Journal of Petroleum Geology最新文献

{"title":"STRUCTURE AND EVOLUTION OF THE TEREK-CASPIAN FOLD-AND-THRUST BELT: NEW INSIGHTS FROM REGIONAL SEISMIC DATA","authors":"Konstantin Sobornov","doi":"10.1111/jpg.12793","DOIUrl":"10.1111/jpg.12793","url":null,"abstract":"<p>The Terek-Caspian fold-and-thrust belt along the northern flank of the Greater Caucasus Mountain Range together with the adjacent foreland basin is one of the oldest oil-producing regions in Russia. Despite the long history of exploration, recently acquired seismic data has provided new insights about the structural architecture and evolution of this area. Its structural development during the Neogene was constrained by a syn-extensional tectonic fabric inherited from Jurassic rifting and extension of the Great Caucasus Basin. The structural framework of this basin controlled the distribution of syn-extensional deposits, and the Cenozoic reactivation of lateral ramps resulted in along-strike variations in structural style. Thus western, central and SE segments of the Terek-Caspian foldbelt are recognised and are referred to here as the Terek-Sunzha fold zone, the Dagestan Promontory, and the Maritime Zone in southern Dagestan.</p><p>Three principal episodes of Cenozoic compression in the Terek-Caspian fold-and-thrust belt took place. The first episode in the Oligocene resulted in the inversion of pre-existing normal faults with the coeval development of a foreland basin to the north of the thrust belt. The dominance of sediments of northerly provenance in the foreland basin suggests there was only moderate uplift of the Greater Caucasus at this time. However, significant uplift of the orogenic belt took place during later phases of Sarmatian (Late Miocene) and Akchagylian (Late Pliocene) compression. Erosion of the uplifting Greater Caucasus gave rise to the development of large-scale, northerly prograding clinoforms which are clearly observed on seismic profiles in the foreland basin. Shortening was largely accommodated by wedge-shaped thrusting facilitated by the presence of mechanically weak stratigraphic units.</p><p>Structural development of the Terek-Sunzha fold zone in the west of the Terek-Caspian fold-and-thrust belt was largely controlled by a Tithonian salt layer which provided an efficient basal detachment surface and which also supplied material to squeezed diapirs in front of the belt. To the east, the plan-view shape and internal architecture of the Dagestan Promontory were influenced by the areal extent of the Lower-Middle Jurassic depocentre of the palaeo-Volga delta which is up to 10 km thick. In the Maritime Zone, the style of shortening was mostly controlled by the presence of a pre-existing structural high composed of folded Palaeozoic-Triassic strata in front of the fold-thrust belt.</p>","PeriodicalId":16748,"journal":{"name":"Journal of Petroleum Geology","volume":"44 3","pages":"259-286"},"PeriodicalIF":1.8,"publicationDate":"2021-06-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/jpg.12793","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45249975","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"STRATIGRAPHY, SEDIMENTOLOGY AND GEOCHEMISTRY OF THE OLIGOCENE – LOWER MIOCENE MAIKOP GROUP IN DAGESTAN, NE CAUCASUS","authors":"Yu. Gavrilov,&nbsp;R. Nedumov,&nbsp;E. Shchepetova,&nbsp;E. Shcherbinina,&nbsp;E. Kozlova,&nbsp;O. Golovanova,&nbsp;B. Pokrovsky","doi":"10.1111/jpg.12798","DOIUrl":"10.1111/jpg.12798","url":null,"abstract":"<p>A relatively complete section of the Maikop Group (Oligocene – lower Miocene) is exposed along the Sulak River valley in Dagestan (NE Caucasus) and contains a depositional record for this part of the Eastern Paratethys. At the Sulak River outcrop, the Maikop Group is ca. 1200 m thick and can be divided into six lithologically-defined formations: these are from the base up the Khadum Formation (Rupelian), the Miatly Formation, the Lower Clayey Formation, the Mutsidakal Formation (Chattian), the Riki Formation and the Zuramakent Formation (lower Miocene). The Khadum Formation rests on the upper Eocene Belaya Glina Formation and the boundary is marked by a sharp lithological transition from pale-coloured, bioturbated limestones below to black organic-rich shales above.