Introduction

Yemen is considered as one of the most prolific hydrocarbon countries in the world. In Yemen, oil seeps were found in Sab’atayn Province and on Tihamah plains since 1921 (Morton 2013). However, Yemen is the last Middle Eastern country to assert itself on the world’s petroleum map due to political reasons. Alif-1, the first oil well in Yemen, was drilled in 1984 by the American company Hunt Oil in the Marib sector of the Sab’atayn Basin, onshore North Yemen. This was followed by another West Ayad-1 oil well in 1986 which was drilled by the Russian company Techno Export in Sab’atayn sector, south of Marib sector in Sab’atayn Basin. Accordingly, the 1990s and 2000s witnessed a boom in oil production due to the extensive exploration of the large petroleum companies in Yemen (As-Saruri et al. 2013).

Yemen is composed mainly of 12 sedimentary rift basins which are categorized according to their geological era into 3 Paleozoic, 5 Mesozoic, and 4 Cenozoic sedimentary basins (Fig. 1). Several cross sections had been presented to show the structural and stratigraphic signature of these sedimentary basins along Yemen (Fig. 2). So far, petroleum explorations were succeeded in only two of these sedimentary basins: the Sab’atayn and the Say’un–Masilah basins (As-Saruri et al. 2010; As-Saruri and Sorkhabi 2014). These two rift basins are considered to be a failed rift arm of a triple junction that occurred in Late Jurassic time (Husseini 1989; Redfern and Jones 1995; Csató et al. 2001). However, their origin can be traced back to the reactivation of Pre-Cambrian fault systems (Ahlbrandt 2002).

Fig. 1
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Geological section map showing the Paleozoic, Mesozoic, and Cenozoic sedimentary basins in Yemen (modified after Sachsenhofer et al. 2012 and As-Saruri and Sorkhabi 2014)

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Fig. 2
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Regional cross sections showing the structural and stratigraphic signature of each sedimentary basin in Yemen. Locations of the lines shown in Fig. 1. A–A′, B–B′ are modified after Beydoun (1966); C-C′ modified after JUNGWIRTH and AS-SARURI (1990), and D–D′ modified after Davison et al. (1998)

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Say’un–Masilah basin is one of the most petroleum productive basins in Yemen. Thus, it draws the attention of several petroleum companies and literatures (Beydoun 1966; Redfern and Jones 1995; Beydoun et al. 1996; Beydoun and As-Saruri 1998; Alareeq 2008; Hakimi et al. 2011; Shelander et al. 2014; Lashin et al. 2016; Areeq et al. 2016; Al-Areeq et al. 2019, 2020). Sunah-1, the first exploratory well in this basin, was drilled by the Canadian Occidental oil company at block 14 in 1991 (Fig. 3) (Lashin et al. 2016). This well was approved to be an oil-producing well with an average production of about 3767 barrel oil per day (BOPD) and specific gravity of about 36° API (As-Saruri and Sorkhabi 2014). This exploration was followed by several successful discoveries in the Himiar, Nazih, Camal, Qataban, Bainoon, Dhahban, Wadi Taribah, Kharir, Tasour, Bayoot, and Sharyoof oil fields (Lashin et al. 2016; Hakimi and Al-Sufi 2018).

Fig. 3
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a Exploration and production blocks of Yemen also showing the midstream and downstream infrastructure (modified after As-Saruri and Sorkhabi 2014). b Location map showing the borders of the area of study (red rectangle) and the surrounding fields with the location of wells and seismic lines used in this study

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Sharyoof field, which is located at block 53 (Fig. 3), is considered as a very significant hydrocarbon field in the Say’un–Masilah basin in Yemen. Rudood-1, the first well in this field, was drilled by DNO petroleum company in 1999, targeting the Cretaceous Qishn Formation (Hakimi et al. 2015). This was followed by drilling 8 development wells which maximized the oil production in 2003 to about 35 million barrels with an average daily production rate of about 18,911 BOPD (DNO 2003). Therefore, several studies were interested in defining the geological characterization of the Qishn reservoir along this field (As-Saruri and Sorkhabi 2014; Hakimi et al. 2015; Lashin et al. 2016; Khamis et al. 2017). However, most of these studies covered mainly the petrophysical and facies characterizations of the reservoir with a few concerns about the structural characterization and the 3D subsurface representation of the Sharyoof field.

