Achim Bechtel

Evolution of lacustrine systems along the Miocene Mur-Mürz fault system (Eastern Alps, Austria) and implications on source rocks in pull-apart basins

Tectonically controlled lakes developed during Miocene lateral extrusion of the Eastern Alps. Mineralogy, and the inorganic and organic geochemistry of rocks from three boreholes were investigated to reconstruct the evolution of Lake Ingering and Lake Groisenbach and to study the distribution of source rocks in pull-apart basins. Gas-prone coal and oil-prone sapropelic shale accumulated during the initial, shallow stages of Lake Ingering. Thereafter, the lake deepened rapidly. 125-m-thick prodelta shale containing a type II kerogen was deposited in the brackish, several hundred meter deep, hydrologically closed lake. Afterwards, decreasing subsidence allowed the filling of the lake by prograding deltas. During the advance of the deltaic systems, the lake became shallower, hydrologically open, and the brackish influence terminated. Source rock quality decreased significantly during the filling stage of the lake, a consequence of dilution of autochthonous organic matter and of enhanced input of land plants. Despite its considerable dimensions, formation and filling of Lake Ingering lasted only two million years. Lake Groisenbach was considerably smaller and more susceptible to high-frequency changes in lake chemistry. Although the water body was temporarily oligosaline, brackish conditions did not occur. High sulphur contents were due to anoxic events and the inflow of Ca-rich waters. Abundant dissolved silica favoured diatoms blooms.

Tectonic and climate control of oil shale deposition in the Upper Cretaceous Qingshankou Formation (Songliao Basin, NE China)

Oil shales were deposited in the Songliao Basin (NE China) during the Upper Cretaceous period, representing excellent hydrocarbon source rocks. High organic matter (OM) contents, a predominance of type-I kerogen, and a low maturity of OM in the oil shales are indicated by bulk geochemical parameters and biomarker data. A major contribution of aquatic organisms and minor inputs from terrigenous land plants to OM input are indicated by n-alkane distribution patterns, composition of steroids, and organic macerals. Strongly reducing bottom water conditions during the deposition of the oil shale sequences are indicated by low pristane/phytane ratios, high C14-aryl-isoprenoid contents, homohopane distribution patterns, and high V/Ni ratios. Enhanced salinity stratification with mesosaline and alkaline bottom waters during deposition of the oil shales are indicated by high gammacerane index values, low MTTC ratios, high β-carotene contents, low TOC/S ratios, and high Sr/Ba ratios. The stratified water column with anoxic conditions in the bottom water enhanced preservation of OM. Moderate input of detrital minerals during the deposition of the oil shale sequences is reflected by titanium concentrations. In this study, environmental conditions in the paleo-lake leading to OM accumulation in the sediments are related to sequence stratigraphy governed by climate and tectonics. The first Member of the Qingshankou Formation (K2qn1) in the Songliao Basin, containing the oil shale sequence, encompasses a third-order sequence that can be divided into three system tracts (transgressive system tract—TST, highstand system tract—HST, and regressive system tract—RST). Enrichment of OM changed from low values during TST-I to high-moderate values during TST-II/III and HST-I/II. Low OM enrichment occurs during RST-I and RST-II. Therefore, the highest enrichment of OM in the sediments is related to stages of mid-late TST and early HST.

The type section of the Maikop Group (Oligocene-Lower Miocene) at the Belaya River (North Caucasus): Depositional environment and hydrocarbon potential

The type section of the Oligocene to lower Miocene Maikop Group, considered the main source rock in the eastern Paratethys, has been studied using geochemical proxies to gain insights into depositional setting and hydrocarbon potential. The Maikop Group at the type section is approximately 600 m (2000 ft) thick. Deposition commenced after a major late Eocene sea level drop and a subsequent early Oligocene sea level rise. The Maikop Group is composed mainly of carbonate-free pelitic rocks. Calcareous rocks are limited to the lower Oligocene succession, including the Polbian Bed that forms a basin-wide marker horizon deposited during a time with significantly decreased salinity (Solenovian event). Anoxic conditions prevailed and were only interrupted for longer periods during deposition of the lower part of the lower Oligocene Pshekha Formation, the Polbian Bed, and the lower Miocene Olginskaya Formation. Total organic carbon (TOC) contents range up to 3.5 wt. %. Hydrogen index values are typically less than 300 mg hydrocarbons (HC)/g TOC but reach 420 mg HC/g TOC in black shales overlying the Polbian Bed (lower Morozkina Balka Formation). Organic richness of this level, approximately 10 m (33 ft) thick, is controlled by low salinity and high bioproductivity. The Maikop Group could generate approximately 2.0 t HC/m2 surface area. A significant part (0.45 t/m2) comes from the lower Morozkina Balka Formation, which generates a high-wax paraffinic–naphthenic–aromatic mixed oil. The Pshekha, upper Morozkina Balka, and Batalpashinsk Formations would generate low-wax oil or condensate. The hydrocarbon generation potential of the overlying formations is minor. Overall, the generation potential of the Maikop Group is surprisingly low.

