Forward seismic modeling as a guide improving detailed seismic interpretation of deltaic systems:
Example of the Eocene Sobrarbe Delta outcrop (South-Pyrenean foreland basin, Spain), as a
reference to the analogous subsurface Albian-Cenomanian Torok-Nanushuk Delta of the Colville
Basin (NPRA, USA)

Nicolas Grasseau<sup>abc*</sup>, Carine Grélaud<sup>b</sup>, Miguel López-Blanco<sup>c,d</sup>, Philippe Razin<sup>b</sup>

ABSTRACT

A forward seismic model of the Middle Eocene Sobrarbe delta outcrop (South-Pyrenees, Spain) has
been made to investigate its seismic signature. The Sobrarbe delta complex corresponds to a

<footnote><sup>a</sup> Institut des Sciences de la Terre de Paris - UMR 7193, Sorbonne Universités, 4 place Jussieu 75005 Paris, France.</footnote>

<footnote><sup>b</sup> Ecole Nationale Supérieure en Environnement, Géoressources et Ingénierie du Développement durable, Institut National Polytechnique de Bordeaux, Equipe d'accueil 4592, Géoressources & Environnement, 1 Allée F. Daguin 33607 Pessac, France.</footnote>

<footnote><sup>c</sup> Departament de Dinàmica de la Terra i de l'Oceà, Universitat de Barcelona, Carrer de Martí i Franquès 08028 Barcelona, Spain.</footnote>

<footnote><sup>d</sup> Institut Geomodels, Universitat de Barcelona, Carrer de Martí i Franquès s/n 08028 Barcelona, Spain.</footnote>

<footnote>* Corresponding author at : Institut des Sciences de la Terre de Paris - UMR 7193, Sorbonne Universités, 4 place Jussieu 75005 Paris, France. Tel. : +33699142395.</footnote>

<footnote>E-mail addresses : Nicolas.Grasseau@free.fr (N. Grasseau), grelaud@ensegid.fr (C. Grélaud), m.lopezblanco@ub.edu (M. López-Blanco), razin@ipb.fr (P. Razin).</footnote>
syntectonic sedimentary wedge locally thicker than 1000 m. This 15 km-long, highly-progradational and moderately aggradational sedimentary system presents sigmoid and oblique clinoforms, characterized by slopes ranging from 0.2° to 5.7°. Along the clinoforms, the classical deltaic depositional environments grade progressively from one to the other: alluvial plain, delta plain, delta front and prodelta. Locally, specific carbonate deposits occur, corresponding to transgressive pulses. The transition from delta front to prodelta is locally marked by large slide scars affecting clinoform morphology.

A detailed outcrop transect of the Sobrarbe delta was converted to an acoustic impedance model by assigning petrophysical properties to each geological facies. This model was then used to create synthetic seismic lines characterized by various input wavelets. The resulting various peak frequency synthetic seismic lines have been interpreted, and the seismic interpretation compared to the outcrop description to calibrate the seismic interpretation and assess the “limits” of that interpretation.

This work shows that the “seismic stratigraphy” methodology is consistent and allows the correct interpretation of most seismic sequences. However, composite reflections from rapid lateral facies transitions and higher-frequency sequence boundaries can clearly lead to seismic interpretation mistakes. The seismic facies of seven facies associations have been described, providing guidelines to recognize them on actual seismic lines. The synthetic seismic image of four selected systems tracts comprising good reservoir bodies has been analyzed to highlight recognition criteria which can be used to interpret analog deltaic systems in the subsurface, and therefore enhance the prediction and characterization of the reservoir bodies that they hold. These guidelines were systematically applied to a direct subsurface analog: the Torok-Nanushuk Delta (Colville Basin, NPRA, USA).

Keywords: Forward seismic modeling, Sobrarbe deltaic complex, Seismic facies, Seismic stratigraphy
# 1. Introduction

In most provinces of the world, oil and gas reservoirs are experiencing a mature stage of production with remaining reserves located in subtle stratigraphic traps. One of the main challenges in petroleum exploration and production is to predict and characterize the thickness and extent of reservoir bodies that are below or at the limit of seismic resolution. The well data of a few centimeters of vertical resolution (1-D) is generally too sparse to allow an accurate lateral correlation between facies within a computer model of a petroleum reservoir. By contrast, seismic surveys allow the investigation of the 2-D and 3-D stratigraphic architecture of reservoir units at a seismic resolution of 10 to 20 meters. Seismic resolution prevents or impairs the interpretation of high- and mid-frequency sequences (spanning around a few to several hundred thousand years). Thus, if there is no dense network of boreholes, there is no subsurface tool leading to the acquisition of 2-D or 3-D geological information at the meter-scale, i.e. scale used for reservoir modeling in the petroleum industry.

Outcrop studies allow the integration of a whole range of geological observation scales in 2-D or 3-D: from the thin-section to the stratigraphic and structural regional scale (Nichols, 2015). Therefore, outcrop studies provide geological data which fill the resolution gap between well and seismic data. Integrating the results of these very detailed outcrop studies in reservoir models is not automatic. The integration requires a phase of upscaling, back to the seismic scale, and the identification of guides and subtle criteria to improve the seismic interpretation. Forward seismic modeling, which simulates a theoretical seismic line from a geological transect, is the ideal tool to complete this integration step (Rudolph et al., 1989; Biddle et al., 1992; Helland-Hansen et al., 1994; Anselmetti et al., 1997; Janson et al., 2007; Schwab et al., 2007; Falivene et al., 2010, Bakke et al., 2011; Holgate et al., 2014).
Ancient fluvio-deltaic systems hold an important proportion of global hydrocarbon reserves. Several authors have already published seismic deltaic system analysis using forward seismic modeling of clinoforms (Helland-Hansen et al., 1994; Zeng and Kerans, 2003; Janson et al., 2007; Holgate et al., 2014) and deep siliciclastic turbidite systems (Bourgeois et al., 2004; Schwab et al., 2007; Falivene et al., 2010). Other models focused on the interpretation uncertainties of the “seismic-stratigraphy” methodology (Hodgetts and Howell, 2000; Zeller et al., 2015) or on the seismic interpretation limits due to seismic resolution (Rudolph et al., 1989; Schlager, 1991; Biddle et al., 1992; Helland-Hansen et al., 1994; Gartner and Schlager, 1999; Zeng and Kerans, 2003, Janson et al., 2007). The world-class outcrops of the middle Lutetian to Bartonian Sobrarbe deltaic complex (Dreyer et al., 1999) exposed in the South Pyrenean foreland basin are very good analogs for fluvio-deltaic sandstone reservoirs accumulated in active tectonic settings. The exceptional outcrop continuity enables the characterization of small-scale geometry and heterogeneity variations of the sedimentary bodies accumulated from the continental domain to the relatively deep basin environment (Figure 1). The Sobrarbe Delta has already been studied on several occasions: stratigraphic and tectono-sedimentary evolution (Wadsworth, 1994; Dreyer et al., 1999; Moody, 2014; Grasseau, 2016); geometries (Callot et al., 2009; Odonne et al., 2011); sedimentary facies (Labourdette, 2011; Mateu-Vicens et al., 2012; Grasseau, 2016).

The aim of the present paper is to provide guidelines to improve the seismic interpretation of subsurface deltaic systems from the study of synthetic seismic lines generated by forward seismic modeling of the Sobrarbe delta outcrop. The Sobrarbe Delta forward seismic model was built from an outcrop dip section measuring 15 km long and 700 m of average thickness. Entire stratigraphic architecture and lateral facies variations observed in the field were preserved (Figure 2a) to generate a synthetic seismic line similar to an acquired seismic line (Figure 2b). Two synthetic seismic sections were generated with different peak-frequencies (30 and 60 Hz). The comparative analysis of these synthetic seismic lines with the outcrop transect allowed the definition of guidelines or subtle criteria to minimize interpretation uncertainties and better predict the: (1) low to high-frequency sequences
and related sequence boundaries, (2) seismic facies from the alluvial plain to the deep-marine basin and specific geometries such as submarine slide surfaces which are relatively frequent in a syntectonic setting, (3) particular systems tracts and the distribution and characterization of related reservoir bodies.

The synthetic seismic lines have also been compared to the acquired seismic lines of the Early-Cretaceous deltaic system located in the National Petroleum Reserve of Alaska (USA) (Figure 2b). The deltaic system made up by the Nanushuk and Torok Formations (Brookian Sequence) is characterized by large-scale clinoforms which filled progressively the Colville foreland basin during the development of the Brooks Mountain Range (Molenaar, 1983, 1985, 1988; McMillen, 1991; Bird and Molenaar, 1992; Houseknecht, 2001; Houseknecht, Bird, et al., 2009; Houseknecht, Burns, et al., 2009). The Sobrarbe deltaic complex and the Nanushuk/Torok delta were both accumulated in syntectonic compressive settings and display similar geometrical and sedimentological features. The analogy between the synthetic and the acquired seismic lines allows to directly apply the results of this study on a concrete example.

## 2. Outcrop transect: the Sobrarbe Delta

### 2.1. Geological setting

The Pyrenean chain is an E-W intracontinental mountain range resulting from the collision between the Iberian and European plates from the Late Cretaceous to the Tertiary times. The Pyrenees constitute a divergent asymmetric orogenic wedge with associated foreland basins to the south and north (Muñoz, 1992). The studied stratigraphic unit is located in the South-Pyrenean zone which is bordered northward by the axial zone and southward by the non-deformed Ebro foreland basin (Figure 1b). The South-Pyrenean zone is represented by a piggy-back sequence of southwards-displaced Mesozoic thrust sheets (Bóixols-Cotiella, Montsec-Peña Montañesa and Gavarnie-Sierras
Exteriores) which developed a series of thrust-top sedimentary basins from the Upper Cretaceous to the Paleogene (Ori and Friend, 1984). The north-south contraction and shortening is accompanied by a migration of deformation from east to west due to a counterclockwise rotation of the Iberian plate (Choukroune, 1974; Arthaud and Séguret, 1981; Roest and Srivastava, 1991; Muñoz, 1992). Present day north-south trending synsedimentary anticlines and synclines also developed at or near the lateral/oblique ramps of these thrusts (Figure 1b). However, some of them have suffered important vertical axis rotations during the emplacement of different thrust sheets (Muñoz et al., 2013).

