Cratonic Basins Research Articles

The pre-Silurian Riachão basin: a new perspective on the basement of the Parnaíba basin, NE Brazil

Abstract We present a new interpretation of the basement immediately below the pre-Silurian unconformity in the mid-western part of the Parnaíba cratonic basin. Regional seismic reflection data, constrained by well and gravity data, have revealed the remnants of a pre-Silurian basin named the Riachão basin. It has a north–south trend and is about 120 km wide with a minimum area of c. 35 000 km 2 and a maximum thickness of 4 km. Seismic interpretation shows the Riachão basin is defined by erosional unconformities at its top (the pre-Silurian unconformity) and base (the top of the pre-Riachão basement). The basin fill is formed of three stratigraphic sequences – Riachão I, II and III – separated by seismic discontinuities and laterally bounded by monoclinal folds and basement uplifts related to thrust faults. Exploration wells show that the Riachão sediments are Ediacaran to Lower Cambrian, clastic and a minimum of 1 km thick, unconformably overlying a heterogeneous metamorphic basement. The mechanism of formation of the basin is unclear. However, our seismic stratigraphic interpretation suggests that Riachão II and III are remnants of marine, clastic, foreland basin sequences of opposed polarity, related to late Neoproterozoic tectonics. The banded, high-amplitude, Riachão I sequence is interpreted as a carbonate platform preceding the main subsidence and clastic input of the Riachão basin.

Implications of preliminary subsidence analyses for the Parnaíba cratonic basin

Abstract Cratonic sedimentary basins are underlain by thick lithosphere and subside over hundreds of millions of years. The driving mechanism for these basins remains enigmatic due to the paucity of crustal-scale constraints. To address this shortcoming, an integrative study of the Parnaíba cratonic basin of NE Brazil is underway. We summarize field observations, well-log information, seismic reflection profiles and subsidence analyses with the objective of shedding light on possible basin-forming mechanisms. Lithological descriptions and borehole logs reveal a stratigraphic succession that consists of interbedded shallow marine and continental deposits. On seismic reflection profiles, this succession is divided into packages of relatively undisturbed reflections bounded by basin-wide disconformities. These disconformities are manifest as bright, rugose reflections that probably represent buried palaeo-landscapes. Backstripped and water-loaded subsidence curves calculated from boreholes distributed throughout the basin show that tectonic subsidence decreases exponentially over c. 350 Ma. A simple model suggests that this subsidence pattern agrees with a thermal time constant of 70–100 Ma, placing a significant constraint on the likely basin-forming mechanism. This background trend is punctuated by minor increases in subsidence that follow basin-wide erosional disconformities. These departures could be indicative of transient epeirogenic uplift events caused by changing patterns of dynamic topography.

Deep crustal architecture of the Parnaíba basin of NE Brazil from receiver function analysis: implications for basin subsidence

Abstract We investigated the crustal architecture of the Parnaíba basin of NE Brazil by analysing receiver functions along a c. 600 km long transect crossing the central portion of the basin. The transect consisted of nine broadband stations interspaced at c. 70 km distance recording continuously for a period of 15 months, with the goal of improving our understanding of the origin and evolution of this large cratonic basin. Our results show that crustal thickness varies between 39 and 45 km along the transect, gradually thickening towards the depocentre, and that bulk V p / V s ratios vary between 1.70 and 1.78. The crust can be divided into a 2.0–3.5 km thick layer of low-velocity sediments, a 15–20 km thick upper crust (3.5 < V s < 3.6 km s −1 ) and a 18–22 km thick lower crust (3.7 < V s < 3.8 km s −1 ). Near the depocentre, where the crust is thickest, the bottom 10–12 km of the crust are characterized by fast S-velocities (4.0–4.2 km s −1 ). Our findings confirm that stretching of the lithosphere is minimal and compatible with flexural subsidence. However, loading from a thick, high-density layer of mafic intrusive rocks pervading the lower crust – as recently proposed for the basin – is found to be inconsistent with our bulk V p / V s ratios and lower crustal S-velocities. Flexural bending by a deeper load, perhaps related to deep mantle convection, seems more plausible.

