Biomarker Thermal Maturity Research Articles

Geochemical characteristic of the Lutetian black shale (Bayburt, NE-Turkey): amount, nature, origin of organic matter and palaeo-environment conditions

Abstract To define the amount, thermal maturity and type of organic matter (OM), a comprehensive analysis of major and trace elements, organic carbon isotope and organic geochemistry was conducted on Lutetian black shales found in the Everek (Bayburt) region of northeastern Turkey. Total organic carbon (TOC) levels in the shale samples range from 0.62% to 6.75%, and type II–III to type III kerogen is generated, displaying a combination of high terrigenous and low marine OM. The δ13C values (ranging from –28.22‰ to –28.23‰), aromatic hydrocarbon compounds (methyl phenanthrene, dibenzothiophene, tri-aromatic and monoaromatic steroids), saturated hydrocarbon compounds (sterane and terpane), acyclic isoprenoids, n-alkane distribution (n-C13–n-C36) and inorganic geochemical characterization support that the black shales were deposited in a terrestrial-marine transition environment and had a high proportion of terrestrial OM with small amounts of marine OM preserved in relatively arid to hot climate and oxic to suboxic conditions. The analysis of biomarker thermal maturity markers, Tmax (ranging from 449–458 oC) and estimated vitrinite reflectance (varying from 0.92 to 1.08%) values suggest that the black shales have reached the oil window. As a result, black shales are thought to contain low to high amounts of TOC, have a mixed kerogen type, reach a high thermal maturity level and produce little hydrocarbons.

History of earthquakes along the creeping section of the San Andreas fault, California, USA

Abstract Creeping faults are difficult to assess for seismic hazard because they may participate in rupture even though they likely cannot nucleate large earthquakes. The creeping central section of the San Andreas fault in California (USA) has not participated in a historical large earthquake; however, earthquake ruptures nucleating in the locked northern and southern sections may propagate through the creeping section. We used biomarker thermal maturity and K/Ar dating on samples from the San Andreas Fault Observatory at Depth to look for evidence of earthquakes. Biomarkers show evidence of many earthquakes with displacements >1.5 m in and near a 3.5-m-wide patch of the fault. We show that K/Ar ages decrease with thermal maturity, and partial resetting occurs during coseismic heating. Therefore, measured ages provide a maximum constraint on earthquake age, and the youngest earthquakes here are younger than 3 Ma. Our results demonstrate that creeping faults may host large earthquakes over longer time scales.

Evidence of Seismic Slip on a Large Splay Fault in the Hikurangi Subduction Zone

AbstractThe Hikurangi subduction zone is capable of producing moderate to large earthquakes as well as regularly repeating slow slip events. However, it is unclear what structures host these different slip styles along the margin. Here we address whether splay faults can host seismic slip at shallow (<1 km) depth by investigating the Pāpaku fault, sampled during International Ocean Discovery Program Expedition 375. We use biomarker thermal maturity to search for evidence of frictional heating within turbiditic sediments of the Pāpaku fault. Four zones of localized high temperature are found near the top of the fault zone, which are interpreted to be zones of localized seismic slip. Thermal modeling shows that the most likely maximum displacement on the shallow Pāpaku fault during each event was 14–17 m. Our results demonstrate that the Pāpaku fault, and potentially other splay faults along the margin, host coseismic slip and have the potential to produce large tsunami (e.g., runup heights of >1 m as observed in the 1947 Poverty and Tolaga Bay earthquakes.

Earthquake slip surfaces identified by biomarker thermal maturity within the 2011 Tohoku-Oki earthquake fault zone

Extreme slip at shallow depths on subduction zone faults is a primary contributor to tsunami generation by earthquakes. Improving earthquake and tsunami risk assessment requires understanding the material and structural conditions that favor earthquake propagation to the trench. We use new biomarker thermal maturity indicators to identify seismic faults in drill core recovered from the Japan Trench subduction zone, which hosted 50 m of shallow slip during the Mw9.1 2011 Tohoku-Oki earthquake. Our results show that multiple faults have hosted earthquakes with displacement ≥ 10 m, and each could have hosted many great earthquakes, illustrating an extensive history of great earthquake seismicity that caused large shallow slip. We find that lithologic contrasts in frictional properties do not necessarily determine the likelihood of large shallow slip or seismic hazard.