Static modeling is considered as the most efficient technique for the 3D representation of the subsurface reservoir. Static model is a geocellular mathematical grid which is based mainly on integrating information resulted from interpreting all the available sources of data including well logs, seismic, and core data (Tyson 2007; Barakat and Dominik 2010; Barakat et al. 2019). It also includes structural modeling which represents the subsurface structural architecture of the reservoir and property modeling which includes the distribution of petrophysical and facies properties between the available wells along the field (Abdelmaksoud et al. 2019; Elmahdy et al. 2020b). The three-dimensional representation of the reservoir aims to enhance the volumetric calculations of the reservoir, well planning, uncertainty analysis, and flow simulations. These data improved the prediction of the economic values of reserves and coordinating on best decisions regarding the production plane of the field.

The aim of this work is to construct a 3D structural model for the sandstone reservoir of the Cretaceous Qishn Formation along the Sharyoof field. This model is used to represent the structural architecture of the reservoir and apply quantitative analysis of the faults. Construction of this model requires an integration of the interpretations resulted from the available wireline logging and 2D seismic data.

Geological setting

The Sharyoof field is one of the oil-producing fields that are located in the Say’un–Masilah basin in the Hadramawt Province. The Say’un–Masilah basin is located along the eastern–central portion of Yemen between latitudes of about 14° 41′′ and 16° 11′′ N and longitudes of about 47° 40′′ and 51° 51′′ E. The basin is bounded from the east and southeast by Fertaq High, from the North by Hadramawt Arch, and from the west by Jahi–Mukalla High (Fig. 1).

Say’un–Masilah basin which is WNW–ESE oriented rift basin was shaped from the reactivation of the Infra-Cambrian Najd fault system (As-Saruri et al. 2013). This reactivation was due to the rifting between Afro–Arabian and Indian–Madagascar plates due to the split up of Gondwana during the Jurassic to the Early Cretaceous ages (Haitham and Nani 1990)(Redfern and Jones 1995). This rifting was followed by Late Cretaceous separating between the Indian and Madagascar plates which give rise to the opening of the Arabian Sea (Mountain and Prell 1990). The eustatic changes in the sea level accompanied with this rifting played a significant role in the basin initiations, sedimentary deposition, and hydrocarbon accumulation (Haq and Al-Qahtani 2005).

Based on the previous tectonic events, the stratigraphic section of the Say’un–Masilah basin was divided into pre-, syn-, and post-rift sequences (Fig. 4) (Beydoun 1966; Haitham and Nani 1990; Al-Matary and Ahmed 2011). Pre-rift sequence is represented by Lower–Middle Jurassic, nonmarine to shallow marine clastics (Kuhlan Formation) overlain by Callovian/Oxfordian, shallow marine carbonates (Shuqra Formation) (Beydoun et al. 1998; Sachsenhofer et al. 2012) and overlying unconformably the Precambrian basement rocks (Lashin et al. 2016; Hakimi and Al-Sufi 2018; Al-Johi et al. 2019).

Fig. 4
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Stratigraphic nomenclature in the Say’un–Masilah basin, eastern Yemen (after Al-Areeq 2008)

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Syn-rift sediments range from Upper Jurassic to Lower Cretaceous, and it is represented by Madbi and Nayfa formations. Madbi Formation (Kimmeridgian/Lower Tithonian) is composed mainly of organic-rich shale and highly fossiliferous limestone sediments, which refer to deep, partly restricted environment (Brannan et al. 1999). Shale sediments of the Madbi Formation are considered as the main hydrocarbon source rock along the Say’un–Masilah basin (Mills 1993). Madbi Formation is overlain by shallow marine limestone sediments of Nayfa Formation (Upper Tithonian/Lower Berriasian) which is overlain by the clastics of Sa’ar Formation (Upper Berriasian/Valanginian). Seaborne (1996) proposed a presence of major depositional hiatus along the top of Sa’ar Formation due to either sea level fall or gentle uplift associated with the rifting process.

Post-rift sequence is represented by an about 3270-m thickness of sediments related to Tawilah and Hadramawt groups which range from Lower Cretaceous to Middle Eocene ages. The Tawilah Group has about 1500-m thickness, and it is composed of nonmarine to shallow marine Qishn Formation (Hauterivian/Barremian) and continental clastics Harshyat (Early Albian to Early Cenomanian) and Mukalla (Late Cenomanian to Turonian) formations (Haitham and Nani 1990; Seaborne 1996). Hadramawt Group of Paleocene to Middle Eocene ages is about 1770-m thick, and it is composed of shelf carbonate sediments of Umm Er Radhuma Formation (Upper Paleocene–Lower Eocene), marine shale and limestone sediments of Jiza Formation (Lower Eocene), gypsum deposits in a sabkha environment of Rus Formation (Lower Eocene), and platform carbonate and thin sandstone sediments of Habishiya Formation (Sachsenhofer et al. 2012).

