Petroleum Inclusions Research Articles

Fluid Inclusion Evidence for the Migration of Anomalously Hot Fluids in the Silurian/Devonian Hunton Group below the Pre-Mississippian Unconformity Surface, Anadarko Basin, Oklahoma: ABSTRACT

Fluid inclusion assemblages in vug- and fracture-filling diagenetic minerals in the carbonates of the Hunton Group record anomalously high homogenization temperatures and salinities. Temperature profiles of single wells display maxima directly below or close to the unconformity with the overlying Woodford Shale, and decrease gradually or abruptly downward. In a few wells, suitable diagenetic minerals directly above the unconformity in the Woodford Shale contain fluid inclusions with homogenization temperatures comparable to those below the anomaly. Highest temperatures coincide with a high-permeability zone developed towards the top of the Hunton carbonates. The thickness and permeability of this zone depends on the development of a paleokarst after Hunton deposition and are controlled by the paleogeographical position within the basin. Depending on the location within the basin, fluids evolve generally from meteoric or moderately saline (up to [approximately]10 weight% NaCl equivalent) with temperatures of 80[degrees] to 120[degrees]C to highly saline (up to [approximately]26 weight% NaCl equivalent) with temperatures up to 260[degrees]C, followed by a decrease in temperature with either increasing or decreasing salinities. Associated petroleum inclusions suggest that the hottest temperatures coincide with peak petroleum migration. Temperature anomalies are highest in the north near the Hunton subcrop on the basin shelf where themore » total Hunton thickness is generally <100 feet, and the strongly karsted carbonates are sandwiched in between the Woo and Sylvan Shales resulting in a highly focused flow.« less

Reservoir diagenesis and hydrocarbon migration under hydrostatic palaeopressure conditions

AbstractFluid inclusion studies from two contrasting fields in the Northern North Sea reveal hydrostatic pressure conditions during the formation of secondary quartz overgrowths which trapped both brine and petroleum inclusions. In these fields, dating of authigenic illite, part of the diagenetic sequence and broadly coeval with quartz cementation, permits comparison with modelled pressure conditions. Pressure modelling indicates the likelihood of several periods of overpressure in the past, interspersed with normal pressure. The timing of diagenesis coincides with one of these periods of hydrostatic pressure. Furthermore, as both fields are now highly overpressured, the cause and timing of overpressure generation post-date the diagenesis. A model is proposed to link the pressure history and diagenesis.

Hydrocarbon composition of authigenic inclusions: Application to elucidation of petroleum reservoir filling history

Geochemical analysis of petroleum inclusions trapped in authigenic feldspar and quartz in the Ula Formation in the North Sea Ula oil field revealed a petroleum of markedly different composition than the oil presently in the reservoir. Using microthermometry and the burial history as a dating tools, it is concluded that the petroleum in the K-feldspar inclusions was present in the more porous and permeable parts of the Ula Formation as early as 45–75 My Bp when the field was at a depth of about 1.0–1.5 km, as compared with the current depth of 3.4 km. This early petroleum, which was trapped as inclusions in authigenic K-feldspar, shows a distinctly different distribution of tricyclic terpanes and pentacyclic triterpanes from that of the current petroleum charge in the Ula Formation, which was derived from the Mandal Formation source rock in late Neogene time. Molecular parameters show that the oil in the K-feldspar inclusions is significantly less mature than the crude oil in the present reservoir. The approximate 90°C temperature increase occurring after entrapment of the early petroleum in Kfeldspar (the field is currently at 143°C) appears not to have reset the low maturity signature of the oil in the K-feldspar inclusions. This could suggest that the temperature in the inclusions is too low for isomerization/selective thermal degradation to occur (lack of catalysts?), or that there are other controls on the ratio of some of these parameters. Still, parameters like the ratio of C 21 to C 28 triaromatic steroids, and those based on dimethyl- and trimethyl-naphthalenes, are comparatively similar in both the inclusions and in the reservoir crude. The oil inclusions in authigenic quartz and albite, formed from about 10 My BP (burial depth ≈ 2.5 km) until the present (burial depth = 3.4 km), are interpreted as representing a palaeo-petroleum charge having a composition intermediate between the oil found in K-feldspar inclusions and the oil charge in the present reservoir. Our data suggest strongly that the petroleum trapped in the inclusions in quartz and plagioclase represents progressive dilution of the early reservoired oil, now found only in K-feldspar inclusions, with oil from the Mandal Formation. Analysis of solution gas in these inclusions suggests that the first petroleum-associated gas to arrive at the trap from the progressively maturing Mandal Formation had a much wetter composition than the current reservoir solution gas. To explain the present comparatively drier and isotopically lighter reservoir solution gas composition, the inference is made that there has been a recent (<5 My Bp) influx of comparatively dry thermogenic gas to the field. This interpretation is independently supported by the noncogenetic δ13 C isotope signature of C 1 relative to δ 13 C 2−4 in the present reservoir solution gas. During formation of K-feldspar inclusions (45–75 My BP), maturation of the Upper Jurassic Mandal Formation, the main source rock in this region, was limited possibly to the Graben axis. This suggests that the petroleum found in the K-feldspar inclusions was most likely derived from a deeper post Caledonian Palaeozoic formation.