The south-Pyrenean foreland basin is made up of several sub-basins delimited by major north-south-trending folds perpendicular to the orogen. The Middle Eocene succession of the Sobrarbe deltaic complex progressively filled the Ainsa sub-basin located at the top of the Gavarnie-Sierras Exteriores nappe in the central part of the South-Pyrenean foreland basin (Figure 1) (Puigdefàbregas et al., 1992; Muñoz et al., 1994). The geometry of the Ainsa basin is mainly controlled by the NNW/SSE trending Buil syncline located between the synsedimentary anticlines of Boltaña (west) and Mediano (east).

The Sobrarbe deltaic complex corresponds to a syntectonic sedimentary wedge locally thicker than 1000 m. The progradation of this sedimentary system over more than 15 km towards the NNW (Figures 1 and 2a) was controlled by the orientation of the Ainsa sub-basin (Puigdefàbregas, 1975; De Federico, 1981; Wadsworth, 1994; Dreyer et al., 1999; Grasseau, 2016). The Sobrarbe Delta consists of a succession of depositional environments characteristic of a fluvio-deltaic sedimentary system mainly dominated by fluvial currents (Garrido-Megias, 1968; Van Lunsen, 1970; Puigdefàbregas, 1975; De Federico, 1981; Wadsworth, 1994; Dreyer et al., 1999; Moody, 2014; Grasseau, 2016).

## 2.2. Transect and depositional environments
The great exposure of strata along the western limb of the Buil syncline allowed the realization of an accurate fieldwork used as input to forward seismic modeling (Figure 2a). Between the villages of Mondot and Santa Maria de Buil, a highly detailed proximal (SSE) to distal (NNW) correlation transect was built from 40 field-measured stratigraphic log sections (Scale: 1: 100) physically correlated between them. The fluvio-deltaic system base is defined on the Buil syncline west limb by the black-marl marker level of San Lino (Arbués et al., 2011; Grasseau, 2016). The top of the Sobrarbe deltaic complex is the Olsón stratigraphic discontinuity interpreted as a minimum of accommodation within the Escanilla Formation (Bentham, 1992; Dreyer et al., 1999; Muñoz et al., 2013).

In the Sobrarbe deltaic complex five depositional environments and 16 sub-environments have been differentiated (Grasseau, 2016) (Figures 1, 2a and Table 1).

The Prodelta environment is characterized by blue-gray marls incised by fine to coarse sandstones and heterolithic sandstones interpreted as turbidite channels and lobes (Figure 2a). These deposits belong to the San Vicente Formation (Van Lunsen, 1970). Two kinds of turbidite systems were defined: a type 1 turbidite system characterized by continuous thin deposits extending from the delta front to the prodelta environment, a type 2 turbidite system which is thick and isolated within the prodelta marls in more distal parts of the basin. Type 1 turbidites consist of an alternation of meter-thick fine- to coarse-grained sandstone beds with planar bedding, and heterolithic intervals of several-centimeters-thick fine-grained sandstone beds and marl. Type 2 turbidites consist of stratified fine- to medium-grained sandstone with planar bedding and megaripple cross-bedding. These deposits grade laterally to fine-grained sandstone layers presenting planar and current-ripple bedding, which alternate with laminated silt intervals.

Towards the continent, the deep marine San Vicente Formation grades progressively to the deltaic Sobrarbe Formation characterized by delta front to delta plain deposits. Delta front environment is characterized by sandstone deposition on deltaic foreset and topsets. Silts and bioturbated fine- to medium-grained sandstone, locally carbonate-rich (due to a high concentration of nummulites), form
the distal to middle delta front deposits deposited on foreset areas (De Federico, 1981; Wadsworth, 1994; Dreyer et al., 1999; Grasseau, 2016). A succession of erosive medium- to coarse-grained sandstone beds with planar bedding and megaripple cross-bedding, interpreted as mouth-bars and terminal distributary channels, form the proximal delta front deposits of the topset where local tidal reworking was recorded by sediments. Submarine gravity slides, more than hundreds of meters wide, locally incise the delta front and prodelta deposits cutting down more than 100 m of thickness (Figure 2a) (Wadsworth, 1994; Dreyer et al., 1999; Callot et al., 2009; Grasseau, 2016). Carbonate shallow marine deposits, accumulated during transgressive episodes, are characterized by (1) localized carbonate platforms made up of reef or peri-reefal facies, or (2) sandy-carbonate shoals rich in nummulites (Figure 2a). Towards the proximal part, upper delta front deposits grade to shale-rich lower delta plain deposits, locally incised by fine- to coarse-grained lenticular sandstone bodies with megaripple cross-bedding, corresponding to isolated fluvial distributary channels.

The Escanilla Formation (red facies association in Figure 2a) is the alluvial lateral equivalent of the Sobrarbe Formation. It consists of an association of conglomeratic sandstone and fine- to coarse-grained sandstone layers, corresponding to amalgamated fluvial channels incising the alluvial plain reddish shales (Garrido-Megias, 1968).

## 2.3. General stratigraphic architecture

The Sobrarbe delta complex is a large-scale highly-progradational and moderately aggradational system where three scales of sequential arrangement have been defined (Grasseau, 2016). Nine mid-frequency stratigraphic sequences S1 to S9 (sensu Catuneanu et al., 2017) have been identified (Figure 2a), each spanning around a few hundred thousand years. Most of these sequences (S4 to S9) are defined from the four-tract model (Hunt and Tucker, 1992) including Lowstand, Transgressive, Highstand and Falling-stage systems tracts. However, in some cases (S1 to S3) a regressive systems tract has been defined when the differentiation among Lowstand, Falling-stage and Highstand
systems tracts was not possible. In this last case, the mid-frequency stratigraphic sequences are two-
tract type (T-R or Transgressive-Regressive sequences from Johnson et al., 1985, Embry and
Johannessen, 1992; López-Blanco, 1993; López-Blanco 2000). Deltaic sequences S1 and S2 were only
deposited on the southeastern area of the Buil syncline. Thus, the western Buil syncline transect
(correlation panel) shows a detailed architecture of the Sobrarbe deltaic complex including only
sequences S3 to S9 (Figure 2a). These sequences can be subdivided into a series of higher-frequency
sequences and parasequences, each spanning around tens of thousands of years, which stacking
controls the different systems tracts trends.

The stacking pattern of the nine mid-frequency stratigraphic sequences defines three low-frequency
Transgressive-Regressive composite sequences SI to SIII, each spanning around several hundred
thousand years (Figure 2a). Sequence SI records the progradation of the deltaic system over more
than 12 km towards the NNE and the accumulation of more than 200 m-thick deposits in the distal
part. The 300 m-thick Sequence SII is characterized by two regressive phases separated by a minor
transgressive stage. The deltaic system prograded and retrograded over distances ranging from 5-6
km. Sequence SIII is mainly characterized by 40 m-thick Nummulite-rich deposits recording a major
transgression of the deltaic system over more than 12 km. The backstepping of the deltaic system is
followed by the progradation of facies belts over 15 km and the accumulation in the distal part of at
least 250 m of deposits. The original dip of clinoforms varies from 0.2° to 5.7° (Grasseau, 2016). The
sedimentary organization of the Transgressive-Regressive composite sequences was clearly
influenced by the variations of the subsidence rate of the Ainsa sub-basin. The development of
tectonic structures of more or less wide amplitude locally hid the effects of subsidence during certain
periods. These parameters are recorded by particular depositional geometries such as wedging
towards active tectonic structures, increase of clinoform dip angles and delta progradation rate, or
by an important accumulation of sediments in the topset of the deltaic system in areas affected by
an increase of subsidence (Grasseau, 2016).
### 3. Subsurface analog: the Nanushuk-Torok fluvio-deltaic system

The Albian-Cenomanian Nanushuk-Torok deltaic system was deposited in the Colville foreland basin (National Petroleum Reserve of Alaska, USA). The foreland basin mainly formed during the Cretaceous and Tertiary, by flexural subsidence in response to the simultaneous development of the Brooks Range orogenic belt to the south, of the Herald Arch thrust belt to the west and of the Beaufort rift shoulder to the north (Figure 3) (Bird and Molenaar, 1992; Moore et al., 1994).

The Nanushuk-Torok system corresponds to a syntectonic prograding-aggrading fluvio-deltaic complex within a first-order depositional sequence (Molenaar, 1985, 1988; Mull et al., 2003; LePain et al., 2009). The base of this deltaic succession corresponds to the Hue Shale Member, known as the gamma-ray zone (GRZ) or highly radioactive zone (HRZ), a highly organic-rich condensed shale interval deposited in the most distal part of the basin (Carman and Hardwick, 1983; Molenaar, 1983, 1985, 1988; Molenaar et al., 1987; Bird and Molenaar, 1992; Creaney and Passey, 1993). The overlying prodelta deposits correspond to the Torok Formation, mainly made of silty shales, siltstone, mudstone and argillaceous sandstone, incised by turbidite sandstone channels and lobes (Molenaar, 1985, 1988; Bird and Molenaar, 1992; Houseknecht, 2001; Houseknecht and Schenk, 2007; LePain et al., 2009). On seismic lines, the Torok Formation corresponds to the foreset and bottomset parts of well-imaged clinoform reflections (Figure 2b). In the topset part of the sigmoid clinoforms, the Torok Formation grades laterally to the Nanushuk Formation, mainly characterized by an alternation of shale and sandstone beds. The sandstone beds are coarsening-up and thickening-up amalgamated delta front deposits, characterized by storm wave-generated structures. These delta front deposits grade laterally to a complex facies assemblage of sandstone, mudstone, coal and conglomerates, interpreted as bay-fill, delta plain and alluvial plain deposits (Fisher and McGowen, 1969; Ahlbrandt et al., 1979; Huffman et al., 1985, 1988, Molenaar, 1985, 1988; LePain and Kirkham, 2001; Houseknecht, Bird, et al., 2009; LePain et al., 2009). In the studied area, the sediment distribution
within the deltaic system appears to have been mainly controlled by fluvial processes. Shoreface
deposits were however clearly reworked by wave action (Ahlbrandt et al., 1979; Huffman et al.,
1988; LePain and Kirkham, 2001; LePain et al., 2009).

To the north of the Colville Basin, the total thickness of prograding wedges ranges between 600 and 1000 meters, whereas in the southern part of the basin, the total thickness reaches 1700 to 2500 meters. The average length of clinoform slope is approximately 30 km in the north and 70 km in the south of the Colville Basin. The average clinoform dip varies between 0.6° and 2.5°. In the foresets and bottomsets, sedimentary accumulations resedimented from gravity slides or destabilizations are frequently imaged on seismic lines by low-amplitude contorted reflections truncating underlying reflections (Houseknecht et al., 2009).

