Middle Jurassic Coals Research Articles

Distribution, occurrence, and enrichment of gallium in the Middle Jurassic coals of the Muli Coalfield, Qinghai, China

Gallium (Ga) is known as the backbone of the electronics industry, but its independent mineralization is rarely found in nature. The discovery of large Ga-bearing coal deposits suggests that the coal-bearing series can be used as the main source for the future development and utilization of Ga, and the organic matter of coal may play an important role in the aggregation of Ga. Based on the analysis of geological prospecting and study of coal samples, this paper discussed the distribution, occurrence, and enrichment of Ga in the Middle Jurassic coals of the Muli coalfield. The results demonstrate that coals from eastern mines are enriched in Ga, where the Ga concentration reaches the propositional cut-off grade (30μg/g). The Ga concentration increases along with an increasing distance from the sediment source; the areas with the highest Ga concentration are the deposit-center and the location with the highest content of mudstone. Sediments were derived from the acid and intermediate magmatic rocks of the Tuolai Mountain in the northeastern Muli coalfield, partly from the crust weathering of the Upper Triassic formation. Ga was first transported in the form of colloids or solutions and finally deposited in authigenic kaolinite. In addition, the positive correlation between the Ga concentration and the inertinite constituents of coal indicates that fibrous tissue might be the dominant component absorbing Ga and that organic matter had a positive impact on the migration of Ga to peat swamps. Ga is prone to deposition in partially oxidized, acidic coal-forming environments with strong hydrodynamic forces. Hydrothermal activity might also stimulate the enrichment of Ga in coal.

Genetic types and origins of natural gases from eastern Fukang Sub-depression of the Junggar Basin, NW China: Implication for low-mature coal-derived gases

Although the Fukang Sub-depression is the largest hydrocarbon generation depression in the Junggar Basin and promises favorable potential for oil and gas exploration, only several small–medium oil/gas accumulations have been discovered so far. After analyzing the molecular and stable carbon isotopic compositions of natural gases and its associated low molecular weight hydrocarbons (LMWHs) from eastern Fukang Sub-depression, their genetic types and origins are discussed in this study. Natural gases are wet gases with an average dryness coefficient of 0.84. Stable carbon isotopes of methane and ethane are well correlated with the thermal evolution trend of coal-derived gases. The C5–7 LMWHs are enriched in methyl cyclohexane but they lack n-alkanes, indicating a predominance of higher plant input. Gases have maturities ranging 0.76%–0.93%Ro and are well consistent with their low n-heptane (H) and iso-heptane (I) contents. The geochemical characteristics of natural gases from eastern Fukang Sub-depression are quite similar with those from the Turpan–Hami Basin. Their gases are considered to be low-mature gases derived from the Lower–Middle Jurassic coal measures. In contrast, they show remarkable differences with those from the Wucaiwan gas field; they are considered to have high-mature gases derived from the Carboniferous coal measures.

Influence of coal petrology on methane adsorption capacity of the Middle Jurassic coal in the Yuqia Coalfield, northern Qaidam Basin, China

The lower coalbed methane (CBM) adsorption capacity of the low rank coals is not only related to its lower maturity, but also determined by the coal maceral compositions. In this study, a total of 13 samples including 10 coals and 3 carbonaceous mudstones, were collected from the Middle Jurassic Dameigou Formation in the borehole YQ-1 of the Yuqia Coalfield, northern Qaidam Basin, NW China. Coal lithotypes, maceral compositions, coal ranks, coal facies and methane adsorption characteristics of these samples were investigated using microscopic observation, proximate analysis, porosity analysis, and isothermal adsorption experiments. The results show that the maceral composition has a great influence on the methane adsorption capacity, and the influence of vitrinite on the methane adsorption is generally stronger than that of inertinite at a similar coal rank. For most samples, there is a positive correlation between the vitrinite contents and the Langmuir volume (VL), a negative correlation between the inertinite contents and the VL, and no obvious correlation between the exinite contents and the VL. Furthermore, the vitrinite/inertinite (V/I) ratio also shows a positive relationship with the VL. However, some samples containing large amounts of unfilled fusinite and/or semifusinite have more pore spaces favoring methane adsorption and can also adsorb significant quantities of methane. Consequently, coals with higher vitrinite contents, in association with some unfilled fusinites or semifusinites, should have the greatest adsorption capacity. Coal maceral compositions vary with the types of coal facies, and thus the methane adsorption capacity of coals may be closely related to coal facies. It is found that the methane adsorption capacities of the coals in a wet forest swamp (Type I) and an intergradation forest swamp (Type II) are greater than those in a drained forest swamp (Type III) and an open water peat swamp (Type IV). It is suggested that the area developed with the wet forest swamp and in the intergradation forest swamp with high porosity should be the best target areas for the CBM exploration in the Yuqia Coalfield.

