Due to the global oil price crisis in 2014, one of the MOL's preventive/reactive measures was to identify geologically or commercially risky elements within their portfolio. This involved reevaluation of all geologic data from Field A in the Volga-Urals Basin. In re-evaluating Field A, several unexpected challenges, problems and pitfalls were faced by the interdisciplinary team performing the task of building a new database, quality checking, and interpreting data dating back to 1947. To overcome these challenges related to this mature field, new approaches and fit-for-purpose methods were required in order to achieve the overall goal of obtaining a reliable estimation of remaining hydrocarbon potential. In the first phase a first-pass 3D geologic model was constructed, along with wrangling, cleaning and interpreting 70 years of subsurface data. This paper focuses on the main challenges involved in evaluating or reevaluating reservoir aspects of a mature field. The primary challenges were related to the estimation of remaining in-place hydrocarbon volumes, the optimization of infill well placement, the identification of primary and secondary well targets, the identification of critical data gaps, and the planning of new data acquisitions. The hands-on experience gained during the development of the geologic model provided invaluable information for the next steps needed in the redevelopment of the field.

This study presents the sequence stratigraphic interpretation of subsurface data in a Lower Carboniferous lagoon setting in the northern boundary zone of the Precaspian Basin. The proposed stratigraphic and facies distribution model of the lagoon fill enables the reconstruction of the relative sea-level history and refines the understanding of the geological evolution and eustatic changes in the Caspi region during the Lower Carboniferous. Semi-circular carbonate mounds were built in the Devonian period and Tournaisian stage enclosing a lagoon with an opening towards the Caspi Sea in a southeast direction. A sea-level fall at the end of the Tournaisian stage terminated the growth of the carbonate buildups; during the Lower Visean (Lower Carboniferous) time, the lagoon was filled with mixed siliciclastic and carbonate sediments. The lagoon fill is interpreted as a third-order composite sequence composed of three fourth-order sequences representing sea-level changes with an increased magnitude and frequency relative to the quiet conditions that prevailed during the previous carbonate buildup in Famennian-Tournaisian. The highstand systems tracts of the fourth-order sequences are likely Waulsortian-type carbonate banks, while the falling-stage and lowstand systems tracts are mostly clastic progradational wedges. The transgressive systems tracts include retrograding shale and grain shoal facies. A prominent falling-stage systems tract was detected in the third fourth-order sequence in the lagoon fill representing a significant sea-level fall in lower Visean time. The falling-stage systems tract is composed of three downstepping-prograding lobes indicating a continuously falling sea level punctuated by minor pauses. Seismic imaging revealed a series of distinct prograding wedges inside the falling stage lobes that are either accretionary bedset or high-frequency nested sequences. The observed fall in sea level during the Lower Visean may be associated with, and it may provide stratigraphic evidence for the beginning of Paleozoic glaciation on Gondwana.

The paper presents a study of a Lower Carboniferous (Visean) clastic sequence commonly called Bobrikovsky Formation, deposited in the Volga-Ural Petroleum Province, Orenburg Region. Our investigation included sedimentological description of core samples from hydrocarbon wells and well log correlations. Facies were identified by well log patterns and calibrated by core sedimentology. The Bobrikovsky Formation is proposed to be interpreted as an overall transgressive-regressive succession in a nearshore-tidal environment. Transgressive lagoon-estuary and barrier island facies became regressional lagoon fill-type settings.