New insights into the Parnaíba Basin: results of investments by the Brazilian National Petroleum Agency

Abstract The Parnaíba Basin is a Paleozoic cratonic basin located in NE Brazil. Despite exploration in the Parnaíba Basin having undergone a long period of stagnation, it is now one of the most prolific natural gas producing basins in Brazil. Production from this basin is c. 5.6 million m 3 day −1 from three established gas fields, with another four fields in development and eight ongoing discovery assessment plans (in January 2017). This success is the result of activity on exploration concessions, both the investments of the Brazilian National Petroleum Agency (ANP) and concessionaires operating in the basin. ANP's investments were decisive in the definition of the offered areas in the bid rounds and, more importantly, in our understanding of the structural and stratigraphic framework through systematic data acquisition, which helped to define an exploration model for the basin. The Parnaíba Basin received investments of c. US $100 million from ANP and the preliminary results from projects located in the SE of the basin suggest the need for a new exploration model for this area, which was probably affected by the Trans Brazilian Lineament. In this model, the petroleum system would include Ordovician–Silurian components.

Main sedimentary sequences and stages of major cratonic basins during the breakup of Rodinia

This paper summarizes the main sedimentary sequences of major cratonic basins during the breakup of Rodinia and the assembly of Gondwana and extracts evolutionary stages of the Neoproterozoic basins around the Paleo‐Pacific Ocean and Iapetus Ocean tracts. During the breakup of Rodinia, sedimentary sequences and evolutionary stages of the main basins in major cratonic blocks, such as Yangtze, Australia, Eastern American, Western American, and Western Africa cratons, mostly experienced intracratonic rifting, drifting or passive continental margin, carbonate platform construction, and foreland basin stages. The time of rifting to drifting transition in Congo‐São‐Francisco precisely marked the closure of the Adamastor Ocean between Congo‐São‐Francisco and Rio de la Plata and presaged the opening of the Iapetus Ocean. The time of rifting to drifting transition of Eastern American of Laurentia and West Africa initiated at ~580 Ma also signified the opening of the Iapetus Ocean. The time of drifting stage of the main basins around the Paleo‐Pacific Ocean tracts is not only earlier than those of the Iapetus Ocean tracts but also is characterized by diachronism. The time of drifting onset of basins in the Yangtze Block is ~720 Ma, the time of drifting initiation of the Adelaide Fold Belt in South Australia is 680 Ma, and the time of drifting stage of the south‐western margin of Laurentia occurred at 550 Ma. The different times of rifting to drifting transition of the Adelaide Fold Belt in South Australia, the Yangtze Craton, and Western American of Laurentia signified the nonisochronous opening of the Neoproterozoic Paleo‐Pacific Ocean.

Iron Ore Mineralization of Ramagiri Greenstone Belt, Anantapur District, Andhra Pradesh, India

Archaean provinces have rocks of 2500 Ma to 3000 Ma that contain, i. the granite – greenstone association, ii. The high-grade association and the cratonic basin association. Amongst the above said features, the granite - greenstone association is more in abundance and is characterized by the presence of mafic volcanic that has dominated the other units and is known as greenstone belts. These belts are surrounded by variant granitoids. These rocks are metamorphosed to greenschist to amphibolites facies and are hosts for mineralization of gold, copper, silver and iron etc. The main thrust in respect of mineralization is on the iron ore that is associated with the belt. It is mainly of “Algoma” type having alternate bands of magnetite and chert. The mineralization is in the Banded Iron formation of the Ilkal formation (Greenstone Group) and is very proximal to Ramagiri village. The thesis deals with all the details like distribution, possible origin and economic viability of the iron ore.

