Oil Expulsion Efficiency Research Articles

Effects of smectite on the oil-expulsion efficiency of the Kreyenhagen Shale, San Joaquin Basin, California, based on hydrous-pyrolysis experiments

The amount of oil that maturing source rocks expel is expressed as their expulsion efficiency, which is usually stated in milligrams of expelled oil per gram of original total organic carbon (). Oil-expulsion efficiency can be determined by heating thermally immature source rocks in the presence of liquid water (i.e., hydrous pyrolysis) at temperatures between 350°C and 365°C for 72 hr. This pyrolysis method generates oil that is compositionally similar to natural crude oil and expels it by processes operative in the subsurface. Consequently, hydrous pyrolysis provides a means to determine oil-expulsion efficiencies and the rock properties that influence them. Smectite in source rocks has previously been considered to promote oil generation and expulsion and is the focus of this hydrous-pyrolysis study involving a representative sample of smectite-rich source rock from the Eocene Kreyenhagen Shale in the San Joaquin Basin of California. Smectite is the major clay mineral (31 wt. %) in this thermally immature sample, which contains 9.4 wt. % total organic carbon (TOC) comprised of type II kerogen. Compared to other immature source rocks that lack smectite as their major clay mineral, the expulsion efficiency of the Kreyenhagen Shale was significantly lower. The expulsion efficiency of the Kreyenhagen whole rock was reduced 88% compared to that of its isolated kerogen. This significant reduction is attributed to bitumen impregnating the smectite interlayers in addition to the rock matrix. Within the interlayers, much of the bitumen is converted to pyrobitumen through crosslinking instead of oil through thermal cracking. As a result, smectite does not promote oil generation but inhibits it. Bitumen impregnation of the rock matrix and smectite interlayers results in the rock pore system changing from water wet to bitumen wet. This change prevents potassium ion () transfer and dissolution and precipitation reactions needed for the conversion of smectite to illite. As a result, illitization only reaches 35% to 40% at 310°C for 72 hr and remains unchanged to 365°C for 72 hr. Bitumen generation before or during early illitization in these experiments emphasizes the importance of knowing when and to what degree illitization occurs in natural maturation of a smectite-rich source rock to determine its expulsion efficiency. Complete illitization prior to bitumen generation is common for Paleozoic source rocks (e.g., Woodford Shale and Retort Phosphatic Shale Member of the Phosphoria Formation), and expulsion efficiencies can be determined on immature samples by hydrous pyrolysis. Conversely, smectite is more common in Cenozoic source rocks like the Kreyenhagen Shale, and expulsion efficiencies determined by hydrous pyrolysis need to be made on samples that reflect the level of illitization at or near bitumen generation in the subsurface.

Experiments on the primary migration of oil from source rocks

Simulation experiments on the primary migration of oil were carried out on massive samples. The results proved that oil generated from source rocks was expelled in the form of an independent oil phase. High oil-expulsion efficiency was observed. It follows that the primary migration of oil is not directly dependent on the quantity of oil generated from the source rocks. Therefore, the oil-expulsion proportion was high though some source rocks yielded only a limited amount of oil. A great deal of gas was produced at the same time of oil-generation. Thus, it can be concluded that the main expulsion energy for oil primary migration came from these gases.

DEPOSITIONAL ENVIRONMENTS, ORGANIC MATURITY AND PETROLEUM POTENTIAL OF THE CRETACEOUS COAL-BEARING ATANE FORMATION AT QULLISSAT, NUUSSUAQ BASIN, WEST GREENLAND

