Asphaltene-Like Material in Siljan Ring Well Suggests Mineralized Altered Drilling Fluid (includes associated papers 20322 and 20395 )
Authors: Jeffrey Alan , Kaplan Isaac 
Published: December 2007

Black asphaltenic-type material removed from the drillstem at 5945 m [19,505 ft] in Well Gravberg-1 from the Precambrian granite, Siljan, Sweden, was investigated to determine its origin. The chemical characterization showed that this material contains small amounts of hydrocarbons maximizing in the diesel range. No heavy hydrocarbons were identified, except for trace amounts of polycyclic aliphatics. From the chemical and stable isotopic characterizations, we concluded that the black gelatinous material is derived predominantly from the alteration of biodegradable nontoxic lubricant (BNTL) additives by caustic soda, admixed with diesel oil and trace amounts of polycyclic hydrocarbons from recirculating local lake water. No evidence for an indigenous or deep source for the hydrocarbons could be justified.

Introduction

Well Gravberg-1 was drilled by the Swedish Power Co. (Vattenfall)to test for hydrocarbons in the Siljan Ring meteorite impact crater. The location of the well in the granitic Precambrian Shield of Scandinavia precludes the possibility of finding commercial quantities of hydrocarbons of conventional origin, from the burial and maturation of organic matter in sedimentary basins. However, if methane or other hydrocarbons are migrating into the crust from the mantle, the fractured rocks beneath the Siljan Ring impact site could be a preferred migration pathway in the otherwise impervious granite of the Precambrian Shield. The well has reached a total depth of 6.96 km [4.32 miles], but so far, hydrocarbon gases have been detected only in minor quantities, dissolved in the drilling fluid.

On July 11, 1987, during fishing for lost wireline, a sticky, black, mineralized material was recovered on the fishing tools. When the drillpipe was brought to the surface, the interior of the bottom section was filled with about 60 kg [132 Ibm] of the black material. One week before this discovery, a mixture containing one sack of caustic soda, XCD polymer, and 1.1 m3 [7 bbl] of a commercially used BNTL was spotted at the drillbit to free the drillstring, which had stuck at 5945 m [19,505 ft] during a trip from the drilling depth of 6377 m[20.922 ft]. After this addition, fluid was circulated for 1 day, after which the drillpipe became plugged. The nozzles in the drillbit were then closed, and the fluid and additives remained at the formation temperature (∼100 degrees C at ∼6 km [∼212 degrees F at ∼3.7 miles]) until the drillstem was freed 6 days later.

Preliminary analysis of the black material established that the major component was magnetite, with smaller amounts of solventsoluble materials, including a suite of straight-chain paraffinic hydrocarbons from C11 to C21, and biological marker compounds very similar to those found in surface hydrocarbon seeps in the vicinity of the Siljan Ring. There was no direct evidence of BNTL in the black material.

Using the analysis data, Gold proposed that the black material is the product of a deep subterranean population of bacteria that use abiogenic hydrocarbons migrating up from the mantle. This has been widely reported in the popular and scientific literature as evidence of the validity of the mantle methane hypothesis. We began a detailed analysis of the black material and well additives to determine whether the nonmineral component in the black material might have a less novel origin than Gold suggested and to see whether it may arise from the various chemicals added to the drilling fluid. In particular, geochemical analysis companies frequently caution against misinterpretation of diesel-oil contamination as oil shows.

Because of the lack of information in the literature on drilling-mud artifacts, this study represents an opportunity to examine the effects of "drillbit metamorphism" on the production of downhole hydrocarbons. It is particularly well suited because the granite rock contains very small quantities of organic carbon (less than 0.5%) and extractable hydrocarbons.

Results and Discussion

The black material was analyzed for total organic carbon content, which averaged 11.2% (*0.3%). The remainder consisted of a mineral matrix composed primarily of magnetite. An aqueous extract of the black material analyzed by gas-chromatography/ mass-spectroscopy (GC/MS) was found to contain a complex mixture of fatty acids and fatty-acid derivatives.

The black material was extracted with dichloromethane, commonly used to isolate bitumen from source rocks. This extract represented 6.3% of the black material, and the GC (using DB-5 column and flame detector)of the total extract (Fig. 1) is dominated by a single broad peak, eluting between C21 and C22 n-alkanes. There is a subordinate suite of compounds, chromatographially identical to n-alkanes in the C11 to C21 range. The dichloromethane extract was separated by liquid chromatography on a silica-gel column into an aliphatic hydrocarbon fraction (17%), an aromatic hydrocarbon fraction (5%), and a polarcompound fraction (65%). tions

JPT

P. 1262^