Sedimentary Processes and Depositional Characteristics of Coarse-grained Subaqueous Fans along Steep Slopes in a Lacustrine Rift Basin:A Case Study from the Dongying Depression,Bohai Bay Basin,China

Coarse-grained subaqueous fans are vital oil and gas exploration targets in the Bohai Bay basin,China.The insufficient understanding of their sedimentary processes,depositional patterns,and controlling factors restricts efficient exploration and development.Coarse-grained subaqueous fans in the Yong′an area,Dongying Depression,are investigated in this study.These fans include nearshore subaqueous fans,and sublacustrine fans,and their sedimentary processes,depositional patterns and distribution characteristics are mainly controlled by tectonic activity and paleogeomorphology.Nearshore subaqueous fans developed near the boundary fault during the early–middle deposition stage due to strong tectonic activity and large topographic subsidence.Early sublacustrine fans developed at the front of the nearshore subaqueous fans in the area where the topography changed from gentle to steep along the source direction.While the topography was gentle,sublacustrine fans did not develop.During the late weak tectonic activity stage,late sublacustrine fans developed with multiple stages superimposed.Frequent fault activity and related earthquakes steepened the basin margin,and the boundary fault slopes were 25.9°–34°.During the early–middle deposition stage,hyperpycnal flows triggered by outburst floods developed.During the late deposition stage,with weak tectonic activity,seasonal floods triggered hyperpycnal flows,and hybrid event beds developed distally.

Research advances on transfer zones in rift basins and their influence on hydrocarbon accumulation

Transfer zones are structural areas of faults interactions where fault motion or displacement can be transferred from one fault to another, regional strain maintains laterally constant. Transfer zones are widely developed in rift basins and have significance on hydrocarbon accumulation. In this review article, we attempt to summarize recent advances on the types, distance-displacement curves, evolutionary stages and controlling factors of transfer zones in rift basins and their effects on sedimentary systems, reservoir properties, trap formation and hydrocarbon migration. The formation of transfer zones is genetically related to the segmented growth of normal faults. Depending on the degree of interaction between these normal faults, transfer zones in rift basins could be divided into two types: soft-linked and hard-linked, which are further subdivided into transfer slope, oblique anticline, horst and transfer fault based on the combination patterns of normal faults. In general, the development of transfer zones experiences several stages including isolated normal faulting, transfer slope forming, complicating and breaking. During the interaction and growth of segmented normal faults, stress-strain and spatial array of faults, pre-existing basement structures, and mechanical conditions of rocks have a great influence on the location and development processes of transfer zones. A transfer zone is commonly considered as a pathway for conveying sediments from provenance to basin, and it hence exerts an essential control on the distribution of sandbodies. In addition, transfer zone is the area where stresses are concentrated, which facilitates the formation of various types of structural traps, and it is also a favorable conduit for hydrocarbon migration. Consequently, there exists great hydrocarbon potentials in transfer zones to which more attention should be given.

Key factors controlling shale oil enrichment in saline lacustrine rift basin:implications from two shale oil wells in Dongpu Depression,Bohai Bay Basin

Comparative analyses of petroleum generation potential,reservoir volume,frackability,and oil mobility were conducted on 102 shale cores from the Dongpu Depression.Results show the shale has high organic matter contents composed of oil-prone type I and type II kerogens within the oil window.Various types of pores and fractures exist in the shale,with a porosity of up to 14.9%.The shale has high brittle mineral contents,extensive fractures,and high potential for oil mobility due to high seepage capacity and overpressure.Although the petroleum generation potential of the shale at Well PS18-8 is relatively greater than that at Well PS18-1,oil content of the latter is greater due to the greater TOC.The porosity and fracture density observed in Well PS18-1 are greater and more conducive to shale oil enrichment.Although the shales in Wells PS18-1 and PS18-8 have similar brittle mineral contents,the former is more favorable for anthropogenic fracturing due to a higher preexisting fracture density.Besides,the shale at Well PS18-1 has a higher seepage capacity and overpressure and therefore a higher oil mobility.The fracture density and overpressure play key roles in shale oil enrichment.

