Csanád Sajgó

Hydrocarbon generation in a super-thick Neogene sequence in South-east Hungary. A study of the extractable organic matter

Abstract In young sediments, petroleum generation takes place at relatively high temperatures, but no detailed data concerning petroleum generation in Pliocene sediments have been published so far. Based on the chloroform-soluble (CBA) and benzene-ethanol-soluble (BABA) bitumens the threshold value of intense petroleum generation appears to be 142°C in the well studied. The corresponding sedimentary formation is about 8 million yr old and its depth is 3450 m. Intense hydrocarbon generation is observed down to the well bottom at 5842.5 m (15–16 million yr, 233°C), with an apparent gap of about 400 m at 5050–5450 m. In spite of the high temperature, the cracking and decreasing abundances of bitumens and hydrocarbons are barely observed. The quantitative increase in the lighter C 14 C 20 has been produced partly by a change in the type of the organic matter.

Primary productivity and early diagenesis in the Toarcian Tethys on the example of the Mn-rich black shales of the Sachrang Formation, Northern Calcareous Alps

Organic, inorganic and isotope geochemical investigations have been performed on 25 samples - 20 laminated marl samples and 5 re-sediment samples - taken from a 27m thick black shale section of early Toarcian age, cropping out in the Northern Calcareous Alps. On the basis of MnO, Fe, organic carbon (TOC) contents and Hydrogen Index (HI) values, the laminated marls of the section can be divided into two parts. TOC contents and HI values are below 2% and 500 mg HC/g TOC in the lower, Mn-rich (and Fe-rich) part (MnO between 2 and 16%, Fe between 3.9 and 6.4%), while they range between 5 and 9% and 600-700 mg HC/g TOC, respectively in the upper, Mn-poor (and Fe-poor) part (MnO < 1%, Fe between 1.5 and 3.8%). Both original amount of organic carbon (TOC or ), calculated from TOC, MnO and sulphur contents, and CaO concentration show a strong upward increase in the lower, Mn-Fe-rich part of the section. TOC or and CaO are believed to represent planktonic particles hence their simultaneous upward increases are interpreted as the result of an at least 2.5 fold increase of productivity during the first part of the black shale event. Following the same logic, in the second part of the black shale event productivity is believed to have slightly decreased from the previously reached high level. Stratigraphic variation of the sulphur isotopic ratio support this scenario. Comparison of organic geochemical and δ 34 S data of the re-sediments with those of the neighbouring marls suggests that increase and decrease (?) of productivity was paralleled by expansion and withdrawal of oxygen-depleted waters. During the black shale event the extent of the oxygen-depleted bottom water was governed by changes in intensity of productivity. Changes in rates of deposition of Mn and Fe were not related to those of the productivity but they deeply influenced nature and intensity of bacterial degradation of organic matter and especially the incorporation of sulphur into kerogen. The high contribution of Mn- and Fe-minerals in the case of the lower Mn-rich part prevented by both dilution and degradation of OM due to Mn-reduction an intense incorporation of sulphur into OM. In the upper, Mn- (and iron) poor part the high initial C org /Fe ratio led to an important incorporation of sulphur into OM and an early termination of sulphate reduction.

White micas with mixed interlayer occupancy: a possible cause of pitfalls in applying illite Kübler index ( crystallinity ) for the determination of metamorphic grade

