Kerogen Assemblages Research Articles

Evaluation of mixed organic-rich carbonate and shale rocks of Meyal oilfield using an integrated palynofacies, geochemical and petrophysical approaches

This study integrated palynological, geochemical, and petrophysical analysis that maybe used to analyze petroleum rocks. Shales and sediments rich in carbonate are essential in assessing the geological history of source rocks for the generation of petroleum. This emphasizes the important role of carbonate source rocks on a global level. This study evaluated the source and reservoir rocks of Meyal oilfield, Potwar Basin. Twenty-five well cutting samples from two wells and four fresh outcrop samples were studied. Results of TOC and Rock-Eval of Sakesar Formation samples showed fair to good TOC, Patala Formation in well had very good TOC contents in well both wells. S2 values of both formations’ samples showed fair to good generative potential. Both formations lie in the maturity window having primarily mixed Type II/III and Type III kerogen with good oil/gas-generation potential. Palynological analysis also shows Type III kerogen in productive oil/gas window and in good agreement with pyrolysis data. Three forms of kerogen assemblages were identified: Structured and Bio-structured phytoclasts, opaque phytoclasts, and AOM. Petrophysical analysis of Chorgali formation showed average porosity ∼12.01%, average water saturation ∼37.71%, and average hydrocarbon saturation ∼62.28%, indicating average to good reservoir potential. Findings of this research will help and guide exploration and production companies.

An integrated palynofacies, geochemical and petrophysical analysis for characterizing mixed organic-rich carbonate and shale rocks of Dhulian oilfield Potwar Basin, Pakistan: Insights for multiple source and reservoir rocks evaluation

The current study aimed to evaluate potential of multiple source and reservoir rocks of Potwar Basin, Pakistan. This study presents palynological, geochemical, and petrophysical data from source and reservoir rocks of Eocene and Paleocene sequences of the Dhulian oilfield. A total of 34 well cutting samples from two wells and four fresh outcrop samples were studied. Results of total organic carbon and Rock-Eval pyrolysis of the Sakesar Formation sediments shows good to very good TOC contents, whereas the Patala Formation sediments from well A shows very good to excellent and very good TOC contents in well B. S2 values of both formations’ samples showed fair to good generation potential. Both formations appear mature and have primarily type III and mixed type II-III kerogen with good gas-generation potential. Palynological analyses are in agreement with the pyrolysis data and showed that sediments have type III kerogen in the oil/gas window having excellent gas generation potential. Three kerogen assemblages were identified: palynomorphs, phytoclasts, and amorphous organic matter. Petrophysical analysis of the Chorgali Formation limestone showed an average porosity of ∼10.6%, with an average water saturation of ∼33.72%, and an average hydrocarbon saturation of ∼66.28%, thus indicating average to good reservoir potential. The findings of this research will help and guide exploration and production companies in the Potwar Basin in terms of source rock organic richness and maturity.

Palynofacies as a palaeoenvironment and hydrocarbon source potential assessment tool: An example from the Cretaceous of north Western Desert, Egypt

Optical examination employing transmitted light and UV-fluorescence microscopy of palynological preparations of eighteen cutting samples representing the Alam El Bueib Member (Hautervian-Barremian), Kharita/lower Bahariya (Cenomanian), and Abu Roash (Turonian-Santonian) formations collected from the Faghur Hj5-1 well, north Western Desert, Egypt, allows the identification of three different palynological assemblages from the studied rock units. These assemblages are mainly non-marine but apparently marine at the base of the Alam El Bueib Member, as evidenced by dinocyst occurrence. In addition, the presence of the Pediastrum and chlorophycean algae ecozone, recognised in previous works, is a good datum for the Abu Roash Formation in the north Western Desert of Egypt. Three associations of palynofacies linked to lithofacies changes are recognised and employed in identification of depositional environments. The Alam El Bueib samples yielded mixed kerogen assemblages of non-marine and marine organic facies. The Kharita/lower Bahariya interval is mostly barren, possibly due to prevailing sandstone lithofacies, except for one sample at its upper part which contains a diverse palynological assemblage. The overlying Abu Roash Formation has a homogeneous kerogen composition comprising mainly granular fluorescent AOM and algae as well as rare palynomorphs. Qualitative as well as quantitative variations of palynofacies allow the reconstruction of the depositional environment. The obtained data have the potential for discriminating spatial and redox status differences and providing also information about terrestrial/freshwater influxes. Results support the model that the Alam El Bueib Member was deposited in a marginal dysoxic-anoxic to distal suboxic-anoxic basin. The Kharita/lower Bahariya unit in the studied well was deposited under marginal dysoxic-anoxic conditions whereas the overlying Abu Roash Formation in a distal suboxic-anoxic basin. Palynofacies results also show that the studied material comprises two distinct facies of kerogen. First, Type II > I kerogen (AOM-rich) is overwhelmingly dominant in the Abu Roash Formation and a few samples from the Alam El Bueib Member which are presumed highly oil-prone, whereas Type III kerogen (phytoclast-rich) is particularly common in the Alam El Bueib Member and Kharita/lower Bahariya unit which are considered gas-prone. Thermal maturity determinations obtained from colour changes of smooth-walled palynomorphs reveal that Alam El Bueib samples belong to immature to mature stages; however, Kharita/lower Bahariya and Abu Roash samples are within the immature phase.

