Formation of methane-related authigenic carbonates in a highly dynamic biogeochemical system in the Krishna–Godavari Basin,Bay of Bengal

We report the abundant occurrence of authigenic Fe-rich carbonate, high Mg-calcite (HMC) and low Mg-calcite from 11 cores recovered from the Krishna–Godavari Basin (K–G Basin), Bay of Bengal. The cores were collected as part of the Indian gas hydrate exploration program on board R/V Marion Dufresne (MD-161: May, 2007) in different environments, including mounds (mud diapirs), mass flows, and hemipelagic sediments over a range of water depths from 647 to 2079 m. Authigenic carbonates range in size from 1 mm to 12 cm and display various morphologies like roundish or platy (micro-) nodules and tube-like forms. From the cores, 173 carbonate samples have been investigated for their depth distribution, mineralogy, geochemical and stable isotopic composition. The stable carbon isotopic composition of 46 out of 88 measured carbonate samples are around −50‰ which allows the differentiation into methane-related carbonates (HMC), especially at Sites 8 and 15, but also in low abundance at Sites 1, 5, 9 and 12. Results indicate that the carbonates at Site 8 and 15 represent paleo methane seepage locations. The Fe-rich carbonates occur abundantly at many sites in the K–G Basin. Their varying carbon isotopic composition indicates that probably not only sulfate reduction through organic matter degradation but also methanogenesis are the responsible processes for their formation.     

Provenance-related characteristics of beach sediments around the island of Menorca, Balearic Islands (western Mediterranean)

The island of Menorca, one of the Balearic Islands (Spain) located in the western Mediterranean, is characterised by a contrasting geology and landscape with two major geographic domains: (1) a southern region called Migjorn, comprised of Late Miocene calcarenites and limestones, and (2) a northern region known as Tramuntana, which is composed of folded and faulted Palaeozoic, Mesozoic and Tertiary (Oligocene) siliceous and calcareous rocks. Both domains are lined by numerous pocket beaches exhibiting a high variety of surficial sediment assemblages. Grain-size and compositional analyses revealed that cliff erosion and nearshore Posidonia oceanica meadows are the main sources of sediments consisting mostly of medium- to coarse-grained carbonate sands of marine biogenic origin, with variable amounts of terrigenous rock fragments and quartz. Based on distinctly different contributions of bioclastic material, biogenic carbonates and quartz, 320 sediment samples from 64 beaches were grouped into different facies associations dominated by either (1) biogenic sands, (2) biogenic sands with terrigenous contributions or (3) terrigenous sands with quartz. Nevertheless, there is a marked regional variability in sediment texture and composition. Thus, variable mixtures of carbonate and siliciclastic sediments characterise the beaches of the northern region, whereas the beaches of the southern region are composed mostly of carbonate sands of marine biogenic origin. An exception is the central sector of the south coast, which is enriched in quartz sand (~10 %); this can be related to outcrops of quartz-rich basement rock and also to rocks exposed in some northern drainage basins captured by southern streams since the Plio-Quaternary.     

礼乐盆地碳酸盐岩时空分布特征及构造意义

对碳酸盐岩(台地与生物礁)的研究具有重要的油气地质意义和科学价值。渐新世以来, 礼乐盆地及周缘开始了大规模的碳酸盐建造, 至今还有一定规模生物礁持续发育。文章通过拖网约束、井震对比和地震相类比方法对礼乐盆地周缘碳酸盐岩的发育特征、构造背景和时空分布规律进行系统研究。结果表明, 礼乐盆地主要发育孤立、前隆、掀斜断块型台地及生物礁。碳酸盐岩时空发育特征为: 1) 渐新世—早中新世以构造前隆和掀斜断块控制的开阔台地为主, 生物礁零星发育; 2) 早中新世—中中新世表现为大量孤立台地和生物礁; 3) 中中新世之后, 发生台地和礁的淹没, 盆地内主要发育点礁。结合构造背景和重磁特征分析表明, 古隆起、掀斜断块、前隆迁移和岩浆活动为控制礼乐盆地不同时期碳酸盐岩发育的主要构造要素。现今礼乐盆地及周缘大量生长的珊瑚礁, 主要发育在上新世—更新世岩浆活动形成的海山之上。     

