J. Connan

Properties of gases and petroleum liquids derived from terrestrial kerogen at various maturation levels

This study deals mainly with shale-sandstone series in which the disseminated kerogen is mostly composed of land-derived debris. Organic matter was characterized by microscopic and chemical techniques. The kerogen maturity was assessed by microscopic studies, mainly by means of vitrinite reflectance measurements. The chemical properties of shale kerogens and of oil and gas shows, were examined in several sedimentary basins in different parts of the world. Oil and gas properties were tentatively interpreted in terms of maturity, using a comparison of oil properties with the kerogen features of shales interbedded in the impregnated sandstone reservoirs. The synthesis is documented in several case histories including those from New Zealand, Colombia, Australia, Indonesia. In low maturity stages (immature zone), dry gas with minor condensate is observed, whereas in higher maturity levels (oil window), wet gas with high paraffinic crudes is generally recorded. Pristane to n-C17 ratios allow a distinction to be made between immature (often > 1.0) and mature ( 0.80) and those produced with high wax crudes or mature condensates (i-C4n-C4 < 0.80). Shallow depth condensates and their related gases have been identified as immature fluids. Properties of some mature condensates are given as references. This study offers a maturity framework as a guide for oil and gas show prediction in shale-sandstone sequences containing land-derived kerogen.

The microbial input in carbonate-anhydrite facies of a sabkha palaeoenvironment from Guatemala: A molecular approach

Abstract Nine representative rock samplesincluding four anhydrites, an oil-stained dolomite and a laminated mudstone (source rock) from a sabkha-type palaeoenvironment (La Felicidad 1 well, Guatemala) have been compared in order to tentatively characterize each facies by a geochemical signature. Each sample has been studied by using basic parameters (TOC, Rock-Eval pyrolysis yields) and analyses of alkanes and aromatics by computerized GC-MS. The following families of compounds have been reviewed: n-alkanes, branched and cyclic alkanes, n-alkylbenzenes, benzohopanes, monoaromatic steroids, D-ring aromatic 8,14-secohopanoids, alkylnaphthalenes, alkylphenanthrenes, alkyldibenzothiophenes and triaromatic steroids. Despite close geochemical relationships of all the sample, a more in-depth investigation taking into account molecular patterns and semi-quantitative data reveals significant differences among the samples. Anhydrites may be differentiated from carbonates which illustrates the efficiency of organic geochemistry in environmental palaeoreconstruction even in organiccarbon-lean sediments (TOC = 0.04–0.08% in anhydrites). In these anoxic, marginally mature sediments the strong predominance of triterpanes over steranes (TT to ST > 20) associated with other molecular features leads to the conclusion that the main contribution to the organic matter of all facies is by bacteria. Several prominent components including novel families (iso-C23 and iso-C27 alkanes, tetracyclic terpanes, D-ring aromatic 8–14-secohopanoids, 1-phenylpentadecane, undecylnaphathalene and their mono- and dimethylated counterparts) have been interpreted as derived from halophilic bacteria. These conclusions may be extended to all crude oils from the same area which may be considered as true bacterially-derived petroleums. The La Felicidad 1 well data are discussed with reference to other palaeosabkha-type sequences from the SW Aquitaine Basin (France) in which the bacterial input is less important. However, in these sabkha sequences 1-phenylpentadecane, undecylnaphthalene and related structures have also been identified.

Novel families of tetra- and hexacyclic aromatic hopanoids predominant in carbonate rocks and crude oils

Two novel families of monoaromatic geomarkers related to hopanoids have been separated and characterized in sediments and crude oils: the benzohopanes (C32–C35) 1 and the ring D aromatized 8,14-secohopanoids (C27–C35) 4 and 5. Both series are particularly abundant in many carbonate samples, where they amount to approximately 100 p.p.m. per component in some crude oils, but can also be detected in other environments. The benzohopanes have been separated as pure compounds by high performance liquid chromatography and identified by X-ray diffraction. The structure of the 8,14-secohopanoids mostly relies on spectroscopic data (NMR and MS) and comparison with synthetic analogs. Both families must be formed in the subsurface from bacterial C35 hopanoid precursors, but the benzohopanes seem to occur at an earlier stage of sedimentation. The two series appear in varying distributions in the different samples and may therefore be potentially useful as geochemical correlation parameters.

Alkylated phenanthrene distributions as maturity and origin indicators in crude oils and rock extracts

Abstract Methylphenanthrene (MP), dimethylphenanthrene (DMP), and trimethylphenanthrene (TMP) distributions have been determined in crude oils and rock extracts from different origins at various stages of thermal maturity. A methodological approach combining Correspondence Factor Analysis and Nonlinear Mapping (NLM) was used for extracting origin/maturity information from these data. It allowed to benefit from the advantages of both methods. The use of such a multivariate data analysis appeared much more efficient than the use of molecular ratios that can be too restrictive and mask specific distribution patterns. This approach performed on the set of natural samples clearly demonstrated the discrimination between the samples through the presence of specific methyl-, dimethyl-, and trimethylphenanthrene isomers as origin/maturity markers. Based on MP, DMP, and TMP distributions, it is possible to distinguish the variations in organic matter type from the effects of thermal maturation. Some substituted phenanthrenes in each isomer series appear as characteristic of the two studied systems: The Aquitaine basin as representative of a marine carbonate environment and the Mahakam delta as representative of a terrestrial environment (higher plants). These compounds could be tentatively related to natural precursors such as triterpenoids or hopanoids.

