Stanley B. Keith1 and Monte M. Swan2
1 MagmaChem, LLC, P. O. Box 672, Sonoita, Arizona, USA, 85637
2 MagmaChem, LLC, 7072 Singing Springs, Evergreen, Colorado, USA, 80439
Hydrocarbon origin theories have focused on: 1) generation of gas and crude oil via burial diagenesis of biogenic, organic-rich sedimentary rocks, and 2) abiogenic hydrocarbon generation in the mantle. We suggest a third possibility--the generation of methane and heavier hydrocarbons through reactions that occur during cooling, fractionation, and deposition of dolomitic carbonates, metal-rich black shales, and other minerals from hydrothermal metagenic fluids. These fluids are proposed to be the product of serpentinization of carbon-rich peridotites under hydrogen-rich, reduced conditions. The association of hydrothermal ore genesis and accessory hydrocarbon has been known for years in economic metal deposits (for example, Gize, 1999). Recent application of mineral industry research to petroleum geology suggests that petroleum accumulations could be considered, at least in part, mineral deposits—products of hydrothermal, geochemically-zoned fluid plumes that possess identifiable paragenetic sequences (Keith and Swan, 2005).
Direct evidence for hydrothermal hydrocarbons continues to mount. Some of the more relevant observations include:
An important corollary feature of the last point is that hydrocarbon-enriched hydrothermal seepages into the lithosphere-hydrosphere interface provide a vital energy/nutrient source that attracts and sustains the biosphere component. Indeed, chemosynthesis in the presence of metal at hydrothermal vent sites like Lost Cities may have contributed to polymerization and production of large organic molecules (e.g. DNA) necessary to spawn life very early in the planet’s history. Continued interaction of the biosphere with the hydrocarbon-enriched hydrothermal nutrient sites may have led to its rapid evolution, expansion, and preservation (via mineral replacement at seepage sites) at critical thresholds during oxidation at the earth’s hydrosphere-biosphere system (for example the Burgess shale in British Columbia).
In this sense, inorganic hydrocarbon and organic hydrocarbon sources have inexorably, inevitably, and continuously co-mingled since the planet’s inception. In effect, the biosphere has been consuming inorganic carbon for over four billion years. Once formed, the black shale strata may become source rocks for generation of conventional hydrocarbons. However, we believe the origination of such an environment has more to do with the initial importation of hydrothermal hydrocarbon products generated from serpentinization of distinct types of carbon-rich peridotite sources in the inner planet. The serpentinization of peridotite may take place within any number of tectonic settings, including sedimentary burial or orogenic thrust loading, alteration in an ocean basin rift environment in the presence of brines, or flat subduction. A generic model for hydrothermal hydrocarbon that incorporates all of the foregoing observations is presented below (Figure 1).
Carbon-rich peridotite at green schist facies conditions is initially with low pH, chloride and bicarbonate brines generated from metagenic devolatization of oceanic crust at the zeolite facies-green schist facies grade change. Xenocrystic nanodiamonds particulates from the original peridotite are incorporated into a hydrogen and methane-charged hydrothermal brine product, which leaves the serpentine source under high geopressure due to the great volume expansion of the serpentine relative to its peridotite precursor. The methane charged hydrothermal brine products ascend through a reduced crust using deep-seated fracture conduits. The conduit system typically experiences a combination strike-slip or reversal/thrust-slip kinematics driven by orogenic horizontal compression. Where the fluids encounter shelf limestones near the base of basin cover sequences. Extensive replacement reactions occur in which large volumes of hydrothermal dolomite are formed. Because this reaction uses a large amount of oxygen to make solid-state dolomite, the residual fluids become even more reduced and additional hydrocarbons are formed—especially where metal-rich sulfides are precipitated.
The hydrocarbon-charged plume continues to ascend through the basin where it may deposit oil and gas in appropriate stratigraphic and/or structural traps. Inevitably, the hydrothermal hydrocarbon-charged plume ‘exits’ the lithosphere as metalliferous, hydrocarbon enriched seep(s), which may become exhalatively incorporated in black shale orogenic basins above the deep-seated conduit system. The black shales provide stratigraphic time lines that date the emplacement of the petroleum system. It is in these black shales that the hydrocarbon plume interacts with and co-mingles with the earth’s biosphere--probably a fundamental source of energy and nutrient. Where liquid state petroleums are formed within the crustal column affected by the hydrocarbon-charged hydrothermal plume, nanodiamonds which survive the depositional process, are co-deposited with the petroleum and provide a tracer back to the diamond-stable peridotite source. Diamonoid overgrowths now reflect the thermal regime present during oil formation with higher-C number diamondoids indicating more oxidative, hot formational conditions. Hydrothermal hydrocarbons have formed in this way since early earth history. Indeed, hydrothermal hydrocarbon continues to actively form and replenish (for example, the aforementioned Eugene Island 300 reservoir).
If the hydrothermal hydrocarbon model is viable, it will engender new exploration strategies, with applications from regional to reservoir scale. Petroleum systems will have to be viewed as crustal-scale features with a geology extending from the serpentinized peridotitic source to the seepage sites at the top of the crust. Petroleum geology is much larger than the basins within which petroleum has been traditionally found. The petroleum geologist of the future will need to acquire new skill sets derived from igneous and metamorphic petrology as-well-as economic geology of metal deposits. Hard rock geology and soft rock geology will no longer exist as separate disciplines as geologists view basins from the basement up.
For example, stratigraphic traps in cratonic cover sequences above basement peridotitic sources could become exploration targets. And, because a given hydrothermal plume fluid-flow pathway is much greater in extent than the fractionated hydrocarbon zones, it may be possible to predict the position of hydrocarbon-rich hydrothermal plumes (i.e., potential reservoirs) by examining more distal parts of the plume. In other words, because the fractionation pattern is regular and predictable, the pattern of hydrothermal fractionation may be predictable on regional, play, and prospect scales, and it may be possible to identify the most likely location of sweet spots. The concept of serpentinite hydrocarbon sources may helps identify new exploration leads through its magnetic-high, gravity-low geophysical signature. Identifying the migration path of fluids generated by serpentinization may identify plays. The associated reservoir architecture may be delineated through geokinematic analysis of structure combined with vectoring of HTD and black shale geochemical mineralization.
New exploration techniques will be required. Integrated, three-dimensional, fluid-flow fractionation models of target reservoir possibilities can be generated using:
Because some of these exploration techniques are more familiar to the metals exploration industry than they are to the petroleum exploration industry (for example, the multi-element geochemical techniques), a partnership between experts in both sets of disciplines will be highly effective in developing new exploration techniques. Ultimately, the hydrothermal hydrocarbon point of view offers a broader set of exciting exploration opportunities. New petroleum resources (especially thermogenic gas and light, low-sulfur oil) may exist in any number of trap configurations beneath and between know hydrocarbon accumulations and the underlying deep seated serpentinite sources.
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Figure 1. Conceptual Model for Generation of Hydrothermal Hydrocarbons During Serpentinization of Peridotitic Source and Subsequent Hydrothermal Dolomitization and Exhalative Block Shale Formation.