Subaqueous Fans Research Articles

Seismic geomorphology study of the Paleogene Hetaoyuan Formation, central-south Biyang Sag, Nanxiang Basin, China

The main objective of this article is to demonstrate the utility of stratal slice images for exploring the sequence stratigraphy and sedimentology of complex depositional systems. A seismic sedimentological study was performed to map sediment dispersal characteristics of the Paleogene Hetaoyuan Formation in the central-south Biyang Sag, located in the Nanxiang Basin of China. The Biyang Sag is underlain by a productive, petroleum-prone, non-marine Cenozoic-age stratigraphic section. Data including wireline logs and 3D seismic profiles are available for this area. The main study strata, the upper member 3 of the Hetaoyuan Formation, were divided into four fourth-order sequences. Four types of depositional systems, near-shore subaqueous fans, braided deltas, slumped turbidite fans and shallow to deep lacustrine systems, were identified from well-based analysis of sedimentary facies. The seismic sedimentological study of the Hetaoyuan Formation in this study area highlights the distribution of the most important reservoirs. The gentle northward-sloping tectonic zone is composed of braided deltas. The steeply sloping south tectonic zone is composed of near-shore subaqueous fans. In the deep central depression of the Biyang Sag, the Hetaoyuan Formation is composed of slumped turbidite fans and shallow to deep lacustrine deposits. The predicted most favorable exploration sedimentary facies are braided delta fronts, middle fans of near-shore subaqueous fans and slumped turbidite fans.

Seismic sedimentology of conglomeratic sandbodies in lower third member of Shahejie Formation (Palaeogene) in Shengtuo area, East China

The sand-conglomerate fans are the major depositional systems in the lower third member of Shahejie Formation in Shengtuo area, which formed in the deep lacustrine environment characterized by steep slope gradient, near sources and intensive tectonic activity. This work was focused on the sedimentary feature of the glutenite segment to conduct the seismic sedimentology research. The near-shore subaqueous fans and its relative gravity channel and slump turbidite fan depositions were identified according to observation and description of cores combining with the numerous data of seismic and logging. Then, the depositional model was built depending on the analysis of palaeogeomorphology. The seismic attributes which are related to the hydrocarbon but relative independent were chosen to conduct the analysis, the reservoir area of the glutenite segment was found performing a distribution where the amplitude value is relatively higher, and finally the RMS amplitude attribute was chosen to conduct the attribute predicting. At the same time, the horizontal distribution of the sedimentary facies was analyzed qualitatively. At last, the sparse spike inversion method was used to conduct the acoustic impedance inversion, and the inversion result can distinguish glutenite reservoir which is greater than 5 m. This method quantitatively characterizes the distribution area of the favorable reservoir sand.

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.P. to 24.6 Ma B.P., thus the formation of diagenetic traps was from 32 Ma B.P. to 24.6 Ma B.P. 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.

Paleocene sequence stratigraphy and depositional systems in the Lishui Sag, East China Sea Shelf Basin

The East China Sea Shelf Basin (ECSSB) is a typical back-arc rift basin and is divided further by a series of sags and basement ridges. The Lishui Sag on the southwest margin of the ECSSB, is underlain by a productive, gas-prone, marine Paleocene section. In combination with sequence stratigraphy and seismic geomorphology, an integrated approach was applied to map the high-frequency sequences and systems tracts of the Lower E1m (57.2–56.5 Ma) and Upper E1l strata (60–57.2 Ma) by using well, two-dimensional and three-dimensional seismic data. Our approach involves documentation of seismic stratal geometries, seismic facies and core lithofacies. Seismic truncation, offlap, onlap and downlap geometries were used for identifying two depositional sequences with four systems tracts in the Upper E1l and Lower E1m strata. Two sedimentary facies were recognized from core and log data and identified as tidal sand ridges and delta mouth bar deposits. The depositional facies were imaged from a series of strata slices obtained by 3D survey. Five types of depositional systems were recognized based on the geometry change expressed on strata slices. In stratigraphic order these are (a) incised channels on the inner shelf and subaqueous fans on the middle shelf, (b) diffuse, elongated linear tidal sand ridges on the continental shelf and turbidity flows on the slope, (c) incised valley, prograding shelf edge delta on the continental shelf and shelf edge and delta front turbidites on the slope, (d) incised valley fills, fluvial aggradation and fluvial dominated mouth bar on the continental shelf, and (e) shoreface barrier bar and sand ridges that are parallel to the shoreline. This integrated analysis provides us an opportunity to evaluate the exploration significance in the stratigraphic framework and improve the accuracy of petroleum plays prediction.