Crustal structure, gravity anomalies, and subsidence history of the Parnaíba cratonic basin, Northeast Brazil

AbstractCratonic basins cover more than 10% of Earth's continental surface area, yet their origin remains enigmatic. Here we use a 1400 km long, deep (20 s two‐way travel time) seismic reflection profile, five wide‐angle split‐spread receiver gathers, gravity anomaly, and well data to constrain the origin of the Parnaíba Basin, a cratonic basin in Northeast Brazil. In the center of the basin, the depth to pre‐Paleozoic basement is ~3.3 km, a zone of midcrustal reflectivity (MCR) can be traced laterally for ~250 km at depths between 17 and 25 km and Moho depth is ~42 ± 2 km. Gravity and P wave modeling suggests that the MCR represents the upper surface of a high density (2985 kg m−3) and Vp (6.75–7.0 km s−1) lower crustal body, likely of magmatic origin. Backstripping of well data shows a concave up decreasing tectonic subsidence, similar in form to that observed in rift‐type basins. We show, however, that our seismic and gravity data are inconsistent with an extensional origin. Other basin forming mechanisms are therefore required. We show that an intrusive body in the lower crust that has loaded and flexed the surface of the crust, combined with sediment loading, provides a satisfactory fit to the observed gravity anomaly. A buried load model is consistent with seismic data, which suggest that the Moho is as deep or deeper beneath the basin center than its flanks and accounts for at least part of the tectonic subsidence through a viscoelastic stress relaxation that occurs in the lithosphere following load emplacement.

Late Cretaceous spore-pollen zonation of the Central African Rift System (CARS), Kaikang Trough, Muglad Basin, South Sudan: angiosperm spread and links to the Elaterates Province

ABSTRACTThis paper presents a first detailed study on and new zonation scheme for the Kaikang Trough, Muglad Basin of South Sudan. The Kaikang Trough lies within the central sediment locus of the Muglad Basin, a non-marine African cratonic basin within the Central African Rift System (CARS) Two wells, KW-1 and K-4, were examined leading to the recognition of five palynological assemblage zones: K-I (early–middle Cenomanian), K-II (late Cenomanian), K-III (Turonian?–Santonian), K-IV (Campanian–Maastrichtian) and K-V (Maastrichtian). The elaterate group are peculiarly absent in otherwise rich Cenomanian palynological assemblages of the Muglad Basin. Why this is so, and the implications for the global middle Cretaceous Elaterate Province, needs some explanation. The CARS provided a range of diverse freshwater alluvial and lake-shore substrates. Its cross-continent sweep may have played a role in the evolution and dispersal of floras including early angiosperms as the rift basin of the proto-South Atlantic became a permanently flooded fjord.

Control of tectonic differentiation on the formation of large oil and gas fields in craton basins: A case study of Sinian–Triassic of the Sichuan Basin

Craton basins are the main sites where marine carbonate oil and gas reservoirs develop in China. However, the previous studies scarcely focused on the controlling effects of tectonic differentiation on the formation of large oil and gas fields in the ancient craton basins. In such circumstances, it is difficult to evaluate and select the favorable zones for exploring carbonate oil and gas. In this paper, based on the research results of the Sinian–Triassic prototype basin and lithofacies paleogeography of the Sichuan Basin, the tectonic differentiation pattern of craton basins and its controls on the accumulation elements and the distribution of hydrocarbons were analyzed with reference to the concept that structures control the sedimentation and distribution of oil and gas. The study reveals the results in three aspects. First, the intra-cratonic graben controls the high-quality source rocks and the hydrocarbon generation center, which forms good source-reservoir assemblage with the high-quality reservoir in the platform margin at the flank, showing excellent near-source accumulation conditions. Second, the three types of paleo-uplifts (i.e. differential denudation, syn-sedimentary, and folding) and the deep-large faults developed in the craton are favorable for the formation and distribution of large-scale carbonate reservoirs. Third, the superimposed areas with tectonic differentiation in multiple periods and multiple types are favorable for forming large oil and gas fields. It is concluded that the marine carbonate rocks show a huge oil and gas exploration potential in the Sichuan Basin. Attention should be paid to the new exploration targets of natural gas, such as the Sinian Dengying Fm. in the platform margin at the east flank of Deyang–Anyue graben, the Lower Cambrian Longwangmiao Fm. on the slope of paleo-uplift in central Sichuan Basin, and the Middle Permian Maokou Fm. in central–west Sichuan Basin, where oil and gas accumulation conditions are favorable.