The seismic data base used for our research consists of a grid of 27 2D seismic lines acquired in 1981 by the Federal Government of the United States in the northeastern part of the Colville Basin in the National Petroleum Reserve of Alaska (NPRA). The seismic lines have been processed and migrated via modern technics of signal processing, and then made available to the public domain by the USGS (United States Geological Survey). These seismic lines are characterized by a peak frequency of 40 Hz and a vertical seismic resolution of approximately 20 meters in the studied interval (Figure 2b).

**4. Forward seismic modeling: methodology**

A forward seismic model of the Sobrarbe Delta outcrop was made by assigning petrophysical
properties to the different geological facies, allowing the creation of an impedance model. Synthetic
seismic lines were then generated from this model, using different input wavelets.

The detailed stratigraphic framework of the Sobrarbe delta (Figure 2a) was first drawn in the GXII
software (ION Geophysical), and then filled with lateral facies changes that were assigned
petrophysical properties to make an impedance model of the outcrop (Figure 4a). The parts of the
correlation transect that were not visible at outcrop were restored by paleogeographic
reconstruction (Grasseau, 2016). A homogeneous unit of 600 meters, corresponding to fluvial
deposits, was added above the Sobrarbe fluvio-deltaic succession in order to take into account the
effects of burial (overburden) on the synthetic seismic image (Figure 2a vs. Figure 4a), making it
comparable to the Torok/Nanushuk delta burial depth. Based on their recurrence and their reservoir
potential, seventeen facies have been selected (out of the twenty-four identified on the outcrop) to
be used in the impedance modeling (Table 1). Each facies was assigned a value of density and a value
of seismic wave propagation velocity, interpreted from the analysis of well logs intercepting the
Torok and Nanushuk Formations in the subsurface analog (NPRA). For that purpose, a comparative
analogy was made between the facies of the two deltas (Table 1). In the acoustic impedance model,
all the beds measuring more than 5 meters in thickness were represented, even if they were thinner
than seismic resolution. The model includes a maximum of geological detail in order to simulate a
more realistic composite response of the seismic signal passing through a similar geologic setting
(Schwab et al., 2007).

The synthetic seismic lines were generated from the acoustic impedance model by vertical incidence
ray-tracing. This method generates the equivalent of post-stack, processed and fully migrated
synthetic seismic sections (Figures 4b and 4c), directly comparable with acquired processed seismic
lines. The synthetic seismic was created using zero phase Ricker wavelets of varying peak frequencies
(30 and 60 Hz), a sampling rate of 4 ms and a Source/Geophone spacing of 6.25 m. These standard
parameters of modeling are similar to the acquisition parameters of industrial 2-D or 3-D seismic
data. A positive standard polarity is used on the synthetic seismic lines, i.e. a positive amplitude
(peak reflection, red color) corresponds to a positive reflection coefficient, and a negative amplitude
(trough, blue color) corresponds to a negative reflection coefficient. The theoretical vertical
resolution of the synthetic seismic lines is calculated from Rayleigh's criterion of 1/4 of the
wavelength. The average velocity used in the Rayleigh's criterion, i.e 2880 m.s⁻¹, is the calculated
average velocity of facies along two measured stratigraphic logs located in the proximal and distal
parts of the geological model. The theoretical vertical resolution corresponds to ca. 24 meters for a
peak frequency of 30 Hz and ca. 12 meters for a peak frequency of 60 Hz.

**5. Results**

The synthetic seismic profiles (30 and 60 Hz) are characterized by a succession of clinoform reflections, presenting different geometries (oblique and sigmoid), variable continuity and amplitude due to the variations of acoustic impedance contrasts in the Sobrarbe delta stratigraphic architecture (Figures 4b, 4c and 5). Within the topsets of sigmoid clinoforms (sequences 4 to 7, Figure 5b), the reflections are mainly tabular, wavy and discontinuous, presenting either strong or weak amplitude. Foreset reflections are characterized by continuous, medium- to low-amplitude dipping clinoform reflections, which locally end by distal erosional truncation (sequences 5 and 6, Figure 5b). The bottomset part of the clinoforms is mainly characterized by a transparent seismic facies, within which isolated high-amplitude seismic reflections are visible (sequences 3 to 6, Figure 5b) and locally end with onlap terminations towards the proximal part (sequences 2 and 3, Figure 5b).

**5.1. Seismic stratigraphy**

**5.1.1. Blind analysis**

The analysis of reflection terminations and clinoform offlap-break trajectories (seismic stratigraphy) has allowed the blind interpretation of nine mid-frequency seismic sequences on the 60 Hz synthetic seismic line (Figure 5b). The 30 Hz synthetic seismic line was not used in this section because its low vertical resolution prevents a detailed seismic stratigraphic study.

The first interpreted seismic sequence is characterized by a succession of low-amplitude negative reflections (trough) organized as oblique clinoforms, which end with toplap terminations below a
continuous high-amplitude peak reflection. This high-amplitude reflection has been interpreted as
the seismic sequence boundary.

The second interpreted seismic sequence is also characterized by medium- to low-amplitude oblique
clinoform reflections, which end with toplap terminations under a rather continuous peak reflection
of high-amplitude, interpreted as the seismic sequence boundary.

The third seismic sequence consists of similar oblique clinoform reflections as in sequence 2. The
clinoforms seem to end with toplap terminations under a continuous high-amplitude peak reflection,
dipping towards the proximal area and marked by a « step-like » undulating geometry around the
10th km in the transect. This reflection is characterized by an incised geometry marked by erosional
truncation terminations around the 12th and 13th km in the transect. This peak reflection, interpreted
as the top of sequence 3, is also the top of sequences 1 and 2 between 10.5 km and 14 km.

The fourth interpreted seismic sequence is mainly characterized by several medium-amplitude
sigmoid to oblique clinoform reflections. The offlap break trajectory, upward and toward the distal
area, records a phase of progradation-aggradation of the deltaic system. The clinoforms end with
onlap terminations on the reflection corresponding to the top of sequence 3, or with toplap
terminations under a high- to medium amplitude continuous reflection interpreted as the top of
sequence 4.

The fifth interpreted seismic sequence is relatively thin, consisting of a few tabular high- to medium-
amplitude reflections, which correspond to the proximal topset part of sigmoid clinoforms. A very-
high-amplitude reflection triplet (peak-trough-peak) is particularly visible in the middle part of this
sequence (between 5.5 and 9 km). These reflections become discontinuous with a lower and variable
amplitude after 9 km (proximal part of the topset). The upper boundary of this seismic sequence is
marked by a continuous reflection of high to medium positive amplitude, characterized by erosional
truncations below and onlap terminations above.
The sixth interpreted seismic sequence is characterized by peak reflections of medium to high
amplitude, locally undulating, corresponding to sigmoid clinoforms that are truncated distally and
end with onlap terminations proximally. The trajectory of the clinoform offlap-breaks appears mainly
vertical which corresponds to an aggradation. The upper boundary of this seismic sequence is a
discontinuous peak reflection of very variable amplitude, marked by erosional truncations and onlap
terminations. This sequence boundary is difficult to pick, particularly towards the proximal part.

The seventh interpreted seismic sequence is characterized in its lower part by two high-amplitude
reflections corresponding to sigmoid clinoforms, marked by a steep offlap break and a concave-up
profile. They end with onlap terminations proximally. The upper part of this seismic sequence is
made of high-amplitude, very discontinuous, undulating reflections between the 8th and 14th km.
Between the 3rd and 8th km, these reflections become more continuous, tabular and present a lower
amplitude. They end distally with erosional truncations below a very low-amplitude, concave-up
reflection (red slide scar on Figure 5b). The upper boundary of this seismic sequence is interpreted
on a continuous reflection of strong negative amplitude (trough), along which several reflections end
with onlap or toplap terminations.

The interpreted seismic sequence 8 is characterized by two reflections of medium to low amplitude,
which geometries resemble sigmoid clinoforms. The top of this seismic sequence is interpreted on a
continuous low-amplitude reflection, marked by onlap terminations.

The ninth interpreted seismic sequence is characterized by low-amplitude concave-up reflections.
They end with toplap terminations below a high-amplitude reflection (between km 0 and 6),
interpreted as the sequence boundary. This reflection grades laterally to a very low-amplitude to
transparent reflection between km 6 and 11, and then becomes discontinuous of medium amplitude
between km 11 and 14.
### 5.1.2. Discussion

Reservoir geologists need a good model to limit the uncertainties linked with the distribution and continuity of reservoir bodies, which are the essential parameters to predict spatial variations of petrophysical properties and net-to-gross ratio. This model comes from the interpretation of subsurface data, mostly based on the seismic stratigraphy method.

In theory, the interpretation of seismic sequences on the synthetic seismic profiles should allow the reconstruction of a sequence architecture similar to the sequence architecture of the geological transect, i.e. with the same interpreted number and same location of sequences and sequence boundaries (SB). On the 60 Hz synthetic seismic line, the picking of the top of the seismic sequences 1, 2, 4, 6, 7 and 8 (Figure 5b) corresponds to the projected position of the sequence boundaries (SB) described on the geological transect (Figure 5c). However, the interpreted seismic sequences 3 and 5 do not correspond to original mid-frequency stratigraphic sequences on the geological transect. These differences and their causes will be discussed below in further detail.

The projected real geological stratigraphic surfaces on an enlargement of seismic sequence 3 (Figure 6) highlight the misinterpretation of its top sequence boundary along a high-amplitude reflection characterized by toplap and erosional truncation terminations. In fact, Figure 6e shows that this reflection results from the combined image of several very close surfaces, creating a composite reflection, relatively continuous, locally wavy, presenting a step-like geometry. This geometry is particularly well-imaged on the positive high-amplitude reflection presenting a dip towards the proximal area between kilometers 10 and 11 (Figures 6c and 6d). The seismic imaging, limited by the vertical resolution, generates false reflection terminations and pseudo-discontinuities via tuning effects (interference) when the thickness between stratigraphic surfaces is thinner than the vertical resolution value of the seismic line (Rudolph et al., 1989; Schlager, 1991; Biddle et al., 1992; Helland-Hansen et al., 1994; Gartner and Schlager, 1999; Zeng and Kerans, 2003). In this case, the lateral transition between two facies can be abnormally imaged by a reflection with a step-like geometry,
which resolution increases with the peak frequency of the seismic profile (Schlager, 1991; Zeng and Kerans, 2003). In the impedance model of the Sobrarbe delta, the lateral facies variations between delta plain and delta front deposits occur within 5 to 10 m-thick stratigraphic units in the clinoform topsets of sequences S3 to S5 (Figures 2b and 6b). These thickness values are below the 12 m-tuning thickness calculated for this 60 Hz synthetic seismic line, which explains the interferences and the presence of this ambiguous step-like composite reflection, wrongly interpreted as a sequence boundary.