Sequence stratigraphy and paleogeography of the Middle Jurassic coal measures in the Yuqia coalfield, northern Qaidam Basin, northwestern China

Detailed stratigraphic and paleogeographic analyses of data from 72 boreholes for the Middle Jurassic intermontane fluvial-lacustrine coal-bearing sequences were conducted in the Yuqia coalfield of the northern Qaidam Basin, northwestern China. Three third-order sequences lasting in total ca. 10.6 m.y., and an internal lowstand systems tract (LST), transgressive systems tract (TST), highstand systems tract (HST), and falling-stage systems tract (FSST) have been identified. A series of sequence-specific paleogeographic maps have been constructed based on the contours of lithological parameters. The paleogeographic units include alluvial fan-braided (meandering) fluvial plain, upper delta plain, lower delta plain, subaqueous delta, shore-shallow lake, and deep lake. The preferred sites of coal accumulation are interdelta bays, upper delta plains, lower delta plains, and fluvial back swamps. The sequence stratigraphic and sedimentological analysis of the Middle Jurassic coal-bearing measures of the Yuqia coalfield provides a basis for a comprehensive coal accumulation model that involves a six-period evolution from the LST, early TST, late TST, early HST, late HST to FSST. The major coal seams were accumulated in the early and late TST of the sequences S1 and S2. These results are of practical significance for coal resources exploration and enhance geological effects of prospecting engineering in the northern Qaidam Basin.

Sedimentology and Coalbed Methane Geology of Middle Jurassic Coal Measures in Juhugeng Mine, Qinghai Province

Field-based lithofacies in the Middle Jurassic coal measures of the Juhugeng coal mine area of Qinghai Province have been identified employing lithology, geometry and lateral facies relationships. The coal measures were deposited in a range of environments spanning from braided river, delta to lake, and economic coal seams have been formed in the deltaic interdistributary bay and fluvial flood basin environments. The coal has a rank ranging from gas coal to coking coal, and the vitrinite macerals have a relatively high content, being from 60%~80%. Cleats and fractures of coal reservoirs are well developed. The isothermal adsorption experiments reveal that coal in the Juhugeng mine has relatively low Langmuir volume and relatively high Langmuir pressure with low to medium absorbability. All these conditions are beneficial to the formation and preservation of the coalbed methane.

Unusual resinite-rich coals found in northeastern Greenland and along the Norwegian coast: Petrographic and geochemical composition

Middle Jurassic coals unusually rich in resinite/liptinite are known in northeastern Greenland from a few outcrops and from several erratic pieces. In addition, since the mid-1800's a number of loose pieces of resinite-rich coal, the so-called Kvæfjord coal, have been found along the Norwegian coast in or close to the shore zone in places where it would have been impossible for the coal to originate from passing ships. This study presents a comprehensive organic petrographic and geochemical investigation of two new pieces of resinite-rich coal found on Store Koldewey in northeastern Greenland, a resinite-rich core sample from the Norwegian island Andøya, and two pieces of the Norwegian Kvæfjord coal. The coals are thermally immature and contain an extraordinary high content of liptinite and in particular resinite (34.2–93.6vol.%). The two coal pieces from Store Koldewey and the core sample from Andøya are compositionally comparable to the resinite/liptinite-rich Middle Jurassic coals that crop out on Hochstetter Forland and Kuhn Ø in northeastern Greenland. The two loose pieces of Kvæfjord coal are exceptionally rich in resinite and appear to be slightly different from the other coals, albeit they are compositionally related. The documentation of in situ resinite/liptinite-rich coals on Andøya and the finding of loose pieces along the Atlantic coast of Norway indicate that formation of these unusual coals occurred regionally in the pre-rift North Atlantic region, probably in peat-forming mires with a yet unknown strongly resin-producing vegetation established at the margins of a gradually northward prograding embayment formed during Middle Jurassic rifting between Greenland and Norway. The unique maceral composition makes the coals excellent potential petroleum source rocks with HI values >400mg HC/g TOC, but the sketchy knowledge of the regional distribution of the coals combined with the probably restricted areal extent of the coal deposits limits their significance as an additional source rock in the North Atlantic area.