The Phytoclast Group as a tracer of palaeoenvironmental changes in the early Toarcian

Abstract In this paper, we present a detailed review of upper Pliensbachian–lower Toarcian kerogen assemblages from the southern areas of the West Tethys shelf (between Morocco and northern Spain) and demonstrate the use of the Phytoclast Group as a tracer of palaeoenvironmental changes in the early Toarcian. The kerogen assemblages in the studied sections from the southern areas of the West Tethys shelf are dominated by the Phytoclast Group and terrestrial palynomorphs, although punctual increases in amorphous organic matter, freshwater ( Botryococcus ) and marine microplankton (dinoflagellate cysts, acritarchs and prasinophyte algae) were observed at specific stratigraphic intervals. The opaque/non-opaque phytoclasts ratio was used to trace changes in palaeoclimate and other palaeoenvironmental parameters and reflect climate gradients associated with water availability during early Toarcian. During the Pliensbachian–Toarcian and Jenkyns events, changes in kerogen assemblages in the southern areas of the West Tethys shelf correlated with changes in the northern Tethys and Panthalassa shelf. The acceleration of the hydrological cycle associated with the aforementioned events was less intense in the northern Gondwana, southern and western Iberian basins, a reflection of the palaeogeographic position of these basins within the semi-arid climate belt when compared with the northern Iberian region and other northern areas of the West Tethys and Panthalassa shelf, inserted in winter-wet and warm temperate climate belts. Amorphous organic matter enrichment associated with the Pliensbachian–Toarcian and Jenkyns events reflects an increase in primary productivity linked with increased continental weathering, fluvial runoff and riverine organic matter, and nutrient input into marine areas, inducing water column stratification and promoting the preservation of organic matter.

Permian source rocks of the onshore and nearshore Carnarvon Basin

A new sampling program of Permian potential source rocks was undertaken to improve knowledge of the Permian petroleum prospectivity in new parts of the Southern Carnarvon and inboard Northern Carnarvon Basins. Presented here are new Rock-Eval data from previously unsampled wells, drillholes and outcrop and new infill sampling between existing data points. Kerogen assemblages of selected intervals were also determined from palynofacies analysis or organic petrography, which suggests the good Permian source rocks are generally dominated by gas-prone kerogens. Possibly terrestrial-derived oil-prone kerogen can also be frequently found in thin intervals of the upper Permian and more rarely in lower Permian in the onshore northern Carnarvon Basin.

Kerogen assemblages and δ13CKerogen of the uppermost Pliensbachian–lower Toarcian succession of the Asturian Basin (northern Spain)