A theoretical model of a prograding-aggrading deltaic sequence was drawn on the GXII software, and then converted into a synthetic seismic profile in order to analyze in detail the seismic response of the facies transition between delta plain and delta front deposits in the topset of sigmoid clinoforms (Figure 7). On the synthetic seismic line generated with a 30 Hz peak frequency (Figure 7b), the transition zone between the delta plain facies and the delta front facies is characterized by a high-amplitude positive reflection presenting a wavy, step-like geometry, under which pseudo-oblique clinoform reflections terminate with pseudo-toplap terminations. With a 60 Hz peak frequency (Figure 7c), the reflections, imaging stratigraphic surfaces, can be better followed across the facies transition, but they show a significant amplitude variation in this area. These high-amplitude parts of the reflections can catch the eye of the interpreter and be misinterpreted as a proper reflection (pseudo-unconformity, Schlager et al., 1991; Stafleu and Schlager, 1995). This "false reflection" or "step-like reflection", dipping towards the proximal part, is comparable with the composite reflection interpreted as the top of seismic sequence 3 in the synthetic seismic line of the Sobrarbe delta (Figures 6d and 7d). The theoretical model confirms that the top sequence 3 reflection corresponds to the composite seismic image of a facies variation between delta plain and delta front deposits and not to the seismic image of a stratigraphic surface / sequence boundary.

The same type of reflection, but characterized by a negative amplitude, locally wavy and presenting a step-like geometry, associated with pseudo-downlap terminations of the clinoforms, is observed
within seismic sequence 2 (black arrows on Figure 6e). By analogy with previous results and comparison with the geological model, this step-like reflection is interpreted as the seismic image of a facies transition between silt and sandstone delta front deposits and prodelta marls.

In this kind of deltaic clinoform systems, the main criteria to differentiate between the image of a real stratigraphic surface (true unconformity) and a facies transition (pseudo-unconformity) are: the step-like geometry of the reflection and its dip towards the proximal area, associated with the presence of pseudo-toplap terminations below (or downlap terminations above). Reflections presenting these particular geometries were actually observed in the seismic lines of the Colville Basin analog (Figure 7e). In this acquired seismic line, all the geometrical criteria are visible, but the amplitude of the composite reflection is not as high as in the synthetic seismic lines. It may be due to a lower contrast of acoustic impedance between the delta front and the delta plain facies in the Torok/Nanushuk system. The integration of the forward seismic modeling results allows a clear improvement of the interpretation of this seismic line, with potential implications at the reservoir scale.

The top of seismic sequence 5 on the interpreted synthetic seismic line is characterized by several erosional truncations below and onlap terminations above the picked reflection between kilometers 3 and 4 (Figure 5b). According to seismic facies analysis (see paragraph 5.2.4), this reflection can be interpreted as the seismic image of a major submarine collapse surface, filled by draping distal delta front deposits (Figure 8d). These slide surfaces are particularly well developed in highly-aggrading to retrograding stratigraphic sequences, imaged by: the presence of a very-high-amplitude reflection (triplet peak-trough-peak) in the distal part of the topset of sigmoid clinoforms; a vertical or proximally migrating offlap-break trajectory (Figure 5b). According to the geological transect (Figure 2a), the slide surface becomes conformable proximally, only showing a minor downward shift of delta front facies. This interpreted seismic sequence boundary is therefore not a mid-frequency stratigraphic sequence boundary, but a higher frequency sequence boundary.
## 5.2. Seismic Facies

The lithological units corresponding to the different depositional environments of the outcrop geological model are characterized by parameters influencing their seismic image, such as: the geometry, thickness and extent of the beds and the type of acoustic impedance contrasts. The comparative analysis between the synthetic seismic profiles and the outcrop transect allows the association of each lithological formation with a seismic facies, i.e. a group of reflections differentiated by their seismic parameters: configuration, amplitude, continuity and frequency (Mitchum et al., 1977). In hydrocarbon plays, the correct interpretation and the mapping of the different seismic facies, coupled with well data analysis, constrain the distribution of sediment bodies within the different sequences or systems-tracts, and infer the properties of potential reservoir systems.

The seismic facies imaging the alluvial plain, delta plain, delta front, slide-scar fill, type 1 and 2 turbidites, isolated carbonate platform deposits and nummulite beds have been characterized in detail in the 30 Hz and 60 Hz synthetic seismic lines. An analogy with the seismic facies observed on the acquired seismic lines of the Colville Basin (NPRA) is then proposed (Figures 8 and 9).

### 5.2.1. Alluvial plain seismic facies

Alluvial plain deposits are characterized by fluvial channels, laterally and vertically amalgamated within flood plain red shale (Escanilla Formation). In the synthetic seismic lines, this lithological unit is imaged in the topsets of the progradational-aggradational deltaic complex by overall tabular and discontinuous reflections, presenting high but variable amplitudes and an average frequency (Figure 8a). These reflections locally show a concave-up, convex-up or undulating geometry. The concave-up geometry can be interpreted as the seismic expression of the original geometry of a channel-base or
as a push-down effect due to a velocity anomaly above. The convex-up geometry is the result of a
pull-up effect, due to a reverse velocity anomaly above. These push-down and pull-up effects are
generated by high positive or negative velocity contrasts between the lens-shaped channel-fill and
the surrounding rocks. In the acquired seismic lines from the Colville Basin (NPRA), this seismic facies
is observed in the proximal part of the topsets, at least 20 km before the offlap break of the
clinoforms (Figure 8a).

**5.2.2. Lower delta plain seismic facies**

The distributary channels, isolated within delta plain shales are imaged differently according to the
frequency used to generate the synthetic seismic lines. On the 60 Hz synthetic seismic section, the
seismic facies is characterized by very discontinuous, high-amplitude concave-up reflections,
presenting a slightly higher frequency than in the alluvial plain seismic facies. These reflections are
localized within a transparent facies, induced by the lack of lithological contrasts within the shale
intervals (Figure 8b). This seismic facies is also located in the topset, between the alluvial plain facies
and the offlap break. On the 30 Hz synthetic seismic line, the delta plain seismic facies is
characterized by sub-tabular reflections, more continuous than in the alluvial plain facies and with
amplitudes varying between low and medium. On the acquired seismic lines of the Colville Basin
(NPRA), the identical seismic facies has not been observed, but a similar facies characterized by low-
amplitude, tabular and continuous reflections could correspond. This facies is also located in the
topset, between the alluvial plain facies and the offlap break.

**5.2.3. Delta front seismic facies**

The massive and relatively thick sandstone units corresponding to the proximal delta front deposits
are poorly imaged on the synthetic seismic lines. They are characterized by low-amplitude reflections
or by an almost transparent facies located under the step-like composite reflection which marks a lateral transition of facies (see paragraph 5.1.2) from delta front to lower delta plain. The distal delta front sandstone units, organized in thinner layers interbedded with prodelta shale, are imaged by continuous dipping reflections (clinoforms) of medium to low amplitude and average frequency (Figure 8c). These sandstone bodies are thinner than seismic resolution and therefore each bed is not imaged by a distinct seismic facies. However, the alternation of sandstone and shale is imaged by a succession of clinoform reflections, resulting from the interference between the image of the top and the base of the sandstone beds (peak-trough couplets). Towards the distal domain, the amplitude of these reflections decreases gradually, highlighting the tuning effect associated with the thinning of the sandstone beds towards the basin. These reflections finally disappear within a transparent facies unit, imaging the low internal reflectivity within the prodelta marls.

On the seismic lines from the Colville Basin (NPRA), a high-amplitude positive reflection (red peak) is often visible over a few kilometers proximally before and around the offlap break of the clinoforms (Figure 8c). The composite blue trough reflection located below could correspond to the image of the proximal delta front massive sandstone. The foreset of the clinoforms presents either a transparent facies, or a facies characterized by reflections of medium amplitude, marked by a peak-trough couplet which could indicate the presence of distal delta front sandstone bodies in the upper part of the clinoform slope. The transparent facies would then correspond to the image of more homogeneous, argillaceous or silty deposits on the slope, which have also been observed at specific stratigraphic levels in the Sobrarbe delta (Sequence S5, Figure 2).

### 5.2.4. Fill of submarine slide-scars seismic facies

Submarine slide surfaces are poorly imaged on the synthetic seismic lines. Where they can be recognized, they are reflections with a concave-up geometry, steeper than the overlying and underlying clinoforms, marked by erosional truncations (below) and onlap terminations (above).
These reflections are discontinuous and difficult to follow towards the distal part, since the collapse surface does not act as a reflector anymore when the fill of the depression becomes similar to the substratum. The clinoforms affected by these slide surfaces present an irregular, undulating geometry, marked by an abrupt and locally discontinuous break of slope (offlap break) (Figure 8d). The fill of these submarine slide scars by distal delta front sandstone and prodelta marls is sometimes imaged by a medium-amplitude reflection corresponding to the interference between the base and the top of the sandstone unit.

In the seismic analog (NPRA), several clinoforms present all the criteria described above: a reflection with an irregular, concave to undulating geometry, marked by an abrupt offlap break, erosional truncations and onlap terminations (Figure 8d). These reflections can thus be interpreted as the image of submarine slide scars on the slope of the delta. Their fill is characterized either by medium-amplitude dipping reflections which can be interpreted as the image of an alternation of distal delta front sandstone and prodelta marls, or by a transparent facies corresponding to the image of homogeneous pro-delta marls.

### 5.2.5. Turbidite seismic facies

Type 1 turbidites form thin, laterally extensive sandstone bodies, interbedded within sandy-silty distal delta front deposits and prodelta marls. On the synthetic seismic lines, they are imaged by continuous reflections which can only be differentiated from the distal delta front seismic facies by slightly higher amplitude and a slightly lower dip between the foreset and the bottomset (Figure 9b). Towards the foreset, these seismic reflections sometimes end with onlap terminations on the reflections of the distal delta front facies. The reflections from the type 1 turbidites dim out or disappear abruptly within a transparent interval (prodelta marls) towards the distal domain. Their geometric characters, similar to those of the distal delta front seismic facies, make it difficult to differentiate them at 60 Hz and almost impossible at 30 Hz. On the acquired seismic lines of the
Colville Basin, this seismic facies could correspond to the medium- to low-amplitude, relatively continuous reflections which mark the distal, less dipping part of the clinoforms.

