Basin-floor Fans Research Articles

(Dis)Order in Submarine Fans at Different Observational Scales: An Example From Unit a of the Laingsburg Formation, Karoo Basin

Perceived order in submarine fans is traditionally used to support simple sequence stratigraphic interpretation of allogenic forcing but evidence is typically limited to qualitative evaluation of the geological dataset. Quantitative analysis of the well-studied ‘Unit A’ basin-floor fan from the Laingsburg Formation, Karoo Basin, South Africa suggests a more complex interpretation. We use Markov facies analysis to test for order in the facies successions, and runs tests and power spectral analyses to assess order in the succession of bed thicknesses. Some bed-by-bed order is detectable in the facies successions, but thickness successions are more complex; no order is detectable as simple consistent bed-by-bed runs of thickening or thinning-upwards. Spectral analysis does detect some significant periodicities, but the periods are different in each vertical section analysed, ranging from 35-45 beds, 50-55 beds and from 60-70 beds in respective vertical sections. This perhaps indicates that complex autogenic compensational stacking processes control the details of how patterns emerge at specific locations, even when larger-scale trends do occur in regional stacking of lobes and lobe complexes and there is no evidence for a single, dominant allogenic control the Karoo submarine fan strata. More comprehensively, this analysis suggests that sequence stratigraphic interpretations of outcrop and core deep-water strata will be more robust with integrated quantitative bed thickness and facies trend analysis as a standard approach. Sequence stratigraphic interpretations of deep-water strata should avoid a model-driven single dominant control approach, and instead use the quantitative analysis to more fully explore the typical complexity of these strata.

Interpreting environments of deposition from facies analysis of outcrop versus seismic reflection data: A cautionary tale from the Mount Messenger Formation, Taranaki Basin (New Zealand)

The lower part of the Mount Messenger Formation, which crops out along the North Taranaki coast (North Island, New Zealand), is widely regarded to display classic examples of exhumed basin floor fan strata. This interpretation is supported by numerous publications on the sedimentary facies and depositional environments inferred for these strata, which are exposed in coastal cliff exposures about 20 m high with limited lateral extent. Consequently, for almost three decades, this succession has served as an analogue and training ground for geoscientists investigating the features of basin floor fan deposits and their reservoir characteristics. Here, we bring together seismic stratigraphy and seismic geomorphology of 2D and 3D seismic reflection datasets, as well as photogrammetric models based on images collected using uncrewed aerial vehicle (UAV) drones from selected locations along the outcrop belt to challenge this paradigm. We analyzed seismic reflection data in an area immediately offshore and down-dip of the strata exposed in the coastal outcrop and found no objective criteria for submarine fan depositional elements based on accepted seismic stratigraphic criteria. Furthermore, mapping of seismic reflectors revealed large submarine channel systems (up to 3 km wide and up to 200 m deep) within the lower part of the Mount Messenger Formation. Submarine channel depositional elements (less than 250 m wide and 25 m thick) are also observed to incise through the outcrops, but they are generally below seismic resolution. In the context of the regional geology, we conclude that the lower Mount Messenger Formation accumulated as continental slope sediments cross-cut by channels rather than as basin floor fans. This study issues a cautionary tale about applying facies analysis to outcrop with limited 3D exposure to interpret sedimentary environments. The spatial distribution of facies and architectural elements viewed in seismic data are invaluable for testing the veracity of facies interpretations.

Seismic attributes and instantaneous spectral analysis-based temperature-porosity simulations: An example of imaging shale gas-bearing basin floor fans of early-cCretaceous delta sequences, Southeast Asian Basin