The Toarcian Oceanic Anoxic Event (TOAE) is characterised by a perturbation of the global carbon cycle and the widespread deposition of organic matter (OM)-rich strata/sediments in the northern areas of the West Tethys Shelf, in several northern European epicontinental basins. Contrariwise, in the southern areas of the West Tethys Shelf such as western and southern Iberian, and northern Gondwana margins, OM-rich strata/sediments are spatially and temporally restricted. To better understand the relationship between OM production, depositional conditions, and palaeoceanographic controls on the occurrence of OM-rich facies in the northern Iberian margin, we collected and analysed 18 samples from the Rodiles section (Asturian Basin) for geochemical (total organic carbon-TOC, total sulfur-TS, δ13CKerogen, and δ13CWood) and palynofacies analysis.The studied upper Pliensbachian–lower Toarcian succession has TOC values reaching up to 2.9 wt%. The relative dominance of non-opaque phytoclasts and the δ13CKerogen values (average of ~ −24‰) in the lowermost Toarcian (Tenuicostatum Zone) agrees with previous interpretations indicating that this area was located in a dominantly winter-wet climate zone. A change in the kerogen assemblages is observed at the Tenuicostatum/Serpentinum zones boundary (onset of TOAE), related to an increase in amorphous OM (AOM). A slight increase in non-opaque phytoclasts and terrestrial palynomorphs, and a negative δ13CKerogen excursion (reaching −31.5‰) is recorded in the same interval. These observations suggest a increase in continental weathering and fluvial runoff associated with more humid conditions, likely resulting in increased superficial productivity and deposition and preservation of OM under, at least, suboxic conditions. Above the TOAE interval, the δ13CKerogen record (values around −24‰) and the drastic drop in TOC and AOM content indicate that bottom water oxygenation improved after the TOAE.Our results demonstrate a link between early Toarcian climate dynamics and the deposition of OM-rich strata/sediments.

Sedimentary organic matter and early Toarcian environmental changes in the Lusitanian Basin (Portugal)

The objective of this study is to investigate the relationship between early Toarcian climatic events and the composition of kerogen assemblages in the Lusitanian Basin (Portugal). In particular, we aim to understand how the Pliensbachian–Toarcian Event (Pl–Toa Event) and Toarcian Oceanic Anoxic Event (TOAE) affected the continental areas of the western Iberian margin and how possible variations in continental sources of organic matter (OM) were expressed in marginal-marine and hemipelagic depositional environments during the early Toarcian.We present here a characterisation [total organic carbon (TOC), total sulphur (TS), δ13C in kerogen concentrate (δ13CKerogen), and palynofacies] of kerogen assemblages from several uppermost Pliensbachian (emaciatum ammonite Zone)–lower Toarcian sections in the Lusitanian Basin (western Iberian margin), including the Peniche section, which contains the Toarcian GSSP. In all sections, TOC concentration is low (average 0.4 wt.%), with the highest values reaching up to 2.1 wt.% in a discrete level located approximately 10.5 m above the base of the levisoni Zone at Peniche. The TOAE negative carbon isotope excursion is observed in kerogen concentrate at the base of the levisoni Zone throughout the basin. Palynofacies analysis demonstrates that the kerogen assemblages are mostly of terrestrial affinity, with the dominance of the Phytoclast Group and terrestrial palynomorphs, and with punctual increases in amorphous organic matter, freshwater (e.g. Botryococcus sp.) and marine microplankton (dinoflagellate cysts, acritarchs, and prasinophyte algae) in specific stratigraphic intervals. A change in palynofacies assemblages associated with the TOAE is observed around the base of levisoni Zone. Although with slight differences between sections, the TOAE interval records an increase in non-opaque, translucent, phytoclasts (NOP) and cuticle fragments, and is also associated with an increased contribution of terrestrial palynomorphs (increase in sporomorphs and Classopollis in tetrads and agglomerates) and decrease in marine palynomorphs. The increases in NOP and terrestrial palynomorphs support the postulated enhancement of the hydrological cycle and increased export of terrestrial OM into marine environments during the early Toarcian, especially during the TOAE, in the western Iberian margin. Understanding the impact of the early Toarcian climatic events on land-based ecosystems may provide important insights into current climate change.

Total organic carbon and pyrolysis analysis of the Lower Cretaceous in Compton Bay and Atherfield, Isle of Wight (England)

The Wessex Basin (United Kingdom) includes hundreds of meters of Lower Cretaceous clays, silts, and sands deposited in a wide range of depositional environments. Studies have investigated these depositional systems from the organic matter (OM) perspective. However, questions remain concerning the composition, source, and the overall depositional constraints on the distribution of sedimentary OM in this area. Elemental (carbonate % and total organic carbon - TOC) and pyrolysis analyses were conducted on representative lithofacies of the Lower Cretaceous from the Wessex Basin at the Compton Bay and Atherfield sections, Isle of Wight. The highest TOC contents were determined in the upper part of the Ferruginous Sands and Sandrock formations. These elevated TOC intervals are associated with predominantly estuarine deposition. Except for one sample from the Vectis Formation, Hydrogen Index (HI) in all studied units is low and indicates Type IV kerogen assemblages, interpreted to be linked with strongly variable climates (with pronounced dry periods) and significant water table fluctuations in the source area and during transport. The one sample with a Type II-III kerogen assemblage from the lagoonal Vectis Formation supports previous studies which suggested that OM in the Vectis Formation varied vertically as a function of fluvial sediment and terrestrial organic matter input to the lagoonal environment with changes in salinity, sediment resuspension, and turbulence as a result controlling the abundance of dinoflagellate cysts.