Type 2 turbidites form relatively thick and well-bedded lens-shaped sandstone bodies. On the synthetic seismic lines, they are imaged by high-amplitude discontinuous reflections which can locally be undulating or concave-up, within a transparent interval corresponding to homogeneous prodelta marls in the bottomset of the clinoforms (Figure 9c). The amplitude is maximum in the center of the reflections and dim out towards their extremities. Dipping reflections from the distal delta front facies (or from the type 1 turbidite facies) locally end with downlap terminations on this Type 2-turbidite seismic facies. In the seismic analog, this type of discontinuous, undulating/concave-up, high-amplitude reflections is frequently observed in the bottomsets of the clinoforms (Figure 9c). These reflections can therefore be interpreted as type 2 turbidites.

### 5.2.6. Seismic facies of the isolated carbonate platform

In the Sobrarbe delta, the isolated carbonate platform unit is characterized by a very high acoustic-wave-propagation velocity compared to the surrounding rocks, and by a thickness lower than or at the limit of seismic resolution. This sediment body is thus mainly imaged by only one composite seismic reflection (peak-trough couplet) of very high amplitude and low frequency, located near the clinoform offlap break, in the distal part of the topset (Figure 9a). This reflection ends with a toplap termination towards the proximal area, because its dip is slightly higher than the overlying reflections. Distally, this well-marked reflection ends abruptly around the offlap break and is replaced by steeper reflections (clinoforms), slightly offset vertically, presenting a lower amplitude and a higher frequency. At 60 Hz, all these parameters are clearly visible, whereas at 30 Hz the high-amplitude reflection interferes with the one above. At 30 Hz, the abrupt termination of the reflection at the offlap break is the only visible criteria, which can however be wrongly interpreted as a fault. This type of facies can be differentiated from the proximal delta front facies by its position close to
the offlap break and by the lack of continuity with the foreset. A slight pull-up effect may be
observed below the high-amplitude reflection imaging the carbonate platform, due to the positive
velocity anomaly which characterizes this sediment body **(Figure 9a)**. The seismic facies of the
isolated carbonate platform was locally observed on the seismic lines of the NPRA analog **(Figure 9a)**.
All the criteria could be recognized: high-amplitude and low-frequency reflection located at and
before the offlap break, which ends with a toplap termination proximally and ends abruptly at the
offlap break. Unfortunately, no well was drilled through this seismic facies and therefore this
interpretation could not be tested.

**5.2.7. Nummulite beds seismic facies**

Nummulite beds are characterized by relatively thick intervals made up of alternating homogeneous sandy/marly-limestone beds rich in nummulites and silt/marls, presenting small acoustic impedance contrasts. On the 60 Hz synthetic seismic line, this lithological unit is imaged by continuous, very-low amplitude and slightly divergent reflections (Figure 9d). Around the offlap break location, these reflections are truncated by another steep reflection, poorly visible, imaging a submarine slide surface. On the 30 Hz synthetic seismic line, the reflections interfere and the seismic facies is therefore only represented by one low- to average-amplitude reflection which separates into two distally, while their amplitude dims out. On the seismic lines of the NPRA analog, a similar seismic facies was observed locally, near the distal margin of relatively thick topsets (Figure 9d). The presence of clinoforms within these topsets indicates a phase of retrogradation of the sedimentary system, making environmental factors favorable to the development of a more carbonate-rich system. However, according to the Cretaceous age of the Torok/Nanushuk Formations, there would be no nummulites associated with this facies in the Colville Basin.
## 6. Seismic response of four selected systems tracts containing good potential sandstone reservoir bodies

Four examples of different mid-frequency systems tracts have been selected and studied in detail because of their main geometrical characteristics, particular variation of accommodation and sediment supply recorded by a related stratal stacking pattern, and the potential interest of sandstone reservoir bodies (extent, thickness and net-to-gross ratio). These are three Regressive systems tracts from sequences 4, 5 and 6 interpreted as Highstand systems tract (HST) or Falling stage systems tract (FSST), and a thick Transgressive systems tract (TST) from sequence 6 (Figure 10). The synthetic and acquired seismic image of the selected systems tracts comprising good potential reservoir bodies are analyzed, aiming to highlight some recognition criteria which can be used to interpret analog deltaic systems in the subsurface, and therefore to enhance the prediction and characterization of the reservoir bodies that they hold.

### 6.1. S4 Regressive systems tract: Highstand and Falling stage systems tracts

The regressive systems tract of sequence S4 is characterized by a rapid thickening (over a distance of 1 km) of the sedimentary wedge to reach 50 meters-thick near the delta front (Figure 10a) where a Highstand systems tract (HST) first and a Falling stage systems tract (FSST) later are developed. The thickness stays relatively constant along the slope and towards the deepest part of the basin. The total length of this S4 regressive systems tract is more than 11 kilometers in our SSE-NNW section.

The topset deposits are poorly preserved and overlain in the most proximal area of the HST by an erosional unconformity identified as a sequence boundary. The erosion of the subaerially exposed sediments records : (1) a decrease in the amount of accommodation space due to a synsedimentary local uplift near the southern area of the section (Grasseau, 2016); (2) a high sedimentation rate. The S4 Regressive systems tract is characterized by sigmoid and oblique clinoforms with progressively
increasing dip angles (from 1.4° to 5.7°) associated with a downward shift of facies belts. The sands are accumulated mainly in the delta front (foreset) and/or in the turbidite systems of the FSST. The FSST delta front deposits form a homogeneous sandstone wedge extending laterally over 3 kilometers. The thickness increases towards the basin to reach locally 50 meters. Its net-to-gross ratio ranges between 40 % and 95 % and decreases along a proximal-distal axis, the mean value being 80 %. The type 1 turbidites, interstratified within delta front and prodelta deposits, consist of heterolithic marl-sandstone units 10 meters thick and 4 kilometers wide, made up of amalgamated channels. These turbidite units are vertically and laterally heterogeneous, with a net-to-gross ratio ranging from 70 % to 95 % (mean = 83 %).

On the synthetic seismic lines, the regressive systems tract of sequence S4 is mainly imaged by oblique clinoform reflections, characterized by positive amplitude and a limited lateral extent (Figure 11).

At 60 Hz, the first aggrading-prograding deltaic unit, corresponding to a Highstand systems tract, is imaged by two oblique clinoform reflections, two peaks of average to low amplitude (arrow 1, Figure 11c). In the proximal direction, these clinoform reflections end with toplap terminations under an undulating positive reflection (red peak) corresponding to the abnormal image of a facies transition between delta plain and proximal delta front facies. The thin stratigraphic thickness (< 10 m) of the clinoform topsets is below the tuning thickness for both 30 Hz and 60 Hz synthetic seismic lines, thus producing false reflection terminations and pseudo-discontinuities as discussed in the paragraph 5.1.2. Close to the offlap break (arrow 1, Figure 11c), a subtle amplitude decrease images the local low inner reflectivity of the homogeneous delta front sandstones, characterized by a high net-to gross ratio. On a larger scale, the amplitude of the clinoform reflections progressively dims out toward the distal part due to a progressive lateral facies transition between distal delta front deposits and marls. In the bottomsets, the clinoform reflections are overlain with onlap terminations by higher amplitude reflections imaging type 1 turbidites (arrow 2, Figure 11c). The amplitude of the
reflections imaging the turbidite deposits dim out toward the distal part, due to a reduction of the net-to-gross ratio distally. At 30 Hz, the reflection characterizing the type 1 turbidites is difficult to identify among the clinoform reflections (Figure 11b).

The second deltaic unit within this systems tract, a Falling-stage systems tract characterized at outcrop by oblique clinoforms, is imaged at 60 Hz by two oblique clinoform reflections of very low to medium positive amplitude (arrow 3, Figure 11c). Toward the proximal part, these reflections end with toplap terminations under a peak reflection imaging a sequence boundary. The proximal part of the foreset is imaged by low-amplitude reflections forming a relatively transparent facies due to the lack of inner reflectivity within the homogeneous delta front deposits. These deposits have a high net-to-gross ratio (Figure 11d). The distal part of the foreset is characterized by an increase of amplitude in the reflections, related to a stronger inner reflectivity, due to (1) the presence of type 1 turbidite deposits, (2) a higher heterogeneity in the distal delta front deposits. The clinoform reflections end with downlap terminations on an undulating « step-like » trough reflection imaging a facies transition between delta front silty-sandstone deposits and prodelta marls (arrow 4, Figure 11c).

On the acquired seismic lines from the Colville Basin, similar seismic systems tracts could be differentiated by the recognition of the same criteria (Figure 11e). They are characterized by oblique clinoform reflections of medium amplitude and small lateral extent (arrow 1, Figure 11e). By analogy, the best, high net-to-gross, reservoir facies should be located within the transparent seismic facies, imaging the low inner reflectivity of the thick delta front deposits (arrow 3, Figure 11e).

## 6.2. S5 Regressive systems tract: highly progradational Highstand systems tract

The regressive systems tract of sequence S5 (Figure 10b) has a typical sigmoid shape highlighted by a series of clinoforms: it is 20 meters thick in the topset part, thickens over 6 km long to reach more
than 250 meters in the foreset to finally thin toward the bottomset. The sedimentary wedge presents a prograding-aggrading trend (normal regression) where most of the delta front sands were accumulated preferentially in the topset areas, whereas the foreset and slope are made up by prodelta marls and minor turbidite units. The dip angle of the clinoforms progressively increases (from 0.5° to 2°) during the deposition of the lower part, and decreases (from 2° to 0.5°) in the upper part of the systems tract. The stacking pattern organization and the related convex-up shoreline trajectory are interpreted as a Highstand normal regression recording a change from aggradation to progradation. In the lower part, the delta front deposits would make poor reservoirs due to a high level of facies heterogeneity. In the upper part, the reservoir properties are better due to the superposition of well-sorted delta front sandstone beds, extending 2 to 4 km laterally and stacked over up to 35 meters of thickness. The net-to-gross ratio of this delta front body ranges from 55% to 85%, with a mean of 65%.

A set of type 1 turbidite deposits is present on the slope of the first low-amplitude and low-angle clinoforms. They consist of marly-sandstone bodies, 2 to 3 km wide and less than 5 m thick, characterized by a very heterogeneous net-to-gross ratio (vertically and horizontally) ranging from 40% to 70%, with a mean of 55%. The marly foresets and bottomsets of the overlying higher-angle clinoforms are locally incised by isolated and confined turbidite channels, a few meters thick and a few meters wide.