Biomarker Thermal Maturity Reveals Localized Temperature Rise From Paleoseismic Slip Along the Punchbowl Fault, CA, USA

AbstractThe Punchbowl Fault, California, USA, is an example of a simple fault zone that has a relatively narrow fault core with further slip localization to principal slip zones. We sampled the principal slip zone, fault core, and wall rocks and conducted hydrous pyrolysis experiments to analyze biomarker thermal maturity within the Punchbowl Fault. Using biomarker maturity as a proxy for temperature rise, we show that the existing principal slip zone experienced greater temperature rise than the surrounding fault core and wall rock, and therefore, we infer that earthquake slip localized along this layer. Furthermore, evidence of slight thermal maturity within the ultracataclasite suggests that the fault core is made up at least in part of reworked former principal slip zones. Using a wide range of possible layer thicknesses, we find that the maximum temperature range during a single earthquake could have varied from ~460 to 1,060 °C at 1‐m/s slip velocity. However, not all samples from within the principal slip zone show a temperature rise, indicating that layer thickness, slip, or shear stress varied during slip. Our temperature estimate also allows us to constrain the frictional energy dissipated during the earthquake to 2.2–25 MJ/m2. Our study demonstrates that localized slip structures can be directly linked to seismicity and that small changes in earthquake or fault parameters can lead to changes in temperature (and likely fault strength) at small scales.

Biomarker thermal maturity experiments at earthquake slip rates

Evidence of temperature rise is useful for identifying where within a fault zone earthquake slip has occurred, as well as some attributes of the earthquake such as energy output. Biomarker thermal maturity can elucidate structures within a fault zone that were hotter than surrounding rock, and therefore likely to have hosted earthquakes. We performed a series of friction experiments on thin gouge layers of Woodford shale at slip rates of 1 m/s. Our results show that biomarker thermal maturity can clearly delineate zones of slip that are as thin as 150 μm. We establish that the kinetics of methylphenanthrene reaction previously determined at longer durations and lower temperatures are applicable at seismic slip rates and timescales. Temperature estimates show that once the slipping zone reached ∼500 °C, biomarker reaction was significant, as predicted by the reaction kinetics. Our results demonstrate that temperature rise in fault zones is a function of both the work density and power density during the earthquake. Biomarker reactions, along with other Arrhenius-style reactions such as those that lead to dynamic shear weakening, are both a function of slip speed and frictional work.

Source Rock Evaluation and Lateral Changes in Thermal Maturity of the Sargelu Formation (Middle Jurassic) in Kurdistan Region-Northern Iraq

In this study, source rock characteristics and lateral changes in thermal maturity of the Sargelu Formation (Middle Jurassic) in three outcrops were studied. The formation’s outcrops can be found in the High Folded, and Imbricated Zones of Iraqi Tectonic Division. In order to achieve the main goals of this study, the Gas chromatography mass spectrometry (GC-MS) and Rock-Eval pyrolysis were performed on the organic matter (OM) of the Sargelu sediments. Pristane/Phytane ratios for analyzed samples indicate reducing conditions (anoxic) during sedimentation. Moreover, based on C29/C30 hopanes ratios the sediments of the Sargelu Formation associated with clay- rich source rock. Biomarker thermal maturity parameters display that all samples are thermally in Oil Window at least. The biomarker findings reveal that the samples of Walasimt and Barsarin are seem more mature than Banik section.
 The values for TOC% of the Sargelu Formation may show Excellent, Very Good, and Poor quality source rock. The Pyrolysis executed for studied samples revealed the kerogen in Banik section belongs to Types II and III (Probably Oil/Gas- prone), while Barsarin and Walasimt sections obtained Types III-IV Kerogen (Gas- prone). Pyrolysis parameters suggested Early Mature Stage in Banik, While Overmature in Barsarin and Walasimt. The data also shows that organic matter of the Sargelu Formation in Banik section is in Oil Widow, while in Barsarin and Walasimt is in Gas Window.