A remote coal deposit revisited: Middle Jurassic coals at Kulhøj, western Germania Land, northeast Greenland

Abstract In 1908, members of the “Danmark Expedition” discovered a coal deposit in a very remote area in western Germania Land, close to the margin of the inland ice in northeast Greenland. The deposit was, however, neither sampled nor described, and was revisited in 2009 for the first time since its discovery. The outcrops found in 2009 amount to approximately 8 m of sediment including a coal seam of 2 m thickness. More outcrops and additional coal deposits most certainly are to be found, pending further fieldwork. The deposits are Middle Jurassic, Callovian, in age and were deposited in a floodplain environment related to meandering river channels. Spores and pollen in the lower fluvial deposits reflect abundant vegetation of ferns along the river banks. In contrast, a sparse spore and pollen flora in the coals show a mixed vegetation of ferns and gymnosperms. Based on proximate and petrographic analyses the coals are classified as medium-rank high-grade coal. Their composition is dominated by inertinite and vitrinite, and they represent deposits laid down in a freshwater mire. No evidence of marine incursion has been found. The coal seam studied does not include liptinite-rich coals such as those present in the same lithostratigraphic unit elsewhere in northeast Greenland, but loose blocks in the area suggest their presence in unknown parts of the succession.

The petrology and provenance of coal artifacts from Thule settlements in north-eastern Greenland

Coal petrographic techniques have been used to trace the origin of coal used to produce artifacts by the Thule people in north-eastern Greenland. The coal artifacts were collected from the north-east coast settlements between 76°55'–80°18'N and date back to the 15th century A.D. The petrographic data suggest that they have a common source, the Middle Jurassic coals outcropping south of 75°15' in north-eastern Greenland. It is thus evident that the Thule people used local material rather than bringing the coal from the known “mines” in Arctic Canada. It also implies that contemporaneous Thule people groups along the east coast of Greenland were in contact and traded.

HYDROCARBON POTENTIAL OF MIDDLE JURASSIC COALY AND LACUSTRINE AND UPPER JURASSIC – LOWERMOST CRETACEOUS MARINE SOURCE ROCKS IN THE SØGNE BASIN, NORTH SEA