Interpretations of subsurface data and computer simulations of basin-fill history in the late Neogene Pannonian Basin of eastern Hungary established a regional stratigraphic model presented in this paper. Three fluvial systems overfilled the basin: a large system from the NE, another from the NW and a third smaller system transported sediments from the territory of Romania, from the SE. Large part of the basin had been filled by Messinian time (late Miocene); the remaining lake was located in southeastern Hungary and neighboring countries. In the Tortonian stage, the three fluvial systems advanced concomitantly (with small-scale, high-frequency cycles), toward the basin center. A significant relative lake-level drop occurred in the Messinian, which induced large-scale erosion and fluvial incision and generated an unconformity surface that provides a major division in the Pannonian strata; it appears as a significant stratigraphic divide that separates two different stages in the history of the Pannonian Basin. In the upper Messinian-Pliocene stage, the basin underwent a tectonic inversion; the shallow lake was filled up rapidly by lowstand turbidites and prograding delta-slope sediments. The inversion continues up to the present day. Turbidites and lowstand prograding wedges associated with the Messinian event may be targets for hydrocarbon exploration. Four main stages can be identified in the Upper Miocene–Pliocene basin fill history: (1) Tortonian highstand regression with several low-rank relative lake-level cycles; (2) Messinian lake-level drop, accompanied by the formation of the “Messinian unconformity” and the deposition of a falling-stage systems tract (FST). The “Messinian unconformity” is a composite surface, represented by a subaerial unconformity at the margins, and a basal surface of forced regression in the depocenters; (3) relative rise of lake level, when the FST turned to a lowstand systems tract (LST); (4) tectonic inversion (basinward tilt and marginal uplift) during or at the beginning of the LST.

The Messinian Salinity Crisis (MSC) was an extraordinary geological event that affected the whole Mediterranean region, as well as global marine circulation between 5.97 and 5.33 Ma. In the Mediterranean, the crisis was mainly characterized by isolation from the Atlantic Ocean, resulting in a new hydrological budget leading to the accumulation of large amounts of evaporites and large-scale erosion of its margins. The nature of the majority of the evaporites and erosional surfaces is still unknown, thus representing a challenge which forms the subject of debate in the scientific community.The Neogene Mallorca basins (Balearic Islands) are well situated to provide a stratigraphic record which expresses the sequence of events related to the MSC, being especially suitable for testing possible geological models or scenarios. New research on the island of Mallorca provides data about the MSC and Zanclean reflooding in the central part of the Western Mediterranean (Balearic Basin), with obvious repercussions for both academia and industry.The main aim of this study is to describe the features which characterize sedimentation during the Miocene-Pliocene transition on the island of Mallorca, to establish how this stratigraphic record relates to the different proposed MSC scenarios for the whole of the Mediterranean. In this regard, a stratigraphic analysis was carried out on a total of 100 exposures and boreholes corresponding to the Neogene basins of Mallorca. The results indicate that, on the island of Mallorca, the development of the MSC is most consistent with models that represent the main phase of desiccation occurring after the accumulation of marginal evaporites (Primary Lower Gypsum), the Terminal Carbonate Complex and the Lago Mare deposits.

Systems tracts are defined on the basis of stratal stacking patterns. This paper explores quantitatively the possible relationships between different systems tracts and the relative sea-level in siliciclastic depositional systems. The development of systems tracts depends on the manifestation of forced regressions, normal regressions (lowstand and highstand) and transgressions, irrespective of the magnitudes of relative sea-level rise preceding and succeeding a stage of relative fall. As such, the transgressive and highstand systems tracts of one sequence may form when the elevation of the relative sea level is lower than the highstand or even lowstand in relative sea level of the previous sequence. Similarly, the lowstand systems tract of one sequence may form when the relative sea-level is higher than the highstand elevation of the previous sequence. Because subsidence/uplift and depositional rate may change considerably along strike of the coastal zone, all the systems tracts may form synchronously along strike. The maximum diachroneity of a systems tract boundary along the coast is half the duration of the accommodation cycle.