Tectonics and Petroleum Potential of the East China Sea Shelf Rift Basin

There are two Cenozoic sedimentary basins in the East China Sea. They are the East China Sea shelf basin and the Okinawa Trough basin. The former can be divided into a western and an eastern rift region. The development of the shelf basin underwent continental-margin fault depression, post-rift and then tectonic inversion stages. Available exploration results show that the distribution of source rocks is controlled by the basin architecture and its tectonic evolution. In the Xihu depression, mudstones and coals are the main source rocks. The eastern rift region has good geological conditions for the formation of large oil and gas fields.

Hydrocarbon Accumulation in Lacustrine Turbidite in the Rift Basin, Bohai Bay

Three types of turbidites are identified in the studied area, including proximal turbidite along the northern steep slope （alluvial fans, fan delta, and subaqueous fans via short-distance transportation）, distal turbidites along the southern gentle slope （stacked sliding of delta and fan delta front）, and fluxoturbidite in the central depression. Detailed studies of several case histories and the relationship between sedimentary facies and faults suggest a significant role of tectonic setting and faults in the development of turbidite, which created source areas, effected slope topography, controlled the climate and paleo-environment, and formed enough slope angle and slope break for sedimentary instabilities and massive block movement to form turbidite. According to statistics, 0.86%, with an average value of 40.3%, of the trap volumes in the 69 identified Tertiary lacustrine turbidites in the Jiyang Superdepression in the Bohai Bay Basin are filled with oil. The porosity and permeability of turbidite sands vary widely. The productive reservoirs are generally those from the braided channels of both distal and proximal turbidite, and from the main channel of proximal turbidite, with a low carbonate content and the porosity and permeability higher than 12% and 1 mD respectively. Most of the lithologic oil pools in the Jiyang Superdepression are enveloped by the effective source rocks, and the percentage of the trap volume generally increases with the hydrocarbon expulsion intensity of source rocks. This is in contrast with structural-lithologic traps （i.e. proximal turbidite along the steep slope and distal turbidite along the gentle slope）, in which, graben-boundary faults play an important role in oil migration, as the turbidites are not in direct contact with effective hydrocarbon source rocks.

Quaternary tectonic history of seismically active intraplate Kachchh Rift Basin,western India:A review

Since the recorded historical period,the Kachchh Rift Basin(KRB)has encountered numerous moderate to large magnitude earthquakes.According to the series of seismicity research so far,there are several important points of debate regarding the tectonic history and evolution of the KRB,especially during the Quaternary period.Therefore,the main objective of the present research is to inspect and perceive the association amongst the strain build-up,earthquake provenance,landform evolution and progression as archived by the Quaternary deposits of the KRB.The previous studies demonstrated the evolutions of various landforms,such as the uplifted fluvial terraces,formation of the gorges,uplifted alluvial fan sequences,which can be ideally used to reconstruct the neotectonic history along active faults of KRB.Considering this,the analysis of the accessible and supportive data,including geochronology provided by earlier studies along with some new dataset for a superior knowledge on the Quaternary tectonic forces prevailing in the KRB,have been carried out.Furthermore,we also emphasized the differences and directions for future potential research issues.The observations of variability in uplift rates across the various active faults in the KRB suggest a complex geological history during the Quaternary period.The results show that the vertical uplift rate along the significant active faults range from 0.8 to 2.8 mm/yr,demonstrating the variable tectonic stress regime prevailing in the KRB.The uplift rates constricted from geomorphic and chronological aspects suggest that the tectonic movements within the Kachchh intraplate region is regulated by the fault segments and the present tectonic stress field is in accordance with the encompassing tectonic stress field associated with the northward movement of the Indian plate corresponding to the Eurasian plate.

Orderliness of Hydrocarbon Accumulation Distribution in Rift Basins of Eastern China

The Cenozoic rift basins in eastern China show a clear temporal and spatial zonation and episodic tectonic evolution, which control their episodic hydrocarbon generation and zonal accumulation. In this paper, based on the study of depositional architecture, hydrocarbon migration system and dynamic evolution in the rift basins, combinations of hydrocarbon accumulation elements were analyzed using sequence stratigraphy. Hydrocarbon distribution in system tracts with different sequence orders was further studied. And we summarized stacking patterns and horizontal combination relationships for different types of reservoirs, such as lithological, tectonic-lithological, tectonic and stratigraphical reservoirs which can be observed from depression center to basin margin. The result reveals that various scales of pools exhibit significant distribution and evolution orderliness in different pool-forming units, i.e., depositional systems, plays and depressions. The regular distribution of various scales of pools is closely related to tectonic evolution and depositional filling in the rift basins. The result can be applied to the fine petroleum exploration in rift basins in eastern China. It will promote the scientific prediction and evaluation of reservoir types and their spatial distribution, lead to the active shifts of exploration targets in different zones, and thus support the stable progress of fine exploration in mature exploration areas.