Geochemical characteristics of He and CO2 from the Ordos (cratonic) and Bohaibay (rift) basins in China

According to the compositional and isotopic analyses of He, CO2 and CH4 of 95 gas samples from the Ordos Basin and 76 gas samples from the Bohaibay Basin in China, this study carried out a detailed investigation on the geochemical characteristics of helium and CO2 from the cratonic and rift basins, especially the cratonic Ordos Basin and rift Bohaibai Basin in China. Compared to the rift Bohaibay Basin, the cratonic Ordos Basin has lower level of helium and CO2, lower R/Ra ratio and lower δ13CCO2 maximum value. When the content of CO2 is <10%, both cratonic and rift basins have organic CO2 associated with the hydrocarbon generation process and inorganic CO2 from the thermal decomposition of carbonates or dissolution of organic acid. Rift basin can also have CO2>20% and δ13CCO2 of −6±2‰ or −4‰–−9‰, which is of volcanic-magmatic or mantle-derived origin, while cratonic basin has no such type of CO2. The cratonic basins (Ordos, Sichuan) have CO2<5% and relatively smaller variation range of δ13CCO2, i.e., 25.6‰ (−0.8‰–−26.4‰). The rift basins have higher level of CO2, R/Ra ratios and δ13CCO2 values. The content of CO2 can reach 100%, the R/Ra ratio can be up to 6.45 and the variation range of δ13CCO2 is 34‰ (7‰–−27‰). Bigger variation range of δ13CCO2 implies stronger tectonic activities, which is not favorable for the gas accumulation. While smaller variation range means more stable structure, which is favorable for the discovery of large gas fields. Sample in the cratonic basin in China generally has CH4/3He of 109–1012 and R/Ra<0.1 and in the rift basin it has CH4/3He of 105–1013 and R/Ra of 0.1–6.45. It is concluded that methane with CH4/3He≤108 is of inorganic origin and methane with CH4/3He≥1011 is of organic origin. The regression equation for the CH4/3He-R/Ra in the cratonic basin is: y=3E+11e−46.28x and it is: y=4E+10e−1.791x in the rift basin. Both Ordos and Bohaibay basins found gas wells with He>0.05%, which has significant commercial values and provides a clue for the exploration of helium-rich gas fields. Traps with R/Ra>2 is a favorable clue for the discovery of CO2 gas fields. Seven CO2 gas fields have been found in the Bohaibay Basin, but no systematic development has been carried out and only some liquid CO2 has been injected for tight oil from the Huagou CO2 gas field.

Early Cretaceous flora of India- A review

Earth’s terrestrial ecosystem during the early Cretaceous was marked by the dominance of naked seeded plants and appearance of flowering plants. Tectonic changes and evolutionary processes affected southern floras of the globe during this time. Review of Indian early Cretaceous flora distributed in peri and intra–cratonic basins signify homogenity of composition with regional variations. The flora composed of pteridophytes, pteridospermaleans, pentoxylaleans, bennettitaleans, ginkgoaleans, coniferaleans, taxaleans and taxa of uncertain affinity along with sporadic occurrence of flowering plants represent a unique Indian Early Cretaceous flora. Similitude of basinal floras with marginal differences can be attributed to taphonomic limitations and taxonomic angularity. A perusal of available data brings out an opportunity for novelty in floral composition and variable associations dictated by prevailed environmental conditions. The eastern, western and central regions of India hold distinct litho units encompassing plant mega fossils represented by leaf, wood / axis, seed, fructification and associated marker forms. Remarkable tenacity of certain plant groups, which even found in modern flora and vulnerability of many taxa constitute a blend of extinct and extant. The appearance and extinction of certain taxa can be explained as a cumulative affect of evolutionary and climatic factors. Perpetuation of gondwanic floral elements during the early Cretaceous along with newly evolved floral components testifies evolutionary innovations and changing ecological constraints.