Late Cretaceous marine arthropods relied on terrestrial organic matter as a food source: Geochemical evidence from the Coon Creek Lagerstätte in the Mississippi Embayment.

The Upper Cretaceous Coon Creek Lagerstätte of Tennessee, USA, is known for its extremely well-preserved mollusks and decapod crustaceans. However, the depositional environment of this unit, particularly its distance to the shoreline, has long been equivocal. To better constrain the coastal proximity of the Coon Creek Formation, we carried out a multiproxy geochemical analysis of fossil decapod (crab, mud shrimp) cuticle and associated sediment from the type section. Elemental analysis and Raman spectroscopy confirmed the presence of kerogenized carbon in the crabs and mud shrimp; carbon isotope (δ13 C) analysis of bulk decapod cuticle yielded similar mean δ13 C values for both taxa (-25.1‰ and -26‰, respectively). Sedimentary biomarkers were composed of n-alkanes from C16 to C36 , with the short-chain n-alkanes dominating, as well as other biomarkers (pristane, phytane, hopanes). Raman spectra and biomarker thermal maturity indices suggest that the Coon Creek Formation sediments are immature, which supports retention of unaltered, biogenic isotopic signals in the fossil organic carbon remains. Using our isotopic results and published calcium carbonate δ13 C values, we modeled carbon isotope values of carbon sources in the Coon Creek Formation, including potential marine (phytoplankton) and terrestrial (plant) dietary sources. Coon Creek Formation decapod δ13 C values fall closer to those estimated for terrigenous plants than marine phytoplankton, indicating that these organisms were feeding primarily on terrigenous organic matter. From this model, we infer that the Coon Creek Formation experienced significant terrigenous organic matter input via a freshwater source and thus was deposited in a shallow, nearshore marine environment proximal to the shoreline. This study helps refine the paleoecology of nearshore settings in the Mississippi Embayment during the global climatic shift in the late Campanian-early Maastrichtian and demonstrates for the first time that organic δ13 C signatures in exceptionally preserved fossil marine arthropods are a viable proxy for use in paleoenvironmental reconstructions.

Geochemical characterization of Neogene sediments from onshore West Baram Delta Province, Sarawak: paleoenvironment, source input and thermal maturity

AbstractThe Neogene strata of the onshore West Baram Province of NW Borneo contain organic rich rock formations particularly within the Sarawak basin. This basin is a proven prolific oil and gas province, thus has been a subject of great interest to characterise the nature of the organic source input and depositional environment conditions as well as thermal maturation. This study is performed on outcrop samples of Lambir, Miri and Tukau formations, which are of stratigraphic equivalence to the petroleum bearing cycles of the offshore West Baram delta province in Sarawak. The investigated mudstone samples are organic rich with a total organic carbon (TOC) content of more than 1.0 wt.%. The integration of elemental and molecular analyses indicates that there is no significant variation in the source input between these formations. The investigated biomarkers parameters achieved from acyclic isoprenoids, terpanes and steranes biomarkers of a saturated hydrocarbon biomarkers revealed that these sediments contain high contribution of land plants with minor marine organic matter input that was deposited and preserved under relatively oxic to suboxic conditions. This is further supported by low total sulphur (TS), high TOC/TN ratios, source and redox sensitive trace elements (V, Ni, Cr, Co and Mo) concentrations and their ratios, which suggest terrigenous source input deposited under oxic to suboxic conditions. Based on the analysed biomarker thermal maturity indicators, it may be deduced that the studied sediments are yet to enter the maturity stage for hydrocarbon generation, which is also supported by measured vitrinite reflectance values of 0.39-0.48% R

Reaction kinetics of alkenone and n‐alkane thermal alteration at seismic timescales

AbstractRecent experiments and field observations have indicated that biomarker molecules can react over short timescales relevant to seismic slip, thereby making these compounds a useful tool in studying temperature rise in fault zones. However, short‐timescale biomarker reaction kinetics studies have previously focused on compounds that have already experienced burial heating. Here, we present a set of hydrous pyrolysis experiments on Pleistocene‐aged shallow marine sediment to develop the reaction kinetics of long‐chain alkenone destruction, change in the alkenone unsaturation ratio ( ), and change in the n‐alkane chain length distribution. Our results show that biomarker thermal maturity provides a useful method for detecting temperature rise in the shallow reaches of faults, such as subduction zone trench environments. Through the course of our work, we also noted the alteration of total alkenone concentrations and values in crushed sediments stored dry at room temperature for durations of months to years but not in the solvent extracts of these materials. This result, though parenthetical for our work in fault zones, has important implications for proper storage of sedimentary samples to be used for alkenone paleotemperature and productivity analysis.