Geochemical Characterizations of Source and Depositional Environment of Hydrocarbons in the Lake Albert Rift Basin

Despite the upsurge in hydrocarbon exploration in the Lake Albert Rift Basin(LARB)over the past three decades,systematic characterization of hydrocarbon compositions remains lacking,leading to uncertainties in source rock and oil generation determination.We characterized crude oil compositions and oil sand samples in the northern and southern subbasins of LARB.The relative abundance of normal and branched linear alkanes,hopanes,steranes,and aromatic hydrocarbon suggest that northern and southern hydrocarbons were deposited in anoxic to suboxic lacustrine environments and share similar biological source compositions(i.e.,a mixture of plants and aquatic algae and bacteria).Relative to southern samples,northern samples show more negative δ^(13)C values for oils,saturates and aromatics,indicating longer migration paths,and exhibit higher MPI-1,DNR-1 and 4-/1-MDBT ratios,indicating higher maturity.Between the two possible sets of source rocks(upper Miocene and Jurassic strata),the positive δ^(13)C values of saturated hydrocarbons(average=-20.5‰)suggest that the upper Miocene lacustrine shale is the most likely candidate.Oleanane index(<5% in our samples)does not exclude either source rock possibility,and C_(28)/C_(29) regular sterane(average=0.63)may be biased by high terrestrial inputs in a lacustrine setting.Together,our data show that northern and southern oils originate from the same source rocks but different oil kitchens.Given the similar geochemical characteristics of southern and northern oils,previous exploration successes in the northern subbasin likely suggest similar potential in the southern sector,while other elements influencing exploration success must be also evaluated.

Geological structure and dynamic mechanism of the Termit rift basin in West African rift system

Based on seismic and drilling data in the study area,the geological structure and kinematic process of the Termit rift basin were studied using seismic profile interpretation and balanced restoration to find out the dynamic mechanism of the basin.(1)The geological structure of the Termit Basin is represented as a narrow rift basin,with development of series of structural styles in extensional,extensional strike-slip and compressional stress setting.On plane,it is narrow in the north and wide in the south,and transitions from graben to half-graben from north to south;it features a graben controlled by the boundary faults in the north and a fault-overlapped half-graben in the south.(2)Before the Cretaceous,a series of hidden faults developed in the West African rift system,which laid the foundation for the development location and distribution direction of the Termit Basin;during the Cretaceous to Paleogene periods,the basin experienced two phases of rifting in Early Cretaceous and Paleogene,which controlled the initial structure and current structural shape of the basin respectively;during the Neogene to Quaternary,the basin was subjected to weak transformation.(3)In the Precambrian,the Pan-African movement gave rise to a narrow and long weak zone within the African plate,which provided the pre-existing structural conditions for the formation of the Termit Basin.In the Early Cretaceous,affected by the South Atlantic rifting,the Pan African weak zone was reactivated,resulting in the first stage of rifting and the basic structural framework of the Termit Basin.In the Paleogene,affected by the subduction and convergence of the Neo-Tethys Ocean,the African-Arabian plate extended in near E-W trending,and the Termit Basin experienced the second stage of rifting.The oblique extension in this period caused intense structural differentiation,and the current structural pattern of alternate uplifts and depressions took shape gradually.

Comprehensive evaluation on hydrocarbon-bearing availability of fault traps in a rift basin: A case study of the Qikou sag in the Bohai Bay Basin, China