A comparative study of salient petroleum features of the Proterozoic–Lower Paleozoic succession in major petroliferous basins in the world

The Proterozoic–Lower Paleozoic marine facies successions are developed in more than 20 basins with low exploration degree in the world. Some large-scale carbonate oil and gas fields have been found in the oldest succession in the Tarim Basin, Ordos Basin, Sichuan Basin, Permian Basin, Williston Basin, Michigan Basin, East Siberia Basin, and the Oman Basin. In order to reveal the hydrocarbon enrichment roles in the oldest succession, basin formation and evolution, hydrocarbon accumulation elements, and processes in the eight major basins are studied comparatively. The Williston Basin and Michigan Basin remained as stable cratonic basins after formation in the early Paleozoic, while the others developed into superimposed basins undergone multistage tectonic movements. The eight basins were mainly carbonate deposits in the Proterozoic–early Paleozoic having different sizes, frequent uplift, and subsidence leading to several regional unconformities. The main source rock is shale with total organic carbon content of generally greater than 1% and type I/II organic matters. Various types of reservoirs, such as karst reservoir, dolomite reservoir, reef-beach body reservoirs are developed. The reservoir spaces are mainly intergranular pore, intercrystalline pore, dissolved pore, and fracture. The reservoirs are highly heterogeneous with physical property changing greatly and consist mainly of gypsum-salt and shale cap rocks. The trap types can be divided into structural, stratigraphic, lithological, and complex types. The oil and gas reservoir types are classified according to trap types where the structural reservoirs are mostly developed. Many sets of source rocks are developed in these basins and experienced multistage hydrocarbon generation and expulsion processes. In different basins, the hydrocarbon accumulation processes are different and can be classified into two types, one is the process through multistage hydrocarbon accumulation with multistage adjustment and the other is the process through early hydrocarbon accumulation and late preservation.

Play types, geologic characteristics and exploration domains of lithological reservoirs in China

Abstract By dissecting thirty-six representative oil-gas reservoirs and analyzing accumulation conditions of giant oil and gas provinces in four prototype basins, including the Songliao Rift and Depression Basin, the Mesozoic Ordos Depression Basin, the Bohai Bay Rift and Depression Basin, the Mesozoic Junggar Depression Basin, the West Sichuan Foreland Basin and the Tarim Craton Basin, the hydrocarbon plays classification scheme of lithologic reservoirs is established and improved from the perspective of oil-gas accumulation zone and exploration target. The results reveal the accumulation characteristics and controlling factors of lithologic reservoirs including three kinds of slopes, three kinds of sag centers, three kinds of hydrocarbon accumulation assemblages and multiple genetic types. Based on the understandings on reservoir formation mechanisms and enrichment regularities of different hydrocarbon provinces, six major assessment indexes, parameter system and grading standard of lithologic plays are established, including hydrocarbon source rock condition, transportation pathway, reservoir condition, reservoir-cap assemblage, fluid property, and time-space configuration etc. Through comprehensive analysis of burial history, hydrocarbon-generation history, digenesis history, tectonic history, trap-forming history and accumulation history, plays of lithologic reservoirs are evaluated and optimized. The results show that the middle-shallow layers in the slope zones of the Songliao and Bohai Bay Basins oil-rich sags, the Mesozoic in the Ordos Basin, the Permian-Jurassic in the Junggar Basin, etc. are the major oil exploration areas in the future, with oil resources of about 42×10 8 t–67×10 8 t; the deep volcanic rocks of the Songliao Basin, the Carboniferous of the Junggar Basin, the Cambrian of the Tarim and Sichuan Basins, the Permian-Triassic reefs and shoals within the platform and on the platform margin of the Sichuan Basin are the major gas exploration areas in the future, with gas resources of about 1.5×10 12 m 3 −2.0×10 12 m 3 .