</p><p>Biostratigraphic studies of calcareous nannoplankton in samples from the Sulak River section allowed the position of the Eocene – Oligocene boundary at the base of the Maikop Group to be defined. The boundary occurs within the CP16 Zone near the division between the CP16a and CP16b subzones. This is consistent with the age of the boundary at a reference outcrop along the Kheu River in Kabardino-Balkaria in the Central North Caucasus, some 200 km west of Dagestan. A positive oxygen stable isotope anomaly occurs at the top of the Belaya Glina Formation.</p><p>Samples of the Maikop Group are characterized by variations in TOC content ranging between 0.14 and 11.06 wt. %. The highest values were measured in both carbonate- and clay-rich samples from the Khadum Formation, and the lowest (less than 0.5 wt.%) in sandstones from the overlying Oligocene Miatly, Lower Clayey and Mutsidacal Formations. Samples of the lower Miocene Riki and Zuramakent Formations have moderate TOC values (on average more than 1.5 wt.%). Results of Rock-Eval pyrolysis show that Maikop samples contain kerogen Types II and III which is distributed unevenly throughout the formations. Clay-rich rocks in the upper part of the Khadum Formation (Solenovian Member) with Type II kerogen have the greatest oil-generating potential, with initial hydrogen index values estimated at 400-600 mg HC/g TOC. The Miatly Formation sandstones and siltstones contain migrated bitumen which is recognized from increased values of Rock-Eval S1 and the high Production Index (S1/(S1+S2). Overlying Oligocene – lower Miocene rocks contain mainly Type III kerogen, although increased TOC values obtained from samples of the Riki Formation indicate that it may have minor gas source potential.</p><p>Samples of Maikop Group sediments from the Sulak section were analysed for their contents of Mo, S, Fe, Mn, V, Ni and other elements. A stagnation coefficient (Mo/Mn x 100) was calculated and was interpreted as a measure of the intensity of anoxia in the Maikop palaeobasin. Anoxic conditions are interpreted to have reached a maximum in the Rupelian and Aquitanian during deposition of the Khadum and Riki Formations respectively. However, geochemica","PeriodicalId":16748,"journal":{"name":"Journal of Petroleum Geology","volume":"44 3","pages":"385-412"},"PeriodicalIF":1.8,"publicationDate":"2021-06-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/jpg.12798","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44559873","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"PETROLEUM SYSTEMS IN THE RIONI AND KURA BASINS OF GEORGIA","authors":"R. F. Sachsenhofer,&nbsp;A. Bechtel,&nbsp;R. Gratzer,&nbsp;O. Enukidze,&nbsp;A. Janiashvili,&nbsp;W. Nachtmann,&nbsp;A. Sanishvili,&nbsp;N. Tevzadze,&nbsp;M. A. Yukler","doi":"10.1111/jpg.12794","DOIUrl":"10.1111/jpg.12794","url":null,"abstract":"<div>\u0000 <p>The Neogene Rioni and Kura foreland basins in Georgia are located between the converging Greater and Lesser Caucasus fold-and-thrust belts. The Rioni Basin continues westward into the Black Sea whereas the Kura Basin extends eastward into Azerbaijan and the Caspian Sea. “Pre-” and “post-salt” petroleum systems are distinguished in the Rioni Basin separated by an Upper Jurassic evaporite succession of regional extent. The pre-salt petroleum system in the northern Rioni Basin is still poorly understood. Bathonian shales have generated oil which has been recorded in Middle Jurassic sandstones. However, as the origin of the oil in Upper Jurassic sandstones (e.g. at the Okumi oil discovery) is still problematic, the pre-salt petroleum system remains poorly constrained. Gas-rich, high volatile bituminous coals of Bathonian age may represent a CBM play.</p>\u0000 <p>The post-salt petroleum system in the Rioni Basin is charged by two prolific source rock units: the Middle Eocene Kuma Formation and the Oligo-Miocene Maikop Group. The petroleum potential of the Kuma Formation, which is about 40 m thick, is classified as good to very good. The Oligocene part of the Maikop Group is several hundred metres thick and contains source rocks with up to 5 wt.% TOC in its lower part. Additional source rocks are present in Cretaceous and lower Paleogene levels. Oil is produced from fractured Upper Cretaceous carbonates in anticlinal structures below the Neogene unconformity and from Mio-Pliocene siliciclastics in fault-related anticlines. Trap formation and hydrocarbon accumulation is interpreted to have occurred since Maeotian time. Proven oil reserves are very low (∼2 million tons) and suggest low charge efficiency.