We investigate whether depositional sequences can form on 1000 y or millennial scale, and what stratal architecture can develop as the result of these short term variations. Abrupt climate changes are caused by a complex interplay between atmospheric, oceanic, and cryospheric processes. Dansgaard-Oeschger (D-O) cycles of similar to 1000 y and Bond cycles of similar to 7000 y have been identified in climate studies since the early 1990s. A 3D forward stratigraphic model, Dionisos, was used in this study to analyze the possible stratigraphic architecture that may evolve in response to the millennial-scale climatic cycles. According to current knowledge, no detectable eustatic changes occur in a D-O cycle, but sea level may change slightly through several D-O cycles. An abrupt similar to 20 m fall and subsequent rise characterize the Heinrich events between Bond cycles. Our modeling included three experiments: (i) stable sea level, (ii) slightly rising sea level, and (iii) slightly falling sea level between Heinrich events. The applied fluvial water discharge and sediment supply varied according to the millennial climatic variations in each experiment. The modeling experiments lead to the formulation of a conceptual model for millennial-scale stratigraphy relevant to glacial periods. The millennial-scale sequences belong to a two-fold hierarchy defined by a series of short D-O cycles nested within longer Bond cycles, which, in turn, are separated by the sharp Heinrich events. The stacking patterns predicted between Heinrich events include: (i) alternating thicker and thinner bedsets of normal regressive highstand progradation (HST) on D-O scale, if sea level is stable; (ii) highstand systems tract-transgressive systems tract (HST-TST) sequences on D-O scale, if the sea level is rising; and (iii) thickening and thinning forced regressive bedsets on D-O scale, if the sea level is falling. In case iii, the Bond-scale falling-stage systems tract (FST) has two distinct parts: a proximal slightly and gradually downstepping unit, followed by a strongly offlapping unit deposited offshore. The intra-FST surface that separates the two units corresponds to the Heinrich sea-level drop, and is referred to in this paper as the "Heinrich discontinuity.'' This type of sequence consists of FST-LST-TST, and no HST may form.

The recent world-class gas discoveries in Early Miocene sand units offshore Israel raises the question of their origin. Apparently, the simplest explanation is to relate them to a fluvial system that arrived from Arabia at that time. This system predated the modern (Pliocene) Nile River supply and existed until captured by the Dead Sea valley. Interestingly, however, very little sedimentation occurred along the Levant continental margin before the Pliocene in spite of its stepped structure that provided much space for accommodation. The only way that sediments could have bypassed the continental margin and arrive at the deep basin without being trapped in the middle is through submarine channels that crossed the continental margin. Here we explore this possibility using 3-D stratigraphic modeling techniques that quantify the sediment load and the water discharge required to fill the basin by pushing enough sediment through submarine channels. We show that such a scenario requires a fluvial system in the order of the largest rivers that exist today on earth in terms of drainage area and water discharge. Alternatively, it requires extreme hydraulic conditions in terms of diffusion coefficients and an elevated drainage basin that could not have existed in the study area. We therefore challenge the traditional view of Arabia as the main source for Oligo-Miocene deposits in the Levant Basin and suggest that the basin was mainly fed by a proto-Nile system that transported clastic material to the North African margin and then farther east by ocean currents. In a wider view we demonstrate how numerical modeling can constrain sediment transport through submarine channels as a function of basin geometry and hydraulic conditions, and how paleogeographic knowledge can be combined with current data on world rivers to evaluate if modeling results are plausible.

A three-dimensional quantitative stratigraphic forward model is employed to investigate the controls leading to the Messinian events in the lacustrine Pannonian Basin of Central Paratethys, and the link between the Messinian salinity crisis in the Mediterranean and the late Miocene-Pliocene stratigraphy of the Pannonian Basin. Subsurface geological data show that a prominent unconformity surface formed during Messinian time in the Pannonian Basin associated with a sudden forced regression, abrupt basinward shift of facies and a subsequent, prolonged lowstand normal regression. The lowstand prograding series filled up the shallow basin fast, while, at the same time, the marginal areas of the basin were subject to tectonic inversion. The Dionisos program used in this research is built on a nonlinear water-driven sediment diffusion process, and it employs multiple sediment classes, basin flexure and compaction. Four different scenarios were built in the experiments to test possible basin histories with different rates and timing of tectonic inversion. Each scenario was modelled in two versions: including and not including a lake-level fall in the Messinian. The results confirm that the Pannonian Basin in the study area has undergone a tectonic inversion since the Messinian, although the exact rates of uplift at different locations remain uncertain. The unconformity and the observed stratigraphic architecture and facies pattern could be modelled adequately only in the versions that applied a Messinian lake-level fall. Our research concludes that the Messinian unconformity in the Pannonian Basin was caused by an absolute lake-level drop, likely linked to the desiccation of the Mediterranean, followed by subsidence and normal regression in the basin centre and concomitant tectonic inversion and uplift along the basin margins.

