Immature Rocks Research Articles

Evidence for regional fluid migration in the Eagle Ford Formation, Austin Chalk, and Buda Limestone of south-central and west Texas, USA

Calcite veins hosted by extension fractures and faults are common in the outcrop belt of the Austin Chalk – Eagle Ford – Buda stratigraphic section across south-central and west Texas, and contain aqueous and liquid hydrocarbon fluid inclusions. The hydrocarbon fluid inclusions in these thermally immature rocks indicate long-distance migration of oils from mature source rock down-dip and possibly from deeper units along faults. The widespread spatial distribution and extensive variety of oil types indicated by the fluid inclusion fluorescence suggest a protracted series of migration events – possibly from multiple source areas – rather than a single event. Oil inclusions in fault and fracture hosted calcite veins provide evidence of both strata-bound and cross-stratal oil migration along fault and fracture networks that are pervasive in the Austin Chalk, Eagle Ford, and Buda, as well as stratigraphically deeper units, in the subsurface and outcrop belt.Aqueous fluid inclusion microthermometry of the calcite veins provides evidence for significant exhumation of rocks in the Austin Chalk – Eagle Ford – Buda outcrop belt. Approximately 1.25–1.79 km of overburden was removed along the Balcones fault zone outcrop belt of south-central Texas, while in outcrop belt of the Devils River Uplift as much as 2.44–3.50 km of overburden was removed. In the productive region of the deeper basin down-dip from the outcrop belt, fluid inclusions in a deep core indicate that fluids were trapped during burial, but do not record maximum burial conditions.

The diagenetic continuum of hopanoid hydrocarbon transformation from early diagenesis into the oil window

Hopanoid molecules are ubiquitous in sedimentary rocks and make up a significant proportion of sedimentary organic matter, oils, and bitumen. Due to their relatively stable chemical structure, they are generally well preserved in the rock record, making them extremely useful as biomarkers or geomarkers. Despite extensive industry and scientific interest, the early phase of their eogenetic and early diagenetic transformation has not been fully demonstrated or examined in a geologic setting. Most previous studies have focused on the oil window range from about 0.5–1.0 %VRo, and neglected transformations that happen at maturity levels below this. Additionally, the geological transformation of hopanoids has been extensively applied as a geochemical indicator of thermal maturity, however the mechanisms that result in these transformations are not fully understood.A major stumbling block in the study of hopanoid diagenesis is the availability of well constrained samples along a single stratigraphic horizon from very low maturity and through the oil window. This study utilizes a natural thermal maturity transect along a single stratigraphic horizon which stretches from thermally immature rocks, up to the middle of the oil window. The purpose of this study is to document the progressive changes in abundance of hopanoid hydrocarbon compound classes and between individual isomers with increasing thermal transformation. Samples for this study were taken from cores of the Upper Cretaceous Second White Specks Formation, a major source rock within the Western Canadian Sedimentary Basin, that has been extensively mapped and studied. This sample set represents a unique opportunity to examine the changes in hopanoid class with increasing thermal maturity.69 samples from 8 wells were examined for biomarkers. Thermal maturity was determined using HAWK programmed pyrolysis analysis of 87 samples from 9 wells. These geochemical methods were combined with a well established stratigraphic framework to ensure that sampling methods were stratigraphically controlled. Neohopenes, hopenes, ββ-hopanes, βα-hopanes, αβ-hopanes, and rearranged hopanes were differentiated and drastic changes in the proportions of these compound classes were observed with increasing thermal maturity. Thermally immature rocks are dominated by hopenes, neohopenes, and ββ-hopanes, whereas the middle of the oil window is dominated by αβ-hopanes and rearranged hopanes. These results allow a detailed determination of the maturity ranges that correspond to the transformation or disappearance of specific hopanoid molecule classes, for example the loss of hopenes in the maturity range of 0.28–0.35 %VRE. Additionally, this study allows the stages of episodic or punctuated transformations of hopanoid classes to be better delineated and narrowed down to specific maturity ranges. This work represents a novel examination of a natural thermal maturity transect and helps to consolidate our understanding of the early diagenetic transformations of hopanoid compounds.