An evaluation system of hydrocarbon-bearing availability of fault traps was established based on the comprehensive analysis of fault segment growth history,fine reservoir anatomy and geochemistry tracing,with the Qikou sag in the Bohai Bay Basin as target area.The displacement/separation transform and displacement gradient method were used to prove the interpretation reliability of fault traps.The method of maximum throw subtraction was used to recover the history of fault growth and determine the availability of the forming period of fault traps.Based on the quantitative relationship between shale gouge ratio and cross-fault pressure difference of known reservoirs in southern Qikou sag,the critical shale gouge ratio of fault lateral sealing was calculated at 20%,and the quantitative evaluation chart based on the relationship of"fault throw–sand-formation ratio and hydrocarbon column height"was constructed.Based on the results of reservoir fine anatomy and quantitative fluorescence tracing test shale smear factor method is suitable for evaluating the vertical sealing of faults in the caprock of the middle submember of first member of Paleogene Shahejie Formation,and the shale smear factor critical value is 3.5.The juxtaposition thickness method is suitable for evaluating vertical sealing of faults in the caprock of the second member of Paleogene Dongying Formation,and the critical juxtaposition thickness of fault is 70–80 m.By combining four factors,the availability of fault trap interpretation,the availability of the forming period of fault trap,the availability of fault lateral sealing and the availability of fault vertical sealing,the comprehensive evaluation chart on hydrocarbon-bearing availability of fault traps in Qikou sag has been established,which provides a reasonable basis for risk assessment of fault traps.

Occurrence of Laminated Dolomitic Exhalative Rocks in Continental Rift Basins of Northwest China

Objective Researchers have recently discovered that sublacustrine sedimentary exhalative mechanism associated with volcanism,is the principle way to form lacustrine exhalative rocks.These rocks differentiate themselves from normal sedimentary rocks in their specificpetrofabric and material composition.

Quantitative Modelling of Multiphase Lithospheric Stretching and Deep Thermal History of Some Tertiary Rift Basins in Eastern China

The stretching process of some Tertiary rift basins in eastern China is characterized by multiphase rifting. A multiple instantaneous uniform stretching model is proposed in this paper to simulate the formation of the basins as the rifting process cannot be accurately described by a simple (one episode) stretching model. The study shows that the multiphase stretching model, combined with the back-stripping technique, can be used to reconstruct the subsidence history and the stretching process of the lithosphere, and to evaluate the depth to the top of the asthenosphere and the deep thermal evolution of the basins. The calculated results obtained by applying the quantitative model to the episodic rifting process of the Tertiary Qiongdongnan and Yinggehai basins in the South China Sea are in agreement with geophysical data and geological observations. This provides a new method for quantitative evaluation of the geodynamic process of multiphase rifting occurring during the Tertiary in eastern China.

Anatomy of Eastern Niger Rift Basin with Specific References of Its Petroleum Systems

An attempt is made in this paper to present the dynamics of the Eastern Niger Rift Basin (ENRB) with references to the key features and processes of petroleum systems based on published information. The Eastern Niger Basin is a superimposed rift basin with sedimentary structures emplaced during two rifts episodes. The Cretaceous episode is characterized by large, tilted normally faulted blocks trending NW-SE, that were reactivated in the Paleogene, while the Paleogene episode is characterized by normal faulted blocks that trend NNW-SSE. The rifting resulted in different basin structures with the north section dominated by asymmetric half-grabens while the south section is dominated by full-grabens. Three source rocks each belonging to three different play fairways exist: 1) The Paleogene Sokor-1 Member source belongs to second cycle syn-rift play associated with fluvial/deltaic facies;2) Cretaceous Yogou and Donga sources from first cycle post-rift play associated with alluvial/fluvial/deltaic and marine clastic and carbonate facies;and 3) Cretaceous Yogou source from first cycle transitional play associated with mudstone and shale of transitional facies. The ENRB comprises two source-reservoir-seal assemblages: a lower assemblage of Upper Cretaceous and an Upper assemblage of the Paleogene. Except for the Yogou source which possesses a self-contained petroleum system, the rest of the source rocks release their oils into the Paleogene Sokor-1 Member reservoir sealed regionally by the Oligocene Sokor-2 Member. The Paleogene assemblage is charged from the Upper Cretaceous Yogou Formation through fractures emplaced during the rifting episodes.

