Juvenile Petroleum Pathway: From Fluid Inclusions via Tectonic Pathways to Oil Fields
Alexander A. Kitchka
CASRE, Nat’l Acad. Sci. Ukraine, firstname.lastname@example.org
Fluid inclusions are natural occurrences in rock bodies that were in the fluid state at the time of entrapment. They may exist as vapor, liquid or solid or any combination thereof. Hydrocarbons, mainly methane and its homologues, are not rare constituents of fluid inclusions, which also include liquid oils and bitumens. The suggestion that hydrocarbon-rich, primordial fluid inclusions concentrated in the basal lithosphere are released and migrate from time to time on pathways opened tectonically suggests massive inorganic petroleum discharges. This suggested scenario for the origin of petroleum is based on the theory that hydrocarbon accumulations in sedimentary basins, with the exceptions of diagenetic gases, coalbed methane and similar proxies of organic origin, are derivatives of fluid inclusions from the underlying crystalline, petroleum-prone basement. Crushing of crystalline rocks in zones of regional, mantle-driven faulting is interpreted to explain the release of deep fluids that may have been trapped in miarolitic clusters of negative crystals for hundreds of millions of years. Thereby, the fluid depletion of the tectonosphere is a direct consequence of dynamo-metamorphism that triggers and supports the “mild”, non-volcanic outgassing of the Earth’s interior. Deep faults systems provide conduits and reservoirs for the migration and paroxismic streaming through intermediate fractured reservoirs or temporal barriers at seismogenic depths. This approach enables us to trace petroleum as it evolves from microscopic fluid inclusions in the crystalline sub-basement upward into the widely diverse oil and gas accumulations in sedimentary basins.
Segregation of juvenile fluids in crystalline rocks
The segregation of hydrothermal fluids could occur in at least two stages: first in fluid inclusions during crystallization of magma or during metamorphism, and second as a result of faulting and accumulation in fractured basement and sedimentary reservoirs. Because temperatures of homogenization for hydrocarbon-bearing, hydrothermal, “proto-petroleum” solutions, the “ultimate volatiles,” are less than for “consanguineous” ore-bearing solutions, host minerals in the end tend to capture them while leaving spaces of sequestration within the crystalline lattice. This phenomenon occurs early, while transit channels and other voids are still being developed by faulting. At the outset of volcanic emanations hydrocarbons usually comprise only a trifling fraction compared with water vapor and other transmagmatic gases. During processes of crystallization, light hydrocarbons are concentrated in the last molten fractions and subsequent pegmatites. In this regard one may expect occasional mutual superposition of precipitated ore and petroleum. Depending on type of mineral and rock there may be 104-106 fluid-saturated inclusions per single cm3 on average. Fluid inclusions in igneous rocks can occupy up to 1-2% of their volume. The basement fluid content originates from volatile compounds homogenized under supercritical conditions by rock melting, transformation and crystallization of magmas. Indeed, hydrocarbons in gas-fluid inclusions in igneous and metamorphic rocks are micro-accumulations of real petroleum. Crystalline basement enriched with such inclusions is a potential source for petroleum generally. However, for this to be the case, large-scale defluidization of the crystalline sub-terrane is required. There must be effective linkage of fluid inclusions comprising a fracture system to effect drainage. Investigations of minerals like quartz, calcite, etc. deposited within fault zones for their hydrocarbon contents has shown that the maximal temperatures of fluids migrating in the fractures were much higher than temperatures derived from vitrinite reflectance studies. The palaeo-temperatures determined from the real fluid paths (infilled extension fractures, veins, lodes, etc.) are more dependable as indications of the “absolute geothermometer” in tight domains of low permeability sediments, because they reflect mainly the temperatures imposed by conductive heat flow transfer. Therefore, the “kitchen” for oil and gas should be sought along deep faults leading to the crystalline basement. Oil and gas fields that have been discovered in the crystalline basement to this date produce a strong argument in a favor of this concept (Krayushkin 2000). It is important in this context to make it clear that there is no problem with the thermodynamic stability of hydrocarbons under mantle conditions, as predicted and substantiated by Prof. E.B. Chekaliuk and proved by recent studies (Kenney et al., 2002).
Trace elements in petroleum and non-hydrocarbon accumulations
With activity of rift-oriented tectono-magmatism fluids from inclusions are bled away, migrating from the crust and upper mantle as a homogeneous fluid with high HC-concentration. Under similar conditions hydrothermal ore-bearing fluids arise; although this is a relatively rare event. Among the most volatile components of these fluids are the stable trace impurities found in oil and gas deposits, compounds of sulfur and boron, halogenides and their derivatives, noble gases, mercury vapour, some complex organo-metallic compounds, including transition metals, platinoids and rare earth elements. When the bulk composition of fluid inclusions has been depleted in respect of hydrocarbons, the makeup of fluid that accumulate above a basement fault zone in productive fields will vary, especially with respect to sulfur hydrogen, nitrogen, and carbon dioxide. In view of the dominant role of water in the inclusions, it is inevitable that there must be solute anomalies over large areas in formation waters of a sedimentary basin, low-salinity waters and marked variations of isotopic ratios. However, geologically-rapid degassing, contamination, and co-evolution with surrounding fluid systems invokes tightening of the primary contours of solute anomalies and finally their almost total disappearance, leaving hydrocarbon (plus nitrogen, CO2, etc.) as residual pools with contrasting, anomalous water fringes along with superimposed epigenetic mineralization in reservoirs.