Organic geochemical characteristics of black shales across the Ordovician–Silurian boundary in the Holy Cross Mountains, central Poland

Black shales in the Holy Cross Mountains area of Poland provide a record of environmental change across the Ordovician–Silurian boundary. The changing depositional conditions have generated a variation in organic matter contents above and below the boundary. Investigating the organic constitution of these black shales has the potential to reveal how their organic matter contents were generated and how suitable these rocks and their lateral equivalents may be for exploitation as shale gas reservoirs. One outcrop at the Holy Cross Mountains with a continuous section across the Ordovician–Silurian boundary occurs near the village of Bardo Stawy in the Kielce region, and contains sandy-silty mudstones, as well as grey and black shales. Organic geochemical analyses of samples at Bardo Stawy reveals low Total Organic Carbon (TOC) contents for Ordovician samples and higher TOC values for Silurian samples. Organic biomarkers indicate that the Ordovician rocks were deposited in a shallow-marine shelf setting, while the Silurian rocks were deposited in a deeper marine environment. The progressive increase in TOC from the uppermost Ordovician to the lowermost Silurian rocks reflects increasingly oxygen-poor depositional conditions during the post-glacial transgression. Following the deposition and preservation of organic matter in the Ordovician and Silurian rocks, these rocks were buried and subjected to thermal maturation. Rock Eval and biomarker thermal maturity parameters all indicate that the organic matter is mature and lies within the oil window. The Ordovician and Silurian shales have direct relevance to recent attempts to discover and exploit shale gas reservoirs in Poland. Our data and interpretations suggest that the relatively low TOC values (<2%) and low maturities for gas generation render these rocks unsuitable for commercial shale gas production. The progressive improvement in conditions for preserving organic matter across the Ordovician–Silurian boundary does, however, leave the possibility that more suitable deposits occur in Early Silurian rocks in the other parts of the basin.

Organic geochemical characteristics of the Lower Cretaceous Abu Gabra Formation in the Great Moga oilfield, Muglad Basin, Sudan: Implications for depositional environment and oil-generation potential

Organic-rich sediments within the Abu Gabra Formation from three wells in the Great Moga oilfield were analyzed using organic geochemistry and organic petrology. The analyzed samples generally contain more than 2.0wt.% TOC and have a very good to excellent hydrocarbon generative potential. This is supported by high bitumen extract and hydrocarbon (HCs) yields with values exceeding 4000 and 2000ppm, respectively. The Abu Gabra also have moderate to high hydrogen index (HI) values of 287–865mg HC/g TOC and large amounts of amorphous organic matter and alginite, consistent with oil-prone Types I and Type II kerogen. Vitrinite reflectance (0.59–0.72) %Ro and pyrolysis Tmax (430–438°C) indicate an early oil window stage. This is supported by bitumen/TOC ratios (0.04–0.09) and biomarker thermal maturity parameters with equilibrium C32 homohopane 22S/(22S+22R) ratios (0.50–58), moretane/hopane (0.11–018) and C29ββ/(ββ+αα) (0.53–0.73) and 20S/(20S+20R) ratios (0.26–0.48).The biomarkers are characterized by a dominance of low to medium molecular weight n-alkane compounds with significant waxy alkanes (n-C25–n-C34), moderately high Pr/Ph ratios (1.17–2.51), high abundance of C27 regular steranes, high C27/C29 regular sterane ratios, the presence of tricyclic terpanes and relatively low sterane/hopane ratios. These data indicate that the organic-rich sediments of the Abu Gabra Formation contain a mixture of aquatic (algal and bacterial) and terrigenous organic matter, deposited in a lacustrine environment and preserved under suboxic conditions.