The Søgne Basin in the Danish‐Norwegian Central Graben is unique in the North Sea because it has been proven to contain commercial volumes of hydrocarbons derived only from Middle Jurassic coaly source rocks. Exploration here relies on the identification of good quality, mature Middle Jurassic coaly and lacustrine source rocks and Upper Jurassic – lowermost Cretaceous marine source rocks. The present study examines source rock data from almost 900 Middle Jurassic and Upper Jurassic – lowermost Cretaceous samples from 21 wells together with 286 vitrinite reflectance data from 14 wells. The kerogen composition and kinetics for bulk petroleum formation of three Middle Jurassic lacustrine samples were also determined.Differences in kerogen composition between the coaly and marine source rocks result in two principal oil windows: (i) the effective oil window for Middle Jurassic coaly strata, located at ∼3800 m and spanning at least ∼650 m; and (ii) the oil window for Upper Jurassic – lowermost Cretaceous marine mudstones, located at ∼3250 m and spanning ∼650 m. A possible third oil window may relate to Middle Jurassic lacustrine deposits. Middle Jurassic coaly strata are thermally mature in the southern part of the Søgne Basin and probably also in the north, whereas they are largely immature in the central part of the basin. HImax values of the Middle Jurassic coals range from ∼150–280 mg HC/g TOC indicating that they are gas‐prone to gas/oil‐prone. The overall source rock quality of the Middle Jurassic coaly rocks is fair to good, although a relatively large number of the samples are of poor source rock quality.At the present day, Middle Jurassic oil‐prone or gas/oil‐prone rocks occur in the southern part of the basin and possibly in a narrow zone in the northern part. In the remainder of the basin, these deposits are considered to be gas‐prone or are absent. Wells in the northernmost part of the Søgne Basin / southernmost Steinbit Terrace encountered Middle Jurassic organic–rich lacustrine mudstones with sapropelic kerogen, high HI values reaching 770 mg HC/g TOC and Ea‐distributions characterised by a single dominant Ea‐peak. The presence of lacustrine mudstones is also suggested by a limited number of samples with HI values above 300 mg HC/g TOC in the southern part of the basin; in addition, palynofacies demonstrate a progressive increase in the abundance and areal extent of lacustrine and brackish open water conditions during Callovian times. A regional presence of oil‐prone Middle Jurassic lacustrine source rocks in the Søgne Basin, however, remains speculative.Middle Jurassic kitchen areas may be present in an elongated palaeo‐depression in the northern part of the Søgne Basin and in restricted areas in the south.Upper Jurassic – lowermost Cretaceous mudstones are thermally mature in the central, western and northern parts of the basin; they are immature in the eastern part towards the Coffee Soil Fault, and overmature in the southernmost part. Only a minor proportion of the mudstones have HI values >300 mg HC/g TOC, and the present‐day source rock quality is for the best samples fair to good. In the south and probably also in most of the northern part of the Søgne Basin, the mudstones are most likely gas‐prone, whereas they may be gas/oil‐prone in the central part of the basin. A narrow elongated zone in the northern part of the basin may be oil‐prone. The marine mudstones are, however, volumetrically more significant than the Middle Jurassic strata.Possible Upper Jurassic – lowermost Cretaceous kitchen areas are today restricted to the central Søgne Basin and the elongated palaeo‐depression in the north.

The origin of natural gas in the Hutubi gas field, Southern Junggar Foreland Sub-basin, NW China

The Hutubi gas field is the largest field discovered in the Southern Junggar foreland sub-basin at present. The origin and generation of gas in the field have been discussed based on the geochemical data. The carbon isotopic composition of methane and ethane in the Hutubi gas field is relatively enriched in 13 C with δ 13 C 1 values ranging from −32.8‰ to −30.5‰ (average value = −31.2‰) and δ 13 C 2 values ranging between −23.0‰ and −21.6‰ (average value = −22.0‰). The hydrogen isotope is lighter with δD 1 from −200‰ to −196‰. Methylcyclohexane is abundant and the content is the highest among C 7 light hydrocarbons, ranging from 49.6% to 59.4%, with an average value of 55.5%. The δ 13 C values of individual C 6 –C 7 light hydrocarbons are heavier with the δ 13 C values from −25.2‰ to −17.9‰. The results show that gas in the Hutubi field is coal-derived gas and sourced from the Middle and Lower Jurassic coal measures. The result of gas generation kinetics from Jurassic coal measures show that the gas generation began 10 Ma and the methane generation transformation ratio is very high (up to 87%) at present. The time of gas generation is consistent with that of the Hutubi structural trap and shows that the formation time of the Hutubi gas field is very late. It is concluded that gas preservation in the Southern Junggar foreland sub-basin is excellent and gas exploration potential is favorable. ► The isotope shows that gas in Hutubi gas field is coal- derived and sourced from Jurassic rock. ► Secondary cracking existed in Hutubi gas field. ► The filling history was determined according the gas generation kinectics.