Geophysical evidence of pre-sag rifting and post-rifting fault reactivation in the Parnaíba basin, Brazil

Abstract. This study investigated the rifting mechanism that preceded the prolonged subsidence of the Paleozoic Parnaíba basin in Brazil and shed light on the tectonic evolution of this large cratonic basin in the South American platform. From the analysis of aeromagnetic, aerogravity, seismic reflection and borehole data, we concluded the following: (1) large pseudo-gravity and gravity lows mimic graben structures but are associated with linear supracrustal strips in the basement. (2) Seismic data indicate that 120–200 km wide and up to 300 km long rift zones occur in other parts of the basins. These rift zones mark the early stage of the 3.5 km thick sag basin. (3) The rifting phase occurred in the early Paleozoic and had a subsidence rate of 47 m Myr−1. (4) This rifting phase was followed by a long period of sag basin subsidence at a rate of 9.5 m Myr−1 between the Silurian and the late Cretaceous, during which rift faults propagated and influenced deposition. These data interpretations support the following succession of events: (1) after the Brasiliano orogeny (740–580 Ma), brittle reactivation of ductile basement shear zones led to normal and dextral oblique-slip faulting concentrated along the Transbrasiliano Lineament, a continental-scale shear zone that marks the boundary between basement crustal blocks. (2) The post-orogenic tectonic brittle reactivation of the ductile basement shear zones led to normal faulting associated with dextral oblique-slip crustal extension. In the west, pure-shear extension induced the formation of rift zones that crosscut metamorphic foliations and shear zones within the Parnaíba block. (3) The rift faults experienced multiple reactivation phases. (4) Similar processes may have occurred in coeval basins in the Laurentia and Central African blocks of Gondwana.

Classification and characteristics of tight oil plays

Based on the latest conventional–unconventional oil and gas databases and relevant reports, the distribution features of global tight oil were analyzed. A classification scheme of tight oil plays is proposed based on developed tight oil fields. Effective tight oil plays are defined by considering the exploiting practices of the past few years. Currently, potential tight oil areas are mainly distributed in 137 sets of shale strata in 84 basins, especially South America, North America, Russia, and North Africa. Foreland, craton, and continental rift basins dominate. In craton basins, tight oil mainly occurs in Paleozoic strata, while in continental rift basins, tight oil occurs in Paleozoic–Cenozoic strata. Tight oil mainly accumulates in the Cretaceous, Early Jurassic, Late Devonian, and Miocene, which correspond very well to six sets of global-developed source rocks. Based on source–reservoir relationship, core data, and well-logging data, tight oil plays can be classified into eight types, above-source play, below-source play, beside-source play, in-source play, between-source play, in-source mud-dominated play, in-source mud-subordinated play, and interbedded-source play. Specifically, between-source, interbedded-source, and in-source mud-subordinated plays are major targets for global tight oil development with high production. In contrast, in-source mud-dominated and in-source plays are less satisfactory.