Basin floor fans (BFF) of deltaic depositional systems are important stratigraphic traps, which are sourced by the regionally developed meandering channel belts and associated deposits of point bar facies. The vertical and lateral changes in the tectonics and sea level adversely affect the temperature (θ) and porosity constraints of petroleum plays within BFF of the lowstand system tracts. These fluctuations impact the sedimentary controls on thickness, lithology-impedance contrast, phase, and frequency of BFF. As a result of the reliance on full-spectrum seismics, quantifying thin-bedded stratigraphic wet gas-bearing intervals (WGI) and light oil-bearing intervals of petroleum systems becomes extremely difficult. This study utilizes seismic attributes and instantaneous spectral analysis-based temperature-porosity reservoir simulations (TPRS) to quantify the BFF of the Early-Cretaceous delta of the Miano gas field, Central Pakistan. The 47-Hz peak-tuned frequency component of spectral decomposition [SpecDecomp] images a 154 km2 NEE-SWW traversed and regionally developed stratigraphic prospect. Linear regression of R2 > 0.80 between spectral decomposition-based tuning frequency of 47-Hz and gross reservoir thickness of BFF validates the presence of gas-bearing shales. The 47-Hz wavelet-based TPRS has resolved the phase reversals and seismic-doublet signatures for thin-bedded shale PETROS at θ of 170°F to 200°F and porosities of 22 % to 32 % of WGI within BFF. TPRS at 47-Hz SpecDecomp profile between the frequency bandwidth spectrums of 12–62-Hz has reappraised the exact morphology of the continental shelf, shelf-slope, and BFF. A 21 m thick aggradational parasequence of BFF was deposited at an angle of 2° during the onset of subsidence and rapid fall, followed by a subsequent standstill of sea-level at θ of 170 to 200°F, ∼26 to 35 % porosities, and laterally fractured subsidized regimes of WGI.. A 20 m thick retrogradational fluvial-dominated facies was deposited at an angle of 35° during a rising followed by a standstill of sea level at a θ of 140 to 165°F, ∼08 % to 18 % porosity, and poorly fractured uplifted compressional regimes of light oil-bearing intervals. The aggradational WGI of Lowstand System Tracts was completely embedded by retrogradational shales of transgressed system tracts from the top of the aggradational and laterally. The uncertainty analysis has prevailed with a moderate-to-good R2 > 0.75 for BFF and a very poor R2 < 0.1 for fluvial-dominated point bar traps. Our workflow has implications in terms of exploring the tectonics and stratigraphic frameworks within the petroleum systems for imaging the most productive zones in a single snapshot of 47-Hz tuning frequency.

A Method for Defining Sedimentary Characteristics and Distributions and Its Application in Qinnan Depression, Bohai Bay Basin

A new method incorporating geophysical analysis and geological analysis is proposed to define the sedimentary characteristics and distributions in basins with few drilling wells to promote the exploration of reservoirs. This method is applied to a study, through which its principles, closed-loop workflow and technologies are introduced in detail and the sedimentary characteristics and distributions of the study area are accurately defined. During the application process of the method, a compatible geological model is established, based on which the seismic data are interpreted and the results derived from the interpretation are further verified via seismic forward modeling. The study results exhibit a successive sand-rich deposition from the retrogradational gully-filling gravity flow deposition including near-shore fans, slope fans and basin-floor fans delimited by different slope break belts in transgressive sequences to the progradational delta deposition in a retrogressive sequence including braided river deltas with a long extension distance and fan deltas developed along a steep slope belt. And the potential reservoirs are located at the point-out sites of sand bodies with lower average P-wave velocities than those of muddy sediments. The proposition and application of this method are of great significance for oil and gas exploration.

Flow transformations, mud partitioning and the variable stratigraphic architecture of basin-floor fan fringes

Highly efficient sediment gravity flows can bypass mid fan channels and lobes and deposit significant volumes of sand, mud and particulate organic matter in outer fan and basin plain settings. The Serpukhovian to Bashkirian fill to the Shannon Basin, western Ireland, includes deep-water fan deposits (Ross Sandstone Fm) that gradationally overlie basin floor shales (Clare Shale Fm). As part of a broader progradational succession, the upward transition from muddy basin floor to sandy fan preserves the stacked deposits of settings present prior to and outboard of mid-fan channels and lobes. Three fully cored boreholes and associated wireline data constrain the facies tracts in an 18 km long panel orientated oblique to original depositional dip. Two distal successions dominated by hybrid event beds (HEBs) are recognised, separated by a prominent condensed section. The lower Cosheen system includes m-thick, tabular HEBs with prominent linked debrites that pass down dip into much thinner sandstones overlain by sand-speckled mudstone caps that thicken distally before thinning. The latter are interpreted as secondary mudflows released following reconstitution of more thoroughly mixed sections of the up-dip linked debrites. Significant bypass and accumulation of mud by this mechanism helped heal local topography and maintain a relatively flat sea floor promoting an overall tabular geometry for the deposits of larger volume hybrid flows reaching the distal sector of the basin. The overlying distal Ross system fringe is characterised by very fine to fine-grained sandstones and is lateral to compensationally-stacked lobes further to the west. It has a progradational (at least initially) stacking pattern, facies transitions developed over shorter length scales, and includes outsized event beds but these are thinner than those in the Cosheen system. Common banding and evidence for turbulence suppression by dispersed clay rather than entrained mud clasts indicate these were transitional flows. In this case, event beds are inferred to taper distally, with significant mud emplaced by plug flow retained as caps to sandy event beds rather than bypassing down-dip. Different flow transformation mechanisms thus impacted how mud was partitioned across the fringe of the two systems and this influenced bed geometries, larger scale bed stacking patterns and stratigraphy. Whereas the flow efficiency concept stresses the ability of flows to carry sand in a basinward direction, it is also imperative to consider the variable efficiency of mud transport given the operation of clay-induced flow transformations. These can either promote bypass or trigger premature fallout of mud with implications for how systems fill accommodation, bed -scale facies transitions and the burial and preservation of particulate organic carbon fractionated along with the clay in deep-water system fringes.