The impact of organic fluids on the carbon isotopic compositions of carbonate-rich reservoirs: Case study of the Lucaogou Formation in the Jimusaer Sag,Junggar Basin,NW China

The processes involved in the interaction between organic fluids and carbonates, and the resulting effect on reservoir quality during the evolution and maturation of organic matter remain unclear despite the fact that these processes influence the carbon and oxygen isotopic compositions of carbonates. Here, we provide new insights into these processes using data obtained from a detailed analysis of a mixed dolomitic–clastic and organic-rich sedimentary sequence within the middle Permian Lucaogou Formation in the Junggar Basin of NW China. The techniques used during this study include drillcore observations, thin section petrography, scanning electron microscopy (SEM) and electron probe microanalysis, and carbon and oxygen isotope analyses. Oil grades and total organic carbon (TOC) contents represent the amount of oil charging and the abundance of organic fluids within a reservoir, respectively, and both negatively correlate with the whole-rock δ¹³C and δ¹⁸O of the carbonates in the study area, indicating that organic fluids have affected the reservoir rocks. Secondary carbonates, including sparry calcite and dolomite overgrowths and cements, are common within the Lucaogou Formation. Well-developed sparry calcite is present within dark mudstone whereas the other two forms of secondary carbonates are present within the dolomite-rich reservoir rocks in this formation. Comparing thin section petrology with δ¹³C compositions suggests that the carbon isotopic composition of matrix carbonates varies little over small distances within a given horizon but varies significantly with stratigraphic height as a result of the development of secondary carbonates. The net change in whole-rock δ¹³C as a result of these secondary carbonates ranges from 1.8‰ to 4.6‰, with the secondary carbonates having calculated δ¹³C compositions from −18.6‰ to −8.5‰ that are indicative of an organic origin. The positive correlation between the concentration of Fe within matrix and secondary carbonates within one of the samples suggests that the diagenetic system within the Lucaogou Formation was relatively closed. The correlation between δ¹³C and δ¹⁸O in carbonates is commonly thought to be strengthened by the influence of meteoric water as well as organic fluids. However, good initial correlation between δ¹³C and δ¹⁸O of whole rock carbonates within the Lucaogou Formation (resulted from the evaporitic sedimentary environment) was reduced by organic fluids to some extent. Consequently, the δ¹³C–δ¹⁸O covariations within these sediments are not always reliable indicators of diagenetic alteration by organic fluids or meteoric water.The characteristics and δ¹³C compositions of the sparry calcite within the formation is indicative of a genetic relationship with organic acids as a result of the addition of organic CO₂ to the reservoir. Further analysis suggests that both carbonate and feldspar were dissolved by interaction with organic CO₂. However, dissolved carbonate reprecipitated as secondary carbonates, meaning that the interaction between organic fluids and dolomites did not directly improve reservoir quality, although this process did enhance the dissolution of feldspar and increase porosity. This indicates that the δ¹³C and δ¹⁸O of secondary carbonates and their influence on whole-rock carbonate isotopic values can be used to geochemically identify the effect of organic fluids on closed carbonate-rich reservoir systems.     

Methane-derived carbonates and chemosymbiotic communities of Piedmont (Miocene,northern Italy): An update

Stable carbon isotope values of authigenic carbonate rocks in the Miocene terrigenous sediments of Piedmont indicate a methane-related origin. Some of these methane-derived carbonates (Lucina limestone) are characterized by the presence of abundant lucinid remains. Carbonate dissolution/precipitation and development of lucinid communities were related to bacterial methane oxidation, both aerobic and anaerobic. Anaerobic oxidation led to carbonate precipitation and production of sulfide, which sustained lucinid communities through chemosynthetic symbiotic bacteria. Aerobic oxidation of methane likely resulted in dissolution of carbonate skeletal grains. Several phases of carbonate precipitation, characterized by slightly different isotopic compositions, are recognizable in the limestones.     