Noninvasive Methods for Hydraulic Characterization of Density Currents with Application to Submarine Fan Experiments

AbstractLaboratory experiments are a convenient way to study morphodynamic processes associated with subaqueous fans. However, the flows driving such systems can be difficult to hydraulically characterize due to the shallowness of the flows, the spatial and temporal dynamics of the autogenic channels, and the mixing at the interface of the ambient fluid and density current. The objective of this study is to develop a set of methods to quantify hydraulic variables in laterally unconstrained and morphologically active density currents. Herein, a methodology is presented capable of characterizing a density current flow field with direct application to submarine fan experiments. This is done with a combination of: (1) particle tracking velocimetry (PTV) analysis of tracers within the density current, (2) flow width measurements from overhead images, and (3) solution of the mass and volume conservation equations in one-dimension to obtain layer-averaged velocity, depth, and fractional excess density.

Refining the pattern and style of Cordilleran Ice Sheet retreat: palaeogeography, evolution and implications of lateglacial ice‐dammed lake systems on the southern Fraser Plateau, British Columbia, Canada

Decay of the last Cordilleran Ice Sheet (CIS) near its geographical centre has been conceptualized as being dominated by passive downwasting (stagnation), in part because of the lack of large recessional moraines. Yet, multiple lines of evidence, including reconstructions of glacio‐isostatic rebound from palaeoglacial lake shoreline deformation suggest a sloping ice surface and a more systematic pattern of ice‐margin retreat. Here we reconstructed ice‐marginal lake evolution across the subdued topography of the southern Fraser Plateau in order to elucidate the pattern and style of lateglacial CIS decay. Lake stage extent was reconstructed using primary and secondary palaeo‐water‐plane indicators: deltas, spillways, ice‐marginal channels, subaqueous fans and lake‐bottom sediments identified from aerial photograph and digital elevation model interpretation combined with field observations of geomorphology and sedimentology, and ground‐penetrating radar surveys. Ice‐contact indicators, such as ice‐marginal channels, and grounding‐line moraines were used to refine and constrain ice‐margin positions. The results show that ice‐dammed lakes were extensive (average 27 km2; max. 116 km2) and relatively shallow (average 18 m). Within basins successive lake stages appear to have evolved by expansion, decanting or drainage (glacial lake outburst flood, outburst flood or lake maintenance) from southeast to northwest, implicating a systematic northwestward retreating ice margin (rather than chaotic stagnation) back toward the Coast Mountains, similar in style and pattern to that proposed for the Fennoscandian Ice Sheet. This pattern is confirmed by cross‐cutting drainage networks between lake basins and is in agreement with numerical models of North American ice‐sheet retreat and recent hypotheses on lateglacial CIS reorganization during decay. Reconstructed lake systems are dynamic and transitory and probably had significant effects on the dynamics of ice‐marginal retreat, the importance of which is currently being recognized in the modern context of the Greenland Ice Sheet, where >35% of meltwater streams from land‐terminating portions of the ice sheet end in ice‐contact lakes.