</p>\u0000 <p>Several stratigraphic horizons containing potential source rocks are present in the Kura Basin of eastern Georgia. Although oil-source correlations have yielded unsatisfactory results, the Maikop Group is the most likely source rock, despite its relatively poor petroleum potential which is at best “fair” in the Tbilisi area in the west of the basin. Additional potential source rocks include Middle and Upper Eocene shales. Fractured Middle Eocene volcaniclastic rocks are the best producing reservoirs for hydrocarbons, but oil accumulations are also found in fractured Upper Cretaceous carbonates and in Lower and Upper Eocene, Oligocene and Neogene siliciclastics. Biomarker data suggest a Cenozoic (or Upper Cretaceous) source rock containing abundant terrigenous organic matter. Anticlines and positive flower structures related to compressional tectonics in front of the Greater and Lesser Caucasus fold-and-thrust belts form the main trap types. Samgori-Patardzeuli-Ninotsminda in the Tbilisi region is by far the largest oil field in Georgia and accounts for nearly 90% of the cumulative production of the country (28.5 million tons). The field was probably charged from a kitchen area located to the north. Strike-sl","PeriodicalId":16748,"journal":{"name":"Journal of Petroleum Geology","volume":"44 3","pages":"287-316"},"PeriodicalIF":1.8,"publicationDate":"2021-06-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/jpg.12794","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43612711","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"EOCENE VOLCANICLASTICS IN THE KARTLI BASIN, GEORGIA: A FRACTURED RESERVOIR SEQUENCE","authors":"G. Tari,&nbsp;A. Vrsic,&nbsp;T. Gumpenberger,&nbsp;E. Mekonnen,&nbsp;W. Hujer,&nbsp;M. Fallah,&nbsp;N. Tevzadze,&nbsp;A. Janiashvili,&nbsp;P. Pace,&nbsp;A. Ricciato,&nbsp;V. Alania,&nbsp;O. Enukidze","doi":"10.1111/jpg.12795","DOIUrl":"10.1111/jpg.12795","url":null,"abstract":"<div>\u0000 <p>In the broader Caucasus region, multiple extrusive volcanic units are present within the Jurassic, Cretaceous, Eocene and Miocene sedimentary successions. Partial reworking of volcanic material from various provenance areas into Eocene, Oligocene and Miocene reservoir units is commonly observed in the Eastern Black Sea and in the Rioni, Kartli and Kura Basins of onshore Georgia. Reservoir quality has in general been negatively affected by volcanic rock fragments which may have undergone complex diagenetic alteration. However, despite concerns regarding reservoir quality, oil at the Samgori field, the largest field in Georgia (∼200 MM brl recovered), is hosted in altered Middle Eocene volcaniclastic sandstones interbedded with deep-water turbidites. Previous studies of core material from numerous wells in this field showed that most of the oil is contained in altered, microfractured, laumontite-rich tuffs which have fracture and cavernous net porosities averaging 12% and average permeability of 15 mD. The laumontite tuffs comprise only up to 20% of a tuffaceous sandstone section and occur as isolated lenses or pods on a sub-seismic scale (i.e. 5-10 m thick), causing highly variable oil productivity from one well to another.</p>\u0000 <p>The petrographic analysis of samples of Middle Eocene volcaniclastic sandstones from outcrops in the central part of the Kartli Basin around Tbilisi broadly confirms the main conclusions of studies completed some 30 years ago which were based on the analysis of subsurface samples. However, the surface samples analysed show that zeolitization events typically did not improve, but actually reduced, reservoir quality due to extensive zeolite cementation. The poor reservoir properties of the plug samples, which are age-equivalent to the proven subsurface Middle Eocene reservoir interval, highlight fracturing as a key factor controlling the presence of exceptional producers (up to 9000 b/d) in the Samgori field complex. The study therefore underlines the critical role of fracturing of the Middle Eocene volcaniclastic reservoir sequence in the Kartli Basin.