A contribution to the reconstruction of Miocene seepage from authigenic carbonates of the northern Apennines (Italy)

Authigenic carbonates from outcrops of the northern Apennines consist of small and irregular lenses and exhibit numerous features indicative of cold-seep settings. Detailed petrographic, mineralogical and geochemical studies from two Miocene deposits are presented. The first carbonate outcrop, named Fosso Riconi, is located in the foredeep basin of the Apenninic chain, whereas the second deposit represents a satellite basin called Sarsetta. The stable isotope data from specific carbonate minerals show a wide range of values well known from other palaeoseeps of the Apennine Mountains. The majority of seep carbonates are formed by low-Mg calcite and ankerite. Those minerals have δ¹³C values between ?7 and ?23‰ V-PDB, suggesting variable amounts of carbonate derived from oxidized methane, seawater (dissolved inorganic carbon) and sedimentary organic matter. Dolomite samples have the lowest δ¹³C values (?30.8 to ?39.0‰ V-PDB), indicating methane as the main carbon source. The findings suggest an evolutionary formation of the seeps and development of authigenic carbonates influenced by the activity of chemosynthetic organisms, of which large lucinid clams are preserved. Bioirrigation by the clams controlled the sediment–water exchange, and is here considered as an explanation for the anomalous Mg content of the calcite. We hypothesize that the seep carbonates were formed during periods of active methane-rich seepage, whereas during periods of slow seepage carbonate formation was reduced. Despite different geological settings, the two examined deposits of Sarsetta and Fosso Riconi show similar features, suggesting that a common pattern of fluid circulation played a major role in carbonate formation at both seep sites.     

Carbonate platforms in wedge-top basins: An example from the Gunung Mulu National Park,Northern Sarawak (Malaysia)

The Melinau carbonate platform initiated during the Mid-Eocene on a rotating slice of the Rajang accretionary prism. The differential sedimentary loading enhanced a rotation of the mobile substratum and created an elongated, asymmetrical wedge-top basin. The extensional southern margin of the basin consists of a 2100–2200-m-thick section of Eocene-to-Oligocene carbonates. These thin laterally towards the northern margin of the basin, where a carbonate factory was active on a postulated underlying thrust. Backstepping and dismemberment of the carbonate system started during the latest Oligocene and deep-marine sedimentation became prevalent over the entire region during the Early Miocene.     

Influence of substrate rocks on Fe–Mn crust composition

Principal Component and other statistical analyses of chemical and mineralogical data of Fe–Mn oxyhydroxide crusts and their underlying rock substrates in the central Pacific indicate that substrate rocks do not influence crust composition. Two ridges near Johnston Atoll were dredged repetitively and up to seven substrate rock types were recovered from small areas of similar water depths. Crusts were analyzed mineralogically and chemically for 24 elements, and substrates were analyzed mineralogically and chemically for the 10 major oxides. Compositions of crusts on phosphatized substrates are distinctly different from crusts on substrates containing no phosphorite. However, that relationship only indicates that the episodes of phosphatization that mineralized the substrate rocks also mineralized the crusts that grew on them. A two-fold increase in copper contents in crusts that grew on phosphatized clastic substrate rocks, relative to crusts on other substrate rock types, is also associated with phosphatization and must have resulted from chemical reorganization during diagenesis. Phosphatized crusts show increases in Sr, Zn, Ca, Ba, Cu, Ce, V, and Mo contents and decreases in Fe, Si, and As contents relative to non-phosphatized crusts. Our statistical results support previous studies which show that crust compositions reflect predominantly direct precipitation from seawater (hydrogenetic), and to lesser extents reflect detrital input and diagenetic replacement of parts of the older crust generation by carbonate fluorapatite.     

Fluid sources and stable isotope signatures in authigenic carbonates from the Northern Apennines,Italy