Organic thermal maturity as a proxy for frictional fault heating: Experimental constraints on methylphenanthrene kinetics at earthquake timescales

Biomarker thermal maturity is widely used to study burial heating of sediments over millions of years. Heating over short timescales such as during earthquakes should also result in measurable increases in biomarker thermal maturity. However, the sensitivity of biomarker thermal maturity reactions to short, higher-temperature heating has not been established. We report on hydrous pyrolysis experiments that determine the kinetic parameters of methylphenanthrene maturation at timescales and temperatures relevant to earthquake heating. Samples of Woodford Shale were heated at temperatures up to 343°C over 15–150min. The thermal maturity of the samples as measured by the methylphenanthrene index-1 (MPI-1) increased with heating time and temperature. We find that MPI-1 increases with time and temperature consistent with a first-order kinetic model and Arrhenius temperature relationship. Over the timescales tested here, MPI-1 is strongly affected by maximum temperature and less sensitive to heating duration. Production of new phenanthrene isomers and expulsion of a liquid pyrolyzate also occurred. Differential expulsion of methylphenanthrene isomers affected the apparent maturity of the rock at lower temperatures and may need to be considered for organic-rich fault rocks. Our results demonstrate that the overall MPI-1 reaction extent in both the rock and pyrolyzate are a useful measure of thermal maturity and reflect temperature history during rapid heating.

Biological markers and carbon isotope composition of organic matter in the Upper Cretaceous coals and carbonaceous shale succession (Jiza–Qamar Basin, Yemen): Origin, type and preservation

Bituminous coals and carbonaceous shales, located in the Jiza–Qamar Basin, on the eastern Yemen, were analysed based on a combined investigation of organic geochemistry and petrology to define the palaeodepositional environment condition, organic matter inputs and their thermal maturity. Carbon isotope compositions and various biomarkers were performed on coals and carbonaceous shales within the Upper Cretaceous Mukalla Formation in the Jiza–Qamar Basin. The δ13C values range from −29.2‰ to −24.5‰ with average values around −26.7‰, whereas the organic carbon contents in most sediment samples are consistently high (>3%), indicating that the organic matter in the Mukalla coal and carbonaceous shale sediments has a predominantly terrigenous origin with slightly marine influence during burial. This has also been identified from kerogen microscopy, which is characterized by dominance of vitrinite and liptinite organic matter.The investigated biomarkers indicated that the Mukalla coals and carbonaceous shales contain high contribution of land plants with a minor aquatic organic matter input that was deposited in a fluvial to deltaic environments and preserved under relatively oxic conditions. This has been achieved from acyclic isoprenoids, terpane and sterane biomarkers of a saturated hydrocarbon and dibenzothiophene, alkylnaphthalene and methylphenanthrene biomarkers of an aromatic hydrocarbon.Based on the analysed biomarker thermal maturity indicators, it may be deduced that the Mukalla coals and carbonaceous shales in the Jiza–Qamar Basin have entered an early-mature to mature stages for hydrocarbon generation. This is also supported by vitrinite reflectance values of 0.63–0.83% Ro indicative that these sediments have reached oil window maturity.

Biological markers and organic petrology study of organic matter in the Lower Cretaceous Abu Gabra sediments (Muglad Basin, Sudan): origin, type and palaeoenvironmental conditions

The organic-rich claystone and shale sediments of Lower Cretaceous Abu Gabra Formation are considered an important regional source rock in the Muglad Basin, Sudan. A total of 20 cutting samples from these organic-rich sediments were collected from six wells drilled at oilfields in the Fula sub-basin, Muglad Basin, in order to geochemically assess the type of organic matter, thermal maturity, and palaeoenvironmental conditions. Results reveal that Abu Gabra organic-rich sediments contain high organic matter more than 2.0 wt% TOC and have a very good to excellent oil-generation potential. This is supported by high bitumen extractions and hydrocarbon yields with values exceeding 5,000 and 3,000 ppm, respectively. The investigated biomarkers indicated that the Abu Gabra sediments contain a mixture of algal and amorphous organic matter that were deposited in a lacustrine environment setting. This has been identified from kerogen microscopy and normal alkane distributions, which are characterized by dominance of types I/II kerogen and low-medium molecular weight n-alkane compounds, respectively. This is also supported by high abundance of C27 regular sterane concentrations, high C27/C29 regular sterane ratios and relatively low value of the biomarker sterane/hopane ratio as well as the presence of tricyclic terpanes. A mainly suboxic to relatively anoxic preservation conditions is inferred from Pr/Ph ratios (1.01–2.51). This is further supported by lower amounts of acyclic isoprenoids compared to n-alkanes (e.g., pristane/n-C17 and phytane/n-C18 ratios). Based on the analyzed biomarker thermal maturity indicators, it may be deduced that the Abu Gabra sediments have entered an early mature stage for oil generation. This is also supported by vitrinite reflectance values of 0.58–0.72 % Ro indicative that these sediments have reached oil window maturity.