</p>\u0000 </div>","PeriodicalId":16748,"journal":{"name":"Journal of Petroleum Geology","volume":"44 3","pages":"413-433"},"PeriodicalIF":1.8,"publicationDate":"2021-06-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/jpg.12795","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47808480","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"EXPLORATION PLAYS IN THE CAUCASUS REGION","authors":"G. Tari,&nbsp;G. Blackbourn,&nbsp;D.R.D. Boote,&nbsp;R.F. Sachsenhofer,&nbsp;A. Yukler","doi":"10.1111/jpg.12791","DOIUrl":"10.1111/jpg.12791","url":null,"abstract":"<p>Exploration efforts around the Greater Caucasus region started towards the end of the 19th century and established a wide range of petroleum play types in various basin segments around the orogen. All these plays are associated with the flanks of the inverted thrust-fold belt and the adjacent foreland basin systems, but display significant variation among the basin segments depending on the tectonostratigraphic units involved and the degree of exploration maturity. Whereas the same main source rocks have generated most of the hydrocarbons in all the basins (namely organic-rich shales in the Oligocene – Lower Miocene Maykop Group and the Eocene Kuma Formation), it is primarily the trapping style, both proven and speculative, which is responsible for the broad spectrum of play types observed. Eleven play type diagrams across six main petroleum provinces of the Greater Caucasus region are presented in this paper and summarize the current exploration understanding of the existing discoveries and potential new play targets. These play cartoons offer a prospect-scale summary of both mature producing and underexplored basin segments in a coherent visual manner, and are therefore intended to promote future exploration efforts in the Caucasus region. The testing of new play types requires the proper risking of the two most critical elements in the region: hydrocarbon kitchen effectiveness, and post-charge trap modification. The de-risking of these factors will require properly designed, fit-for-purpose acquisition of modern geological and geophysical data sets.</p>","PeriodicalId":16748,"journal":{"name":"Journal of Petroleum Geology","volume":"44 3","pages":"213-236"},"PeriodicalIF":1.8,"publicationDate":"2021-06-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/jpg.12791","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42091176","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"SOUTH CAUCASUS PALAEOGEOGRAPHY AND PROSPECTIVITY: ELEMENTS OF PETROLEUM SYSTEMS FROM THE BLACK SEA TO THE CASPIAN","authors":"G. A. Blackbourn,&nbsp;N. Tevzadze,&nbsp;A. Janiashvili,&nbsp;O. Enukidze,&nbsp;V. Alania","doi":"10.1111/jpg.12792","DOIUrl":"10.1111/jpg.12792","url":null,"abstract":"<p>Nine Mesozoic and Cenozoic palaeogeographic maps are presented to illustrate the petroleum prospectivity of the South Caucasus from a fresh perspective and as part of the wider Caucasus region. Previously, elements of petroleum systems – reservoir, source and sealing lithologies, and the timing of their formation – have mostly been examined within individual sub-basins or licence blocks, and regional understanding has been limited. Emphasis is placed here on the onshore prospectivity of Georgia and Azerbaijan; the well-known Pliocene Productive Series of eastern Azerbaijan and the southern Caspian is not considered.</p><p>The Great Caucasus Basin (GCB) formed in the Early Jurassic following closure of PalaeoTethys, and remained a significant feature, despite structural modifications, until end-Eocene underthrusting and uplift converted the basin into the Greater Caucasus mountains. By the Toarcian a major delta system had developed along its northeastern margin, while the Transcaucasus block to the south was mostly covered by a shallow sea with limited sediment supply. Bajocian volcanism across the South Caucasus was accompanied by modification of the structure of the Great Caucasus Basin with the intrusion of tholeiitic dykes, possibly associated with onset of northward NeoTethyan subduction. Rising sea levels led to the abandonment of the GCB delta system. Relative uplift of the South Caucasus in the Bathonian created lowlands surrounded by marginal settings in which paralic deposits and coals were laid down. Jurassic hydrocarbon source rocks include deep-marine shales deposited within the Great Caucasus Basin together with coals; their potential is confirmed by numerous seeps within both Georgia and Azerbaijan. Various Middle Jurassic sandstones are potential reservoirs.</p><p>Carbonates dominated by the late Callovian, with widespread development of Oxfordian reefs and of Late Jurassic evaporite basins in the North Caucasus. Bedded anhydrites in Georgia comprise potential seals. Shallow-marine clastics again became widespread across the Caucasus in the Cretaceous, later replaced by carbonates including chalk-like limestones. Deeper-marine conditions persisted in the Great Caucasus Basin, which became less well-defined and split into separate depocentres. Fractured chalks are known reservoirs in the North Caucasus and prospective reservoirs in the South Caucasus.