Relationship between reservoir diagenetic evolution and petroleum emplacement in the Ula Field, North Sea

The diagenetic and petroleum filling histories of the Ula Field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements. This study shows that diagenesis continues actively after the arrival of petroleum in the sandstones, although the reaction rates and petroleum saturation remain obscure. Microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110°C (i.e. ≈ 2.5 km) and that this cementation has continued to the present day. Synchronously with quartz cementation, authigenic albite was formed as inter- and intragranular cements. Large amounts of petroleum inclusions locally occur in the albite and quartz cements. These inclusions were trapped in the cements as the main charge of petroleum arrived at the structure. Diffusion, through thin water layers between the petroleum and the mineral surface, would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite. It is evident from analysis of the petroleum inclusions in early formed K-feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth, before the rapid subsidence in the last 25 my. Compared with petroleum in the free porosity of the Ula Formation, the petroleum in the K-feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature. Later trapped petroleum inclusions, in quartz and albite, have characteristics found both in K-feldspar and in the Ula Formation DST oil and, thus, is likely to reflect the progressive change in the Ula Field petroleum charge which occurred during the time period of quartz diagenesis.

Petroleum migration in the Miocene Monterey Formation, California, USA: constraints from fluid-inclusion studies

AbstractThe Miocene Monterey Formation constitutes a fracture-controlled petroleum reservoir, with intercalated calcareous and fine-grained siliceous rocks serving as both the source and reservoir for oil accumulations. Petroleum is produced from macroscopic fractures, and numerous tar and asphalt seeps at the surface attest to the present-day movement of hydrocarbons through fractures in the Monterey Formation. Many fractures are filled with carbonate (mostly calcite and dolomite), quartz, baryte and anhydrite. These same fractures often contain tar or oil filling openings, and occasionally a thin layer of oil can be seen coating growth surfaces between two generations of vein-filling minerals.Evidence for migration of fluids through these fractures in the geological past is provided by aqueous and petroleum fluid inclusions contained within vein-filling minerals. Vein-filling dolomite from Jalama Beach contains three different types of primary petroleum inclusions (based on fluorescence characteristics)—indicating that oils with significantly different API gravities flowed through the fractures. Petrographic and microthermometric analyses of oil and coexisting aqueous inclusions indicate that the fracture-filling minerals precipitated from aqueous solutions of seawater salinity at ∼75–100°C, and that oil was introduced into the fracture system episodically during mineral growth. A sample from the Lion's Head area consists of early calcite and late quartz, both of which contain aqueous inclusions with seawater salinity. Inclusions in quartz homogenize at slightly higher temperatures than those in calcite. These data are consistent with calcite deposition during an early heating event, followed by quartz deposition during cooling. No petroleum inclusions were observed in the Lion's Head sample.