THE EVOLUTION OF LANPING RIFT BASIN FROM LADINIAN IN MIDDLE TRIASSIC EPOCH TO EARLY JURASSIC EPOCH

Lanping basin was a massif (land massif) in late Palaeozoic Era. The ocean of Jinshajiang separated it from Yangtze plate in east. Lancangjiang ocean separated it from Yunnan—Tibet plate in west. From late Permian Epoch, the oceanic crust of Jinshajiang subduced the west from east, the one of Lancangjiang down went the east from west, and then the Yunnan—Tibet ancient land gradually closed to the Yangtze. In the end of the Permian Period, two continents and Lanping plate touched together, and the evolution history of the Paleotethys was end. Hercynian orogenic belt in the east and west sides of Lanping had volcanic rock colliding in early—middle Triassic Epoch. In Ladinian in middle Triassic and Carnian in late Triassic, the north side of Lanping basin formed the serial volcanic rock of spilite—quartz keratophyre because mantle\|derived magma causing by delamination rose and mixed with the constituent of continental crust. The volcanic rock overlapped the middle Triassic and late Palaeozoic stratum in angular unconformity. It was the feature of double peak or evolution from the basic to the acid. The race element distribution of volcanic rock was same as the one of tholeiite in island and inter\|arc basin. The rate of lead isotope of the volcanic rock was much higher. These points all distributed above the NHRL in Pb\|Pb. This indicated that the Pb of volcanic rock was the mantle\|derived magma mixed with crust one. The large\|area progression in Lanping rift basin begun in late Carnian.. The east side in Lanping basin developed the sedimentary system that was granule gravel (grit) rock in border facies—limestone in beach facies—black shale, and the middle had black shale, banded siliceous rock, brecciform limestone in late Carnian to Norian. The geochemistry research of siliceous rock showed that the genesis of the chert was hot water. The development of brecciform limestone was related with action of central\|axis rift. The Lanping rift basin went into consuming stage in Rhaetian Epoch of the late Triassic. The basin developed clastic rock bearing coal of continental\|oceanic alternation facies. In early Jurassic, the sedimentary area atrophied further, and the fine lacustrine sediment whose thickness was not great developed in the east of central\|axis. The west stratum of the basin in late Triassic Epoch touched directly with the one in middle Jurassic. Lanping basin was going into another evolution stage that was down\|warped basin.

A stratigraphical-geochemical study on the Chaco Paraná continental rift basin—An approach study based on regional sedimentology and drill-hole core analyses, South América