Qishn Formation, the lower most part of Tawilah Group, is proposed to be deposited in tidal estuarine, nonmarine, coastal plain, and shelf environments (Leckie and Rumpel 2003). Qishn Formation is divided into Qishn Clastic Member (Hauterivian–Middle Barremian) overlaid by Qishn Carbonate Member (Late Barremian to Aptian) (Fig. 4). Qishn Clastic Member is the main reservoir rock along the Say’un–Masilah basin, with an average porosity of about 19 % and an average permeability of about 413 mD (Hakimi et al. 2015). This member is composed mainly of alternating beds of sandstone and carbonates contaminated with low amount of shale and coal intercalations. Petroleum geologists divided the Qishn Clastic Member into three units, basal (S3), middle (S2), and upper (S1), based on the depth of the sandstone reservoir came across while drilling (Beydoun et al. 1993; King et al. 2003; Omran and Alareeq 2014; Lashin et al. 2016).

Qishn Carbonate Member is composed mainly of carbonate and shale sediments which deposited in a deep water environment, characterized by alternating opened and closed marine conditions (Beydoun et al. 1998). This member is considered as the cap rock that seals the oil migration from the deep-seated, thermally mature source rock of the Madbi Formation (Ahlbrandt 2002).

Data and methods

The detailed reservoir characterization of the present study was carried out by the integration of the seismic and well logs data. A set of 15 lines of 2D seismic data and well logging data of 6 wells were provided by the Data Bank Development project in Yemen for this study (Fig. 3).

The seismic data are post-stack seismic profiles of SEG-Y format, with a sample rate of about 1ms. The seismic lines cover an area of about 482 km2 with an average distance of about 900m between the seismic lines. Seismic lines were designated taking in consideration the orientation of the existence structural elements. Thus, 8 lines of the seismic data were oriented NW–SE direction, 4 lines to NE–SW direction, 2 lines near to E–W direction, and only one line to N–S direction (Fig. 3).

The logging data of six wells along Sharyoof field were used for the interpretation of the different lithologies and the determination of the formation well tops. The details of the types of well logging data along each well are represented in Table 1. Well log correlation was performed to illustrate any lateral variation in the lithologies along the field or any break in the continuity of the deposits.

Table 1 Available well logging data along Sharyoof field
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The workflow for this study began by creating synthetic seismogram from sonic and density logs with the available check shot data in order to tie the formation tops that were determined from the wells with their specific horizons along the seismic lines (White and Simm 2003). Several processes have been applied for tying the well log synthetic seismogram to seismic data (White and Hu 1998; Duchesne and Gaillot 2011). The most common steps for this technique are the following:

  1. 1-

    Calibrating the sonic log in order to adjust the sonic integrated times to match the smoothed check shot times.

  2. 2-

    Integrating the calibrated sonic log with the density log in order to determine the acoustic impedance (AI) which is required for defining the reflectivity coefficient.

  3. 3-

    Applying the deterministic wavelet extraction technique which includes using the Extended White wavelet extraction algorithm (White 1980) to resolve phase mismatches between the final processed seismic data and the created synthetic seismogram.

Tying the formations well tops with the seismic horizons was followed by the application of structural smoothing attribute on the 2D seismic lines of Sharyoof field in order to highlight the appearance of faults affecting the selected seismic reflectors in the cross-sectional view (Chopra and Marfurt 2005) (Fig. 5). Finally, a manual interpretation of faults and horizons picking were performed in order to produce the two-way time (TWT) structural contour maps expressed in milliseconds (ms) and represented the subsurface structures of the main formations along the Sharyoof field. These TWT maps were converted to depth structural contour maps by using domain conversion technique.