Integrated microstructural observations, X-ray powder diffractometric (XRPD) modal composition, illite Kubler index and chlorite “crystallinity” determinations and vitrinite reflectance measurements were carried out on marly slates from selected profiles of the Helvetic zone of the Central Alps, Switzerland. The studied profiles were: Upper Jurassic from the Wildhorn nappe, Brienz, Upper Jurassic from the Parautochthonous of the Aar massif, Glarus Alps, Eocene from the Griesstock nappe of the Glarus Alps and Upper Jurassic from the Axen nappe, Rhine Valley. In some of the localities studied, illite Kubler index (“crystallinity”) values were anomalously high, yielding only diagenetic conditions, while chlorite “crystallinity” and vitrinite reflectance showed anchi- and epizonal metamorphic conditions. Detailed XRPD observations carried out on Ca-saturated and glycolated mounts indicated subordinate amounts of swelling (smectitic) interstratifications in white mica. In addition to the dominant K-white mica, traces of discrete paragonite and paragonitic phases and tobelitic impurities in the form of either regular interstratifications or micas with mixed (K≫Na>NH4) interlayer compositions could be detected from the XRPD (00,10) basal reflections. On the basis of the organic maturity assumed from vitrinite reflectance, probable partitioning of N and H between organic and inorganic phases, and the results of elemental (C, H, N, S) analyses carried out on the <2 μm fraction decarbonated and oxidized samples, the amount of NH4+ fixed in inorganic phases could be estimated. Small, but systematically appearing, absorption bands between 1400 and 1440 cm−1 in the FTIR spectra unequivocally proved the presence of ammonium in the rocks studied. Small amounts of N within the mica flakes were detected by electron energy loss spectrometry (EELS), confirming that NH4+ is indeed fixed in the interlayer site position. Energy dispersive spectroscopy (EDS) using scanning transmission electron microscopy (STEM) revealed heterogeneities in the interlayer cation occupancies. Although K is always dominant, irregular, local, domain-like enrichments in Na could be seen. The most accurate model to describe the disequilibrium state of the incipient metamorphic white micas studied is that of a dioctahedral mica structure with irregularly varying interlayer occupancies combined with subordinate amounts of swelling mixed-layers. The present work shows that, if these white micas are to be used for metamorphic petrogenetic purposes, special attention should be paid to their detailed characterization, especially in organic matter rich lithologies often characterized by high Al/Si bulk chemical ratios. Seemingly, white micas with mixed interlayer occupancies may be more widespread than has generally been anticipated so far.

Variations in organic geochemistry and lithology of a carbonate sequence deposited in a backplatform basin (Triassic, Hungary)

Abstract The variations in the organic matter quantity and quality were studied with respect to the mineral composition of the carbonate sequences accumulated on a gentle slope (Zl-1 well) and at the toe of the slope (Rzt-1 well) located between a Late Triassic carbonate platform and a backplatform basin. Parallel variations observed in mineral composition and organic geochemical features of the successions appeared to be controlled by the change in climate and by sea-level fluctuations. The repetitive sea-level changes resulted in a variation in the carbonate-rich basin facies and in the mineralogically heterogenous slope and toe-of-slope ones. According to Rock Eval pyrolysis, organic petrography and carbon isotope ratios, the immature organic matter is of predominantly marine origin and composed of mainly liptinites in both of the studied boreholes. The results of the GC and GC/MS analyses of the saturated hydrocarbon fractions of bitumens together with the composition of kerogen pyrolysates reveal a predominant algal input with a minor variable bacterial and subordinate terrestrial contribution for the Rzt-1 borehole. In the Zl-1 borehole a significantly higher proportion of the bacterial biomass contributed to the organic precursors. The δ 13 C values and the composition of the kerogen pyrolysates together with the results of the maceral analysis and GC data suggest a relatively higher, but moderate, higher plant derived contribution in the slope facies and at the top of the toe-of-slope facies. The elementary composition of kerogens and Rock Eval data display type II-S kerogen in the basin and the slope facies, and type I-II-S one in the toe-of-slope facies. Variations in the hydrogen content of the organic matter mainly reflect variations in the preservation conditions and in primary productivity. The presence of the 2,6,10,15,19-pentamethyleicosane and the extremely low pristane/phytane ratios indicate a relatively high methanogenic bacterial activity and strongly anoxic depositional conditions in the Rzt-1 well, especially in two most organic-rich toe-of-slope facies.

Comparison of Pliocene organic-rich lacustrine sediments in twin craters

The organic matter of Pliocene oil shales from maar-type twin craters (Egyhazaskeszoand Varkeszo¨ ) in Hungary was studied by diAerent analytical techniques (Rock-Eval pyrolysis, bitumen analysis, FTIR, elemental analysis and pyrolysis of the insoluble material). The organic-rich, alginitic layers were deposited at the same time, under the same palaeoclimatic conditions and have basically similar lithologies. Despite this, the oil shale deposits from each crater show distinct diAerences. Furthermore, within each crater, the older oil shale deposits are diAerent from the younger. This phenomenon can be explained both by variations in organic matter input and changes in the depositional envir- onment. The principal source of the organic matter is the microalgae Botryococcus braunii, but the terrestrial con- tribution is also significant. The prevalence of the algal material is supported by the elemental composition and kerogen pyrolysis data. The pyrograms show that there is considerably more algal material in the Varkeszosamples. Kerogens in the Egyhazaskeszocrater contain much more organic sulphur and pyrite is more abundant. The nominally Type II kerogens in the twin craters are the products of diverse processes. Varkeszokerogens are in fact mixtures of Type I and Type III organic matter and are preserved relatively well. Egyhazaskeszokerogens must have suAered biological degradation and chemical alteration during pyrite formation, resulting in medium sulphur-rich Type II kerogen formation. # 2000 Elsevier Science Ltd. All rights reserved. During the Pliocene several oil shales were deposited in small maar-type volcanic craters in Hungary (Solti, 1990). Four to five million years ago in the Pannonian lake system, very intense and repeated volcanic erup- tions disturbed the sedimentation. The basic magma gave rise to stratavolcanoes and the formation of tuA rings. The craters formed by basalt volcanism filled with water of the Pannonian Lake after volcanic activity had ceased. The small, separate lakes were current free and oligohaline. The depositional environment — warm water caused by postvolcanic geysers and an abundant nutrient supply due to the intense weathering of the