Characteristics and origin of abnormally high porosity zones in buried Paleogene clastic reservoirs in the Shengtuo area, Dongying Sag, East China

There are three abnormally high porosity zones developed in buried Paleogene nearshore subaqueous fan and sublacustrine fan clastic reservoirs at 2,800–3,200 m, 3,250–3,700 m and 3,900–4,400 m, respectively, within the Shengtuo area of the Dongying Sag. Here the porosity of reservoirs buried deeper than 4,000 m can still be greater than 20%. Investigation of these three abnormally high porosity (AHP) zones in the 3rd to 4th member of the Paleogene Shahejie Formation in the Shengtuo area was carried out with utilization of core observation, thin section identification, SEM observation, image analysis, core physical property testing and other technical methods. The results show that, the AHP zones in 2,800–3,200 m and 3,250–3,700 m are visible pores primary AHP zones dominated by significant primary intergranular pores (more than 50% of the total porosity), while secondary pores and micropores in authigenic clays may develop in some reservoirs. AHP reservoirs in the AHP zone of 3,900–4,400 m are dominated by micropores in matrix, visible pores are mainly grain dissolution pores but with low absolute content (< 1%), so this zone belongs to the micropores primary AHP zone. The genesis of the three AHP zones was studied to distinguish between porosity enhancement and porosity preservation. Our research shows that, in deeply buried clastic reservoirs in the Shengtuo area, mineral dissolution occurred in a relatively closed diagenetic system with high temperature and high salinity. Reservoir rocks underwent extensive feldspar dissolution, while detrital carbonate grains and carbonate cements show no evidence of extensive dissolution. Although significant feldspar dissolution pores developed, feldspar dissolution enhanced porosity only a little due to the precipitation of almost isovolumetric dissolution products in the nearby primary intergranular pores in forms of authigenic clays and quartz cements. Net enhanced porosity originating from feldspar dissolution is generally less than 0.25%. Thus, the subsurface dissolution has little impact on the mid-deep buried high porosity reservoirs. Reservoirs in braided channels of middle fans in sublacustrine fans and reservoirs in the middle-front of fan bodies of nearshore subaqueous fans provide the basis for the development of AHP zones. The shallow development of fluid overpressure and early hydrocarbon emplacement have effectively retarded compaction and carbonate cementation, so that the high porosity in the superficial layers is preserved in the mid-deep layers. These are the main controlling factors in the development of AHP zones.

Depositional System of Aershan Formation in Tanan Sag of Erlian Basin

Erlian Basin was composed of many small faulted lacustrine basins in early Cretaceous period. Delta systems were developed and were the most important sandstone reservoir in the gentle slope of faulted lacustrine basins. The paper took an example of Tanan sag, which was one of the most representative faulted sags in Erlian Basin. This paper analyzed the depositional characteristics of nearshore subaqueous fan systems and fan deltas systems, based on core data, logging data and grain size probability curve. Fan delta-lacustrine deposition system and near shore turbidite fan-lacustrine deposition system were the most developed deposition system in the study area. The lake basin belonged to “narrow basin deep water” type. Mud stone of Deep lake facies and sand stone of fan delta facies in Aershan formation are the best lithology association which would be explored oil and gas in Tanan sag.

Subsurface evidence for late Mesozoic extension in western Mongolia: tectonic and petroleum systems implications

AbstractNew seismic reflection profiles from the Tugrug basin in the Gobi‐Altai region of western Mongolia demonstrate the existence of preserved Mesozoic extensional basins by imaging listric normal faults, extensional growth strata, and partially inverted grabens. A core hole from this region recovered ca. 1600 continuous meters of Upper Jurassic – Lower Cretaceous (Kimmeridgian–Berriasian) strata overlying Late Triassic volcanic basement. The cored succession is dominated by lacustrine and marginal lacustrine deposits ranging from stratified lacustrine, to subaqueous fan and delta, to subaerial alluvial‐fluvial environments. Multiple unconformities are encountered, and these represent distinct phases in basin evolution including syn‐extensional deposition and basin inversion. Prospective petroleum source and reservoir intervals occur, and both fluid inclusions and oil staining in the core provide evidence of hydrocarbon migration. Ties to correlative outcrop sections underscore that, in general, this basin appears to share a similar tectono‐stratigraphic evolution with petroliferous rift basins in eastern Mongolia and China. Nevertheless, some interesting contrasts to these other basins are noted, including distinct sandstone provenance, less overburden, and younger (Neogene) inversion structures. The Tugrug basin occupies an important but perplexing paleogeographic position between late Mesozoic contractile and extensional provinces. Its formation may record a rapid temporal shift from orogenic crustal thickening to extensional collapse in the Late Jurassic, and/or an accommodation zone with a Mesozoic strike‐slip component.

Sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice‐contact fan deposits: A Pleistocene example (Ireland)

AbstractA pit located near Ballyhorsey, 28 km south of Dublin (eastern Ireland), displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice‐contact fan deposits. The coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies. The lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space, mainly controlled by the position of an overlying ice roof during ice‐bed decoupling. These sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments. The sorted sediments consist of fine‐grained laminated facies, cross‐laminated sand and channelized gravels, and are interpreted as subglaciofluvial sediments deposited within a subglacial de‐coupled space. The subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds, triggered by fluid overpressure and shear stress during episodes of ice/bed recoupling (clastic dykes and folds). The upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake, forming a thick sedimentary wedge erosively overlying the subglacial deposits. Gravel facies and large‐scale trough bedding sand are observed within this proximal wedge, while normally graded sand beds with developed bedforms are observed further downflow. The building of the prograding ice‐contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading (normal faults). This study facilitates the recognition of subglacial/submarginal depositional environments formed, in part, during localized ice/bed coupling episodes in the sedimentary record. The sedimentary sequence exposed in Ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation, the impact on the subglacial drainage system and the consequences on the Irish Sea Ice Stream flow mechanisms.

Bolla Bollana boulder beds: A Neoproterozoic trough mouth fan in South Australia?

AbstractThe Bolla Bollana Formation is an exceptionally thick (ca 1500 m), rift‐related sedimentary succession cropping out in the northern Flinders Ranges, South Australia, which was deposited during the Sturtian (mid Cryogenian) glaciation. Lithofacies analysis reveals three distinct facies associations which chart changing depositional styles on an ice‐sourced subaqueous fan system. The diamictite facies association is dominant, and comprises both massive and stratified varieties with a range of clast compositions and textures, arranged into thick beds (1 to 20 m), representing stacked, ice‐proximal glaciogenic debris‐flow deposits. A channel belt facies association, most commonly consisting of normally graded conglomerates and sandstones, displays scour and fill structure of ca 10 m width and 1 to 3 m depth: these strata are interpreted as channelized turbidites. Rare mud‐filled channels in this facies association bear glacially striated lonestones. Finally, a sheet heterolithics facies association contains a range of conglomerates through sandstones to silty shales arranged into clear, normally graded cycles from the lamina to bed scale. These record a variety of non‐channelized turbidites, probably occupying distal and/or inter‐channel locations on the subaqueous fan. Coarsening and thickening‐up cycles, capped by dolomicrites or mudstones, are indicative of lobe build out and abandonment, potentially as a result of ice lobe advance and stagnation. Dropstones, recognized by downwarped and punctured laminae beneath pebbles to boulders in shale, or in delicate climbing ripple cross‐laminated siltstones, are clearly indicative of ice rafting. The co‐occurrence of ice‐rafted debris and striated lonestones strongly supports a glaciogenic sediment source for the diamictites. Comparison to Pleistocene analogues enables an interpretation as a trough mouth fan, most probably deposited leeward of a palaeo‐ice stream. Beyond emphasizing the highly dynamic nature of Sturtian ice sheets, these interpretations testify to the oldest trough mouth fan recorded to date.