The composition, origin and fate of complex mixtures in the maltene fractions of hydrothermal petroleum assessed by comprehensive two-dimensional gas chromatography

Sedimentary organic matter in hydrothermal systems can be altered by high temperature fluids to generate petroleum. The saturated and aromatic fractions of these hydrothermal oils are compositionally similar to conventional oil with the exception that they often contain higher concentrations of polycyclic aromatic hydrocarbons (PAH) as well as substantial mixtures of coeluting organic compounds that produce dramatically rising signal on the baseline of gas chromatograms termed unresolved complex mixtures (UCMs). Little is known about the compounds that compose UCMs and why or how they form. This is in part due to an inability to discriminate between in situ and migrated components that characterize the petroleum generated in hydrothermal systems. However, UCMs are also a product of the limitations imbedded in analytical separation techniques. With the advent of comprehensive two-dimensional gas chromatography (GC×GC), a revision of what should constitute molecular complexity needs to be considered. We address these problems by comparing the molecular compositions of the maltene fractions of three previously published hydrothermal petroleum samples using time of flight-mass spectrometry (GC×GC–ToF-MS) and 12 hydrothermal petroleum samples in cores from three locales using comprehensive two-dimensional gas chromatography with flame ionization detection (GC×GC–FID). The sediment cores were collected from Middle Valley, located off the axis of the Juan de Fuca Ridge, and the Escanaba Trough, along the Gorda Ridge, both in the NE Pacific Ocean, as well as from the Guaymas Basin in the Gulf of California. We define a UCM in GC×GC data to be a condition in which ⩾25% of the detected peaks within a chromatographic area coelute in either the first or second dimension. In turn, complex (CM) and simple mixtures (SM) are defined as having 5–24% and <5% coelution, respectively. All CM and UCMs were dominated by an array of configurational isomers, which becomes increasingly aromatic with higher molecular weight. We relate this to a multi-molecular complexity metric (MCM) by quantitatively comparing the difference in total peak variance and peak density for a GC×GC chromatogram. MCM values correlate with biomarker thermal maturity ratios for the Escanaba Trough and Guaymas Basin samples indicating that molecular complexity in these hydrothermal environments is in part a function of burial temperatures. Partial Least Squares (PLS) linear regression was applied to the total number of peak retention times as a proxy for the bulk molecular differences between each hydrothermal oil sample. Differences in the sample regressions correlate with the thermal maturity and the degree of PAH alkylation, indicating that this technique can be used to assess the degree of oxidative weathering due to dehydrogenation and hydrocarbon cracking. Subtracted chromatograms were then used to quantitatively track all of the individual molecular changes within the pyrolytic regime at Escanaba Trough. These subtracted chromatograms indicate that high molecular weight PAHs are highly mobile in hydrothermal fluids and may represent a phase partitioning that is occurring at greater depths. This phase condenses just below the seafloor to form an UCM in the near surface sediments. Saturated hydrocarbon biomarkers, such as hopanes, steranes and biphytanes are less mobile and more prone to being cracked and/or aromatized prior to migration toward the ocean floor. Together these techniques suggest that the molecular complexity of hydrothermal petroleum maximizes during the early stages of thermal maturation. The diversity of compounds forming these UCMs then decreases with increasing dehydrogenation, dealkylation and condensation reactions associated with elevated thermal stress and exposure to oxidants within the hydrothermal fluids.