Coals and coaly mudstones of the Cretaceous Atane Formation are exposed along the north coast of the island of Disko and the south coast of Nuussuaq peninsula, West Greenland. Numerous oil seepages have been found in the region, but the so-called Kuugannguaq oil type only occurs at the north coast of Disko. The oil is presumed to have been generated from coaly (Type III) source rocks in the Vaigat strait where the Atane Formation is thermally mature due to deep burial. The exposed coals and coaly mudstones may thus be thermally immature equivalents of the active source rocks. The exposed section at Qullissat on the island of Disko is composed of four sedimentary facies associations: delta plain, distributary channel, delta front, and transgressive sand sheet. Samples of coals and coaly mudstones from the delta plain association were analysed for their total organic carbon (wt % TOC) and total sulphur (wt % TS) contents, and their source rock potential was determined by Rock-Eval pyrolysis. The organic matter composition was analysed by reflected light microscopy and the thermal maturity was established by vitrinite reflectance measurements. The Qullissat samples were supplemented with source rock screening data from coals and coaly mudstones from the Atane Formation at Paatuut on the south coast of Nuussuaq. The coals and coaly mudstones from Qullissat are dominated by huminite, but several samples have a considerable content of inertinite. The mineral content is high in some samples. Inundations of the peat-mires may have been quite frequent resulting in the formation of the coaly mudstones. TS contents (0.13–8.97 wt %) and the presence of framboidal pyrite suggest that the precursor peats were influenced by seawater, and that peat formation probably occurred during rises in relative sea-level. The organic matter is thermally immature, and a constructed vitrinite reflectance gradient for the region suggests that the Qullissat section prior to exhumation was buried to 1,500–1,600 m depth. Hydrogen Index (HI) values from both Qullissat and Paatuut are generally low; estimated maximum HI values for three Qullissat coals yield values of 140–190 mg HC/g TOC. The coals are gas-prone and only marginally oil-prone, and may in addition possess a limited oil expulsion efficiency. The effective oil window extends from approximately 1.0–1.6%Ro and the start of the effective oil window is located at about 3,000 m depth. Very thick sedimentary successions in the Vaigat strait indicate that such burial depths have been reached for the Atane Formation offshore, and up-dip migration of hydrocarbons from these source rocks may have generated the Kuugannguaq oil seepage.

Characterization of oil generation and expulsion from coals and source rocks using diamond anvil cell pyrolysis

A visual technique has been developed using a diamond anvil cell to characterize oil-prone vs. gas-prone coals and source rocks, and evaluate their oil expulsion efficiency. The experiments, conducted using 15 humic or shaly coals, two boghead coals, and eight source rock samples from Taiwan, Turkey, North America, and Australia, show that the technique has potential to distinguish oil-prone coals from gas-prone coals. The oil potential of humic coals was estimated from the amounts of total liquid generated during pyrolysis, which show correlations with the Hydrogen Index (HI) from Rock-Eval pyrolysis and the gas-to-oil ratio (GOR) determined by Py–GC analysis. The changes of visual area (or volume) of neoformed liquid as a function of temperature at four heating rates were used to calculate the extents of kerogen-to-oil transformation for determining the kinetic parameters. The results confirm the kinetic controls on the measured transformations but reveal unusually high apparent activation energies and frequency factors. The use of this technique as a quantitative tool to predict oil generation kinetics should be treated with caution until this discrepancy has been resolved. In addition, this study has demonstrated that the expulsion of oil-like liquid from coals is generally more efficient than from shales, possibly due to the much higher organic content in coal.

Guidelines for assessing the petroleum potential of coaly source rocks using Rock-Eval pyrolysis

Coaly source rocks are sufficiently different from marine and lacustrine source rocks in their organic matter characteristics to warrant separate guidelines for their assessment using Rock-Eval pyrolysis. The rank threshold for oil generation is indicated by the increase in BI (S1/TOC) at Rank(Sr)∼9–10 (Tmax ∼420–430 °C, Ro ∼0.55–0.6%), and the threshold for oil expulsion is indicated by the peak in QI ([S1+S2]/TOC) at Rank(Sr)∼11–12.5 (Tmax ∼430–440 °C, Ro ∼0.65–0.85%). The pronounced rank-related increase in HI (S2/TOC) prior to oil expulsion renders the use of immature samples inappropriate for source rock characterisation. A more realistic indication of the petroleum generative potential and oil expulsion efficiency of coaly source rocks can be gained from samples near the onset of expulsion. Alternatively, effective HI′ values (i.e. HIs near the onset of expulsion) can be estimated by translating the measured HIs of immature samples along the maturation pathway defined by the New Zealand (or other defined) Coal Band. Coaly source rocks comprise a continuum of coaly lithologies, including coals, shaly coals and coaly mudstones. Determination of the total genetic potential of coaly source rock sequences is best made using lithology-based samples near the onset of expulsion.