Geology and evolutionary stages of the Late Precambrian Hammamat sediments at Gebel Um Tawat, North Eastern Desert, Egypt

The Um Tawat Late Precambrian immature molasse-type Hammamat sedimentary rocks, located in the Northern part of the Eastern Desert of Egypt, were deposited at the late stage evolution of the Pan-African Orogeny. The Um Tawat molasse sediments are mainly derived from Dokhan volcanics and rest unconformably on these volcanics. They are intruded by the younger granites of Sala’at El-Belih granites and Gebel Gattar batholith. Hammamat sediments consist of repeated cycles of conglomerates, sandstones, siltstones, and minor mudstones deposited in an inter-mountainous basin by braided stream and alluvial fan regime constituting a stratigraphic section of about 500 m thick. Petrographically, the conglomerates are of polymictic type. Sandstone could be termed arkosic greywacke and shows ripple marks and in some places cross bedding. Siltstone is characterized by clear lamination and graded bedding. Mudstone is characterized by very fine grains of silt and clay size particles. The evolutionary stages of the study area is divided into five stages: (1) compressional regime resulted in the formation of the metavolcanics and metagabbro-diorite complex; (2) tension and relaxation crustal regime and uplift of the old basement forming G. Dokhan; (3) relaxation period, weathering, and erosion processes of the Dokhan volcanics formed the Hammamat molasse-type sediments; (4) tensional regime or weak rift-like processes led to the eruption of the younger granites and (5) regional compressional deformation subjected to the Hammamat sediments. This phase form G. Um Tawat and forming one regional open anticline fold whose fold axis is trending N47°E–S47°W and plunges 5° toward SW.

Sulfur isotope composition of individual compounds in immature organic-rich rocks and possible geochemical implications

We applied compound-specific sulfur isotope analysis (CSSIA) to organic matter (OM) extracted from ancient and immature organic-rich rocks from the Cretaceous Ghareb (Shefela Basin locality, Israel) and Miocene Monterey (Naples Beach locality, California, USA) Formations. Large variations in the δ34S values of different organosulfur compounds (OSCs), that reach up to 28‰ and 36‰, were observed in the Ghareb and Monterey samples, respectively. Additionally, some common OSCs in both locations showed consistent 34S trends relative to each other. The consistent enrichment in 34S of C35 hopane thiophene relative to iC20 thiophene in the studied sections probably resulted from differences in the timing of OM sulfurization. Reactive organic precursors quickly consume the most 34S-depleted reduced S, while less reactive species incorporate the heavier residual S at a later time. Despite the differences in the depositional environments, ages, and the initial δ34S values of the reduced S (represented by the δ34S of pyrite) between the Ghareb and the Monterey Formations, the sulfurization order of common organic compounds seems to be similar. All of the δ34S values of OSCs are 34S enriched relative to that of the coexisting pyrite with the exception of the C25 highly branched isoprenoid (HBI) thiophene in several samples from the Monterey Formation. The existence of 34S-depleted sulfurized HBI may point to OM sulfurization that occurred at or near the sediment-water interface during the deposition of the Monterey. Moreover, the δ34S of steroid sulfides shows an inverse trend with the pristane/phytane ratio, which may indicate that the sulfurization mechanism of these OSCs are affected by redox conditions. Further investigation of CSSI values in immature rocks from other basins may help constrain the OM sulfurization process, timescale, and depositional conditions and their possible use as paleoenvironmental proxies.