This paper is focused on a geologic “regional rift basin system pattern” and its stratigraphical-geochemical relationship. This is mainly based on the littoral shallow marine sedimentary succession paleogeography and deposits. These successions characterize the large extensional intracratonic Chaco Paraná Basin rift system. The basin is located in South America west of the Brazilian Shield. The analyzed rift basin system evolved from the Upper Cretaceous (Late Campanian-Senonian-Maastrichtian-Early Paleocene) to Quaternary time. The siliciclastic littoral shallow marine successions were deposited from Early Senonian-Maastrichtian to Late Miocene during three main successive littoral shallow marine transgressions of continental extension.; These transgressions happened over the wide pediplanized terrains of South America. These lands exist west of the more positive areas, between the Brazilian Shield and the foreland massifs that were settled in the more westernwards areas. Later, these regional foreland massifs were coupled and raised to the Andean Orogen Belt during the last 5 million years.; The extensive intracratonic pediplanized low topographic relief areas were the reservoirs of siliciclastic littoral shallow marine succession deposits during the three successive widespread vast continental littoral shallow marine transgressions.; The first transgression began at the Latest Campanian-Senonian and/or Early Maastrichtian time. After this episode, the sedimentary depositional systems continued during the Cenozoic until the Latest Miocene. These successions constitute a major allostratigraphic unit. The limit with underlying units is the regional unconformity between the regional volcanic event (Jurassic-Cretacic and interleaved eolianite sandstones) at the base and the undifferentiated Quaternary sediments (called as the Pampeano and Post-Pampeano Formations sensu lato). Based on many facies analyses there had been checked out different levels in the eustatic sea level variations within the allostratigraphic unit.; Three major stages of extensional climax were recognized and related to the stages of conspicuous eustatical sea-level variations. They happened during the Latest Senonian-Paleocene, Eocene and Miocene.; The first transgression occurred during the Upper Cretaceous-Paleocene although the sedimentary deposits related to this event are scarce, which are only a few meters in thickness. However, the Upper Cretaceous-Paleocene succession is very well recognized in the actual pre-Andean zone in the north-west of Argentina and Bolivia (the Sierras Subandinas and the meridional imbricated fault systems just joint to the actual orogen, i.e., Quebrada de Humahuaca outcrops).; During the Eocene and Middle to Latest Miocene occurred the second and third extensive regional littoral shallow marine transgressions. They are present either in well log registers as in most widespread outcrops on the entire Southamerican continent.; The regional analysis led to the deduction of long periods of tectonic quiescence, at least three of them. They may be inferred and synchronously related with eustatic highstand sea level variations that occurred during the Late Paleocene-Early Eocene, Latest Eocene-Early to Mid Oligocene and Middle-to-Late Oligocene-Early Miocene.; The structural style is related with major extensional N-S strike faultings (regional tilted and faulting blocks). On the other hand, quite a number of strike-slip faults (mainly of regional characteristic) are present crossing the area. They have a clear influence on the accommodation and transfer zones of the rift basin system. The strike is north-west to south-east on the border of the basin, to the west, in the contact with the Pampean Ridges and the narrow-meridionally-extense Sub-Andean folded trend (mainly Paleozoic units belonging to the so-called Sierras Subandinas geological province). Also, at the western edge of thestudied area, there exist many large shear zones and upthrust faults. The strike-slip regional faults dislocated the Pampean and Sub-Andean blocks due to the interaction of crossing regional tilted and fault blocks. For this reason, an en echelon regional block model is characteristic. Incipient contaminated igneous activities were associated with this cortical weak zones.; Domes, needles and necks of volcanic and sub-volcanic origin appear as the landscape of the region. A part of the igneous activity was dated on Latest Pliocene although mainly corresponding to Pleistocene and Holocene. This deduction is obvious because their morphological constitution was never eroded. The volcanic aparatous are morphologically unmodified from their extrusion to present days.; All the studied successions seem to resemble a long persisting erosive, transportation and deposition episode. This phenomenon is linked to a large regional (continental) unconformity dated at Late Cretaceous. The entire analyzed sedimentary succession deposits and their siliciclastic facies associations correspond clearly to a “heterolithic facies succession” which is very common within persisting tide-dominated depositional systems (passive margins). In fact, this is what happened during Cenozoic times (Torra, 1998b, 2001a). The heterolithic Miocene facies deposits constitute one of the best continental exposed examples.; Paleogeographical evidence showed that the Paranense and Amazonic Sea transgressions had been a littoral shallow marine connection during long time from Middle to Late Miocene. During the Late Cretaceous and Eocene periods marine connections were also active in the region. This fact is strongly supported by the tectonic and geomorphological framework of the proto-Southamerican continent, fossil remains and similar sedimentary deposits.; The geochemical results showed an outstanding similarity among the three sandy-muddy successions herein studied. Both major and trace elements always show the same geochemical patterns. Specially mentioned are the elements gallium, cesium, chromium, barium, vanadium, thorium, zirconium, rubidium and strontium because they present very constant values through all successions.; The Paranense and Amazonic epicontinental seas had been connected to the Pacific Ocean during the three marine episodes. The connections were formed by narrow inter-mountain valleys, present in the pre-Andean foreland massifs. These events occurred prior to the main orogenesis elevation of the Andean orogen belt in the last 5 to 1 Ma (Pliocene-Latest Pleistocene).; This paper shows, for the first time, a synthetic stratigraphical-geochemical “regional model” for the Chaco Paraná Basin rift system which should be largely improved in later studies. The Chaco Paraná Basin carries many unexamined-unexplored natural resources which need more regional and local studies for their evaluation. This is in spite of the area that has the problem of a significative vegetation coberture and scarce good outcrops. The development of modern techniques of data acquisition will help to overcome these difficulties.

Seismic response analysis of organic reef in gentle slope of rift basin

Analyzing the formation and sediment characteristics of gentle slope, the authors elaborate formation mechanism of organic reef and characteristics of reservoir in gentle slope of rift basin. Using the forward model of seismic exploration, the study provides the objective judgment for the exploration of organic reef reservoir in gentle slope of rift basin.

Mechanism of diagenetic trap formation in nearshore subaqueous fans on steep rift lacustrine basin slopes—A case study from the Shahejie Formation on the north slope of the Minfeng Subsag, Bohai Basin, China