Late Pliensbachian–Early Toarcian palaeoenvironmental dynamics and the Pliensbachian–Toarcian Event in the Middle Atlas Basin (Morocco)

Geochemical and palynofacies analysis of 38 samples from the Middle Atlas Basin (Aït Moussa and Issouka sections, Morocco) allowed to investigate sea-level changes, climate, and tectonics and impact on depositional environments in the westernmost Tethyan Gondwana-margin during the Late Pliensbachian–Early Toarcian and the Pliensbachian–Toarcian Event (Pl–Toa Event).The studied sections from the Middle Atlas Basin have low total organic carbon contents. Overall, kerogen assemblages are dominated by the Phytoclast Group, thus showing a strong terrestrial affinity and some degree of proximity to the source. Deposition occurred dominantly in oxic and proximal environments.Upper Pliensbachian kerogen assemblages from Aït Moussa are dominated by terrestrial particles and agree with the overall regressive character of the sedimentary succession. Uppermost Pliensbachian–lowermost Toarcian kerogen assemblages from Aït Moussa show an increase in marine particles. Considering the overall sedimentological context of the Middle Atlas Basin, these kerogen assemblages are interpreted to reflect local variation in accommodation space (driven by different sedimentation rates or tectonic compartmentalization) or ecological conditions (such as nutrient availability, temperature, turbidity, etc). In the Early Toarcian, above a major regional discontinuity and at the beginning of the Pl–Toa Event, increases in Botryococcus sp., AOM, and terrestrial palynomorphs suggest episodes of coastal erosion associated with transgression, likely driven by a combination of eustatic/tectonic changes and warmer and more humid climates leading to an increase in continental weathering and fluvial runoff. On the other hand, increases in sporomorphs and pollen grains occurring in tetrads and agglomerates indicate regressive episodes associated with increased sedimentation rates driven by enhanced continental weathering and fluvial runoff.This study shows the strong relationship between sea-level and the combined response of litho-, hydro-, and biosphere during the Early Toarcian, with implications to understand organic productivity and organic matter accumulation and preservation in the Tethys Ocean during the Pl–Toa Event.

Sedimentary organic matter and δ13CKerogen variation on the southern Iberian palaeomargin (Betic Cordillera, SE Spain) during the latest Pliensbachian–Early Toarcian

The Early Toarcian Oceanic Anoxic Event (T-OAE) is characterised by major palaeoenvironmental and palaeoceanographical changes (ocean anoxia, global warming, ocean acidification, among other), and a severe perturbation of the global carbon cycle. Although widespread oceanic anoxia was a significant control on the occurrence of organic-rich facies during the T-OAE in several locations of the northern Tethyan margin, the effects of anoxia, oceanic productivity, and dilution are poorly understood in other worldwide locations, such as the southern Tethyan and Iberian margins.In this study, we report new geochemical [total organic carbon (TOC), total sulphur (TS), total carbonate content (CaCO3), δ13C from kerogen isolates (δ13CKerogen)], palynofacies and thermal maturation data from the Upper Pliensbachian–Lower Toarcian successions in La Cerradura and Fuente Vidriera, External Subbetic domain, Betic Cordillera (SE Spain). The obtained results indicate that these successions have low TOC contents (~ 0.3 wt.%), with TOC reaching up to 0.46 wt.% around the Polymorphum–Serpentinum ammonite zone boundary in La Cerradura. δ13CKerogen negative carbon isotope excursions (CIE) are observed at the base of the Polymorphum Zone, the Pliensbachian–Toarcian Event (Pl-Toa), and base of the Serpentinum Zone (T-OAE). Overall, the low maturity kerogen assemblages are dominated by terrestrial OM and with a minor marine component. Small increments in terrestrial OM, non-opaque phytoclasts, and palynomorphs are observed just before and during the T-OAE in the studied area.During the Early Toarcian, in the External Subbetic domain, accumulation and preservation of OM was low and with a significant terrestrial contribution, similarly to other locations around the southern margins of Iberia and Tethys. The dominance of the opaque phytoclast subgroup, the occurrence of Classopollis, and the relatively more positive δ13C of terrestrial kerogen suggest that OM was mostly sourced from an area characterised by a semi-arid climate (i.e. with pronounced arid periods) during most of the Polymorphum Zone. The T-OAE negative CIE is accompanied by an increase in terrestrial OM, with a slight increase in the non-opaque phytoclast subgroup and terrestrial palynomorph subgroup. The increase of non-opaque phytoclast and terrestrial palynomorphs are in line with the posited enhancements of the hydrological cycle and export of terrestrial OM into marine environments during the T-OAE. This study suggests a link between 13C fractionation during photosynthesis in C3 plants and climate during the Early Toarcian. This link suggests coupling of the water and terrestrial carbon cycles before and during the T-OAE.