Trehalose, mannitol and arabitol as indicators of fungal metabolism in Late Cretaceous and Miocene deposits

Trehalose, mannitol and arabitol are the main saccharides of extant fungal metabolism, but their occurrence and distribution in geological materials have rarely been considered. Here, we identify these sugars in Miocene lignites and for the first time in Late Cretaceous mudstones and coals. The co-occurrence of trehalose, mannitol and arabitol in the sedimentary rocks investigated suggests their fungal origin, because these three saccharides are major compounds present in most modern fungi, including the very common mycorrhizal and wood-rotting groups. Therefore, we conclude that these sugars should be treated as new fungal biomarkers (biomolecules) present in geological rocks. Trehalose and mannitol are major compounds in total extracts of the samples and a sum of their concentration reaches 4.6 μg/g of sample. The arabitol concentrations do not exceed 0.5 μg/g, but in contrast to trehalose, the concentration correlates well with mannitol (R2 = 0.94), suggesting that they have the same, translocatory role in fungi. Based on the trehalose vs. mannitol and arabitol distributions in Cretaceous samples and their comparison with data for modern fungi, we preliminarily conclude that the coal seams from the Rakowice Małe (SW Poland) section were formed during warmer climatic periods than the overlying sediments. Furthermore, no DNA could be isolated from the samples of lignites and overlying sediments, whereas it was abundant in the control samples of maple, birch and oak wood degraded by fungi. This indicates an absence of recent fungi responsible for decay in lignites and implies that the saccharide origin is connected with ancient fungi.Other sugar alcohols and acids like D-pinitol, quinic acid and shikimic acid, were found for the first time in sedimentary rocks, and their source is inferred to be from higher plants, most likely conifers. The preservation of mono- and disaccharides of fungal origins in pre-Palaeogene strata implies that compounds previously thought as unstable can survive for tens to hundreds of millions of years without structural changes in immature rocks unaffected by secondary processes.

Paleozoic Granitoids of the Southern Part of the Voznesenka Terrane (Southern Primorye): Age, Composition, Melt Sources, and Tectonic Settings

This paper presents data on the geological position, geochemistry, age, and isotopic characteristics of the granitoids of the southern part of the Voznesenka terrane, Southern Primorye (Muraviev–Amursky Peninsula and its vicinities). All of the studied granitoids were formed in three stages: the Ordovician, Silurian, and Permian. The Silurian and Permian ages of the granitoid intrusions have been previously determined (Ostrovorussky Massif, 432–422 Ma, and 250 ± 4 Ma, early and late associations, respectively; Sedanka massif, 261 ± 3 Ma). The granites of the Artem and Nadezhdinsky massifs define an U–Pb zircon age of 481 ± 6 and 452 ± 4 Ma, respectively. The geochemical and isotope data show mainly the crustal nature of the granitoids. Their formation was related to melting of relatively immature rocks of the continental crust (mafic–intermediate volcanic rocks). The Nd isotope composition of the granitods (TNd(DM–2) = 1.3 Ga) indicates the absence of the mature ancient crust at the basement of the southern Voznesenka terrane. The maximum contribution of mantle sources to the granite formation is recorded in the Permian associations. A comparison of the peaks of intrusive magmatism in the southern part of the Voznesenka terrane and adjacent territories suggests that the formation of the granitoids of the Muraviev–Amursky Peninsula and its vicinities was caused by the interaction of continental blocks with two oceanic basins: the Paleoasian (and its fragments) and Paleopacific ones.