Expulsion of oil from petroleum source rocks: inferences from pyrolysis of samples of unconventional grain size

Oil expulsion efficiencies from different lithologies have been investigated and compared employing pyrolyses of samples of unconventional grain-size (from < 0.106 mm up to 8 mm) in the expectation of getting a little closer to natural systems. Standard (regular) pyrolyses, including Rock-Eval, employ finely ground samples (< 0.25 mm). Although this facilitates the escape of indigenous and generated hydrocarbons for analytical purposes, it changes rock texture and largely precludes much information about expulsion. It seems far from natural. Accordingly, we performed Rock-Eval pyrolysis and open-anhydrous programmed-temperature pyrolysis gas chromatographic (Py-GC) analyses, on five grain-size splits of samples — up to 8 mm. This does reveal more information about expulsion and permits inferences about subsurface natural expulsion. The expulsion of oil from a source rock depends upon the release of the generated oil from kerogen and movement (primary migration) of the released oil within the source rock. Thermovaporization and pyrolysis results suggest that neither the release of liquid hydrocarbons from kerogen nor the movements of these hydrocarbons within source rock, is the major limiting factor on the expulsion of oil from H-rich kerogen-containing source rocks (torbanite, lamosite, marinite, liptinite-rich humic coals). Thermovaporization of indigenous liquid (S1) hydrocarbons from tight carbonates and liptinite-poor humic coals is adversely affected by increasing grain size of the rock, suggesting that primary migration of liquid hydrocarbons is relatively restricted. Retention of liquid hydrocarbons may lead to oil to gas cracking. Py-GC analyses support this inference. Source rocks containing H-poor kerogen (e.g. brown coal, humic coal) are not good expellers of liquid hydrocarbons probably because both the release of oil from the kerogen and the movement of oil within a coal are limiting factors on expulsion. However, pyrolysis results on liptinite (sporinite)-rich immature humic coal (HI > 300 mg HC/g C org) show that liquid range hydrocarbons are expelled from coal when the generated oil exceeds the coal sorption capacity. This suggests that expulsion of oil from liptinite-poor, or in general H-poor, humic coals is primarily limited by the release of generated liquid hydrocarbons from kerogen because such coals, generally, do not generate sufficient amounts of liquid hydrocarbons to overcome the coal sorption capacity.

Development of a detailed model of petroleum formation, destruction, and expulsion from lacustrine and marine source rocks

A variety of laboratory experiments, including programmed micropyrolysis, isothermal fluidized-bed pyrolysis, oil evolution from a self-purging reactor, pyrolysis-mass spectrometry, and hydrous pyrolysis are analyzed to derive chemical kinetic expressions for pyrolysis of lacustrine and marine kerogens. These kinetic parameters are incorporated into an improved, detailed chemical-kinetic model which includes oil and gas generation from kerogen, oil degradation by coking and cracking, gas generation from residual kerogen, and hydrogen consumption reactions. Oil is described by eleven boiling-point fractions of two chemical types. The model includes equation-of-state calculations of vapor/liquid equilibria and PVT behavior. The model can simulate closed, open and leaky systems, and the open system can include an inert-gas purge. The porosity is calculated for both unconstrained conditions as well as conditions simulating natural compaction and fracturing during sedimentary burial. Model calculations are compared to results from a variety of laboratory experiments, including hydrous pyrolysis. Oil expulsion efficiencies and properties are also calculated for a variety of geological conditions. The relative amounts of water and hydrocarbon phase(s) expelled are governed by saturation-dependent relative permeabilities. Gas/oil ratios in the expelled petroleum are related to organic content and geological heating rate.