Depositional environment and source potential of Jurassic coal-bearing sediments (Gresten Formation, Höflein gas/condensate field, Austria)

Coal-bearing Jurassic sediments (Gresten Formation; Lower Quartzarenite Member) are discussed as source rocks for gas and minor oil in the basement of the Alpine–Carpathian frontal zone (e.g. Höflein gas/condensate field). Core material has therefore been analysed to characterize depositional environment and source potential of the Lower Quartzarenite Member (LQM). Geochemical data from the Höflein condensate are used to establish a source–condensate correlation. The LQM was deposited in a flood basin with transitions to a delta-plain environment. Coal originated in frequently flooded mires and evolved within an oxygenated and acidic environment. It is inferred from geochemical data that organic matter from aquatic macrophytes and gymnosperms contributed to coal formation. Wildfires were abundant and oxidation of plant remains occurred frequently. This resulted in the formation of dull coal with very high inertinite contents. Bituminous shales were formed in deeper waters under dysoxic conditions. Apart from abundant algae and micro-organisms, it is concluded that there was an increased contribution of higher land plants relative to macrophytes to the biomass of the shales. Despite high inertinite contents, coal within the LQM has a significant oil potential. Bituminous shales contain a Type III–II kerogen. According to pyrolysis–gas chromatography data, coal and shale generate a high wax paraffinic oil. The organic matter is immature to marginal mature (0.55% Rr). Bituminous shales are considered a potential source for the Höflein condensate. Coal may be the source for gas and minor oil in the Klement Field, but is not the source for the condensate. The equivalent vitrinite reflectance of the condensate is 0.8%, suggesting condensate generation at 4–4.5 km depth. The Gresten Formation reaches this depth near its depocentres, implying southward-directed migration of the Höflein condensate.

Thermochemical and geochemical characteristics of sulphur coals

Abstract Gas chromatography–mass spectrometry (GC-MS) method was applied for investigation of the extracts obtained from three pairs of Donets bituminous coals (76–79% of C daf ) of similar rank but differing in sulphur content. The elemental characteristic of the coals and hydrocarbon composition of their extracts reflect the differences in the environments of sulphur coals formation and differences in their structure. The thermal and natural coalification pathways of low- and high-sulphur coals formed under low-reduced and reduced conditions during early diagenesis were determined.

From shale oil to biogenic shale gas: Retracing organic–inorganic interactions in the Alum Shale (Furongian–Lower Ordovician) in southern Sweden

Methane-rich gas occurs in the total organic carbon–rich Alum Shale (Furongian to Lower Ordovician) in southern Sweden. The lower part of the thermally immature Alum Shale was impregnated by bitumen locally generated by heating from magmatic intrusions from the Carboniferous to the Permian. Organic geochemical data indicate that the migrated bitumen is slightly degraded. In the upper Alum Shale, where methane is the main hydrocarbon in thermovaporization experiments, centimeter-size calcite crystals occur that contain fluid inclusions filled with oil, gas, or water. The Alum Shale is thus considered a mixed shale oil–biogenic shale gas play. The presented working hypothesis to explain the biogenic methane occurrence considers that water-soluble bitumen components of the Alum Shale were converted to methane. A hydrogeochemical modeling approach allows the quantitative retracing of inorganic reactions triggered by oil degradation. The modeling results reproduce the present-day gas and mineralogical composition. The conceptual model applied to explain the methane occurrence in the Alum Shale in southern Sweden resembles the formation of biogenic methane in the Antrim Shale (Michigan Basin, United States). In both models, melting water after the Pleistocene glaciation and modern meteoric water may have diluted the contents of total dissolved solids (TDS) in basinal brines. Such pore waters with low TDS contents create a subsurface aqueous environment favorable for microbes that have the potential to form biogenic methane. Today, biogenic methane production rates, with shale as the substrate using different hydrocarbon-degrading microbial enrichment cultures in incubation experiments, range from 10 to 620 nmol per gram and per day.

Stable isotopes of organics and inorganics, clay mineralogy and chemical environment of an Aptian lacustrine succession in northeastern Brazil