The effect of organic matter type on formation and evolution of diamondoids

A series of anhydrous pyrolysis experiments, using sealed gold tubes, were performed on three types of kerogen to investigate the effect organic matter type has on the generation and evolution of thermogenic diamondoids. Based on the compositional variation of pyrolysis products, the cracking of kerogens can be divided into three stages: oil generation (0.6%–1.5% EasyRo), wet-gas generation (1.5%–2.1% EasyRo) and dry-gas generation (>2.1% EasyRo). The experimental results indicate that diamondoids were mainly generated in the oil and wet-gas generation stages and decomposed in the dry-gas generation stage. In addition to thermal maturity, the formation of diamondoids is also influenced by the type of organic matter. Type I and IIA kerogens produced more diamondoids than Type III kerogen, and diamondoids generated from Type III kerogen were dominantly adamantanes. Therefore, the concentration and concentration ratios of diamondoids can be used to assess the maturity of source rocks (1.0%–1.5% EasyRo) and determine the type of organic matter (1.0%–2.0% EasyRo). Isomerization ratios of diamondoids depend mainly on thermal maturity and the type of organic matter has little effect. The use of isomerization ratios to determine thermal maturity is best for source rocks at higher maturity levels (1.5%–3.0% EasyRo). Therefore, bivariate diagrams of concentration versus isomerization indices of diamondoids (e.g., DMAs/MDs vs. DMAI-1 and DMAs/MDs vs. TMAI-1) can be used to evaluate the source rock maturity over a wider EasyRo range (1.0%–3.0% EasyRo) than single diamondoid parameters. As there are differences in the concentration and distribution of diamondoids in the extracts of three source rocks, the possibility exists to use diamondoid indices of immature rocks to determine the type of source rock.

Petrography and Geochemistry of Archaean Greywackes from Northern Part of the Dharwar-Shimoga Greenstone Belt, Western Dharwar Craton: Implications for Nature of Provenance

ABSTRACT Greywackes (Dharwar greywackes) are the most abundant rock types in the northern part of the Dharwar-Shimoga greenstone belt of the western Dharwar craton. They are distinctly immature rocks with poorly-sorted angular to sub-angular grains, comprising largely quartz, plagioclase feldspar and lithic fragments of volcanics (mafic+felsic), chert and quartzite, with subordinate biotite, K-feldspar and muscovite. They are characterized by almost uniform silica (59.78-67.96 wt%; av. 62.58), alkali (4.62-7.35 wt%; av. 5.41) contents, SiO2/Al2O3 (3.71-5.25) ratios, and compositionally are comparable to the andesite and dacite. As compared to Ranibennur greywackes, located about 100 km south of Dharwad in the Dharwar-Shimoga greenstone belt, the Dharwar greywackes have higher K2O, CaO, Zr, Y, ΣREE, Th/Sc, Zr/Cr, La/Sc and lower Sr, Cr, Ni, Sc, Cr/Th values. The chondrite normalized patterns of Dharwar greywackes are characterized by moderately fractionated REE patterns with moderate to high LREE enrichment, with almost flat HREE patterns and small negative Eu anomalies, suggesting felsic dominated source rocks in the provenance. The frame work grains comprising felsic and mafic volcanics, feldspars and quartz suggest a mixed source in the provenance. The moderate CIA values ranging between 57 and 73, indicate derivation of detritus from fresh basement rocks and from nearby volcanic sources.The mixing calculations suggest that the average REE pattern is closely matching with a provenance having 40% dacite, 30% granite, 20% basalt and 10% tonalite. These greywackes were deposited in a subduction related fore-arc basin than a continental margin basin. Their La/Sc ratios are high (av. 4.07) compared to the Ranibennur greywackes (1.79), suggesting that the greywackes of the northern part of the basin received detritus from a more evolved continental crust than the greywackes of the central part of the Dharwar-Shimoga basin.

Sulfur Species in Source Rock Bitumen before and after Hydrous Pyrolysis Determined by X-ray Absorption Near-Edge Structure

The sulfur speciation of source rock bitumen (chloroform-extractable organic matter in sedimentary rocks) was examined using sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy for a suite of 11 source rocks from around the world. Sulfur speciation was determined for both the native bitumen in thermally immature rocks and the bitumen produced by thermal maturation of kerogen via hydrous pyrolysis (360 °C for 72 h) and retained within the rock matrix. In this study, the immature bitumens had higher sulfur concentrations than those extracted from samples after hydrous pyrolysis. In addition, dramatic and systematic evolution of the bitumen sulfur moiety distributions following artificial thermal maturation was observed consistently for all samples. Specifically, sulfoxide sulfur (sulfur double bonded to oxygen) is abundant in all immature bitumen samples but decreases substantially following hydrous pyrolysis. The loss in sulfoxide sulfur is associated with a relative increase in the fra...