Diagenetic traps in conglomerate in nearshore subaqueous fans in the steep slope zones of rift basins have been important exploration targets for subtle reservoirs in eastern China. However, the mechanism of how those traps were formed is not clear, which inhibits further exploration for this type of reservoir. In order to solve the problem, we take as an example nearshore subaqueous fans in the upper part of the fourth member of the Shahejie Formation （Es4s） on the north slope of the Minfeng Subsag in the Dongying Sag. Combining different research methods, such as core observation, thin section examination, scanning electron microscope （SEM） observation, fluid-inclusion analysis, carbon and oxygen isotope analysis of carbonate cements, and analysis of core properties, we studied the genetic mechanisms of diagenetic traps on the basis of diagenetic environment evolution and diagenetic evolution sequence in different sub/micro-facies. Conglomerate in Es4s in the north Minfeng Subsag experienced several periods of transition between alkaline and acidic environments as ＂alkaline-acidic-alkaline-acidic-weak alkaline＂. As a result, dissolution and cementation are also very complex, and the sequence is ＂early pyrite cementation / siderite cementation / gypsum cementation / calcite and dolomite cementation- feldspar dissolution / quartz overgrowth quartz dissolution / ferroan calcite cementation / ankerite cementation / lime-mud matrix recrystallization / feldspar overgrowth carbonate dissolution / feldspar dissolution / quartz overgrowth / pyrite cementation＂. The difference in sedimentary characteristics between different sub/micro-facies of nearshore subaqueous fans controls diagenetic characteristics. Inner fan conglomerates mainly experienced compaction and lime-mud matrix recrystallization, with weak dissolution, which led to a reduction in the porosity and permeability crucial to reservoir formation. Lime-mud matrix recrystallization results in a rapid decrease in porosity and permeability in inner fan conglomerates in middle-to-deep layers. Because acid dissolution reworks reservoirs and hydrocarbon filling inhibits cementation, reservoirs far from mudstone layers in middle fan braided channels develop a great number of primary pores and secondary pores, and are good enough to be effective reservoirs of hydrocarbon. With the increase of burial depth, both the decrease of porosity and permeability of inner fan conglomerates and the increase of the physical property difference between inner fans and middle fans enhance the quality of seals in middle-to-deep layers. As a result, inner fan conglomerates can be sealing layers in middle-to-deep buried layers. Reservoirs adjacent to mudstones in middle fan braided channels and reservoirs in middle fan interdistributaries experienced extensive cementation, and tight cemented crusts formed at both the top and bottom of conglomerates, which can then act as cap rocks. In conclusion, diagenetic traps in conglomerates of nearshore subaqueous fans could be developed with inner fan conglomerates as lateral or vertical sealing layers, tight carbonate crusts near mudstone layers in middle fan braided channels as well as lacustrine mudstones as cap rocks, and conglomerates far from mudstone layers in middle fan braided channels as reservoirs. Lime-mud matrix recrystallization of inner fan conglomerates and carbonate cementation of conglomerates adjacent to mudstone layers in middle fan braided channels took place from 32 Ma B.R to 24.6 Ma B.P., thus the formation of diagenetic traps was from 32 Ma B.R to 24.6 Ma B.R and diagenetic traps have a better hydrocarbon sealing ability from 24 Ma B.P.. The sealing ability of inner fans gradually increases with the increase of burial depth and diagenetic traps buried more than 3,200 m have better seals.

Tracking sources and paleotectonic settings of Mesozoic sandstones in interlinked rift basins of western India:An integrated approach using petrography and heavy mineral chemistry

Based on integration of field,petrographic and heavy mineral chemical data,this study highlights the source and tectonic setting of the Mesozoic sandstones of Kutch,Saurashtra,Narmada and Cambay basins at the western margin of India,formed by the progressive splitting of the eastern Gondwanaland.The Kutch Basin is dominated by arkosic sandstone,whereas Saurashtra,Narmada and Cambay basins show the predominance of sub-arkose and sub-litharenite.The modal analyses of framework grains in Kutch sandstones indicate basement uplift and transitional continent settings.In contrast,the sandstones of Saurashtra,Narmada and Cambay basins bear imprints of recycled orogenic and craton interior belts.The presence of abraded and detrital quartz overgrowth and rounded zircons in most sandstones reveal the recycling of sediments in these basins.Tourmaline and rutile mineral compositions constrain the possible lithology of source rocks.The tourmaline mineral chemistry(Ca-Fe_(tot)-Mg plot)suggests the derivation of sediments from various sources,including Li-poor granitoids associated with pegmatites,aplites,Ca-poor metapelites,metapsammites and quartz-tourmaline-rich granitic rocks.The compositions of rutile grains(Cr vs.Nb plot)in sandstones indicate metapelitic sources.The gamut of all mineral chemical data supports the predominance of sediment sources from quartzo-feldspathic rocks with minor inputs from mafic rocks.Based on available paleocurrent data and correlation of source compositions,we infer that the Mesozoic sediments of Kutch,Saurashtra,Narmada and Cambay basins were primarily sourced by various lithologies of the Aravalli Craton.The Narmada Basin possibly received additional sediment input from the Bundelkhand Craton.