Fig. 5
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Seismic line 5, with wells SH-2 and SH-4 (profile location at Figure 2). a Seismic line before the application of any attributes. b The same seismic line after the application of the structural smoothing attribute to clarify the position of the faults

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The domain conversion technique allows the conversion of data from time domain to depth domain and vice versa. This technique has been used for decades in order to correlate seismic interpretation data with well data and perform volume calculations (Mott-Smith 1939; HUBRAL 1977; Al-Chalabi 1997; Cameron et al. 2006). Domain conversion can be performed by either direct time–depth conversion or by velocity modeling (Etris et al. 2001). The velocity modeling is the most common domain conversion technique used in the oil industry (Shelander et al. 2014) (Amaru et al. 2017) (Meisingset et al. 2018) (Ogbamikhumi and Aderibigbe 2019). Thus, the velocity model of this study was constructed by using the interval velocities assigned for each formation as a constant value and determined from the nearest local well. The interval velocity method allows spatial variation of velocity between well locations, and it gives reasonable details for the layers (Etris et al. 2001). Once a velocity model has been created, it can be used to convert TWT maps into depth maps.

The resultant depth structural contour maps were integrated with the interpreted well logs to build a 3D structural model for the Sharyoof field using Petrel software. The Petrel software uses the corner point gridding technique in modeling. This technique is divided into three sub-processes: (a) fault modeling for representing the faults in the field; (b) pillar gridding for producing the 3D grid based on the fault model; and (c) vertical layering for creating horizons, zones, and layers in the 3D grid. A set of operations were performed on the constructed 3D geocellular grid of the model to estimate quantitative properties for the faults like fault dips value and direction and fault throws (Elmahdy et al. 2020a).

Results and discussion

Well logging interpretation

Integrating seismic data and well logs helped in the determination of the subsurface geological characterization of Qishn reservoir along Sharyoof oil field. Well logging data of only three wells (SH-1, SH-2, and SH-4), which covered the Upper Cretaceous Sharwayn and Mukala formations, were used to determine their depths along Sharyoof field. Wireline logging interpretation of the all 6 wells defined the depths of Lower–Middle Cretaceous Fartaq, Harshiyat, Qishn, and Sa’ar formations (Fig. 6). The Qishn Formation was divided into Qishn Carbonate and Qishn Clastic members. It is composed mainly of sandstone sediments accompanied by alternating beds of siltstones, shales, and carbonates. The Qishn Carbonate Member contains red shale layer which was used as a marker bed for well correlation and seismic interpretations. The Lower Qishn Clastic Member was divided into S1, S2, and S3 based on the depths of the sandstone encountered in the member (Fig. 6).

Fig. 6
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Stratigraphic correlation between Sharyoof wells showing the main formations and members of the Sharyoof field

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Synthetic seismograms were estimated SH-4 well. Seismic well tying showed a great concurrency between the resulted synthetic seismogram and the seismic section (Fig. 7). Plotting the formations and members along the seismic sections resulted in determining the specific horizon of each formation, where a total number of 8 horizons could be detected representing the stratigraphic units of Sharwaya, Mukalla, Fartaq, Harshiyat, Qishn (including Qishn Carbonate, Red Shale Marker, and Qishn Clastic members), and Sa’ar formations (Fig. 8).

Fig. 8
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Stratigraphic correlation between SH-1, SH-2, and SH-4 showing the main members and layers of Qishn Formation along the Sharyoof field

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Fig. 7
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Created synthetic seismogram with the seismic line 5 (profile location at Figure 2) showing the formation tops tying between the well and the seismic horizons

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Fig. 9
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NW–SE (line 5) and N–S (line 15) seismic lines (profile location at Figure 2) showing the main faults and horizons along the Sharyoof field

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Fig. 10
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SW–NE (line 13) and W–E (line 14) seismic lines (profile location at Figure 2) showing the main horizons along the Sharyoof field

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Fig. 11
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Structure contour maps of 3 main horizons (Fartaq, Qishn carbonates, and Qishn clastics) in time and depth domains along the Sharyoof field

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Fig. 12
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Three-dimensional structural modeling representing the main faults and horizons of the Sharyoof field

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Seismic interpretation

Among the available 2D seismic lines, manual seismic interpretation was used to define the main structural elements of the Sharyoof field. In Qishn Carbonate Member, red shale layer is characterized by its strong seismic amplitude, which was used as seismic marker for interpreting the overlying and underlying horizons.

Fault interpretation of the 2D seismic data oriented to NW–SE and N–S directions resulted in the determination of 6 planar, gentle dipping normal faults, affecting the Cretaceous seismic horizons from Sharwaya to Sa’ar formations. Two faults (F2 and F3) are characterized by opposite dip directions, forming a major horst along the center of the area. Other faults have the same dip directions, forming a step-like fault structures (Fig. 9).