Maturation of the HC-enriched lithospheric domains
Myriad juvenile fluids are scattered very irregularly in any monolith as microscopic cavities - primordial and secondary, authigenic and xenogenic inclusions of endogenous origin such as magmatic, metamorphic, pneumatolytic and hydrothermal-types. But these are fluids only potentially, since the liquid or gas captured in the inclusions cannot flow like a continuous medium. Their movements are extremely slow, being along defects of crystalline lattices of minerals with solid-state diffusion of heat, impuritive and stoichiometric vacancies, and owing to re-crystallization slip upon the boundaries of sub-grains. In combination with movement of vacuoles toward the heat source under a contrasting thermogradiental field and coalescence of primary inclusions due to their cracking under possible overheating, these processes lead to higher concentrations of the dispersed fluid phase in the subcrustal space of a sedimentary basin (Kaliuzhny 1982) and to gradual formation of volatile-saturated zone (VSZ) with a high fluid-inclusion population density. Because of the fact that most large sedimentary basins originated initially as single or multi-phased rift systems, the VSZ undergoes a complex development along with a rising mantle/crustal interface (intruding mantle diapir) and can form a lens of abnormal mantle with numerous multilevel magmatic chambers. It should be noted that due to continuous tectonically-generated PT-disequilibrium the primordial fluid inclusions move from the surrounding media to such lens-shaped chambers that feed back the process.
Principal stages of petroleum emigration
Atrophication of rifting is caused by its conversion into passive margin, while re-orientation of the regional stress field and depletion of the abnormal mantle leads to transformation of the VSZ into a transition zone of mixed crustal-mantle with a regional halo of residual fluid inclusions captured during the latest riftogenic pulses. That important stage of basin development with respect to hydrocarbons corresponded with segregation of newborn proto-petroleum systems and their carrier waters and gases in post-rift fluid inclusions. These post-rift residual fluid halos represent a vertical domain of high fluid concentrations, which is traced upwards amplified in primary and secondary fault zones. The next necessary stage of VSZ development corresponds with “primary” petroleum migration, a process similar by analogy with that supposed for sedimentary rocks. Progressive decreasing rock elasticity in the residual post-rift crustal-mantle mixture caused by downwarping of a sedimentary basin floor, secondary overheating of fluid inclusions, and three-dimensional re-fracturing under augmentation of lithostatic loading, the widespread swarm of micro-cracks propagate their own nucleation/annihilation and supply the coalescing fault zones with new surges of juvenile fluids. Subsidence rates are a very important parameter governing the mobilization ratio for juvenile fluids (Artyushkov 1993). These fluids periodically accumulate in quasi-stable fractured chamber that emerge at the transition interface from unstable frictional faulting to localized quasi-plastic shearing. Such intermediate places can exist and accumulate fluids only under the following specific conditions: First of all, where stress waves interfere constructively they can produce a resonant standing wave domain that prevents collapse of interlinked fractures. Secondly, such a domain must be isolated from the direct influence of magmatic sources so as to bypass the annealing of hydrocarbons. And thirdly, to attract and accumulate fluids, this highly fractured medium should be generated in a location of long-term tectonic stability. Ascending dilational “clouds” diverging from the aforementioned chambers can feed into temporary habitats that are within drilling depths. Typically, the temporal sites are associated with active or passive multilevel detachment surfaces in the crust that enable lateral migration of juvenile fluids along suprahydrostatic-circulating systems. Lastly is the crucial stage coinciding with the tectonic reactivation of VSZ that supplies the proto-petroleum mass discharge (Figure 1). This answers the traditional objection to the inorganic theory: why crystalline shields are unfavorable for commercial oil and gas accumulations. Crystalline shields are tectonically passive areas where a subcrust VSZ with immobilized fluid inclusions has not been revitalized, so the above-listed stages are missing or effectively passed long ago.