Authigenic carbonates are frequently associated with methane cold-seep systems, which extensively occur in various geologic settings worldwide. Of interest is the relation between the fluids involved in their formation and the isotopic signals recorded in the carbonate cements. Along the Northern Apennines foothills (Italy), hydrocarbons and connate waters still seeping nowadays are believed to be the primary sources for the formation of fossil authigenic carbonate found in Plio-Pleistocene marine sediments. Four selected outcrops of dolomitic authigenic carbonates were analysed to compare signature of seeping fluids with fractionation of stable carbon and oxygen isotopes recorded in the carbonate.Along the foothills, deep methane-rich fluids spontaneously rise to the surface through mud volcanoes or are exploited in wells drilled nearby to the fossil Plio-Pleistocene authigenic carbonates. The plumbing system providing fluids to present-day cold seeps was structurally achieved in Late Miocene and Plio-Pleistocene. δ¹³C values of methane, which vary from −51.9 to −43.0‰ VPDB, indicate that gas composition from the deep hydrocarbon reservoirs is relatively uniform along the foothills. On the contrary, δ¹³C in fossil authigenic carbonates strongly varies among different areas and also within the same outcrop.The different carbon sources that fed the investigated carbonates were identified and include: thermogenic methane from the deep Miocene reservoirs, ¹³C-enriched CO₂ derived from secondary methanogenesis and microbial methane from Pliocene successions buried in the Po Plain. The δ¹³C variability documented among samples from a single outcrop testifies that the authigenic carbonates might represent a record of varying biogeochemical processes in the hydrocarbon reservoirs. The sources of stable oxygen isotopes in authigenic carbonates are often ascribed to marine water. Oxygen isotopic fractionation in the dolomite cements indicates that marine pore water couldn't be the sole source of oxygen. δ¹⁸O values provide a preliminary evidence that connate waters had a role in the carbonates precipitation. The concomitant occurrence of active cold seepages and fossil record of former plumbing systems suggests that generation and migration of hydrocarbons are long-lasting and very effective processes along the Northern Apennines foothills.     

Middle to Late Cenozoic tectonic events in south and central Palawan (Philippines) and their implications to the evolution of the south-eastern margin of South China Sea: Evidence from onshore structural and offshore seismic data

Using recently gathered onland structural and 2D/3D offshore seismic data in south and central Palawan (Philippines), this paper presents a new perspective in unraveling the Cenozoic tectonic history of the southeastern margin of the South China Sea. South and central Palawan are dominated by Mesozoic ophiolites (Palawan Ophiolite), distinct from the primarily continental composition of the north. These ophiolites are emplaced over syn-rift Eocene turbidites (Panas Formation) along thrust structures best preserved in the ophiolite–turbidite contact as well as within the ophiolites. Thrusting is sealed by Early Miocene (∼20 Ma) sediments of the Pagasa Formation (Isugod Formation onland), constraining the younger limit of ophiolite emplacement at end Late Oligocene (∼23 Ma). The onset of ophiolite emplacement at end Eocene is constrained by thrust-related metamorphism of the Eocene turbidites, and post-emplacement underthrusting of Late Oligocene – Early Miocene Nido Limestone. This carbonate underthrusting at end Early Miocene (∼16 Ma) is marked by the deformation of a seismic unit corresponding to the earliest members of the Early – Middle Miocene Pagasa Formation. Within this formation, a tectonic wedge was built within Middle Miocene (from ∼16 Ma to ∼12 Ma), forming a thrust-fold belt called the Pagasa Wedge. Wedge deformation is truncated by the regionally-observed Middle Miocene Unconformity (MMU ∼12 Ma). A localized, post-kinematic extension affects thrust-fold structures, the MMU, and Late Miocene to Early Pliocene carbonates (e.g. Tabon Limestone). This structural set-up suggests a continuous convergent regime affecting the southeastern margin of the South China Sea between end Eocene to end Middle Miocene. The ensuing structures including juxtaposed carbonates, turbidites and shallow marine clastics within thrust-fold belts have become ideal environments for hydrocarbon generation and accumulation. Best developed in the Northwest Borneo Trough area, the intensity of thrust-fold deformation decreases towards the northeast into offshore southwest Palawan.     