The M essinian S alinity C risis is well known to have resulted from a significant drop of the M editerranean sea level. Considering both onshore and offshore observations, the subsequent reflooding is generally thought to have been very sudden. We present here offshore seismic evidence from the G ulf of L ions and re‐visited onshore data from I taly and T urkey that lead to a new concept of a two‐step reflooding of the M editerranean B asin after the M essinian S alinity C risis. The refilling was first moderate and relatively slow accompanied by transgressive ravinement, and later on very rapid, preserving the subaerial M essinian E rosional S urface. The amplitude of these two successive rises of sea level has been estimated at ≤500 m for the first rise and 600–900 m for the second rise. Evaporites from the central M editerranean basins appear to have been deposited principally at the beginning of the first step of reflooding. After the second step, which preceeded the Z anclean G lobal S tratotype S ection and P oint, successive connections with the P aratethyan D acic B asin, then the A driatic foredeep, and finally the E uxinian B asin occurred, as a consequence of the continued global rise in sea level. A complex morphology with sills and sub‐basins led to diachronous events such as the so‐called ‘ L ago M are’.This study helps to distinguish events that were synchronous over the entire M editerranean realm, such as the two‐step reflooding, from those that were more local and diachronous. In addition, the shoreline that marks the transition between these two steps of reflooding in the P rovence B asin provides a remarkable palaeogeographical marker for subsidence studies.

The Messinian Salinity Crisis is well known to have resulted from a significant drop of the Mediterranean sea level. Considering both onshore and offshore observations, the subsequent reflooding is generally thought to have been very sudden. We present here offshore seismic evidence from the Gulf of Lions and re-visited onshore data from Italy and Turkey that lead to a new concept of a twostep reflooding of the Mediterranean Basin after the Messinian Salinity Crisis. The refilling was first moderate and relatively slow accompanied by transgressive ravinement, and later on very rapid, preserving the subaerial Messinian Erosional Surface. The amplitude of these two successive rises of sea level has been estimated at 500 m for the first rise and 600–900 m for the second rise. Evaporites from the central Mediterranean basins appear to have been deposited principally at the beginning of the first step of reflooding. After the second step, which preceeded the Zanclean Global Stratotype Section and Point, successive connections with the Paratethyan Dacic Basin, then the Adriatic Correspondence: F. Bache, GNS Science, P.O. BOX 30368, Lower Hutt 5040, New Zealand. E-mail: f.bache@gns.cri.nz © 2011 The Authors Basin Research © 2011 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 125 Basin Research (2012) 24, 125–153, doi: 10.1111/j.1365-2117.2011.00521.x EAGE

Systems tracts are linkages of contemporaneous depositional systems that accumulate under specific conditions of accommodation and sediment supply. They are identified by their stratal stacking pattern and position within the sequence. In practice, it is often difficult to assign accurately relative sea-level position to stacking patterns. Observations also show that systems tracts may succeed each other in different orders, not necessarily according to the prediction of an ideal model. The purpose of this study is to analyze the quantitative conditions for different possible combinations of systems tractswithin a cycle of relative sea-level change. Icehouse and greenhouse eustatic trends, active and passive tectonics, and varying depositional rates are considered as conditions. The systems tracts were defined based on the rate of accommodation change and depositional rate.Our quantitative calculations revealed that if the tectonics is active and the basement motion-time curve is segmented with sharp turning points between segments, eight possible successions of systems tracts may occur. If the basement motion is transitional between segments of subsidence and uplift, six scenarios are possible. Only three possible combinations of systems tracts can form in a sea-level cycle if the basement tectonics is passive. Variable depositional rate alone can be responsible for four different combinations of systems tracts under similar accommodation conditions. These combinations of systems tracts may occur primarily in greenhouse world. Eustasy under icehouse climatic conditions has rapid rates of change (up to 4000 m/myr), which are likelymany times faster thanwhat subsidence/uplift rates realistically can reach.As a consequence, only the full series of systems tract in the order of forced regression-lowstand normal regression-transgression-highstand normal regression can be predicted in icehouse, although the thickness of systems tracts in the real rock record may be rather small because of the fast rate of sea-level change.