Fault-valve behavior and upward fluid transfer
Tectonically quiet upward fluid migration occurs on a minor scale due to ascending movements of dilational “clouds” of re-opened fracture network. This is caused by a wave-like pattern of stress maxima in the vicinity of deep faults and represents propagation of local fronts of dilation associated with them. Even for inactive cratonic areas small, periodic variations of the stress field caused by opening-closing cycles of micro-fissures and cracks have been established using seismic wave velocity monitoring. During stages of relative tectonic rest, active tectonic “pumping” transfers water, gases, and hydrocarbons by stages from level to level toward the surface. It might be imagined that such a movement proceeds through successive, temporally quasi-stable, chambers that follow one another like railroad cars. This process corresponds to the stage of pervasive “cold” outgassing through the basement. It is a matter of different-length orders of stress fluctuation. Another situation is typical for mobile belts owing to their active seismogenic environment. The “suction-pump” model proposed by Richard Sibson (1994) to explain dilational jogs and bends evolving along propagating ruptures, an earthquake slip can provoke a breaching of impermeable barriers and thus cause suction of deep fluids from the fluid-saturated crust and transfer them by a “fault-valve” action. Application of this model to petroleum migration is shown on Fig. 2. It might be supposed that mass discharge allowing cumulative fluid hydrocarbon streaming toward the surface is a result of implosive collapse of interlinked fractures due to seismogenic violence.
Injection of juvenile oil and gas into the sedimentary cover does not necessarily imply that a stable pool will appear in the first reservoir above the crystalline basement. In fact, the dynamics of their injection shows a complexity so pools are usually formed in reservoir compartments of higher porosity and permeability having the best fluid capacity to perform. That is why the lowermost pools with higher initial pressure in a multistage field may not be preserved and can outflow to stationary pools as long as a free area of the conduit exists. So the field as such is a result of non-linear redistribution between reservoirs, and absence (or traces) of hydrocarbon pools near the basement/sediments interface is not a paradox, but the exception proving the rule. Being a potent solvent, oil (and gas condensate) extracts high-molecular organic matter along its vertical (chimneys) and lateral pathways during secondary and tertiary migration and is even able to contain (depending on the density) fossil pollen and micro-particles of reservoir matrix. It can effectively capture and incorporate a wide spectrum of fragmental relics from dispersed organic matter and from all host rocks being traversed. It is therefore no surprise that the shorter the distance between a hydrocarbon field and its so-called source rock in the base of sedimentary column the closer is the correlation between “biomarkers”. The recent discovery of a deep biota of thermophyllic bacteria (Pedersen 1993) allows biological contamination to impact juvenile petroleum even in the crystalline basement.
Practical consequences and conclusions
As known, many various geological phenomena are responsible for oil and gas migration. The latter can be effected through deep faults and be due to different tectonic processes as well. However, there is an opportunity to reduce some of these processes to single common denominator. Such is the universal mechanism of influence of allochthonous masses on the deep sub-basement of oil and gas-prone basins. Propagation of allochthonous masses, for example, foldbelts, flooding basalts, glaciers, and deltaic fans continuously and differentially change a lithostatic loading onto the underlying bedrock and can form different structures in the sedimentary cover. But in the same time the crystalline basement can be plunged in consecutive order with simultaneous sagging of its individual tectonic blocks. Cyclic and forward motion of allochthonous slabs to a foreland provokes a progressive and gradual augmentation of lithostatic pressure onto the underlying terrane. Subsequent fault reactivation and subsidence result when a major tectonic block is close to isostatic equilibrium at a new hypsometric level. Due to this process, the system of faults, fractures and fissures become temporary high permeable for fluids by every subsequent stage of allochthonous slab propagation. Thrusting plunges and deflects the ancient “keyboard” of the faulted foreland. Thus, the basement is subjected by a “mild,” short-time, “crypto-rifting” kind of diastrophism. Thereby an ascending effusion of plutonic fluids, including drainage of VSZ, becomes possible. This is the tectonic stage corresponding with formation of oil and gas fields in foredeep basins particularly. This hypothesis substantiates the new approach to explain a powerful petroleum potential for deltaic sequences worldwide (Kitchka 1998) that may be enhanced by sharp changes in sea level.
It is not difficult to calculate that within an active fault zone, such as a regional strike-slip belt, that the crushing and mylonitization of rocks propagate to significant length, width and depth in the crust and involves thousands of cubic kilometers of host rocks and releases enormous fluid volumes from juvenile inclusions. Therefore, proof of the petroleum potential for any sedimentary basin is given if its crystalline basement is enriched with inclusions of HC-dominated composition. This approach can substantiate why one sedimentary basin with lack of thick or rich organic source rocks is, nevertheless, a prolific petroleum province but another one, with excellent conditions, is quite sterile in respect of commercial accumulations of hydrocarbons. Recognition of a feature such as a HC-saturated site – an anisotropic depth domain - is a challenge ahead to explorationists and geophysicists and the intra-basement traps within testable depths of drilling are the deep fractured reservoirs where giant virgin hydrocarbon accumulations await discovery. This revelation should disencumber exploration practice that is now overstressed and infatuated with the bio-organic origin of petroleum paradigm. Lastly, there is a single circumstance one may want to consider in contemplating the truth of the paradigm: simple calculations based on average present-day rates and volumes (a conservative estimate) of hydrocarbon seepages on land and sea testify that at the present rate of seepage the world’s conventional oil reserves (proven to this date) should disappear in no more than one million years.
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