Phosphorite from the Continental Margin off Madras, Bay of Bengal

A recent survey reveals that phosphorites occur over an area of 800 km2 between 100 and 500 m depths on the outer continental shelf and upper slope ESE off Madras. Appreciable subsurface continuity of phosphorites is not indicated. They mostly occur over two terraces recorded between the 100 and 200, and 350 and 400m isobaths. Sampled phosphorites may be classified under four types: (1) dense conglomeratic phosphorites in the size range 2 5 cm, (2) dark gray phosphatic nodules in the size range 1 3 cm, (3) creamy white calcareous algal nodules with phosphatic laminations in the size range 0.5 3 cm, and (4) recent gastropod shells with phosphatic studs. Conglomeratic phosphorites confined mostly to the upper terrace were recov ered along with calcareous pelecypod and gastropod fossils belonging to Cretaceous to Oligocene age. Gray phosphatic nodules occur over upper as well as lower terraces. The algal nodules are mostly confined to the upper terrace and are less abundant. Acoustic profiles (3.5 kHz) and the recovery of fossils indicate that the upper terrace is a karstic surface of a carbonate platform, probably of Cretaceous to Oligocene age. Petrographic study of the conglomeratic phosphorite shows skeletal matter set in phosphatic matrix and what appears to be the phosphatized limestone. Selective phosphatization of foraminiferal tests is common. Dark gray phosphatic nodules show brownish-gray laminations containing phosphatized skeletal fragments set in phosphatic matrix. Creamy white calcareous algal nodules exhibit alternate gray phosphatic and white calcitic laminations. All the phosphorite types are composed of carbonate fluorapatite and calcite as major minerals, followed by quartz and aragonite. Phosphorous pentoxide content of conglomeratic type varies from 15% to 20%, dark gray nodules from 16% to 19.98%, and calcareous algal nodules from 8.10% to 17.54%. The significant enrichment of Mo and Pb in phosphorites is attributed to their fixation in organic matter under reduced condition. SEM studies indicate a microbial origin for the gray phosphatic and algal nodules. Studies suggest four episodes of phosphatization with intervening reworking and redoposition, probably commencing from the Eocene period. Phosphatization during the late Pleistocene and early Holocene period also can be ruled out.     

Phosphate inclusions in tuffites from the Krishtofovich Rise (Sea of Japan)

Tuffites of the core taken from the underwater Krishtofovich Rise in the southeastern Sea of Japan contain phosphate inclusions that were studied by scanning electron microscopy and chemical methods. The tuffites relatively enriched in phosphorus represent products of Miocene alkaline volcanism and are overlain by a thin layer of sediments partly transformed into smectites and containing microfossils presumably of the Late Miocene age. The phosphate inclusions variable in shapes and consistency are largely confined to the lower sedimentary layer, although occurring as single fragments of compact rock and nonlithified small inclusions in voids of altered tuffs as well. By their structure, microtextures, and main chemical composition, the phosphate inclusions are similar to the phosphorites from other areas of the ocean, which indicates their similar biogenic-diagenetic origin. Judging from the REE composition in the most phosphatized matter, they originate largely from seawater and, to a lesser extent, from tuffites. The redistribution of mobile phosphorus enclosed in the tuffites results in a small-scale phosphatization insufficient for ore formation.     

Molecular signals for anaerobic methane oxidation in Black Sea seep carbonates and a microbial mat

Linked to gas seeps on the Ukrainian shelf (northwestern Black Sea), massive authigenic carbonates form as a result of anaerobic methane oxidation. Lipid distributions in these ‘cold seep’ carbonates and an associated microbial mat were investigated for process markers reflecting the presence and metabolic activity of distinctive methane-related biota. The samples contain free, irregular isoprenoid hydrocarbons, namely the tail-to-tail linked acyclic C₂₀-isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), its C₂₅-homologue 2,6,10,15,19-pentamethylicosane (PMI), and several unsaturated derivatives thereof. Furthermore, specific acyclic and cyclic C₄₀-isoprenoids were released upon ether cleavage of the polar fraction from the carbonate. The abundance of these compounds indicates a pronounced role of particular Archaea in the biogeochemical cycling of carbon at methane seeps. Stable carbon isotopic analyses of these lipids reveal extraordinary depletions in ¹³C corresponding to δ-values in the range of −100±30‰ PDB, whereas other compounds show isotopic compositions normally observed for marine lipids (around −30‰ PDB). The isotope data imply that the biosynthesis of the archaeal isoprenoids occurred in situ and involved the utilization of isotopically depleted, i.e. methane-derived, carbon. Apart from archaeal markers, the carbonate and the mat contain authigenic, framboidal pyrite and isotopically depleted fatty acids, namely iso-, and anteiso-branched compounds most likely derived from sulphate-reducing bacteria (SRB). The indications for a tight association of these normally competitive organisms support a model invoking a syntrophic relationship of SRB with Archaea responsible for the anaerobic oxidation of methane. The biomarker patterns obtained from the Black Sea samples were further compared to those from a Oligocene seep carbonate (Lincoln Creek Formation, WA, USA) in order to evaluate their biomarker potential for ancient settings. The prominent occurrence of isotopically light crocetane (−112‰) and PMI (−120‰) meets the findings for the contemporary materials. Thus, isotopically depleted isoprenoids provide diagenetically stable fingerprints for the reconstruction of carbon cycling in both, modern and ancient methane seep systems.     

