Search

Most of the oil in low temperature, non-uplifted reservoirs is biodegraded due to millions of years of microbial activity, including via methanogenesis from crude oil. To evaluate stimulating additional methanogenesis in already heavily biodegraded oil reservoirs, oil sands samples were amended with nutrients and electron acceptors, but oil sands bitumen was the only organic substrate. Methane production was monitored for over 3000 days. Methanogenesis was observed in duplicate microcosms that were unamended, amended with sulfate or that were initially oxic, however methanogenesis was not observed in nitrate-amended controls. The highest rate of methane production was 0.15 μmol CH4 g-1 oil d-1 , orders of magnitude lower than other reports of methanogenesis from lighter crude oils. Methanogenic Archaea and several potential syntrophic bacterial partners were detected following the incubations. GC-MS and FTICR-MS revealed no significant bitumen alteration for any specific compound or compound class, suggesting that the very slow methanogenesis observed was coupled to bitumen biodegradation in an unspecific manner. After 3000 days, methanogenic communities were amended with benzoate resulting in methanogenesis rates that were 110-fold greater. This suggests that oil-to-methane conversion is limited by the recalcitrant nature of oil sands bitumen, not the microbial communities resident in heavy oil reservoirs.

The Lower Cretaceous McMurray Formation is the primary host of the Athabasca oil sands deposit, one of the largest petroleum deposits in the world. Regional studies show that within the McMurray Formation, bitumen-saturated reservoir sands are encountered within the western, central and northeastern sections of the northeastern Athabasca deposit, while the southeastern part of the deposit has never been charged by petroleum, and that, the lateral contact between petroleum- and water-saturated reservoir sands is in some instances characterized by rapid changes in bitumen saturation, even between closely spaced wells. In the literature, a number of petroleum entrapment schemes have been proposed to explain how the bitumen accumulated through different trapping mechanisms, however controversy remains. This paper investigates the concept of inter-compartmental petroleum “fill and spill” charge and entrapment in a setting where compartments are clearly defined by mud-filled channel deposits. Classical molecular maturity parameters based on gas chromatography – mass spectrometric analysis of hydrocarbon compounds that are extracted from the bitumen show notable changes in composition between the geologically defined reservoir compartments. Relatively higher maturity sourced oil resides in the western compartment while maturity decreases to the eastern most compartment suggesting that the eastern compartment was filled by petroleum of lower maturity which has been displaced via fill and spill as petroleum migration from an increasing maturity source enters compartments to the west. Oil saturation and hydrocarbon geochemistry results also suggest the oil charge was very limited locally and individual compartments located towards the eastern edge of the Athabasca may not have seen the multiple charges evident in the oil sands to the west. The concept of inter-compartmental fill-and-spill provides new insights regarding the often overlooked intra-formational geological controls on reservoir charge in the McMurray Formation. The concept explains that apparently sharp lateral oil–water contacts are in fact due to mud-filled channel deposits which at least locally serve as lateral seals. This concept should be applicable to other meandering fluvial belt reservoirs worldwide and suggests a necessity to revise existing stratigraphic trap schemes by including the point bar stratigraphic play type of trap as one of the trapping mechanisms. Additionally, geochemical gradients can be used as a tool for defining the presence and extent of individual compartments as well as the level of fluid communication as an aid to well placements.