A simple analysis of primary oil migration through oil-propagated fractures

The conditions for oil-induced propagation of fractures which already exist are examined. The following conditions are necessary for oil-induced fracture propagation: (a) Organic matter is concentrated into discrete, isolated flakes. If kerogen is finely disseminated, oil-induced fracture propagation is not possible. (b) Kerogen flakes are thin and elongate with a large length to thickness ratio. The length of a kerogen flake must be 6 mm, if it is 0.1 mm thick. (c) Kerogen does not acquire permeability until a small amount of oil is generated. (d) The oil-source rock permeability is not more than that of a compact shale or argillaceous limestone. A continuous fracture network throughout the oil-source rock is necessary for primary oil migration. About 1–2% by volume of organic matter is sufficient to create a fracture network if the source rock is nearly impermeable and kerogen flakes are longer than 1 cm. Oil-generation may cause existent flaws in oil-source rocks to propagate and form a fracture network which acts as a pathway for primary oil-migration. However, before a definite conclusion can be reached, further study is necessary of the relationship between the kerogen size distribution and oil expulsion efficiency of source rocks, and the permeability behaviour of kerogen flakes during source rock maturation.

Principles of Geochemical Prospect Appraisal

Our current understanding of the geochemical processes responsible for the creation of commercial accumulations of oil and gas can be reduced to simple rules and equations to allow the petroleum geologist to use geochemistry to estimate--before drilling--the likely volumes and compositions of oil and gas delivered to the trap. In most sedimentary basins, oil is expelled from source rocks between 120° and 150°C, whereas most gas and gas condensate are released between 150° and 230°C. When the initial potential of source rocks exceeds 10kg/MT, oil expulsion efficiencies are between 60 and 90%. Expelled oil and gas migrate as petroleum-rich phases driven by fluid potential gradients. Most flow is laterally updip in beds with effective horizontal permeabilities greater than 1 md. In lower permeability rocks, the petroleum fluids move vertically, up or down, along the path of least resistance (i.e., the most permeable or thinnest beds) separating them from a high-permeability lateral carrier bed. The residual saturations of petroleum left behind along the migration pathway are of the order of a few percent of the porosity. Geochemical analysis of source rocks helps determine the yields and compositions of petroleum fluids expelled in the catchment area of a prospect. These fluid masses are converted to volumes from knowledge of the phase relations and densities of petroleum fluids. Evaluation of the total volume of rock through which the petroleum potentially can migrate and the mean porosity of the rocks that constitute the migration pathway allow migration losses to be assessed. These losses constrain both the range of migration of petroleum fluids away from the mature source rock and the volumes of petroleum remaining for entrapment in the prospect.

Hydrocarbon generation and migration from Jurassic source rocks in the E Shetland Basin and Viking Graben of the northern North Sea

In the E Shetland Basin oil generation began 65 Ma ago; peak oil generation maturity occurs today at 3250 m (0.7% R 0 ) and was first reached 40–50 Ma ago; the oil generation threshold is at 2500 m. Highest oil saturations in the Kimmeridge Clay occur at 0.8% R 0 ; oil expulsion efficiencies are &gt;20–30%. Oil phase migration has probably occurred through oil wet kerogen laminae, and through interconnected large pores aided by low oil/water interfacial tensions. Oil migrated along strong lateral fluid pressure gradients, from overpressured source rocks in half grabens to Jurassic reservoirs in tilted fault blocks. In the Viking Graben the Kimmeridge Clay is at oil floor maturity below 4500 m; oil and peak oil generation began 7C–80 and 55–65 Ma ago respectively; 40 Ma ago the Kimmeridge Clay passed through peak generation, and gas generation by cracking of oil had begun. Peak dry gas generation from Brent coals occurs today below 5000 m, and began 40 Ma ago. The Frigg Field gas, probably generated from late Jurassic source rocks, migrated through microfractures in overpressured mudstones below 3500 m; above 3500 m methane probably migrated in aqueous solution and was exsolved in the early Tertiary aquifer.