Hydrogen isotope type-curves of very hot crude oils

Several crude oil accumulations in the Pannonian Basin are trapped in uncommonly hot (>170°C) reservoirs. Their maturities range from mature to very mature on the basis of cracking parameters of their biological marker homologous series (ratio of products to reactants). A stable carbon isotopic study of these oils, poor in biological markers commonly used for correlation purposes, did not provide a reliable oil-to-oil correlation. As an alternative tool, the hydrogen isotope compositions of oil fractions separated on the basis of different polarities were measured, and hydrogen isotope type-curves were generated for a set of mature to very mature crude oil samples. This method of presenting hydrogen isotope composition of fractions as type-curves is novel. Nineteen samples (17 crude oils from SE-Hungary, 1 oil condensate and 1 artificial oil) were chosen for the present study. The aim was to examine the applicability of hydrogen isotope type-curves in oil-to-oil correlation and to test the simultaneous application of carbon and hydrogen isotope type-curves in the field of petroleum geochemistry. We have shown that, although the conventionally used co-variation plots proved to be inadequate for the correlation of these hot and mature oils, the simultaneous use of carbon and the newly introduced hydrogen isotope type-curves allows us to group and distinguish oils of different origins.

Basin-centred gas in the Makó Trough, Hungary: a 3D basin and petroleum system modelling investigation

ABSTRACT World-wide exploration for unconventional hydrocarbon accumulations has been increasing in the last decade. The deep Makó Trough of the Pannonian Basin in southeast Hungary has become a new target for companies looking for unconventional gas resources, but so far the exploration drilling has been unsuccessful. To investigate the size of the assumed basin-centred gas accumulation we have studied the hydrocarbon potential of the source rocks in the Makó Trough using 3D basin and petroleum system modelling technology. The thermal and maturity history and timing of hydrocarbon generation were assessed, and the generated volume of hydrocarbons estimated and compared with the pore volume of the assumed unconventional reservoirs. The estimated mean volume of gas generated in the drainage area (490–650 × 109 Sm3) [given at surface conditions] is much less than the volume (>14 × 1012 Sm3) needed to fill the basin-centred gas accumulation. Therefore, the Makó Trough is unlikely to contain a large tight-gas sand accumulation. The Endrőd Marl is a fair quality, gas-prone source rock with average original TOC values of 0.75 wt%, reaching 1.5 wt% in the Hód-I and Makó-7 wells. These are below the TOC values of the proven gas shales of North America.

Upward increase of kerogen δ13C in the Peru Margin Upper Oligocene: possible implications for the Cenozoic evolution of atmospheric CO2

Abstract Carbon isotopic composition of predominantly marine kerogen in latest Oligocene mudstones of the Peru Margin ODP 682A Hole shows an about 3.5‰ increase with decreasing age. Py-GC and elemental (C/N ratio) analysis of the kerogen plus sulphur isotopic study together with earlier knowledge on geological setting and organic geochemistry results in a better understanding of depositionary environment and allows to separation of the influence of concentration of water dissolved carbon dioxide (ce) on kerogen δ13C from that of other factors (bacterial degradation, sea surface temperature, DIC δ13C, productivity, and admixture of land plant OM). Based on this analysis, the major part of the kerogen shift is considered as a result of the latest Oligocene decrease of marine photosynthetic carbon isotopic fractionation in the Peru Margin photic zone, which in turn possibly reflects a simultaneous drop in atmospheric CO2 level. Uncertainties in the evaluation of the factors affecting the marine photosynthetic carbon isotopic fractionation and the extent of ocean–atmosphere disequilibrium do not permit calculation of the decrease of the atmospheric CO2.