Facies Evolution and Distribution of Subaqueous Fans Complex in Biyang Sag, Eastern China

Hetaoyuan formation of Palaeogene in Biyang sag experienced the process of sedimentation during the main depressing period. Lithological traps were formed by the Sandstone-conglomerate bodies as favorite targets for oil and gas exploration in the southeast of Biyang sag. In this study, seismic profile, cores and well loggings as main data are used to analyze the micro-facies of subaqueous fan complex system. Methods such as Phasing concertion, spectrum decomposition, and strata slice play important roles in the study of facies evolution and distribution in high-resolution sequence framework of the upper member of the third Hetaoyuan formation. Subsequently, All of the coarse-grained turbid sandstone and the distributary channels sediments are potential reservoirs for oil and gas storage.

Middle Wisconsin glacial advance into the Appalachian Plateau, Sixmile Creek, Tompkins Co., NY

Areal mapping of the middle Wisconsin varved clay site along Sixmile Creek near Ithaca, New York, has revealed a much more widespread and varied array of sediments than previously thought. Lacustrine clays, some varved, are interbedded with sands and gravels interpreted as sub-aqueous fan deposits, and both are overlain by a deformation till. Nine radiocarbon dates indicate a 34–37 14C ka BP age for the lacustrine sediment, with the deformation till less than a few thousand years younger. Beneath this sequence is a deposit dated at ±42 14C ka BP. Both strata represent a tundra climate with a mean July temperature of about 10°C. The Sixmile Creek deformation till must correlate with the 35 14C ka BP till along the Genesee River, 125km to the NW, and defines a Cherrytree stade glacial advance into the Appalachian Plateau, much further south than what has generally been accepted. Such an advance would require drainage from a proglacial lake in the western Ontario basin to flow westward instead of northeastward. The Sixmile strata suggest a colder than accepted middle Wisconsin stage. Recent data indicate that this stage is one of progressive cooling, with large climatic fluctuations.

Lateral and vertical facies relationships of bedforms deposited by aggrading supercritical flows: From cyclic steps to humpback dunes

The preservation of bedforms related to supercritical flows and hydraulic jumps is commonly considered to be rare in the geologic record, although these bedforms are known from a variety of depositional environments. This field-based study presents a detailed analysis of the sedimentary facies and stacking pattern of deposits of cyclic steps, chutes-and-pools, antidunes and humpback dunes from three-dimensional outcrops. The well exposed Middle Pleistocene successions from northern Germany comprise glacilacustrine ice-contact subaqueous fan and glacial lake-outburst flood deposits. The studied successions give new insights into the depositional architecture of bedforms related to supercritical flows and may serve as an analogue for other high-energy depositional environments such as fluvial settings, coarse-grained deltas or turbidite systems.Deposits of cyclic steps occur within the glacial lake-outburst flood succession and are characterised by lenticular scours infilled by gently to steeply dipping backsets. Cyclic steps formed due to acceleration and flow thinning when the glacial lake-outburst flood spilled over a push-moraine ridge. These bedforms are commonly laterally and vertically truncated and alternate with deposits of chutes-and-pools and antidunes. The subaqueous fan successions are dominated by laterally extensive sinusoidal waveforms, which are interpreted as deposits of aggrading stationary antidunes, which require quasi-steady flows at the lower limit of the supercritical flow stage and high rates of sedimentation. Humpback dunes are characterised by downflow divergent cross-stratification, displaying differentiation into topsets, foresets and bottomsets, and are interpreted as deposited at the transition from subcritical to supercritical flow conditions or vice versa. Gradual lateral and vertical transitions between humpback dunes and antidune deposits are very common.The absence of planar-parallel stratification in all studied successions suggests that the formation of these bedforms is suppressed in flows characterised by hydraulic jumps under highly aggradational conditions. The large-scale lateral and vertical successions of bedforms are interpreted as representing the temporal and spatial evolution of the initial supercritical flows, which was strongly affected by the occurrence of hydraulic jumps. Small-scale facies changes and the formation of individual bedforms are interpreted as controlled by fluctuating discharge, bed topography and pulsating unstable flows.