Effect of thermal maturation on plant-derived terpenoids and leaf wax n-alkyl components

Plant biomarkers, such as terpenoids and leaf wax components (n-alkanes, n-alkanoic acids and n-alkanols), are frequently found in sediments and can be used, often in association with stable carbon (and hydrogen) isotope measurements, as paleovegetation and paleoclimate proxies. However, few controlled studies have monitored plant biomarker alteration to determine if certain plant biomarkers are preferentially lost relative to more recalcitrant forms. To investigate the role of selective alteration and degradation of plant biomarkers, hydrous pyrolysis was used to artificially mature leaves from four plant species, including the deciduous angiosperms Acer rubrum and Platanus occidentalis, the deciduous conifer Taxodium distichum and the evergreen conifer Pinus sylvestris. Leaves were artificially matured at temperatures ranging from 150 to 330°C for 72h to simulate maturation. With increasing temperature, functionalized di- and triterpenoid yields decreased, with a greater loss of triterpenoids at lower temperature. Both diterpene and triterpene yield increased during maturation up to 310–320°C. A greater amount of diterpenes and triterpenes was generated for P. sylvestris and A. rubrum, respectively, and might be related to differences in terpenoid starting composition. Terpenols were preferentially converted to terpenes over terpenoic acids. Taken together, hydrous pyrolysis of plant biomarkers indicates that paleovegetation reconstruction from terpenoids can be informative, but may only be a qualitative vegetation proxy under many conditions. The n-alkane yield largely increased up to 320°C, whereas the n-alkanol yield mainly decreased with increased maturity. The n-alkanoic acids initially increased, but then decreased. The stable carbon isotopic composition (δ13C) of the n-alkanes was generally, though not universally, constant up to 200°C. Above this, the δ13C values of individual chain length hydrocarbons, for some species, changed by ca. 2‰. This suggests that n-alkane δ13C values should be unaltered in immature rocks, but can vary in the catagenic stage of maturation (oil window).

Classes of organic molecules targeted by a methanogenic microbial consortium grown on sedimentary rocks of various maturities.

Organic-rich shales are populated by methanogenic consortia that are able to degrade the fossilized organic matter into methane gas. To identify the organic fraction effectively degraded, we have sequentially depleted two types of organic-rich sedimentary rocks, shale, and coal, at two different maturities, by successive solvent extractions to remove the most soluble fractions (maltenes and asphaltenes) and isolate kerogen. We show the ability of the consortia to produce methane from all rock samples, including those containing the most refractory organic matter, i.e., the kerogen. Shales yielded higher methane production than lignite and coal. Mature rocks yielded more methane than immature rocks. Surprisingly, the efficiency of the consortia was not influenced by the removal of the easily biodegradable fractions contained in the maltenes and asphaltenes. This suggests that one of the limitations of organic matter degradation in situ may be the accessibility to the carbon and energy source. Indeed, bitumen has a colloidal structure that may prevent the microbial consortia from reaching the asphaltenes in the bulk rock. Solvent extractions might favor the access to asphaltenes and kerogen by modifying the spatial organization of the molecules in the rock matrix.

Change in hydrogen isotope composition of n-alkanes, pristane, phytane, and aromatic hydrocarbons in Miocene siliceous mudstones with increasing maturity

The variation in the hydrogen isotope composition ( D/ H ratio values) of n-alkanes, pristane, phytane, and aromatic hydrocarbons with increasing maturity was investigated using a set of immature to mature Miocene siliceous mudstones. The D/ H values of aromatic hydrocarbons change at the earliest stage (vitrinite reflectance, R o = 0.4–0.5%), by way of higher values in immature rocks. A dramatic increase in values is observed for pristane and phytane during the mature stage ( R o = 0.6–0.7%), attributable to hydrogen exchange with sediment porewater. The decrease in the values of short chain n-alkanes and the disappearance of carbon number predominance in n-alkane D/ H values with increasing maturity is a result of the thermal cracking of aliphatic structures in bitumen and kerogen. The long chain n-alkanes derived from higher plant wax can maintain their initial values, even in the zone of extensive oil generation ( R o < 0.7%).