Palynofacies, micropalaeontology, and source rock evaluation of non-marine Jurassic–Cretaceous boundary deposits from northern Germany - Implications for palaeoenvironment and hydrocarbon potential

Abstract The Jurassic–Cretaceous boundary interval of northern Germany is characterised by non-marine deposits attributed to the Purbeck and Wealden facies. These organic-rich sediments were deposited under restricted, brackish-lacustrine conditions in a palaeogeographically isolated basin (Lower Saxony Basin). Rock-Eval and δ13Corg measurements were performed to characterise the composition and distribution of the organic matter, to determine the kerogen type chemically, and to define the quantity and maturity of the organic matter. A strong correlation between the optical characteristics, documented by the palynofacies analysis, and the chemical kerogen analyses have led to a better understanding of the palaeoenvironment and the processes which caused the organic matter enrichment. The samples studied from the basin center show a high petroleum generation potential, their estimated maturity is close to the onset of oil generation. Total organic carbon (TOC) concentrations of up to 18% have been measured, hydrogen index values reach up to 1185 mg HC/g TOC. The kerogen assemblages in two basinal successions are dominated by granular amorphous organic material (AOM), dinoflagellate cysts, and Botryococcus algae. In a more marginal succession continent-derived debris (phytoclasts, pollen and spores) is dominant. The AOM is interpreted to have originated either from phytoplankton (probably dinoflagellate cysts and Botryococcus), or its bacterial/archaeal degradation products formed under anoxic conditions. TOC-rich samples which are dominated by these amorphous kerogens show the highest petroleum potential, the AOM therefore plays a key role for the petroleum generation. The long-term climatic and hydrological evolution of the Lower Saxony Basin, which is based on palynomorphs, ostracods, and benthic foraminifera, is here interpreted by a five-stage model. Fluctuations in the salinity of the water bodies are directly reflected by the ratio between different freshwater and brackish-marine organisms as well dinoflagellate cyst diversity; five different cyst morphogroups are used for palaeoenvironmental interpretations. The deposition of TOC-rich beds in the German Wealden can be best explained by both high primary productivity and the establishment of prolonged phases of bottom water anoxia in a brackish-lacustrine depositional environment. Ecological conditions supporting enhanced productivity and preservation of the algal/bacterial-derived organic matter were most favourable during the mid–late Berriasian (Wealden 1–4).

Kerogen typing using palynofacies analysis in Permian Barren Measures Formation in Raniganj sub–basin, East India

The objective of the study is the simple assessment of kerogen type in Barren Measures Formation shale using palynofacies. The Barren Measures Formation sandwiched between the coal–rich formations consist of organic–rich black carbonaceous shales. These ̴ 1000 m thick, black shales are in the productive gas window having kerogen types III with excellent gas generation potential as proven in recent pyrolysis studies. In this study, the palynological analysis of the four core samples of the Barren Measures Formation shale was done. The analysis has revealed three types of kerogen assemblages, viz. palynomorphs, phytoclasts and amorphous organic matter. The palynomorph assemblage consists of spores and pollen; phytoclasts consist of secondary xylem wood macrophyte plant debris and amorphous organic matter is a higher plant decomposed product. Based on the application of published kerogen classification and correlation of Tyson (1995), the pure kerogen type in these shales is categorized as mixed type; type II (oil prone) and type III (gas prone). However, the gas–prone kerogen assemblage gets relatively dominant in the samples from greater depth in the studied borehole. Our results are analogous to the previously published outcomes of kerogen typing evaluated using Rock–Eval pyrolysis experiments.