Static Reservoir Simulations and Seismic Attributes Application to Image the Miocene Deep-Water Reservoirs in Southeast Asia

Globally, deep-water reservoir systems are comprised of a variety of traps. Lateral and downdip trapping features include sand pinch-outs, truncation against salt or shale diapirs, and monoclinal dip or faulting with any combination of trapping designs; the potential for massive hydrocarbon accumulations exists, representing significant exploration prospects across the planet. However, deep-water turbidites and submarine fans are two different types of traps, which are developed along the upslope and the basin floor fans. Among these two traps, the basin floor fans are the most prolific traps as they are not influenced by sea-level rise, which distorts the seismic signals, and hence provides ambiguous seismic signatures to predict them as hydrocarbon-bearing zones for future explorations. Therefore, the deep-water channel-levee sand systems and basin floor fans sandstone define economically viable stratigraphic plays. The subsurface variability is significant, and hence, characterizing the thick (porous) channelized-basin floor fans reservoir is a challenge for the exploitation of hydrocarbons. This study aims to develop seismic-based attributes and wedge modeling tools to accurately resolve and characterize the porous and gas-bearing reservoirs using high-resolution seismic-based profiles, in SW Pakistan. The reflection strength slices better delineate the geomorphology of sand-filled channelized-basin floor fans as compared to the instant frequency magnitudes. This stratigraphic prospect has an area of 1180 km2. The sweetness magnitudes predict the thickness of channelized-basin floor fans as 33 m, faults, and porous lithofacies that complete a vital petroleum system. The wedge modeling also acts as a direct hydrocarbon indicator (DHI) and, hence, should be incorporated into conventional stratigraphic exploration schemes for de-risking stratigraphic prospects. The wedge model resolves a 26-m thick hydrocarbon-bearing channelized-basin floor fans lens with a lateral distribution of ~64 km. Therefore, this wedge model provides ~75% correlation of the thickness of the LSL as measured by sweetness magnitudes. The thickness of shale that serves as the top seal is 930 m, the lateral mud-filled canyons are 1190 m, and the thick bottom seal is ~10 m, which provides evidence for the presence of a vibrant petroleum play. Hence, their reveals bright opportunities to exploit the economically vibrant stratigraphic scheme inside the OIB and other similar global depositional systems.

Seismic characteristics and AVO response for non-uniform Miocene reservoirs in offshore eastern Mediterranean region, Egypt

The Eastern Mediterranean region, extending from the Offshore Nile Delta Cone of Egypt to the Levant Basin, is a confirmed hydrocarbon-rich territory with several giant gas discoveries. Numerous gas fields have been discovered in the Miocene reservoirs within the Nile Delta Cone, and the Levant Basin. The Miocene sedimentary sequences in this region are extremely heterogeneous, consisting mainly of turbiditic slope deposits, channels, and basin floor fans that were capped by evaporites formed during the Messinian Salinity Crisis. As a result, the seismic characteristics and interpreted properties of this heterogeneous section are ambiguous. The study area is located in the Offshore North Sinai Basin, where a thick Early Miocene section was deposited midway between the Nile Delta province, which includes the El-Fayrouz discovery, and the Levant Basin, which includes Tamar, Tanin, and several other discoveries. This study uses quantitative seismic interpretations methods, such as amplitude variations with offset and fluid replacement modeling, to assess the seismic acoustic impedance trend with depth. Also, determine the seismic amplitude response for the brine and gas sands reservoir of the Early and Late Miocene section to link the unexplored study area within the North Sinai Offshore Basin with the explored Nile Delta and Levant Basins. In addition to evaluate direct hydrocarbon indicator (DHI) of the dimming seismic amplitude that is compatible with the structure’s last closed contour of the Syrian Arc anticline of the Early Miocene reservoirs (EMT-1 prospect). Different vintages of 2D and 3D seismic data, six wells, and various published data were used in this study. The quantitative interpretation shows the pitfalls of the acoustic impedance trend and seismic response dependency on depth for gas and brine sand, which led to the drilling of the EMT-1 dry well. Also, the fluid replacement, P-wave velocity (Vp), and density (ρ) modeling confirmed that the seismic dimming amplitude was due to a seismic processing artifact, which was corrected by readjusting the overburden Messinian salt processing velocity model. This research concludes that the seismic quantitative interpretations are successfully used to assess the acoustic impedance versus depth and understand DHI pitfalls, as well as the processing workflow that could enhance the seismic image.