Petroleum geochemistry of the Potwar Basin, Pakistan: 1. Oil–oil correlation using biomarkers, δ 13C and δD

Geochemical characterisation of 18 crude oils from the Potwar Basin (Upper Indus), Pakistan is carried out in this study. Their relative thermal maturities, environment of deposition, source of organic matter (OM) and the extent of biodegradation based on the hydrocarbon (HC) distributions are investigated. A detailed oil–oil correlation of the area is established. Gas chromatography–mass spectrometry (GC–MS) analyses and bulk stable carbon and hydrogen isotopic compositions of saturated and aromatic HC fractions reveals three compositional groups of oils. Most of the oils from the basin are typically generated from shallow marine source rocks. However, group A contains terrigenous OM deposited under highly oxic/fluvio-deltaic conditions reflected by high pristane/phytane (Pr/Ph), C 30 diahopane/C 29Ts, diahopane/hopane and diasterane/sterane ratios and low dibenzothiophene (DBT)/phenanthrene (P) ratios. The abundance of C 19-tricyclic and C 24-tetracyclic terpanes are consistent with a predominant terrigenous OM source for group A. Saturated HC biomarker parameters from the rest of the oils show a predominant marine origin, however groups B and C are clearly separated by bulk δ 13C and δD and the distributions of the saturated HC fractions supporting variations in source and environment of deposition of their respective source rocks. Moreover, various saturated HC biomarker ratios such as steranes/hopanes, diasteranes/steranes, C 23-tricyclic/C 30 hopane, C 28-tricyclic/C 30 hopane, total tricyclic terpanes/hopanes and C 31( R + S)/C 30 hopane show that two different groups are present. These biomarker ratios show that group B oils are generated from clastic-rich source rocks deposited under more suboxic depositional environments compared to group C oils. Group C oils show a relatively higher input of algal mixed with terrigenous OM, supported by the abundance of extended tricyclic terpanes (up to C 41+) and steranes. Biomarker thermal maturity parameters mostly reached to their equilibrium values indicating that the source rocks for Potwar Basin oils must have reached the early to peak oil generation window, while aromatic HC parameters suggest up to late oil window thermal maturity. The extent of biodegradation of the Potwar Basin oils is determined using various saturated HC parameters and variations in bulk properties such as API gravity. Groups A and C oils are not biodegraded and show mature HC profiles, while some of the oils from group B show minor levels of biodegradation consistent with high Pr/ n-C 17, Ph/ n-C 18 and low API gravities.

Molecular composition and organic petrographic characterization of Madbi source rocks from the Kharir Oilfield of the Masila Basin (Yemen): palaeoenvironmental and maturity interpretation

The upper part of Madbi Formation organic-rich shale is considered an important regional source rock in the Masila Basin, Yemen. Ten cutting samples from this Upper Jurassic organic-rich shale were collected from wells drilled in the Kharir Oilfield, Masila Basin in order to geochemically assess the type of organic matter, thermal maturity and depositional environment conditions. Results reveal that Upper Jurassic organic-rich shale samples contain high organic matter more than 2.0 wt.% TOC and have very good to excellent hydrocarbon potential. Marine algae organic matter is the main source input for the Upper Jurassic shale sequence studied. This has been identified from organic petrographic characteristics and from the n-alkane distributions, which dominated by n-C14-n-C20 alkanes. This is supported by the high value of the biomarker sterane/hopane ratio that approaches unity, as well as the relatively high C27 sterane concentrations. A mainly suboxic depositional environment is inferred from pr/ph ratios (1.75–2.38). This is further supported by relatively high homohopane value, which is dominated by low carbon numbers and decrease towards the C35 homohopane. The concentrations of C35 homohopane are very low. The depositional environment conditions are confirmed by some petrographic characteristics (e.g. palynofacies). Detailed palynofacies analysis of Madbi shales shows that the Madbi shale formation is characterised by a mix of amorphous organic matter, dinoflagellates cysts and phytoclasts, representing a suboxic, open marine setting. The Upper Jurassic marine shale sequence in the Masila Basin is thermally mature for hydrocarbon generation as indicated by biomarker thermal maturity parameters. The 22 S/22 S + 22R C32 homohopane has reached equilibrium, with values range from 0.58 to 0.62 which suggest that the Upper Jurassic shales are thermally mature and that the oil window has been reached. 20 S/(20 S + 20R) and ββ/(ββ + αα) C29 sterane ratios suggest a similar interpretation, as do the moretane/hopane ratio. This is supported by vitrinite reflectance data ranging from 0.74% to 0.90%Ro and thermal alteration of pollen and spore. The thermal alteration index value is around 2.6–3.0, corresponding to a palaeotemperature range of 60–120°C. These are the optimum oil-generating strata. On the basis of this study, the Madbi source rock was deposited under suboxic conditions in an open marine environment and this source rock is still within the oil window maturity range.