The Hirnantian glacial landsystem of the Sahara: a meltwater-dominated system

During the Hirnantian (443 Ma), ice sheets expanded towards the present-day north over North Africa and the Arabian Peninsula when the western part of the Gondwana supercontinent straddled the South Pole. A glacigenic succession is exposed around the flanks of several Saharan cratonic basins, including Murzuq and Al Kufrah basins in Libya. As a result of the present-day hyper-arid context of the outcrops, a suite of palaeo-glacial landforms has been identified and, in some cases, mapped. These landforms occur along glacial erosion surfaces, defined as unconformities derived through direct ice sculpting, meltwater erosion or a combination of these processes. Unconformities, mostly resulting from ice sculpting and including glacially striated pavements and mega-scale glacial lineations (MSGLs), are identified at a number of locations along these erosion surfaces, in association with drumlinized features. The majority of reported examples are of ‘soft sediment character’; that is, generated through the deformation of unconsolidated sediment. Features mainly associated with meltwater erosion include tunnel valleys which occur in swarm-like belts, defining palaeo-grounding-line positions as the ice fronts retreated. A ‘smeared’ series of palaeo-ice-stream footprints, resulting from multiple advance–retreat cycles, is characteristic of the landsystem. Deformation of sediments into a series of push moraines, expressed as kilometre-scale fold–thrust belts, allows the locations of the former ice front to be identified. The Hirnantian glacial record of North Africa is a classic example of a continent-scale archive of an ancient palaeo-ice sheet. Expeditions to the Sahara during the late 1960s, focusing on the Algerian record, revealed a full suite of glacial indicators including striated pavements, pingos, roches moutonnees and a variety of meltwater channels (Beuf et al. 1971). Subsequently, largely as a result of their significance as hydrocarbon reservoirs, more recent studies re-evaluated these outcrops from a sedimentological perspective. Much effort focused on unravelling the glacial stratigraphy in neighbouring …

Speculations on the formation of cratons and cratonic basins

Surface wave tomography using Rayleigh waves has shown that Tibet and the surrounding mountain ranges that are now being shortened are underlain by thick lithosphere, of similar thickness to that beneath cratons. Both their elevation and lithospheric thickness can result from pure shear shortening of normal thickness continental lithosphere by about a factor of two. The resulting thermal evolution of the crust and lithosphere is dominated by radioactive decay in the crust. It raises the temperature of the lower part of the crust and of the upper part of the lithosphere to above their solidus temperatures, generating granites and small volumes of mafic alkaline rocks from beneath the Moho, as well as generating high temperature metamorphic assemblages in the crust. Thermal models of this process show that it can match the P, T estimates determined from metamorphic xenoliths from Tibet and the Pamirs, and can also match the compositions of the alkaline rocks. The seismological properties of the upper part of the lithosphere beneath northern Tibet suggest that it has already been heated by the blanketing effect and radioactivity of the thick crust on top. If the crustal thickness is reduced by erosion alone to its normal value at low elevations, without any tectonic extension, over a time scale that is short compared to the thermal time constant of thick lithosphere, of ∼250 Ma, thermal subsidence will produce a basin underlain by thick lithosphere. Though this simple model accounts for the relevant observations, there is not yet sufficient information available to be able to model in detail the resulting thermal evolution of the sediments deposited in such cratonic basins.

Discovery of a Devonian mafic magmatism on the western border of the Murzuq basin (Saharan metacraton): Paleomagnetic dating and geodynamical implications

Intraplate deformation is most often linked to major stress applied on plate margins. When such intraplate events are accompanied by magmatism, the use of several dating methods integrated within a multidisciplinary approach can bring constraints on the age, nature and source mobilized for generating the magma and in turn on the nature of the intraplate deformation. This study focuses on the large gabbro Arrikine sill (35 km in extension) emplaced within the Silurian sediments of the western margin of the Murzuq cratonic basin in southeastern Algeria. Its emplacement is dated during the early Devonian (415-400 Ma) through the determination of a reliable paleomagnetic pole by comparison with the Gondwana Apparent Polar Wander Path (APWP). This age can be correlated with deep phreatic eruptions before Pragian time thought to be at the origin of sand injections and associated circular structures in Algeria and Libya. For the sill, the K–Ar age of 325.6 ± 7.7 Ma is related to a K-rich aplitic phase that has K-enriched by more than 20% the Devonian gabbro. Laser-ICP-MS U–Pb method dates only inherited zircons mostly at c. 2030 Ma with additional ages at c. 2700 Ma and younger ones in the 766-598 Ma age range. The Arrikine sill is a high-Ti alkaline gabbro having the geochemical composition of a hawaiite akin to several intraplate continental and oceanic provinces, including the contemporaneous Aïr ring complexes province in Niger, but also to the Mauna Loa volcano in Hawaii. This peculiar composition akin to that of the contemporaneous Aïr province is in agreement with a lower Devonian age for the Arrikine sill.The lower Devonian Arrikine sill emplacement is related to a “Caledonian” transtensive reactivation of the western metacratonic boundary of the Murzuq craton. This event also generated in the Saharan platform the so-called “Caledonian unconformity” of regional extension, the Aïr ring complexes and magmatic rocks that produced sand injections. It could be related to rifting of the Hun terranes that occurred at the plate margin to the north (Stampfli and Borel, 2002, Blackey, 2008 and references therein). The mid-Carboniferous (c. 326 Ma) reactivation corresponds to Variscan compression on NW Africa generating aplitic fluids, but also to the major “Hercynian unconformity” of regional extension. The generation of the Arrikine magma is attributed to partial melting through adiabatic pressure release of uprising asthenosphere along tectonically reactivated mega-shear zones, here bordering the relictual Murzuq craton enclosed in the Saharan metacraton.