</p><p>Uplift of the southern South Caucasus during the Paleogene led to northward transport of sediment into evolving E-W to ESE-WSW basins in eastern Georgia and western Azerbaijan. Marine deposits within these basins form reservoirs, including thick fractured volcanogenic turbidites in eastern Georgia. Reduced sediment supply here at the start of the Late Eocene allowed organic-rich restricted-marine source rocks to accumulate.</p><p>Rapid uplift of the GCB associated with underthrusting at the end of the Eocene led to emergence of the Greater Caucasus mountains. The prolific Mai","PeriodicalId":16748,"journal":{"name":"Journal of Petroleum Geology","volume":"44 3","pages":"237-257"},"PeriodicalIF":1.8,"publicationDate":"2021-06-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43085153","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"METHODS TO ESTIMATE EROSION IN SEDIMENTARY BASINS","authors":"Karthik Iyer,&nbsp;Ebbe H. Hartz,&nbsp;Daniel W. Schmid","doi":"10.1111/jpg.12782","DOIUrl":"10.1111/jpg.12782","url":null,"abstract":"<p>Net erosion, the difference between the present-day and maximum burial depths of a reference unit, may have a major impact on hydrocarbon prospectivity in a sedimentary basin. Erosion may affect all the components of a petroleum system, from source rock to reservoir to seal. In most cases, vitrinite reflectance (VR), temperature and sonic velocity data, which are often readily available, can be used to determine net erosion in a region based on the thermal and mechanical evolution of sedimentary layers with burial. This paper revisits these methods and discusses the determination of net erosion from these datasets. Furthermore, it is shown that a closer look at the data is warranted if the estimates derived from complementary VR/temperature and velocity datasets significantly diverge. Such differences can be reconciled by critically examining the datasets and the regional geology, resulting in erosion estimates from both datasets which are within 100 m of each other. Lastly, a fully automated, process-driven method combined with multi-objective optimization algorithms and that takes all three datasets into account is showcased while determining net erosion for three wells located in the Norwegian Barents Sea. One of the benefits of this method is that it explores a wide range of likely scenarios that would best match the different datasets. Furthermore, the method can also automatically flag datasets that are inconsistent with each other by returning an overall low fit score. These datasets can then be critically examined to determine their reliability and to arrive at a more consistent erosion estimate, reducing the error margin to about 100 m.</p>","PeriodicalId":16748,"journal":{"name":"Journal of Petroleum Geology","volume":"44 2","pages":"121-144"},"PeriodicalIF":1.8,"publicationDate":"2021-03-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/jpg.12782","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46915179","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"John Ramsay, 1931–2021","authors":"","doi":"10.1111/jpg.12786","DOIUrl":"10.1111/jpg.12786","url":null,"abstract":"","PeriodicalId":16748,"journal":{"name":"Journal of Petroleum Geology","volume":"44 2","pages":"209-210"},"PeriodicalIF":1.8,"publicationDate":"2021-03-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/jpg.12786","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"64114597","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"THE ROLE OF MULTIPLE WEAK LITHOLOGIES IN THE DEFORMATION OF COVER UNITS IN THE NORTHWESTERN SEGMENT OF THE ZAGROS FOLD-AND-THRUST BELT","authors":"Hemin A. Koyi,&nbsp;Howri Mansurbeg","doi":"10.1111/jpg.12783","DOIUrl":"10.1111/jpg.12783","url":null,"abstract":"<div>\u0000 <p>The geometry, kinematics and dynamics of fold-and-thrust belts are strongly influenced by the mechanical behaviour of the basal décollement. However, many fold-and-thrust belts also include mechanically weak lithologies such as evaporites and marls or mudstones at different levels within the shortened stratigraphy. The kinematics and dynamic evolution of these thrust belts are controlled by the mechanical behaviour both of the basal décollement and of the weak units embedded within the overlying stratigraphic succession. In the Zagros fold-and-thrust belt (ZFTB), the shortened sedimentary cover is between 7 and 12 km thick and mechanically weak lithologies compartmentalize the stratigraphic column at shallow and intermediate levels. In this paper, satellite, field and seismic data from the Kurdistan Region of Iraq are used to identify structures of different sizes and surface traces. The observations are used to underline the role of mechanically weak horizons within the Zagros stratigraphy and the decoupling of deformation both laterally and with depth in the belt.