Selected trace and minor elements in Asunción soft rocks

Geochemical processes in soft immature rocks of the Asuncion area are studied through X-ray fluorescense analysis of some trace and minor elements used as provenance indicators. Good correlation between light rare earth elements and other refractories were found and that the analytical data suggest inter alia an overall stable continental area setting.

Simulation experiments for evolution of fatty acids in immature source rocks

The anhydrous, hydrous and bitumen-extracted simulations were carried out for the immature source rocks from the Liaohe sag. It has been shown from the result that with increasing temperature in simulation experiments, the fatty acids content decreased at first and then increased. The decrease of fatty acids in immature rocks is presumably related to alkanes generation in immature oils, whilst the increase may be related to the fact that some additional fatty acids are generated from kerogen and the tightly bound fatty acids in kerogen are released as bound fatty acids in kerogen and unbound fatty acids in bitumen. The fact that the bitumen generated from kerogen contains fatty acids has demonstrated that some bound and tightly bound fatty acids in kerogen can be transferred into bitumen. The preferential fatty acids in the immature source rocks are found to be mono-carboxylic acids with longer chains, whilst krogen contains relatively more di-carboxylic acids. It has been found that the fatty acids in immature source rocks can be changed from that with more longer chains to that with more shorter chains when evolution extent has been increased. Based on simulation results and the fact that the majority of fatty acids in immature oils are those with longer chains, it is inferred that the contribution of fatty acids to forming alkanes in immature oils mainly takes place at the evolution stage withR 0 <0.6%. The simulation experiments have also demonstrated that H2O could promote the generation of fatty acids with more di-carboxylic acids and delay alkanes formation from fatty acids.

O Cambro-Ordoviciano da Provincia Borborema

This paper describes some volcano-sedimentary deposits that characterize the time interval from Neoproterozoic IE to the early Ordovician, typical of the Borborema Province, Northeast Brazil. They are usually designated molassic deposits and comprise coarse-grained immature rocks with common lateral gradation to pelitic lithologies of continental environments. The volcanism varies, but is mostly acid to intermediate. Two major types of basins may be identified: The first type is connected with the formation and evolution of pull apart depressions along the major shear zones (or lineaments) as part of the escape-tectonics phase of the Borborema Province (LL type basin). The second type is formed by foreland (foredeeps) and intradeeps (distal rifts) related to the late and post-tectonic evolutionary processes of the Sergipano Belt, a "miogeoclinal" orogen positioned at the northeast border of the São Francisco Craton. In both cases, the preservation of these deposits - usually scarce and scattered throughout the province - was only possible due to the immediate protection by younger Paleozoic covers (Silurian and younger) of syneclises and Mesozoic rifts (generated above these syneclises). Presently, the thickest and largest occurrences of these deposits do not crop out as they are beneath the Paleozoic basins according to geophysical and other subsurface data. Some of these Eo-Paleozoic rifts have been pointed out as precursor rifts for the development of the Parnaiba Syneclise.

Genetic correlation of biodegraded crude oils from Lower Assam, India using biomarker compositions

Crude oils occurring in the Tertiary reservoirs of the Assam basin are waxy, of deltaic origin and similar to a number of oil occurrences in South-east Asia. In Lower Assam, however, crude oils occurring at shallow depths in various structures have been biodegraded and are heavy, highly aromatic and depleted in n-alkanes and isoprenoid alkanes. The loss of these alkanes makes geochemical interpretation of these data with regard to source, maturity and correlations difficult. The biomarkers of the sterane, terpane and phenanthrene series in these oils remain unaffected by biodegradation and their distributions show that the oils have originated from predominantly terrestrial organic matter. Higher plant contributions, and deltaic environments of deposition, can be inferred from the high concentrations of oleanane and bicadinanes. The oils are genetically correlatable on the bsis of their biomarker fingerprints. They are mature and were generated from source rocks during the late categenetic phase. The occurrence of the oil pools in immature rocks suggests migration of oils from deep-seated mature source rocks, probably through faults acting as conduits, followed apparently by alteration in shallow accumulations.