Characteristics and Enrichment Factors of Supergiant Lower Cambrian Longwangmiao Gas Reservoir in Anyue Gas Field: The Oldest and Largest Single Monoblock Gas Reservoir in China

An ultra-deep monoblock marine carbonate gas reservoir with the highest maturity and largest individual scale in China has been discovered in Lower Cambrian Longwangmiao Formation (LF) of Anyue gas field in the last few years. Based on a thorough study on Longwangmiao gas reservoir in the Gaoshiti-Moxi(GM) area, this paper presents the characteristics of such formation in Anyue gas field as follows: ① it is a lithologic-anticlinal gas reservoir developed on a gentle paleo-uplift; ② the gas reservoir is characterized by high temperature and pressure, contains sulfur, and belongs to a dry gas reservoir; and ③ the upper part of the gas reservoir has a good connectivity along with a free of bottom water and edge water, but poor in the lower part, where small scope of enclosed CaCl2 water exists locally. This paper further studies the main control factors for forming this supergiant gas reservoir: ① synsedimentary paleo-uplift controlled the distribution of large-area particle shoal, superimposed by multiple corrosion, which high quality reservoir was formed; ② syngenetic extended aulacogen was filled by mudstone and shale rapidly, resulted in widely distributed thick source rock; ③ synsedimentary gypsum rock and densified carbonate rock formed lateral sealing, multiple type of cap rocks superimposed vertically, therefore excellent closure conditions were formed; ④ large scale of gas reservoir trap was formed at early stage, and it was in a favorable hydrocarbon accumulation pointing area for a long time; and ⑤ the effective time-space matching of numerous hydrocarbon accumulation elements is the key to the formation of the supergiant gas reservoir. The understanding on the formation mechanism and enrichment rule of this ultra-deep, high temperature and high pressure gas reservoir will theoretically enrich the petroleum geology theory, and practically bring important reference to the exploration of large gas fields in the deep ancient craton basins with high maturity and multiple tectonic evolutions in the world.

About this title ‐ Precambrian Basins of India: Stratigraphic and Tectonic Context

This Memoir provides a comprehensive review of the Precambrian basins of the four Archaean nuclei of India (Dharwar, Bastar, Singhbhum and Aravalli-Bundelkhand), encompassing descriptions of the time–space distribution of sedimentary–volcanic successions, the interrelationship between tectonics and sedimentation, and basin histories. Studies of 22 basins within the framework of an international basin classification scheme deepen an understanding of the basin architecture especially for cratonic basins. Most Indian sedimentary successions formed as cratonic to extensional-margin rift and thermal-sag basins, some reflecting mantle plume movement, subcrustal heating or far-field stress. This Memoir shows that Phanerozoic plate-tectonic and sequence stratigraphic principles can be applied to the Precambrian basins of large Archaean provinces. The differences between the stratigraphic architecture of the Indian Precambrian and examples of Phanerozoic basin-fill successions elsewhere are ascribed to variable rates and intensities of the controls on accommodation and sediment supply, and changes inherent in the evolution of the hydrosphere–atmosphere and biosphere systems.