Wolfgang Stahl

Near-Surface Evidence of Migration of Natural Gas from Deep Reservoirs and Source Rocks

Gaseous hydrocarbons migrate to the surface in small quantities from deep source rocks and natural gas or crude oil accumulations. This process leads to anomalous concentrations of hydrocarbons in near-surface sediments. The hydrocarbon anomalies (amount and composition) are useful in exploration, because they may point to oil and gas reservoirs. The origin of hydrocarbon anomalies in shallow sediments is not a simple one. Methane in near-surface sediments may be caused by a combination of bacterial production of methane, leakage from reservoirs, and leakage of methane from thermal maturation of organic matter in source rocks. Organic geochemistry, especially carbon isotope techniques, have been used to recognize secondary fractionation processes caused by degassing or bacterial oxidation. Examples of geochemical surveys in onshore and offshore areas indicate secondary fractionations so small that gases from near-surface sediments and from reservoirs can be correlated by the geochemical data. The application of stable carbon isotopes is a significant step toward improving geochemical hydrocarbon surface exploration methods.

Isotopic tracing of methane in water and exchange with the atmosphere

Sediments of Lake Constance are rich in bacterial methane (yield CH 4 -85‰). However, methane flux from the sediment into the water was not detected. Deep water (below ca. 50 m) has background signature (yield < 50 nL/L ; -40< δ 13 CH 4 < -30‰). Surface water is enriched in methane relative to atmospheric equilibrium, most probably the methane is produced by methanogens in the surface layers. The surface water is a source for atmospheric methane ; calculated flux rates exceed high values reported for ocean water. Deep water of the Bay of Bengal is generally low in methane (background signature). At two locations at the continental slope, bacterial methane (δ 13 CH 4 < -50‰) seeps from the sediments into the water. Methane concentration in the surface water is close to atmospheric equilibrium in the equatorial area but higher in the Ganges/Bramaputra mouth, where methane flux to the atmosphere reaches up to 30 kg km -2 yr -1 .

Geochemistry of Gaseous Hydrocarbons Adsorbed in Shallow Sediments Offshore Santa Barbara, California: ABSTRACT

An offshore geochemical surface survey was conducted west of Santa Barbara, California. Two hundred sediment samples were taken in an area of approximately 400 sq km. The samples were degassed and the gases analyzed by gas chromatography and isotope mass spectrometry. GC-MS techniques were applied to the extracts of several samples to look for C28 and C29 steranes. In addition, grain size determinations and fluorescence analyses were carried out on selected samples. The analytical results, mainly the 13C/12C ratios of the methane, allow discrimination between (1) an area of exclusively bacterial methane and (2) an area where thermogenic (petroleum-related) hydrocarbons are present. The area of biogenic gases is characterized by isotopically light methane in the range of ^dgr13C1 = 90 ppt. High methane yields occur up to 50,000 ppb. The area of thermogenic gases is characterized by ^dgr13C1- values in the range of -44 ppt which correspond to methane originating from predominantly marine source rocks (type II kerogen) within the oil window. The methane yields are approximately 200 ppb. Three oil fields, not yet in production, are situated within the area exhibiting the shallow, thermogenic methane. This observation supports the conclusion that hydrocarbons can migrate to the surface from deeper source rocks or from deep gas and oil reservoirs where they can be identified by isotope geochemistry. The application of this methodology to the field of hydrocarbon exploration is obvious. End_of_Article - Last_Page 997------------

Das Klimasystem der Erde

Das irdische Klimasystem ist in unserem Sonnensystem einzigartig. Es hat dazu gefuhrt, das sich vor mehr als 3 Mrd. Jahren in den Ozeanen der Erde Leben entwikkeln konnte. Diese Lebewesen waren zunachst sehr primitiv. Im Laufe der Erdgeschichte fand allmahlich eine Hoherentwicklung statt. Das entstehende Leben auf der Erde hat die Bedingungen, unter denen es sich spater entwickelte, selbst entscheidend gepragt: Erst durch die Entwicklung der Photosynthese und damit der Produktion von freiem Sauerstoff konnte sich hoheres Leben entfalten, bis vor ca. 230 Mio. Jahren die ersten Saugetiere auftraten.

Carbon Isotope Measurements of Hydrocarbons Adsorbed in Near-Surface Sediment Samples: ABSTRACT

Geochemical surface exploration for hydrocarbons can be markedly improved by carbon isotope analyses of methane which is adsorbed in near-surface sediments. This technique allows isotopic determinations on samples of 25 µL of methane with an overall ^dgr13C1 reproducibility of approximately ± 1 part per thousand. End_of_Article - Last_Page 698------------

Lagerung von Abfällen

Archaologen verwenden bei der Rekonstruktion der Lebensweise unserer Vorfahren auch die bei Ausgrabungen gefundenen Abfalle und Gebrauchsgegenstande. Die Scherben von Topferwaren hatten, auch wenn sie sich uber Jahrtausende nicht zersetzt haben, fur die Nachwelt keinen dauernden Schaden angerichtet.

Eine Erde für alle

An der Schwelle zum 21. Jahrhundert orientieren sich die internationalen Kontakte des Geologischen Dienstes der Bundesrepublik Deutschland mit den Landern der Dritten Welt verstarkt am Prinzip der partnerschaftlichen Kooperation. Um die Existenzgrundlage der Menschheit auf unserer Erde zu erhalten, mus der Teufelskreis von Bevolkerungswachstum, Umweltzerstorung und Armut auf globaler Ebene aufgebrochen werden.

Carbon Isotopes--New Tool in Hydrocarbon Correlation and Exploration: ABSTRACT

The recently developed isotopic type-curve technique is based on the 13C/12C ratios of saturates, aromatics, heterocyclics, asphaltenes, and kerogen. This technique has been applied to problems of correlation of oil with oil and oil with source rocks, and has also been used to identify bacterial degradation in crude oils. Correlation of gas with source rock is essentially related to the determination of the carbon isotope ratio of methane. The ratio links the gas to the type and maturity of its organic source material. This technique was applied to head space analyses of canned drillhole samples. The type and maturity of source rocks can be evaluated as shown by isotope-depth profiles from different basins. Migration of gaseous hydrocarbons from deep sources to higher strata becomes evident. End_of_Article - Last_Page 532------------

Bacterial Degradation of Crude Oils--Identification by Carbon Isotopes: ABSTRACT

The recently developed isotope type-curve technique for oil/oil and oil/source rock correlation is based on the determination of the carbon isotope ratios of different crude oil fractions. This method can be used for the identification of bacterial oxidation of crude oils and might be useful in controlling the rate of degradation after environmental catastrophes such as oil spills. Laboratory experiments show the change in compositional and isotopic data of a crude oil and of gases which are dissolved in the oil under attack. The results are in agreement with geochemical data of naturally degraded crude oils. End_of_Article - Last_Page 701------------

Origin of Gases Adsorbed in Near-Surface Sediments Identified by Carbon Isotopes: ABSTRACT

Carbon isotope ratios of gases desorbed from nearshore sediment samples collected from the Anadarko basin and from the North Sea have been determined by a technique recently developed in the Hannover laboratories. The results show a relation to geochemical data of the deep pooled hydrocarbons in both areas. Because of this relation, isotope analyses of adsorbed gases are of great interest in crude oil exploration. End_of_Article - Last_Page 701------------