Palynofacies and TOC analysis of marine and non-marine sediments across the Middle–Upper Jurassic boundary in the central-northern Lusitanian Basin (Portugal)

The Middle-Upper Jurassic transition is a geodynamic benchmark in the evolutionary history of several peri-Atlantic basins. Contrary to the vast Tethyan and peri-Tethyan areas, in the Lusitanian Basin (Portugal), this interval corresponds to a major basin-wide disconfor- mity preceded by a complex forced regression that induced sharp facies variations across the depositional systems. The Middle Jurassic units below are fully marine, whereas the Upper Jurassic sediments (Lower?-Middle Oxfordian), correspond to freshwater/brackish lacustrine, grading into punctuated brackish-restricted lagoonal and shallow-mar- ine paleoenvironments. This study presents total organic carbon and palynofacies data of 34 samples (total analyzed thickness about 85 m) collected from three key sections encompassing the Middle-Upper Jurassic transition in the central-northern sector of the Lusitanian Basin. The pal- ynofacies of the analyzed part of Middle Jurassic units (Cabo Mondego Formation) are characteristic of marine environments at their base, evidencing upwards a regres- sive trend of the depositional systems. Total organic carbon content is generally low, reaching up to 1.94 wt%. The Upper Jurassic Cabacos Formation presents kerogen assemblages mostly of continental origin, although punc- tuated by minor intervals of marine influence. Total organic carbon content is more variable, reaching up to 30.56 wt%. Intraclasts of re-deposited fragments of microbial mats were found incorporated in the kerogen assemblages, which point to highly dynamic erosional and depositional processes. Diversity of Botryococcus sp. occurrences was confirmed as an indicator of the degree of paleoenvironmental stability. The vertical distribution and comparison of the kerogen assemblages of the different sections indicate major changes of these parameters among relatively close settings and along narrow vertical intervals, attesting to the high sedimentary dynamics observed in the Lusitanian Basin.

HIGH‐RESOLUTION STRATIGRAPHY, PALYNOFACIES AND SOURCE ROCK POTENTIAL OF THE ÁGUA DE MADEIROS FORMATION (LOWER JURASSIC), LUSITANIAN BASIN, PORTUGAL

In the Lusitanian Basin (central‐western Portugal), the Lower Jurassic carbonate‐dominated succession is thought to have significant source rock potential. One of the most important units is the Água de Madeiros Formation (Upper Sinemurian – lowermost Pliensbachian) which is composed of alternating organic‐rich marls and limestones including black shale horizons. This paper is based on a study of this formation at its type locality at S. Pedro de Moel in western Portugal. Data includes Total Organic Carbon (TOC) measurements, palynofacies analyses and results of Rock‐Eval pyrolysis presented within a high‐resolution lithostratigraphic framework.TOC contents were measured in some 200 samples from the Água de Madeiros Formation covering a stratigraphic interval of 58 m, and vary widely up to a maximum of about 22 wt %. Kerogen assemblages are dominated by marine amorphous organic matter with varying contributions by phytoclasts and palynomorphs. A majority of the 85 samples analyzed by Rock‐Eval pyrolysis have S2 values above 10 mg HC/g rock, reaching a maximum of 78 mg HC/g rock. These high S2 values are correlative with maximum values of the Hydrogen Index which averages 355 mg HC/g TOC (maximum of 637 mg HC/g TOC). However in spite of these indicators of source‐rock potential, the Água de Madeiros Formation in the study area is thermally immature or very early mature, as indicated by Tmax values below 437 °C and average vitrinite reflectance values of 0.43 % Ro.

