The Marathon spokesman declined to provide operating rates for the pipeline.
To be sure, Capline's latest data still show Bakken crude ranks higher in volatility than most other crudes, based on vapor pressure tests conducted by the company. At the 5.94 psi, for example, Capline's latest Bakken sample still ranks more than double that of Light Louisiana Sweet crude, which tested at 2.38 psi in May 2013.
Still, the wider range of readings and the infrequency of the testing suggests there remains much uncertainty about the quality of Bakken crude.
Other data sets have suggested not only that the crude is light by its nature - Bakken reserves are rich in so-called "light ends" like butane, propane and other byproducts of petroleum - but that it is also growing lighter.
Refiner Tesoro Corp. said in a 2013 presentation that its purchases of crude sourced from North Dakota's Bakken region have increased in volatility, topping readings of 12 psi in 2013.
Some lawmakers have asked for more data to aid regulatory efforts on this issue.
"It is not in anyone's best interest to knee-jerk a response without data," North Dakota Senator Heidi Heitkamp said in an interview this week.