Sequence stratigraphic architecture, evolution, and geological controls of an Eocene isolated carbonate platform, NE Sirte Basin, Libya

The sequence stratigraphic architecture, evolution, and controlling mechanisms of a lower and middle Eocene isolated carbonate platform in the Sirte Basin in north-central Libya were interpreted from two- and three-dimensional (2D and 3D) seismic and well data. The platform contains six stratigraphic surfaces that divide the platform sedimentary successions into two stratigraphic sequences, namely Sequence 5 and Sequence 6, and five systems tracts. The five systems tracts vary in thickness and differ in depositional systems. The lowstand systems tract (LST) of Sequence 5 forms at the base of the focus interval and is composed of a basin floor fan system with hummocky to undulated seismic facies. The highstand systems tracts (HST) of Sequence 5 has the greatest thickness and comprises a well-defined slope system showing a funnel-shaped trend and suggesting progradational stacking pattern. The LST of Sequence 6 contains a middle ramp system formed during the lowest sea-level. The two transgressive systems tracts (TST) are the thinnest, several meters thick, and separating the LST from the HST. Log curves show an overall funnel-shaped motif through all three depositional systems, indicating a progradational stacking pattern. The main factor controlling the platform growth and demise is sea-level fluctuations. Additional factors, such as, subsidence, antecedent topography, and prevailing currents have also contributed to the evolution of the platform. Sea-level changes coupled with subsidence create accommodation for the platform growth and terminate deposition by drowning. The pre-existing topography creates a topographic relief and influences the growth pattern of the platform. Prevailing local currents influence the direction of platform progradation. This study provides insights on the evolution of isolated carbonate platform in a rift setting, which can help predict the lithofacies and depositional systems of economic significance.

Stratigraphic expression of the Paleocene-Eocene Thermal Maximum climate event during long-lived transient uplift—An example from a shallow to deep-marine clastic system in the Norwegian Sea

Seismic geomorphology and stratigraphic analysis can reveal how source-to-sink systems dynamically respond to climatic and tectonic forcing. This study uses seismic reflection data from the Norwegian Sea to investigate the stratigraphic response to a short-lived (0.2 Myr) period of climate change during the Paleocene-Eocene Thermal Maximum (PETM), superimposed on a long-lived (∼8 Myr) period of hinterland uplift. The data show that long-term uplift resulted in ∼300 m of relative sea-level fall, forced regression and formation of incised valleys during the latest Paleocene-earliest Eocene. The short-lived PETM climate perturbation at ∼56 Ma changed the transport dynamics of the system, allowing sediment to be bypassed to wide channel complexes on the basin floor, feeding a large mud-rich basin-floor fan more than 50 km into the basin. Our analysis also suggest that sediment supply was up to four times higher during the PETM compared to earlier and later periods. Maximum regression at ∼55.5 Ma resulted in the formation of a subaerial unconformity. The style of subaerial incision was dictated by shelf accommodation and proximity to the area of direct sediment input. Out-of-grade shelves and slopes sourced by littoral drift were prone to incision, but direct-fed and graded shelves and slopes were not. Despite maximum regression, sediments were not transported significantly beyond the toe-of-slope aprons, suggesting that rapid climate change was more efficient in bypassing sediment to the deep-water than low stands of sea level. As long-term accommodation increased after the PETM, deltas were still able to reach shelf edge, but periods of maximum regression were not associated with deep incisions along the outer shelf and only smaller canyons and gullies formed. The shelf-slope wedge was finally transgressed at ∼51 Ma. The age of deep valley incisions overlaps with the time of subaerial erosion in the East Shetland and Faroe-Shetland basins, suggesting a common mechanism for North Atlantic uplift around 55–56 Ma. Other seismic stratigraphic surfaces do not seem to be regionally time-equivalent, highlighting the importance of local controls on internal architecture of shelf-slope wedges. This study demonstrates the high-resolution stratigraphic response to long- and short-term external forcing together with intrinsic processes and can help identify similar relationships in other areas.

