THE SEARCH FOR MANTLE MARKERS;

 

EXAMINATION OF THE GRAVBERG 1 “BLACK GUNK.”

 

J. F. Kenney

Gas Resources Corporation, Houston, Texas.

 

 

 

Abstract:

 

            Measurement of the abundance of the rare-earth platinide element iridium has been evaluated as an alternate, or complement, to measurement of the helium isotope ratio ³He/⁴He as an indicator of a contribution of mantle origin material to a tested sample.  Tests have been run upon the anomalous black, hydrocarbon-based material observed in the Swedish deep gas exploration well Gravberg 1 at depths of approximately 18,000 and 22,700 feet, respectively.  Extraordinarily high abundances of iridium have been measured at both depths, with the greater magnitude at the greater depth.  Examination of the surface rocks in the Siljan Ring area eliminated possibility of contamination from the surface which might be attributable to the original meteorite impact;  alternate possible sources of error are discussed.  The iridium tests effectively eliminated speculation that the black, hydrocarbon-based material observed in Gravberg 1 could have originated from any alteration of either diesel oil or a commercial additive to the drilling fluid.

 

 

            In this talk, I report some progress of an ongoing project of investigations connected with the origins of petroleum hydrocarbons and the results of some measurements which have been made on samples taken from the drilling site of the deep gas exploration well Gravberg 1 in the Siljan Ring district of Sweden.  I start with a bit of background by way of perspective for this particular set of tests and report some very tantalizing and some quite exasperating results.  The results reported here must be accepted as preliminary only.  There are additional tests which must be made before a definitive conclusion can be taken from these initial results reported here today.  Reasons for these reservations are discussed in the last sections.

 

 

1.         Introduction.

 

            By the term “mantle marker” we define any substance, - molecule, atom or nucleus, - which by its presence gives evidence of a contribution, in whatever sample is under scrutiny, of some material which originated in the mantle of the Earth.

 

            The drilling of the deep gas exploration well Gravberg 1 in the Siljan Ring seeks a commercial deposit of mantle origin hydrocarbons, specifically one of mantle origin methane.  The existence of mantle origin hydrocarbons has received considerable attention since 1951, when the great Russian geologist Nikolai Kudryavtsev¹ first enunciated the modern Russian-Ukrainian theory of deep, abiotic petroleum origins.  Hydrocarbons of mantle origin have been observed in the gases emerging from the deep ocean trenches and “smokers” in places like the East Pacific Rise where such are observed in the fluids discharged from vents, and also trapped within the matrices of rocks that have been rapidly erupted from great depths.  As recently as last Summer, the KTB deep crystalline rock scientific drilling project at Windischeschenbach in the Oberfälz observed an entire series of hydrocarbons, of weight up to C₁₁H₂₄, which were identified as having a mantle origin contribution by reason of the ratio of the nuclear isotopic weights of the helium gas which was entrained in the hydrocarbons.

 

 

2.         Helium-3, the traditional mantle marker.

 

            The most common test used to measure the contribution of fluids of mantle origin employs the isotopic ratio of helium nuclei, ³He/⁴He.  However, the helium isotope measurement test suffers a number of disadvantages, not the least of which is that often there is simply no helium to measure (of any isotope).  Additionally, the helium test seems to suffer almost universally the problems of air contamination or other sample contamination.  The helium isotope test is a fine measurement when it works;  however, it is delicate, and it can give a “false negative” result (i.e., a low ratio of ³He/⁴He for what does in fact contain a component of mantle origin fluids).  It deserves note that the helium isotope tests for the gases from Gravberg 1 suffered from contamination and imprecise sampling which led to confusion about the interpretation.

 

 

3.         Iridium, Ir, and the group VIII, platinide, elements.

 

            For somewhat more than six years, our company has been actively concerned to develop a reliable test for materials of mantle origin, particularly for mantle origin hydrocarbons, which could be used alternatively, or in conjunction with, the helium isotope tests.  During the past few years, we have been evaluating intensely the use of the very rare group VIII metal iridium as a mantle marker.  We have applied some of the first trials for this new test upon the Gravberg 1 “black gunk,” as I shall explain farther on.

 

            Iridium is a member of the group VIII platinide series of rare metals, which includes also ruthenium, rhodium, osmium and palladium.  The element iridium is very rare throughout the universe generally and is found only in certain stony, iron meteorites, particularly in carbonaceous chondrites.