Authigenic Mg-calcite at a cold methane seep site in the Laptev Sea

Authigenic minerals were studied in Holocene shelf sediments of the Laptev Sea (cold methane seep site, water depth 71 m). The study presents the first finds of large hard carbonate concretions with Mg-calcite cement in recent sediments of the Arctic shelf seas. These concretions differ from previously reported glendonites and concretions from bottom sediments of the White Sea, Kara Sea, Sea of Okhotsk, etc. A study of the morphology, microstructure, and composition of these newly reported concretions revealed the multistage formation of carbonates (structural varieties of Mg-calcite and aragonite). It was shown that organic matter played an important role in the formation of authigenic carbonates, i.e., in the formation of sedimentary–diagenetic Mg-calcite. The role of methane as a possible source for authigenic carbonate formation was estimated. It was found that methane-derived Mg-calcite accounts for 17–35% of concretion materials. Mg-calcite had δ¹³С-С_carb values between–24 and–23‰ and δ¹³С-С_org values between–44.5 and–88.5‰.     

Relating background fractures to diagenesis and rock physical properties in a platform–slope transect. Example of the Maiella Mountain (central Italy)

Fractured reservoirs are of prime interest as fracture networks control most of the fluid flow and/or accumulation. However, characterizing 3D fracture patterns from subsurface data remains challenging. Studying fractures on outcrops is a good substitution to 1D data from subsurface exploration tools. In addition, outcrops allow deciphering the nature, origin and conditions for fracture formation through the geodynamic history. In this paper, we aim at characterizing the true 3D fracture patterns and determining the genetic role of facies, diagenesis and rock physical properties. We targeted a platform–slope transect within a carbonate reservoir analog, the Maiella Mountain in central Italy, where implications for analog hydrocarbon reservoir can be discussed.Fracture patterns are sorted based on geometric and kinematic criteria from field measurements and petrographic analyzes on thin-sections. Sedimentary facies, pore types and rock physical properties have been characterized in order to establish the impact of early diagenesis on rock evolution. Diagenetic sequences have been unraveled and correlated to the fractures. Fracture sequences have been determined considering the cross-cutting relationships and compared with burial–uplift history. In the two studied formations (platform and slope carbonates), we interpret a stage of fracturing perpendicular to bedding, formed at shallow depth and occurring prior to major regional tectonic events. The studied carbonates have undergone early diagenesis during fast and shallow burial, conferring early brittle behavior. The amount of stylolites is not correlated to burial depth but to fracture density, porosity and free air P–wave velocity. It means that fracture development, mechanical and petrophysical properties are acquired during early diagenesis.Both studied formations have undergone the same geodynamic history and their brittle response is different and not related to folding but to burial and early cementation. Deciphering the close relationship between sedimentary facies, diagenetic and geodynamic history has allowed unraveling the controling factors on rock properties and therefore on fracture pattern.     

Organic geochemistry and reservoir characterization of the organic matter-rich calcilutite in the Shulu Sag,Bohai Bay Basin,North China