Geometry analysis and oil reservoirs prediction in the Lower Cretaceous clinoforms of the northern Priobskoye field

. Developing of the exploration criteria for oil reservoirs and non-structural traps in the clinoform successions are the key target as for detailed field appraisal in the West-Siberia basin well as for exploration in the clinoform complexes in the new sedimentary basins without drilling. Based on complex analysis of the seismic, well logs, well tests and core data in the northern Priobskoye field, the correlation between clinoform geometry, edge trajectory and distribution pattern of the reservoirs in the productive formation AS is established. The highest flow rates and reservoir properties are typical for bar deposits, proximal fan and slope channels. Deposits of bars are formed at the high stand system tract and at the beginning of the falling system tract on the edges of tangential clinoforms with gently ascending, flat and descending edge trajectory. Deposits of basin floor fans and slope channels are confined to the drop in the relative sea level and its low stand; it is advisable to search for them at the bottomset of tangential clinoforms with a descending edge trajectory and in gently cross-bedded clinoforms.

The Sediment Budget Estimator (SBE): A process model for the stochastic estimation of fluxes and budgets of sediment through submarine channel systems

AbstractTurbidity currents transport vast amounts of sediment through submarine channels onto deep-marine basin-floor fans. There is a lack of quantitative tools for the reconstruction of the sediment budget of these systems. The aim of this paper is to construct a simple and user-friendly model that can estimate turbidity-current structure and sediment budget based on observable submarine-channel dimensions and general characteristics of the system of interest. The requirements for the model were defined in the spirit of the source-to-sink perspective of sediment volume modeling: a simple, quantitative model that reflects natural variability and can be applied to ancient systems with sparse data availability. The model uses the input conditions to parameterize analytical formulations for the velocity and concentration profiles of turbidity currents. Channel cross section and temporal punctuation of turbidity-current activity in the channel are used to estimate sediment flux and sediment budget. The inherent uncertainties of geological sediment-budget estimates motivate a stochastic approach, which results in histograms of sediment-budget estimations, rather than discrete values. The model is validated against small-scale experimental turbidity currents and the 1929 Grand Banks turbidity current. The model performs within acceptable margins of error for sediment-flux predictions at these smallest and largest scales of turbidity currents possible on Earth. Finally, the model is applied to reconstruct the sediment budget related to Cretaceous slope-channel deposits (Tres Pasos Formation, Chile). The results give insight into the likely highly stratified concentration profile and the flow velocity of the Cretaceous turbidity currents that formed the deposits. They also yield estimates of the typical volume of sediment transported through the channels while they were active. These volumes are demonstrated to vary greatly depending on the geologic interpretation of the relation between observable deposit geometries and the dimensions of the flows that formed them. Finally, the shape of the probability density functions of predicted sediment budgets is shown to depend on the geological (un)certainty ranges. Correct geological interpretations of deep marine deposits are therefore indispensable for quantifications of sediment budgets in deep marine systems.

Delineation of stratigraphic traps within the basin floor fans of Miocene sedimentary sequences, offshore Indus, Pakistan using inverted acoustic impedance simulations

AbstractDeep‐water clastic settings frame major stratigraphic plays the world over. The advanced seismic interpretation tools provide prediction of accurate physical characteristics of undiscovered reservoirs, including their lithology, thickness, and porosity. The designed amplitude spectrum (DAS)‐based processing tool on Full spectrum seismic may yield a remarkable stratigraphy. This study deals with the prediction of thick and porous (possibly gas‐rich) litho‐facies using the DAS tool on high‐resolution seismal profiles within the offshore Indus Basin (OIB), SW Pakistan. The seismic and trace envelope magnitudes bound the demarcation of stratigraphic traps. The DAS processing tool on relative acoustic impedance inversion (RAI) magnitude demarcates the aggradational porous sandstone‐filled reservoirs, which are encased within a transgressive non‐porous shale‐filled channel‐levee system. The thickness predicted by band‐limited seismic acoustic impedance (AI) simulation for channels, point bars, and levees is 28, 27, and 24 m, respectively. The DAS‐based RAI processing on AI reservoir simulation accurately resolves the lenses of channel sandstone, point bars and levees, and confirms the channelized basin floor fans (BFFs) with an average 11, 34, 24, and 52 m thickness and a lateral extent between 10 and 25 km. The western zones have implications for basin development, which have accumulated the porous and thickest reservoir facies inside the aggradational parasequences of BFFs of Miocene sedimentary sequences. Hence, the presented scheme of exploration serves as a comparable example for exploring the hydrocarbon‐bearing plays inside OIB and global deep‐water depositional systems.