Structural investigation of nonpolar sulfur cross-linked macromolecules in petroleum

Abstract A novel hexane-soluble nonpolar macromolecular fraction (NPMF) has been found to occur in substantial amounts (up to 32%) in sulfur-rich crude oils and a rock extract. It is highly aliphatic and has a molecular weight culminating at several thousand mass units, as proven by spectroscopic and molecular weight studies. C-S bond hydrogenolysis of NPMF with Raney nickel as a catalyst yields high proportions of aliphatic hydrocarbons in which long linear, acyclic polyisoprenoid and carotenoid chains usually predominate (except in one case) over polycyclic structures, such as steroids and hopanoids. Hence, NPMF consists mainly of macromolecules composed of low molecular weight hydrocarbon subunits cross-linked with sulfide bridges. Use of deuterated Raney nickel indicated in one case (Rozel Point oil) that the long chains and some hopanoids are multiattached to the macromolecular network, whereas other structural subunits, such as steroids or gammacerane, are essentially monoattached. Detailed structural determinations of the hydrocarbon “building blocks” of NPMF give information on their origin and the mode of formation of these macromolecules in the subsurface. Indeed, most of the building blocks can be related to algal (e.g., long linear chains, steroids, β-carotene, and related carotenoids) or bacterial (e.g., acyclic and monocyclic carotenoids, long-chain acyclic isoprenoids) precursors which essentially exist in living organisms as monounsaturated or polyunsaturated species or are easily transformed into such species by diagenetic processes (e.g., steroids). It appears that these alkenes or polyenes become selectively trapped into a macromolecular network by reaction with inorganic sulfur species produced by bacteria in a kind of natural, low-temperature, vulcanization process. This process could start at early diagenesis already in the water column or eventually continue in the bottom sediment. Although its exact nature is yet unknown, it seems likely that the cross-linking reaction can be initiated by the cleavage of sulfur species in a radical type mechanism. The alkanes formed upon desulfurization of NPMF usually represent much higher amounts than the free alkanes of the samples and show a dramatically different composition. They may deliver very useful, complementary information in studies related to source and palaeoenvironment.

Formation of ultralaminae in marine kerogens via selective preservation of thin resistant outer walls of microalgae

Abstract Transmission Electron Microscopy (TEM) observations recently revealed the common occurrence of very thin lamellar structures, termed ultralaminae, in kerogens from source rocks and oil shales so far considered as amorphous. Ultralaminae in lacustrine kerogens were shown to derive from the selective preservation of the algaenans occurring in the very thin outer walls of various freshwater Chlorophyceae. The chemical correlation between such algaenans and fossil lacustrine ultralaminae was chiefly based on the production on pyrolysis of n -alkylnitriles with a typical distribution (bimodal, maxima at C 28 and C 16 ). The origin of marine ultralaminae is investigated in this work. To this end, spectroscopic and pyrolytic studies were carried out on (i) the algaenan forming the very thin outer walls of Nanochlorum eucaryotum (an extant marine Chlorophycea) and (ii) two ultralaminae-containing marine kerogens from the Lower Toarcian shales of Paris basin (Fecocourt and Bray). A high contribution of polymethylenic chains probably linked via ehter bridges was observed in these bio- and geopolymers. Furthermore, on pyrolysis they afforded the same n -alkylnitriles with the same distribution (unimodal, maximum at C 13 , lack of C 17+ compounds). Fossil ultralaminae in marine kerogens therefore derive from the selective preservation of the algaenan-composed very thin outer walls of marine Chlorophyceae such as N . eucaryotum. Algaenans from marine and freshwater Chlorophyceae and, therefore, marine and lacustrine ultralaminae, should be characterized by sharply distinct n -alkylnitrile distributions.

Occurrence and origin of «ultralaminar» structures in «amorphous» kerogens of various source rocks and oil shales

Abstract Forty low maturity, type I–II kerogens from source rocks and oil shales, ranging in age from Infra-Cambrian to Miocene, were examined by transmission electron microscopy (TEM). When previously observed by light microscopy and/or UV fluorescence microscopy the above samples, like a very large number of kerogens, seemed homogeneous and amorphous. In fact, TEM revealed the presence of “ultralaminar” structures in 22 of these “amorphous” kerogens. The abundance of “ultralaminae” depends on the considered sample and some of the tested kerogens are nearly exclusively composed of “ultralamina” accumulations. Based on thickness, three main groups of “ultralaminae” can be identified. Parallel studies on extant algae indicated that some of the above “ultralaminae” likely derived from the resistant outer walls of microalgae. The fossilization of such outer walls should occur via the “selective preservation” pathway and result in the formation of highly aliphatic “ultralaminar” kerogens. The lack of “ultralaminae” in Infra-Cambrian kerogens is consistent with a cyanobacterial origin.