Although extensive studies have been conducted on unconventional mudstone (shales) reservoirs in recent years, little work has been performed on unconventional tight organic matter-rich, fine-grained carbonate reservoirs. The Shulu Sag is located in the southwestern corner of the Jizhong Depression in the Bohai Bay Basin and filled with 400–1000 m of Eocene lacustrine organic matter-rich carbonates. The study of the organic matter-rich calcilutite in the Shulu Sag will provide a good opportunity to improve our knowledge of unconventional tight oil in North China. The dominant minerals of calcilutite rocks in the Shulu Sag are carbonates (including calcite and dolomite), with an average of 61.5 wt.%. The carbonate particles are predominantly in the clay to silt size range. Three lithofacies were identified: laminated calcilutite, massive calcilutite, and calcisiltite–calcilutite. The calcilutite rocks (including all the three lithofacies) in the third unit of the Shahejie Formation in the Eocene (Es₃) have total organic carbon (TOC) values ranging from 0.12 to 7.97 wt.%, with an average of 1.66 wt.%. Most of the analyzed samples have good, very good or excellent hydrocarbon potential. The organic matter in the Shulu samples is predominantly of Type I to Type II kerogen, with minor amounts of Type III kerogen. The temperature of maximum yield of pyrolysate (T_max) values range from 424 to 452 °C (with an average of 444 °C) indicating most of samples are thermally mature with respect to oil generation. The calcilutite samples have the free hydrocarbons (S₁) values from 0.03 to 2.32 mg HC/g rock, with an average of 0.5 mg HC/g rock, the hydrocarbons cracked from kerogen (S₂) yield values in the range of 0.08–57.08 mg HC/g rock, with an average of 9.06 mg HC/g rock, and hydrogen index (HI) values in the range of 55–749 mg HC/g TOC, with an average of 464 mg HC/g TOC. The organic-rich calcilutite of the Shulu Sag has very good source rock generative potential and have obtained thermal maturity levels equivalent to the oil window. The pores in the Shulu calcilutite are of various types and sizes and were divided into three types: (1) pores within organic matter, (2) interparticle pores between detrital or authigenic particles, and (3) intraparticle pores within detrital grains or crystals. Fractures in the Shulu calcilutite are parallel to bedding, high angle, and vertical, having a significant effect on hydrocarbon migration and production. The organic matter and dolomite contents are the main factors that control calcilutite reservoir quality in the Shulu Sag.     

Mineral carbon MinC(%) from Rock-Eval analysis as a reliable and cost-effective measurement of carbonate contents in shale source and reservoir rocks

The abundance of carbonate minerals in rocks is an important element for petroleum geochemists, geologists and engineers to consider during hydrocarbon exploration and production. Carbonate minerals can be related to the depositional environments of the source rocks and hence the type of organic matter that can be subsequently modified through diagenetic processes. The total carbonate content in shales can also be used to deduce their fracability as reservoir rock for shale gas/oil extraction. At present, mineralogical analysis by X-ray diffraction (XRD) is the most widely used technique in the oil and gas-related petrophysical and geochemical laboratories for identifying and quantifying carbonate minerals in rock samples. In contrast, the use of the total mineral carbon content parameter MinC (%) from Rock-Eval analysis has been limited despite (1) the parameter's effectiveness as demonstrated through instrument and methodology development; and (2) the presence of a large volume of Rock-Eval results due to its widespread use in the oil and gas industry.In this study, we acquired XRD and Rock-Eval analytical results on over nine hundred rock samples ranging from Ordovician to Cretaceous age from four petroleum sedimentary basins in Canada. Least-squares regression analyses produce empirical equations with R² values greater than 0.92 for the correlations between XRD total carbonate contents and the Rock-Eval MinC (%) values of six different suites of samples. This indicates that the MinC (%) values generated by Rock-Eval 6 and other instruments of similar capability can be used with confidence for estimating the amounts of total carbonates in sedimentary rock samples. While the correlation between the two types of carbonate content measurements is geology-specific, a global linear regression model (R² of 0.97) based on the large combined data set has been proposed for approximating the total carbonate abundances based on Rock-Eval MinC (%) values.     

Oligocene–Miocene carbonates and their role for constraining the rifting and collision history of the Dangerous Grounds,South China Sea

The attenuated continental crust of the Dangerous Grounds is located in the southeastern part of the South China Sea. It was affected by unconformities as identified by several authors (Cullen et al., 2010, Hinz and Schlüter, 1985, Hutchison, 2010, Hutchison and Vijayan, 2010). In the northeastern Dangerous Grounds, a prominent reflector in seismic data is associated with the top of a widespread Oligocene to Early Miocene (18–20 Ma) carbonate platform. This reflector and the underlying carbonates can be used to constrain the timing of the unconformities and the rifting history of the Dangerous Grounds. By carefully interpreting seismic reflection lines we trace the platform carbonates based upon their appearance in the seismic image. This platform is continuous in the Palawan–Borneo trough and gets patchy toward the Dangerous Grounds. In the Dangerous Grounds the image of this key reflector changes and here it merely forms the top of a clastic layer. Carbonates remain abundant but mainly as isolated reefs that grew on top of tilted fault blocks. In the southwestern Dangerous Grounds the prominent unconformity sealing the tectonic activity is known as the Middle Miocene Unconformity. This in fact is an Early Miocene unconformity, which represents a sequence boundary in the Borneo–Palawan trough and in various parts of the Dangerous Grounds, while in other parts of the Dangerous Grounds, it represents a major angular unconformity.The unconformity characteristics supplemented with tentative ages indicate that Luconia and the southern Dangerous Grounds were sub-aerial during the Early Miocene, while the Reed Bank, the northern Dangerous Grounds and parts of the central Dangerous Grounds were mostly submerged except for some islands concentrated on the western edge of the Borneo–Palawan trough. This trough is interpreted as a foreland basin where the flexural forebulge provided shallow marine conditions that promoted reef growth. As the carbonate deposition migrated from the Borneo–Palawan trough toward the Dangerous Grounds we suggest that the flexural forebulge provided shallow water conditions for further reef growth on the eastern Dangerous Grounds.     