Stratigraphic analysis of XES02: Implications for the sequence stratigraphic paradigm

ABSTRACT Sequence stratigraphy has the potential to provide a consistent method for integrating data, correlating strata, defining stratigraphic evolution, and generating quantifiable predictions. However, the consistent application requires a precise definition of concepts, stratigraphic units, bounding surfaces, and workflow. Currently no single generally accepted approach to sequence stratigraphic analysis exists, nor are there any robust tests of models and methods. Applying conventional sequence stratigraphic analysis to strata from an analog laboratory experiment (eXperimental EarthScape02, XES02) with known boundary conditions and chronology provides some initial robust testing of the models and methods. Despite stratigraphic architectures apparently consistent with those expected within the sequence stratigraphic paradigm, blind-test applications yield: 1) deducted erroneous base-level curves, 2) systems-tract classification mismatches, 3) disconnected systems-tracts type and actual base level, 4) time-transgressive basin-floor fans, and 5) missing systems tracts. Stratigraphic forward models using base-level curves derived from Wheeler diagrams cannot match the timing, redeposited-sediment volume, and depositional environments observed in the XES02 experiment. These mismatches result from common Wheeler diagram construction practice, producing poorly resolved base-level minima timing and base-level fall durations, hence inaccurate fall rates. Consequently, reconstructions of controlling factors based on stratal architectures remain uncertain, making predictions similarly uncertain. A reasonable path forward is to properly acknowledge these uncertainties while performing stratigraphic analysis and to address them through multiple scenario analysis and modeling.

Mesozoic hydrocarbon accumulation model in the Northern South China sea

The ChaoShan Depression is the largest Mesozoic depression covering an area of 3.7 x 104 km2 where the relict Mesozoic strata are up to 5000 m thick. It has experienced disruption since the late Mesozoic. In early survey, the oil-gas migration condition and reservoiring mechanism are poorly expounded from the Meso-Cenozoic tectonic superposition due to poor seismic imaging to the deep Mesozoic layers. New seismic surveys using long streamers and quasi three-dimensional layouts has improved obviously the deep images of the Mesozoic formations, enabling analysis of tectono-stratigraphic features and petroleum geology. Correlating to the regional Mesozoic stratigraphic and facies characteristics from the well LF35-1-1 and onshore outcrops, two sets of source layer tested with high organic carbon content are interpreted within the upper Triassic to mid-Jurassic semi-closed gulf sequences which become thicker toward the east side of Dongsha Island. Three sets of potential reservoir beds are also interpreted, which are a basin-floor fan sandstone layer of the Triassic-Jurassic boundary, a limestone layer atop the mid Jurassic, and a sandy layer in the Upper Jurassic. Over the low bulge of the ChaoShan Depression, a big anticline is found bounded by two sets of faults which played as migration passage for hydrocarbon from the deep Mesozoic source to the reservoir layers. Likely, one set of the fault act reverse barrier to block the petroleum escape, thus form the fault-bounded trap, favorable for future exploration.

Provenance of Oligocene lithic and quartz arenites of the East Carpathians: Understanding sediment routing systems on compressional basin margins

AbstractWe present new sedimentological, petrographical, palaeontological and detrital zircon U–Pb data on late Oligocene–early Miocene sedimentary rocks of the thin‐skinned thrust belt of East Carpathians. These data were acquired to reconstruct the sedimentary routing system for two compositionally different turbidite fans made of the regionally extensive Kliwa and Fusaru formations. On the eastern margin of the Moldavides foreland basin, large low‐gradient river systems draining the East European Platform provided well‐sorted quartz‐rich sand forming deltas on wide shallow shelves and thick Kliwa submarine fans. Due to the westward subduction of a thinned continental plate, the western basin margin was characterized by short, steep‐gradient routing systems where sediment transport to deep water was mainly through hyperpycnal flows. The Getic and Bucovinian nappes of the East Carpathians and the exhumed Cretaceous–Early Palaeogene orogenic wedge fed Fusaru fans with poorly sorted lithic sand. The Fusaru fans trend northwards in the foredeep basin having an elongate depocentre, interfingering and then overlapping on the distal part of the Kliwa depositional system due to the eastward advance of the Carpathian fold‐and‐thrust belt. A smaller sediment input is supplied by southern continental areas (i.e. Moesian Platform, North Dobrogea and potentially the Balkans). In general, the sandstone interfingering between distinct basin floor fan systems is less well documented because the facies would be similar and there are not many systems that have a distinct sediment provenance like Kliwa and Fusaru systems. This case study improves the understanding of regional palaeogeography and sedimentary routing systems and provides observations relevant here or elsewhere on the interfingering turbidite fan systems.