What we know (and what we don't) about the petroleum prospectivity of the Northland Basin, North Island, New Zealand

The offshore Northland Basin is a major sedimentary accumulation lying to the west of the Northland Peninsula of New Zealand. It merges with the Taranaki Basin in the south and its deeper units are separated from Deepwater Taranaki by a buried extension of the West Norfolk Ridge. Sedimentary thicknesses increase to the northwest and the Northland Basin may extend into Reinga. Its total area is at least 65,000 km2 and if the Reinga Basin is included, it may be up to 100,000 km2. As in Taranaki, petroleum systems of the Northland Basin were thought to include Cretaceous to Recent sedimentary rocks. Waka Nui–1 was drilled in 1999 and penetrated no Cretaceous sediments, but instead drilled unmetamorphosed Middle Jurassic coal measures. Economic basement may be older meta-sediments of the Murihiku Supergroup. Thick successions onlap the dipping Jurassic unit and a representative Cretaceous succession is likely to be present in the basin. Potential source rocks known to be present include the Middle Jurassic coal measures of Waka Nui–1 and the Waipawa Formation black shale. Inferred source rocks include Late Jurassic coaly rocks of the Huriwai Beds, the Early Cretaceous Taniwha Formation coaly sediments, possible Late Cretaceous coaly units and lean but thick Late Cretaceous and Paleogene marine shales. Below the voluminous Miocene volcanoes of the Northland arc, the eastern margin of the basin is dominated by a sedimentary wedge that thickens to more than two seconds two-way travel time (TWT), or at least 3,000 m, at its eastern margin and appears to have been thrust to the southwest. This is interpreted to be a Mesozoic equivalent of the Taranaki Fault, a back-thrust to subduction along the Gondwana Margin. The ages of sedimentary units in the wedge are unknown but are thought to include a basal Jurassic succession, which dips generally to the east and is truncated by an erosional unconformity. A southwestwards-prograding succession overlies the unconformity and its top surface forms a paleoslope onlapped by sediments of Late Cretaceous to Neogene ages. The upper succession in the wedge may be of Early Cretaceous age—perhaps the equivalent of the Taniwha Formation or the basal succession in Waimamaku–2. The main part of the basin was rifted to form a series of horst and graben features. The age of initial rifting is poorly constrained, but the structural trend is northwest–southeast or parallel to the Early Cretaceous rifting of Deepwater Taranaki and with the Mesozoic Gondwana margin. Thick successions overlie source units which are likely to be buried deeply enough to expel oil and gas, and more than 70 slicks have been identified on satellite SAR data suggesting an active petroleum system. Numerous structural and stratigraphic traps are present and the potential of the Northland Basin is thought to be high.

Simulation experiment on hydrocarbon generating potential of various source rocks on the northern margin of the Qaidam Basin, Qinghai, China

The northern margin of the Qaidam Basin is one of the main oil-gas exploration areas in western China, where source rocks are composed mostly of Middle and Lower Jurassic dark mud shale, carbargillite and coal. A large number of subsurface and outcrop samples differing in lithology with different types of organic matter were selected for resource evaluation, research and calculation. And among them, 13 samples were used for simulation experiment on hydrocarbon generating potential of various source rocks. At first, two kinds of heating modes were compared through simulation experiment, including single temperature-step heating and continual heating. Perhaps, the process of hydrocarbon generation and expulsion occurred naturally between a close system and an open system. In addition, the first heating mode was convenient, and all its reaction products were involved in the whole thermal evolution, and the final simulation experimental results were basically in consistency with the natural evolution trend. Therefore, the first heating mode was adopted and the hydrocarbon yield of every sample was worked out. According to the type and lithology of organic matter and the hydrocarbon yield of samples for simulation experiment, hydrocarbon generation and expulsion mode with three kinds of lithology and five types of source rock has been established to provide the basis for hydrocarbon generation evaluation, research and resource calculation.

Distribution Characteristics of Sulfur and the Main Harmful Trace Elements in China's Coal

Abstract: To promote the rational development and use of clean coal resources in China, data on the regional and age distribution of sulfur, arsenic and other harmful elements in Chinese coal was broadly collected, tested for content, and analyzed. Coal in northwestern China is characterized by low to extremely low levels of sulfur; the coal of the Taiyuan Formation in northern China mainly has high‐sulfur content; that of the Shanxi Formation is mainly characterized by low sulfur coal; and the Late Permian coal in southern China has overall higher sulfur content; other regions have low sulfur coal. The average content of harmful trace elements in the bulk of China's coal is similar to the corresponding content in the coal of the North America and the rest of the world, whereas the content of various elements (Hg, Sb and Se) is different in magnitude to the corresponding percentage in the crust. The average content of the elements Cr, Se, Co, Be, U, Br in Late Permian coal in S China ranks first in the country whereas the average content of Hg and CI in the coals of Late Carboniferous to Early Permian age in N China are the highest. The average content of Mn in Early and Middle Jurassic coal is higher in NW China. The high content of harmful elements in some coal should cause particular concern both in the development and utilization of coal.