New field observations and fossil analyses complete and clarify the strong impact of the Mediterranean sea-level changes linked to the peak of the Messinian Salinity Crisis on the Dacic Basin in southwestern Romania. In addition to the Gilbert-type fan delta already evidenced along the Danube River in the area of Turnu Severin, a new Gilbert-type fan delta is described northward. Early Zanclean bottomset beds are evidenced and dated based on nannofossils at the junction of the two coalescing Gilbert-type fan deltas. A clear sedimentological, morphological and chronologic differentiation is established in the area between the Carpathians Late Miocene piedmont alluvial fans and the early Zanclean Gilbert-type fan deltas. The early Zanclean age of the Hinova clays, where the bottomset beds of the Gilbert-type fan deltas are mostly developed, is confirmed by the occurrence of nannofossil markers of Subzone NN12b and a Bosphorian mollusk macrofauna. Early Zanclean inflow of Mediterranean marine waters into the Dacic Basin is also supported by the record of planktonic foraminifers. In the Dacic Basin, the Messinian Salinity Crisis resulted in the cutting of the Iron Gates by a Carpathians river. Fluvial erosion also affected the residual Pannonian Basin and probably catched the paleo-Tisza River which contributed to the erosion of the Iron Gates and to the fluvial drainage of the partly desiccated Dacic Basin. Arguments are reinforced in favor of a marine gateway between the Mediterranean and Dacic Basin through the Balkans before and after the Messinian Salinity Crisis.

Earlier studies revealed a prominent unconformity in the Messinian stratigraphic record of the lacustrine Pannonian Basin, but it was unclear what factors were responsible for the formation of this unconformity, and thus its origin remained controversial. The problem was further complicated by the fact that stratal patterns in the Messinian varied significantly along the paleo-margins of the lake. Our study introduces quantitative stratigraphic simulations to analyze the role of subsidence, sediment supply, and lake-level changes in the formation of the unconformity and the various architectural patterns that evolved in response to the Messinian events. The results suggest that a relative lake-level fall occurred in the Messinian, accompanied and followed by tectonic inversion in several parts of the basin. Since the rate of tectonic subsidence and/or uplift varied in space and time, the size of the relative lake-level fall varied significantly across the basin, while various strata architectural patterns formed in the same time interval. The age of the unconformity was estimated, based on seismic correlations with paleomagnetic chronozones, to be between 5-6 Ma, which was refined by stratigraphic simulations. Galeacysta etrusca cysts were found in the unconformity, which confirmed its late Messinian age. The surface is called the Intra-Messinian Unconformity (IMU) in this paper. Seismic interpretations and simulations of stratal architecture led to the formulation of a basin fill model and identification of four stratigraphic architectural patterns related to Messinian events including a major lake-level fall and coeval and subsequent uplift events in certain parts of the basin. (C) 2007 Elsevier B.V. All rights reserved.

The purpose of this study was to decipher the combined effect of extensional tectonics and halokinesis on formation of structural features in the Marib-Shabwa basin of Yemen. Based on seismic interpretations, a close relationship has been found between the tectonic position in half-grabens and full-grabens and the developed salt structures in these subbasins. The hinge margin and hanging wall of half-grabens are characterized by salt rollers and salt pillow zones. Diapir zones, tilted basinward by uplift, dominate the footwall sides. Elongated diapirs formed in the axial zone of full-grabens that display changing structural character along strike: they are reverse fault-bounded at their middle portions in response to local contractional effects in the overburden; whereas toward their ends, the diapirs become normal fault-flanked indicating local extensional stresses. Accommodation zones among small-scale half-grabens are associated with normalfaulted asymmetric or non-faulted symmetric diapirs.