Characterization and evolution of a carbonate basin floor fan on a leeward side of an isolated carbonate platform, Sirte Basin, Libya

The recognition of carbonate basin floor fans and the interpretation of their formation and evolution are extremely crucial to the hydrocarbon exploration since they form important oil and gas reservoirs. There has been little research on the geometries and controlling factors of the carbonate basin floor fans from subsurface three-dimensional (3D) seismic and wireline log data. Most studies have been conducted on siliciclastic basin floor fans. Thus, this study characterizes the internal configuration and processes controlling the deposition of a lower Eocene carbonate basin floor fan system in the Sirte Basin in a sequence stratigraphic context through integrated 3D seismic and wireline log interpretations. The carbonate fan complex shows hummocky to undulated seismic geometries on vertical seismic sections and bounded by an unconformable sequence boundary at the base and a downlap surface at the top. It has an aerial extension of around 9 km from south to north along depositional dip and at least 8 km from east to west along strike. In well data, the fan system predominantly comprises limestone units intercalated with thin shale intervals. Log curves display an overall funnel-shaped trend, suggesting progradational stacking pattern. The vertical thickness of the fan complex varies from 120 to 165 m. The evolution of the carbonate fan was influenced by the interplay of accommodation space and sediment supply. A pre-existing topography, tectonic subsidence, and sea-level rise generated accommodation space for the deposition of the fan system, whereas sediment-gravity flows transported the fan sediments from the platform interior and margin to the basin floor. This study provides sedimentological and stratigraphic criteria to recognize and interpret the evolution of basin floor fans along isolated carbonate platforms from 3D seismic and wireline log data and may serve as an analog for ongoing and future studies on carbonate basin floor fans elsewhere.

Tectonostratigraphic evolution of the Sohar Basin, exploration concepts and emerging plays offshore on the UAE's east coast

The Sohar Basin, located offshore the East Coast of the Arabian Peninsula, is an area that has seldom been the focus of international oil companies. While all commercial discoveries in the UAE and Oman have been made to the west and south of the Oman Mountains, the Sohar Basin is underexplored and remains prospective. This lack of activity is primarily because the Thamama reservoir, which has yielded huge success to the west of the Oman Mountains is likely not present within the Sohar Basin. However, potential traps, reservoirs, and source rocks are present within the Tertiary succession. The Sohar Basin formed in the late Cretaceous after the obduction of the Semail Ophiolite onto the Arabian Plate. Following a period of tectonic quiescence, which lasted until the Early Eocene, the gravitational collapse occurred along the eastern edge of the present-day Oman Mountains.Interpretation of both 2D and 3D seismic reveals that the subsurface is comprised of three tectonostratigraphic units, defined as pre-, syn-, and post-tectonic packages. North-South trending thrust faults generate distinct mini-basins within the pre- and syn-tectonic sequences. The post-tectonic sequence is defined by the development of prograding systems in a tectonically quiescent margin formed in the Lower Miocene and which persists to the present day. Deep-water depositional systems dominate the undrilled far-east and south of the basin where the presence of submarine canyon systems and associated basin floor fans is supported by 3D seismic data. In recent times, the Sohar Basin has been subject to extensional faulting likely driven by sediment load caused by a prograding delta system.The results of this work reveal that multiple untested play types exist within the Sohar Basin, which are comprised of both structural and stratigraphic elements. This paper integrates both a structural and sequence stratigraphic interpretation to provide insights into the tectonostratigraphic evolution of the Sohar Basin. Consequently, the fundamentals of play fairway mapping have been employed to unlock the hydrocarbon prospectivity offshore the UAE's East Coast.

Sedimentation of the Oligocene-Lower Miocene clinoforms of the Maikop formation in the Eastern and Central Pre-Caucasus region as a key criteria for reservoir exploration