Unusual, oil-bearing inclusions in fluorite from Baluchistan, Pakistan

AbstractPhenomenally large (up to 2 mm) oil-bearing and associated brine inclusions in fluorite from carbonate-hosted, epigenetic, fluorite-calcite-(baryte) deposits of the Koh-e-Maran area are investigated using a combination of microthermometry, UV-microscopy and FTIR microspectroscopy. The liquid hydrocarbon phase in primary ‘oil’ inclusions is brown in colour and is dominated by saturated, low molecular weight, aliphatic hydrocarbons.Two types of mixed aqueous and ‘oil’ inclusions occur. Aqueous/oil types represent co-eval trapping of immiscible drops of oil and brine during primary growth. In oil/aqueous inclusions the oil appears to ‘wet’ the aqueous phase resulting in an odd ‘dish-shaped’ meniscus. The oil or liquid hydrocarbon part of the inclusion is primary but the aqueous part is thought to represent a secondary infill. The fluid inclusion evidence suggests that fluorite precipitated from a dilute (3.5 wt.% NaCl) brine at temperatures around 110–140°C in the presence of an immiscible liquid hydrocarbon phase dominated by saturated, light hydrocarbons. This ‘oil’ was present as an emulsion in the aqueous fluid and the phenomenal size of the inclusions is thought to reflect the large droplet size in the emulsion. Infiltration of a more saline, calcium-enriched brine into pre-existing oil inclusions resulted in complex oil-water inclusions showing a reversal in the nature and shape of the oil-water interface due to the presence of unspecifed surfactants in the brine which affected the wetting characteristics of the oil.The homogenization temperatures and the presence of liquid petroleum inclusions are characteristic of Mississippi Valley-type (MVT) and manto-type fluorite deposits in many other parts of the world.

Goethite-bearing brine inclusions, petroleum inclusions, and the geochemical conditions of ore deposition at the Jumbo mine, Kansas

Petroleum-bearing fluid inclusions occur in sphalerite, calcite, dolomite, and barite at the Jumbo mine, a Mississippi Valley-type deposit in eastern Kansas. In addition to petroleum, Na-Ca-Mg-Fe chloride brines were present during deposition of calcite and sphalerite in which primary inclusions contain ≳23 equivalent wt.% NaCl. Dolomite- and barite-hosted inclusions are more dilute, possibly because of mixing between hydrothermal fluids and groundwater during mineralization. Primary oil inclusions in sphalerite have homogenization temperatures ( Th) between 85 and 95°C. Aqueous inclusions have Th values ranging from ~90 to 130°C for sphalerite to below ~50°C for barite. Primary brine inclusions in calcite at the Jumbo mine contain goethite, apparently as a daughter mineral (although it is possible that the goethite formed as a result of diffusion of H 2 from the inclusions like the hematite daughter described by Roedder and Skinner, 1968, from Bingham, Utah). Goethite has also been tentatively identified in inclusions from the Fletcher mine of Missouri. If goethite is a true daughter phase, it implies the presence of oxidized fluids during mineralization. This suggests that ore deposition resulted from interactions between hydrothermal fluids and dilute groundwater ( e.g., by dilution or cooling of ore fluids, or oxidation of organometallic complexes that may have transported metal constituents).

Temperatures of mineralization by liquid inclusions, Cave-in-Rock fluorspar district, Illinois

Temperatures of vapor bubble disappearance of liquid inclusions in fluorite, sphalerite, and quartz from the Deardorff and Victory mines of the Cave-in-Rock fluorspar district were determined. Inclusions were studied in minerals of both principal stages of deposition indicated in the deposits. The data obtained indicate that ore deposition took place in the temperature range of 94 degrees C. to 142 degrees C., assuming a pressure correction of +13 degrees C., and that temperature oscillated during mineralization. Fluorite, the first mineral to be deposited in each stage, was deposited in the upper part of the temperature range, and sphalerite and quartz, which followed fluorite in each stage, were deposited at lower temperatures. Petroleum inclusions in fluorite show considerably lower temperatures of vapor bubble disappearance than aqueous inclusions, but petroleum inclusions cannot be used for temperature determinations, because pressure corrections cannot be estimated. The basic assumptions of the method are evaluated with respect to the deposits and it is concluded that no errors of first order magnitude are expected. The apparent variation in temperature during mineralization is tentatively interpreted as the result of 2 pulsations of hydrothermal solutions from a single source.