The subsurface microbiology of an Athabasca oil sands reservoir in western Canada containing severely biodegraded oil was investigated by combining 16S rRNA gene- and polar lipid-based analyses of reservoir formation water with geochemical analyses of the crude oil and formation water. Biomass was filtered from formation water, DNA was extracted using two different methods, and 16S rRNA gene fragments were amplified with several different primer pairs prior to cloning and sequencing or community fingerprinting by denaturing gradient gel electrophoresis (DGGE). Similar results were obtained irrespective of the DNA extraction method or primers used. Archaeal libraries were dominated by Methanomicrobiales (410 of 414 total sequences formed a dominant phylotype affiliated with a Methanoregula sp.), consistent with the proposed dominant role of CO2-reducing methanogens in crude oil biodegradation. In two bacterial 16S rRNA clone libraries generated with different primer pairs, > 99% and 100% of the sequences were affiliated with Epsilonproteobacteria (n = 382 and 72 total clones respectively). This massive dominance of Epsilonproteobacteria sequences was again obtained in a third library (99% of sequences; n = 96 clones) using a third universal bacterial primer pair (inosine-341f and 1492r). Sequencing of bands from DGGE profiles and intact polar lipid analyses were in accordance with the bacterial clone library results. Epsilonproteobacterial OTUs were affiliated with Sulfuricurvum, Arcobacter and Sulfurospirillum spp. detected in other oil field habitats. The dominant organism revealed by the bacterial libraries (87% of all sequences) is a close relative of Sulfuricurvum kujiense – an organism capable of oxidizing reduced sulfur compounds in crude oil. Geochemical analysis of organic extracts from bitumen at different reservoir depths down to the oil water transition zone of these oil sands indicated active biodegradation of dibenzothiophenes, and stable sulfur isotope ratios for elemental sulfur and sulfate in formation waters were indicative of anaerobic oxidation of sulfur compounds. Microbial desulfurization of crude oil may be an important metabolism for Epsilonproteobacteria indigenous to oil reservoirs with elevated sulfur content and may explain their prevalence in formation waters from highly biodegraded petroleum systems.

Quantitative data from petroleum systems in China (Liaohe basin) and Canada (Athabasca tar sands) support the theory that 25-norhopanes are produced during biodegradation of petroleum in the subsurface. Within a single oil column, both case histories showed increasing severity of degradation, indicated by destruction of hopanes and production of 25-norhopanes downward to the oil–water contact. In the Athabasca samples between the [Peters, K.E., Moldowan, J.M., 1993. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, New Jersey, p. 363] scale of biodegradation levels 5–9, concentrations of C28 20S triaromatic steroids and other biodegradation-resistant compounds increased by 35%, reflecting a concentration effect as a consequence of removal of more degradable compounds. Over the same interval, the concentrations of C28 17α 25-norhopane and C29 17α 25-norhopane increased by an order of magnitude, thus requiring that the balance be met by their net production during degradation. A detailed molecular investigation of the Athabasca bitumen revealed that C30 17α hopane degrades faster than C29 17α hopane, whilst the rate of formation of both C29 17α 25-norhopane and C28 17α 25-norhopane are similar, complicating a straightforward interpretation of demethylation of hopanes to form 25-norhopanes. Hopane degradation in the Athabasca tar sand may also occur without the production of 25-norhopanes. The results show that even within a single petroleum accumulation, a number of mechanisms control changes in the abundance and composition of hopanes and 25-norhopanes.

Most of the world's remaining petroleum resource has been altered by in-reservoir biodegradation which adversely impacts oil quality and production, ultimately making heavy oil. Analysis of the microorganisms in produced reservoir fluid samples is a route to characterization of subsurface biomes and a better understanding of the resident and living microorganisms in petroleum reservoirs. The major challenges of sample contamination with surface biota, low abundances of microorganisms in subsurface samples, and viscous emulsions produced from biodegraded heavy oil reservoirs are addressed here in a new analytical method for intact polar lipids (IPL) as taxonomic indicators in petroleum reservoirs. We have evaluated the extent to which microbial cells are removed from the free water phase during reservoir fluid phase separation by analysis of model reservoir fluids spiked with microbial cells and have used the resultant methodologies to analyze natural well-head fluids from the Western Canada Sedimentary Basin (WCSB). Analysis of intact polar membrane lipids of microorganisms using liquid chromatography-mass spectrometry (LC-MS) techniques revealed that more than half of the total number of microorganisms can be recovered from oil-water mixtures. A newly developed oil/water separator allowed for filtering of large volumes of water quickly while in the field, which reduced the chances of contamination and alterations to the composition of the subsurface microbial community after sample collection. This method makes the analysis of IPLs (or indirectly microorganisms) from well-head fluids collected in remote field settings possible and reliable. To the best of our knowledge this is the first time that IPLs have been detected in well-head oil-water mixtures.