Comparison of torbanites of various origins and evolutionary stages. Bacterial contribution to their formation. Causes of the lack of botryococcane in bitumens

Abstract Torbanites are kerogen-rich deposits chiefly of the remains of a colonial alga with a morphology similar to the one of extant Botryococcus braunii. A previous comparison of an immature torbanite (NE 70) and of the resistant biopolymer isolated from the outer walls of this green alga indicated closely related structures. Accordingly, it appeared that torbanite formation is mainly based on the selective preservation of this resistant constituent. In the present work the structure of a torbanite within the oil window (BJ 248) was examined by FTIR and solid state 13C NMR on bitumen-free, unheated material and pyrolysis residues, with identification of the hydrocarbons and fatty acids released on 400°C pyrolysis. Comparison with NE70 indicated that maturation entails a partial oxygen elimination, an extensive aromatization, a relative increase in tertiary aliphatic carbons and an important release of hydrocarbon chains. Nevertheless, these chains (saturated, unbranched, up to C31) still make a major contribution to BJ 248 structure. The ester functions, surviving in BJ 248 for protected from external attacks by the polymeric network, generate fatty acids on pyrolytic treatment. However, these ester functions are less abundant in BJ 248 than in NE 70 and some variations in distribution are noted (even predominance rather than exclusive formation of even products, lower level of unsaturated acids). The fatty acids released on pyrolysis of various Recent sediments and of Green River Shale kerogen correspond to degraded biolipids incorporated into the kerogen during diagenesis. In sharp contrast the fatty acids from torbanites correspond to sterically protected esters already present in the initial resistant biopolymer. The bitumen of BJ 248 was also analysed for its free fatty acids and branched/cycle alkanes, along with the bitumen of less mature torbanites including NE 70. Maturation of BJ 248 results in a considerable increase in the level of fatty acids in the bitumen. The distribution of these unbound acids reveals an important alteration (lack of unsaturated, weak even predominance). Very high hopane to sterane ratios were noted in the bitumen of the torbanites analysed as well as in BJ 248 and NE 70 pyrolysates. A high hopane to sterane ratio is usually indicative of an intense bacterial contribution; however, the formation of at least a part of the hopanes of torbanite bitumen from hopanoid precursors present in Botryococcus cannot be ruled out. No botryococcane was detected in the bitumen of the twelve torbanites examined. This lack may reflect, either a complete removal of botryoccocane precursors under partly oxic conditions of sedimentation, or, in some bitumens, a primary property linked to torbanite formation from the A race. It should be remembered that there are two races of B. braunii with similar resistant biopolymers but differing in hydrocarbon content: non-isoprenoid hydrocarbons in A race, botryococcenes in B race. Selective preservation of the resistant biopolymer would generate, from both races, torbanites with similar structures, but when derived from the A race, torbanite bitumens should not contain any traces of botryococcane. Consequently, non-occurrence of botryococcane in crude oils cannot be used as a definite proof of the lack of Botryococcus contribution in source rocks.

Origin and occurrence of 25-norhopanes: a statistical study

Abstract The alkane fraction of more than 200 rocks, biodegraded oils and non-biodegraded oils, have been analysed by means of computerized GC-MS, in order to investigate the effect of natural biodegradation on the occurrence of “demethylated hopanes”, i.e. 17α-25-norhopanes. The results obtained indicate that 25-norhopanes are preexisting biomarkers the concentration of which is enhanced by selective biodegradation of more readily degradable homologs, i.e. regular hopanes, rather than by demethylation of hopanes in reservoirs. However, the use of 25-norhopane enrichment as a palaebiodegradation indicator in apparently non-biodegraded oils is still valuable providing the initial background content in the corresponding source rocks is known. Furthermore, 25-norhopanes appear to be diagnostic of specific environmental conditions (marine and lacustrine source rocks, dysoxic and not very hypersaline). Lastly, one other (novel) bacterially resistant rearranged hopanoic compound, namely a C29 neohopane, is applicable for both biodegradation and maturation evaluation.

Distribution of aromatic steroids in geological samples: their evaluation as geochemical parameters

Abstract The distribution of ring-C monoaromatic and triaromatic steroid hydrocarbons of about fifty crude oils and sediments of various origins and ages were studied in order to test their validity as geochemical source and maturity parameters. Three ratios appeared to be useful maturity parameters, i.e. A/B cis to trans isomers in non-rearranged monoaromatics, mono/triaromatics and shortvs long-chain homologues. Four other variations may be considered as source parameters depending upon lithology or other palaeo-environmental conditions, i.e. rearranged vs non-rearranged monoaromatics, 5β/5α -methyl rearranged monoaromatics, carbon number distribution and occurrence of a yet unknown series of methylated triaromatic steroids. 5α-Methyl rearranged monoaromatics seem to be particularly important in anhydrites from sabkha environments. Recent identification of most aromatic steroid constituents occurring in geological samples confers on this class of compounds an increasing value as correlation parameters in geochemical studies.