Palaeogeographic reconstruction of a fluvio-marine transitional system in Narmada rift basin, India——Implications on Late Cretaceous global sea-level rise

Rising sea-levels in tectonically active epicontinental basins often lead to varied depositional settings and palaeogeography, mostly influenced by the net accommodation resulting from mutual interference of the extent and nature of landward encroachment by the sea and the net sedimentation. The Cenomanian Nimar Sandstone Formation, Bagh Group, Narmada rift basin, uniquely portrays the effect of sea-level rise within an intra-cratonic setting and attributes to the corresponding palaeogeographic changes in west-central India. An integrated sedimentological–sequence-stratigraphic study of the broadly fining-upward Nimar Sandstone Formation(thickness~ 20–30 m) depicts the actual nature of changeover from a fluvial to a marine-dominated transitional depositional setting. Detailed sedimentological study reveals total seventeen facies, grouped in five facies associations, viz., the channel-fill facies association(FA-1), the overbank facies association(FA-2), the fluvial-dominated fluvio-tidal facies association(FA-3), the tide-dominated fluvio-tidal facies association(FA-4), and the shoreface facies association(FA-5). Overall facies architecture indicates a west-to-eastward marine encroachment, resulting in stacking of three distinct palaeo-depositional conditions:(i) an initial fluvial system with channel and overbank, changing into a tideinfluenced fluvial bay-head delta in the inner estuary, followed by(ii) marine encroachment leading to a tidedominated central estuary with inter-to sub-tidal settings, and finally,(iii) with further intense marine encroachments, a wave-reworked open shore condition in the outer estuary zone. The overall fining-up succession with a systematic change from fluvial to marine-dominated depositional systems points to a landward shift of the shoreline, signifying a major transgressive event correlated to the Cenomanian global sea-level rise. Characteristic stratal stacking patterns point to four coarsening-and fining-up hemicycles, embedded within the major transgressive succession. These high-frequency cycles attest to the varied interplay of sedimentation, tectonics and sea-level changes, and the resultant net accommodations. A palaeogeographic model is proposed based on the high-frequency transgressive–regressive hemicycles, which envisages the evolution of the depositional environments in relation to the Cenomanian eustatic rise in the intra-cratonic riftogenic fluvio-marine transitional basinal setup.

A near-shore clastic-carbonate mixing mode in a continental rift basin(early Oligocene,eastern Shijiutuo Uplift,Bohai Bay Basin,China):sedimentology,reservoir characteristics and exploration practice

A comprehensive sedimentary and reservoir analysis was conducted based on seismic,well logging,core and relative test data,taking Members 1 and 2 of Shahejie Formation of the early Oligocene in the steep slope belt,eastern Shijiutuo Uplift(STU),Bohai Bay Basin(BBB)as a case.The study indicates that a near-shore mixed fan deposit formed in the study area and developed characteristics and pattern of a high-quality reservoir.The mixed clastic-carbonate rocks constitute Members 1 and 2 of Shahejie Formation which developed along the steep slope belt and is named as a near-shore mixed fan.The mixed fan of the study area is mainly composed of microfacies of proximal channel,mixed deposited channel,mixed clastic beach,mixed bioclastic(grain)beach,with vertical multi-stage superimposition feature,and basically a similar shape as modern near-shore fans.It constitutes a new depositional type developing in the steep slope belt of a characteristic and complex lacustrine rift basin in the study area.This mixed fan in the steep slope of eastern STU is controlled by comprehensive factors including tectonics,clastic material supply,climate,palaeogeomorphology and hydrodynamic conditions.The reservoir quality of Members 1 and 2 of Shahejie Formation of eastern STU is,however,actually controlled by the sedimentary environment and diagenesis processes.Coarse-grained mixed rocks of near-shore fans,rich in bioclastics,can form excellent reservoirs,characterized by resistance to compaction,easy to dissolution,little influenced by burial depth and high production of oil and gas,which enable them become key exploration targets of medium-deep strata of BBB.Analyses of high-quality reservoir,its controlling factors and the oil and gas exploration implications of the near-shore mixed fan developing in the study area give a deeper insight into discussions of the same type of mixed rocks of other lacustrine rift basins worldwide.