Organic facies of the Oligocene lacustrine system in the Cenozoic Taubaté basin, Southern Brazil

Abstract A quantitative study of palynofacies and organic facies was carried out in the Cenozoic sedimentary succession of the Taubate basin. The palaeoenvironmental distributions of particulate organic components were examined in relation to various potential controlling factors (lithology, depositional environment, and stratigraphic sequence). About 30 samples were analyzed from a single borehole drilled in Sao Paulo State. This involved rigorous kerogen counts supported by 30 TOC and sulphur, 20 Rock-Eval pyrolysis and 20 biochemistry analyses. In the studied area the Oligocene deposits are approximately 600 m thick. Data was obtained by counting 300 to 500 kerogen particles per sample, and then recalculating the percentages of the different constituent groups (phytoclast, palynomorph, and amorphous organic matter — AOM). The kerogen assemblage is palynomorph-dominated (> 70%), consisting mainly of freshwater lacustrine algae ( Pediastrum and Botryococcus ). Non-opaque phytoclasts predominate in most of the kerogen samples. However, significant variations in the proportion of biostructured and non-biostructured brown phytoclasts occur in some intervals. The fluorescent AOM content occurs in some levels and is composed of two main components: an intensively fluorescing structureless organic matter, which also has high lipid and protein content indicating a probable bacterial origin for the organic matter (microbial mats); and terrestrially-derived amorphous organic matter, which has been reworked to a different extent exhibiting a high content of carbohydrates. Comparison of the mean kerogen assemblages from different lithologies revealed that the finer-grained lithologies generally have higher palynomorph percentages, whereas coarser-grained lithologies present higher percentages of phytoclasts, emphasizing the important role of grain size in organic-matter distribution. The TOC content is generally less than 10%, but reaches as high as 44% in some intervals, due mainly to the presence of coaly seams. The T max , Spore Coloration Index, and spectral fluorescence measurement values obtained from these samples indicate a low maturity stage. The integration of these results, together with the organic facies parameters, indicate that the analyzed sedimentary section represents a variation from a freshwater lacustrine environment with an oxidant regime to a restricted saline, lacustrine environment with a dysoxic–anoxic regime. There was an excellent correlation between the optical and geochemical parameters.

Early Tertiary basin formation and the development of Lacustrine and quasi-lacustrine/marine source rocks on the Sunda Shelf of SE Asia

Abstract The Tertiary basins of the Sunda Shelf of SE Asia were formed in the ?Mid- to Late Eocene and accumulated thick intervals of syn-rift lacustrine and low salinity organic rich shales throughout the Late Palaeogene. Towards the end of the Oligocene and into the Early Miocene marine transgression occurred throughout the region. A change to deep open marine sedimentation followed by a general shallowing and deposition throughout the Neogene, of great thicknesses of marginal marine and paralic sediments. The syn-rift sediments are the most important hydrocarbon source rocks in this region, although precise dating of them is difficult due to the long-ranging nature of the palynomorphs and the paucity of the microfauna. However characteristics of the depositional environments and nature of the source rocks deposited within separate provincial basins that were wholly or partially landlocked can be assessed from a detailed examination of the isolated palynomorph and kerogen assemblages. A classification of lacustrine depositional environments within the region on this basis is presented.

Cretaceous Source Rock Characterization of the Atlantic Continental Margin of Morocco

Characterization of the petroleum potential for the Atlantic margin of Morocco has been based primarily on limited, antiently acquired organic geochemical data. These indicate the area of drilling behind the paleoshelf edge to be only fair in organic carbon and C15+ extract values with predominantly terrestrial kerogen types. Recently acquired geochemical data obtained from relatively recent drilling both behind and beyond the paleoshelf edge indicate 4 depositional facies containing hydrogen rich amorphous kerogen assemblages. These are: (1) Lower to Mid Jurassic inner shelf facies probably deposited in algal rich lagoon-like, (2) Lower Cretaceous non marine coaly facies probably deposited in algal rich swamplike environments, (3) Middle Cretaceous facies characterized by restrited anoxic environment with sediments rich in marine kerogen types deposited under sluggish wather circulation, (4) Upper Cretaceous to Tertiary outer-shelf to Upper slope facies probably deposited under algal-rich upwelling systems. Of these, the Cretaceous facies is the most widespread and represents the best source rock potential characteristics. Correlation of these facies to recently acquired good quality seismic packages allows for extrapolation of probable organic facies distribution throughout the continental margin. This should enhance the hydrocarbon potential of the Mesozoic and Cenozoic sediments both landward and seaward of the paleoshelfmore » edge and thus permits refinement of strategies for hydrocarbon exploration in the area.« less

Environments and palynofacies of a Dinantian (Carboniferous) littoral sequence: the basal part of the Navan group, navan, country meath, Ireland

The basal, dominantly littoral, part of the Courceyan (Dinantian, Early Carboniferous) Naval Group is described in borehole N866 from the Tara Mines area, Navan, County Meath, Ireland. The kerogen assemblages of 17 samples from the sequence are described in terms of the palynofacies they represent. Both the depositional environments and palynodebris deposited within them reflect distance from terrestrial source, which became more distant with time. The change in littoral sedimentation from dominantly siliciclastic to dominantly carbonate, near tthe top of the Liscartan Formation, represents drowning of the principal siliciclastic source areas on the margins of the Irish Midlands during the Courceyan (PC Miospore Biozone time). Spore colours and UV fluorescence indicate that the Navan area was not heated as much as other areas of the Irish Midlands.