</p>\u0000 <p>The decoupling between shallow and deeper structures observed in seismic profiles from the Kurdistan Region of Iraq is also reported from field observations from the Iranian part of the Zagros fold-and-thrust belt, where folds with different surface traces occur. Decoupling between shallow and deep layers by incompetent lithologies at intermediate depths (e.g. marls, mudstones and evaporites) results in the formation of disharmonic folds. The geometry, size and location of such folds may differ between outcropping and subsurface structures. Decoupling may have a significant impact on hydrocarbon exploration in different parts of the Zagros fold-and-thrust belt due to potential offsets between outcropping and subsurface structures and their associated traps.</p>\u0000 </div>","PeriodicalId":16748,"journal":{"name":"Journal of Petroleum Geology","volume":"44 2","pages":"145-166"},"PeriodicalIF":1.8,"publicationDate":"2021-03-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/jpg.12783","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41850302","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"GEOCHEMICAL CHARACTERISTICS AND CHARGE HISTORY OF OIL IN THE UPPER CRETACEOUS M1 SANDSTONES (NAPO FORMATION) IN BLOCK T, ORIENTE BASIN, ECUADOR","authors":"Ma Zhongzhen,&nbsp;Chen Heping,&nbsp;Yang Xiaofa,&nbsp;Zhou Yubing,&nbsp;Tian Zuoji,&nbsp;Wang Dandan,&nbsp;Liu Yaming,&nbsp;Zhao Yongbin","doi":"10.1111/jpg.12784","DOIUrl":"10.1111/jpg.12784","url":null,"abstract":"<p>Major oil discoveries have recently been made in Block T in the north of the Oriente Basin, Ecuador. The oil is reservoired in the M1 Sandstones of the Upper Cretaceous Napo Formation. To investigate the origin and charge history of the petroleum, a detailed geochemical study was carried out on 43 crude oil samples from 42 producing wells in Block T together with fluid inclusion analyses of three core samples from two wells.</p><p>According to the results of GC/GC-MS analyses of the oil samples, the oils contain n-alkanes with a peak carbon number at C₁₅-C₁₇ and a subordinate peak at C₂₅-C₃₀. The nC_21-/nC₂₂₊ ratio ranges from 0.64 to 1.69, and the Carbon Preference Index from 0.95 to 1.23. The odd-over-even predominance is 1.02–1.27. A cross-plot of C₂₂/C₂₁ versus C₂₄/C₂₃ tricyclic terpanes indicates that the source rock is a marine marl mixed with a small amount of terrigenous material. C₂₇ regular steranes are more dominant than C₂₈ ≈ C₂₉ and the C₂₉/C₂₇ ratio ranges from 0.67 to 0.94 indicating a source rock dominated by marine algal material with minor terrigenous input. R_c calculated using the MPI index was 0.83% to 1.11%, indicating that the oils were generated during the early to peak oil generation stage. A cross-plot of C₂₉ααα20S/(20S+20R) versus C₂₉αββ/(ββ+αα), and ratios of C₃₁L-hopane 22S/(22S + 22R) and C₃₂L-hopane 22S/(22S + 22R), gave similar maturity results.</p><p>The presence in the same oil samples of a complete n-alkane series together with an unresolved UCM hump and 25-norhopanes indicates at least two stages of oil charging, with severe biodegradation of the early-stage oil and a later charge of fresh, unaltered oil.</p><p>The homogenization temperatures of 36 fluid inclusions in samples from Block T wells F20 and F67 range from 81 to 95°C. A reconstructed burial and geothermal history of well F20 indicates that the M1 Sandstones in this area reached a temperature of 81°C at 19–16 Ma, after which temperatures increased continuously to 95–100°C at the present day. The homogenization temperatures of the analysed fluid inclusions combined with the geothermal history indicate that oil charging into the M1Sandstones began in the early Miocene and continues at the present day.</p>","PeriodicalId":16748,"journal":{"name":"Journal of Petroleum Geology","volume":"44 2","pages":"167-186"},"PeriodicalIF":1.8,"publicationDate":"2021-03-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/jpg.12784","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46724109","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}