Occurrence of Aninopteris (Matoniaceae) from the Middle Jurassic of East central Iran

Aninopteris Givulescu and Popa 1998, a matoniacean fern originally known from the Lower Jurassic deposits of Romania, has been recently reported from the Middle Jurassic coal bearing deposits of Mazino mine, south of Tabas region, east central Iran. This is the second record of this genus, excepting its original occurrence from the Anina region of Romania. Besides the younger age, the Iranian material differs from the type material in having a higher number of sporangia, and less branched secondary veins. Thus, Aninopteris formosa var. persica var. nov. is proposed for the studied material.

Petrology, rank and evidence for petroleum generation, Upper Triassic to Middle Jurassic coals, Central Alborz Region, Northern Iran

Abstract Upper Triassic to Middle Jurassic coals from the Alborz region of northern Iran were analyzed by reflected light-fluorescence microscopy and Rock Eval 6® pyrolysis to evaluate their regional rank variation, degree of hydrothermal alteration, and petroleum generative potential. The coal ranks in the region range from a low of 0.69%RoR in the Glanddeh-Rud area to a high of 1.02%RoR in the Gajereh area. Tmax (°C) values (Rock Eval 6 pyrolysis) also increase progressively with increasing vitrinite %Ro values, however Tmax is suppressed lower than would be expected for each rank ranging from 428 °C for the Glandeeh coal to 438 °C for the Gajereh coal. Tmax suppression may be caused by maceral composition and soluble organics within the coal. Moderately high hydrogen indices, persistent and oily exudations from the coals during UV exposure, and traces of hydrocarbon fluid inclusions suggest that liquid petroleum was likely generated within some of the coals.

Origin and accumulation model of the AK-1 natural gas pool from the Tarim Basin, China

The AK-1 gas pool represents the first commercial gas discovery in the Kashi Depression, northwest Tarim Basin. The pool is characterized by dry hydrocarbon gas (dryness index > 0.995), heavy methane carbon isotopic value ( δ 13C 1 = −25‰), and a lighter δ 13C 2 ( δ 13C 2 − δ 13C 1 = 3.9‰). High CO 2 content ( δ 13C CO 2 = −8.6‰) is observed in the main production reservoir. These characteristics indicate that the hydrocarbon gas originated from multiple sources, and that the CO 2 is of mixed inorganic and organic origin. There are two possible source areas: the lower block of the AK-1 overthrust structure and the central area of the Kashi Depression. Two possible source rocks are present in both areas: Lower Carboniferous mudstone and Lower and Middle Jurassic coal measures, typically with type II and type III kerogens, respectively. Both reservoir fluid inclusion data and trap structure evolution indicate that the gas pool was formed during the Pliocene–Quaternary. We constructed geological models of methane generation and carbon isotopic fractionation for the two potential source rocks using different geothermal histories of the two possible source areas. By comparing modeled results with the geochemical characteristics of the gas pool, we conclude that the source rock of the gas pool is the Lower Carboniferous mudstone, and the main source area is in the lower block of the AK-1 overthrust structure with a secondary source area in the center of the Kashi Depression. The pool gas is interpreted as a late stage cumulative gas. The structure trapped gas generated from source rock in the lower block of the AK-1 overthrust at a methane fractional conversion of 0.64–1.0 together with inorganic CO 2 from the thermal decomposition of Carboniferous carbonate rocks in the central area of the Kashi Depression.