Thermal Maturity Patterns in Alaska: Implications for Tectonic Evolution and Hydrocarbon Potential

Nearly 10,000 vitrinite reflectance and conodont color alteration index determinations from sedimentary rocks in Alaska were used to produce a thermal maturity map of rocks exposed at the surface and to evaluate subsurface thermal maturity relations in the Colville and Cook Inlet basins. Rocks exposed at the surface of the Tertiary interior basins and in the Aleutian forearc and backarc basins uniformly are of very low thermal maturity, indicating that these basins are at or near maximum burial, have seen little uplift and exhumation, and are probably thermally immature with respect to hydrocarbon generation. In contrast, many sedimentary basins show elevated levels of thermal maturity at the surface, with the highest values at basin margins. This geometry suggests a patt rn of greater uplift along basin margins, possibly reflecting isostatic readjustments as crustal loads are removed by erosion. We investigated thermal maturity relations in three sedimentary basins (Colville, Cook Inlet, and Kandik) in more detail. Thermal maturity patterns in the Colville basin are broadly asymmetric, indicating systematic differential uplift ranging from a minimum of no uplift in the north (Point Thomson area) to 9-13 km of uplift and exhumation in the central Brooks Range; even greater uplift further to the south is indicated by the presence of greenschist facies and higher grade metamorphic rocks. This pattern may reflect the deflexing of the lithosphere subsequent to the principal episode(s) of crustal convergence and thickening. These patterns further suggest a similar thermal history for the proximal Colville basin and the northern foothills belt, suggesting the possibility of hydrocar on accumulations in the foothills. Thermal maturity isograds within the Brooks Range cut major thrust faults, indicating that maximum burial postdated the principal phases of thrusting. In contrast, isograds in the foothills belt to the north are warped broadly by local structure, indicating continued north-south shortening subsequent to maximum burial. Such deformation could have remobilized hydrocarbons in early traps. A broad southward extension of thermally immature rocks in the central portions of the foothills belt suggests relatively young east-west shortening (parallel to the strike of the orogene), a feature that to date has not been included in regional tectonic syntheses. Alternatively, the thermal maturity pattern could be explained by tectonically unrelated episodes of uplif in the eastern and western parts of the Brooks Range. In the Cook Inlet basin, vitrinite reflectance isograds also are indicative of relatively greater uplift at the basin margins than at the basin center, which appears to be presently at its maximum burial depth. Uplift in the Cook Inlet basin may reflect compression along the faults bounding the basin. Relatively high thermal maturity along the western margin of the basin also may reflect magmatic heat sources from the Alaska Peninsula-Aleutian volcanic arc. The Seldovia arch, a major structural feature of the End_Page 1874------------------------------ basin, does not appear to deform vitrinite reflectance isograds, implying that deformation on that structure ceased prior to maximum burial. In the Kandik basin, a thermal maturity anomaly (thermally mature younger rocks in fault contact with thermally immature older rocks) provides clues to the nature and timing of east-west thrusting. Mesozoic foreland basin deposits associated with thrusting buried Paleozoic rocks of the easternmost part of this fold-and-thrust belt to relatively shallow depths, driving potential hydrocarbon source rocks into the oil-generation window. The western foreland basin deposits were overridden by advancing thrusts, and tectonically buried to as deep as 10 km. These disparate thermal domains are juxtaposed along the Glenn Creek fault, which may represent a terrane boundary in east-central Alaska.