            Iridium is one of a class of metallic elements which chemists call siderophilic (or “iron-loving”).  The element is so called because, in any melt containing silicon and iron compounds, whatever iridium may be present manifests itself always in association with the iron, and essentially not at all with the silicates, regardless of the temperature and pressure.  For reason of this chemical affinity, at the time of the separation of the core from the mantle of the Earth, almost all the iridium in the Earth went into the core together with iron.  Thus, what was originally a very rare metal in the Earth became extremely rarer still.

            The usual abundance of iridium in crystalline rocks of the Earth's crust is typically about 10-20 parts per trillion, (ppt).  The typical abundance of rocks of mantle origin as measured by such as certain mantle xenoliths, or from the basalt magma from deep “hot spot” volcanoes which are assumed (for other reasons) to be erupting from the mantle, is several orders of magnitude greater, ranging between approximately 50-300 ppt.  The iridium abundance of the upper mantle has been estimated to be between 200-120,000 ppt.  That of the iron meteorites which bear iridium is greater by almost another order of magnitude, typically 350,000-700,000 ppt.  Presumably the typical abundance of iridium in the Earth's core must lie somewhere between that of the original iron meteorites and the rocks in the mantle.

            Thus there are three distinct regimes of iridium abundance:  that of the iron meteorites from which it originated, which is the highest;  that of material originating from the mantle of the Earth, which is less than the first by at least an order of magnitude;  and that of the Earth's crustal material, which is by far the smallest.  Therefore, using this test, the presence in a rock or other material of this rare platinum-group element, iridium, in elevated abundance can serve to mark that material as, at the least, having a component of substance which originated in the mantle, - or else possibly as evidence of meteoric impact.  This final point had to be taken carefully into consideration when evaluating the results of the measurements from Gravberg 1 because the Siljan Ring district is precisely a meteorite impact structure.

 

 

4.         The Gravberg 1 “black gunk.”

 

            During the Summer of 1987, there was observed on and concentrated inside the bottom of the drill string a thick, oily, black hydrocarbon material which was magnetic and initially bore a strong stench.  This was observed at a time after which the drill string had been stuck in the hole for a number of days and during the period when the drilling operation had been using a water based mud.  This material has come to be known throughout much of the scientific community and the petroleum industry as the Gravberg 1 “black gunk.”  There has subsequently been much speculation and argument over the composition and origin of the “black gunk.”  The “black gunk” has been occasionally described as an “asphaltine-like material.”  This description is quite incorrect and misleading.  There is no asphalt in the Gravberg 1 “black gunk”.  The black appearance of the Gravberg 1 is attributable to the fine grained Magnetite contained in it.  The Gravberg 1 “black gunk” is a mixture of approximately 90% Magnetite suspended in a light oil consisting mostly of alkanes in the molecular range of C-8 through C-16.

            The increasing shows in Gravberg 1 of cutting gas and of connection or trip gas, which increased in magnitude with increasing depth, had been (at least to some persons) startling enough;  the occurrence of a heavy liquid hydrocarbon material at a depth of approximately 18,000 feet in a plutonic granite environment was indeed a surprise !

 

            When the now-famous Gravberg 1 “black gunk” was first observed, there was much argument about how it came to be inside the drill pipe, and whether it might represent some alteration of a drilling additive bearing the trade-name “Torque-Trim” and so on.  There was even one especially silly announcement that the Gravberg 1 “black gunk” represented a breakdown, or alteration, of diesel oil in the drilling fluid, - until the man putting forth that opinion was reminded that the drilling fluid was a water-based mud containing no diesel oil.  I wanted to test the material in a way which, if the test produced a definite answer, would obviate all of the arguments about the significance of the “black gunk.”  Thus an investigation for iridium abundance seemed immediately relevant.  If the Gravberg 1 “black gunk” showed elevated amounts of iridium, the concerns about how that substance got into the drill string would become irrelevant.  Certainly no one has ever claimed yet that Torque-Trim originates in the mantle of the Earth, nor likewise the soybean oil and organic alcohols from which it is made.

 

 

5.         The measurements Made by Dr. Frank Asaro and the University of California Lawrence Berkeley Laboratory.

 

            I promptly took up the matter of the Gravberg 1 “black gunk” with Dr. Frank Asaro of the University of California Lawrence Berkeley Laboratory.  I had already been discussing this problem of the search for reliable mantle markers and the potential use of iridium as such an indicator with Dr. Asaro and his group at Berkeley;  and together we had drawn up a tentative program to investigate for the presence of iridium in crude oil, drill cuttings from deep gas wells, and from other samples which could give indication of the origin of oil or gas.