Conifer Woods of the Middle Jurassic Hojedk Formation (Kerman Basin) Central Iran

This paper documents the first record of permineralised wood from Middle Jurassic coal bearing deposits to the north of Kerman, Iran, deposited c. 170 million years before present. The coniferous woods have character combinations resembling, and thus have been assigned to, the genera Xenoxylon Gothan and Agathoxylon Hartig. Since Xenoxylon is essentially Laurasian and Agathoxylon has been recorded from the Northern Hemisphere during the Mesozoic, these woods help confirm conclusions from recent geological studies that place the Kerman Basin of Iran in southern Laurasia during the Jurassic.

04/01183 Paleoenvironments and paleogeography of the Lower and lower Middle Jurassic coal measures in the Turpan-Hami oil-prone coal basin, northwestern China: Shao, L. et al. AAPG Bulletin, 2003, 87 (2), 335–355

04/01183 Paleoenvironments and paleogeography of the Lower and lower Middle Jurassic coal measures in the Turpan-Hami oil-prone coal basin, northwestern China: Shao, L. et al. AAPG Bulletin, 2003, 87 (2), 335–355

Modelling of the hydrocarbon generation history and volumetric considerations of the coal-sourced Lulita Field, Danish North Sea

The hydrocarbon generation history of the Middle Jurassic Lulita Field in the Søgne Basin in the northeastern part of the Danish Central Graben has been predicted by modelling the Lulita-1Xc well located on-structure and a pseudowell located in the kitchen area. The Lulita Field consists of east-dipping Middle–Upper Jurassic sediments in a structural trap. Waxy oil, sourced from Middle Jurassic coals and type III kerogen (and possibly type I kerogen) shales, occurs in Middle Jurassic sandstone reservoirs sealed by Upper Jurassic marine shales. Maturity modelling (vitrinite reflectance) was carried out by using the PresRo ® kinetic model and a heat flow history calibrated from both the tectonic-subsidence history and maturity data. An effective oil window (‘oil expulsion window’) between 0.85% and 1.8%R o for the Middle Jurassic coal and type III kerogen source rocks was applied. Timing for onset of oil generation differs between the Lulita-1Xc well and the pseudowell. The top of the source rocks becomes mature during the Late Cretaceous in the kitchen area and during the Oligocene in the Lulita-1Xc well. However, oil expulsion from the entire Middle Jurassic source rocks is modelled to have started from the Paleocene in the kitchen area and appears to have continued through to the present day. No, or little, expulsion is modelled in the Lulita-1Xc well, indicating that the Lulita Field was charged by updip migration of hydrocarbons from the Søgne Basin kitchen to the east. Hydrocarbon charging pre-dates the time of potential diagenetic modification of the reservoir, thus limiting reduction in reservoir porosity and permeability. Based on the known coal source rock volume and quality, calculated maximum generated and expelled hydrocarbons cannot account for the estimated in-place volumes of oil. The additional hydrocarbons were probably generated from coaly shales and shaly coals that form the lateral equivalent of the Middle Jurassic coals in the kitchen area, possibly with contributions from lacustrine to brackish type I kerogen shales.

Coal facies studies in Denmark and Greenland

Coal of Jurassic age occurs onshore Denmark and in the Danish North Sea, whereas coal of Carboniferous and Miocene age only occurs in the Danish North Sea and onshore Denmark, respectively. The Lower–Middle Jurassic coals are the most widespread and best documented. Onshore Denmark the coals are of low rank while the Jurassic coals in the North Sea are thermally mature and hydrocarbon generating in the Søgne Basin. The Jurassic coals have a maximum thickness of ∼2 m and were formed in coastal plain mires and in inland fresh water mires. The Miocene brown coals, up to ∼2 m thick, were formed in an overall deltaic setting. Lower Carboniferous and Palaeogene coals occur in northern Greenland, Middle Jurassic coals in northeast Greenland, and Cretaceous coals in western Greenland. The Middle Jurassic low rank coals have been investigated in detail. The up to ∼3.5-m-thick coal seams accumulated in coastal mires and they may have an extraordinary resinite-enriched composition. Only a single Cretaceous coal seam has been investigated with regard to the depositional environment; the seam records drowning of a peat mire. The Lower Carboniferous and Palaeogene coals have not been investigated.