In the distal part of this systems tract crops out a set of type 2 turbidite deposits of 2 to 5 m-thick well-bedded sandstone, alternating with laminated silt intervals of few-meters to few-tens-of-meters thick. Locally, 10 m-thick massive sandstone interpreted as turbidite channels incise the type2 turbidite deposits. This whole turbidite unit forms a 3 km-wide and up to 25 m-thick turbidite system (on this 2D transect). Its net-to-gross ratio ranges from 50 to 85%, with a mean of 70%.

On the synthetic seismic lines, the regressive systems tract of sequence S5 is mainly imaged by oblique clinoforms of large lateral extent and medium to low positive amplitude, interfingering
distally within a transparent seismic facies (arrows 1 to 5, 9; **Figure 12**). High-amplitude undulating reflections characterizing a type 2 turbidite system are also visible within the bottomsets (arrows 6 and 7; **Figure 12**).

At both 30 Hz and 60 Hz, the clinoform reflections end with toplap terminations under an undulating « step-like » peak-trough reflection couplet of high amplitude (**Figure 12**). This reflection is the image of a facies transition between delta plain and delta front facies. This delta front facies is mainly characterized by sandstone with a high net-to-gross ratio. The clinoform reflections 1 and 2, of medium to low amplitude, end distally with downlap terminations on the sequence boundary (arrows 1 and 2, **Figure 12**). These two peak reflections are the image of type 1 turbidite facies deposits accumulated along the slope. Such reflections are difficult to differentiate from the seismic reflections imaging distal front delta deposits (arrows 3 to 5, **Figure 12**). At 60 Hz, an area with a very low amplitude/almost transparent facies is visible within the topset and the foreset (arrow 8, **Figure 12c**), revealing the low inner reflectivity of the homogeneous, high net-to-gross, proximal delta front sandstone bodies. At 30 Hz, this same interval is imaged by the trough part of the peak-trough reflection couplet imaging the top of the systems tract (arrow 8, **Figure 12b**). This seismic facies grades distally to high/medium amplitude clinoform reflections (arrow 9, **Figure 12**), parallel and continuous, imaging a high inner reflectivity due to the progressive intercalation of distal delta front deposits within the prodelta marls. The net-to-gross ratio decreases distally. Reflections 6 and 7 (arrows 6 and 7, **Figure 12**), located within a transparent facies in the bottomsets of the clinoforms, typically correspond to the image of type 2 turbidites. A local increase of amplitude associated with a lenticular geometry highlights the presence of a turbidite channel (arrow 7, **Figure 12**). The net-to-gross ratio decreases towards the margins of the turbidite systems, the maximum value corresponding to the central part of the channels.

On the acquired seismic line from the Colville Basin, a similar regressive systems tract is observed (**Figure 12d**). The apparent oblique clinoform reflections (arrows 3 to 5, 9, **Figure 12d**) may
correspond by analogy to the reflections imaging the distal delta front deposits. The amplitude of the reflections decreases within a transparent seismic facies in the foreset and in the topset. However, the clinoform 3 (arrow 3, **Figure 12d**) shows similar reflection characteristics and terminations than the type 1 turbidite synthetic seismic facies (arrows 1 and 2, **Figures 12b and 12c**), and thus may be interpreted as imaging turbidite deposits accumulated along the slope. Undulating high-amplitude reflections in the distal part of this systems tract can be interpreted as type 2 turbidites (arrows 6 and 7, **Figure 12d**). Finally, the arrow 8 highlights a transparent seismic facies which may be interpreted as imaging the proximal delta front deposits (arrow 8, **Figure 12**).

The best potential reservoirs, with a high net-to-gross ratio, are imaged by either (1) a seismic reflection of medium to low negative amplitude corresponding to the lower part (trough) of the reflection imaging the lateral facies transition between the delta plain and delta front deposits, or (2) a transparent or very low amplitude seismic facies highlighting the homogeneity of delta front deposits, or (3) a seismic facies corresponding to type 2 turbidites.

### 6.3. S6 Transgressive systems tract

The Transgressive systems tract of Sequence S6 (**Figure 10c**) is characterized by a constant thickness of 20 to 30 meters over more than 7 km and is truncated by a submarine slide surface basinward.

It consists of a retrograding sedimentary succession in which a series of low-angle (0.5°) clinoforms developed. S6 TST deposits are characterized proximally by proximal delta front sandstone with tidal reworking. These deposits laterally grade towards the distal area to a fining-up succession of distal delta front sandstone rich in nummulites overlain by prodelta marls. Delta front sandstone bodies are very extensive, over more than 6 km, and interfinger toward the SSE with the delta plain argillaceous deposits. The net-to-gross ratio increases from the distal to the proximal part and
decreases from the base to the top of the sedimentary succession. The net-to-gross ratio ranges from 40% to 65% with a mean of 50%.

On the synthetic seismic line, the transgressive systems tract of sequence S6 is imaged by a reflection couplet (peak-trough) of high to very high amplitude, continuous, subhorizontal and slightly undulating, intercalated within the topset seismic facies (white arrows, **Figure 13**). This reflection is bent by several velocity anomalies (pull-up effects) caused by the presence of lenticular sandstone bodies within the shale interval above (**Figures 13b and 13c**). A slight amplitude dimming can be observed in the distal direction. This reflection is better characterized at 60 Hz, because it stands out from the transparent facies imaging the delta plain shale. Within this transgressive sandstone body, the net-to-gross ratio increases in the proximal direction and decreases upward.

On the acquired seismic lines from the NPRA, similar high-amplitude, continuous and slightly undulating reflections intercalated within the topset seismic facies (black arrows, **Figure 13d**) have already been interpreted as the image of a transgressive systems tract (Houseknecht et al., 2009). These high-amplitude reflections present the same criteria as described in the synthetic seismic line.

The whole transgressive systems tract of the S6 sequence corresponds to a potential reservoir body, comprising however an important silty fraction.

## 6.4. S6 Aggrading to prograding Highstand systems tract

The regressive systems tract of Sequence S6 (Figure 10c) has a homogeneous thickness of about 60 m over a distance of a few kilometers in the topset. It thickens abruptly in the foreset due to a complex of submarine gravity slides deposits which reaches up to more than 120 meters in thickness. This systems tract is then completely truncated by a major submarine slide surface which marks the top of the sequence.
The systems tract is made up of a succession of tens-of-meters thick aggrading-prograding sedimentary units within which a series of low-angle clinoforms (0.5°) develop. The stacking pattern organization and the convex-up shoreline trajectory were interpreted as a Highstand normal regression recording a rapid change from a well-developed aggrading stage to a very prograding stage. Along the clinoforms, proximal delta front sandstone reworked by tidal currents grade laterally to coarsening-up or fining-up medium-distal delta front sandstone rich in Nummulites, an then to prodelta marls. The Highstand systems tract contains meter-thick argillaceous or silty levels, which limit the vertical and lateral connectivity of the delta front sandstone bodies. The sandstone bodies are 20 to 40 m thick and 2 km long (on this 2D transect). A succession of gravity instabilities truncated these deposits towards the basin. The resulting depressions were filled in their proximal part by fine-grained distal delta front deposits (draping the paleotopography), and in their distal part by some turbidite deposits interbedded within prodelta marls. These elements give a very complex and heterogeneous architecture to the Highstand systems tract of Sequence S6. The net-to-gross ratio remains however high, since it ranges from 40% to 85% with a mean of 71%.

On the synthetic seismic line, the aggrading-prograding regressive systems tract of sequence S6 is imaged by a gently dipping and laterally extensive seismic wedge (Figure 14). This systems tract is mainly characterized by the seismic facies corresponding to the fill of submarine slide scars in the distal direction.

At 60 Hz, the regressive systems tract of sequence S6 is imaged by onlapping clinoform reflections which are rather discontinuous and characterized by high to average positive amplitude (arrow 1 and 2, Figure 14c). The amplitude dims out in the distal direction, attesting of a lower lithological contrast, partly due to a lateral facies transition between delta front deposits and marls. Although the delta front deposits appear rather thick, massive and homogeneous at outcrop (Figure 14d), they present a high reflectivity due to the occurrence of marl intervals interbedded in the sandstone units and thick-enough to have an impact on the seismic image. These argillaceous levels limit the vertical
connection between the potential sandstone reservoir bodies and decrease the net-to-gross ratio. In the distal direction, the clinoform reflections end with erosional truncations against steeper reflections imaging submarine slide scars. The seismic facies corresponding to the fill of these slide scars is characterized by high-amplitude, undulating reflections which end with onlap terminations on the base of the slide scar reflection (arrow 2, **Figure 14c**). At 30 Hz, the delta front seismic facies is characterized by discontinuous, almost chaotic positive high- to medium-amplitude reflections (arrow 3, **Figure 14b**). The low reflectivity of this seismic facies is due to the low seismic resolution that prevents imaging the thin interbedded argillaceous/marly beds into the thick delta front sandstone units. The infill of the submarine slide scars is only imaged by a sharp increase of amplitude along the reflections, creating a discontinuity that might be wrongly interpreted as a fault (arrow 2, **Figure 14b**).

On the acquired seismic lines from the Colville Basin (**Figure 14e**), this kind of systems tract can be differentiated by analogy with the synthetic seismic lines. The overall geometry is a sigmoid clinoform with a relatively thick topset. In the topset, the reflections are continuous, tabular and locally wavy, with a low to medium amplitude (arrow 4). Their amplitude increases around the offlap break, and more distally the reflections become steeper, discontinuous and undulating, with local onlap terminations. This seismic facies probably corresponds to the image of the fill of submarine slide scars (arrows 1 and 2).

The best potential reservoir facies, with the highest net-to-gross ratio, is the delta front facies, located in the distal part of the topset and around the offlap break, before the erosional truncations induced by the submarine slides.

## 7. Conclusion
The main objective of this work was to provide guidelines helping to improve the seismic interpretation of deltas from the analysis of synthetic seismic lines generated from a deltaic outcrop analog. The Middle Eocene Sobrarbe deltaic complex exposed in the South Pyrenean foreland basin is an excellent candidate for forward seismic modeling due to the extraordinary continuity and exposure of its outcrops. The Sobrarbe delta is also a very good analog for fluvio-deltaic sandstone reservoirs accumulated in active tectonic settings. A stratigraphic dip-section, measuring 15 km long and 700 m of average thickness, was built from this detailed outcrop dataset, and then converted to an acoustic impedance model by attribution of petrophysical properties (from NPRA subsurface analog) to the sedimentary facies of the geological model. Two stacked and migrated synthetic seismic lines were generated with 30 Hz and 60 Hz Ricker wavelets.