Oligocene-Lower Miocene clinoforms of the Maikop formation are productive in the Eastern and Central Pre-Caucasus region. In spite of commercial discoveries and long exploration history, Maikop formation is poor characterized by borehole data in the deep basins such as Terek-Caspian trough. Furthermore, there are challenges of the detail stratigraphic subdivision in the deep basins, correlation, sedimentary conditions and criteria of reservoirs exploration. Based on seismic and borehole data analysis, sequent-stratigraphic framework, distribution area, progradation direction, seismic unconformities and thicknesses of the 17 Maikop sequences are established. Estimation of the clinoforms height allowed reconstructing paleobathymetry and paleogeography of the Pre-Caucasus region in the Oligocene – Early Miocene. Clinoforms M1-M7 prograded from the northeast, sea depth increased from 300 to 450 m. During clinoforms M8 sedimentation, new provenance area appeared on the west, sea depth increases up to 800 m. Clinoforms M12-M16 prograded from the north, sea depth decreases from 480 to 270 m. To the end of the M17 sequence deposition, starved deep basin was totally compensated. Three morphologic types of the clinoforms are identified. Type I represents by tangential clinoforms up to 75 m of height, that interpreted as subaqueous delta. Type II are the sigmoid shelf-edge delta clinoforms up to 800 m of height. Clinoforms of type III are low-angle wedges, confined to the slope and floor of the deep basin. Height of the wedges does not exceed 270 m. In the topset of the shelf-edge deltas, shallow-marine sandstones and structural traps predicted, while basin floor fans and stratigraphic traps expected in the bottomset. Subaqueous deltas are regarded to stratigraphic traps, while the wedges are predominantly mud-prone.

Seismic-informed carbonate shelf-to-basin transition in deeply buried Cambrian strata, Tarim Basin, China

We develop an enhanced interpretation workflow to improve facies interpretation in ultradeep (5000–8000 m), high-velocity (6000–7000 m/s) carbonate rocks using low-frequency (as low as 18 Hz) seismic data. Our study area includes two seismic surveys north and south of the Cambrian platform margin that extends more than 500 km in strike direction. Relative impedance (−90° data) and frequency fusion are applied to predict impedance and lithology in carbonate platform-basin architecture. Outcrop seismic modeling clarifies the facies architecture of subtle subseismic clinoforms. Stratal slices are made for seismic geomorphology. We recognized eight seismic sequences and seven seismic facies in 1200-m prograding sediments. As a strongly prograding accretionary margin, the stratal relationships and transitions from platform interior to shelf margin to slope and basin are consistent throughout the basin. Platform interior is characterized by intertidal, supratidal, and salt flat facies; the shelf margin develops microbial reef and shoal facies; the slope and basin facies are dominantly gravity flow and deepwater sediments. The northern and southern areas of the margin, however, are characterized by different slope-to-basin geomorphology and environments. In the north, the shelf edge was cemented, with little reworking of shallow-water sediments to the slope and basin. Gravity failure–related facies, such as gullies and slump aprons, dominate in the slope. No turbidite fans are found in the sediment-starved deep basin. In the south, a series of submarine channels transported shallow sediments long distance to form slope fans and basin-floor fans. Turbidite sediments contain a significant amount of low-impedance sediments from the shelf edge. The syndepositional tectonic trench faults, by initiating channels, are hypothesized to be a new mechanism for sediment transport in a deepwater environment. Such a mechanism provides a clue to help researchers recognize and map deepwater reservoir facies in the basin.

A 3D geological model of a structurally complex relationships of sedimentary Facies and Petrophysical Parameters for the late Miocene Mount Messenger Formation in the Kaimiro-Ngatoro field, Taranaki Basin, New Zealand

The present study investigates the reservoir characteristics of the Mount Messenger Formation of Kaimiro-Ngatoro Field which was deposited in deep-water environment. A 3D seismic dataset, core data and well data from the Kaimiro-Ngatoro Field were utilized to identify lithofacies, sedimentary structures, stratigraphic units, depositional environments and to construct 3D geological models. Five different lithologies of sandstone, sandy siltstone, siltstone, claystone and mudstone are identified from core photographs, and also Bouma sequence divisions are also observed. Based on log character Mount Messenger Formation is divided into two stratigraphic units slope fans and basin floor fans; core analysis suggests that basin floor fans show better reservoir qualities compared to slope fan deposits. Seismic interpretation indicates 2 horizons and 11 faults, majority of faults have throw less than 10 m, and most of the faults have high angle dips of 70–80°. The Kaimiro and Ngatoro Fields are separated by a major Inglewood fault. Variance attribute helped to interpret faults, and other seismic attributes such as root-mean-square amplitude, envelope and generalized spectral decomposition also helped to detect hydrocarbons. The lithofacies model was constructed by using sequential simulation indicator algorithm, and the petrophysical models were constructed using sequential Gaussian simulation algorithm. The petrophysical parameters determined from the models comprised of up to ≥ 25% porosity, permeability up to around 600mD, hydrocarbon saturation up to 60%, net to gross varies from 0 to 100%, majority of shale volumes are around 15–20%, the study interval mostly consists of macropores with some megapores and 4 hydraulic flow units. This study best characterizes the deep-water turbidite reservoir in New Zealand.