Our earlier studies on active continental extensional basins revealed that high-frequency changes in sea/lake-level, spatially heterogeneous subsidence/uplift and multiple sources of sediment supply with varying rates and directions are the main factors that influence the formation of stratigraphic architectural patterns. Thus, the evolving stratal geometry results from the sea/lake-level--[time], subsidence [time, space] and sediment supply--[time, space, direction, composition] functions. The stratigraphic simulation results presented here demonstrate that Type 1 and 2 unconformities and conformities may develop in segments of an extensional basin during a sea/lake-level fall depending on the local subsidence rates. These different surfaces can be time-equivalent along the basin margin. A sea/ lake-level rise can either form a highstand systems tract (HST) or a transgressive and an overlying highstand systems tract (HST), depending on the sediment supply. A decrease and a subsequent increase in sediment supply during a stable base level stand can produce the same architectural patterns (backstepping of transgressive systems tract (TST) and aggradational or progradational patterns of HST) as a rise in the base level at constant sediment supply. Stratigraphic architectures built from multiple sediment sources show a variety of patterns as responses to spatial and temporal changes in intensities of individual sediment supply sources. Some of the possible end members of these interfingering architectural patterns have been simulated and classified. Changes in direction of sediment supply may lead to the formation of onlap-surfaces similar to unconformities.

A chronostratigraphic framework has been developed to date the late Tertiary and Pleistocene fill of the southern Dead Sea basin. This framework is based on a sequence stratigraphic interpretation of seismic data tied to eustatic sea-level curves. The limited biostratigraphic data available for the Dead Sea basin stratigraphy make this chronostratigraphy tentative, but nine third-order sequence boundaries have been identified and related to climatically driven lake-level falls that appear to correlate with eustatic events. In contrast, higher-order sequence boundaries were interpreted to reflect changes in both climate and local tectonics and the consequent sediment supply.The seismic stratigraphic interpretation indicates that fan-delta sediments accumulated at the southern and northern ends of the basin, while lacustrine elastic sediments filled the rest, Interfingering of southern and northern source sediments has been recognized in the northern part of the basin. The changing position of interfingering was interpreted as a response to lake-level changes, the lake-level falls inducing increased sediment flux from the northern source, and lake-level highstands favoring the influx and progradation of sediments from the south.

The rifting in the extensional Pannonian basin occurred in the Middle Miocene, the subbasins within the Pannonian system formed under the control of expansional tectonics during this time. Sequence stratigraphic analysis on well logs revealed a single third-order synrift sedimentary sequence developed due to tectonically enhanced eustatic sea-level change in shallow-marine environments.The Pannonian basin became an isolated lake by its postrift phase during the Middle-Late Miocene boundary. The first postrift sequence formed under peculiar conditions, when the basin lost its marine connections and simultaneously a considerable differential subsidence began. The large volume of freshwater input supplied by rivers and the rapid subsidence of the basement caused the basin to be a deep freshwater lake.The lacustrine postrift infill was studied by sequence stratigraphy using seismic reflection, well log and magnetostratigraphic data. Numerous unconformities can be revealed which represent relative lake-level fluctuations of different magnitudes. The water level and volume of the large inland lake was very sensitive to different effects, the most important ones are lake-water budget, sediment influx and differential subsidence of the basement. The style of extension and sedimentation show differences with respect to passive continental margins that are reflected in the sedimentary architecture.A significant third-order water-level change has been discovered in the eastern part of the Pannonian basin, while the western side did not experience such a great change in lake level. This fact gives credit to the view of different interplay of lake-level-influencing effects in different areas of the basin. This major third-order lake-level change has been age dated using magnetostratigraphy and a very close match was found with the age of the Mediterranean salinity crisis.The Pannonian basin offers the possibility to study the infilling style and stratigraphic evolution of an inland rifted basin. Detailed sequence analysis allows the recording of the apparent sea/lake-level motion which can be compared to those observed in the surrounding seas. Further tectono-stratigraphic evaluations of the Pannonian basin may reveal hidden facts on the nature of lake-level fluctuations and contribute new data to the discipline of sequence stratigraphy.