Mesozoic and Cainozoic rocks dredged from the South China Sea (Reed Bank area) and Sulu Sea and their significance for plate-tectonic reconstructions

Outcrops of pre-Quaternary rocks were sampled by dredging and coring during two cruises of RV ‘Sonne’ in the South China Sea and Sulu Sea (1982, 1983). Late Triassic deltaic sandstones wih plant remains are the oldest sediments recovered. Late Jurassic/Early Cretaceous regional metamorphism is indicated by gneiss and garnet-mica schists exposed in the northwestern flanks of Reed Bank. These rocks, and occurrences of gabbro, diorite, dacite, rhyolite and siliceous shale prove that the Dangerous Grounds and Reed Bank are underlain by a continental fragment. During the period of seafloor spreading in the South China See a Late Oligocene to Early Miocene carbonate platform developed on this southwards-drifting continental fragment, which subsided in the Middle Miocene. Middle Miocene andesite and Early to Middle Miocene reefal carbonates were recovered from the Cagayan Ridge in the Sulu Sea. The new data are in accord with the plate-tectonic history developed by Taylor and Hayes (1980, 1983) and Holloway (1981).     

Organic geochemical study of source rocks and natural gas and their genetic correlation in the eastern part of the Polish Outer Carpathians and Palaeozoic–Mesozoic basement

Geochemical characteristics of organic matter in the profiles of Dukla, Silesian, Sub-Silesian and Skole units of the Polish Outer Carpathians and of the Palaeozoic–Mesozoic basement in the Dębica-Rzeszów-Leżajsk-Sanok area were established based on Rock-Eval, vitrinite reflectance, isotopic and biomarker analyses of 485 rock samples. The Oligocene Menilite beds have the best hydrocarbon potential of all investigated formations within the Dukla, Silesian, and Skole units. The Ordovician, Silurian, Lower Devonian and locally Middle Jurassic strata of the Palaeozoic–Mesozoic basement are potential source rocks for oil and gas accumulated in Palaeozoic and Mesozoic reservoirs. Thirty one natural gas samples from sandstone reservoirs of the Lower Cretaceous-Lower Miocene strata within the Outer Carpathian sequence and eight from sandstone and carbonate reservoirs of the Palaeozoic–Mesozoic basement were analysed for molecular and isotopic compositions to determine their origin. Natural gases accumulated both in the Outer Carpathian and the Palaeozoic–Mesozoic basement reservoirs are genetically related to thermogenic and microbial processes. Thermogenic gaseous hydrocarbons that accumulated in the Dukla and Silesian units were generated from the Menilite beds. Thermogenic gaseous hydrocarbons that accumulated in the Sub-Silesian Unit most probably migrated from the Silesian Unit. Initial, and probably also secondary microbial methane component has been generated during microbial carbon dioxide reduction within the Oligocene Menilite beds in the Dukla Unit and Oligocene-Lower Miocene Krosno beds in the Silesian Unit. Natural gases that accumulated in traps within the Middle Devonian, Mississippian, Upper Jurassic, and Upper Cretaceous reservoirs of the Palaeozoic–Mesozoic basement were mainly generated during thermogenic processes and only sporadically from initial microbial processes. The thermogenic gases were generated from kerogen of the Ordovician-Silurian and Middle Jurassic strata. The microbial methane component occurs in a few fields of the Dukla and Silesian units and in the two accumulations in the Middle Devonian reservoirs of the Palaeozoic–Mesozoic basement.