Development of Pleistocene glaciomarginal lake in the foreland of the Sudetes (SW Poland)

Numerous glaciomarginal lakes existed during the Pleistocene in the Sudetes and their foreland (SW Poland) because of damming off by an ice sheet. In the mountain area they were formed in the valleys or intermountain basins during the phase of maximum ice extent. In the mountain foreland they were developed at the front of the more active ice margin in the period preceding the maximum ice sheet advance. The latter lakes were less confined by the topography and their contact with the ice sheet was often considerably larger. This caused their different development.The sedimentological development of a small lake of this type, located near Mokrzeszów, was studied. The lake was characterized by high variability in water level and high energy depositional processes. Because of its small size, the lake responded very quickly to all changes in external factors. The lake became filled mainly by a coarse-grained delta. This happened in several phases because the water level fluctuated considerably owing to oscillations in the ice sheet extent. During lowstands, the delta became incised. Locally, subaqueous fans were formed. The lake basin eventually became overridden by the ice sheet.

The Depositional System of Dongying Formation and Subtle Reservoir Exploration in Sha’nan Structural Belt Qikou Sag of Bohai Bay Basin, China

The subtle reservoir has become an important exploration target within the Bohai Bay Basin, especially in the Qikou sag. In order to guide subtle reservoirs exploration in offshore area geological analysis to Sha’nan structural belt is carried out in this paper. Qikou sag has large thickness, high abundance, ideal type hydrocarbon source rock. By unconformity recognition technique and paleotectonic analysis technique, results show that tectonic uplift had provided sedimentary source to the depression around it after Es1. By 3-D seismic interpretation technique and Seismic facies analysis, the sedimentary facies was confirmed. Geological analysis results show that sand bar, subaqueous fan is developed around it in Ed3. With the new geological analysis data sedimentary model of Sha’nan structural belt has been built in this paper. The sand bar and subaqueous fans showed a good reservoir potential, which are advantageous to oil accumulations. The southern part of Sha’nan structural belt has good hydrocarbon exploration potential.

Construction of an evolutionary deglaciation model for the Irish midlands based on the integration of morphostratigraphic and geophysical data analyses

AbstractAlternative, established models for the deglaciation of the midlands of Ireland are tested against an interpretation of a suite of deglacial sediments covering an area of 600 km2 in the east central midland area. Interpretation of the sediments is based on geomorphological mapping, lithostratigraphic characterization of exposures and geotechnical data supported by electrical resistivity tomography (ERT) and ground penetrating radar (GPR). GPR depicted small‐scale sedimentological and deformational structures within low‐conductivity soft sediments, such as cross‐bedding, planar bedding, channel‐like features and faulting planes, and revealed the internal architecture of eskers, glaciodeltas, subaqueous fans and raised bogs. ERT data permitted the detection of depth to bedrock and the lithological characterization of unconsolidated sediments. The ten sites presented were surveyed by traditional mapping methods and/or geophysical techniques. This allowed the construction of a local model of the deglaciation of the area which recognized five main stages. An ice sheet covering most of Ireland withdrew as a single body as far as the midlands. At this stage, two main directions of ice retreat are identified from the spatial distribution of meltwater/overflow channels, esker and morainic ridges, and ice‐marginal glaciolacustrine deposits. A pattern of deglacial sedimentation into an expanding ice‐marginal glacial lake is depicted. The glacial lake was dammed to the west by two ice dome fronts, one decaying to the north‐west and another to the south‐west, and by the Shannon Basin watershed to the east. Glacial lake outlets identified along the watershed and the altitude of the topset/foreset interface zone depicted in glaciodeltaic deposits allowed the identification of three lake water levels. The highest level is at 87–89 m Ordnance Datum (OD), the second lake level at 84 m OD and the third at 78 m OD. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Genetic types of Daxing conglomerate bodies and their controls on hydrocarbons in the Langgu Sag, Bohai Bay Basin, East China