Analysis and occurrence of C 26-steranes in petroleum and source rocks

The C 26-steranes previously reported in oils and source rocks (MOLDOWAN et al., 1985) have been identified as 21-, 24-, and 27-norcholestanes (1A, 1B, and 1C). Various 24-norcholesterols or stanols, possible precursors for the 24-norcholestanes, occur widely at low levels in marine invertebrates and some algae, and 24-norcholestanes occur in marine petroleums of Tertiary through Paleozoic age. There are reports of 27-norcholesterols and stanols in recent sediments, but the precursor organisms have not been identified. The natural occurrence of the 21-norcholestane structure is unprecedented. Unlike 24- and 27-norcholestane, 21-norcholestane is in low concentration or absent in immature rocks and increases substantially relative to the other C 26-steranes in thermally mature rocks, oils, and condensates. This suggests an origin involving thermal degradation of a higher molecular weight steroid. The ratio of 21-norcholestane to the total C 26-steranes is shown to be an effective maturity parameter in a series of Wyoming (Phosphoria source) and California (Monterey source) oils. Molecular mechanics MM2 steric energy calculations indicate a relative stability order of 21 ⪢ 27 > 24- norcholestane for the major stereoisomers. Authentic 21-, 24-, and 27-nor-5α-cholestanes and 24- and 27-nor-5β-cholestanes were synthesized and subjected to catalytic isomerization over Pd/C to yield the full suite of stereoisomers for each. In immature rocks the 5α,14α,17α(H),20R isomers predominate. Mixtures of 24- and 27-norcholestane in oils and mature rocks show a familiar elution pattern of four major peaks presumed to be 14α,17α(H),20S, 14β,17β(H),20R, 14β,17β(H),20S, and 14α,17α(H),20R in order of elution, as well as putative 20S and 20R rearranged steranes [13β,17α(H)-diasteranes]. However, the 21-norcholestanes are represented by a single peak in mature sediments under normal GC conditions, which is shown to consist of a major 14β,17α(H) peak and a minor 14α,17α(H) peak using an extended injection-temperature-hold-time gas Chromatographic technique.

Variation of graptolite reflectance with depth of burial

Changes in reflectance and bireflectance of graptolite with increasing depths of burial were studied in four sequences of samples, two from boreholes in Poland and Sweden, and from two outcrop sections in western and northwestern Canada. Graptolites show similar optical changes (in reflectance and bireflectance), with increasing depths of burial to those of vitrinite. Maximum reflectance of low-maturity graptolites (% R o max) 1.1–1.7) increases gradually with depth of burial (at 0.14 per 100 m for the Polish samples) and at a similar but more variable rate (at 0.12–0.43 per 100 m for the Canadian samples) for high-maturity graptolites (% R o max8.0−9.0) in the chlorite zone of metamorphism. Indirect correlation between the optical properties of graptolites and those of vitrinite has been established using conodont alteration indices. In general, graptolites show higher reflectance levels than vitrinite for the same conodont indices of thermal maturity. For immature rocks (% R o vitrinite = 0.2−0.5, CAI: 1.5), graptolite maximum reflectances range from 0.6 to 1.2; for the oil generation zone % R o vitrinite 0.5−1.30, CAI: 1.5–2.5) from 1.2 to 2.2.

Study on diagenesis of organic matter in immature rocks

Since many immature oils have been found in a number of Tertiary basins of China, a series of cores (Oligocene) and several immature rocks after thermal simulation have been investigated for their biomarker distributions by GC and GC-MS. The presence of biomarkers in the cores seem to follow a rule of less to greater stability of hopenes, ββ-hopanes, diasterenes with increasing the depth of cores, and subsequently the 22R, 22S configuration of hopanes reaches equilibrium. The thermal simulation experiments with immature rocks demonstrated that it is possible to generate some immature oils from immature rocks during the diagenesis stage. The tricyclic terpanes generated from source rocks during diagenesis stage tended to be enriched in the oils compared to their source rocks and the relative abundance of lower molecular weight tricyclic terpanes to their higher molecular weight homologues may be useful for the subdivision of diagenesis.