Abstract The studied Aptian lacustrine succession of the continental Jatobá Rift Basin varies mainly between pure carbonates (predominantly laminated limestones), marls and shales, with some intercalations of presumably deltaic sandstone complexes. In accordance with geochemical data, the occurrence of dolomite indicates intensive microbial activity in a stratified water column with slightly enhanced salinity. Petrographic data prove mild weathering conditions. Aggregates of authigenic smectite observed in sandstones, probably representing transformed volcanogenic glass particles, strongly indicate explosive volcanic activity. Occasionally occurring dolomite seems to have been formed due to intensive microbial activity under moderately increased salinity conditions. Potential hydrocarbon source rocks containing Type I organic matter (OM) were deposited during various phases of the Aptian. Enhanced biological productivity is indicated by bulk organic geochemical data (Corg, hydrogen index, extraction yield) and δ13C values of carbonates within peletoidal and laminated limestone layers. Carbon-isotope ratios of carbonates argue for OM cycling and remineralization. High δ18O values of carbonates are attributed to periods of lower lake levels. A major contribution of aquatic organisms (green algae, microalgae, zooplankton) and minor input from macrophytes and land plants to OM accumulation is indicated by n-alkane distributions, steroid composition and δ13C values of individual biomarkers. Microbial communities included heterotrophic bacteria and cyanobacteria, as well as purple and green sulphur bacteria. The sediments were deposited in an alkaline palaeolake. Highly reducing (saline) bottom water conditions and a stratified water column existed during OM accumulation of the Crato Formation. This is indicated by low pristane/phytane, gammacerane index, and the presence of β-carotane and aryl isoprenoids. Differences in OM composition and stable isotope data reflect the evolution of the basin from a stratified saline lake to a freshwater environment with limited potential for OM preservation.

Palaeozoic source rocks in the Dniepr–Donets Basin, Ukraine

ABSTRACT The Dniepr–Donets Basin (DDB) is a major petroleum province in Eastern Europe. In order to understand the regional and stratigraphic distribution of source rocks for the dominantly gas-prone petroleum system, 676 fine-grained rocks from 30 wells were analysed for bulk parameters (total organic carbon (TOC), carbonate, sulphur, RockEval). A subset of samples was selected for maceral and biomarker analysis, pyrolysis-gas chromatography and kinetic investigations. Organic-rich sediments occur in different intervals within the basin fill. Maximum TOC contents (5.0 ± 1.9%) occur in the Rudov Beds, several tens of metres thick. The oil-prone rocks (Type III–II kerogen) were deposited in basinal settings above an unconformity separating Lower and Upper Visean sections. While maximum TOC contents occur in the Rudov Beds, high TOC contents are observed in the entire Tournaisian and Visean section. However, these rocks are mainly gas condensate-prone. Highly oil-prone black shales with up to 16% TOC and hydrogen index values up to 550 mgHC g–1TOC occur in Serpukhovian intervals in the northwestern part of the DDB. Oil-prone Lower Serpukhovian and gas condensate-prone Middle Carboniferous coal is widespread in the southern and southeastern part of the basin. Although no source rocks with a Devonian age were detected, their presence cannot be excluded.

Depositional environment, organic matter characterization and hydrocarbon potential of Middle Miocene sediments from northeastern Bulgaria (Varna-Balchik Depression)

Abstract The depositional environments and hydrocarbon potential of the siliciclastic, clayey and carbonate sediments from the Middle Miocene succession in the Varna-Balchik Depression, located in the south-eastern parts of the Moesian Platform, were studied using core and outcrop samples. Based on the lithology and resistivity log the succession is subdivided from base to top into five units. Siliciclastic sedimentation prevailed in the lower parts of units I and II, whereas their upper parts are dominated by carbonate rocks. Unit III is represented by laminated clays and biodetritic limestone. Units IV and V are represented by aragonitic sediments and biomicritic limestones, correlated with the Upper Miocene Topola and Karvuna Formations, respectively. Biogenic silica in the form of diatom frustules and sponge spicules correlates subunit IIa and unit III to the lower and upper parts of the Middle Miocene Euxinograd Formation. Both (sub)units contain organic carbon contents in the order of 1 to 2 wt. % (median: 0.8 for subunit IIa; 1.3 for unit III), locally up to 4 wt. %. Based on Hydrogen Index values (HI) and alkane distribution pattern, the kerogen is mainly type II in subunit IIa (average HI= 324 mg HC/g TOC) and type III in unit III (average HI ~200 mg HC/g TOC). TOC and Rock Eval data show that subunit IIa holds a fair (to good) hydrocarbon generative potential for oil, whereas the upper 5 m of unit III holds a good (to fair) potential with the possibility to generate gas and minor oil. The rocks of both units are immature in the study area. Generally low sulphur contents are probably due to deposition in environments with reduced salinity. Normal marine conditions are suggested for unit III. Biomarker composition is typical for mixed marine and terrestrial organic matter and suggests deposition in dysoxic to anoxic environments.

New geochemical data on fossil wood from the Albian of the Dolomites (Southern Alps, Italy)

Abstract Information is provided about organic-matter bearing sediments and fossil drift-wood from the Puez area (Col de Puez, Southern Alps) near Wolkenstein (S. Tyrol, Italy). The locality is located on the Trento Plateau which represents a submarine high during the Lower Cretaceous. Its terpenoid hydrocarbon composition indicates that the wood fragment derived from a conifer belonging to the family Podocarpaceae or Araucariaceae. Intense degradation of OM argues for lengthier drifting. Long-term drifting is also indicated by the infestation of the bivalve Teredo (“shipworm”). The finding of a fossil tree trunk sheds some light on the early Lower Cretaceous tectonic history of the Trento Plateau and the Dolomites.