Palynological zonation and palynofacies investigation of the Fjerritslev Formation (Lower Jurassic - basal Middle Jurassic) in the Danish Subbasin

A detailed study of the palynology and palynofacies of the Fjerritslev Formation (Lower Jurassic - basal Middle Jurassic) has resulted in the definition of four spore/pollen zones and four dinoflagellate cyst zones. The spore/pollen zones are the Corollina - Ricciisporites Zone (Late Rhaetian), the Cerebropollenites macroverrucosus Zone (Sinemurian - Pliensbachian), the Spheripollenites - Leptolepidites Zone (Toarcian), and the Perinopollenites elatoides Zone (Middle Jurassic). The dinoflagellate cyst zones are the Rhaetogonyaulax rhaetica Zone (Rhaetian), the Dapcodinium priscum Zone (latest Rhaetian - earliest Sinemurian), the Liasidium variabile Zone (Sinemurian), and the Nannoceratopsis gracilis Zone (Late Pliensbachian - ?Bajocian/Bathonian). These zones, and the Pinuspollenites - Trachysporites Zone Lund 1977 (Hettangian), are proposed for use in the Danish Subbasin. The combined spore/pollen and dinoflagellate cyst zonation has resulted in a detailed biostratigraphical subdivision of the sequences studied. A new combination, Manumia delcourtii (Pocock 1970) nov. comb. et emend., is proposed here, and the species description emended. New photographs of the holotypes of some of the species erected by Nilsson (1958) are included in the plates. The kerogen assemblages recorded from the Fjerritslev Formation, indicate a marine depositional environment, with a high but variable influence of terrestrially-derived organic particles. Stratigraphic variation in the kerogen assemblages generally correlate with the lithostratigraphical subdivision, and support previous environmental interpretations of the Fjerritslev Formation. Indications of strongly reducing conditions in the bottom waters were found in the Stenlille-2 borehole, in samples here referred to the Early Toarcian. The variations in the kerogen assemblages in the sequence investigated from the Gassum-1 borehole are not correlatable with the other boreholes, but seem primarily to reflect a distinct decrease in bioturbation in the Late Sinemurian. The Fjerritslev Formation does not generally represent a potential source rock for oil. Some levels (parts of the F-111 member) show, however, the characteristics of a fair to good source rock. The organic matter is generally immature or only at the earliest stage of maturity.

The genesis and palynofacies characteristics of marine petroleum source rocks

Summary This paper attempts to demonstrate that the multi-disciplinary philosophy behind palynofacies investigations is of great value to the meaningful interpretation of the origins and palaeoenvironment of marine petroleum source rocks (MPSRs). The key control on the formation of MPSRs is identified as bottom water oxygenation (correlated with the location of the Eh interface and the intensity of macrobenthic activity). Particular emphasis is placed on the necessity for greater accuracy in the terminology used to describe levels of oxygenation. Aerobic environments are characterized by organically lean sediments (0–3.0% TOC) with Type III kerogen assemblages composed of relatively refractory land plant debris or highly degraded, marine-derived, amorphous organic matter (AOM). Their organic richness is largely dependent on sediment accumulation rate and proximity to sources of terrestrial organic matter supply. They produce mainly gas at maturation. Dysaerobic to anoxic environments are characterized by MPSR facies with high TOC values and Type II kerogen assemblages dominated by relatively lipid-rich AOM. These represent the classic ‘black shale’ source rocks and are the main source of petroleum in marine basins. Except where they are redeposited, dinocysts are characteristically absent or rare in marine ‘black shales’. They are mainly produced in unstable, seasonally mixed water masses and may consequently be regarded as indices of hydrographic stability. Prasinophycean phycomas, which differ from dinocysts in their function, are often the dominant, or most conspicuous, marine palynomorphs of pelagic sediments and stably stratified ‘black shale’ basins.