Seismic horizons along the 2D seismic lines oriented to NE–SW and E–W directions were characterized by their clear appearance, and they were easily interpreted to represent the syn- and post-rift Cretaceous sequence. Interpretation of these horizons showed a conformable extension of the whole stratigraphic units without a clear appearance of faults or discontinuities affecting these units (Fig. 10).

Picking of the seismic horizons resulted in the formation of 8 TWT structural contour maps related to Fartaq, Harshiyat, U. Qishn Carbonates, Red Shale, Qishn Clastics S1, Qishn Clastics S2, Qishn Clastics S3, and Sa’ar horizons (Fig. 11). These maps along the Sharyoof field represent the syn- to post-rift deposits which belong to the Late Jurassic to Early Cretaceous ages. The constructed velocity model was used in order to convert the resultant TWT maps into depth maps (Fig. 11).

3D structural modeling

A 3D structural model was constructed for the Sharyoof field covering an area of about 40 km2 and a thickness of about 1100 m (Fig. 12). The model was composed of the constructed 8 conformable maps and the interpreted six normal faults which considered as the main pillars of this model. Structural architecture of the Sharyoof field is represented by major horst structure along the central area of the Sharyoof field bounded with a step fault structure. These faults extended from the Early Cretaceous Sa’ar Formation until reaching Late Cretaceous Mukalla Formation. Quantitative analysis of these faults through the model showed that the faults had a NE–SW strike direction with an average length of about 3.4 km, average dip value of about 65°, and average fault throws of about 2.7 m.

According to the quantitative well logging analysis of the Sharyoof wells estimated by Lashin et al. (2016), S1 unit of the Qishn Clastics contain the main hydrocarbon reservoir along the Sharyoof field with an average porosity of about 15% and average oil saturation of about 65%. However, faults in the current study extended to the Late Cretaceous Mukalla Formation and were not restricted only to the Sa’ar–Qishn formations as mentioned in Lashin et al. (2016).

Faults extended along the Sharyoof field were initiated during the Infra-Cambrian as a part of the left lateral tear faults known as “Najd fault system.” The system was resulted from an E–W compression (Moore 1979) and gives rise to a left lateral, NW–SE–oriented faults (Giraud et al. 1986).

Faults of the Sharyoof field oriented NE–SW indicating a NW–SE extensional direction. This fault orientation aligned with the Early Cretaceous rifting between the African–Arabian and Madagascar–Indian plates. This Early Cretaceous rifting was followed by the Late Cretaceous rifting between the Indian and Madagascar plates which was resulted in the extension of the previous normal faults until reaching the Late Cretaceous Mukalla Formation.

It is worth mentioning that most of the producing wells along the Sharyoof field were drilled along the central zone of the field. Faults of the opposite dip directions resulted in the formation of a central horst which was the main structural trap along the field. The petroleum system along the Sharyoof field was represented by a reservoir rock contained in the Qishn Clastic Member and seal rock represented by the Qishn Carbonate Member. The type of hydrocarbon is marine type II (Al-Johi et al. 2019), and the main source rock along the field is the shales of the Lower Madbi Formation. The 3D structural modeling of the field shows extensional tectonic effects on all Cretaceous sequences from the Fartaq Formation to the Sa’ar Formation. The increase of the hydrocarbon productivity along the Sharyoof field requires drilling a new development well, which suggested being north–east the current wells, as the hydrocarbon potentiality is mainly concentrated along this area.

Conclusion

This paper represents the structural architecture of the Jurassic to Tertiary formations along the Sharyoof field. Wireline logging interpretation determined the depths of four main formations along the Sharyoof field (Fartaq, Harshiyat, Qishn, and Sa’ar formations). In addition, wireline logging interpretation aid in dividing the Qishn Formation into members that were based on their lithological content. Applying the structural smoothing attribute enhanced the visualization of faults along the 2D seismic lines. Seismic interpretation resulted in the determination of 6 NE–SE, planer, and normal faults that were extended along the whole area of the field. The normal faults along the Sharyoof field are characterized by gentle dipping with an average dip value about 65° and an average fault throws about 2.7 m. Most of these normal faults have an opposite dip directions forming a set of horsts and grabens. The Early Cretaceous rifting between the African–Arabian and Madagascar–Indian was the main responsible for the reactivation of the Najd fault system and the initiation of these NE–SW normal faults along the Sharyoof field. In addition, Oligocene–Eocene rifting of the Red Sea and Gulf of Aden were responsible for the extension of most of these faults to the Tertiary sections.