The main results from the analysis of the synthetic seismic lines are: (1) that the seismic sequence interpretation using the seismic-stratigraphy method revealed significant differences with the sequences identified in the field by classical sequence stratigraphy; (2) a seismic facies catalogue presenting the seismic reflection parameters of alluvial plain to deep basin deposits; (3) guidelines to identify different systems tracts within the seismic stratigraphic sequences and thus better predict sub-seismic reservoir facies distribution and quality. Finally, close similarities between the synthetic seismic lines and acquired seismic lines from the Torok/Nanushuk deltaic system (Colville Basin, NPRA, USA) allowed the direct application of these results.

* For this type and scale of deltaic system, a 60Hz seismic peak frequency is necessary to interpret the mid-frequency (4th order) seismic sequences. The "seismic stratigraphy" methodology is consistent and allows the correct interpretation of most of these mid-frequency seismic sequences. However, two possible interpretation mistakes have been highlighted: (1) some reflections are imaging lateral facies transitions instead of stratigraphic surfaces, especially in low-frequency seismic lines. The criteria of identification are the wavy and step-like geometry of the reflection and its dipping toward the proximal area (opposite to clinoform dip direction); (2) some higher-frequency
sequence boundaries can be misinterpreted as mid-frequency stratigraphic sequence boundaries,
due to the presence of large slide-scar surfaces. These surfaces generate erosional geometries
(erosional truncations) and infill geometries (onlap terminations) which are usual criteria for the
identification of sequence boundaries on seismic lines.

* The seismic facies of seven facies associations observed at outcrop have been described in this paper. The "alluvial plain" seismic facies occurs in the topsets of progradational-aggradational clinoforms and is characterized by overall tabular and discontinuous reflections, presenting high but variable amplitudes, an average frequency, and local concave-up, convex-up or undulating geometry. The "lower delta plain" seismic facies is also located in the topset, between the alluvial plain facies and the offlap break. It is characterized by very discontinuous, high-amplitude concave-up reflections, presenting a slightly higher frequency than the "alluvial plain" seismic facies. The "delta front" seismic facies is characterized in its proximal part by low-amplitude reflections or by an almost transparent facies, and in its distal part by continuous dipping reflections of medium to low amplitude and average frequency. "Submarine slide surfaces" can be recognized when the clinoforms present an irregular, undulating geometry, marked by an abrupt and locally discontinuous break of slope (offlap break). The "turbidite" seismic facies is either characterized by continuous clinoform reflections of slightly higher amplitude and slightly lower dip than the distal delta front facies (type 1 turbidites), or by high-amplitude discontinuous reflections, locally undulating or concave-up, located in the bottomsets within a transparent interval (type 2 turbidites). The "isolated carbonate platform" seismic facies is characterized by one composite seismic reflection of very high amplitude and low frequency, located near the clinoform offlap break. Finally, the "nummulite beds" seismic facies is characterized by continuous, very-low amplitude and slightly divergent reflections located in the topsets and truncated just before the offlap break. These criteria can now be used to interpret the seismic facies imaging the different lithological units on actual seismic lines.
* In order to predict the distribution and quality of reservoir bodies from the interpretation of seismic data in this type of deltaic systems, it is necessary to consider two parameters: (1) the recognition of the type of systems tract which can sometimes be complicated by interferences, composite systems tract boundaries affected by frequent lateral facies variations, etc.; (2) the differentiation of the seismic facies corresponding to the potential reservoir bodies within these systems tracts.

In the falling stage systems tracts, the seismic facies of the best reservoir bodies (high net-to-gross ratio) can be recognized by: (a) a subtle decrease of amplitude imaging the low inner reflectivity of delta front sandstone within the foreset of the clinoforms, or (b) by a transparent seismic facies, poorly imaged by a composite trough reflection, due to the low inner reflectivity of relatively thick delta front deposits.

In the highstand systems tracts, the seismic facies of the best reservoir bodies can be recognized by:
(a) a delta front sandstone seismic facies marked by a trough reflection of moderate to low amplitude, corresponding to the lower part (trough) of the composite reflection imaging the lateral facies transition between delta plain deposits and delta front deposits, and (b) a type 2 turbidite seismic facies, within which a local increase of amplitude associated with a lens-shape geometry suggests the presence of a turbidite channel.

Transgressive systems tracts are imaged by a continuous very-high to high amplitude reflection (peak-trough-peak triplet), subhorizontal to slightly undulating, interfingering within the topset seismic facies. The net-to-gross ratio increases along a distal-proximal axis and decreases along a vertical (base to top) axis.

* The direct application on acquired seismic lines of the Colville Basin (NPRA, USA) shows that most of the seismic attributes and geometries observed in the 30 Hz and 60 Hz synthetic seismic lines have also been recognized in the 40 Hz actual seismic lines of the Torok-Nanushuk fluvio-deltaic system, even if the peak frequencies between these two datasets are different. This is certainly due to the
different scales of these two deltas, the Torok-Nanushuk delta being thicker and much more extensive (progradational distance) than the Sobrarbe delta. For example, several Transgressive systems tracts, made of high-potential reservoir bodies interbedded within impermeable shale, have been recognized within the Nanushuk Formation topsets. Type 2 turbidites, characterized by good-quality isolated sandstone bodies, have been observed in the bottomsets of the Torok Formation. They could form interesting plays in this deltaic system. The interpretation of slide scar surfaces may allow the prediction of good-quality gravity deposits in the bottomsets of the clinoforms. The differentiation between delta front and type 1 turbidite seismic facies along the Nanushuk-Torok clinoforms may lead to a better prediction of the extent of good quality delta front sandstone bodies, essential to improve the distribution of reservoir properties in reservoir models.

The results of this study can now be applied to other subsurface deltaic systems, in different geological settings, in order to test these recognition criteria and maybe reduce uncertainties during the reservoir modeling phase. The continuation of this project will consist in studying the effects of different types of reservoir fluids (water/oil/gas) on the seismic image of this deltaic system.

## ACKNOWLEDGEMENTS

This paper presents part of the results of a joint Ph.D. project between Bordeaux University, France and Barcelona University, Spain. We are grateful to Total (GSG Nigéria) for the funding of this research. Support from the Spanish Ministry of Economía, Industria y Competitividad (Project SEROS, CGL2014-55900-P) and from Generalitat de Catalunya (2014SGR467) is acknowledged. We also thank ION Geophysical for providing free research University licenses for the seismic modeling software GXII, and the USGS for sharing the NPRA subsurface database. We are very grateful to Anne Schwab and two other anonymous reviewers, whose comments and corrections have greatly improved the quality of this manuscript.
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<figure><img src="image_1.png" /><figcaption>**Figure 1:** Geological setting of the Sobrarbe deltaic complex. (a) Simplified geological map of the Lutetian-Bartonian deposits of the Sobrarbe deltaic complex (Grasseau, 2016) showing the main facies associations and geological structures. The thick black line corresponds to the location of the</figcaption></figure>
correlation transect used for synthetic seismic modeling (Figures 2a and 4). (b) General map of the Pyrenean mountain range with the main geological units and the study area.

<figure><img src="image_2.png" /></figure>

**Figure 2:** Outcrop-based transect and analog seismic profile of prograding deltaic complexes. (a) Simplified stratigraphic architecture of the Sobrarbe deltaic deposits along a correlation transect built on the western flank of the Buil syncline (Figure 1). Three orders of sequential arrangement have been distinguished: high-frequency sequences and parasequences, mid-frequency stratigraphic sequences (S3 to S9) and the transgressive-regressive composite sequences (SI to SIII). (b) Acquired seismic profile C-81 (USGS NPRA subsurface data set), imaging the Albian-Cenomanian Nanushuk-Torok fluvial-deltaic system, used as an analog in this study. This prograding-aggrading deltaic system
developed in the Colville foreland basin, within the National Petroleum Reserve of Alaska (NPRA),
USA (see **Figure 3** for location).