 

            The first appearance of the Gravberg 1 “black gunk”, however bizarre and unexpected, seemed like a worthy candidate for iridium abundance investigation.  The first quick test of that material rendered a formidably startling result:  the iridium abundance was between 250-300 parts per trillion.  This was, almost, the highest measurement of iridium abundance ever observed in any terrestrial material.  Only certain of the iron meteorites had ever previously manifested such a concentration of that rare metal.

            When the “black gunk” was observed again in the Winter of 1989 from a depth of approximately 22,000 feet, we obtained immediately a sample of that batch also, which will be referred to as “black gunk-2”.  Both a sample of the recently observed “black gunk” from 22,000 feet and a sample from the batch first observed at approximately 19,000 feet (“black gunk-1”) were subjected to intensive and thorough new chemical isolation, analysis for presence and abundance of other metals, and neutron activation and nuclear spectroscopic analysis for iridium abundance.  Those tests ascertained that the iridium abundance in the Gravberg 1 “black gunk” first observed (“black gunk-1”) was 294 ppt;  the same in the “black gunk” observed the second time “black gunk-2”) was 570 ppt.  These results are shown on the first slide which shows also for comparison the relative abundances of iridium in crustal rock, basalt, the upper mantle, and carbonaceous chondrites.  Note that the scale of abundances is logarithmic.

 

 

6.         Addressing the speculation that the high levels of iridium abundance observed at Gravberg 1 might be attributable to the Siljan meteorite.

 

            An immediate quandary about the high iridium count in those samples arose because the site of Gravberg 1 is a recognized meteorite impact zone.  Recognizing that the highest abundances of the rare Group VIII elements are observed in meteorites, it might seem reasonable to conjecture that the high iridium abundance in the Gravberg 1 “black gunk” could be attributable to that which was delivered by the meteorite whose impact created the Siljan ring.

 

            For such hypothesis, there are two independently determining tests:

 

(1)        Does the local surface (or near surface) granite or other rock in the Siljan ring district manifest such high iridium abundance ?

            and

(2)        Does the iridium abundance diminish with depth (as it must if its source were from the meteorite impact at the surface), or does it increase with depth (as it might be expected to do if its origin were from the mantle of the Earth) ?

 

            The initial tests upon the surface rocks from the Siljan district do not support a suggestion of meteorite origin for the iridium observed in the “black gunk.”  The first measurements taken from surface and quarry rocks from the Siljan district show only a slight difference in iridium content from the average for such in rocks of the crust of the Earth.

            The startling increase in the iridium abundance from the sample taken from approximately 19,000 feet to that taken at about 22,000 feet, which amounts to almost a doubling of that abundance, argues persuasively against any suggestion that the iridium observed in the “black gunk” originated from the meteor impact at the surface of the Earth.

 

            From these arguments it might appear that we should conclude that this truly extraordinary amount of the very rare element, iridium, must have originated in the mantle of the Earth.  Not so;  for such conclusion would be premature.  In order to come to a definitive conclusion, one must investigate exhaustively all possible sources of contamination of iridium from any source(s) other than the rock and fluids in the well.  This subject is taken up briefly in the following section.

 

 

7.         Sources of possible error;  and the requirement of further investigation.

 

            These startling results demand further investigation.  When one is dealing with measurements of a substance in such extremely small trace quantities as a few parts per trillion, or a few per 10^-12, the greatest problem is, after having detected the material which is the subject of the search, to make certain that the subject material detected is not a result of contamination, either of the original sample or of the test equipment and procedure.  The greatest amount of work connected with iridium investigation obtains from this necessary control testing.  The use of iridium as a "well site" testing procedure is entirely new;  this set of experiments has been the first.

 

            Dr. Frank Asaro is a very conservative and painstakingly careful scientist.  Both he and we are determined to test every reasonably conceivable source of contamination before concluding that the source of the large abundance of iridium is from some material of mantle origin.  Neither our company nor the University of California's Lawrence Berkeley Laboratory entertain any intention to initiate another opera buffa like that which several less responsible persons generated at the University of Utah last Spring with their folderol about "cold (kitchen table) fusion.”  We must test every possible source of contamination.  Partly for exactly this purpose an extra trip to Gravberg 1 was made early last Summer to gather approximately seventy samples of material which might have influenced the iridium count.

            For instance, one possible source of iridium contamination could be any chromium which might have been used in the construction of any of the drilling equipment and which might have entered the drilling fluid system.  Any stainless steel in the fluid handling system must be automatically suspect.  Such possible sources of contamination can be measured in several ways.  First, samples of the material to be tested can be taken from the well site.  For example, we took a flame torch and cut away a section of a liner in the mud pumps which were used during the drilling through the section of hole where the “black gunk” was observed.  Second, a thorough knowledge of the drilling equipment allows one to trace the constituent metal back to the manufacturer and ultimately to the steel makers (e.g., U.S. Steel, Lone Star Steel) who maintain “lot” samples of steel that has gone into their products.  Such can be tracked down for independent testing for iridium.  It deserves mention that neutron activation testing for iridium is entirely nondestructive.  Third, samples of the drilling fluid itself can be tested directly to determine whether it is carrying iridium.