The genetic types, porosity and permeability characteristics, as well as oil-gas production capacity of the Daxing conglomerate bodies in the lower and middle sections of Paleogene Sha 3 Member, in the Langgu Sag of the Bohai Bay Basin are studied by combining various data such as seismic, logging, core analysis and formation testing. Three genetic models of conglomerate bodies including faulted-trough gravity flow, grain flow dominated nearshore subaqueous fans, and debris flow dominated nearshore subaqueous fans are built based on core, seismic and logging phases figured out by analyzing the sedimentary settings. The conglomerate bodies generated in faulted-trough flow are composed of matrix supported conglomerates and grain supported conglomerates, in which various kinds of secondary pores are developed; the conglomerate bodies generated in grain flow dominated nearshore subaqueous fans primarily consist of grain supported conglomerates, of which the reservoir space is mainly dissolved pores within gravels, intercrystal pores and fractures; the conglomerate bodies generated in debris flow dominated nearshore subaqueous fans are mainly composed of matrix supported conglomerates and are dominated by dissolved pores within gravels. The conglomerate bodies with different genesis have different reservoir characteristics and oil/gas production capacities. Among them, the conglomerate bodies generated in grain flow dominated nearshore subaqueous fans have the best porosity, permeability and the highest oil/gas production, thus are the key targets for petroleum exploration.

Sequence Filling Patterns and Petroleum Implications of Lower Cretaceous Series in South Tamtsag Depression

Sequence stratigraphic study of Lower Cretaceous strata in the South Tamtsag Depression of Mongolia identifies one second-order and four third-order sequences. These sequences are controlled by syndepositional fault activities at the slope-break zone of the lacustrine basin. At the initial rift stage of the basin, creation of accommodation space outpaced sediment accumulation and the lake level fell below the structural slope-break zone. Sequences formed in this stage can be divided into lowstand, transgressive and highstand system tracts. At the main rift stage, lake level did not fall below the structural slope-break zone and sequences formed in this stage consist of transgressive and regressive system tracts. Vertical stacking patterns within sequences are expressed by cyclic occurrences of alluvial fan, fan delta and delta deposits in the gentle slope belt located in the western part of the basin, and fan delta, nearshore subaqueous fan and lake deposits in the steep slope belt in the eastern part of the basin. The distribution of sandstone units in the sequences, which are the major target reservoirs, can be easily recognized through the combination of sedimentology analysis and interpretation of seismic reflection profiles. The slope-break zone of the lacustrine basin, which can be identified from seismic reflection profiles, played critical roles on the thickness and lateral extension of the sandbodies in the basin, and thus is the main controlling factor of petroleum reservoirs in the basin. Because mudstone and oil shale developed during the transgressive system tracts are hydrocarbon source rocks and cap rocks, deltaic sandstone units and incision channel fills are the most important petroleum reservoirs in the Tamtsag basin. Finally, this paper divides four types of subtle traps according to the petroleum accumulation model for future exploration.

A Facies Analysis and Sedimentary Architecture of the Paleogene Dainan Formation in the Gaoyou Depression, North Jiangsu Basin, Eastern China

Sedimentary facies of Paleogene Dainan Formation in the Gaoyou Depression, North Jiangsu Basin, as well as their distribution and evolution, are studied based on comprehensive analysis of cores and logging data. The results show that fan delta, delta, near-shore subaqueous fan, and lacustrine facies deposited in the Dainan Formation. Fan delta and near-shore subaqueous fan deposited in the southern steep slope. Delta deposited in the northern gentle slope. The sedimentary evolution of the Dainan Formation could be divided into two periods, in which the lake changed from shallow to deep, and then to shallow, the sedimentary area gradually enlarged, fan delta, delta and near-shore subaqueous fan prograded first, then retrograded, and finally prograded. It also shows that the fan deltaic subaqueous distributary channel and deltaic subaqueous branch channel have sandstones with well physical properties, which are the major reservoir facies belt of hydrocarbon.