<figure><img src="image_3.png" /></figure>

**Figure 3:** Map of the northern part of the Alaska North Slope, Chukchi Sea and Beaufort Sea, showing the main geologic elements, seismic facies and paleocurrent data from the Lower Cretaceous First-order depositional sequence (modified from Houseknecht et al., 2009). This sequence corresponds to the deltaic system of the Nanushuk Group and Torok Formation, used as subsurface analog for the Eocene Sobrarbe Delta. The proximal domain of the Nanushuk-Torok deltaic system was located to the west, on the Chuckchi platform. This deltaic system prograded toward the East and North-East over several hundreds of kilometers, progressively filling in the Colville foreland basin. Foreset dip directions are from Bird and Andrews (1979) and Houseknecht et al. (2009); topset cross-bed directions are from Hujman et al. (1985). The approximate location of line C-81 (Figure 2b) is indicated. Note that the seismic line is not parallel but slightly oblique to the main sediment transport direction.
<figure><img src="image_4.png" /><figcaption>**Figure 4:** Steps used to build the synthetic seismic lines of the Sobrarbe delta complex. (a) Acoustic impedance model used for generating the synthetic seismic lines. (b) Synthetic seismic line generated with a 60Hz peak frequency wavelet. (c) Synthetic seismic line generated with a 30Hz peak frequency wavelet.</figcaption></figure>
<table><caption>**Table 1:** Table presenting the Sobrarbe delta sedimentary facies that have been selected for the forward seismic modeling, and their petrophysical properties. The values of wave propagation velocity and density have been chosen by facies analogy with well data from the Nanushuk-Torok Formations (NPRA, USA). These acoustic parameters were used as input to build the impedance model (Figure 4a).</caption><thead><tr><th>Depositional environment</th><th>Sub-environment</th><th>Facies</th><th>Model P-waves velocities (m/s)</th><th>Model density (g/cm3)</th></tr></thead><tbody><tr><td rowspan="3">Upper delta plain</td><td>Braided fluvial channel</td><td>Conglomerate</td><td>2900</td><td>2,27</td></tr><tr><td>Meandering fluvial channel</td><td>Very coarse to fine cross-bedded sandstone</td><td>3100</td><td>2,31</td></tr><tr><td>Well-drained floodplain</td><td>Red clay</td><td>2600</td><td>2,21</td></tr><tr><td rowspan="3">Lower delta plain</td><td>Distributary channel</td><td>Coarse to fine cross-bedded sandstone</td><td>3200</td><td>2,33</td></tr><tr><td>Ponded floodplain</td><td>Grey clay</td><td>2700</td><td>2,23</td></tr><tr><td>Ponded floodplain</td><td>Fine silty ripple-laminated sandstone</td><td>3100</td><td>2,31</td></tr><tr><td rowspan="2">Delta front</td><td>Proximal mouth bar and tidal sand ridges</td><td>Medium to coarse cross-bedded sandstone</td><td>3500</td><td>2,38</td></tr><tr><td>Distal mouth bar/Lower nearshore</td><td>Carbonate silty to medium sandstone</td><td>3300</td><td>2,35</td></tr><tr><td rowspan="6">Prodelta</td><td>Low energy delta front</td><td>Argillaceous silt to very fine sandstone</td><td>2900</td><td>2,27</td></tr><tr><td>Slope</td><td>Decimetrical conglomerate or sandstone block in silty matrix</td><td>3100</td><td>2,31</td></tr><tr><td>Turbidite slope channel</td><td>Medium to fine cross-bedded sandstone</td><td>3400</td><td>2,37</td></tr><tr><td>Slope channel levée or turbiditic lobe</td><td>Heterolitic fine sandstone</td><td>3200</td><td>2,33</td></tr><tr><td>-</td><td>Marls/shales</td><td>3000</td><td>2,29</td></tr><tr><td>-</td><td>Black marls</td><td>2700</td><td>2,23</td></tr><tr><td rowspan="3">Carbonate shallow marine</td><td>Isolated carbonate platform</td><td>Oyster, coral and algae-rich wackestone/packstone</td><td>4000</td><td>2,47</td></tr><tr><td>Nummulite beds</td><td>Nummulite floatstone/rudstone</td><td>2900</td><td>2,27</td></tr><tr><td>Calciturbidite</td><td>Nummulite packstone/grainstone</td><td>3200</td><td>2,33</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>**Figure 5:** Synthetic seismic section of the Sobrarbe delta sequential interpretation. (a) Uninterpreted synthetic seismic line with 60Hz wavelet. (b) Blind interpretation of the synthetic seismic line using classical seismic stratigraphy. Nine mid-frequency seismic sequences have been identified. (c) 60Hz synthetic seismic line with real mid-frequency sequence boundaries projected from the geological transect (black lines).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 6:** Detailed 60Hz synthetic seismic image of the interpreted sequences 2 to 4 which covers the facies transition area between the delta front deposits and the delta plain deposits (topsets). (a) Complete 60 Hz seismic section. The black box corresponds to the area of interest to observe the seismic response of lateral facies transitions. (b) Impedance model with stratigraphic surfaces and acoustic impedance distribution. Lateral variations of impedance imaging the lateral facies transitions are pointed (black arrows). (c) Reflection imaging the lateral facies transition in the topsets (black arrows) without interpretation. (d) Seismic sequences interpretation using classical seismic stratigraphic methodology (Figure 5b). (e) Projection of the geological stratigraphic surfaces (reflectors) on the synthetic seismic section. Black arrows highlight the reflection imaging the lateral facies transition in the bottomsets.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

**Figure 7:** Schematic theoretical seismic model used to analyze in detail the lateral facies transitions from delta plain to delta front at different frequencies. (a) Acoustic impedance model. (b) Synthetic seismic line generated with 30Hz peak frequency wavelet. (c) Synthetic seismic line generated with 60Hz peak frequency wavelet. (d) Zoom on the area of interest on the 60Hz synthetic seismic line of the Sobrarbe delta. (e) Analogy with the acquired seismic line 28-81a (NPRA). Arrows indicate the seismic image of a reflection with a step-like geometry, dipping towards the proximal area, possibly imaging a lateral facies transition between delta plain and delta front deposits.
<figure><img src="image_8.png" /><figcaption>**Figure 8:** Seismic facies analysis characterizing the seismic image of each depositional environment, generated with 30Hz and 60Hz peak frequency wavelets, and comparison with equivalent seismic facies observed on acquired seismic lines from the NPRA Torok-Nanushuk analog. The top picture is the impedance model for the synthetic seismic lines, with the location of the detailed seismic facies figures. The next line presents corresponding outcrop pictures of each geologic facies association. (a) Alluvial domain; (b) Delta plain; (c) Delta front. The black arrows highlight the distal limit of the low-amplitude to transparent delta front seismic facies; (d) Fill of slide-scar surfaces. The black arrows highlight the reflections with a concave-up geometry imaging the slide-scar surfaces and related filling.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 9:** Seismic facies analysis characterizing the seismic image of each depositional environment, generated with 30Hz and 60Hz peak frequency wavelets, and comparison with equivalent seismic facies observed on acquired seismic lines from the NPRA Torok-Nanushuk analog. The top picture is the impedance model for the synthetic seismic lines, with the location of the detailed seismic facies figures. The next line presents corresponding outcrop pictures of each geologic facies association. (a) Isolated carbonate platform deposits. The black arrows highlight the very high amplitude and low frequency reflections imaging the carbonate platform; (b) Type 1 turbidite system deposits. The black arrows highlight the continuous reflections imaging the type 1 turbidites; (c) Type 2 turbidite system deposits. The black arrows highlight the high-amplitude discontinuous reflections (type 2 turbidites) located within a transparent interval imaging homogeneous prodelta marls in the bottomset of the clinoforms; (d) Nummulite beds. The black arrows highlight the continuous, very-low amplitude and slightly divergent reflections imaging the Nummulite beds.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 10:** Geometries and reservoir properties of four systems tracts selected within the Sobrarbe deltaic complex for their reservoir potential. (a) S4 Regressive systems tract is made of a Highstand systems tract (HST) and a Falling stage systems tract (FSST), both presenting good-quality delta front and type 1 turbidites reservoir bodies; (b) S5 Highstand systems tract presents its best reservoir bodies in the extensive and relatively thick delta front facies located in its upper part, and within type 2 turbidites; (c) S6 transgressive systems tract (TST) and Highstand systems tract (HST) present interesting reservoir bodies in the delta front facies: relatively thin but laterally extensive sandstone bodies in the retrograding TST, and thick but less extensive sandstone bodies in the aggrading HST.</figcaption></figure>
<figure><img src="image_11.png" /></figure>

**Figure 11:** Synthetic seismic image of Sequence S4 Regressive Systems Tract (Highstand (HST) and Falling stage systems tracts (FSST)) and analogy with acquired NPRA seismic line. (a) Acoustic impedance model; (b) Synthetic seismic line generated with 30Hz wavelet; (c) Synthetic seismic line generated with 60Hz wavelet; (d) Outcrop picture of Highstand and Falling stage systems tracts; (e) Acquired seismic line 33-81a (NPRA). Arrows highlight points of interest detailed in the text. **Arrows:** (1) Oblique clinoform reflections which progressively dim out towards the distal part due to a progressive lateral facies transition between distal delta-front deposits and marls; (2) At 60Hz, a high-amplitude bottomset reflection onlaps the underlying clinoform reflection. The onlapping reflection images the type 1 turbidites. At 30Hz, the reflection characterizing the type 1 turbidites is difficult to differentiate from the clinoform reflections; (3) Very low amplitude to transparent seismic facies located around the toplap termination of oblique clinoforms. This seismic facies images the lack of inner reflectivity within the homogeneous delta front deposits of the FSST; (4) Undulating « step-like » trough reflection imaging a facies transition between delta front silty-sandstone deposits and prodelta marls.
<figure><img src="image_12.png" /></figure>

**Figure 12:** Synthetic seismic image of Sequence S5 Regressive Systems Tract and analogy with acquired NPRA seismic line. (a) Acoustic impedance model; (b) Synthetic seismic line generated with 30Hz wavelet; (c) Synthetic seismic line generated with 60Hz wavelet; (d) Acquired seismic line 27b-81 (NPRA). Arrows highlight points of interest detailed in the text. Arrows: (1) to (5) and (9) oblique clinoforms of large lateral extent characterized by a lateral amplitude variation: from a low negative amplitude (blue) or transparent facies (around the toplap location), imaging delta front deposits, grading to a medium positive amplitude (red), imaging type 1 turbidites, and finally diming distally to a transparent seismic facies, imaging prodelta marls; (6) Type 2 turbidites are imaged by high-amplitude wavy reflections located within a transparent seismic facies in the bottomsets of the clinoforms; (7) A turbidite channel within type 2 turbidite deposits is imaged by a local increase of amplitude, associated with a concave-up geometry; (8) Area within the topset and the foreset with very low (at 60Hz) or negative (at 30Hz) amplitude seismic facies revealing the low inner reflectivity of the homogeneous proximal delta front sandstone bodies.
<figure><img src="image_13.png" /><figcaption>**Figure 13:** Synthetic seismic image of Sequence S6 Transgressive Systems Tract and analogy with acquired NPRA seismic line. (a) Acoustic impedance model; (b) Synthetic seismic line generated with 30Hz wavelet; (c) Synthetic seismic line generated with 60Hz wavelet; (d) Acquired seismic lines 12p-81, 16p-81, 18p-81 (NPRA). **Arrows:** high-amplitude, continuous and slightly undulating reflections intercalated within the topset seismic facies imaging a transgressive systems tract.</figcaption></figure>
<figure><img src="image_14.png" /></figure>

**Figure 14:** Synthetic seismic image of Sequence S6 Regressive Systems Tract and analogy with acquired NPRA seismic line. (a) Acoustic impedance model; (b) Synthetic seismic line generated with 30Hz wavelet; (c) Synthetic seismic line generated with 60Hz wavelet; (d) Outcrop picture of collapse scar surfaces incising the stacked unit of thick delta front sandstones and thin interbedded argillaceous/marly beds covered by vegetation. The collapse scars are filled with delta front sandstone and marls; (e) Acquired seismic line C-81 (NPRA). Arrows highlight points of interest detailed in the text. Arrows: (1) and (2) Onlapping discontinuous clinoform reflections of high to medium positive amplitude imaging the delta front deposits and thin interbedded argillaceous/marly beds; (3) Discontinuous, almost chaotic positive high- to medium-amplitude reflections within a low reflectivity seismic facies, imaging the thick delta front sandstone unit and the thin interbedded argillaceous/marly beds that are below seismic resolution; (4) Continuous, tabular and locally wavy reflections with a low to medium amplitude, imaging the thick topset of this type of Regressive systems tract;