            Furthermore, any iridium which might enter a sample by reason of contamination by another terrestrial source of that rare metal will be present in association with other specific metals, such as platinum, which have themselves characteristic isotopic ratios.  Nuclear spectroscopic analysis for these other associated metals then permits the separation of the iridium attributable to the contaminating source from that of the sample.  These analyses plainly require sophisticated equipment and skills.

 

 

8.         Continuation of these investigations.

 

            At this point, despite these extraordinary observations, the process became entangled with complications attributable to the United States Federal Government's budget deficit and the requirements of a recent statute called the Gramm-Rudman-Hollings Act, - all of which have utterly nothing to do (in any intrinsic sense) with this examination for mantle markers.  The University of California's Lawrence Berkeley Laboratory needs some new and additional equipment to carry out this work.  As written earlier, the greatest problem and the real test of the research scientist examining trace elements in abundances on the orders of parts per trillion are to control and detect possible contamination.  Therefore there must be made repeated tests for an entire suite of different elements, and of their isotopes.  For these tests, the Lawrence Berkeley Laboratory needs some additional equipment.  The University of California applied to the United States Department of Energy for money for this purpose.  However, instead of granting the Lawrence Berkeley Laboratory the funds needed for equipment, because of the requirements of the Gramm-Rudman-Hollings Act, the government has cut back the laboratory's operating budget.  This is a deplorable turn of events and one which I hope will be only temporary.  However, there the investigation stands at this moment.

 

            For the future, the samples from the drill stem test at Gravberg 1 ought certainly to be tested also for iridium content, as should additional samples of the drilling fluid.  It has been difficult to test such flammable liquid materials.

            One step in the experimental procedure requires that the sample be placed in a operating nuclear pile and activated with neutrons of a desired energy for a period of time.  A material placed in any active nuclear pile becomes quite hot, - in the conventional thermodynamic sense of that word as well as in the sense of being radioactive.  A flammable material containing a diesel-based drilling fluid would explode inside the pile.  On the basis of what happened at Chernobyl, such would not be a desired result.  These tests have involved some tricky procedures.  The entire program has been extensively reviewed by the Applied Science Division of the Lawrence Berkeley Laboratory at the University of California.  Among other tasks, the preparation of the samples for neutron activation requires that the volatile compounds be first evaporated off.  However, that evaporative process must be carried out so as not to lose also the element which is sought.  Iridium forms, for instance, gaseous compounds with the halogen elements, such as iridium hexafluoride, IrF₆, which could be lost in a careless separation of the volatile compounds.

 

 

9.         Tentative conclusions.

 

            With acknowledgment of the cautions stated above, the tentative conclusions which may be drawn from these preliminary results are:

 

(1)        The Gravberg 1 “black gunk,” which has been observed at depths of approximately 19,000 and 22,000 feet, manifests elevated abundances of iridium which are observed in association with the formation magnetite in the “black gunk.”

 

(2)        The lack of similar abundances of iridium in the surface and near-surface rocks in the Siljan Ring district, together with the observed large increase in iridium abundance with increasing depth argue strongly that the iridium observed in the “black gunk” did not originate from the meteorite which created the Siljan Ring and lakes.

 

(3.)       Measurements of iridium in geological samples, and specifically in rock cuttings, drilling fluids, cores, and drilling fluids show promise to provide a test for mantle origin material, complementing the helium isotopic ratio test.

 

 

 10.      Acknowledgments.

 

            Our special thanks go to Professor Bengt Collini of the University of Uppsala for his assistance in collecting samples of surface and quarry rocks from the Siljan district, and also to the men of the drilling crew at Gravberg 1 who were very helpful in assisting me to collect those additional samples.

 

 1          N. A. Kudryavtsev, "Against the organic hypothesis of the origin of petroleum," Petroleum Economy [Neftianoye Khozyaistvo], 1951, 9, 17-29.

 

                                                                                                                                   Paper delivered at the International Conference on Deep Gas, Hannover, Germany, June 1990,

                                                                                                                                      and published in the Proceedings of that conference, and (later) in

                                                                                                                                      Geologisches Jahrbuch Reihe D, Heft 107, (1999), pp 165-174.