Effects of hydrous pyrolysis on biomarker thermal maturity parameters: Monterey Phosphatic and Siliceous members

Hydrous pyrolysis of immature Monterey Phosphatic or Siliceous rock at progressively higher temperatures causes systematic changes in biomarker thermal maturity parameters of the generated hydrocarbons. Biomarker ratios based on proposed carbon-carbon cracking or aromatization reactions increase during hydrous pyrolysis along similar pathways for both Siliceous and Phosphatic members. An increase in these biomarker ratios is also observed for oils of increasing thermal maturity from the offshore Santa Maria Basin, although the rates of changes for each parameter differ between the hydrous pyrolysis and natural samples. Changes in some cracking parameters during maturation appear to result from differential thermal stability of the compounds rather than conversion of precursors to products.The behavior of isomerization-based biomarker ratios in these experiments is more complex than ratios based on carbon-carbon cracking or aromatization reactions. During heating, kerogen-bound precursors generate steranes and hopanes showing lower levels of thermal maturity based on isomerization ratios than those extracted from the unheated rock. Asymmetric centers in the kerogen-bound steroids and hopanoids appear to be protected from isomerization compared to those of free steranes or hopanes in the bitumen. The Phosphatic and Siliceous rocks can show different sterane or hopane isomerization ratios when heated under the same time/temperature conditions. Further, these isomerization ratios unexpectedly decrease at high hydrous pyrolysis temperatures (> 330°C) for the Phosphatic, but not for the Siliceous samples. This could be caused by the combined effects of isomerization and differential destruction of epimers, apparently mediated by rock mineralogy.Differences between the biomarker compositions of bitumens and expelled oils in these experiments mimic those caused by natural primary migration. Heavier, more polar compounds are preferentially retained in the bitumen. For example, bitumens are enriched in tri- over monoaromatic steroids, hopanes over tricyclic terpanes, and regular steranes over diasteranes compared to expelled oils. No significant fractionation of stereoisomers was observed, such as 22S vs 22R C32 17α(H),21β(H)-homohopanes or 20S vs 20R and ββ vs αα C29 5α(H)-steranes.

Introduction to exploration geochemistry

Biomarker thermal maturity is widely used to study burial heating of sediments over millions of years. Heating over short timescales such as during earthquakes should also result in measurable increases in biomarker thermal maturity. However, the sensitivity of biomarker thermal maturity reactions to short, higher-temperature heating has not been established. We report on hydrous pyrolysis experiments that determine the kinetic parameters of methylphenanthrene maturation at timescales and temperatures relevant to earthquake heating. Samples of Woodford Shale were heated at temperatures up to 343 °C over 15–150 min. The thermal maturity of the samples as measured by the methylphenanthrene index-1 (MPI-1) increased with heating time and temperature. We find that MPI-1 increases with time and temperature consistent with a first-order kinetic model and Arrhenius temperature relationship. Over the timescales tested here, MPI-1 is strongly affected by maximum temperature and less sensitive to heating duration. Production of new phenanthrene isomers and expulsion of a liquid pyrolyzate also occurred. Differential expulsion of methylphenanthrene isomers affected the apparent maturity of the rock at lower temperatures and may need to be considered for organic-rich fault rocks. Our results demonstrate that the overall MPI-1 reaction extent in both the rock and pyrolyzate are a useful measure of thermal maturity and reflect temperature history during rapid heating.